Eruption ceased in late January 2008; quiet continues in late 2009

The eruptive episode that began with the volcano reawakening in October 2004 (BGVN 29:09) ended in late January or early February 2008. The activity included explosions containing ash that rose up to ~ 3 km above the crater and lava dome growth. Sherrod and others (2008) provide a comprehensive discussion of the 2004-2006 portion of the eruption. This report spans 28 November 2007 through October 2009.

A GPS receiver on the W part of the active spine recorded continued SW advance at a rate of 3-4 mm per day during September through November 2007. During 28 November-4 December 2007, small inflation-deflation events occurred, which the USGS Cascades Volcano Observatory (CVO) interpreted as dome-growth pulses. On 31 December 2007 aerial observers saw a new small, snow-free spine on top of the active lobe.

On 25 January 2008, a steam plume rose from the dome slightly above the crater rim. Though seismicity had persisted at low levels through mid-February 2008, very few earthquakes were recorded after late January. Locatable earthquakes were fewer than one per day, all under M 2.0. Ground tilt measurements showed an overall subsidence in the area of the new dome. A GPS receiver on the previously active spine settled about 2 cm per day on a southward path. During February, the daily ground-tilt events stopped and gas emissions were barely detectable.

Comparison of photographs taken by remote cameras during late January to mid-February 2008 showed no evidence of extrusion. Cynthia Gardner (CVO), in a personal communication, noted that dome growth stopped in late January or early February (January 27 ± 10 days).

During March 2008, the most significant developments were a small, M 2.0 earthquake on 4 March and a very small earthquake swarm on 6 March. The latter started with a roughly M 1.2 event, followed by several smaller tremors over a seven-minute period. No tilt changes were associated with the swarm. On 14 March, the Pacific Northwest Seismic Network recorded four very small earthquakes located near the volcano. There were no tilt changes associated with this activity.

Radar imagery analyzed by Jet Propulsion Laboratory staff during late March 2008 showed that the E and W arms of Crater Glacier were touching, or close to touching, just N of the 1980s lava dome. From 30 May 2008 (figure 71) to 8 July 2008, the W arm of the glacier advanced ~ 20 m. By 8 July, the old and new lava domes in the crater were encircled by ice (figure 72). Further down slope glacier ice descended into the gullies that had been carved by erosion into the Pumice Plain. On 10 July, after nearly 5 months without signs of renewed activity, CVO lowered the Alert Level to Normal and the Aviation Color Code to Green.

Figure 71. Aerial view of the St. Helens crater, as seen from the N. The two arms of the Crater Glacier had by 30 May 2008 fully encircled the dome. USGS photograph by Steve Schilling.

Figure 72. Old and new lava domes (center and upper right respectively) in the St. Helens crater encircled by ice, as seen from the NW. USGS photograph taken on 5 August 2009 by Steve Schilling.

As of October 2009, earthquakes, volcanic gas emissions, and ground deformation had all fallen to levels observed prior to the onset of the eruption.

References. Lahusen, R.G., 2005, Acoustic flow monitor system?user manual: U.S. Geological Survey Open-File Report 02-429, 22 p.

Sherrod, D.R., Scott, W.E., and Stauffer, P.H., eds., 2008, A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006: U.S. Geological Survey, Professional Paper 1750, 856 p.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismic Network, University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

March 1980 (SEAN 05:03) Cite this Report

Steam-and-ash explosions follow earthquake swarm

After [more than] a week of local seismicity, Mt. St. Helens began to erupt steam and ash on 27 March, the first eruption in the contiguous USA since the 1914-17 activity of Lassen Peak, California. Steam-and-ash explosions and earthquakes were continuing on 6 April, but no fresh magma had reached the surface.

Seismic activity [began to increase by 16 March and stronger activity was initiated] on 20 March at 1548 by a M [4.2] earthquake centered [about 4 km below] Mt. St. Helens. Events of magnitudes 3.5 on 22 March, 3.4 on 23 March, and 4.2, 3.4, and 3.4 on 24 March punctuated smaller events that were occurring every few minutes by the 24th at depths of less than 5 km. USGS and University of Washington seismologists installed an array of portable seismographs. USGS personnel who flew over the volcano on 24 March saw no new snow-free patches or fumaroles, but observed a number of snow avalanches triggered by earthquakes. Seismicity became more vigorous on 25 March, when eleven earthquakes of M 3.4-3.8 were recorded, and continued to increase the next day, when fourteen shocks of M 3.4-4.0 accompanied numerous smaller events that continued to occur.

At 1236 on 27 March, a loud noise heard more than 15 km from the summit heralded the initial steam-and-ash explosion. Ash emission continued until a M 4.5 earthquake was recorded at 1401. The explosion prompted the evacuation of about 360 persons from [near] Mt. St. Helens. A team of USGS volcanologists was sent to monitor the volcano. Helicopter reconnaissance revealed a new crater 60-75 m in diameter and about 50 m deep, located within the summit crater [that predated] the previous eruption, in 1857. A second explosion occurred at about 0300 the next morning, producing an ash-laden cloud that reached 2 km above the summit and a non-incandescent ash avalanche that flowed down the NW flank. Shortly thereafter, a steam plume rose to more than 3 km height. Aerial observers reported two nested arcuate fissures [first seen the afternoon of 27 March], one about 5 km long, the other about 1.5 km long, trending approximately E-W across the summit, S of and above the new crater.

By dawn on 28 March, pulses of dark, dense ash were rising to 3 km above the summit and some blocks were being ejected. Small mudflows moved in pulses and surges down the NE flank, reaching timberline by midafternoon. Occasional periods of rhythmic ash emission, lasting 45 minutes to l hour, occurred through the day, and more low-temperature ash avalanches traveled down the N and NE slopes. Many seismic events occurred, including one of M 4.2, concentrated in a zone about 2 km deep in the volcano's NW quadrant. That evening, the water level in Swift Lake Reservoir (figure 1) was lowered by at least 8 m as a precautionary measure, to accommodate any eruption-induced snowmelt runoff or mudflows.

Figure 1. Sketch map of Mt. St. Helens. The positions and lengths of the arcuate summit fissures are approximate, as indicated by question marks at each end. The old summit crater is shown as larger than its actual diameter of about 0.5 km. The two active vents have coalesced at the surface, but maintain their separate identities. Courtesy of Robert Christiansen and Robert Tilling.

A new 30- to 50-m-diameter crater, about 10 m from the one formed [on 27 March], was discovered during overflights 29 March. Blue flame was observed in the vents, at times flickering and jumping from one crater to the other. No strong ash pulses were reported between explosions on 28 March at 2300 and 30 March at 0410. During this period, seismic events appeared to migrate to the SSE along a 25 km-long linear trend, extending from 2 km depth in the volcano's NW quadrant to 15-20 km depth below Swift Reservoir, at the S foot of Mt. St. Helens. However, continuing analysis of the seismic data indicated that the apparent migration may be an artifact of data reduction and the crustal model used. Refinement of epicenter determinations is in progress.

Strong activity resumed on 30 March. At 0740 an anvil-shaped steam and ash cloud grew, producing ashfall as far away as Bend, Oregon, about 250 km to the S. The cloud could be seen on high-resolution NOAA weather satellite imagery, but the altitude of its top could not be estimated from the satellite data. An AP photo, probably of this explosion, clearly shows [an ash veil] moving most of the way down the SE flank. [D. Swanson notes that this phenomenon, described as a "sizeable ash avalanche" in the original report, was in fact only a veil of ash that moved slowly by gravity and deposited very little material.] Six more explosions projected ash to more than 1.5 km above the summit on the 30th.

A wind shift on 31 March sent ash from the continuing explosions W of the volcano onto more populated areas. Ashfall began about noon in the Kelso-Longview area (population 75,000), about 65 km W, leaving a thin layer of light-colored, abrasive material. Only minor venting occurred during the night of 31 March-1 April; three earthquakes of M 4.5-4.7 were recorded, with foci only 1 km beneath the summit, but the number of events was declining. Ash from a large explosion on 1 April was collected in Spokane, 500 km to the E.

Harmonic tremor was recorded for the first time from 1925 to 1930 on 1 April. This brief tremor episode was weak and poorly-defined, but vigorous, high-amplitude tremor, registered on nearly all seismic stations in W Washington, lasted from 1940 to 1955 on 2 April. Two bursts of harmonic tremor were recorded the next day, from 1840 to 1900 and 2100 to 2111; eleven individual earthquakes took place between the two tremor episodes on 3 April and a larger event occurred at 2115. Additional bursts of harmonic tremor occurred on 4 April from 0759 to 0826 and 1110 to 1142, and on 5 April between 1112 and 1130. Steam-and-ash emission continued intermittently through 6 April. Despite the bursts of tremor — evidence of magma movement within [or beneath] the volcano — there has been no evidence of juvenile material in any of the ejecta studied. To date, analysis of tilt and gravity data from the N flank has yielded no statistically significant deformation trends.

Information Contacts: D. Mullineaux, USGS, Denver CO; R. Christiansen, USGS, Menlo Park, CA; S. Malone, R. Crosson, E. Endo, Univ. of Washington; R. Tilling, USGS, Reston, VA.

April 1980 (SEAN 05:04) Cite this Report

Major uplift on N flank; explosions end

Numerous debris flows descended the N and E flanks early in the eruption, reaching nearly to timberline in some areas. These were composed of ice blocks and sparse blocks of rock in a matrix of dark-colored mixed ash and snow. Non-incandescent ash avalanches occasionally flowed down the flanks during the early part of the eruption.

By 8 April, the two initially separate active vents had coalesced into a single crater (figure 2), at least 500 m long x 350 m wide. The crater had deepened to 300 m by 12 April and sometimes contained small quantities of water or ice. Most mid-April eruption clouds were small and consisted primarily of vapor, but ash-rich clouds were occasionally ejected to 1 km or more above the summit. Large ice blocks were sometimes included in the ejecta. The tiltmeter at Timberline, about 4 km NE of the summit, recorded episodes of deformation of as much as a few tens of µrad in a few minutes to a few hours but these episodes oscillated between uplift and subsidence, resulting in little or no net tilt. Periods of uplift appeared to precede explosions, which were followed by subsidence. Explosions decreased in size and ash content through mid-April, with the last confirmed ejection of ash on 22 April. No fresh magma has been identified in any of the ejecta. Episodic explosions were replaced by continuous steaming, which was continuing in early May.

Figure 2. USGS sketch map showing the summit and upper flanks of Mt. St. Helens. The N-flank uplift is delineated by contours with an interval of 80 feet (about 25 m), generated by measuring differences in elevation shown on airphotos taken in August 1979 and April 1980. The boundary faults that define the summit graben wrap around the N-flank uplift. Within the graben the active crater is shown by a hachured line. Glaciers are represented by dashed lines.

A comparison by James Moore of the 1952 topographic map of Mt. St. Helens with airphotos taken 7 and 12 April 1980 indicated that a substantial net uplift had taken place on the upper N flank. Cartographers from the USGS National Mapping Division comparing August 1979 airphotos with the April 1980 sets confirmed Moore's findings, delineating a feature with an area of nearly 4 km² that had risen at least 25 m and had a maximum uplift of about 100 m. [D. Swanson notes that the "uplift" was more apparent than real. It resulted from horizontal displacement of high terrain over low terrain, not vertical displacement of the low terrain.] Subsequent geodimeter readings showed that one area of the uplift had [moved outward] as much as 6 m 24-29 April.

The upper portion of Forsyth Glacier has been extensively distended and cracked by the uplift, and USGS personnel warned of the danger of a major avalanche down the N or NE flank. As a result, Washington's Governor Ray ordered the entire Spirit Lake area N and NE of the volcano, plus the zone immediately W and S of the volcano, closed to all but government workers and scientists. Access to a second zone, extending 8 km beyond the inner zone, was allowed to landowners during daylight hours only. About 300 loggers and 60 permanent residents of the area around Spirit Lake had been evacuated earlier, just after the initial explosions in late March.

The arcuate fissures described last month defined a graben extending more than 3 km through the summit, including the active crater. Both ends of the graben died out as gentle sags after wrapping around the uplift just to the N. The graben contained a median fault, E of and trending toward the active crater. This fault was downdropped on the N side by an amount estimated by Hopson and Melson at less than 30 m. Total subsidence of the graben was difficult to estimate because of its proximity to the uplift.

Although the number of local earthquakes has fluctuated (figure 3), total seismic energy release has remained relatively constant through the eruption. Refinement of the crustal model used to locate the earthquakes has clustered the hypocenters beneath the volcano, 1-10 km below the summit. No tremor has been recorded since 12 April.

Figure 3. Number of earthquakes per day in the Mt. St. Helens area with magnitudes > 3.4 (upper line) and > 4.0 (lower line, with area below shaded for emphasis) recorded at the Newport Observatory (360 km from the volcano) 20 March-5 May 1980. Short vertical lines above the earthquake graph indicate one or more periods of harmonic tremor on that day. Data provided by Robert Tilling and the Newport Observatory.

The USGS continues to monitor the activity in cooperation with the University of Washington, the United States Forest Service (USFS), and others. Numerous instruments, including seismographs, tiltmeters, and gravimeters have been placed around the volcano (figure 4), supplementing continuing visual observations from ground stations and aircraft.

Figure 4. Monitoring equipment around Mt. St. Helens as of mid-April 1980. Data provided by Robert Tilling.

Information Contacts: J. Moore and R. Christiansen, USGS, Menlo Park, CA; D. Mullineaux and D. Crandell, USGS, Denver, CO; R. Tilling, USGS, Reston, VA; C. Hopson, Univ. of California, Santa Barbara; W. Melson, SI; S. Malone, R. Crosson, and E. Endo, Univ. of Washington; Newport Geophysical Observatory.

May 1980 (SEAN 05:05) Cite this Report

Major eruption sends cloud to 23 km, destroys summit, and devastates region

A major eruption destroyed the summit of Mt. St. Helens, projected ash into the stratosphere, devastated the N and NW flanks, and killed dozens of people on 18 May. The initial explosion was heard more than 350 km away. Substantial ashfalls occurred hundreds of kilometers downwind, closing roads, schools, and businesses, and threatening crops in the NW USA.

Explosions similar to those of early to mid-April resumed [7] May and continued until 14 May. Several tens of earthquakes per day of M 3 or greater continued to be recorded through 17 May. Total seismic energy release remained relatively constant through late April, then declined slightly.

The USGS began daily [geodetic] measurements on the periphery of the N flank bulge on 25 April, recording consistent outward [displacement] of 1.5-2 m/day through 17 May (figure 5). The direction of movement was nearly horizontal, to the NNW.

Figure 5. Cumulative outward movement of four different points on the N flank bulge of Mt. St. Helens, 23 April-18 May 1980. Courtesy of Robert Tilling. [Originally from SEAN 05:06.]

18 May eruption. Much of the information on the 18 May eruption is from Robert Christiansen. His detailed narrative of the eruption will appear in the news section of Nature, [v. 285, p. 531-533.]

At 0832 on 18 May, seismographs recorded an earthquake of about M 5 (its unusual wave characteristics prevented a straightforward magnitude calculation). A remarkable series of photographs by Vern Hodgson shows the entire N-flank bulge immediately began to separate from the volcano along a fissure that opened across its upper section. The bulge quickly formed a massive avalanche that raced downslope, displaced the water of Spirit Lake, and struck a ridge about 8 km to the N. Most of the avalanche material then turned W and flowed down the N fork of the Toutle River (the outlet of Spirit Lake).

. . . The mudflows destroyed 123 homes and most of the bridges crossing the Toutle River for tens of kilometers downstream, then continued down the Cowlitz River into the Columbia, where suspended sediment, logs, and other debris filled the ship channel, stranding many vessels in Portland harbor.

A powerful laterally-directed blast emerged from the area formerly occupied by the bulge and overtook the avalanche within seconds. The blast, carrying lithic ash and lapilli, devastated a zone extending 30 km E-W and more than 20 km outward from the volcano in an arc encompassing almost 180° of the N flank (figure 6). Destruction was virtually total in an inner zone nearly 10 km wide, where no trees remained in the previously thickly-forested area. Beyond the inner zone, all trees were blown to the ground, pointing outward from the source of the blast in a nearly uniform radial pattern. In the outer few hundred meters of the blast area, trees were seared but remained standing.

Figure 6. USGS sketch map of Mt. St. Helens. The area devastated by the 18 May 1980 directed blast is shown schematically by sketching radial tree blowdown. The deposit left by the avalanching N flank is stippled.

Almost simultaneously with the ejection of the lateral blast, a large vertical cloud rose rapidly from the pre-existing summit crater to more than 19 km above sea level (as measured by Portland airport's weather radar), passing through an unusually high tropopause at 13.5-14 km (figure 7). Vigorous feeding of the vertical column continued for more than 9 hours, before declining gradually during the late afternoon. Ash clouds moved rapidly NE and E. Large quantities of ash fell on a wide area of Washington, N Idaho, and W and central Montana. Ashfall at Ritzville, Washington, more than 300 km from Mt. St. Helens, totaled at least 7 cm (figure 8). In Spokane, 500 km NE of the volcano, visibility was briefly reduced to only 3 m at about 1500. A trace of ash fell in Denver about noon the next day, and USGS hydrologists detected slight ashfall in parts of Oklahoma.

Figure 7. Oblique airphoto showing Mt. St. Helens erupting at about 1130 on 18 May 1980. View is approximately to the N. Courtesy of Austin Post, USGS.

Figure 8. Isopach map of ashfall from the 18 May 1980 eruption of Mt. St. Helens, prepared from data provided by Albert Eggers and the USGS. Thicknesses are in centimeters.

Pyroclastic flows, generated both by collapse of the vertical column and direct emission through the large northward breach produced by the directed blast, left a fan-shaped pumiceous deposit extending [into] Spirit Lake [and] the Toutle River, overlying debris flow deposits in that area.

Ash cloud. NOAA weather satellite imagery clearly recorded the rise and dissemination of ash clouds, and at least two distinct major pulses of ash ejection (figure 9). Measurements from the images showed that the cloud from the main explosion initially expanded in all directions, with the bulk of the ejecta moving E. [Measurements from satellite images indicated that the rate of horizontal advance of the cloud front averaged 250 km/hr for the first 13 minutes after the eruption's onset. Horizontal velocity soon decreased, remaining at about 100 km/hr for the first 1,000 km of its dispersal to the ENE.] Portland airport reported wind speeds of only 120 km/hr toward the E at 12 km altitude. The second pulse could be seen on the image returned at 1215. [From an aircraft, D. Swanson observed that] the color of the column [gradually] changed from dark gray to pale gray [between about 1200 and 1220].

Figure 9. Mosaic of four images from NOAA's GOES West geostationary weather satellite, in orbit over the equator at 135°W. All of Washington and Oregon, most of Idaho and parts of Montana, California, Nevada and S Canada are shown. Image times (all 18 May 1980) are: upper left, 0845 (13 minutes after the eruption began), upper right, 0915, lower left, 0945, lower right, 1315. The first three images show the rapid dissemination of the Mt. St. Helens ash cloud after the initial explosion. The last image shows the eruption column from the second major burst, about 1 hour after it was ejected. Ash from earlier activity had spread to the Idaho-Montana border. Courtesy of Arthur Krueger and Andy Horvitz.

Ash was widely dispersed in the atmosphere because of varying wind directions at different elevations (figure 10). Murray Mitchell reported that ash had made a complete circuit of the globe by 29 May. Most of the tropospheric material had fallen out by mid-June, but a diffuse dust veil remained in the stratosphere from the latitude of Mt. St. Helens N to the polar region. Bernard Mendonça reported that as of 9 June, NOAA's solar radiation and lidar equipment in Hawaii had detected no St. Helens ejecta. Seasonal arctic haze precluded observations from the Barrow, Alaska station. Stratospheric circulation patterns make aerosol movement to the S very unlikely before autumn.

Figure 10. Paths traveled over North America by 18 May 1980 Mt. St. Helens ash at 3, 9, 16, and 18 km altitudes. Tick marks along each line show position of ash cloud front at that altitude every 24 hours, at noon GMT (0550 local time at Mt. St. Helens). The date at each tick mark is indicated. For clarity, the 18 km path is shown as a dashed line. The 21 May cloud from at 9 km altitude is just off the map to the E. Data provided by NOAA's Air Resources Laboratory.

High-altitude studies of the eruption cloud were carried out using aircraft from several NASA installations, including the Ames and Langley research centers, LASL and NCAR. The pilot of the NASA Ames aircraft saw ash at nearly 23 km altitude while flying E of the volcano 18 May. NASA's SAGE satellite measured particle densities over S Canada on 22 May and the United States 23-27 May. "Ground truth" for the SAGE measurements was gathered by balloon from the University of Wyoming and other locations. Most data from these studies have not yet been analyzed. However, Grant Heiken reported that preliminary results from the LASL aircraft, sampling at about 15 km, show recovery of 1-11 µm particles (the bulk of which were 3-4 µm) that were virtually all glass shards. William Smith will gather a series of reports for the Upper Atmospheric Programs Bulletin, published jointly by the FAA and NASA.

Pollution-monitoring equipment operated by the Alexandria Virginia Health Department collected an unusual quantity of particulate matter (now being analyzed) during a rainstorm late 20 May. The 20 May rain was unusually acidic for that area, with a pH of 4-4.5, and occurred as Mt. St. Helens ash passed overhead.

Because of poor weather conditions, few brilliant sunrises or sunsets were reported in the United States after the 18 May eruption, although Grant Heiken was awakened by a gaudy, blood-red sunrise in Los Alamos, New Mexico. Charles Van Zant observed a ring around the sun from Cancún, Mexico (on the Yucatán Peninsula) at about noon on 23 May. The ring filled about 1/4 of the sky and was rainbow-colored at its edge.

Energetics. The acoustic pressure wave produced by the initial 18 May explosion was recorded on microbarographs operated by NOAA in Boulder, Colorado and Washington, DC, and on infrasound equipment operated by William Donn at the Lamont-Doherty Geological Observatory N of New York City. The waves recorded on these instruments were comparable to those generated by previous 10 megaton nuclear tests. Wave frequencies were very low, about 1 cycle per 5 minutes (0.003 Hz).

Morphologic changes. The [debris avalanche], lateral blast, and vertical explosion created a crater, breached to the N, with a N-S dimension of about 3 km and an E-W dimension of about 1.5 km (somewhat wider at the base of the breach). The summit was destroyed. The maximum elevation of the volcano, on the crater rim, was about 350 m less than the previous summit altitude of 2,975 m. The lower end of the breach extended downward nearly to the 1,500 m level.

Volume. Volume calculations for the eruption were very preliminary, based on the size of the new crater and the amount of ash deposited. Most estimates are in the 1-2 km³ range, but some are as high as 4 km³. Comparison with previous eruptions in the VRF indicated that explosions of this size occur only about once a decade.

Socioeconomic effects. At least 24 people are known to have been killed by the 18 May explosion, most by the avalanche and directed blast. Thirteen others known to have been in the area of maximum devastation at the time of the explosion are missing and presumed dead, including USGS geologist David Johnston (obituary at the end of this report). In addition, 37 persons believed to have been near the volcano on 18 May remain unaccounted for, bringing the probable death toll to 74. [Later estimates yield lower death tolls as the number of missing declined; the most recent data cited by Blong (1984) total 57 deaths.] State of Washington officials estimated the financial losses to private enterprise and state and local government to be at least $2.7 billion. [Blong (1984) tabulates about $1.5 billion in losses and cleanup costs.]

The following is from a report by R.J. Blong, who visited Yakima, Washington (140 km ENE of the volcano) 4 weeks after the 18 May explosion to study the socioeconomic effects of the tephra fall.

"Ashfall at Yakima from the 18 May eruption amounted to 12-18 mm. Automatic street lighting systems came on about 1115 and did not switch off until the following morning, although there was some lightening of the sky just before dusk. No deaths can be directly attributed to the ashfall. Two people died from cardio-pulmonary disease during the clean-up operation. Some people experienced headaches and gastroenteritis the day after exposure to the ash, the symptoms recurring in some of those involved in the cleanup. This may have been a stress reaction, but a virus with similar symptoms was prevalent in the area before the tephra fall. The cleanup operation created a great deal of camaraderie but some hostility developed where cars sped through streets stirring up the ash and where some roofs remained uncleaned. Anxiety but not depression developed.

"The Yakima airport and the airspace above was closed for 7 days. Twenty thousand tons of ash were removed from the 40 hectares of hard surface area by a team of up to 150 people working with 40 pieces of earthmoving equipment. The airport cleanup cost about $75,000 for equipment hire, labor (including overtime), and fuel.

"Pacific Power and Light experienced no problems meeting the peak demand when the darkness fell, probably because it was a warm day and a Sunday. Four or five older-style transformers caught fire. Subsequently, during a rainfall, several poles caught fire either as a result of lightning strikes or from shorting out by ash across contacts. Generally the wind removed the tephra from poles, insulators and transformers and there was less trouble than anticipated by the company.

"Pacific Northwest Bell telephone service experienced an unprecedented demand. The toll network was designed to handle with little delay the busiest days (Mothers Day and Christmas) but there were 70% more attempts to make calls than occurred on Mothers Day, the previous Sunday. The company also had to take steps to keep the tephra out of the electro-mechanical system. Maintenance on public phones has doubled since the tephra fall whereas no increase in maintenance has been necessary for semi-public phones.

"The cost of cleaning the ash from Yakima's approximately 350 km of streets has been estimated at $2-4 million. Downtown businesses suffered a serious loss of revenue through being closed for up to a week. The Greater Yakima Chamber of Commerce estimated on 30 May the cost of the ashfall at $95 million, of which equipment damage and automotive maintenance and repair amounted to $42 million. Motel occupancy rates were down from 80-90% to less than 50%. The tourist and convention industry will continue to suffer severely unless steps are taken to assure townsfolk and potential visitors alike that 20 years of Mt. St. Helens activity does not mean 20 years of 18 May and its aftermath."

Petrology. The following preliminary petrologic data are from William Melson.

"An early dark-colored and later light-colored tephra layer have been noted in airfall from the 18 May explosions (R. Kienle, personal communication). Near the volcano, the upper layer contains large essential ejecta of pumice. These products have been analyzed on the electron microprobe. Pumice from three separate localities provided by R. Kienle and Bruce Nolf have essentially identical compositions with the following average: SiO₂ = 63.35, Al₂O₃ = 18.38, FeOT = 3.87, MgO = 1.83, CaO = 5.22, K₂O = 2.01, Na₂O = 4.31, TiO₂ = 0.37, P₂O5 = 0.09, Cl = 0.1-0.2, F and S < 0.05%. Analyses of glass inclusions in plagioclase typically have consistently low sums, averaging about 93%, suggesting about 7% dissolved H₂O. Cl in these is typically around 0.15% and F and S less than 0.05%. The dominant phenocryst in the pumice is plagioclase (average An 47-70), which makes up about 35% by weight. Accessory phases, in order of abundance, are hornblende, hypersthene (Fs 35), and subequal amounts of titan-magnetite and ilmenite.

"Tephra from the lower dark layer of the 18 May explosion layer is similar in composition to the pre-18 May tephra, reflecting the presence of material derived largely from the explosive destruction of the central part of the cone. The upper, lighter-colored tephra contains much more pumiceous material, visible megascopically and revealed in bulk analyses, reflecting, finally, the breaching of hornblende hypersthene dacite magma. Bulk analyses of the light-colored tephra show an increasing concentration of glass with distance from the volcano.

"Pumiceous tephra from the 25 May explosion contains accessory augite, according to C. A. Hopson, a phase absent or extremely rare in the 18 May pumice. It appears that the explosions are successively tapping deeper, less water-rich portions of a zoned magma chamber."

Seismicity. Preliminary analysis of seismic and deformation data indicates that there was no immediate warning of the imminence of a large explosion. After the M 5.0 earthquake that apparently triggered the eruption, a brief period of harmonic tremor was recorded, followed by the absence of any earthquakes with magnitudes greater than 2 until about 1145. Seismic activity increased rapidly after 1145, and almost continuous M 3.5-4 seismicity was recorded from 1400 to 1630 at the USGS Newport Observatory. After 1630, seismicity declined.

Tilt. Records of the only surviving tiltmeter, on the S flank, show that rapid inflation began at the same time as the explosion at 0832. Rapid inflation lasted only 10 minutes, succeeded by deflation that continued until about 1630. Moderate inflation then began and has continued.

Post-18 May activity. Eruptive activity declined after 18 May, and by the 21st was limited to episodic ejections from the crater, mostly of vapor. Large fumaroles and secondary explosions were generated from the debris flow deposit, occasionally producing columns of material as high as 2 km. Between 19 and 24 May, only a few earthquakes with magnitudes greater than 3 were recorded, in contrast to the several tens of events that had occurred each day since late March (figure 11). However, harmonic tremor began during that period (exact date not [reported]).

Figure 11. Number of seismic events per hour with magnitudes > 3.2, 20 March-28 May 1980 at Mt. St. Helens. Courtesy of Robert Tilling.

At 0232 on 25 May the amplitude of harmonic tremor began to increase. Within minutes, an ash-rich eruption column had been seen from a surveillance aircraft. By 0245, NWS radar at Portland recorded the top of the column at nearly 14 km. A swarm of small earthquakes, centered about 8 km below the volcano, began at 0249 and continued at a rate of 1-2/hr.

The density of ash in the eruption column started to decrease within 5 minutes, and the height of the column was declining within the first hour. Winds were quite variable, but much of the ash blew toward the W half of the compass. By 0600, ash was falling in the Portland-Vancouver area (80 km SW). Ashfall darkened the early morning in the Kelso-Longview area (55 km W) and the ash cloud extended as far as the Olympic Peninsula of NW Washington. Heavy rain during the eruption mixed with the ash to drop mud on much of the affected region. Many airports were closed and ground travel was difficult.

By 0800, harmonic tremor amplitude had declined and the earthquake swarm had begun to subside. However, the eruption continued through most of the day, with the altitude of the top of the column ranging from 4 to 6 km. The eruption declined during the evening, and activity was limited to emission of steam clouds containing varying amounts of ash by 0100 the following morning.

Most of the tephra ejected 25 May was juvenile material. Some pyroclastic-flow deposits were emplaced on the N flank. H.H. Lamb's preliminary estimate for the total Dust Veil Index from the 18 and 25 May eruptions is 600-1500.

The ash content of the plume declined during the next several days, and by late 28 May, the plume was entirely composed of vapor. Small incandescent areas were seen on the crater floor during the night of 28-29 May and on several occasions thereafter. Careful inspection showed that the incandescence was caused by the heating of parts of the crater floor by venting gases, not the presence of magma at the surface. Vigorous steaming continued through early June, with vapor rising to about 3.5 km altitude. SO₂ content of the plume continued to be an order of magnitude greater than before the 18 May eruption. An L-shaped lake about l km across was observed in the crater 10 June [but see SEAN 05:06], away from the area of active steam venting. Harmonic tremor continued, at varying amplitudes, through early June, but earthquake activity remained at very low levels.

At press time, a third large explosion occurred. An Eastern Airlines pilot observed the ejection of a dense ash column at 2045 on 12 June. The cloud blew S and SW, dropping marble-sized tephra on Cougar (18 km SW of the pre-18 May summit). Ashfall began in Portland (about 80 km SW) by 2300, and more than 0.5 cm accumulated. Portland airport radar recorded pulsating echoes to altitudes of 10.6 to nearly 17 km. New bursts of ash were observed at about 2-minute intervals from a USFS monitoring aircraft. An earthquake of about M 4.0 was recorded by University of Washington seismographs at 2110. About 1,500 people were evacuated, without injury, from a designated danger zone within about 30 km of the volcano. Ash and accompanying rain made roads in NW Oregon and SW Washington muddy and treacherous. Portland airport was closed. After this explosion, on 15 June, the presence of a growing lava dome in the center of the crater was confirmed by the USGS.

Historical activity. Mt. St. Helens was last active between 1831 and 1857, when a series of eruptions were separated by intervals of up to 7 years. Most were small explosive events, and none approached the size of the 18 May activity. Crandell and Mullineaux (1978) describe a 4,000-year old eruption, [considerably greater than] that of 18 May, in their definitive paper. Pyroclastic flows and mudflows extended more than 30 km down the Toutle River, and more than 20 cm of tephra fell tens of kilometers NE of the volcano.

Very few volcanologists throughout history have lost their lives by eruption, but last year Robin Cooke and Elias Ravian were killed at Karkar and now we must report the death of David Johnston at Mt. St. Helens. At the time of the 18 May eruption, Dave was monitoring the volcano from a position just 8 km NNW of the summit. No one knew better than Dave the risk involved in his St. Helens work, and no one contributed more to the understanding of this volcano's eruptive mechanisms. Although only 30 years old, his PhD work on St. Augustine, and subsequent work with the USGS had already established his position among the leading young volcanologists in the world. His enthusiasm and warmth will be missed at least as much as will his scientific strength.

Further Reference. Blong, R.J., 1984, Volcanic Hazards: A Sourcebook on the Effects of Eruptions: Academic Press, Sydney, 424 p. [See also references following SEAN 05:12.]

Information Contacts: R. Christiansen, USGS, Menlo Park, CA; R. Tilling, USGS, Reston, VA; D. Mullineaux, D. Crandell, USGS, Denver, CO; A. Krueger, NOAA/NESS; M.P. McCormick, NASA Langley Research Center; W.S. Smith, FAA; G. Heiken, LANL; M. Mitchell, NOAA, Silver Spring, MD; R. Dalton, Alexandria, VA Health Dept.; B. Mendonça, L. Machta, NOAA/ARL; W. Donn, Lamont-Doherty Geological Observatory; C. Hopson, Univ. of California, Santa Barbara; B. Nolf, Central Oregon Community College; R. Blong, MacQuarie Univ.; A. Eggers, Univ. of Puget Sound; D. Dzurisin, HVO, HI; H.H. Lamb, Univ. of East Anglia; W.G. Melson, SI; S. Malone, R. Crosson, E. Endo, Univ. of Washington; Newport Geophysical Observatory; C. Van Zant, Richardson, TX; UPI, New York Times.

June 1980 (SEAN 05:06) Cite this Report

12 June explosion, then lava dome extrusion

After the 12 June explosion, a lava dome began to grow in the crater, accompanied by occasional weak bursts of harmonic tremor. No significant explosions have taken place since the start of dome emplacement, nor has the dome growth generated any nuées ardentes.

Richard Janda calculated that about 2 km³ of debris was emplaced in the N fork of the Toutle River and Spirit Lake by the 18 May eruption. Richard Waitt estimated the total 18 May airfall component as 1.1 km³ of unconsolidated material, with a mean bulk density of 0.5 g/cm³. Poor weather has prevented the aerial photography necessary for the production of a topographic map of the post-18 May volcano. After a map is produced, the amount of material removed from the edifice on 18 May can be accurately determined.

After the 25 May eruption most of the few individual earthquakes recorded on local seismographs were centered 10-15 km N of Mt. St. Helens and were thought likely to be related to regional crustal stresses rather than volcanic activity (figure 11). Harmonic tremor continued, increasing in amplitude early 3 June, but dropping to a very low level by 5 June before ending shortly thereafter. A persistent steam column rose from the crater to altitudes of 3-4.5 km in early June. A small amount of ash was occasionally included in the lower portion of the plume. Improved visibility on 10 June revealed that a roughly crescent-shaped lake, about 300 m long and 100 m wide, had formed on a portion of the [S] part of the crater floor within the past 5 days. Evidence of recent rock avalanches from the crater walls was also visible.

At about 1300 on 12 June, Steven Malone noted the onset of weak harmonic tremor and an increase in small discrete events on seismographs operated by the University of Washington USGS. A marked increase in harmonic tremor amplitude occurred at 1905 and an eruption plume rising to 4 km altitude was observed 5 minutes later. Portland airport radar tracked the eruption column to 11 km altitude shortly thereafter. Eruptive activity fluctuated in intensity for the next 2 hours. Harmonic tremor continued at roughly the same amplitude until about 2000, when it declined to a low level. A dramatic increase in tremor amplitude at 2111 was accompanied by an eruption column that quickly reached almost 16 km altitude, producing pulsating echoes on Portland airport radar. An explosion was heard more than 200 km away in Toledo, Oregon. Activity remained vigorous until about midnight, when cloud height and tremor amplitude began to decline. By 0200, the altitude of the top of the plume had dropped to about 4.5 km.

Infrared images returned by NOAA's GOES West weather satellite show the eruption cloud drifting SW. Marble-sized pumice and ash fell on Cougar accompanied by a strong sulfur odor. Ashfall in Portland began about 2300, reaching accumulations of around 0.5 cm. Infrared imagery shows that the cloud reached the Pacific coast of N Oregon by 2330 and continued to drift SW for several more hours, before becoming impossible to distinguish from heavy weather clouds in the area. Ash fell on more than 11,000 km² of NW Oregon and SW Washington, reaching Lake Oswego, Oregon, 110 km SW of Mt. St. Helens.

About 1,500 people were evacuated from a zone within about 30 km of Mt. St. Helens. No deaths or injuries were reported near the volcano. Portland airport was closed for several hours during the night, but few flights were affected. Rain before and after the ashfall in the Portland area turned the ash into a slippery mud, as it did to much of the 25 May ash. However, blowing ash continued to cause occasional disruptions to travel more than a week after the eruption.

Several pumice and ash flows were emplaced on the N flank by the 12 June eruption, one stopping roughly 20 m from the shore of Spirit Lake, which was still partially covered by floating debris. The deposits ranged from 2-10 m in thickness, and internal temperatures as high as 600°C were measured.

The explosions of 25 May and 12 June were of similar size. Both sent most of their ash toward the heavily populated areas W of the Cascade Range, although analysis of wind directions at various altitudes indicates that prevailing winds blow toward the E half of the compass almost 90% of the time (Crandell and Mullineaux, 1978).

On 15 June, USGS personnel confirmed the presence of an active lava dome, roughly 200 m in diameter and 40 m high, at the bottom of the crater. Water vapor and other gases rose from a shallow lake surrounding the dome. Red glow was visible at night through cracks in the dome's surface. Poor weather often made observations of the dome and its rate of growth difficult. Clear weather on 18 and 19 June revealed that the dome was growing upward about 6 m/day, reaching a height of 65 m by the 19th. Harmonic tremor had stopped by 15 June and did not resume until two episodes of very weak tremor, lasting 30 and 65 minutes, were recorded during the night of 24-25 June. Sporadic tremor continued through late June, but very few discrete earthquakes were recorded. Clear visibility 28-29 June indicated that no vertical dome growth had occurred since 19 June. The dome was about 200 m in diameter and was crossed by a 12 m-long fissure in which cherry red incandescence could be seen.

Information Contacts: R. Christiansen, D. Peterson, R. Waitt, USGS, Menlo Park, CA; R. Tilling, USGS, Reston, VA; D. Mullineaux, D. Crandell, USGS, Denver, CO; R. Janda, USGS, Tacoma, WA; A. Krueger, NOAA/NESS; S. Malone, R. Crosson, E. Endo, Univ. of Washington; UPI; AP; New York Times.

July 1980 (SEAN 05:07) Cite this Report

Explosions destroy lava dome; 18 May cloud detected over Europe and Asia

An eruption similar to those of 25 May and 12 June destroyed the June dome on 22 July. After a slightly smaller eruption on 7 August, growth of a new dome began.

Activity was weak from late June through mid-July. A vapor plume rose to a maximum of 3.5-4 km altitude. SO₂ emission, measured by remote sensing equipment, ranged from 1,800 to 2,600 t/d. No lava-dome growth was detected.

After several episodes of harmonic tremor on 27 June, weaker than the tremor preceding any of the eruptions, seismicity recorded by the USGS-University of Washington seismic net was limited to a few small shallow earthquakes per day. In an 11-minute period on 6 July, four earthquakes of M 2.0-3.2 occurred about 8 km below Marble Mountain, 14 km SE of Mt. St. Helens. The first of these shocks was recorded only 17 minutes after the start of an earthquake swarm beneath Mt. Hood, 100 km SSE of Mt. St. Helens (see Hood section). Six events of M 1.4-2.9, centered about 17 km SE of Mt. St. Helens, were recorded in 13.5 hours 19-20 July. A few hours later, one seismograph recorded 50 minutes of very weak harmonic tremor, the first since 27 June. On the day of the eruption, 22 July, shallow events centered under the N flank began at about 1000. Four earthquakes were recorded between 1400 and 1500, nine from 1500 to 1600, and twenty from 1600 to 1700. An increase in SO₂ emission just prior to the eruption caused a sharp drop in the CO₂/SO₂ ratio after a steady increase over the preceding week.

The initial tephra ejection phase began at 1714 and lasted 6 minutes. NWS radar at Portland airport measured the top of the eruption cloud at slightly less than 14 km. Clear weather allowed the cloud to be seen from as far away as Seattle, Washington and Salem, Oregon, both about 150 km distant. An overflight by Donald Swanson revealed that the activity had blown a hole in the lava dome that had begun to grow in the crater shortly after the 12 June eruption, but had not destroyed it.

A second, larger, tephra cloud emerged from the volcano at 1825 and was fed vigorously for 22 minutes, destroying the lava dome. Pyroclastic flows moved down the N flank, nearly reaching Spirit Lake. A NOAA weather satellite infrared image at 1845 (2 minutes before the end of this active phase) yielded a temperature of -52°C at the top of the column, corresponding to an altitude of about 13 km (figure 12). Portland airport radar recorded a maximum cloud altitude of more than 18 km during this phase. Arthur Krueger estimates the base of the stratosphere to have been at about 16 km altitude.

Figure 12. Series of three enhanced infrared images returned 22 July 1980 by NOAA's GOES West geostationary weather satellite. Left, image at 1845 shows the cloud from the Mt. St. Helens explosion at 1714 drifting NE as the column from the explosion that began at 1825 rises from the volcano. A temperature of -52°C at the top of the latter column was calculated from this image, corresponding to an altitude of about 13 km. Center, image 1 hour later, at 1945, shows material from the eruptive pulses that began at 1901 rising from the volcano, as two distinct earlier clouds move NE. Right, image at 2145 shows that feeding of the cloud has nearly ended, and the distal end of the plume has crossed into Canada at the NE corner of Washington. Courtesy of Arthur Krueger and Andy Horvitz.

The final series of eruptive pulses began at 1901, lasting about 2 hours and 40 minutes. The highest cloud ejected during this period reached almost 14 km at 1907 and pyroclastic flows were most numerous around that time. NOAA weather satellite images show that feeding of the eruption column had declined substantially by 2115, and that the ash plume was definitely detached from Mt. St. Helens on the next image, at 2145. The plume moved NE across Washington into British Columbia, Canada, remaining visible on the images until about 0100, when it merged with a cold front over W Alberta. Ash fell as far away as Montana and Alberta. Ashfalls were light, the thickest reported at Colville, Washington (nearly 450 km NE of Mt. St. Helens) where 1.5 mm were measured.

The USGS informed federal, state, and local government agencies of the earthquake swarm during the early afternoon of 22 July. People working on the flanks of the volcano, including about 120 USFS personnel, wereevacuated before eruptive activity began. No casualties were reported.

Gary Heckman reports that a class M-8 solar flare that occurred on 22 July began 37 minutes after the volcano's first explosion (at 1751), peaked at 1803, and ended at 1909. Class M-8 flares, near the top of the intermediate intensity scale, normally occur about once a month. However, solar activity is presently near the peak of its 22-year cycle and intense flares have become more frequent.

Aerial observers reported glowing red areas in the crater during the night of 22-23 July. Overflights the next morning revealed a new crater, about 250-300 m in longest dimension and 100 m deep, in the area formerly occupied by the lava dome. A thick blanket of ash surrounded the new crater, which emitted a small vapor column similar to those of mid-July. SO₂ emission dropped to about 800 t/d on 23 July, but returned to mid-July levels the following day. Temperatures of up to 705°C were measured at 1.5 m depth in the new pyroclastic flows on 24 July, while surface temperatures were 70-80°C.

On 28 July at 0608, a little ash rose slowly from the new crater to 3.5 km altitude. Harmonic tremor accompanied the activity, which lasted about 20 minutes. No unusual pre-eruption seismicity was recorded, but glow was observed a few hours earlier from a USFS monitoring aircraft.

SO₂ emission dropped to 700 t/d on 28 July and did not exceed 1,100 t/d through the end of the month. Red glow continued to be visible in the crater as of early August. The USGS attributed the glow to superheated rock, not a new lava dome.

At press time, another eruption was followed by renewed lava-dome growth. At about noon on 7 August, volcanic tremor began, increasing in intensity as a M 2.3 earthquake occurred at 1238 in the vicinity of Marble Mountain, site of earthquakes in early and mid-July. Remote sensing of CO₂ and SO₂ gas emissions showed that the CO₂/SO₂ ratio had dropped from twelve to fourteen on 6 August to about three on 7 August. However, the lower ratio was caused by a decrease in CO₂ rather than the increased SO₂ emission that caused the similar ratio decline before the 22 July eruption. Because of the changes in seismicity and gas emission, the USGS withdrew its personnel from the area closest to the volcano and notified the USFS Emergency Coordination Center that an eruption might be imminent. Other persons working on the volcano's flanks were evacuated, including fire fighters battling persistent smoldering debris in the 18 May eruption deposits.

Tremor intensity continued to increase, and another M 2.3 earthquake was recorded in the Marble Mountain area at 1458. An eruption began at 1623, producing an ash-laden column that rose quickly to about 13.5 km. A small pyroclastic flow moved down the N flank, leaving a thin deposit, but did not reach Spirit Lake. At 1910, an increase in tremor amplitude coincided with a series of pulsating ash ejections that sent clouds to a maximum altitude of less than 7 km. Seismic and eruptive activity waned briefly about 2200, then a stronger explosion at 2232 sent tephra to 11 km altitude, accompanied by more tremor. Variable winds sent ash over a wide area, but ashfalls were very light. Ash was reported in Grays Harbor County, about 150 km NW; Mt. Vernon, 250 km N; Wenatchee, 200 km NE; and Portland, 80 km SW. Small pulses were seen on the USGS monitoring camera the next day at 0935, 1007, and 1734. Ash rose to about 3.5 km altitude as surrounding steam columns reached nearly 5 km. A new lava dome was visible in the vent on 8 August, with its top about 40 m below the vent rim. About 8 m of vertical growth appeared to have taken place by 9 August, but no further growth was evident on 11 August.

Tracking of the 18 May eruption cloud continues at several locations worldwide. Ronald Fegley reported that NOAA's lidar equipment at Mauna Loa, Hawaii (19.5°N, 155.6°W) detected a significant volcanic layer at 18.4 km altitude on 15 July. Weaker layers had been detected on 10 and 13 June and 2 and 8 July at similar altitudes.

M.P. McCormick provided the following information. On 4 June, the ground-based lidar at NASA's Langley Research Center in SE Virginia (37°N, 76.4°W) first detected a persistent layer at 18 km altitude (no data were obtained 30 May-3 June). H. Jäger and R. Reiter observed layers near 12 km altitude (slightly above the local tropopause) over Garmisch-Partenkirchen, Germany (47.5°N, 11.0°E) on several days beginning on 25 May. Material extending from 13 to 15.5 km altitude was detected there on 12 June. Beginning 11 June, G. Visconti and G. Fiocco reported material between 15 and 20 km, and possibly at 14 km, over L'Aquila, Italy (42.4°N, 13.4°E). M. Fujiwara of Kyushu University, Japan (33.6°N, 130.4°E) reported a layer on 3 June just below 15 km, clearly higher than the local tropopause. NASA's SAGE satellite will move through the mid-latitudes of the northern hemisphere in August and will collect data on ejecta remaining in the atmosphere. H.H. Lamb reports that he observed a colored ring, which he believes to be an example of Bishop's Ring, around the sun at sunset on 12 June at Norwich, England (52.6°N, l.3°E).

Issue No. 80-3 of the Upper Atmospheric Programs Bulletin, published jointly by NASA and the FAA, contains ten pages of reports of high-altitude atmospheric research and satellite study of the 18 May eruption cloud.

JMA reports that many of their stations, from the NE to the SW ends of Japan, recorded the pressure wave from the 18 May explosion on microbarometers. Arrivals ranged from 1447 to about 1628 (local time at Mt. St. Helens) on 18 May. Calculated apparent wave velocities ranged from 305 to 309 m/s.

Strong electrical effects were associated with the heavy ashfall of 18 May. Rocke Koreis reported observations of ball lightning at elevations from near ground level into the ash cloud over Yakima, Washington (140 km ENE of the volcano). Strong thunder continued throughout Yakima's ashfall, which was entirely dry and lasted from 1115 to 2300 on 18 May. A distinct second pulse of ash was noted in late afternoon. Koreis also reported that a light plane fleeing the initial 18 May explosion at an air-speed of more than 270 km/hr was briefly being overtaken by the eruption cloud moving at an estimated 320 km/hr.

Information Contacts: R. Decker, HVO, Hawaii; R. Tilling, C. Zablocki, USGS, Reston, VA; D. Swanson, R. Christiansen, USGS, Menlo Park, CA; A. Krueger, NOAA/NESS; S. Malone, R. Crosson, E. Endo, University of Washington; M.P. McCormick, NASA Langley Research Center; R. Fegley, NOAA/ERL; H. Jäger, Fraunhofer-Institut für Atmosphärische Umweltforschung; G. Visconti, University dell'Aquila; G. Fiocco, Istituto di Fisica dell'Atmosfera; M. Fujiwara, Kyushu University; H.H. Lamb, University of East Anglia; Seismological Division, JMA; W.S. Smith, FAA; R.Koreis, Yakima, WA; AP; UPI.

August 1980 (SEAN 05:08) Cite this Report

One small explosion; lava dome growth stops

USGS personnel measured temperatures in the 7 August pyroclastic-flow deposits of 647°C near the crater and 639°C near their distal margin the day after emplacement. By 8 August a lava dome had filled the inner crater formed during the 22 July explosions to about half its former depth of almost 100 m. More than 20 m of additional dome growth had taken place by the morning of 9 August. Occasional bursts of vapor and ash rose to 3-6 km altitude on 8 August, accompanied by small seismic events. SO₂ emission increased from about 900 t/d on 8 August to at least 2,000 t/d on the 9th, but explosive bursts had ended and seismic activity was very low.

During the next several days, the rates of CO₂ and SO₂ emission fluctuated substantially (table 1). The dome appeared to have risen slightly between 9 and 11 August, but no growth has been observed since. The deformation monitoring line between the crater and a ridge W of Spirit Lake shortened about 3 cm/day from 8-13 August, a rate typical of previous inter-eruption periods since measurements resumed in mid-June. No significant changes in ground tilt around the base of the volcano have been recorded since June.

Table 1. Daily rates of CO₂ and SO₂ emissions from Mt. St. Helens during 10-29 August, measured in t/d by remote sensing equipment. No data are available for 18, 27, and 28 August. Data for 15 August does not include gas released by the explosion.

On 15 August at 1437, an ash-rich cloud rose to about 1 km above the volcano. The cloud became gradually less ash-laden, and dissipated after less than 15 minutes. Volcanic tremor was recorded during the eruption, declining as the eruption waned, but there was no premonitory seismicity. The activity blasted a small crater in the W side of the dome but did not destroy it. A similar eruption had occurred 28 July.

The surface of the lava dome was about 7 m lower on 17 August than it had been before the 15 August explosion. No shortening of the deformation monitoring line occurred between 13 and 17 August, but shortening resumed 19 August, indicating renewed inflation. Seismicity was limited to infrequent very small shallow events, many of which probably represented rockfalls in the crater.

On 22 August, a small quantity of water from Maratta Creek, a tributary of the North Fork of the Toutle River, breached a portion of the nearly 30 km-long debris dam left in the Toutle by the 18 May eruption. Water and debris flowed about 3 km downstream where it formed a small lake with an estimated volume of 3.8 x 10⁵ m³. Five days later this lake overflowed, moving nearly 10 km down the Toutle valley to Camp Baker, site of a partially completed dam project designed to control the much larger floods that could occur in the debris-clogged valley when heavy rains resume in the autumn.

Some equipment was damaged at the construction site. Much of the water was contained by the unfinished check dam, but some continued about 15 km farther to the town of Toutle, damaging or destroying some temporary bridges and access roads. No casualties were reported.

Mt. St. Helens remained quiet through early September. Gas emission continued to fluctuate but no explosions occurred. Deformation monitoring lines began to lengthen slightly on 25 August, indicating slight deflation of the volcano, but an average of 2 cm/day of contraction by early September showed a return to more typical gradual inflation. On 30 August, the lava dome was the same size as on 9 August. Incandescence could be seen through deep cracks in its surface and through the small crater formed by the 15 August explosion. Cracks in the walls and floor of the inner crater containing the dome also revealed incandescent material. Seismicity remained generally weak in late August and early September. A brief earthquake swarm on 4 September began with a M 2.5 event at 2046, followed by four shocks within the next 9 minutes. All were centered 2-4 km beneath an area about 8 km NNW of Mt. St. Helens and were believed by the USGS to be of tectonic origin.

Monitoring of material ejected into the stratosphere by the 18 May eruption continues. Results from NASA's lidar at Wallops Island, Virginia show that the layer at 18 km had become more diffuse by August, occupying a zone between 16 and 20 km. In Tucson, Arizona, Aden and Marjorie Meinel continued to observe a weak layer at this altitude during August sunrises and sunsets. A NASA P-3 aircraft will collect data on the 18 May material during a mid-September cross-country flight, timed to coincide with information-gathering by NASA's SAGE satellite as it passes over the Northern Hemisphere.

Two reports in the 5 September issue of Science present data on the petrology, chemistry, and size distribution of the 18 May tephra. Hooper and others emphasize the bimodal character of ash deposited about 400 km ENE of Mt. St. Helens. The ash changed abruptly from a relatively dark, glass-poor silicic andesite to a lighter-colored glass-rich rhyodacite 3.25-3.5 hours after ashfall began. This interval corresponds quite closely to the timing of changes observed in the character of the eruption column at the volcano SEAN 05:05). Fruchter and others present bulk analyses of ash collected at numerous locations in Washington. In addition to petrology and major and trace element chemical analyses, the report focuses on the tephra's toxic and radioactive components, which do not appear to have been abundant enough to have a significant effect on animal or plant life.

The July 1980 issue of the Washington Geologic Newsletter lists Mt. St. Helens research projects being carried out by 25 groups at 20 institutions and government agencies, in addition to giving sources for pre- and post-eruption maps and airphotos of the volcano. In addition, names and addresses of USGS personnel with the particular Mt. St. Helens study on which each was working as of 20 June are listed. An eruption chronology, and reports on ashfall distribution and petrography are also presented in this publication.

Pre- and post- 18 May photographs and satellite images of Mt. St. Helens and vicinity are available from the EROS Data Center, Sioux Falls SD 57198. Among these are about 1,500 photographs taken by USGS personnel in low-altitude aircraft; color infrared photographs taken at 18 km altitude from NASA's U-2 aircraft on 1 May and 19 June; and cloud-free LANDSAT images.

Information Contacts: D. Peterson, USGS, Vancouver, WA; R.Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; M.P. McCormick, NASA Langley Research Center; A. and M. Meinel, University of Arizona; UPI.

September 1980 (SEAN 05:09) Cite this Report

Harmonic tremor and vapor emission; 18 May material remains in upper atmosphere

September activity was limited to slow outward movement of the N flank, minor seismicity, and weak vapor emission. No eruption was associated with the brief earthquake swarm of 4 September, and seismicity was limited to a few very minor events through mid-month. Outward movement of the rampart N of the inner crater continued, averaging 1.2 cm/day 26 August-9 September (in contrast to the 1.5 m/day movement of the N-flank "bulge" prior to 18 May). Several small jets of ash were ejected from a vent near the base of the lava dome on 9 September. Each burst lasted about 30 minutes, but consisted of only a few cubic meters of ash. Weather and instrument problems limited data on CO₂ and SO₂ emission rates. Early September CO₂/SO₂ ratios ranged from 4.7 on the 5th to 2.4 on the 9th, with a maximum CO₂ emission rate of 5,000 t/d, on the 6th.

Many large avalanches occurred on the crater walls during the afternoon and early evening of 12 September, but no accompanying seismic activity was recorded. Avalanching declined during the night of 12-13 September. Ground deformation measurements indicated that outward movement of the crater's N rampart had slowed to less than 1 cm/day by mid-September. Several radial fissures in the lava dome within the inner crater widened slightly in the 2 weeks since they first developed about 9 September. The USGS believes that this widening was probably related to the N rampart movement. No dome growth has been detected since 11 August, but some incandescent areas were visible on the dome on 23 September. No significant seismic or eruptive activity was recorded in mid-September.

On 24 September at 0917 a gray gas plume rose from the volcano to about 3 km altitude, just clearing the crater rim, then drifted to the S. Vigorous gas emission lasted about an hour, but there was little or no ash in the plume. No seismicity accompanied this event, but low-amplitude harmonic tremor began a few hours later at about 1400. The tremor was intermittent, with episodes lasting from less than a minute to 15 minutes, separated by 2- to 15-minute quiet intervals. Tremor episodes declined in number and duration after about 2 hours, and had nearly ended by 1800. On 26 September, very low-level harmonic tremor began at 0740. Minor steam emission started at 0747, lasting 9 minutes. The tremor ended by 0800. No additional tremor had been recorded as of 30 September. Outward movement of the N crater rim had virtually stopped 24 September, but measurements on the 26th indicated that very slow deformation had resumed. Although rates of outward movement have varied, the average rate for the most active portion of the N flank was slightly less than 1 cm/day during September.

In mid-September, scientists aboard a NASA P-3 aircraft studied the plume at Mt. St. Helens and material remaining in the upper atmosphere from the 18 May eruption. The aircraft left Hampton, Virginia on 17 September, flew S to Georgia, traveled W along the 32nd parallel to about Tucson, Arizona, then continued NW to Portland, Oregon. During this transcontinental flight and the return trip across the northern plains the following week, 2 wavelengths of lidar backscatter data showed a generally consistent broad layer of volcanic material at 14-22 or 23 km altitude. Peak backscatter typically occurred from two levels, 18.5-19 km and about 21.5 km. The lower was usually significantly stronger. Although the 21.5 km peak sometimes approached that at 18.5-19 km, the upper peak disappeared suddenly in certain areas.

Simultaneous data were gathered on two occasions from the P-3 aircraft and NASA's SAGE satellite, once over Sacramento, California and once over Portland. A dust sonde released from Laramie, Wyoming by James Rosen collected information on upper atmosphere particulates as the NASA aircraft made lidar measurements in the same area on its return flight.

Scientists with the NASA-funded Research on Atmospheric Volcanic Emissions (RAVE) project collected a variety of data on the plume from Mt. St. Helens during a 4-hour flight in the NASA P-3 on 22 September. Filter samples were collected and the plume was analyzed directly for SO₂, H₂S, OCS, CS₂, O₃, NO, and total S.

A University of Washington-USGS report provided additional information on the seismicity associated with the 7 August explosions and minor ash emission 8-10 August (SEAN 05:07 and 05:08). Harmonic tremor began at 1207 on the 7th, occurring as 10- to 20-second bursts at randomly spaced intervals. Low-frequency events were recorded at 1529 and 1554. At about 1622, records show a gradual increase in tremor amplitude, followed by a large seismic event at 1626:45 that marked the onset of eruptive activity. An ash-laden cloud rose to 13.5 km altitude and a small pyroclastic flow moved down the N flank. By 1730, tremor amplitude had returned to pre-1207 background levels. A series of pulsating ash ejections reaching a maximum altitude of less than 7 km coincided with a gradual increase in tremor amplitude starting at 1910. Tremor amplitude declined substantially by 2045. A second amplitude increase beginning at 2130 was followed by a large seismic event at 2232:22 as a stronger explosion sent tephra to 11 km altitude. Peak tremor levels were similar to those associated with the first 7 August explosion. At 2328, the first of eight 9-11 km-deep earthquakes occurred, all with magnitudes less than 1.7. Tremor amplitudes declined slowly. Between 8 August at 0325 and 9 August at 0129, approximately 18 events similar to large bursts of tremor and lasting 20-130 seconds were recorded. These can be correlated with discrete periods of ash emission. The last significant ash emission event was recorded on 10 August at 0813. No changes in tremor amplitude preceded these events.

Information Contacts: D. Peterson, USGS, Vancouver, WA; R.Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; M.P. McCormick, NASA Langley Research Center; J. Friend, Drexel University.

October 1980 (SEAN 05:10) Cite this Report

Explosive eruptions and new lava dome

Minor activity — early October. A gradual increase in activity began 7 October with barely detectable harmonic tremor that started a few minutes after midnight and lasted less than an hour. During the day on the 7th, a few plumes, containing no appreciable ash, rose to about 3 km altitude before drifting NE. No harmonic tremor was recorded 8 October, but shallow earthquakes of magnitudes 1.6 and 1.8 occurred at 1535 and 1537. Some 8 October plumes contained smaller amounts of ash. Several minor gas clouds containing a little fine ash were emitted 10 October between 0915 and 1100, accompanied by intermittent low-level harmonic tremor. The initial column on the l0th reached 4.5-6 km altitude and drifted NNE. Two more gas plumes were emitted during the afternoon, again accompanied by minor seismicity. Gas emissions decreased in frequency and intensity 11-13 October. Seismic records during poor weather on the 14th indicated that occasional minor gas emission continued. On 15 October, some cracks that had not been present the preceding week were visible on the crater floor. Three small volcanic earthquakes were recorded, but no harmonic tremor was detected.

Explosive eruptions 16-18 October. Shallow earthquakes, most with magnitudes less than 1, became increasingly frequent on 16 October, reaching one event every few minutes by early evening. A M 3 earthquake, centered about 1 km beneath Mt. St. Helens, occurred at 1902. An hour later, the USGS and the USFS notified state and local officials of the possibility of an eruption, prompting the successful evacuation of 92 persons from around the volcano.

Observers in a USFS aircraft saw strong incandescence in the inner crater area at 2157, then an explosion 1 minute later. Tephra was ejected for only 5-10 minutes, accompanied by strong, regular seismicity probably related to explosive gas release. The top of the eruption column reached 13.5 km, as measured by Portland airport's radar. Ash and pea-sized pumice fell on Cougar. Pumice was reported at Amboy (40 km SW) and a trace of ash fell on the Portland-Vancouver area. A few flights to Portland were rerouted or cancelled and an air pollution alert was issued, but there were no serious disruptions of auto traffic the next morning.

Seismic activity stopped immediately after the 16 October explosion, but low-level seismicity resumed about 0900 the next morning. At 0928, an ash-laden eruption cloud began to rise rapidly from the crater, reaching 9 km altitude at 0932 and its maximum altitude of 14.3 km by 0938. This explosion produced a single pyroclastic flow, at about 0935, that traveled 3-4 km down the N flank to the break in slope, coming to rest after about 5 minutes. Ash emission began a gradual decline about 0940 then decreased abruptly at 0954, although sporadic eruptive activity continued until about 1015, when seismicity ceased. Wind directions differed with altitude, but the main plume drifted SE to SSE, reaching north-central Oregon. Ash from the explosion of late 16 October and early 17 October fell as far away as Eugene Oregon, 280 km SSW. These explosions destroyed the lava dome that was extruded after the 7 August eruption.

Weak seismicity resumed at about 2045 on the 17th, gradually increasing in intensity. Observers in a USFS aircraft noted strong incandescence in the inner crater at 2108, and the emergence of an eruption cloud at 2112 as tremor amplitude increased sharply. An incandescent pyroclastic flow descended the N flank at 2116. The eruption column reached its maximum altitude of 13.7 km at 2119. Eruption intensity declined gradually after 2119, with activity becoming intermittent by 2200. Emission of steam and ash pulses continued until about 2350, accompanied by intermittent seismicity. Ash blew SE.

Occasional seismic activity, consisting of very low-level signals lasting a few seconds to a few minutes, continued until the early afternoon of 18 October. A seismic episode that began on the 18th at 1232 abruptly intensified 3 minutes later as an eruption cloud emerged from the crater, reaching a maximum altitude of 6 km at 1239. A second burst, ejected at 1246, reached 7.6 km altitude by 1249. Vigorous tephra ejection stopped l minute later, but intermittent weak emissions continued for another 15 minutes. Eruptive activity gradually weakened, but sporadic low-level seismicity continued. A new tephra plume was ejected at 1428, reaching 6 km altitude at 1432, but activity declined quickly and the episode had essentially ended by 1500. Light ashfalls from the 18 October explosions were reported from as far as north-central Oregon.

Lava dome growth. By the time visibility returned at 1520, a new lava dome about 30 m across and 6 m high was growing in a saucer-shaped depression in an area formerly occupied by the inner crater and the recently destroyed, post-7 August lava dome. An hour later, the dome was 40 m in diameter and 9 m high. By the next morning it had grown to an elliptical feature 270 m in largest dimension and 50 m high, but it had essentially ceased increasing in volume, with sagging of the summit compensating for continuing increases in width. Fragments continued to spall from the dome's irregular breadcrust-like surface. Noisy, episodic gas emission occurred around the base of the dome. Occasional low-level seismicity continued, but these episodes became briefer and less frequent through 19 and 20 October.

Minor activity — late October. No significant seismicity was recorded during the next several days. When visible through heavy weather clouds the summit of the lava dome appeared to have sagged further and the margins widened slightly, but there was no apparent increase in volume. On 25 October, a series of small, shallow seismic events between 1100 and 1130 accompanied the ejection of individual vapor plumes that reached 3.6 km altitude. The next day at 1720, a shallow volcanic earthquake of about M 2 was followed by several smaller shocks in the next few hours but no plumes were ejected. When weather conditions permitted in late October, measurements showed continued outward movement of the N crater rampart. Gas vents near the dome fumed vigorously through the end of October and cracks in the surrounding crater floor continued to widen very slowly.

Information Contacts: D. Peterson, J. Dvorak, D. Mullineaux, C. Newhall, USGS, Vancouver, WA; R. Tilling, S. Russell-Robinson, USGS, Reston, VA; S. Malone; R. Crosson; E. Endo, University of Washington.

November 1980 (SEAN 05:11) Cite this Report

Harmonic tremor and vapor emission; chemical data summarized

Activity was limited to vapor emission and occasional seismic activity through early December. Most early November seismic events were caused by rockslides from the crater walls. No significant local earthquakes or harmonic tremor were recorded until mid-November, when brief episodes of harmonic tremor began, barely within the detection limits of sensitive seismographs on and near the volcano. Intermittent low-level tremor continued through early December. Stronger tremor started on 25 November at 2054, gradually fading into background noise about 35 minutes later. Observers in a USFS aircraft reported a slightly brighter glow in the dome area after this event. A second burst of stronger tremor began 27 November at 2034, continuing for about an hour, and several more such episodes, lasting only a few minutes each, were detected through 30 November.

USGS monitoring of the N crater rampart revealed a maximum net outward movement of about 23 cm between the October explosions and 26 November. A major reversal to inward movement had occurred in late October before an outward trend resumed in November. Outward growth accelerated in mid-November to slightly more than 1.5 cm/day at times, a rate similar to that recorded during the summer. About 20 cm of expansion was measured between 12 and 26 November.

No major changes have taken place in the volume or ratio of gases emitted by the mountain. Two large fumaroles opened in the crater floor, very close to the margin of the lava dome, one on 18 or 19 November, the other on the 25th. As they opened, both ejected mud (containing no fresh magma) that coated snow on the flank. As of early December, the new fumaroles were 2-3 m across, glowed cherry red, and puffed noisily at half-second intervals.

The following is a report from W.G. Melson: "A small but definite trend toward andesite compositions is revealed by major element analyses of the 18 May-7 August eruptives. A total of 46 samples of probable essential ejecta have been analyzed (table 2 and figure 13), a minimum of five such samples from each eruptive episode. The trend is an irregular one and is most pronounced with regard to MgO and CaO when plotted against time of eruption."

Table 2. Ejecta composition for each eruptive episode of Mt. St. Helens, 18 May-7 August 1980. Electron microprobe analyses of fused powders by W. Melson, T. O'Hearn, and J. Nelen, SI. Samples collected by D. Swanson, C. Hopson, W. Melson, R. Fiske, and C. Kienle. Total Iron calculated as FeO. Values are averages of the total analyses.

Figure 13. Average of MgO, left, and CaO, right, concentration in rocks from each eruptive episode at Mt. St. Helens, 18 May-7 August 1980, plotted against time of each episode. Analyses are of fused powders by the electron microprobe. Numbers in parentheses are the number of samples analyzed and included for average value. Total iron calculated as FeO.

Information Contacts: T. Casadevall, C. Newhall, D. Swanson, USGS, Vancouver, WA; R. Tilling, USGS, Reston, VA; S. Malone, R. Crosson, E. Endo, University of Washington; W. Melson, SI.

December 1980 (SEAN 05:12) Cite this Report

Dome growth, vapor plumes, and earthquake swarms

Renewed dome growth took place in late December, without the large explosions that immediately preceded previous dome-building episodes in June, August, and October. Activity was limited to minor seismicity and weak vapor emission for about a month after the 16-18 October explosions and dome extrusion. Frequent periods of very low-level harmonic tremor, lasting a few minutes to several hours, began to appear on seismic records 19 November. Bursts of higher level tremor, similar to explosion events seen earlier at Mt. St. Helens, could often be correlated with ejections of vapor columns that sometimes contained ash. A few discrete shallow earthquakes were recorded, but remained infrequent until late December.

A series of vapor plumes marked the volcano's behavior throughout much of December. A few minutes of stronger tremor accompanied emission of a vapor plume that rose to 3 km altitude 7 December, and one of several bursts of higher-amplitude tremor on 9 December occurred as a plume was ejected to 2.7 km altitude at 1325. A new thin deposit of ash was noted on the upper S flank early 12 December. Emission of this ash was not observed, but a burst of increased tremor had begun at 0417, lasting about 30 minutes. On 13 December at 2017, a plume reached 5.5 km altitude as higher level tremor was recorded. Inspection of the dome 15 December revealed a new small [explosion] crater in its [SE] edge. Adjacent to the new crater, a roughly triangular section of the dome had been [exploded away], extending about 15 m along its outer edge and 30 m toward the center of the dome. Plumes associated with increased tremor rose to 3.3 km altitude 16 December at 0800 and 17 December at 1520. A plume reaching 6 km altitude was briefly visible through clouds on 21 December at 1409, accompanied by a short burst of tremor. Two days later, at 1258, seismic activity and vapor emission increased simultaneously. Gas and a little tephra rose to almost 3 km, but activity continued for only a few minutes. New crater floor cracks were apparent after this event.

Deformation measurements 6 and 7 December showed a halt or possibly a reversal of the outward movement of the N crater rampart that had resumed in November (SEAN 05:11). Measurements on 18 December revealed little or no change. However, observations on the 23rd showed renewed northward displacement. Fissures in the crater floor appeared to be widening as well as extending radially from the inner crater.

On 25 December, the number of discrete shallow earthquakes began to increase. Seismicity peaked before noon 27 December, averaging five events per hour and occasionally reaching eight/hour during the next 30 hours. University of Washington geophysicists located about two dozen of these events. All were centered at 2 km depth or less and were within l km NW of the October dome. No migration of events was evident.

Aerial observations 26 December were hampered by poor visibility, but there were no apparent changes in the crater. Bad weather prevented additional observations of the crater until 28 December at 0900, when USGS and USFS personnel saw a new extrusion, about 0.25 the size of the October dome and emerging from its SE edge. A spine-like structure protruding 30-60 m from the center of the October dome was noted an hour later. All but 8 m of this structure toppled the next day at 1540. Growth of the new SE lobe and another much smaller new lobe on the NW edge of the October dome had apparently stopped by 3 January but crater floor deformation has continued. The SE lobe measured at least 225 m in an E-W direction and reached a maximum height of about 100 m above the crater floor, although by 6 January the crest was subsiding somewhat. The NW lobe was about 100 m across. The elliptical October dome had been about 230 m in largest horizontal dimension and 50 m high on 19 October. A collapse pit formed on the October dome during the growth of the new lobes, but the pit's dimensions were not available.

Deformation measurements showed that the N crater rampart had moved outward about 85 cm on 23-28 December and another 1.5 m by 2 January. Since then, the crest of the rampart has been uplifted and thrust northward dramatically, as much as 5 m by 6 January. Other thrusts have been observed in relatively level terrain on the crater floor.

By the afternoon of 29 December, seismicity had declined to a rate of one or fewer events per hour. As of 7 January, no harmonic tremor and very few discrete earthquakes were being recorded.

Further References. Foxworthy, B.L. and Hill, M., 1982, Volcanic Eruptions of 1980 at Mt. St. Helens: The First 100 Days; USGS Professional Paper 1249, 125 p.

Lipman, P.W. and Mullineaux, D.R. (eds.), 1981, The 1980 Eruptions of Mt. St. Helens, Washington; USGS Professional Paper 1250, 844 p. (62 papers).

Information Contacts: D. Swanson, C. Newhall, J. Dvorak, USGS, Vancouver, WA; S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

January 1981 (SEAN 06:01) Cite this Report

Dome growth and seismicity; small vapor plumes

Lava extrusion resumed 5 February, adding a substantial quantity of new material to the dome that grew in the crater after the 16-18 October explosions and the two new lobes produced in late December and early January (figure 14).

Figure 14. E-W and N-S cross sections through Mt. St. Helens, January 1981, by Michael Doukas, USGS.

Minor activity — January. After growth of the December-January lobes ceased between 2 and 4 January, outward movement of the N crater rampart gradually declined to an average of about 0.5 cm/day, although rates were variable and data were limited. January seismicity was the quietest of any period since earthquakes began 20 March. Only 40 discrete events were large enough to be recorded on three or more stations of the USGS-University of Washington seismic net, in contrast to 136 in December and 74 in November. Of the January earthquakes, about ten were low-frequency events associated with dome growth early in the month, many others were rock avalanche events, and a few accompanied ejection of steam plumes. A new fumarole opened on 9 January on the E margin of the lava dome. This fumarole was the probable source of small steam-and-ash plumes on 16 January at 1152 (to 3 km altitude) and 20 January at 1204 (to at least 3 km), both accompanied by bursts of seismicity. Similar seismic activity was recorded 24-25 January and field parties saw light ash deposits on fresh snow. Several similar bursts occurred 31 January-1 February, two of which could be correlated with steam-and-ash emission. However, another steam plume was ejected without accompanying seismicity.

Increased deformation and seismicity. Deformation and seismic activity began to increase at the beginning of February. Radial fissures in the crater floor began to widen at a noticeably faster rate and movement of thrust faults accelerated. A larger number of glowing cracks in the surface of the lava dome indicated that its temperature was increasing. On 2 February at 0336, a 4-minute burst of seismicity was followed by a M 2 earthquake at 0340, then low-level harmonic tremor were recorded until 0630. Occasional bursts of seismic activity continued through the day, and 35 minutes of low-level tremor was recorded that night. A gradual increase in discrete earthquakes began 3 February. Occasional low-level tremor was recorded, as were several bursts of seismicity, one of which was associated with a small plume at 1220. By midnight 4-5 February, the number of discrete events had reached 4-5/hour and continued at that rate for about 6 hours.

Lava extrusion. Just before 0500, the USGS and University of Washington issued an advisory predicting an eruption within the next 12 hours. Seismicity began to decline about 0600, probably signaling the beginning of lava extrusion. By 0800, earthquakes were occurring at a rate of only about 1/hr. Very heavy steaming obscured the crater, but new lava could be seen on the October dome during about 30 seconds of visibility. The number of discrete seismic events decreased further by mid-afternoon, remaining at many fewer than 1/hr through 8 February. However, bursts of unusual seismic signals were recorded, possibly caused by lava extrusion.

Improved visibility revealed that the new lava was extruded through the collapse pit in the center of the October dome. The new material appeared to have both uplifted and overridden the October dome, leaving this area about 35 m higher by the time growth apparently stopped during the night of 6-7 February. The small NW lobe emplaced during the December-January activity was pushed about 12 m N and partially overridden by new lava. New thrust faulting also occurred in the SW part of the crater, but was much less extensive than the thrusting associated with the December-January activity. The increase in dome volume produced by the February extrusion was roughly equal to the volume of lava produced by each of the two previous events, but at press time it was not possible to determine how much volume was of new lava on the surface and how much was caused by uplift of pre-existing lobes.

Information Contacts: D. Swanson, C. Newhall, J. Dvorak, USGS, Vancouver, WA; S. Malone, C. Boyko, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

February 1981 (SEAN 06:02) Cite this Report

Minor seismicity; small steam explosions; chemical data summarized

Mt. St. Helens remained quiet as of 10 March, as it has since the end of the lava extrusion episode of 5-7 February. The February lava approximately doubled the volume of the composite dome in the crater, adding about 5 x 10⁶ m³ of new material to the 1.5 x 10⁶ m³ extruded 18-19 October and the 3.5 x 10⁶ m³ extruded 27 December-4 January. All of the pre-existing dome, except for a portion of the December-January SE lobe, was covered by the February lava. Between 8 and 21 February, the February lobe spread 12 m while sagging 3 m, resulting in dimensions for the new lava of 281 m in E-W direction and 119 m in maximum height above the crater floor.

Low-frequency volcanic earthquakes associated with the February lava extrusion ended 9 February. Occasional bursts of seismicity continued to be recorded. One, on 10 February at 0915, coincided with the emission of a steam cloud, containing a minor amount of ash, that rose to 4 km altitude. Field crews reported hearing a boom prior to this event. Some rock-avalanche events were also recorded after dome emplacement ended. A M 5.5 tectonic earthquake occurred late 13 February about 12 km N of Mt. St. Helens. As of 28 February, about 175 aftershocks stronger than M 1 had been recorded. Through the end of February, seismographs continued to record a few rock-avalanche events and bursts of seismicity of the type that has sometimes been associated with steam explosions. Clouds prevented observations of the crater for much of February, but clear weather on the 26th revealed evidence of numerous minor steam explosions on the N side of the lava dome.

Geodetic measurements showed a few centimeters of horizontal contraction of the Mt. St. Helens edifice between 4 February and 5 March. No significant movement of the N crater rampart occurred after the early February dome emplacement, nor has there been any measurable deformation of the crater floor during this period.

The following, from W. G. Melson, is based on microprobe analyses performed on the 1980-81 eruptives. "The SiO₂ content of the essential ejecta underwent a slight increase in the 7 August eruption, peaked in the 17-19 October eruption, but remained lower than for the 18 May tephra. This temporarily reversed a prior trend toward more basic compositions, which resumed with the December-January and February dome enlargements. CaO, FeO, and MgO show an inverse relationship to SiO₂ (figure 15), an expected relationship in a 'normal' fractionation sequence."

Figure 15. Change of SiO2, CaO, FeO, and MgO as a function of time of eruption at Mt. St. Helens. Electron microprobe analyses of fused powders were performed at SI by W. Melson, J. Nelen, and T. O'Hearn. Each data point value is the average of the following number of individual analyses of essential ejecta: 18 May, 9d; 25 May, 11; 12 June, 9; 22 July, 7; 7 August, 10; 17-18 October, 11; December-January dome enlargement, 6; February dome enlargement, 1 (sample from D. Swanson, USGS). Analytical precision for each analysis is about a 2 of: SiO2=0.62, FeO* (all Fe as FeO)=0.43; MgO=0.33, CaO-0.17.

Information Contacts: D. Swanson, C. Newhall, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA; W. Melson, SI.

March 1981 (SEAN 06:03) Cite this Report

Lava extrusion and small explosion follow deformation and seismicity

March eruptive activity was limited to occasional emission of small steam clouds, at least one of which contained ash. However, significant deformation was measured within the crater, and there was a slight increase in volcanic seismicity during the second half of March. Geologists announced that another eruptive episode was likely if the deformation and seismic trends continued, but none had occurred by press time.

The USGS-University of Washington seismic net recorded fifteen bursts of seismicity in March and five more bursts during the first six days of April. In the past, similar signals have often been correlated with episodes of steam emission, but because of poor weather, correlations with only two such episodes could be confirmed in March: a minor puff on 9 March at 1549, and a steam cloud containing some ash on 27 March at 1441. Newly fallen ash (made up of reworked dome material) observed NE of the volcano 25 March may have been ejected during a burst of seismicity the previous day.

The seismic net began to detect small, low-frequency, shallow events on Mt. St. Helens on 21 March. Fifteen of these discrete volcanic events were recorded by the end of March. Numerous aftershocks of the M 5.5 tectonic event that occurred 13 February about 12 km N of Mt. St. Helens, continued to appear on seismic records through March.

Deformation measurements showed that outward movement of the N-crater rampart resumed in March. Between 9 and 17 March, the rampart moved 7 cm to the N; by 22 March it had advanced 6 cm farther northward; and an additional 3.5 cm of movement was measured by 24 March.

A newly-established leveling net on the crater floor showed pronounced uplift near the lava dome, indicating that the dome was rising. Increasing crater-floor deformation was also demonstrated by accelerations in the rate of widening of a fissure from 3 mm/day to l cm/day and the rate of movement of a thrust fault from 0.5 cm/day to 1 cm/day by late March.

Addendum: On 9 April at about 1800, local seismicity began to increase, to about 1 event per hour at first and to about 2/hr after midnight. The USGS-University of Washington team issued an advisory about midnight stating that an eruption was likely within the next day if seismicity continued to increase.

Periods of constant low-frequency seismicity became more common and by 0230 on 10 April low-frequency activity was continuous. Individual events superimposed on this activity had increased to an average of 6-8/hour by 0600 and remained at that level through the day. At 0821, a small explosion produced an ash-bearing plume that rose to 4.5 km altitude. A light ashfall was reported at a ranger station 40 km to the NE. Although clouds prevented observation of the crater, a USGS helicopter crew could see that this explosion had generated no pyroclastic flows.

About 1900, the pattern of seismic activity started to change. The number of discrete events dropped to 4-6/hr, but these events were slightly stronger and total seismic energy release briefly stayed about the same. However, by midnight there had been a notable decline in both the number of events and seismic energy release, and by 0200 only one to two events were being recorded per hour. Seismicity had essentially ended by 2100-2200 on 11 April.

The weather cleared somewhat late 12 April, and geologists were able to view the crater between 1800 and 1900. New lava extended roughly 75 m NNW from the pre-existing dome. Television station videotape taken between 1900 and 1930 showed significant additional lava extrusion.

Information Contacts: D. Swanson, C. Newhall, S. Russell-Robinson, USGS, Vancouver, WA; C. Boyko, A. Adams, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

April 1981 (SEAN 06:04) Cite this Report

Steam and ash emission; more data on dome extrusion

About 5 x 10⁶ m³ of lava were added to the composite dome in the crater of Mt. St. Helens in early April. Because of poor visibility, no precise time for the start of extrusion could be determined, but a small explosion that ejected an ash-bearing plume to nearly 5 km altitude on 10 April at 0821 may have marked the beginning of the episode. Much of the lava had already been emplaced by the time geologists had their first view of the crater early 12 April, and extrusion was essentially complete by evening. Most of the associated seismicity had ended by late 11 April, but occasional discrete low-frequency events continued to be registered by the USGS-University of Washington net through 17 April.

Although deformation measurements showed that the magma rose through a conduit beneath the central collapse pit of the pre-existing dome, the April lava emerged from a vent somewhat N of the central pit, covered roughly the N quarter of the older material, and extended about 160 m NNW from it previous margin. After the April event, the dome had a volume of about 15 x 10⁶ m³, maximum and minimum lateral dimensions of 630 m (NNW-SSE) and 310 m (E-W), and a maximum height above the crater floor of 110 m. A substantial but uncertain amount of uplift of the entire crater floor was associated with the April extrusion, and some points on the crater floor spread away from the dome as much as 1.5 m, with most of the movement occurring during extrusion. One radial fissure exhibited about 55 cm of strike-slip movement during the episode. As of 5 May, only a few mm of additional deformation had taken place within the crater. No net deformation of the volcano as a whole has been associated with any of the extrusion episodes.

In the weeks following the April extrusion, characteristic low-level seismicity was sometimes correlated with witnessed bursts of steam emission. Simultaneous seismicity and ejection of steam containing a little ash occurred on 13 April at 0842; 14 April at 0950, 0953, and 1021; 17 April at 0958; and 24 April at 1018. Seismicity accompanied ejection of plumes of steam (without ash) on 25 April at 0921 and 26 April at 0821. A small amount of ash that fell about 50 km SE of Mt. St. Helens on 6 May between 1500 and 1530 may have been ejected during a period of seismicity at 1415.

Information Contacts: D. Swanson, C. Newhall, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

May 1981 (SEAN 06:05) Cite this Report

Deformation rates and SO₂ emissions increase; eruption expected soon

Small steam explosions, some ejecting a little ash, occurred intermittently through May. Explosions correlated with emergent low-frequency seismic signals occurred on 6 May at 1414; 11 May at 2140; 13 May at 0810 and 1059; 20 May at 0615; 22 May at 0247; 25 May at 2206; and 29 May at 1229. However, increased seismicity did not accompany an increase in steaming at 1040 on 13 May, or three ash puffs ejected between 1425 and 1431 the same day. Furthermore, seismic signals produced by steam explosions are often indistinguishable from those generated by rockfalls in the crater, so the seismic record alone cannot confirm the occurrence of either.

Rates of ground displacement within the crater and the volume of SO₂ emitted by the volcano both began to increase in late May. Until about 20 May, only very slow changes were noted in the position of the N crater rampart and in thrust faults surrounding the dome (figure 16). Measurements 27 May showed an acceleration in the rate of displacement, and reoccupation of rampart stations 5 June showed outward movement of about 1 cm/day. The rate of rampart movement had increased to about 2 cm/day between 11 and 15 June, and the S thrust fault (SE of the dome) moved 8.3 cm/day during the same period. Data telemetered 29 May-9 June by a newly installed bubble tiltmeter just NE of the dome showed substantial uplifts consistent with other deformation data.

Figure 16. Map of the May 1981 composite lava dome in the crater of Mt. St. Helens, prepared by D. Swanson. The margins of the dome are outlined by a stipple pattern. The approximate position of the crest of the N crater rampart is shown by a pattern of diagonal lines on each side of the April lobe. The base station for deformation measurements is located several kilometers to the N, off the map.

The rate of SO₂ emission was measured by COSPEC from fixed-wing aircraft flying under the plume. Between 1 and 16 May, the three-day moving average of SO₂ output decreased from 450 to 150 t/d. This trend reversed in late May, with emission rates rising from 190 t/d on 22 May to about 500 t/d by 11 June.

The USGS and the University of Washington Geophysics Program issued a joint advisory 12 June stating that an eruption, probably of the dome-building type, was likely to begin within the next 1-2 weeks if ground deformation and gas emission trends continued.

Information Contacts: T. Casadevall, D. Dzurisin, C. Newhall, and D. Swanson, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, and C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA; UPI.

June 1981 (SEAN 06:06) Cite this Report

Lava extrusion adds 5th lobe to crater dome

The increased ground deformation and SO₂ emission described last month were followed by an episode of lava extrusion atop the pre-existing composite dome that probably began late 18 June. The new lobe was roughly comparable in volume to lobes emplaced during previous extrusion episodes in October 1980, December 1980-January 1981, February 1981, and April 1981. A final volume figure awaits analysis of aerial photographs.

Seismicity began to increase during the evening of 17 June, and by afternoon seismographs were recording several events per hour. The seismic events were impulsive and of higher frequency than those that had typically accompanied previous eruptive episodes, but were centered directly beneath the crater within 1 km of the surface. The increased seismicity prompted an advisory alert notice, issued by the USGS and University of Washington at 1130 on 18 June, stating that a dome-building eruption was likely within the next day or two.

Between 1600 and 1700 on 18 June, the seismicity changed character to smaller, indistinct (non-impulsive) events, and the direction of tilt recorded by the single bubble tiltmeter (about 30 m from the NE margin of the dome) reversed. These changes were interpreted by the USGS as probably marking the beginning of lava extrusion, but cloudy weather prevented direct observation. As many as twelve of the indistinct seismic events, sometimes merging into bursts of noise, occurred each hour until about midnight, when the character of seismicity changed again to more typical low-frequency events with emergent arrivals. These events, some larger than those of the previous few hours, decreased gradually to number only a few per day by 22 June.

Poor weather prevented access to the crater until the afternoon of 19 June, when geologists observed new lava that had emerged from near the center of the pre-existing dome. The new lava covered an area roughly 300 m in both N-S and E-W dimensions, overriding portions of the lobes extruded in February and April and much of the talus at their margins. The June extrusion increased the height of the composite dome by around 50 m, but the top of the new lobe subsided 5-10 m between the afternoons of 19 and 20 June.

Between 29 May and the probable beginning of lava extrusion on 18 June, the bubble tiltmeter had recorded roughly 2000 µrad of tilt, in a direction consistent with other deformation data showing outward movement radial to the dome. Tilt rates increased from about 100 µrad/day during the first ten days of measurements to 140 µrad/day on 16 June, and to roughly 140 µrad/hr during the final 3 hours before the tilt reversal that probably marked the beginning of lava extrusion. Between the time of the tilt reversal and the rockfall that ended telemetry early 23 June, roughly half of the relative uplift recorded 29 May-18 June had been lost. In contrast to the pre-extrusion period, the post-extrusion record showed numerous features including instantaneous offsets probably associated with rockfalls and short-term tilt fluctuations. Present plans call for the installation of a network of telemetered bubble tiltmeters in the crater.

Information Contacts: T. Casadevall, D. Dzurisin, C. Newhall, D. Swanson, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

July 1981 (SEAN 06:07) Cite this Report

Occasional ash plumes to 3 km altitude

The mid-June lobe (figure 17) was roughly comparable in volume to lobes emplaced during previous episodes last October, December-January, February, and April that built the pre-existing composite dome. A continuously recording tiltmeter about 30 m from the NE margin of the dome provided data on pre-extrusion uplift and probably recorded the beginning of extrusion at about 1700 on 18 June (figure 18). This tiltmeter was destroyed early 23 June, but three new tiltmeters were installed in early July, within 100 m of the NE, E, and SE sides of the dome. None showed any significant changes through July. Other July deformation measurements did not show the accelerating outward movement that has typically preceded extrusion episodes. The volume of SO₂ emissions, measured by COSPEC from fixed-wing aircraft flying under the plume, usually ranged from 100 to 300 t/d during July, averaging about 150 t/d. Through the end of July there was no suggestion of the increase in SO₂ emissions that preceded both the December 1980-January 1981 and the June 1981 lava extrusion episodes by several weeks. Poor weather precluded determination of SO₂ trends before other extrusion episodes.

Figure 17. Sketch map of the June 1981 composite dome in the Mt. St. Helens crater, prepared by D. Swanson. The margins of the Mt. St. Helens dome are outlined by a stipple pattern. The approximate position of the N crater rampart is shown by a pattern of diagonal lines on each side of the talus apron. The tiltmeter just NE of the dome was destroyed by a rockfall 23 June 1981.

Figure 18. Data from the tangential and radial components of the continuously recording bubble tiltmeter, about 30 m NE of the margin of the dome, between its installation 29 May and its destruction by a rockfall 23 June 1981. Courtesy of D. Dzurisin.

Occasional steam-and-ash emissions were observed during July and early August. An ash-laden gas plume rose to nearly 3 km altitude at 1453 on 9 July, accompanied by seismicity. A small ash plume just cleared the rim of the crater at 1138 on 14 July, and other plumes, accompanied by seismicity, were seen by USGS field crews at about 0845, 0948, 1442, and 1805 on 15 July, the largest reaching about 3 km altitude. A plume emerging from the February lobe of the composite dome reached 3 km altitude at 1257 on 16 July. Light ashfall was reported at Cougar between 0800 and 0900 on 27 July; this ash may have been ejected during a period of seismicity recorded at 0750. Five minutes of low-level tremor accompanied weak gas emission at 1605 on 28 July. An ash-laden plume rose to more than 3 km altitude at about 1805 on 30 July, accompanied by a seismic event and followed by about 5 minutes of low-level tremor. Several episodes of very low-level tremor were recorded 1-2 August. A characteristic burst of seismicity accompanied a plume, recorded on USFS video equipment at 0735 on 2 August, that appeared to be ash-laden and rose to about 3.5 km altitude. Several moderate seismic bursts at about 1905 on 3 August accompanied a small ash plume that reached 3.5 km altitude according to Portland Airport radar; 7 minutes of moderate tremor followed this ash emission. A small ash-laden gas emission occurred at 1133 on 4 August.

Information Contacts: D. Dzurisin, C. Newhall, D. Swanson, USGS, Vancouver, WA; W. Rose, Michigan Technological University; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

August 1981 (SEAN 06:08) Cite this Report

Lava extrusion adds new lobe to composite dome

Extrusion of a new lobe onto the NE portion of the composite lava dome started 6 September and had ended by noon on 11 September, after reaching a volume comparable to previous lobes. The eruption was preceded by increases in the crater displacement rate, SO₂ emission, and seismicity. This was the first extrusion episode at Mt. St. Helens in which weather conditions allowed observation of the crater immediately before and during the event.

On 5 July, the USGS installed three continuously recording bubble tiltmeters along a roughly N-S line within 150 m of the E side of the composite dome. The central tiltmeter, directly E of the middle of the dome, began to record gradual inflation on 8 July. The rate of inflation recorded by this tiltmeter increased systematically through July and most of August, reaching 235 µrad/day by 27 August and totaling about 2,800 µrad for that month. Telemetry problems plagued the tiltmeter 175 m to the NE (at the same site as the instrument destroyed by a rockfall during the June extrusion). Field measurements of its output in July and data returned after resumption of telemetry 13 August showed a similar trend of uplift, but at only about half the rate recorded by the central tiltmeter. Before another rockfall interrupted telemetry 29 August, the N tiltmeter had detected about 800 µrad of inflation in 16 days. About 300 m SW of the central tiltmeter (directly E of the S end of the dome), the third tiltmeter, although functioning properly, recorded no significant net tilt through August.

Of the numerous thrust faults in the crater floor around the dome, the most vigorous showed rapid acceleration in late August. The highest measured rate of movement occurred about 100 m SW of the dome, along a fault first observed after the June extrusion. From 1.8 cm/day between 18 and 22 August, thrusting along this fault increased to 6.3 cm/day 27-29 August. Two other thrusts W of the dome showed similar accelerations but less total movement. Near the very active central tiltmeter E of the dome, movement resumed along an older thrust 22 August and accelerated on the 29th.

However, most of the thrust faults that formed prior to the June extrusion remained inactive. The substantial differences in rates of thrust fault movement, combined with the variation in trends shown by the three bubble tiltmeters, indicated to USGS personnel that the crater floor was behaving as a group of independent blocks or plates on a scale of the order of 100 m rather than as a single coherent body.

Rates of displacement along the former N-crater rampart varied considerably during August. At the most active site, the rate of outward movement increased to 2 cm/day between 12 and 18 August, declined to about 0.7 cm/day until late August, then resumed an irregular acceleration. A very gradual increase in the number of shallow volcanic earthquakes began about 29 July. Most of the events were centered near the former site of Goat Rocks Dome, slightly NW of the present composite dome. Several larger events occurred 13-16 August, then seismic activity remained relatively constant until early September. Small steam-and-ash plumes were occasionally ejected during August, sometimes accompanied by brief periods of harmonic tremor. The volume of SO₂ emissions, measured by COSPEC from fixed-wing aircraft flying under the plume, averaged 60 t/d 9-18 August, then increased sharply to about 360 t/d 19-24 August.

Increases in rates of tilting, outward movement of the former N crater rampart, movement along thrust faults, and SO₂ emission, prompted a USGS advisory on 26 August that stated in part "an eruption, probably of the dome-building type, will likely begin in 1-3 weeks." After this advisory was issued, activity in the crater continued to increase, although rates of SO₂ emission dropped to an average of 150 t/d 26 August-5 September. Between 3 and 5 September, outward movement of the former N crater rampart reached 10.4 cm/day at one site and 6.5-7 cm/day at three others. Thrusting along the most active fault about 100 m SW of the composite dome accelerated to 23 cm/day 1-3 September, 28 cm/day 3-4 September, and 48 cm/day 4-5 September. Movement along the older thrust fault near the central tiltmeter reached a rate of 7-8 cm/day by 4 September.

Beginning 2 September, USGS personnel working in the crater noted 1-2 rockfalls per hour and frequent audible and felt earthquakes. However, the earthquakes were probably very shallow, as no significant increase in seismicity was recorded by the University of Washington seismic net through 4 September. Audible and felt earthquakes in the crater were nearly constant on 5 September, and rockfalls increased further, particularly from the overhanging NE portion of the June lobe. Recorded seismicity began to increase shortly after noon, and increased more rapidly during the predawn hours of 6 September, triggering a joint USGS-University of Washington advisory at 0800 on 6 September that predicted a dome-building eruption within the next 12-48 hours.

During this period, sharply varying data were returned by the three continuously-recording tiltmeters. After recording about 80 µrad/day of inflation 1-4 September, tilt at the N station reversed to relatively slow deflation on 5 September. Deflation continued on this instrument until its telemetry was ended by a rockfall during the afternoon of 6 September. Only 175 m to the SE, the central tiltmeter continued to record increasingly vigorous inflation, with rates reaching 700 µrad/hr on 6 September. This instrument recorded more than 10,000 µrad of inflation on 6 September before an incandescent boulder ended its telemetry during the afternoon. The S tiltmeter (about 300 m SW of the central instrument) had recorded no significant tilt previously but began to show deflation 5 September that continued through the 8th.

The seismic character changed to lower frequency events with emergent arrivals after dawn on 6 September, and epicenters moved to the area of the present dome. At about 1000 on the 6th, avalanche events began to dominate the seismic record, with only a few discrete low-frequency events appearing for the next several hours. USGS personnel working in the crater observed huge blocks falling from the NE portion of the June lobe, and were soon forced to retreat to a ridge N of the crater. Avalanche events peaked on the seismic record at about noon, but remained at high levels until about 1700. Clouds of dust from the frequent rockfalls made observation of the crater difficult, but by 1500-1530 it was evident to USGS personnel that the entire NE portion of the June lobe was breaking up. A bulge appeared to be developing on the E side of the lobe, but poor viewing conditions made this observation uncertain. By 1600-1700, an area of tens of cubic meters of fresh lava was clearly visible on the dome, and by 1830 many glowing rockfalls could be seen. . . . The number of seismic events began to decline after 1700. Significant numbers of low-frequency events resumed briefly about 2200, but seismicity dropped sharply at about 2330.

Aircraft crews monitoring the crater during the night of 6-7 September saw numerous glowing rockfalls, and by 0500 on 7 September a new lobe had been emplaced in the area formerly occupied by the NE portion of the June lobe. Most of the NE portion of the June lobe had fallen as talus, but from its high point to its SW margin, the June lobe remained intact. Slow aseismic growth and downslope spread of the new lobe continued through the afternoon of 10 September, but USGS field parties reported that growth had stagnated by noon 11 September. The new lobe was comparable in size to lobes extruded in previous episodes, but precise determination of its volume and daily growth rate await analysis of airphotos and reduction of field data.

Information Contacts: T. Casadevall, D. Dzurisin, C. Heliker, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

September 1981 (SEAN 06:09) Cite this Report

Minor ash emission; slow deformation

When USGS personnel arrived in the crater on the morning of 11 September, there was a characteristic area of smoother lava on the top of the new lobe. Similar features had marked the end of the December 1980-January 1981 and June 1981 extrusion episodes. No further growth was observed. The new lobe had a volume of about 5 x 10⁶ m³, comparable in size to previous lobes, and brought the total volume of the dome to about 30 x 10⁶ m³.

Poor weather plagued monitoring efforts after the extrusion episode. At 1559 on 10 September, just before extrusion ended, gas and fine ash rose to about 3 km altitude in a 15-minute eruptive episode accompanied by seismicity. Other gas emissions, all accompanied by seismicity, occurred at 0705 on 13 September, 1426 on 14 September, and 1028 on 16 September. No additional gas emissions were observed through the end of September.

Deformation within the crater showed a pattern similar to that of previous post-extrusion periods. The rate of thrust fault movement, which had accelerated to nearly 50 cm/day on the most active fault just prior to the September extrusion, decreased rapidly before stabilizing on 10 September. After the September extrusion ended, continued slow movement (about 0.5-1 cm/day) was measured on some thrust faults around the dome, while other thrusts remained inactive. Similarly, outward movement of one station on the N crater rampart reached more than 10 cm/day before extrusion began; after the extrusion episode ended, rates of outward movement had dropped to 0.25-0.6 cm/day.

The volume of SO₂ emission peaked at 660 t/d during the afternoon of 6 September, just prior to the beginning of lava extrusion. During the extrusion episode, emission rates varied from 190 to 310 t/d, then dropped on 11 September to 70 t/d, the lowest measured rate for the month. SO₂ emission increased sharply in mid-September to 530 t/d on the 17th and dropped to 340 t/d on the 18th; then poor weather stopped data collection until the end of the month, when two days of measurements showed a rate of about 250 t/d.

Information Contacts: T. Casadevall, D. Dzurisin, D. Swanson, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

October 1981 (SEAN 06:10) Cite this Report

Lava extrusion adds new lobe to composite dome

Lava extrusion that probably began 30 October added a new lobe to the composite dome in the crater, the 9th extrusive episode since the catastrophic eruption of 18 May 1980. After lava extrusion ended 10 September (SEAN 06:08), rates of displacement in the crater remained low for several weeks, as they had after earlier extrusion episodes. Sulfur dioxide emission ranged from 70 to 190 t/d 9-24 October, but showed no particular trends.

The crater floor was not quiet during this period. Inflation of the dome has caused small thrust faults to form in the surrounding crater floor. In early October, the most active thrust, S of the dome, was moving at about 1.5 cm/day, and stations on the N crater rampart showed outward movement of about 0.5 cm/day. By 24 October, these rates had increased to 14.5 and 3.5-4 cm/day respectively, and levelling profiles perpendicular to the dome showed that crater floor tilt rates had reached 400-500 µrad/day, prompting the USGS to issue an advisory prediction of renewed lava extrusion within the next 2 weeks.

Eleven laser targets were installed on the dome and distances to the targets were measured from fixed points 150-1,000 m away beginning 21-23 October. Rates of lateral expansion of the dome varied considerably, but were as much as 50 cm/day as of 25 October. Three continuously recording tiltmeters located 550, 850, and 1,050 m N of the center of the dome began returning data 13 October. No significant tilt was detected by these instruments until about noon on 26 October, when all three began to record inflation at 25-50 µrad/day. The number of local earthquakes began to increase on 27 October, and gradually became more frequent through the 29th, but most of the events were extremely small. Seismicity increased substantially between midnight and 0700 on 30 October, then all but rockfall events ceased fairly abruptly shortly after 0730. During this period, there were a few events that reached roughly M 1, but none were in the M 1.5-1.8 range typical of the larger events prior to past additions to the dome, and the total seismic energy release was much smaller than that associated with previous extrusion episodes. Inflation registered by the three continuously recording tiltmeters N of the dome flattened out on 29 October and at least one instrument began to show deflation by 0800 on the 30th.

Poor weather prevented any observations of the crater 30 October. When visibility improved early on the 31st, a substantial quantity of lava had already been extruded from the N part of the pre-existing dome's summit area. The new lobe advanced gradually down the N flank over portions of lobes extruded in April, June, and September. Geologists in the crater 31 October noted that several plastic airphoto targets N of the dome had melted and some metal fenceposts had been bent, although boards on the ground were not disturbed, indicating that a directed hot blast may have moved 200-300 m N from the dome. By the evening of 1 November, the new lava had advanced about halfway down the dome's N flank (figure 19), but was moving more slowly than the previous day. The lobe front moved roughly 20-40 m downslope during the next 24 hours, a quarter to half of the previous rate of advance. As of late 4 November, lava had nearly reached the foot of the April lobe, where a substantial pile of talus had accumulated, and appeared to be advancing slowly, but lava extrusion had stopped by 5 November. The volume of the new lobe appeared to be roughly comparable to that of previous lobes (a few million cubic meters).

Figure 19. Sketch of the N side of the Mt. St. Helens composite dome at 1400 on 1 November 1981, looking S from the mouth of the crater. Sketch by Bobbie Myers.

The extrusion was accompanied by major deformation of the dome, particularly at the S end, where dramatic changes took place. Large blocks were thrust upwards 10-20 m and steep outward tilting was obvious. On 31 October, the four remaining laser targets on the dome showed outward movements of 1-8.2 m since the previous measurements six days earlier. However, by 2 November, expansion of the pre-existing dome had slowed to only a few centimeters per day and some areas may have been showing subsidence. Large incandescent cracks radial to the center of the new extrusion extended from top to bottom of the September lobe, and the June lobe also showed large new cracks. As during previous extrusion episodes, thrust faults stopped moving (on 1 November) and significant deformation of the crater floor had not resumed as of 10 November. SO₂ emission during the extrusion peaked at 220 t/d (on 31 October) then dropped steadily, to 160 t/d 3 November, 130 t/d on the 6th, and 80 t/d on the 10th.

Information Contacts: T. Casadevall, D. Dzurisin, D. Peterson, D. Swanson, USGS, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

November 1981 (SEAN 06:11) Cite this Report

Dome expands slowly; rock avalanches

Measurable downslope movement of the new lobe extruded onto the composite lava dome (figure 20) had ended by 4 November. Distance measurements from fixed points on the crater floor to laser targets on the dome showed that very slow expansion of the entire dome continued through November. As of late November, the rate of expansion was stable at roughly 0.25-0.33 cm/day. No movement occurred during November along any of the small thrust faults formed in the crater floor by past expansion of the dome, nor was there any measureable outward movement of the N crater rampart. Gas sampling was infrequent because of poor weather, but no significant changes were evident in the rate of SO₂ emission.

Figure 20. Sketch map of the Mt. St. Helens composite dome before extrusion of the October-November 1981 lobe, prepared by D. Swanson. The dark circles indicate the positions of the eleven laser targets installed on the dome in late October, before the latest lobe was extruded. Some of the small thrust faults in the crater floor around the dome are shown by hachured lines.

During November, seismic instruments recorded 54 earthquakes in the Mt. St. Helens area (some were aftershocks from the Elk Lake earthquake of 13 February; SEAN 06:02), a typical level of seismicity for a month without an extrusion episode. The largest event was slightly stronger than M 1. Many of the earthquakes resembled steam plume or rock avalanche events, but weather conditions prevented visual confirmation. Medium-frequency events with sharp arrivals, similar to some of the August and early September earthquakes, were recorded at a rate of about 1/day in early December.

A series of seismic signals that indicated rock avalanching in the crater began with isolated 30- to 45-minute periods of seismicity late 4 December. Activity peaked the next day, with 14 events of more than 45 seconds duration between 0930 and 1800, the longest a 220-second episode beginning at 1253. Seismic instruments continued to record occasional rock avalanche events during the night of 5-6 December. As of press time, weather conditions had prevented observations of any changes that may have occurred in the crater. None of the December rock-avalanche events were as large as the seismic signal from the 2 October avalanche that deposited as much as 7 m of debris in the S part of the crater.

Information Contacts: D. Swanson, USGS Cascades Volcano Observatory (CVO), Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

December 1981 (SEAN 06:12) Cite this Report

Rock avalanches; no significant deformation

After the entire dome showed very slow expansion through November, it subsided about 5 cm between early December and early January, and spread outward a roughly proportional amount (figure 21). Deformation of the crater floor surrounding the dome has typically begun a few weeks after past lava extrusion episodes, but as of 11 January there had been no significant deformation of the crater or of the edifice as a whole. No movement occurred along any of the small crater-floor thrust faults produced by past expansion of the dome, and no outward movement of the N crater rampart was detected. No changes in crater floor tilt were recorded. The average quantity of SO₂ emitted daily has decreased steadily through 1981 and this trend continued in December and early January. Poor weather limited gas sampling to only 4 days in December, when 50-135 t/d of SO₂ were emitted, with an average of about 100 t/d. SO₂ emission rates were measured at 120 t/d on 7 January and 66 t/d on the 10th.

Figure 21. Sketch map of the 1982 Mt. St. Helens composite dome in the crater, prepared by D. Swanson. The dark circles indicate the positions of the 11 laser targets installed on the dome in late October,a before the latest lobe was extruded. Some of the small thrust faults in the crater floor around the dome are shown by hachured lines.

Seismicity remained at a low level through December. Most of the 64 discrete earthquakes in the Mt. St. Helens area were aftershocks of the 13 February Elk Lake event (SEAN 06:02). Large rock avalanches from the crater walls occurred during periods of poor weather but were detected on seismographs. On 5 December, a 1-hour period of seismicity that began about 0200 included five events of more than 70 seconds duration, and two others of 40-70 seconds in length. Between 1200 and 1700 the same day, seven more events lasting 70 or more seconds were recorded, the longest (150 seconds) at 1607. Geologists observed rockfall debris covering much of the NE floor of the crater after this seismic episode. Debris from another large rockfall, on the NW crater floor, was discovered 23 December after geologists had been prevented from entering the crater by a week of wet weather. Seismic records did not show any activity that could be unequivocally associated with this rockfall. Most rockfalls have occurred during or just after storms and have not apparently been correlated with volcanic activity.

Information Contacts: T. Casadevall, D. Swanson, USGS, CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

January 1982 (SEAN 07:01) Cite this Report

Seismicity increases slightly but little deformation

Although deformation of the crater floor has typically begun a few weeks after past extrusion episodes, USGS monitoring had shown few changes within the crater by early February. The small crater-floor thrust faults produced by past expansions of the dome had shown no significant movement as of 10 February, nor had there been any outward movement of the N-crater rampart or inflation of the edifice as a whole. However, telemetry from a crater-floor tiltmeter 1 km N of the dome resumed 4 February after a 3-month hiatus and showed 17 µrad of inflation in 4 days. The dome continued to spread very slowly outward through December and January, at 1-3 mm/day. The rate of SO₂ emission remained low in January, usually ranging from 50 to 100 t/d, but increased steadily during the first 8 days of February from about 60 to 130 t/d.

During the first half of January, there were only three seismic events large enough to be recorded on more than one seismograph of the Mt. St. Helens net. However, during intermittent periods of increased seismicity in the last 2 weeks of January, several events per day were recorded. These active periods lasted as long as 3-4 days before seismicity declined to background levels. A total of 27 earthquakes in the Mt. St. Helens area, a few as large as M 1, were recorded during the second half of the month. Similar activity continued into early February.

Information Contacts: D. Dzurisin, J. Ewert, D. Swanson, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

February 1982 (SEAN 07:02) Cite this Report

Eruption expected by the end of March

No eruptive activity has occurred since measureable downslope movement of the newest lobe of the composite dome stopped 4 November. However, seismicity beneath the crater and deformation within the crater began in late February, and the USGS expected an eruption by late March.

Only two shallow earthquakes large enough to be detected by more than one instrument of the Mt. St. Helens net occurred 1-23 February. Seismic activity increased on 24 February. Since then, seismographs have recorded about five very small (negative magnitude) events per day centered below the crater at 5-12 km depth, and 10-12 somewhat larger (but usually less than M 1) shocks per day at 1-3 km depth. No earthquakes have been located in the zone between the groups of shallow and deeper foci, and no harmonic tremor has been recorded. The deeper earthquakes were the first since 1980, when they often followed (but never preceded) explosive activity.

Deformation within the crater began at about the same time as the increased seismicity. Outward movement of the N-crater rampart started in late February and had increased to 2.4 cm/day by early March. Horizontal expansion of the lower portion of the composite lava dome increased to 1.6-1.7 cm/day by early March, apparently accompanied by uplift, but conditions prevented measurements of targets on the upper section of the dome. Movement also occurred along the small crater-floor thrust faults, produced by past expansions of the dome, but snow made quantitative monitoring difficult. A dry tilt station about 300 m N of the dome showed inflation during each of three measurement intervals (6-24 February, 24 February-5 March, and 5-8 March), but a tiltmeter 700 m farther N detected no inflation, suggesting to USGS geologists a shallow source near the first tilt station. In contrast, simultaneous inflation was measured at these two sites before the last extrusion episode October-November 1981. No expansion of the edifice as a whole had occurred as of early March. Measurements of SO₂ emission have been infrequent because of poor weather. The rate of SO₂ emission had increased in early February and reached 240 t/d on the 10th. Poor weather prevented further measurements until 21 February. Since then SO₂ emission has remained at background levels of 80-120 t/d.

On 12 March, the USGS and University of Washington issued a joint extended outlook advisory stating that an eruption was likely within the next 3 weeks. Another dome-building episode was rated as the most probable eruption type, but because of the changed seismic pattern the advisory noted that explosions or lava flows were possible.

Information Contacts: D. Dzurisin, J. Ewert, D. Swanson, K. Cashman, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; R. Tilling, USGS, Reston, VA.

March 1982 (SEAN 07:03) Cite this Report

First large explosion since October 1980; two new lobes added to lava domes

The first explosive eruption in 17 months ejected a tephra cloud that briefly rose to more than 13.5 km altitude on 19 March. A directed blast from near the base of the lava dome spawned a multilobate avalanche that flowed several kilometers down the volcano's N flank. A mudflow moved down the N fork of the Toutle River, but caused only minor damage. Clouds produced by explosions 20-21 March were much smaller and contained only a little tephra. Lava extrusion began 21 March, adding a new lobe to the SE side of the crater's composite dome. No injuries resulted. Smaller explosions 4-5 April were followed by the extrusion of a second lobe onto the N side of the dome.

Premonitory activity during 26 February-18 March. Seismic activity began to increase 3 weeks before the March eruption and included a substantial number of deeper events, in contrast to previous dome extrusion episodes, which were typically preceded by only a few days of shallow seismicity. Earthquakes occurred in two zones, at about 1-3 and 4-12 km depth (below average seismic station elevation of about 1 km above sea level). An average of one event per day stronger than M 1.5 occurred in the shallow zone 26 February-12 March, with the rate of energy release remaining relatively constant. Most of the deeper events had negative magnitudes, and energy release from the deeper zone was about 2 orders of magnitude less than from the shallow zone. The end of deeper seismicity 12 March coincided with both an increase in the number of events (to an average of 3/day of m_b > 1.5 through 17 March) and a jump in the rate of energy release.

Deformation in the crater accelerated rapidly in mid-March. Between 17 and 18 March, uplift of an area near the SW base of the dome accompanied about 30 cm of movement along a nearby thrust, higher than any rate of crater-floor thrust movement previously measured at Mt. St. Helens. Outward displacement rate of the N-crater rampart reached 32 cm/day, and a portion of the dome itself expanded 42 cm in the 24 hours ending shortly before the eruption. However, no deformation of the edifice as a whole was detected by measurements outside the crater. For the first 18 days of March, the rate of SO₂ emission averaged 110 t/d, remaining at about the same level as it has since the lava extrusion episode of October-November 1981.

After remaining approximately constant for several days, the rate of seismic energy release increased again about noon 18 March, and 14 events larger than M 1.5 were recorded in the next 24 hours. A few brief (1-2 minutes or less) periods of low-level harmonic tremor were recorded during the afternoon of 19 March, as were 20 discrete events stronger than M 1.5. SO₂ emission doubled to about 230 t/d. Tilt measured about 300 m N of the dome reversed about 1900 and seismic data showed that explosions began at 1928. After 2 minutes of initial seismicity there was a brief hiatus, followed by about 40 minutes of activity that declined gradually.

Explosive eruption on 19 March. A vertical tephra column, probably ejected from a vent near the center of the dome, reached its maximum altitude of more than 13.5 km (as measured by radar at Portland airport) at 1933 on 19 March. By 1950, radar data indicated that the altitude of the top of the column had dropped to 10.5 km. An infrared image returned at 2003 by a NOAA geostationary weather satellite showed a cloud-top temperature of -35°C, yielding an altitude of about 7 km. According to radar data, the eruption column contained 20-60 times less tephra than the cloud produced by the last significant explosion, in October 1980 [but see SEAN 07:04]. Ash blew SE at about 30 km/hr. Light ashfalls were reported as much as 80 km away, but caused only minor disruptions to auto travel. Bombs up to 3 m across fell 200-300 m from the dome. Frothy pumice (density about 0.8) fell 8 km away. Smaller explosions occurred at 0135 the next morning, when radar detected a cloud, containing a little ash, that rose to about 5.5 km altitude, and a small steam-and-ash column was ejected at 0415 on 21 March.

[Further investigation revealed a more complex sequence of events than was originally reported in the Bulletin. The following has been modified by R. Waitt and D. Swanson. A detailed description can be found in Waitt and others, 1983.] The initial avalanche apparently resulted from a directed blast that emerged from near the SW base of the dome. This blast destroyed the dome's SW margin, and struck the S wall of the crater, removing snow cover and rock. The resulting mixture of snow granules (0.5-2 mm in diameter), hot pumice, and lithic material [descended] the [E and] S crater walls, [flowed around] the E and W sides of the dome, joined N of it, then flowed out through the breach in the N side of the crater and continued for several kilometers down the N flank. Fed by water from the avalanche . . . and a [transient] pond [behind the dome], a complex mudflow sequence moved down the N fork of the Toutle River, which flows W . . . at the N foot of the volcano. Upstream deposits showed evidence of two distinct pulses, but gauges downstream registered only one well-defined peak. About 70 families were evacuated from the Toutle valley, but no major damage was reported. The mudflow buried trucks at an earthen flood-control dam and breached its S side. Three storms earlier this winter had produced higher peak river stages at Castle Rock, roughly 70 km downstream. Floods produced by these storms had breached the N side of the dam and the combined damage has essentially destroyed the dam's effectiveness.

Lava extrusion on 20-24 March. Seismographs began to record rockfall events, probably associated with extrusion of a new lobe of lava, during the evening of 20 March. This activity slowly increased, and aerial observers first saw the new lobe during the night. It emerged from a vent at the top of the most recent lobe (extruded October-November 1981) and flowed down the SE side of the dome, barely reaching the crater floor. Growth was fairly rapid through 23 March, but there was little apparent increase in size between the 23rd and 24th, and the number of rockfall events was noticeably declining early 24 March. By the time growth slowed, the volume of new lava appeared to be greater than that for any previous lobe. SO₂ emission increased to 370 t/d 21 March, about 3.5 times background levels, but had dropped to 90 t/d by 24 March.

However, before dawn on 24 March new glowing radial cracks were observed in older portions of the dome. The N-crater rampart and the N side of the dome showed 12 cm of outward movement between the mornings of 23 and 24 March and 16-18 cm during field work 24 March. No unusual seismicity accompanied the movement, nor was any significant tilt measured N of the dome, but at similar stages of previous dome extrusion episodes, little or no deformation of any kind has been observed.

Poor weather prevented geologists from entering the crater again until early April. Seismicity remained at low levels through the end of March. SO₂ emission dropped to about background levels 24 March, but by the next measurement, on 28 March, had increased to about 200 t/d and reached a rate of 440 t/d during a small gas explosion. On 29 March, the rate was still high, at 180 t/d, but weather conditions prevented further measurements until a week later. Seismographs began to record a few very small, brief (20 seconds or less) harmonic events 1-2 April, and these became more numerous 3-4 April. Occasional low-frequency earthquakes began to appear on the seismic records 3 April. A few were recorded the next morning, then these events increased to about 2 per hour after 1400. A further increase in seismicity was noted in the early evening, and at about 2000, University of Washington seismologists alerted USFS and Washington state officials that an eruption was imminent.

Renewed explosions and dome growth during 3-12 April. [A large rock avalanche and] explosive activity began at 2052 on 3 April, and three seismic pulses occurred in 3 minutes. A plume containing a little ash rose to 8.5 km (altitude data from Portland airport radar) and drifted NE. Minor ashfall was reported in Packwood, 65 km away. Seismographs recorded pulsating activity for the next several hours, then a pair of stronger events at 0035 and 0039 that accompanied the ejection of an ash-poor cloud to almost 10 km altitude (as measured by Portland airport radar). A small mudflow emerged from the breach in the N side of the crater and flowed a short distance down the N flank. After 10-15 minutes, seismicity briefly dropped to background levels, but apparent harmonic tremor began about 0230 and continued for the next 14 hours. Gas and/or rockfall events began at roughly 0330 and became increasingly frequent during the next several hours.

Before dawn, geologists observed a new lobe of lava on the N side of the composite dome. Growth of this lobe continued through 8 April, but had slowed considerably by the 9th. The April lava, perched on the N side of the dome, looked very similar to the October 1981 lobe but appeared to be smaller than any previously extruded. Gas emission events, including one that sent a plume to 7 km altitude at 1719 on 5 April, could be seen on seismic records, as well as large avalanche events as large chunks fell off the dome. Seismicity declined gradually as lava extrusion continued and had dropped to low levels by 12 April. By 10 April, deformation in the crater had decreased to levels typical of periods between extrusion episodes. As lava extrusion was beginning early 5 April, the rate of SO₂ emission increased to 900 t/d, dropping to 500 t/d during the afternoon, and to 390 t/d, a typical value during dome extrusion episodes, on 6 April. No gas data were available 7 April but SO₂ emission had returned to background levels 8-10 April.

Further Reference. Waitt, R.B., Pierson, T.C., MacLeod, N.S., and Janda, R.J., 1983, Eruption-Triggered Avalanche, Flood, and Lahar at Mount St. Helens-Effects of Winter Snowpack: Science, v. 221, no. 4618, 1394-1396 p.

Information Contacts: T. Casadevall, R. Janda, C. Newhall, D. Swanson, R. Waitt, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington; O. Karst, NOAA/NESS; D. Harris, University of Alberta; R. Bailey, USGS, Reston, VA.

April 1982 (SEAN 07:04) Cite this Report

Lava extrusion adds new lobe to composite dome

The tephra volume of the 19 March eruption column was of the same order of magnitude as (but probably slightly less than) that of the October 1980 cloud (not 20-60 times less, as reported in SEAN 07:03). Extrusion of lava onto the N side of the composite dome stopped by 10 April and seismicity had dropped to low levels by the 12th, but another dome-building phase began on 14 May.

During the 19 March activity, the crater-floor thrust fault scarps near the dome were either buried or scoured away. Measurement of the rates of movement of these small faults had previously been an important deformation monitoring technique. By early May a number of small new thrusts had formed on the crater floor W and SW of the dome, but had not yet yielded useful deformation data. Data from a continuously recording tiltmeter NW of the dome varied considerably from dry tilt measurements made as little as 100 m away, indicating that (as in previous inter-eruption periods) the crater floor was deforming as a group of relatively small, independent blocks rather than as a coherent unit.

Rates of SO₂ emission have remained at about two times background levels since the end of lava extrusion 10 April. Ejection of small, ash-poor plumes from the dome's summit began 21 April, and geologists observed 1-2/day through early May. Many of the plumes could be correlated with seismic events, some of which were felt in the crater. Comparison of distance measurements made immediately before and after individual plume ejections showed that points on the dome had moved outward and upward by 3-5 cm after each small explosion.

Swelling of the composite dome was relatively slow until early May, then accelerated. By 11 May the SW portion of the dome was moving outward as much as 70 cm/day. Other areas of the dome were less active. Although the SW portion of the dome had also experienced the most rapid expansion prior to the October-November 1981 eruption, the lobe extruded during that episode emerged from the N part of the dome's summit area.

The number of local earthquakes began to build about 8 May and by 12 May had reached about 12 events per day (magnitude greater than about 1). All were shallow and had magnitudes of less than 2. Many were felt by geologists working near the dome.

Because of the increasing seismicity and deformation, the USGS and University of Washington issued a joint extended outlook advisory late 11 May stating that an eruption was likely to begin within the next week, possibly within the next few days. The number of shallow, low-frequency events grew considerably late 13 May. Lava began to emerge from the summit of the composite dome between 0100 and 0200 on 14 May and began to flow down the dome's NE flank. Harmonic tremor increased substantially about 0200, and a steam plume rose about 2 km above the crater rim. Deformation and growth of the dome was continuing on crater rim. Deformation and growth of the dome was continuing on 17 May, but seismicity had gradually decreased.

Information Contacts: T. Casadevall, C. Newhall, D. Peterson, D. Swanson, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington, D. Harris, University of Alberta.

May 1982 (SEAN 07:05) Cite this Report

Frequent gas and ash emission; more on May lobe

Lava began to flow down the NE side of the dome 14 May, but the bulk of the new lava formed a lobe on the dome's NW flank 15-19 May. Since then, ejections of steam and ash, similar to those of July and August 1981, have occurred about once a day. Two of these, on 7 and 8 June, caused light ashfalls on Portland. Gas-emission rates remained high through early June.

The crater-floor deformation before the extrusion was blocky and incoherent, as during previous pre-extrusion periods. While a continuously recording tiltmeter at a new site on the W crater floor recorded increasingly rapid subsidence (as did a dry tilt station at the same location), reoccupation of dry tilt stations less than 100 m away showed accelerating uplift. The development of a small thrust fault was observed between the dome and the continuously recording tiltmeter, leading Dan Dzurisin to suspect that thrusting was responsible for the different tilt directions at nearby sites.

Local seismicity had begun to increase on 8 May (SEAN 07:04). In the 24 hours starting at 0700 on 13 May, 63 earthquakes were recorded (about twice the previous day's number) and some were felt by geologists working in the crater that day. Three radiating fractures, trending NE, N, and NW, were seen in the April lobe, on the N side of the dome. SO₂ emission remained at background levels of about 100 t/d. The rapid subsidence measured by the continuously recording tiltmeter stopped about midnight. Harmonic tremor started shortly thereafter, at 0055 on 14 May, and continued until about 0600. Bursts of seismic energy could be seen within the tremor. During an overflight at 0415, spectacular, nearly continuous cascades of incandescent material could be seen on the NE flank of the dome, but an hour later the rockfalls had ceased almost entirely. After dawn, a jumbled, blocky area could be seen on the dome's summit and upper NE flank, and there was a rockfall apron on the NE side of the dome. The jumbled area was larger by afternoon, but it was not certain whether it was new lava or scoriaceous older material being uplifted by endogenous dome growth. Episodic gas emission was observed on 14 May, and by afternoon the rate of SO₂ release had increased fourfold from the previous day, to about 400 t/d. The number of earthquakes decreased to twenty in the 24 hours starting at 0700 on 14 May.

On 15 May, rockfall activity continued on the dome's NE flank and rockfalls began on its NW side. The surface morphology of the N side of the April lobe was changed, but no movement was visible, and growth appeared to be occurring within the April lobe. However, by afternoon, flow texture had developed on a tiny lobe on the NE flank and a dominant lobe on the NNW flank. The number of recorded earthquakes declined to eleven between 0700 on 15 May and 0700 on the 16th. Downslope movement of the NNW flank lobe continued on 16 May, but the tiny NE flank lobe was stagnant. The number of earthquakes had dropped to only 1-2/day. Poor weather prevented observations 17-18 May, but seismographs detected numerous rockfalls. When geologists returned to the crater 19 May, rockfalls and some downslope movement of the NNW flank lobe were continuing, but little extrusion appeared to have occurred since the 16th. Only minor rockfalls were observed 20 May and these had stopped by evening.

The rate of SO₂ emission remained high 15-19 May, at 250-650 t/d. On 19 May, several fissures, surrounded by a small tephra blanket, were observed in the top of the March lobe. That day, a plume from these fissures quickly increased the SO₂ emission rate from 340 to 2,600 t/d, but the rate dropped back to 500 t/d after about 50 minutes. An average of one large gas and ash ejection per day has occurred through early June. Many caused light dustings of ash near the volcano. The ash consisted of abraded and rounded lithic fragments and crystals, but included no fresh magma, although some of the larger fragments were hot. The highest observed plume rose to 5.5 km altitude late 6 June. Light ashfalls occurred in Portland early 7 and 8 June. SO₂ emission continued at an elevated level of 200-300 t/d through early June.

USGS analyses showed no significant chemical differences between the lobes extruded in 1981 and in March, April, and May 1982. All contain 62-63% SiO₂ and 40-42% phenocrysts (table 3).

Table 3. Analysis from the USGS analytical laboratory, Denver, CO, averages three samples from the Mt. St. Helens May 1982 lobe. Each sample was split into three groups before analysis and all were run with an internal standard. *Total iron as Fe₂O₃. Data provided by Kathy Cashman.

Information Contacts: T. Casadevall, K. Cashman, D. Dzurisin, C. Newhall, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington.

June 1982 (SEAN 07:06) Cite this Report

Gas and tephra plumes; deformation

After downslope movement of lava extruded onto the N flank of the composite dome ended about 19 May, activity was limited to ejection of vapor plumes that sometimes contained tephra. Vigorous gas emission from the top of the dome produced plumes that lasted 5-45 minutes. Old material from the walls of the vent was carried upward by the gas, and blocks 20-30 cm across fell about 125 m from the dome. On 9 June at 1316, the largest of these rose to 6 km altitude, dropping ash a few kilometers to the N and larger tephra in the crater. The accompanying high-amplitude burst of seismicity saturated nearby seismographs but appeared to consist of a series of individual events. Before 9 June, most of the plume emissions were associated with lower amplitude seismic signals, but about half of those after 9 June were accompanied by bursts of stronger seismicity. Plumes were ejected about once a day until 22 June, but none has been observed since then. Before the May extrusion episode, similar gas and tephra ejections were typically preceded by 1-3 minutes of felt earthquakes, and electronic distance measurements showed that they were accompanied by several-centimeter expansions of the dome. However, neither precursory earthquakes nor deformation were associated with the post-extrusion plume emissions. The rate of SO₂ emission had remained high for several weeks after the May lava extrusion (SEAN 07:05), but returned to the normal inter-eruption background level of about 100 t/d by early June.

Electronic distance measurements to targets on the dome and crater floor began to show [swelling] in mid to late June. The rate of [swelling] was a few millimeters per day in early July and was not accelerating significantly. Tilt stations on the crater floor also showed uplift, at a rate about twice as high 7 June-6 July as mid May-early June. The 7 June-6 July rate was similar to that observed several weeks before the May lava extrusion.

Information Contacts: T. Casadevall, W. Chadwick, D. Dzurisin, USGS CVO, Vancouver, WA; C. Boyko, S. Malone, E. Endo, C. Weaver, University of Washington.

July 1982 (SEAN 07:07) Cite this Report

Dome growth begins 17 August

Electronic distance measurements from sites on the crater floor to targets on the composite dome registered [swelling] beginning in mid-June. The rate of [swelling] remained a few millimeters per day until the end of July, when it accelerated to a few centimeters per day. On 27 July local seismicity increased to 3-4 events per day, then stabilized on 2-3 August to 1-2 events per day. By 13 August, 3-4 events per day were again occurring. The rate of SO₂ emission has remained at the inter-eruption background level of about 100 t/d.

The seismic pattern and the rise in displacement rate prompted the USGS and University of Washington to issue an extended outlook advisory on 30 July, predicting an eruption within 3 weeks. Seismicity and displacement rates continued to increase. On 16 August an eruption was predicted within 4 days, then within 24 hours on 17 August.

The dome began to grow endogenously on 17 August. The W and SW sides were expanding at a rate of about 10 m/day. Numerous rockfalls were occurring, but without explosions or changes in rates of deformation, displacement, seismicity, or gas emission. Extrusion of a new lobe on top of the dome began during the day on 18 August, with lava flowing slowly onto the dome's W and S sides.

Information Contacts: W. Chadwick, C. Newhall, USGS CVO, Vancouver; S. Malone, University of Washington.

August 1982 (SEAN 07:08) Cite this Report

New lobe extruded onto composite lava dome

Lava extrusion that began early 18 August added a new lobe to the composite dome during the next 5 days. Seismicity and increasing deformation of the dome and the surrounding crater floor preceded the eruption, which included about a day of strong endogenous-dome growth before lava appeared at the surface.

Distances and vertical angles were measured between five sites on the crater floor and targets (5-6/station) on the dome. Rapid acceleration of dome deformation started about 6 August, but was strongly asymmetrical, concentrated on the W side. Some targets on the W side of the dome were moving outward at 30 cm/day by 15 August, but distances between the crater floor and points on the E and N sides of the dome shortened by no more than 4 and 5 cm/day respectively, and no movement occurred there until a week before the eruption.

The number of earthquakes that could be located by the University of Washington-USGS seismic net (usually events with magnitudes of 1 or greater) began to build in late July (SEAN 07:07), then increased to an average of 6/day 11-13 August and about 12/day 14-16 August. All were centered within a few kilometers of the surface. Smaller events (magnitude less than or equal to 0) showed comparatively steady increases through this period.

Continuously recording tiltmeters 50-60 m W of the dome measured thousands of µrad of tilting. A tilt reversal had been noted a few hours before the start of some earlier extrusion episodes, but no such reversal preceded the August eruption. Rapid tilting was recorded until rockfalls from the new lobe ended transmission.

Rapid thrusting began in mid-August along cracks in the crater floor W of the dome, reaching rates of 2.5 m/day laterally and 1.5 m/day vertically along the most vigorous fault just before the eruption. The thrusting subdivided the W crater floor into numerous small blocks, and one dry tilt station was cut by a 3 m thrust scarp. Tilt stations will be re-established but on a part of the crater floor less prone to thrusting. Rapid endogenous growth of the W side of the dome began 17 August. Parts of the W side of the dome moved 12 m outward that day and 22 m of displacement (13 m [downward]) of one W side dome target was measured the next day. The character of seismicity began a gradual change on 17 August, when only two events occurred that were large enough to be located, and surface (rockfall or gas emission) events started to dominate the seismic records.

New lava was first seen on the surface of the dome at 1130 on 18 August, emerging from the NW side of the summit. After the new lava appeared, the June 1981 and March 1982 lobes began to crack apart, and large blocks rolled down the W side of the dome. When geologists left the crater at about 1800, the new lobe was still small and was confined to the summit area.

By the next morning, lava had flowed onto the NW flank and was moving down the talus slope at the base of the dome. Substantial growth of the new lobe continued through the 19th, and slower extrusion occurred for the next few days before ending between 22 and 23 August. Slow deformation of the E side of the dome (where targets remained in place during lava extrusion) continued through 20 August, then stopped, and had not resumed as of early September.

SO₂ emission remained at the background level of 80 (± 50) t/d through 17 August, but increased sharply, to about 500 t/d, when lava extrusion began early 18 August (figure 22). After reaching a maximum of 530 (± 100) t/d that afternoon, the rate of SO₂ emission declined gradually to 150 t/d on the 22nd, then remained at roughly 130 t/d through the end of August.

Figure 22. Rates of SO2 emission at Mt. St. Helens, August 1982. Courtesy of Robert Symonds.

Information Contacts: D. Dzurisin, C. Heliker, C. Newhall, R. Symonds, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

September 1982 (SEAN 07:09) Cite this Report

Deformation, SO₂, and seismicity remain low

As of 5 October, no significant deformation of the lava dome or the surrounding crater floor had been observed since the August extrusion episode (SEAN 07:07 and 07:08), although some spreading and subsidence of the new lobe continued. SO₂ emission remained very uniform, ranging from 50 to 135 t/d and averaging 95 ± 25 t/d in September. Seismicity remained at background levels through early October.

Information Contacts: T. Casadevall, D. Swanson, USGS CVO, Vancouver, WA; C. Boyko, University of Washington.

October 1982 (SEAN 07:10) Cite this Report

SO₂ emission declines; no deformation

Seismicity remained at background levels through early November, and no significant deformation has been measured. SO₂ emission ranged from 20 to 70 t/d in October, the lowest rate since the eruption of 18 May 1980.

Information Contacts: T. Casadevall, D. Swanson, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

November 1982 (SEAN 07:11) Cite this Report

No deformation; seismicity and SO₂ emission low

As of early December, no significant deformation had been measured. Snow conditions in the crater limited vertical angle and distance measurements between the dome and the crater floor, but comparison of 24 November and 4 December data showed no change. A continuously recording tiltmeter was installed 19 October on the E crater floor, within 25 m of the talus at the base of the dome. This instrument measured daily oscillations in tilt, but had recorded no deformation exceeding the diurnal variation as of 6 December. The rate of SO₂ emission declined to an average of about 35 ± 10 t/d in November, the lowest measured since 18 May 1980. Before the August extrusion episode, the average rate of SO₂ emission between eruptions had been roughly 100 t/d. Seismicity remained at background levels through November.

Information Contacts: T. Casadevall, D. Dzurisin, D. Swanson, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

December 1982 (SEAN 07:12) Cite this Report

Deformation, seismicity and SO₂ emission quiet

Gas emission, deformation, and seismicity remained quiet through the end of December. SO₂ emission ranged from 20 to 35 t/d, similar to the decreased amounts measured in November. No significant deformation of the composite lava dome or the edifice as a whole was detected. Some sagging of the dome continued, but at declining rates. Seismicity remained at background levels. Seismic records showed a few signals that may have been generated by small avalanches, but no gas emission events were detected.

Further Reference. Special Section: Mount St. Helens: Science, 1983, v. 221, p. 1369-1396 (9 papers).

Information Contacts: T. Casadevall, D. Swanson, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

January 1983 (SEAN 08:01) Cite this Report

New lobe extruded onto composite lava dome

Increases in SO₂ emission, deformation, and seismicity preceded a series of small explosions and the extrusion of a new lobe onto the composite lava dome, the first since August 1982.

The rate of SO₂ emission, which had remained very low for several months, tripled between measurements 13 and 15 January and remained between 70 and 120 t/d through the end of the month. About twenty small shallow earthquakes were recorded 17-18 January, but seismicity declined and remained at background levels for the next 2 weeks. Heavy snow in the crater made deformation measurements impossible on the S and E sides of the dome, but very slow acceleration in the rate of outward movement of the dome's N side began in mid-January. A few small gas-and-ash emissions occurred in late January.

Gas monitoring on 30 January showed that SO₂ emission had increased to roughly twice the rate of the previous 2 weeks, and SO₂ flux ranged from 170 to 260 t/d through 7 February. On 31 January, a pronounced acceleration was measured in the outward movement of the N side of the dome. Points on the W side of the dome, usually the area of most rapid outward movement, showed little such activity, but sagged downward several tens of centimeters. A gradual, slight increase in the number of seismic events began 1 February, but seismicity remained relatively weak, reaching about the level of the 17-18 January activity.

At 2339 on 2 February and 0256 the next morning, explosions sent plumes containing small amounts of ash to about 6 km altitude. A pilot reported that the cloud top was at 8 km altitude at 0015 on 3 February. GOES East satellite images showed the plumes moving slowly NW. At 0430 a cloud about 150 km in diameter remained centered over the volcano, but it had begun to diffuse 30 minutes later and by 0530 had reached nearly to Puget Sound, about 100 km from the volcano. Ashfall was reported at Olympia, near the S end of Puget Sound. During a predawn flight 3 February, geologists observed that the explosions had created a small notch in the upper E flank of the dome. Within the crater, the deposits from these explosions showed a complex stratigraphy. Rare breadcrust bombs were found at the top of the deposits. A laterally-directed component from one or both of the explosions melted snow on the E crater floor and wall, producing a mudflow that reached Spirit Lake. The ash column from a third explosion on 4 February at 1728 reached about 4.5 km altitude. This explosion enlarged the flank notch to 60-100 m deep and 80-100 m wide. Deformation data on 3 and 5 February showed continued acceleration of outward movement of the N side of the dome, reaching 5-6 cm/day by the 5th. Visual observations showed severe deformation of the E side of the dome, where a large wedge of rock just S of the notch had tipped up and out several meters. Locatable seismic events stopped 5 February, and only events typically associated with steam emissions and rockfalls were detected during the next several days.

Late 5 February, the USGS and University of Washington issued an extended outlook advisory notice stating that an eruption was likely within the next week and could include some explosive activity. Poor weather prevented observations until about noon on 7 February, when geologists observed the extrusion of a new lobe of lava from the floor of the E flank notch. Lava advanced mainly toward the E, filling the notch, and by afternoon had reached the top of the talus pile at the base of the dome. From the air, geologists estimated that the new lobe extended roughly 100 m E-W and 50 m N-S. A small explosion occurred from the dome at 1640 on 7 February. Weather conditions prevented access to the crater for the next few days, but seismographs recorded rockfall events, suggesting that the new lobe continued to advance. Glimpses of the dome beneath low weather clouds 14 February indicated that the new lobe was still growing.

Information Contacts: T. Casadevall, C. Newhall, D. Swanson, S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington; D. Haller, NOAA/NESDIS.

February 1983 (SEAN 08:02) Cite this Report

Spine added to February lobe, then extrusion stops; seismicity indicates renewed extrusion by late March

Extrusion of a new lobe onto the E flank of the composite lava dome stopped by 2 March. However, a renewed increase in seismicity was evident by 4 March and an eruption was expected by the end of the month. Poor weather hampered observations throughout February and early March, denying geologists access to the crater and views of the dome on most days.

Because of the weather, it was difficult to determine exactly when the February lava extrusion began. Between 30 January and 4 February, 21 gas and ash explosions were observed on seismic records and by FAA radar at Portland airport. Infrared photographs from an overflight during the night of 4-5 February did not appear to show new lava in the notch in the dome's upper E flank, but at 0930 on the 5th it contained a large smooth-sided creased rock (about 10 x 20 m in lateral dimension and 10 m high), probably the new lobe. An overflight during the night of 5-6 February showed that a substantial amount of new lava had been extruded. University of Washington seismologists note that for most extrusion episodes at Mt. St. Helens, surface events (principally rockfalls) have begun to dominate the seismic record at about the time that lava extrusion started. Surface events began to increase noticeably late 4 February, although most were small and the number of surface events did not exceed the number of subsurface earthquakes until early 6 February. Gas-emission events were also recorded [seismically], some of which were associated with [observations of] minor vapor-and-ash plumes. By the afternoon of 7 February, subsurface earthquake activity had decreased to one to five events per day and remained at that level through the end of the month.

After observing the new lobe on 7 February, geologists were next able to see it on the 11th, when it appeared to have grown in size by about 30%. A well-developed, smooth-sided crease was oriented along the long axis of the lobe's surface. Similar features observed during late stages of extrusions in December 1980, and February, June, and September 1981 were thought to represent the last material extruded from a vent and not disrupted by later flow. Although rockfall seismicity continued, indicating that the lobe was advancing, little growth was apparent between observations 11 and 15 February. Measurements on 23 February showed that the new lobe had advanced 23.5 m to the E since the 11th.

A type of seismicity not previously recorded at Mt. St. Helens was first detected 14 February. Hundreds of tiny events that were remarkably similar to each other (many were identical for as much as 20 cycles) occurred at an average interval of 40 seconds. In the 24 hours beginning at 0830, 559 of these events were recorded, but none was large enough to locate and their origin is uncertain.

Television footage 21 February showed a spine, not present on the 19th, growing from the center of the lobe. On 24 February, the spine was roughly 30 m tall, and by the 28th it had roughly doubled in height, extending about 20 m above the dome's summit. The relatively undisturbed growth of the spine indicated that little downslope movement of the lobe was occurring during this period. On 28 February, geologists noted that no rocks had fallen from the lobe front onto the previous night's snowfall. Observations 1 March indicated that extrusion had ended. The new lobe had filled all but 10-15 m of the 60-100 m-deep notch, oozed out its E end, and reached the E foot of the dome (figure 23). Geologists estimated that it was roughly the same size as previous lobes. Total seismic energy released during the February extrusion episode was comparable to that associated with previous extrusions, but occurred over a longer time span.

Figure 23. Sketch showing the Mt. St. Helens composite lava dome in the crater as viewed from the NNE in late February 1983. The February 1983 lobe is shown with its spine, as are lobes extruded in May and August 1982. Sketch by Bobbie Myers.

Deformation data were limited, but indicated that little swelling of the dome was associated with the extrusion. Although the W side of the dome has usually been the area of most rapid outward movement, none was measured until 7 February, and only 0.7 cm of expansion occurred between then and 1 March. During the same period, the N side of the dome moved outward 7-8 cm, but deformation there began before the extrusion (SEAN 08:01) and shortening of measured lines totaled about 20-25 cm. Visual observations indicated substantial deformation of the dome's E side, but no instrumental measurements were possible.

SO₂ emission peaked on 15 February, reaching 400 t/d, about twice the early February rate (SEAN 08:01). By late February, SO₂ emission had dropped to slightly more than 100 t/d and the average for the month was about 170 t/d.

The number of surface seismic events remained steady through early March, but an increase in subsurface earthquakes was evident by 4 March. Six subsurface events were recorded on 1 March and ten on the 3rd; the twelve events on 4 March were larger, so energy release was substantially higher. Energy release continued to accelerate significantly 5-6 March, and 25 subsurface events were recorded on the 6th. Because of the increased seismicity, the USGS and University of Washington issued an advisory notice 6 March stating that renewed eruptive activity could be expected. Poor weather prevented deformation measurements that have previously been successfully used to predict the time of eruption onset.

Seismic energy release declined late 6 March, and only twelve to fifteen events were recorded daily 7-9 March, but both values remained significantly above background levels. An updated advisory notice issued 8 March suggested that an eruption would begin within the next 3 weeks. On 9 March, observers in a helicopter saw that most of the spine had fallen, but did not report the presence of any additional new lava. Measurements on the N and W sides of the dome 10 March did not show large acceleration of displacement rates.

Information Contacts: T. Casadevall, C. Newhall, D. Swanson, B. Myers, S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

March 1983 (SEAN 08:03) Cite this Report

SO₂ emission, seismicity, and changes in morphology of new lobe may indicate continued endogenous growth

CVO personnel report that frequent rockfalls from the toe of the February lobe and changes to its morphology, coupled with elevated SO₂ emission and small seismic events, may reflect continuing endogenous dome growth. Poor weather limited visibility and restricted access to the crater through March and early April.

A new incandescent radial fissure was observed on the NE side of the February lobe during a night overflight 23 March and remained visible through the end of the month. ln the past, such fissures have typically formed during periods of rapid endogenous growth. By 31 March a mound of rubble estimated from brief aerial observations to be roughly 50-60 m in diameter and 20-30 m high had developed on the lobe, in the area where a spine had been extruded in late February. Frequent rockfalls from the E end of the lobe were continuing on 11 April, but conditions in the crater prevented geologists from mapping changes to the lobe front. Gas and ash explosions, some of moderate size, were observed or detected seismically roughly 1-2 times per day through early April. Most appeared to originate from fumaroles at the top of the dome. The main fumarole, a conical pit tens of meters in diameter at the surface and tens of meters deep, was located at the head of the E-flank notch that was the source of the February lobe.

Rates of SO₂ emission remained elevated through early April, averaging 150 ± 95 t/d in March, only 20 t/d less than the February mean. Early April values ranged from 135 to 180 t/d. In contrast, rates in the months prior to January were only about 35 t/d. Deformation of the N and W sides of the dome continued but did not accelerate in March. Measurements of the deformation of the active E side of the dome have not been possible.

Extremely small events, felt by field crews in the crater but recorded only by a single seismometer 1 km N of the dome, continued through March. The number of larger earthquakes remained above background levels, averaging about half a dozen per day. This value oscillated considerably, without obvious trends or apparent correlations with variations in activity at the dome, but dropped substantially after the early March increase (SEAN 08:03). Seismic energy release exceeded previous post-extrusion periods. In the past, 90-95% of cumulative energy release had occurred during extrusion episodes, but this pattern changed in October 1982, and seismic energy equivalent to two typical extrusions has been released since then. However, seismic energy accompanying extrusion of the February lobe was unusually low, comparable to that associated with smaller extrusions such as in October 1981. Energy release in early April exceeded February rates.

Information Contacts: T. Casadevall, C. Newhall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

April 1983 (SEAN 08:04) Cite this Report

Intrusive and extrusive dome growth continue

Since early February, growth of the composite lava dome has been essentially continuous. Accelerating outward movement of the dome had preceded previous extrusion episodes, but stopped as lava reached the surface. However, substantial endogenous growth has continued throughout the current episode. Poor weather continued to hamper observations.

About 1 April, a broad stubby spine began to emerge at roughly 1 m/day from the center of the February lobe, reaching 30 m in N-S dimension, 20 m E-W, and about 25 m in height. Growth of this spine stopped about 15 April, and extrusion of another spine started about 70 m to the SE. The latter spine remained active until about 27 April, when at 60 m height it was the highest point on the dome and roughly the same size as the now-toppled February spine (SEAN 08:02). Between visits to the crater 29 April and 4 May a new lobe began to grow high on the NE flank of the February lobe. This lava had a typical "spreading center" source and scoriaceous carapace. Extrusion continued as of 11 May, but the growth rate was slow and the lobe remained several times smaller than previous ones.

Dramatic deformation has continued on the E and particularly the NE sector of the dome since early-mid March. Because of frequent rockfalls, it was difficult to maintain targets on these areas of the dome, but rates of displacement reached 1.5 m/day, and averaged about 1 m/day over roughly 1-week periods. Between measurements 4 and 11 May, the NE margin of the dome moved 9 m outward and 2.5 m downward. Deformation on the N side of the dome was limited, but significant rate changes were observed. Through March the rate was constant at about 1.5 cm/day, but dropped to about 1 cm/day around 1 April as spine growth started. Deformation slowed further to 7-8 mm/day around the 15th as growth of one spine stopped and extrusion of another began, but returned to about 1 cm/day at the end of the month and remained at that rate as of 11 May. The W side of the dome, site of the most rapid deformation before many previous extrusion episodes, remained quite stable. No significant deformation of the edifice as a whole was detected.

Vapor and tephra emissions continued from the main vent near the source area of the February lobe but were relatively infrequent, occurring one to three times per day. Blocks up to 30 cm in diameter were ejected. Tephra could often be seen in the plumes, which sometimes rose to 1 km above the crater rim; the largest, 18 April at 1259, reached 6 km altitude. There was no apparent correlation between plume emissions and changes in extrusive activity or deformation.

SO₂ emission remained at roughly 150 t/d until about 27 April, when it dropped to 60-90 t/d. A similar rate was measured 30 April and 4 May, but SO₂ emission returned to 150 t/d 11 May.

Seismic activity remained elevated through April. All but nine of the 243 events with locatable foci were of low-frequency with emergent onsets, a similar pattern to that seen in March. Between 1 and 12 April, daily earthquake totals commonly ranged from four to eight, increased to eight to twelve events per day 13-24 April, then dropped slightly to an average of eight per day through the end of the month. Surface and avalanche events showed a similar pattern. Numerous gas-emission events were recorded. Many began with a series of high-amplitude events that ended within a few minutes or gradually faded into brief periods of harmonic tremor. About 20 April, sequences of tiny discrete similar events, previously seen in February (SEAN 08:02), reappeared on one seismometer, but these events could not be located and their significance remained uncertain. The start of extrusion of a new lobe of lava between visits to the crater 29 April and 4 May was not marked by an obvious change in seismicity. Geologists working in the crater 11 May heard loud but relatively small earthquakes, which had not been audible during previous extrusion episodes.

Information Contacts: D. Swanson, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

May 1983 (SEAN 08:05) Cite this Report

Lava dome growth continues

New lava began to emerge high on the NE flank of the February lobe between 29 April and 4 May. Slow extrusion continued for about the next 3 weeks, but little new material appeared to have emerged onto the surface since then. Deformation measurements indicated that intrusive activity continued after extrusion stopped, as the new lobe continued to move outward at an average rate of 15-20 cm/day through early June. Little movement was noted on the N and W sides of the composite dome. The spine that grew on the February lobe from 15 April to about the end of the month and had formed the highest point on the composite dome toppled between visits to the crater 24 and 26 May.

The rate of SO₂ emission averaged 95 ± 35 t/d in May and remained similar in early June. This represented a decline from the April average of roughly 150 t/d, but remained substantially above the September 1982-mid January 1983 quiet rate of 25-50 t/d.

The number of earthquakes with locatable foci dropped from 243 in April to 155 in May. Nearly all were low-frequency events with emergent onsets, as has been observed since the early February explosions. However, strain release for surface events was nearly twice as high as for earthquakes, and nearly twice as many were recorded in May as in April. Most of the rockfalls observed by geologists in the crater occurred from the dome's active NE flank. Large . . . avalanche events were recorded 5, 15, and 18 May.

Information Contacts: T. Casadevall, E. Iwatsubo, USGS CVO, Vancouver, WA; C. Boyko, University of Washington.

June 1983 (SEAN 08:06) Cite this Report

Lava dome growth continues

Lava extrusion continued through early July. The rate of advance of the new lobe was about 4-5 m/day in June, but the net increase in length was substantially reduced by rockfalls from its front, which remained 75-100 m above the base of the dome in early July. Since extrusion began, the thickness of the lobe just downslope from the vent has increased from a few meters to at least 50 m.

Increasingly rapid deformation of the lava dome had preceded previous extrusion episodes but ended as lava reached the surface. However, deformation did not stop when extrusion began in February (SEAN 08:01), and deformation measurements showed that intrusive activity has continued since then. Deformation remained strongest in the NE part of the dome, which moved outward an average of 1 m/day and downward about 30 cm/day in June. Both of these rates gradually decreased in June. In contrast, outward movement of the much less active SE sector of the dome increased from 4-5 cm/day through 28 June to 8 cm/day between then and the following measurement 5 July. Acceleration of the N side of the dome was also noted.

The SO₂ emission rate averaged about 65 t/d 3-16 June, increased by a factor of three to roughly 200 t/d 20-28 June, then dropped to about 75 t/d 4-6 July.

The number of earthquakes and the rate of seismic energy release in June were both similar to May values. Low-frequency shocks continued to dominate the seismic records as they have since extrusion began in February. Surface events, primarily rockfalls from the dome and walls of the crater, were more frequent in June. Much of the change can probably be attributed to warmer weather, which typically increases the number of rockfalls from the crater walls.

Information Contacts: D. Swanson, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

July 1983 (SEAN 08:07) Cite this Report

Lava dome growth continues; plumes emitted

Growth of the composite lava dome continued through early August (figure 24). Net advance of the active lobe was reduced to roughly 10-20 m in July by frequent rockfalls from its front, but one area on the lobe thickened 11 m during the month. Deformation measurements showed continuing intrusive activity. The highest rates of expansion were on the NE side of the dome, where outward movement averaged 60-70 cm/day, although fluctuations by a factor of two to three were observed for periods lasting a maximum of 3-4 days. The station on the SE side of the dome typically moved outward 1-2 cm/day but this rate sometimes briefly increased to several centimeters per day. A short-term acceleration of endogenous growth in mid-July was accompanied by a slowing in extrusion of the new lobe.

Figure 24. Sketch by Bobbie Myers of the Mt. St. Helens composite lava dome, viewed from the N on 17 July 1983. Recent lobes are dated.

Two vents on the summit of the dome were the sources of three to six small steam-and-ash explosions per day in July and early August. Plumes from most of the explosions barely cleared the crater rim, typically causing a fallout of fine ash within the crater, but little or none outside. A larger explosion in late July produced ballistic fragments 1 cm in diameter and a plume with considerable lightning. A steam and ash plume reached 4.5 km altitude 4 August and another rose to 3.5 km on 6 August.

The average rate of SO₂ emission in July was 120 ± 80 t/d. Measurements 4, 5, 6, and 10 July yielded mean values of 85 ± 15 t/d, then the emission rate increased 15, 17, and 21 July to 225 ± 70 t/d before dropping back to 80 ± 50 t/d 22-29 July and roughly 50 t/d 1-9 August. Mid-month increases in endogenous-dome growth and seismicity were also noted (see first and last paragraphs of this report).

Measurements were made of the SO₂ content of two plumes produced by vapor-and-ash ejections. On 1 August, data collection began with the onset of the explosion and continued for 30 minutes after plume ejection ended. No change in the SO₂ emission rate (55 ± 5 t/d) was observed. Two measurements before a plume ejection 9 August yielded rates of about 40 t/d. Values within the plume were initially 105 t/d before dropping gradually back to 40 t/d as it dissipated.

July seismic activity was similar to that of previous months. Vigorous surface activity was recorded, some of which was thought to be caused by avalanching from the crater walls on warm days. Several brief periods of increased seismicity were noted, and the largest, around the middle of the month, produced a noticeable step in the seismic energy release curve.

Information Contacts: D. Swanson, R. Symonds, E. Iwatsubo, B. Myers, USGS CVO, Vancouver, WA.

August 1983 (SEAN 08:08) Cite this Report

Lava extrusion continues; internal dome growth accelerates; small fluidized avalanches; vapor and ash plumes

New lava was still being added to the active lobe in early September and deformation of other parts of the dome was accelerating. The lobe front moved down the NE flank at about 1 m per day in August, roughly the same rate as in July. Rockfalls from the lobe's leading edge appeared to decline in July and August, but continued to remove some material, reducing the lava's net August advance to 20-25 m.

Rates of outward movement of survey targets on the S, SE, and N flanks of the dome began an irregular increase about 8 July and by early September had reached nearly 11 cm/day high on the S side. No acceleration of endogenous growth was observed in the area of most rapid deformation, below the active lobe on the NE flank, where rates averaged 60 cm/day. Movement of crater floor stations N and S of the dome was first detected around early August, gradually increasing into the millimeters per day range by early September. The pattern of increasing deformation was generally similar to periods that preceded extrusion of new lobes in 1981 and 1982. However, Donald Swanson noted that the irregular acceleration of endogenous growth contrasted with the quite steady increases measured before 1981-2 extrusion episodes, and that it was continuing after 2 months without the onset of new extrusion, exceeding the typical 1 month-6 week durations of the 1981-82 premonitory periods.

Numerous rockfalls, some quite large, occurred from a N-flank notch that was propagating upslope toward the dome's extrusive vent. This activity built a large structurally unstable talus slope of hot blocks. Upon reaching the talus, some rockfalls became fluidized, probably by entrainment of heated air from between talus boulders. Early 12 August, Daniel Dzurisin observed a group of large boulders from the notch bounce onto the talus. A few seconds later, a second rockfall reached the talus and fluidized. An ash cloud quickly formed over the avalanche, and moved downslope at the same speed as the entrained boulders, stopping as they came to rest. The avalanche formed a lobate deposit with marginal levees less than 1 m high. Fine particles extended to roughly the distal end of the boulder deposit. Ash clouds formed by smaller avalanches were diffuse enough so that boulders could be seen rolling slowly downslope; these avalanches seemed to be only partially fluidized. The avalanches traveled no more than several hundred meters beyond the base of the talus, into the large breach on the N side of the crater. For several days after a large rockfall, avalanches occurred roughly every 2 hours, but declined to 1-2/day during quiet periods.

Occasional ejection of steam-and-ash plumes continued from several vents in the broad summit region of the dome. The number of plumes varied from day to day, but generally ranged from three to six daily and remained relatively unchanged through the summer. Plumes typically rose about 1 km above the dome and deposits were usually limited to the dome's summit area. No projectiles from these plumes reached the crater floor in August. Tom Casadevall reported that COSPEC measurements indicate that the volcano emits more SO₂ while plumes are being ejected than during quiet periods; on 18 August a plume briefly produced a 4-fold increase in SO₂ emission. However, plume events normally last only 15-20 minutes, and the excess SO₂ values decay exponentially, so they do not have a large effect on daily gas flux. The rate of SO₂ emission averaged 70 ± 50 t/d in August, ranging from 40 to 90 t/d most of the month, but measurements 18-23 August yielded values of more than 150 t/d.

August seismic activity was generally similar to that of July. A substantial increase in surface events was recorded, but was thought to reflect increased avalanching from the crater walls as warm weather melted snow on the rim. For about 10 days in late August, the number of earthquakes and the rate of seismic energy release increased slightly, but declined to previous levels by early September.

Information Contacts: T. Casadevall, D. Dzurisin, D. Swanson, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

September 1983 (SEAN 08:09) Cite this Report

Lava from new vent added to composite dome

Much of the NE-flank lobe has been intruded just below the surface, carrying a thin rubbly carapace of older material downslope. New lava with visible flow structures has occasionally broken through to the surface. Expansion of the S and SE flanks continued to increase slowly until late September but accelerated more rapidly after the 28th and had reached three to four times early September values by 1 October. A USGS-University of Washington advisory notice issued on that date predicted a new extrusion within 10 days.

Lava began to emerge from a new vent about 50 m S of the source of the NE-flank lobe by 7 October. As with the NE-flank lobe, little lava reached the surface, instead lifting a thin layer of older material to form a ridge roughly 300-400 m in E-W dimension, 100 m in N-S dimension, and 20 m thick, at the S edge of the NE flank lobe. The vent area was surmounted by a large pile of material, some of which appeared to be new lava, that by 12 October was 12-13 m higher than the previous summit of the dome. The advance of the NE portion of the May lobe slowed to less than 10% of its previous rate between 5 and 10 October. Deformation of the S portion of the dome stopped accelerating 5-6 October, also approximately coincident with the appearance of the new lava, but remained at rates as high as 105 cm/day. A small thrust fault began to form just beyond the talus pile at the dome's S side about 4-5 October and moved at an approximately constant rate of 2.5-3 cm/day 7-12 October.

Plumes of hot gas (a smell of H₂S was often noted by geologists in the crater) continued to be ejected 2-4 times per day. During a period of very clear weather and strong winds 20-22 September, plumes were observed on satellite imagery, rising roughly 2 km above the summit and extending 100-150 km. Most plumes contained little ash. No significant change in plume size, frequency, or density accompanied the appearance of the new lava. Rates of SO₂ emission averaged 100 ± 50 t/d in September, a slight increase over August values. Rates decreased slightly in mid-September to 70-90 t/d and usually remained in that range in early October. Since early September, seismic energy release has varied only slightly and there was no change in seismicity associated with the emergence of the new lava.

Information Contacts: D. Swanson, T. Casadevall, C. Newhall, S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington; M. Matson, S. Kusselson, E. Legg, NOAA/NESDIS.

October 1983 (SEAN 08:10) Cite this Report

Deformation, SO₂ emission, and seismicity increase as lava extrusion pattern changes

Growth of the composite lava dome continued through October. At the end of September, the pattern of lava extrusion began to change, with lava redirected southward along the margin of the active lobe. Examination of daily airphotos indicated that by 6 October a substantial area of uplift had developed along the S edge of the lobe and a spine was emerging from a point about 50 m WSW of the vent that had fed the active lobe since the beginning of May. The spine grew 1.5-2 m per day until 15 October, then its growth slowed to about 0.5 m/day for the next 2 weeks. The spine reached 30 m in height, 17 m higher than any other point on the dome. From the spine, a sharp, lateral ridge extended about 100 m NE. By 31 October, this feature had crumbled. The spine remained nearly intact, but had stopped growing and some crumbling had occurred.

As the spine grew, lava emerged from a "spreading center" just to the S, pushing the spine slowly NW. Lava advanced S and SE along the periphery of the May lobe at roughly 1-2 m/day through October, thickening this portion of the lobe by a factor of 4-5. As this area of the lobe grew, advance of the NE end of the lobe slowed and had nearly stagnated by the end of October.

Although deformation of the S and SE flanks of the dome stopped accelerating about 6 October as the spine began to emerge, outward movement of this part of the dome continued at high but relatively stable rates of as much as 120 cm/day through October as the active lobe advanced and thickened. The crater floor adjacent to the S and SE flanks of the dome also continued to deform slightly through October, with a maximum uplift of 6-8 cm and maximum horizontal strain of about 20 cm during the month.

SO₂ emission averaged 75 ± 45 t/d in October. Several days of elevated SO₂ emission 1-8 October (reaching 210 t/d on the 1st) accompanied the onset of the changed lava extrusion pattern on the dome. From 8 October through the end of the month, rates dropped to 30-80 t/d. Several small gas-and-ash ejections from an explosion pit near the summit of the dome continued to occur daily, elevating SO₂ flux to 3-4 times background, usually for only a few minutes but occasionally for tens of minutes. Plumes were usually grayish-white and contained only a little tephra. Sand-size and occasionally cobble-size fragments fell near the vent, but only small quantities of very fine material were deposited on the crater rim.

Since November 1980, a drainage system has developed in the 1980 pyroclastic-flow deposits. In the late spring of 1983, steam was noted in the drainage system for the first time. A zone of six to eight small hot springs had developed near the N edge of the crater at the contact between hydrothermally altered ancestral dacite and the pyroclastic-flow deposits. Flow rates were typically less than 1 liter per second. Temperatures measured at the springs in September and on 26 October ranged from 76 to 91°C, pH was 7.1-8.2, and specific conductance was 3,300-5,800 mhos. Travertine was being deposited at one of the springs.

Seismic energy release declined in mid-September, but a gradual increase began in early October, leveling off about 9-10 October at roughly twice the late September rate. A small decrease in the slope of the energy release curve occurred in late October but poor weather may have caused instrumental interference. By early November, the mid-October rates had been regained. Little change in surface events was observed in October. In the summer, many surface seismic events were the result of seasonal avalanching from the crater walls, but in October most were caused by steam ejections.

Information Contacts: D. Swanson, T. Casadevall, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

November 1983 (SEAN 08:11) Cite this Report

Continued lava dome growth

Lava extrusion continued to extend the active lobe SSE through November. Lava advanced about 100 m across the dome's broad, gently sloping summit area during the month, approaching the break in slope at the top of its steep upper flank. The rate of outflow appeared to be roughly the same as in previous months. A new depression, about 50 m x 80 m in horizontal dimensions and 30-40 m deep, formed about 50 m SE of the "spreading center" that fed the lobe in October. The "spreading center" had stagnated and the new depression may have been the source of some of the November lava. Just N of the stagnant "spreading center," the spine extruded in October was crumbling, but was still the high point on the dome at the end of November.

Collection of deformation data was hampered by the loss or inaccessibility of targets on the most active parts of the dome. Outward movement of accessible targets on the dome's SE flank was about 55 cm/day between late October and early November, 35 cm/day 8-21 November, and 48-50 cm/day after 21 November. If deformation patterns remained similar to those measured before the most active targets were lost, the most rapidly moving areas on the dome may have been expanding at roughly 90-100 cm/day, down slightly from the 120 cm/day of early October. Deformation of the dome's NE side had been rapid as the active lobe advanced down the NE flank from early May through late September but was negligible in November.

Gas-and-ash plumes continued to be emitted 3-6 times daily. The plumes, dominantly gas but sometimes containing some tephra, rose a few hundred meters to 1 km above the dome. Poor weather limited airborne gas measurements to five in November. The rate of SO₂ emission averaged 70 ± 45 t/d, nearly identical to October values. However, the 21 November rate of 150 t/d may have been measured in the remnants of a gas-and-ash plume; without this figure, November SO₂ emission averaged 50 ± 20 t/d.

For most of November, seismicity continued at approximately the October rate. Both seismic energy release and the number of events increased at the end of the month to values higher than in October. Average daily earthquake counts ranged from 5-13/day through 18 November, 12-15/day 19-25 November, and 24-33/day 26 November-1 December.

Information Contacts: D. Swanson, C. Mullins, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

December 1983 (SEAN 08:12) Cite this Report

Renewed extrusion on NE flank; breadcrust bombs

Numerous rockfalls occurred through December as the lobe front broke up at the top of the S and SE flanks. Some rockfalls were hot enough to become fluidized, moving down a chute and spreading over a small area of the crater floor at the base of the dome. The fluidized rockfalls generated from the front of the NE flank lobe in August (SEAN 08:08) were larger and had stronger seismic signatures. Maximum displacement rates measured on the S and SE sides of the dome dropped from about 50 cm/day in late November to about 20 cm/day by late December. Between 28 December and 4 January, values increased sharply, to as much as 60 cm/day.

New lava appeared on the NE flank in mid-December, forming jagged spires and ridges, but little downslope advance was observed. Instead, the lava appeared to have broken through the crust of the dome in a zone that extended about 200 m NE from the summit spine and was about 100 m wide (in a NW-SE direction). Night overflights showed an increased number of glowing cracks on the dome's NE flank. Deformation of the NE flank also began to accelerate in mid-December, increasing from 0.5-1 cm/day to a few centimeters per day by the end of the month.

The dome's summit spine crumbled rapidly, but continued to receive some new lava. As of early January, it remained the high point on the dome, but several meters of net height loss appeared to have occurred. Snow had accumulated in the depression that formed in November near the top of the dome. However, its NW rim tilted dramatically away from the remainder of the depression, and a new mound had grown in that area.

Gas-and-ash ejection from vents high on the dome continued to occur several times a day. On 16 December, a substantial number of breadcrust bombs with a maximum diameter of about 4 cm were found in the crater on snow that had probably fallen 2 days earlier. The bombs were more vesicular than any material ejected since the strong explosive activity of 19 March 1982 (SEAN 07:03). December SO₂ emission averaged 105 ± 25 t/d, a 30-40% increase over October and November values, but similar to rates measured April-September.

Seismic events were slightly more numerous in December, but only minor growth was observed in the rate of energy release. Seismicity increased slightly in early December, dropped a little at mid-month, then showed a minor increase at month's end. There were no obvious changes in seismicity that could be correlated with changes in activity on the dome. The number of surface events decreased seasonally because winter conditions inhibit rock avalanching from the crater walls.

Further Reference. Swanson, D.A., Dzurisin, D., Holcomb, R.T., Iwatsubo, E.Y., Chadwick, W.W., Jr., Casadevall, T.J., Ewert, J.W., and Heliker, C.C., 1987, Growth of the Lava Dome at Mount St. Helens, Washington (USA), 1981-1983 in Fink, J. (ed.) The Emplacement of Silicic Domes and Lava Flows; Geological Society of America Special paper 212, p. 1-16.

Information Contacts: D. Swanson, T. Casadevall, USGS CVO, Vancouver WA; R. Norris, University of Washington.

January 1984 (SEAN 09:01) Cite this Report

Deformation and seismicity then new lobe

A new phase of the ongoing activity began with increasing deformation and seismicity in late January 1984, followed by the extrusion of a new lobe near the dome's summit.

Numerous rockfalls occurred in December after the active lobe had reached the top of the dome's steep S and SE flanks. Rockfalls from that area were infrequent in January, suggesting that the advance of the lobe had nearly stopped. Deformation of the dome's SE flank accelerated briefly in late December to more than 50 cm/day, but slowed by an order of magnitude in early January and remained at 5-6 cm/day through the end of the month. On the NE flank, rates of outward movement remained relatively high through 4 January. On 9 January, when weather next allowed access to the crater, NE flank deformation had slowed and a new mound was perched just E of the dome's summit (SEAN 08:12). The surface of the mound was old material, but it was apparently cored by magma. It reached its maximum elevation 20 January, then subsided slowly. Earthquakes remained relatively numerous through 9 January, but the period 10-17 January was the quietest seismically since late September, with the number of events per day dropping from about fourteen to two to six and energy release declining to near background level.

In mid-January, points on the floor of the crater's breach, about 1 km N of the dome, began to move outward, and medium-frequency events started to appear on seismic records. Energy release remained low but the number of medium-frequency events increased gradually through the end of January. Gas ejection episodes increased noticeably in duration and amplitude on seismic records. By 23 January, some were followed by several minutes of weak harmonic tremor. Vigorous plumes were observed and small blocks were deposited on the crater floor. SO₂ emission averaged 90 ± 40 t/d during the first 3 weeks of January (as compared to 105 ± 25 t/d in December), but dropped to 35 ± 20 t/d for the remainder of the month and was below detection limits 26-28 January. A fairly large gas-and-ash ejection on the 28th was not followed by the typical temporary several-fold increase in SO₂ flux.

New cracks were observed on top of the dome 29 January. A graben was evident on its SW side by 1 February, and radial cracks had appeared on the W side. By 3 February, the graben was a few tens of meters wide and a few meters deep, extending across the summit crater. Deformation began to accelerate rapidly. A point halfway up the N flank of the dome that had moved outward no more than a few centimeters per day through most of January showed rates of 11.6 cm/day on 30 January, 46 cm/day on 3 February and about 1 m/day by the 5th. Deformation changes on the SE flank were less dramatic, but rates also increased, from 6 cm/day through 3 February to 20 cm/day on 5 February (all rates are average daily changes since the previous measurement). This activity was accompanied by a rapid increase in the number of earthquakes and seismic energy release beginning 1 February. SO₂ emission increased to 55 t/d 1 February and reached 140 t/d on the 6th.

When geologists arrived at the crater 6 February they observed a new mound filling the NW part of the dome's summit crater. The early January mound, about 100 m to the E, was subsiding. That evening, a small landslide from the E side of the dome moved 50-100 m to the main crater wall, causing minor snowmelt. The next day, the new mound had elongated and extended a short distance down the dome's N flank, while the January mound continued to subside. The surface of the new mound, like the January mound, was old material. No glow from the new mound was observed at night. N flank displacement was 2.4 m between 5 and 6 February, and rates of 3 m per day were measured on the 6th. SO₂ emission increased to 140 t/d on 6 February and 170 t/d the next day. Seismicity peaked during the evening of 7 February, dropping sharply in the next 24 hours to only slightly elevated levels. The number of rockfall events increased but they were smaller and less frequent than during previous extrusion episodes. Poor weather prevented access to the crater after 6 February. A brief glimpse of the dome on 10 February revealed a new lobe perched on its summit. . . . The new lobe appeared to be hot; snow had accumulated on the rest of the dome, but not on the new lobe.

Information Contacts: R. Holcomb, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

February 1984 (SEAN 09:02) Cite this Report

Lava extrusion stops; deformation and seismicity decline to low levels

Increasing seismicity, gas emission and deformation of the composite lava dome culminated in the extrusion of a small new lobe in early February. The extrusion was not observed because of poor weather, but was believed to have started between late 7 and early 8 February (when seismicity decreased sharply from peak levels reached during the evening of the 7th). The roughly circular new lobe, about 140 m in diameter and 25 m thick, spread N from the former apex of the January mound, just E of the dome's summit. The mound, first seen 9 January, had a surface of old material but was probably cored by magma. It had reached its maximum elevation 20 January, then subsided slowly. A new mound, almost as large as the February lobe, formed in the dome's summit crater after 6 February, probably just before the lobe was extruded. Observations of this feature have been limited because it has remained enveloped in vapor, but it appeared to be cored by a highly oxidized monolith or spine rising about 30-40 m from the crater floor and having a similar or slightly smaller width. The USGS estimated that the dome expanded by about 4 x 10⁶ m³ during the period of rapid deformation from late January through early February. The volume of the February lobe was estimated to be about an order of magnitude smaller.

After the end of the February extrusion episode, activity declined to the lowest level since continuous dome growth began in early 1983. From mid-February through early March, deformation on all sides of the dome has been less than 1 cm/day. Although one station moved 5 cm between 3 and 4 March, it was located close to the new lobe and may have been affected by local movement. Seismic energy release, which had increased in early February, dropped abruptly with the sharp decline in the number of events late 7 to early 8 February. Two M 2 events were recorded 10 and 12 February, but there were few earthquakes after the 12th and very little energy release. SO₂ emission dropped gradually through February, from 170 t/d on the 7th to slightly less than 100 t/d at the end of the month. Gas-and-ash ejections, which had occurred several times a day in past months, were not observed after 3 February. A wispy plume emitted continuously from the dome 3-7 February deposited a very thin layer of powdery ash nearby, but this had stopped by the end of the extrusion episode.

Information Contacts: R. Holcomb, C. Mullins, T. Casadevall, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

March 1984 (SEAN 09:03) Cite this Report

New lobe extruded onto the composite lava dome

Strong seismicity and rapid deformation were followed in late March by the addition of a new lobe to the composite lava dome. Deformation, seismicity, and SO₂ emission declined after the extrusion of a small lobe in early February, and remained low through mid-March. Poor weather prevented access to the crater 22-27 March. Deformation on the 22nd was at low levels, but measurements on the 27th showed that targets on the N side of the dome had moved outward at an average rate of 0.5 m/day since the 22nd. Instantaneous rates increased from 1.9 to 2.4 m/day during a 2-hour period on 27 March.

Seismicity began to increase late 22 March, and the number of events doubled each day 24-28 March. The swarm was characterized by type "M" (medium-frequency) events similar to those that preceded the February extrusion episode. The type "M" swarm peaked early 28 March, then declined rapidly, ending by midnight. As the type "M" events diminished, a swarm of small crater events began. By noon, eleven events were being recorded per minute, and by 1600 there were 15/minute. On more distant stations, the individual events could not be resolved, merging into a tremor-like signal with an amplitude that increased through the evening.

A continuously recording tiltmeter about 30 m from the base of the talus N of the dome began telemetering data 27 March. Within a day, outward tilt, initially 200 µrad/hour, increased to more than 400 µrad/hour, accelerating abruptly to more than 1,000 µrad/hr during the afternoon of 28 March, then went off scale by 2000 that evening.

At 0320 on 29 March, an avalanche from the N side of the dome removed much of the February lobe and advanced 0.5-1 km onto the crater floor. Minor snowmelt occurred, but there was no significant mudflow. The March 1984 avalanche was similar in size to the 4 April 1982 avalanche (SEAN 07:03). Fine particles from the avalanche rose to 4.5 km altitude, dusting the E and SE parts of the crater and flanks of the volcano. A large arcuate crack in the February lobe had been observed 27 March, and failure occurred along this crack.

By 29 March, the N side of the dome had decoupled from the rest of the structure and was moving very rapidly outward. The W and probably the SE sides of the dome were virtually stationary, but the N side had moved outward 42 m since 27 March, and instantaneous rates of 15 m per day were observed on the 29th. Since 22 March, the SE side of the dome had moved only a few centimeters outward, while the W side had expanded about 4 m. When the tiltmeter N of the dome was releveled early 29 March, the rate of outward tilt had dropped to 400 µrad/hr. By midnight, the tilt rate was decreasing rapidly.

Tremor gradually separated into individual events early 29 March. During the first couple of hours after the avalanche, large rockfalls were superimposed on the tremor. The tremor gradually evolved into an earthquake swarm that remained vigorous until midnight.

Poor weather prevented frequent measurements of SO₂ emission rates immediately before the extrusion episode. From 3-28 March, SO₂ emission remained relatively constant at about 80 t/d, increasing to 400 t/d 29-30 March.

An overflight at about 2200 on 29 March confirmed that lava had reached the surface, emerging just W of the remnants of the February extrusion. The lobe eventually grew to nearly fill the crater at the top of the dome and reached the edge of the 29 March avalanche chute. Fragments spalled down the chute but lava did not flow beyond the edge of the dome's summit area. Weather conditions prevented direct observations of the extrusion, but deformation and seismic data suggested that lava production ended within a few days.

The earthquake swarm began to decline on 30 March, but did not reach background levels until 4 days later. This slow decline in seismicity contrasts with previous years when seismicity often dropped to background levels within hours of the onset of lava extrusion.

Total outward movement of the N flank was about 3.2 m between 30 March and 2 April, but deformation declined rapidly and had probably nearly stopped by 31 March. Between 22 March and 2 April, the N flank moved outward a total of 55 m. Tilting measured near the N foot of the dome had stopped by the morning of 31 March. Unlike the extrusion episodes of 1981-2, no tilt reversal was detected. Total tilt (assigning a rate of 400 µrad/hr while the instrument was off-scale late 28 to early 29 March) was 28 milliradians, more tilt than had previously been recorded in association with an extrusion episode at Mt. St. Helens (20 milliradians of tilt preceded the September 1981 extrusion). Tiltmeters were redeployed 80 and 250 m N of the dome 6 April and had detected no tilt as of 16 April. Rates of outward movement of the dome were only about 5 mm/day in mid-April.

Information Contacts: S. Brantley, T. Casadevall, D. Dzurisin, C. Newhall, P. Otway, USGS CVO, Vancouver, WA; R. Norris, S. Malone, University of Washington; D. Sowa, Northwest Orient Airlines.

April 1984 (SEAN 09:04) Cite this Report

Mud flow and vertical plume

Deformation, seismicity, and gas emission declined to background levels after the end of the extrusion episode that began in late March (SEAN 09:03). More than half of the April earthquakes were recorded on the 1st. Seismicity reached background level 3 April and remained low through early May. Deformation was very slow through early May. Rates were roughly half the background values measured before deformation began to accelerate prior to the March extrusion episode. Maximum rates were 6 mm/day on the SE side. However, at some points on the dome, no outward movement was measured. Three single-axis telemetering tiltmeters installed along a radial line at 50, 250, and 750 m N of the dome recorded little tilt in April. SO₂ emission averaged 85 ± 50 t/d in April.

. . . On 14 May at 0932 seismicity in the crater began to increase. A pronounced increase was detected at 0935 and a great increase at 0937. No plume was visible at 0934, but by 0939 a plume had risen to an altitude estimated at 5.5-6 km by ground observers. Airplane pilots reported that the eruption cloud had reached 7.5 km altitude at 0950 and 10.5 km at 1000-1005. Radar at Portland airport measured an elevation of 6 km at 0940. Light ashfall was reported at Mt. Rainier National Park, 65 km to the NNE.

In addition to a vertical plume, cold to hot (but not molten) material was ejected laterally from the upper SW portion of the dome. Some cleared the rim of the crater and fell on the outer W flank of the volcano. However, much of the fragmented dome material struck the W wall of the crater, melting heavy snow that had accumulated there during the winter. A flow consisting of about 90% snow and 10% sand-sized lithic fragments plus a few blocks to 1 m in diameter moved through the [crater and the] breach N of the crater. Near the crater, the flow was about 200 m wide. In the outer half of this zone, some melting occurred, but there was little erosion of the underlying snow. Deposits were typically 0.5-1 m thick but locally reached 2 m in thickness. The inner half of the zone was dominantly erosional. Approximately the outer 10 m of the erosional section consisted of fluted snow covered by a lag deposit of fine lithics a couple of centimeters thick. Most of the erosional section was eroded down to [1980 deposits]. The initial flow was apparently primarily snow, followed by [a slushy flow] and then (in the erosional zone) by [a watery flow].

The flow divided into two branches [on the pumice plain (on the volcano's N flank)]. The NE branch . . . reached Spirit Lake (about 5.5 km from the crater) at 0955. The main (NW) branch, a mudflow, entered the North Fork of the Toutle River. Addition of water to the flow caused degeneration to a hyperconcentrate (exhibiting Newtonian behavior, a transition that typically occurs when water concentration reaches about 30% of the flow by weight) by the time it reached Coldwater Lake (about 12 km from the crater). It reached the lake at 1020, carrying boulders to 2 m in diameter, but peak flow there did not occur until 1100. At N-1 debris dam (about 30 km from the crater), peak flow was 135 m³/s above background. Peak flow had attenuated to 27 m³/sec at the mouth of the North Fork (about 90 km from the crater).

Information Contacts: D. Peterson, T. Casadevall, D. Childers, N. MacLeod, C. Mullins, B. Myers, P. Otway, USGS CVO, Vancouver, WA; R. Norris, University of Washington; M. Matson, NOAA/NESDIS; NWS, Portland, OR.

May 1984 (SEAN 09:05) Cite this Report

Explosions from dome; plumes & snow/water flows

Occasional ejections of gas and tephra from the upper W flank of the dome produced plumes and snow/water flows in late May and early June, but were much smaller than the 14 May episode.

Seismicity that was similar in duration and character but less vigorous than that associated with the 14 May episode began on 26 May at 0814 and lasted for about 20 minutes. A plume that contained little ash rose to about 6.5 km altitude. A snow/water flow moved down a channel 20-25 m wide that had been carved by the 14 May episode, and a small amount of material reached Spirit Lake (5.5 km from the crater). The outer portion of the flow was similar to a snow avalanche and the inner zone was more fluid. Although ejecta from the 26 May explosion did not strike the crater wall, the explosion was partly directed and apparently picked up snow as it moved across a deposit left on the crater floor by a large snow and rockfall from the crater wall that occurred on 22 May at 1516. An additional source of snow may have been the 1.5 m that had drifted into the area near the breach in the N side of the crater since 14 May. Warm rock from the interior of the dome was found in the 14 May flow deposit, but all of the rock in the 26 May flow deposit was cold and appeared to be from the exterior of the dome or the 22 May rockfall debris.

The source of the 14 and 26 May episodes was . . . a notch roughly 10-20 m deep and up to 50 m wide that extended roughly 200 m down the W flank from near the dome's summit. Most of the notch cut through the August 1982 lobe, but its head was in the W side of the March 1984 lobe. During an overflight in the evening of 1 June, a prominent long straight chain of glowing spots was visible extending from the head of the notch E across the summit of the dome and the March 1984 lobe. This feature was not observed during the previous night overflight on 17 May.

A 20-minute seismic signal weaker than that of 26 May began on the 27th at 1320. A small white plume rose roughly 600-900 m above the crater rim but was not detected by radar at Portland Airport. No mudflow was reported. An 18-minute seismic signal intermediate in strength between those of 26 and 27 May started at 0351 on 6 June. At 0415, radar at Portland airport detected a plume to about 6 km altitude. Light ashfall occurred on the NE side of the volcano and ash with ballistic fragments struck about 1/3 of the way up the crater wall. Minor snowmelt occurred, and relatively clear water flowed down the 14/26 May channel, causing a small increase in the normal flow into Spirit Lake. The source of the explosion was again the W side of the dome, where a crater had developed since the explosions of 26-27 May.

Outward movement of the dome remained minor but increased slightly, from 2 mm/day in early May to 4 mm/day in early June on the N side (most noticeably at the base of the dome), and also doubled in the same period on the dome's SE side, reaching 10 mm/day in early June. SO₂ emission measured 1-1.5 hours after the 14 May episode was 260 t/d. Two weeks later, rates of SO₂ emission were about 55-60 t/d.

Information Contacts: R. Holcomb, C. Mullins, P. Otway, R. Waitt, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

June 1984 (SEAN 09:06) Cite this Report

New lobe extruded into notch in dome's W flank

A new lobe in the composite lava dome began to emerge in mid-June. Its location, on the W flank, was within the notch that was the source of explosions 14, 26, and 27 May, and 6 June as well as a similar event on 7 June. Growth of the W flank lobe had stopped by 1 July. Accelerating deformation on the dome's N side was measured in late June and early July, but rates of displacement began to decrease after several days and no lava reached the surface.

A small explosion occurred 7 June at 1720, when airline pilots observed an ash cloud that rose to about 9 km altitude. Increased water flow from the crater began at about 1740, and a mudflow about 3 m wide and 15 cm deep reached Spirit Lake at 1802.

Beginning 14-15 June, the number of earthquakes at the nearest crater seismometer (Yellow Rock) increased from about 20-30 to 50-60/day, and the number of surface (rockfall) events began to increase 16-17 June. The small new lobe was first seen during the afternoon of 17 June (by gas-monitoring aircraft pilot Allyn Merris), and was more clearly visible during an overflight at 2230 that night. The dome was photographed from the air 18 June (figure 25).

Figure 25. Photograph of the new lobe on the W side of the Mt. St. Helens composite dome taken on 18 June 1984, early in the extrusion episode. By 1 July, the lobe had overfilled the 50-m-wide notch. Photo by T. Casadevall.

The following information is from Peter Otway.

"When first tracked, on 18 June, the leading edge of the lobe was moving downslope at over 30 m/day although rates near the extrusion site were at least 50% higher. By 22 June, movement of the leading edge had slowed to 13 m/day, and by 25 June, to 6 m/day. When next observed, on 1 July, the flow had stopped.

"The lobe, which overfilled the notch formed by the the May-June explosions, was 60 m wide at its maximum and 150 m long. Its volume was estimated to be of the order of 0.2 x 10⁶ m³.

"The extrusion of 16-17 June was preceded by a slight acceleration of spreading rate on the SE side of the dome from less than 10 mm/day during late May to 13 mm/day between 10 and 19 June, declining to 5 mm/day by the end of June. Although monitoring of the W side has not been possible because of the gas blasts that commenced in May, targets 100 m N and NE of the June lobe moved radially away from the extrusion site at 60 mm/day prior to 22 June.

"During this time, targets on the N side of the dome moved outward at rates that steadily increased from 5 mm/day in early June to 60 mm/day by 25 June, but then accelerated rapidly to peak at 0.8 m/day between 30 June and 2 July. By 10 July, the rates had fallen to 20 mm/day. The direction of movement for all northern targets prior to 25 June was NNE, away from the mid-June extrusion, but swung to the N as the rates rapidly increased beginning 25 June, suggesting that an intrusive event occurred in late June at a site at least 100 m E of the mid-June extrusion. Targets on the NE side of the dome averaged a steady 13 mm/day NNE movement during June indicating that this area has decoupled from the mobile N sector."

Rates of SO₂ emission ranged from 15 to 35 t/d (near the detection limit) during the first 2 weeks of June. The next measurement, the morning of 18 June, was 105 t/d. Rates averaged about 100 t/d through 1 July, but had dropped to 40 t/d by 6 July.

Information Contacts: S. Brantley, T. Casadevall, E. Endo, C. Mullins, C. Newhall, P. Otway, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

July 1984 (SEAN 09:07) Cite this Report

Deformation, seismicity, and SO₂ emission drop

Seismicity, gas emission, and deformation of the composite lava dome declined after the extrusion of a new lobe in June and an intrusive event at the end of the month. The quoted material is from Peter Otway.

"Following late June's record deformation of up to 0.8 m/day at the upper stations on the N side of the dome, rates decreased quickly after 2 July and were down to background levels at a steady 10 mm/day on 16 July. No changes in the rates had been recorded as of early August. On the SE side, displacement rates maintained a constant 6 mm/day throughout July and on the S side remained steady at 1.2 mm/day.

"A number of new survey phototheodolite targets were installed across the top of the dome in July, resulting in a total of 32 targets being surveyed by theodolite several times a month, in addition to the 12 EDM targets tracked 2-3 times a week.

"During the installation of targets on the top of the dome on 30 July, several small steam emissions (accompanying a minor seismic signal) were witnessed at close range. They came from two separate locations: the top of the March lobe, and the old summit crater 80 m to the W. Some ash was present in the emission from the W side. The fume cloud rose less than 300 m from the dome and dispersed slowly. Such emissions appear to occur from time to time even when the general activity is low."

Rates of SO₂ emission increased before the lava extrusion in mid-June, then declined gradually, to 55 t/d on 13 July, and only 15 t/d on the 20th and 12 t/d on the 27th. Seismicity was elevated through early July as dome growth continued, then declined to background levels by the second week of the month and remained low through early August.

Information Contacts: P. Otway, C. Mullins, USGS CVO, Vancouver, WA; R. Norris, University of Washington.

August 1984 (SEAN 09:08) Cite this Report

Intense deformation, then extrusion of new lobe

Strong deformation, vigorous seismicity, and increased SO₂ emission at Mt. St. Helens were followed by extrusion of a new lobe on the NW side of the composite lava dome (figure 26 is a profile showing the dome's 1980-1983 growth).

Figure 26. E-W profile of the composite lava dome, drawn from a succession of photographs taken from 1 km to the N. Successive lobes added to the dome December 1980-83 are shown. Growth since October 1983 is not shown. From Brantley and Topinka, 1984.

Deformation on the N side of the dome gradually accelerated from 1 cm/day on 15 August to about 14 cm/day by the end of the month. A slight increase in the number of recorded earthquakes started on 9 August and steepening of the seismic energy release curve was evident by the 12th as occasional events in the M 1.5-2.3 range began to occur. Another slight upturn in earthquake counts started after 27 August. Rates of SO₂ emission remained low in August, ranging from 12 to 37 t/d.

Deformation accelerated rapidly in early September, rising to 40 cm/day by 2 September and 70 cm/day by the 5th. Instantaneous rates of 1.5 m/day were measured early 8 September and reached 2 m/day late that afternoon. An extensive crack system defined the S margin of the zone of large-scale deformation, which included the dome's NW sector from about due N to about N70°W. On 4 and 5 September, fissuring extended from the W side of the dome across the N end of the June lobe, passing just N of the dome's summit, but it did not continue across the E side of the dome. As deformation became more rapid, rockfalls began to fill the widening fissures.

Seismic instruments began to detect increased rockfall activity on 6 September at about 2200, and earthquakes started to increase shortly thereafter. The number of small earthquakes increased further during the day 8 September. Between 0300 and 0400 on 9 September, the number of low-frequency (type L) events increased, as did the number of very small events previously termed "peppercorns," which appeared to grade upward in magnitude into the type L's. A major increase in earthquake activity continued through the day, saturating many nearby seismic stations set at maximum attenuation. At 1830, the earthquake rate began to drop, and within an hour had declined from one event every few minutes to virtually none. Simultaneously, background tremor-like activity began to build, increasing strongly through the evening. Tremor reached maximum amplitude about 0300, saturating nearby stations, and was detected by seismometers on the S side of Mt. Rainier, about 60 km away. Tremor declined about dawn on the 10th and discrete earthquakes became visible again on seismic records. Because of the tremor saturation during the night, it was not possible to determine at what time the discrete earthquakes had resumed. The number of earthquakes gradually decreased from about one every 2 minutes to 5-10/hour by the 11th.

Deformation measurements suggested that maximum rates of internal dome growth approximately coincided with the strongest seismicity. Measurements of targets on the NW side of the dome early 10 September indicated that outward movement of almost 52 m had occurred since the previous afternoon. Instantaneous rates of 15 m/day were measured between 1000 and 1030, dropping to about 10 m/day in late afternoon and a few meters per day on the 11th.

Aerial observations before dawn 10 September also showed vigorous activity. Large incandescent block-and-ash avalanches originating from three areas, two on the dome's NW flank and one on the N flank, occurred about every 5 minutes. The avalanches moved quickly down the flank and several hundred meters onto the crater floor, building an extensive talus pile. Cascades of individual blocks were continuous, occurring at rates of hundreds per minute. Avalanches had been distinctly smaller and less frequent during an overflight 8 hours earlier, and none was seen on a flight before dawn on the 9th. No new areas of incandescence were seen on the E and S sectors of the dome, although pre-existing incandescent areas had brightened somewhat.

Rates of SO₂ emission also reached maximum values on 10 September, increasing from 14 t/d 30 August, to 54 t/d on 5 September, 440 t/d on the 8th, and 786 t/d on the 10th. Enhanced SO₂ emission was accompanied by increased fuming and small steam-and-ash ejections. The largest rose about 1 km above the crater rim early 9 September, dropping about 1 mm of ash in the crater and a trace SE of the volcano. All of the ash in this plume was from older dome rocks.

On 12 September, geologists observed that new lava had been extruded, extending about 300 m down the NW flank. By the next day, seismicity had declined to background levels and deformation had slowed.

Reference. Brantley, S. and Topinka, L. (eds.), 1984, Volcanic Studies at the U.S. Geological Survey's David A. Johnston Cascades Volcano Observatory, Vancouver, Washington; Earthquake Information Bulletin, v. 16, no. 2, p. 43-122.

Information Contacts: M. Doukas, R. Holcomb, D. Swanson, J. Sutton, USGS CVO, Vancouver, WA; R. Norris, University of Washington; UPI.

September 1984 (SEAN 09:09) Cite this Report

New lobe in collapse zone on composite dome

The new lobe, which began to emerge during the night of 9-10 September and had stopped growing by the 14th, partially filled an elongate E-W-trending collapse zone, about 350 m long by 250 m wide, on the dome's upper W side. The collapse, in what had been the highest part of the dome, destroyed much of the March 1984 lobe and engulfed the notch produced by explosions in May (SEAN 09:05). The SE wall of the collapse zone was defined by the zone of fissuring observed in early September. The volume of collapse was about 1.3 x 10⁶ m³.

The new lobe was about 250 m long by 220 m wide, with an average thickness of 35 m. The USGS calculated its volume at about 2 x 10⁶ m³. It was almost entirely confined within the collapse zone, although it protruded considerably above the rim of the collapse zone. Sublimate deposits were more extensive than on previous lobes, and a large area at the top of the new lobe was a dirty yellow color.

At least 3.4 x 10⁶ m³ of internal expansion of the dome took place during the period of accelerating deformation prior to extrusion of the new lobe. Between the afternoon of 9 September and the next morning, targets on the NW side of the dome moved outward as much as 52 m. Displacement rates dropped sharply on the 10th, and by 11 September outward movement was occurring at only centimeters per day. Deformation remained very slow through early October.

After increasing sharply to 786 t/d on 10 September shortly after the new lobe was first observed, rates of SO₂ emission declined to less than 300 t/d on the 12th, less than 200 t/d the next afternoon, and just under 100 t/d by the 15th. A period of higher SO₂ emission rates occurred in late September, with measurements on the 23rd, 26th, and 28th yielding values of 190, 135, and 148 t/d, but SO₂ values had dropped to less than 50 t/d on 2 October.

Seismicity gradually declined 11-12 September, from about 20 events per hour on the 11th to about 5 per hour late on the 12th, after very vigorous activity the previous two days (SEAN 09:08). Most earthquakes had stopped by 13 September, but surface events, primarily caused by rockfalls, were frequent that day and remained frequent for the next week. The number of surface events declined by the last week in September and seismic activity remained low through early October.

Information Contacts: D. Swanson, C. Mullins, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

October 1984 (SEAN 09:10) Cite this Report

Deformation, seismicity, and gas emission low

Since the extrusion of a new lobe onto the composite lava dome in early September (figure 27 and SEAN 09:08 and 09:09), seismicity, deformation, and SO₂ emission have returned to near background levels. Poor weather has limited observation of the crater, slow settling of the lava dome has continued, and rates of displacement averaged 4-5 mm/day in October. No large landslides were evident from field work or seismic records. Little vapor emission from the dome has been visible, but seismic instruments recorded signals apparently caused by very small steam and ash events, with as many as twenty on a single day (27 October) [but see SEAN 09:11]. Field observations on 1 November found no new tephra in fresh snow layers in the crater. Rates of SO₂ emission averaged 75 ± 25 t/d in October. Very few earthquakes were recorded in October.

Figure 27. Sketch of Mt. St. Helens, by Bobbie Myers, looking S, showing the dome after extrusion of the new lobe in early September 1984.

Information Contacts: D. Swanson, C. Mullins, B. Myers, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

November 1984 (SEAN 09:11) Cite this Report

Seismicity, SO₂ emission, and deformation weak

Seismicity, SO₂ emission, and deformation of the composite lava dome remained at background levels in November. The maximum displacement rate on the N side of the dome was 3 mm/day. SO₂ data were successfully collected from four flights in mid to late November. Rates of emission averaged 25 ± 10 t/d. Very few seismic events were recorded in November and early December.

It has proven difficult to discriminate between the seismic signatures produced by different types of surface events, including tephra emissions, snow avalanches, and rockfalls from the dome and crater walls. Field evidence indicates that the seismic signals reported in SEAN 09:10 were not produced by steam-and-ash emissions. Since 24 September, no emissions of tephra from the dome have been observed by field geologists, nor have any tephra layers been found in the snowpack that has been accumulating in the crater through October and November. USGS geologists noted that each of the three most recent large explosions (March 1982, February 1983, and May 1984) were preceded by a period (1.5-5 months) in which no tephra emission events were observed.

Information Contacts: D. Swanson, C. Mullins, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

December 1984 (SEAN 09:12) Cite this Report

Activity remains at background levels

Deformation of the composite lava dome, seismicity, and rates of SO₂ emission remained at background levels through early January. The maximum rate of outward movement on the N and S sides of the dome in December and early January was 5 mm/day. The rate of SO₂ emission, 10 t/d or less on 3 and 5 December, had increased slightly to 50 ± 10 t/d by the next measurement on 2 January and 25 ± 4 t/d 5 January. No tephra emissions have been observed by geologists working in the crater, nor have fresh tephra layers been found in the snowpack accumulating in the crater. Unusually good weather during the early winter has allowed the emplacement of two telemetering tiltmeters on the dome itself (tiltmeters had previously been installed on the crater floor near the dome), a telemetering strainmeter on a crack near the dome's summit, and a small trilateration network consisting of two benchmarks and five targets in the dome's summit area.

Information Contacts: D. Swanson; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

January 1985 (SEAN 10:01) Cite this Report

Activity remains at background levels

Seismicity, SO₂ emission, and deformation of the composite lava dome remained at background levels through early February. The duration of the current quiet period was comparable to the longest previous interval between extrusion episodes, which separated the August 1982 extrusion from the onset of more than a year of continuous activity beginning in February 1983.

Very few earthquakes or surface events were recorded through early February. Rates of SO₂ emission measured on 5 days in January and early February ranged from 76 ± 4 t/d on 22 January to 12 ± 5 t/d on 4 February. Two telemetering H₂ monitors were installed on the dome in early January, one in a 70°C fumarole, the other in ambient air. Some variation in H₂ values was observed, but more background data will be necessary before the significance of such variations can be determined. No tephra emissions have occurred since 24 September. USGS geologists noted that each prolonged lull in tephra emissions (ranging from 6-20 weeks) between 1982 and 1984 was followed by a moderate explosion.

Information Contacts: D. Swanson; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

February 1985 (SEAN 10:02) Cite this Report

Activity at background levels; longest quiet phase since activity began in 1980

Seismicity and rates of displacement and SO₂ emission remained at background levels in February and early March. Maximum displacement rates on the dome were 2-3 mm/day. SO₂ emission averaged 50 ± 10 t/d in February and did not change significantly in early March. The highest temperature yet measured on the dome, 912°C, was recorded 18 February in a rubble-filled crack high on the N side of the dome. Glow at this site has been seen in night photos since early fall 1984, but no temperature measurements had previously been made there.

No tephra emission has occurred from the lava dome since 24 September, and the last extrusion episode was in early September (SEAN 09:08 and 09:09). This is the longest quiet interval since the volcano became active in 1980.

Information Contacts: D. Swanson; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

March 1985 (SEAN 10:03) Cite this Report

Activity remains at background levels

SO₂ emission averaged 50 ± 10 t/d in March, identical to the February rate, and remained low in early April. Maximum displacement rates were 2-3 mm/day. No gas-and-ash emissions from the dome were observed, or detected by seismographs.

Information Contacts: D. Swanson; J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

April 1985 (SEAN 10:04) Cite this Report

Activity at background levels; magnetic data

No gas-and-ash emissions from the dome were observed, and none was detected by seismographs through early May. Maximum displacement rates were 2-3 mm/day. SO₂ emission averaged 30 ± 5 t/d in April, lower than the average February and March rates of 50 ± 10 t/d.

The following is a report from Daniel Dzurisin and Roger Denlinger.

"Measurements of total magnetic field intensity on and near the lava dome are providing unique insights into the dome's internal structure and cooling history. Permanent changes in magnetic intensity occur on and near the dome during episodes of rapid growth, and secular increases occur on the dome as its exterior cools and becomes permanently magnetized."

"Magnetic intensity data with a precision of 0.25 gamma are collected simultaneously at two base stations on the volcano's flanks, and at fixed stations on the dome and surrounding crater floor. Identical proton precession total field magnetometers are used at the base stations and in the crater. The base stations are automated and transmit data to CVO once each minute; crater stations are measured sequentially by a field crew using portable instruments. The base station record is subtracted from the crater data to remove diurnal variations to an empirical accuracy of about 2 gammas."

"Two types of changes in magnetic field intensity have been detected since measurements began in March 1984. The first type occurs at stations near the dome during rapid endogenous or exogenous growth of the dome. Permanent decreases in magnetic intensity of a few to a few tens of gammas accompanied dome extrusions in March and September 1984. No comparable changes occurred during a relatively passive extrusion in June 1984 [but see SEAN 10:10], or during a period of rapid endogenous growth that followed in early July. Parts of the dome were displaced by several tens of meters during the March and September extrusions, but displacements during the June and July events were considerably smaller. We tentatively attribute this first type of magnetic change to large displacements of the cooled magnetic exterior of the dome, relative to nearby magnetic monitoring stations."

"A second type of change occurs only on the dome, where the magnetic field intensity at most stations has increased steadily since measurements began there in December 1984. Rates of increase vary from 0.1 to 2.2 gammas per day on different parts of the dome, but do not change significantly with time at any one station. We tentatively attribute these secular increases in field strength to cooling and magnetization of the outer parts of the dome."

"To better understand this second type of magnetic intensity change, we have made a preliminary magnetic intensity map of the dome, and have repeatedly measured several short magnetic profiles. The magnetic intensity map shows a broad positive anomaly of about 1,000 gammas amplitude associated with the dome. A magnetic profile with more closely spaced stations across the September 1984 lobe reveals local anomalies with wavelengths of a few to a few hundred meters, with a maximum amplitude of about 700 gammas. We tentatively attribute the long wavelength anomaly to a cooled magnetic exterior enclosing the lobe's hotter, non-magnetic interior. The strength of the long wavelength anomaly increased by as much as 100 gammas from February to April 1985, presumably owing to continued cooling. More detailed magnetic profiles centered at each magnetic monitoring station on the dome tell a similar story. Measurements are made at 1-m intervals along N-S and E-W profiles about 20 m long, at sensor heights of 2.5, 3.7, and 5.0 m above the ground. Typically, short-wavelength (1-10-m) anomalies decay rapidly with increasing height, but they do not change significantly with time. Instead, the magnetic intensity along the entire profile increases uniformly with time, implying the growth of broader anomalies at depths of a few tens of meters."

"During the next year, we plan to improve our data base on the dome, and to begin quantitative modelling and interpretation of our results. The existing magnetic intensity map is not sufficiently detailed to distinguish the magnetic signatures of various lobes comprising the dome, so we will make a more detailed map this summer. We will also numerically model the development of magnetic anomalies associated with the September 1984 lobe and the dome as a whole, to estimate the downward migration rate of permanent magnetization, and the implied volumetric cooling rate of the dome. Combined with results from other concurrent geophysical studies, the goal of this research is to characterize the thermal structure of the lava dome and its temporal evolution. Our results may eventually bear on such diverse topics as the rheology of the dome and the volcanic hazards implications of its continued growth."

Information Contacts: D. Dzurisin, R. Denlinger, D. Swanson, J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

May 1985 (SEAN 10:05) Cite this Report

Strong seismicity accompanies major dome-building episode

During mid-May, seismicity and rates of displacement began to increase from background levels. After 8 months of quiescence, a new lobe was extruded onto the composite lava dome, and a major intrusive event further enlarged the dome.

On 13 May, the number of medium- to high-frequency events recorded from the crater seismic station began to increase. Measurements on 16 May indicated that displacement rates on the dome had increased slightly while seismicity continued to build gradually. Several vigorous gas emissions, the first since 24 September 1984, occurred on 17 May from a vent in the NE part of the dome's summit area. No tephra was ejected, and the largest plume rose to just above the crater rim. Seismicity and displacement rates continued to increase, and on 20 May the USGS and University of Washington issued a volcano advisory notice stating that recent changes at the volcano suggested "renewed eruptive activity will begin within the next 2 weeks, possibly within the next few days."

By the 23rd, seismicity had reached high levels. On the 24th, very low-frequency events increased in size and number, although medium- to high-frequency events decreased. Observations made during a night overflight on the 25th at 0030 indicated increased glow from cracks on the dome. Later that morning, the low-frequency earthquakes that typically immediately precede and accompany extrusions had become more numerous, and deformation patterns suggested to USGS scientists that magma had nearly reached the surface of the dome. Observations during the next week were few because of poor weather conditions.

Between 21 and 23 May, a trough-like fracture system began to develop on top of the dome. By 27 May, this graben was 30-50 m wide and had cut across the entire S portion of the dome. Sometime between the 27th and 30th, a new lobe of gas-poor lava was extruded on the SE flank of the dome, near the E end of the newly formed graben. The 80-m-wide lobe extended out of the graben about 100 m down to the crater floor (figure 28). By the 30th, the graben was 400 m long, 30 m deep, and averaged 90-100 m wide. The dome flank S of the graben had been displaced outward.

Figure 28. Sketch of Mt. St. Helens, by Bobbie Myers, looking NW, showing the composite lava dome after formation of the graben and extrusion of the new lobe in May 1985.

Seismicity continued at very high levels; on the 29th, M 2.5-2.8 events were recorded every few minutes as deep as 1 km below the dome. During past extrusion episodes, seismicity typically decreased sharply when lava reached the surface, but on the 31st, seismicity remained vigorous. Late on 31 May or early on 1 June, a slow decrease in the number of seismic events began. By 5 June seismicity had decreased to moderate levels and had reached background levels by the 17th.

Measurements by USGS scientists on 16 June indicated that the S flank of the dome had moved outward about 70-100 m during the eruption, but that the current displacement rate had by then decreased to about a centimeter per day. A station on the crater floor S of the dome moved 46 m outward and 18 m upward between 17 May and 8 June, but had moved only a few centimeters more by the 16th. These measurements suggested to USGS scientists that about 6-8 x 10⁶ m³ of magma were intruded into the dome during this eruption, the largest single dome-building event since 12 June 1980.

After the vigorous gas emission events of 17 May, vapor emission continued from the same vent during the eruption. However, SO₂ emission rates averaged about 40 t/d during May, about the same as the average April rate of 30 ± 5 t/d. On 30 May, the rate increased from background levels to 90 ± 10 t/d, by 2 June to 160 ± 25, and by the 8th to 220 ± 10 t/d. SO₂ emission decreased to 165 ± 10 t/d on June 10th and to 60 ± 10 t/d on the 12th.

Equipment to measure H₂ emission was installed on top of the dome in January. After 3 months of very low measurements, ambient H₂ around the top of the dome began to increase dramatically on 24 May, continued to rise over the next few days, and reached a very high value before the station was destroyed early on the 29th. Because the instrument was not yet calibrated, the measurements cannot be quantified.

Information Contacts: D. Swanson, C. Newhall, S. Brantley, K. McGee, J. Sutton, B. Myers, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

June 1985 (SEAN 10:06) Cite this Report

Activity returns to background levels

After the extrusion of a new lobe and endogenous growth of the composite lava dome in late May-early June, seismicity, deformation, and SO₂ emission returned to background levels. Displacement rates began to decline on 10 June and had reached background levels by late June. As of early July the dome was stable, with maximum displacement rates of a few millimeters per day. After reaching a high of 220 ± 10 t/d on 8 June, SO₂ emission had decreased to a background level of 60 ± 5 t/d by the 12th, and remained between 25 ± 5 and 45 ± 5 t/d through early July.

Information Contacts: D. Swanson, J. Sutton USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

July 1985 (SEAN 10:07) Cite this Report

Activity at background levels

Seismicity, SO₂ emission, and deformation remained at background levels. Maximum displacement rates on the dome were 1-2 mm/day, and SO₂ emissions continued to decline, averaging 28 ± 5 t/d in July, compared to 45 t/d in late June. No vigorous gas emissions from the dome have been observed since mid-May.

Information Contacts: D. Swanson, S. Brantley, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

August 1985 (SEAN 10:08) Cite this Report

Activity remains at background levels

Displacement rates in August and early September were 1 mm or less per day. SO₂ emissions averaged 30 ± 10 t/d. No gas or ash emissions were observed, and none was detected by seismographs.

Information Contacts: S. Brantley, E. Iwatsubo, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

September 1985 (SEAN 10:09) Cite this Report

Activity at background levels

Through early October, maximum displacement rates were 1-2 mm/day on the S side of the dome, the region where deformation has been concentrated for the past 6 months. The period includes several weeks preceding the May/June lobe extrusion on the S side of the dome (SEAN 10:05). Three measurements during September showed an average rate of 30 ± 15 t/d of SO₂ emission. There were no tephra or energetic gas emissions during the month.

Information Contacts: D. Swanson; S. Brantley, USGS CVO, Vancouver, WA; S. Malone, University of Washington.

October 1985 (SEAN 10:10) Cite this Report

Activity at background level; magnetic data on May-June dome building eruption

Seismicity, deformation, and SO₂ emissions were at background levels throughout October. Seven measurements of SO₂ yielded an average of 35 ± 10 t/d. Maximum displacement rates on the dome were 1-2 mm/day.

A continuously recording stainless-steel wire strainmeter was installed on the dome on 24 August, spanning several cracks just north of the graben produced during the May-June 1985 dome building episode (SEAN 10:05). Since 1 September the strainmeter has shown a constant extension rate of 0.2 mm/day, thought to be caused by gravitational stresses.

The following is a report from Daniel Dzurisin and Roger Denlinger.

"In our last report (SEAN 10:04), we discussed preliminary results of magnetic field intensity measurements since March 1984. Two types of temporal changes had been detected. On the crater floor near the dome, permanent decreases in magnetic intensity accompanied the March, June, and September 1984 extrusions (the report in SEAN 10:04 stated incorrectly that no changes were detected in June 1984). These changes were attributed to displacement of the N part of the dome during rapid endogenous growth. At some stations, smaller reversible changes accompanied the March and June 1984 extrusions. Their cause is not known, but stress-induced piezomagnetic effects could have played a role. On the dome itself, there had been a steady increase in magnetic intensity at most monitored sites since measurements began in December 1984. Those changes were attributed to cooling and magnetization of outer parts of the dome."

"There have been three significant developments since the April 1985 report. First, the largest dome building eruption since activity began in 1980 occurred during May-June 1985. One magnetic site was destroyed and two others were significantly displaced when a graben split the S part of the dome during rapid endogenous growth. A small flow was eventually extruded from the graben floor. Large magnetic changes occurred only at those sites on the S part of the dome that were obviously displaced; elsewhere, earlier trends continued uninterrupted. On the crater floor, the field intensity at the station closest to the dome increased significantly during endogenous growth; other stations showed no changes. Large changes on the dome were almost surely caused by displacement of monitoring sites and nearby rock (one station rode the floor of the new graben downward almost 50 m). The cause of the intensity increase at one crater floor station is not known, but disruption of the dome could have been responsible."

"A second development is that rates of magnetization at stations on the N part of the dome decreased in early June 1985, and rates at stations on the S part of the dome increased. By October, there was a suggestion that rates at some N sites had started to increase again. Two explanations for the changes at N sites are under consideration: 1) a seasonal change in cooling and magnetization rates (less precipitation, less rapid cooling and magnetization in summer); and 2) a heat pulse related to the May-June episode of dome growth. Increased magnetization rates at S sites probably reflect rapid cooling of the walls of the new graben, or of newly-emplaced magma beneath the floor of the graben."

"Finally, a preliminary aeromagnetic survey was conducted above the dome in October, and additional surveys are planned this winter. Our goal is to model the internal thermal structure of the dome, to establish a better framework for interpreting temporal changes. Preliminary results suggest that the magnetic carapace of the dome is 1-10 m thick in most places, with large local variations."

Information Contacts: D. Dzurisin, R. Denlinger, D. Swanson, J. Sutton, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

November 1985 (SEAN 10:11) Cite this Report

New lahar warning gauge; background activity

Mt. St. Helens was quiet through November and early December, with seismic activity, SO₂ emission, and displacement rates remaining at background levels. Displacement rates on the dome were less than 1 mm/day; SO₂ levels continued to be low and quite variable.

The USGS's Water Resources Division has installed a new gauge along the crater's main drainage channel to improve warnings of mudflows and snow avalanches. The new gauge, just N of the crater (in Loowit Channel at The Steps), and an existing one 25 km downstream in the North Fork Toutle River (at Elk Rock) transmit to CVO via satellite every 5 minutes. These stations, in addition to seismometers and other gauges downstream, should provide an early warning of rapidly rising water levels (figure 29).

Figure 29. Location of monitoring sites, Mt. St. Helens, Washington; circles indicate seismometers, triangles indicate gauges. Base map modified from Childers and Carpenter, 1985.

Much of the network was installed prior to November 1982 to monitor levels of the lakes created when the 18 May 1980 avalanche blocked the outflow from Spirit Lake and downstream tributaries to the North Fork Toutle River. Major flooding could occur if one of the debris dams failed, and seismometers were installed in 1983 to supplement the gauges. A characteristic signature is produced by lahars flowing from the crater in a confined channel, as in May and June 1984. Although exact times and discharge volumes could not be determined from the seismic records for those events, future refinements may provide a relationship between seismic signals and discharge volumes (Brantley and others, 1985).

References. Brantley, S., Power, J., and Topinka, L., 1985, Reports from the U.S. Geological Survey's Cascades Volcano Observatory at Vancouver, Washington; Earthquake Information Bulletin, v. 17, no. 1, p. 20-32.

Childers, D. and Carpenter, P.J., 1985, International Symposium on Erosion, Debris Flow, and Disaster Prevention; September 3-5, Tsukuba, Japan.

Information Contacts: G. Gallino, S. Brantley, E. Iwatsubo, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

December 1985 (SEAN 10:12) Cite this Report

Activity remains at background levels

Activity at Mt. St. Helens remained at background levels in December. For the 7th consecutive month, only minor seismicity, geodetic changes, and SO₂ emissions were detected. Displacement rates on the dome were approximately 2 mm/day, while the continuously recording strainmeter just N of the May-June 1985 graben showed step-like extension across cracks averaging 0.2 mm/day. Seismicity consisted mostly of surface events (primarily rockfalls). SO₂ emissions continued to be low and variable.

Further References. Keller, S.A.H. (ed.), 1986, Mount St. Helens, Five Years Later: Proceedings of a Symposium at Eastern Washington University May 16-18, 1985; Eastern Washington University Press, 448 p. (47 papers).

Manson, C.J., Messick, C.H., and Sinnott, G.M., 1987, Mount St. Helens-A Bibliography of Geoscience Literature, 1882-1986: USGS Open-File Report 87-292 (1600+ references).

Special Section: Mount St. Helens: JGR, 1987, v. 92, no. B10, p. 10,149-10,334 (12 papers).

Information Contacts: D. Swanson, S. Brantley, USGS CVO, Vancouver, WA; C. Jonientz-Trisler, University of Washington.

January 1986 (SEAN 11:01) Cite this Report

Brief earthquake swarm and H₂ pulse

Seismicity, deformation rates, and SO₂ emissions remained at background levels through early February. Deformation on the dome averaged <1 mm/day; two successful SO₂ flights in January showed an average emission of 68 plus or minus 22 t/d. Seismometers recorded 27 surface events, all attributed to rockfalls. There was no seismic evidence of energetic steam emission.

On 30 December at 1600, seismometers in the crater began to record a sequence of earthquakes centered on the lava dome. For the next 1.75 hours, one hundred individual earthquakes were recorded above a "tremor-like" background signal with frequencies 3-4 times higher than that of harmonic tremor. Approximately an hour after the onset of seismicity, a gas emission event was detected by the atmospheric hydrogen sensor located on the dome. H₂ was released in one large pulse lasting about an hour, followed by several smaller pulses.

Scientists at CVO interpret the seismicity coupled with H₂ degassing as evidence for sudden contact of groundwater with magma. The resulting pressure change may have produced microfractures in overlying crustal material, which in turn allowed the release of gas to the atmosphere. The background "tremor-like" signal (acoustic emission) is believed to be associated with moving gases rather than the moving magma associated with harmonic tremor.

Information Contacts: K. McGee, E. Endo, E. Iwatsubo, and S. Brantley, CVO; C. Jonientz-Trisler, Geophysics Program, University of Washington, Seattle, Washington 98195 USA.

February 1986 (SEAN 11:02) Cite this Report

Longest period of repose continues

Activity remained at background levels for the 8th consecutive month, the longest repose period since activity began in 1980. February's single successful SO₂ flight (on the 10th) measured 45 plus or minus 10 t/d. Maximum deformation rates were ~1 mm/day. Seismicity also remained low, with no evidence of energetic gas emission.

Information Contacts: D. Swanson, E. Endo, and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.

March 1986 (SEAN 11:03) Cite this Report

Steam and ash plumes; deeper earthquakes

During March, seismicity, rates of SO₂ emission, and deformation remained at background levels. Maximum deformation rates on the dome were 1-2 mm/day, measured across the graben formed during the last extrusive event in May-June 1985. An average of 20 plus or minus 5 t/d of SO₂ were measured in March during three overflights. Seismicity was also at background levels, although 17 small, but deeper than usual, earthquakes were located between 29 January and 1 April. These medium- to high-frequency events had a depth range of 3-8 km and epicenters trended NNW to SSE across the crater.

On 16 April at 1717, a minor gas-and-ash emission event sent a plume to 5.4 km above the dome. An airplane pilot reported that the cloud moved NE from the volcano. A smaller plume rose < 2 km above the crater rim the next day at 1428. Beginning early 15 April, there had been several occasions on which brief bursts of seismicity were associated with minor (1-2 mm) dome strain episodes (measured by continuously-recording strainmeter across a zone of cracks just N of the graben), tilt excursions of a few µrads, and increases in the signal from the dome gas sensor, which detects H₂, H₂S, and SO₂. Earlier (and weaker) episodes of instrumental activity were accompanied by poor weather that prevented observation of associated emissions. Occasional deeper earthquakes, like those located in February and March, continued through mid-April, and there was no significant change in the volcano's seismic energy release. No general increase in the rate of swelling of the dome was detected before the emission of the steam-and-ash plumes.

No episodes of gas-and-ash emission had been seen since mid-May 1985, shortly before the last lava extrusion episode (SEAN 10:05).

Information Contacts: D. Swanson and J. Sutton, CVO; C. Jonientz-Trisler, University of Washington.

April 1986 (SEAN 11:04) Cite this Report

Steam and ash emissions, then new lobe added to the summit lava dome; first activity since May-June 1985

The gas-and-ash emission event of 16 April (SEAN 11:03) marked the beginning of increased seismicity and deformation that culminated in the extrusion of a new lobe on top of the lava dome on 8 or 9 May.

Approximately 50 gas-and-ash emissions occurred in the 3 weeks following the onset of activity, emitting plumes to as much as 6 km altitude. On 19 April at 1950, 50-kg blocks were expelled to > 1 km from the vent; some were thrown over the S rim of the crater. Two small mudflows were generated, which were confined to the crater. A new crater 50-75 m N-S, 20-30 m E-W, and 20 m deep was formed on the top of the dome, and the gas sensor, strainmeter, and tiltmeter that were on top of the dome were destroyed. Between 18 and 20 April the tiltmeter on the N side of the dome registered 1,500 µrads of inflation and a line on the crater floor N of the dome shortened 2 cm.

The last of the unusually deep (3-8 km) earthquakes that began on 29 January (SEAN 11:03) was recorded on 13 April. Seismicity remained at background levels for ~10 days following the 16 April emission, with the exception of 20-23 April, when a slight increase was recorded. On 27 April, activity began to increase again. Shallow events increased rapidly on 5 and 6 May. By 7 May, earthquake activity reached high levels (figure 30), prompting the USGS to issue a Volcano Advisory Notice, forecasting a magmatic event within the next few weeks. Seismicity and tilt increased greatly overnight, so the Advisory was revised on 8 May to predict an event within the next few days.

Figure 30. Seismic strain release at Mt. St. Helens, 2 April-13 May. The dashed line represents surface events, the solid line earthquakes, and the dotted line total E energy release. Courtesy of C. Jonientz-Trisler.

In the morning of 8 May, discrete earthquake activity was almost continuous. Larger events with magnitudes of 2.5-3.0 were recorded every few tens of minutes. The tiltmeter on the N flank of the dome went off scale [see also SEAN 11:05], and one on the crater floor 280 m N of the dome registered as much as 85 µrads/hour of inflationary tilt until it leveled off at about 1500 (figure 31). At about that time, high-frequency earthquakes also decreased, and low-frequency events, commonly associated with lava extrusions, became dominant. Field crews in the crater that day measured 7 cm of contraction along a 66-m line on the N crater floor, suggesting that the crater floor was thrusting away from the dome. USGS geophysicists noted that the change in tilt direction and decrease in seismicity probably occurred when magma intruding the dome gained easy access to the summit area and stopped deforming the NE flank. During the mid-afternoon, the first successful gas flight since the onset of gas emission episodes in mid-April measured an SO₂ emission rate of 700 t/d, an order of magnitude above rates measured in recent months.

Figure 31. Tilt and seismicity at Mt. St. Helens, 14 April-14 May. Tilt plots are from single-axis bubble tiltmeters on the dome's N flank (Ruby, top) and on the crater floor near the N base of the dome (Sauna, middle). Both measure tilt radial to the dome; increasing values correspond to tilt outward from the dome or down to the N. Seismic amplitudes (bottom) are from the Garden. Seismic station, 700 m N of the center of the dome.

At approximately 1950, another major gas-and-ash emission occurred, ejecting juvenile tephra to the E and SE of the crater. Accompanying the event was a large rockfall from the new lobe. The rockfall moved down the N talus chute, destroying the tiltmeter on the N flank of the dome, and generated a hot surge that moved 600-800 m from the base of the dome. Meltwater generated by the slide and surge caused a water and/or mudflow 1 m deep that tripped flood gauge wires just N of the crater. Gauges downstream did not detect the flow. Scientists flying over the volcano that night saw a glow on top of the lava dome.

During midmorning on 9 May, moderate- to high-frequency events stopped, low-frequency events decreased and rockfalls began to dominate the seismic record for the next 24 hours. On 10 May, moderate- to high-frequency events resumed, but overall seismicity dropped to moderate levels and by 13 May levels were only slightly elevated.

On 14 May, field crews were able to reach the crater to confirm the extrusion of a new lobe on top of the dome. The new lobe was 250-300 m wide (E-W), 275-300 m long (N-S), and > 40 m thick in places, covering the E one quarter to one third of the September 1984 lobe. A broad area of the dome's summit was heavily fissured, with many hot radial cracks low on the S flank of the dome, indicating that intrusive growth had also taken place. Monitoring is limited to the N side of the crater because of snow cover, precluding estimates of dome volume change. The maximum deformation measured between 8 and 14 May was 1.9 m along a line from the N-crater floor to a point on the N flank of the dome. Lines were remeasured after 1 hour and showed no additional change.

The April-May activity was the first since the dome building episode in May-June 1985 (SEAN 10:05), the longest quiet period since eruptive activity began in 1980.

Information Contacts: D. Swanson, E. Endo, D. Dzurisin, and K. McGee, CVO; C. Jonientz-Trisler, University of Washington.

May 1986 (SEAN 11:05) Cite this Report

Activity returns to background levels

During extrusion of a new lobe in early May (SEAN 11:01), the entire dome deformed asymetrically, with maximum movement measured at points on the NE and S sides. A point on the top of the dome just E of the new lobe moved outward horizontally 28 m and subsided 3.2 m between 28 April and 14 May, with most of the change between 8 and 10 May. Another station at the W end of the axis of the 1985 graben moved 21.7 m. More typical displacements on the dome were 4-5 m horizontally. By approximately 28 May, deformation rates had returned to background levels.

During the dome's rapid endogenous growth, the tiltmeter on the N flank of the dome recorded ~15,000 µrads of outward tilt before going off scale on 8 May. The tiltmeter on the N-crater floor recorded ~1,000 µrads of outward tilting from 5 May to midday 8 May when the trend reversed. A large rock avalanche, consisting mainly of newly extruded dacite from the N flank of the dome, destroyed this instrument late on 9 May (figure 32). The N-flank tiltmeter was reset on 14 May and recorded slow outward tilting through 11 June.

Figure 32. Sketch by Bobbie Myers, looking S, showing the new lava lobe extruded in May on top of the dome at Mt. St. Helens and the 9 May rockfall deposits on the crater floor N of the dome. Meltwater generated by the hot rockfall caused a water-rich flow to move out of the crater and down onto the Loowit Fan (SEAN 11:04).

During peak activity, poor weather conditions limited successful COSPEC flights to 8 May when an SO₂ emission rate of 700 t/d was measured. During the next flight, on 14 May, SO₂ emissions were down to background levels of 35 plus or minus 5 t/d. In five flights between 14 May and 9 June, an average of 40 plus or minus 5 t/d were measured.

Seismicity also decreased greatly after 10 May and had reached background levels by 20 May (figure 33). The last of the medium- to high-frequency earthquakes, typical of brittle fracture, occurred 17 May. As of 12 June there were a few low-frequency, shallow earthquakes beneath the dome, but the record was dominated by avalanches off the crater wall, associated with summer snowmelt.

Figure 33. Cumulative seismic strain release at Mt. St. Helens, 15 April-30 May. The solid line represents earthquakes and the dashed line surface type events (steam-and-ash emission, avalanches.)

Information Contacts: D. Swanson, D. Dzurisin, J. Sutton, E. Endo, and Bobbie Myers, CVO; C. Jonientz-Trisler, University of Washington.

June 1986 (SEAN 11:06) Cite this Report

Activity at background levels

Deformation, SO₂ emission, and seismicity remained at background levels through early July. EDM lines monitoring the dome's summit area and N and W flanks showed maximum deformation rates of 2-3 mm/day. Continuously recording tiltmeters detected no unusual changes. Seismicity remained at low levels. Seismic instruments recorded a few episodes in which several shocks occurred within a minute. Some events were felt by geologists working on the crater floor and one episode was associated with unconfirmed reports of plume emission. Similar activity has been recorded during previous inter-eruption periods. COSPEC monitoring on 3 days in June yielded only background SO₂ emission rates of 15-25 t/d.

Information Contacts: D. Swanson, E. Endo, and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.

July 1986 (SEAN 11:07) Cite this Report

Activity remains at background levels

Activity remained at background levels through early August. Rates of SO₂ emission ranged from ~10 to 40 t/d. No gas emission episodes were observed, and none were evident on seismic records. Frequent rockfalls occurred in the crater. Deformation of the lava dome remained minimal.

Information Contacts: S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.

August 1986 (SEAN 11:08) Cite this Report

Earthquake swarm and small plume emission

Deformation, seismicity, and rates of SO₂ emission remained at background levels through early September. Deformation of the lava dome did not exceed 1-3 mm/day and no significant changes were detected by tiltmeters. SO₂ emission was measured at 55 plus or minus 15 t/d on 13 August and 35 plus or minus 5 t/d on the 19th, normal values for periods between extrusion episodes. Seismicity also remained at background, with an approximately equal number of low-frequency and medium- to high-frequency events. Previous periods between extrusion episodes had generally been dominated by low-frequency seismicity.

A swarm of tiny earthquakes began late 12 September, with rates reaching 800-1,000 events/day. Maximum focal depths were 2-3 km below the crater, and most were probably shallower. The swarm continued until the onset of seismicity characteristic of steam-and-ash emission at 0939 on 15 September. A white plume rose from the crater to ~3.5 km altitude, and the associated seismicity continued for about an hour. Seismicity then declined to background, although seismometers recorded another few minutes of apparent, but unwitnessed, gas emission starting at 2047 that evening. A tiltmeter on the W side of the dome indicated inflation for 1-2 days before the 15 September plume emission. This apparent inflationary tilt stopped after the morning plume emission, and no other tiltmeters or deformation measurements showed any significant changes.

Information Contacts: D. Swanson and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington.

September 1986 (SEAN 11:09) Cite this Report

New lobe extruded onto composite lava dome

Increasing seismicity and deformation culminated in the extrusion of a new lobe onto the composite lava dome during the night of 21-22 October.

Seismicity and tilt had returned to background levels after an earthquake swarm that started on 12 September and ended after a small steam-and-ash emission 3 days later (SEAN 11:08). Another small seismic swarm, accompanied by minor tilting on the dome's W flank, was recorded on 30 September and 1 October. Seismicity declined and tilt leveled off at about the same time on 1 October, but no associated plume emission was detected.

Late on 3 October, seismicity and tilt on the dome's W flank began to increase again, with some seismic events reaching about M 2. As in the earlier seismicity/tilt episodes, tiltmeters on the dome's S and E flanks detected no deformation, while the N-flank instrument detected smaller changes than were recorded on the W flank. The activity culminated in a large rockfall from the overhanging N face of the dome's May 1986 lobe. The hot rockfall moved down the dome's N flank to ~250 m beyond its base, covering the W side of the deposit left by the large rock avalanche of May 1986. An associated surge melted plastic, lightly charred a wooden antenna post, and deposited several centimeters of hot sand- and silt-sized material several hundred meters beyond the toe of the rockfall. A substantial quantity of dust from the rockfall was entrained in a vertical column, then drifted to the S and SW. Light ashfalls were reported in Vancouver, Washington, 80 km SW. Seismicity dropped to background more than an hour before the rockfall. Tilt reversed at about the time the rockfall occurred, remained flat for a few hours, then inflation resumed at an initially slow but accelerating rate. SO₂ emission, at background rates of 20-30 t/d 3, 11, and 22 September, increased slightly to 65 plus or minus 5 t/d on 6 October, the day after the rockfall, declining to 45 plus or minus 5 t/d two days later.

Seismicity began to build again about 8 October. After a slight decrease between 6 and 9 October, the rate of outward movement at targets on the W flank of the dome increased to 2.5 cm/day between measurements on 9 and 14 October, an order of magnitude above background, and to 15-16 cm/day on the W and NW flanks by 16 October. Inflationary tilt also continued to accelerate, reaching 400 µrads/day by 15 October. A strainmeter across a crack on the W part of the dome's summit (the September 1984 lobe) measured rates of opening of ~1 mm/day, and new cracks were visible in the same area. Seismicity, slightly elevated on 13 October, was at moderate levels by the 15th. Rates of SO₂ emission remained at background levels of 15-25 t/d.

On 16 October, the USGS and the University of Washington issued an advisory notice stating that an episode of rapid dome growth was likely to begin within the next 3 weeks. Until 18 October, as the number of earthquakes increased, the ratio of low-frequency and medium- to high-frequency events remained roughly the same. Beginning late on the 18th, the number of low-frequency events began to increase rapidly. As seismicity increased to high levels, a 19 October notice updated the expected start time for rapid dome growth to 2-10 days. Many new cracks and rockfalls were sighted on the W part of the dome on 20 October. Rates of SO₂ emission, 50 t/d on the 20th, increased to 550 and 375 t/d during two flights on 21 October. Vigorous seismicity continued until mid-afternoon 21 October, when discrete earthquakes suddenly stopped, replaced by several hours of a constant tremor-like signal of much lower amplitude. Deformation rates on the NW side of the dome reached 2.5 m/day on the 21st, but more rapid deformation was probably occurring on the W side. Earthquakes resumed late that night as inflationary tilt at the summit began to flatten gradually.

There were no night observations of the dome, but after sunrise a new lobe was visible just W of the dome's summit, within the area that had been most rapidly deforming. The lobe emerged from an elongate spreading center oriented slightly W of N. It continued to grow through the day, and by 1600 was roughly 250 m in longest dimension (approximately N-S) and 40 m high. Seismicity diminished during the day, and by evening was at only moderate levels. There was no evidence of explosive activity associated with the extrusion. A large thrust fault and many smaller thrusts severely disrupted a wide area of the snow- and ice-covered talus on the SW and WSW crater floor, extending 250-300 m from the base of the dome to the crater wall. Displacements may have been as much as several meters. Thrust faulting had not affected that part of the crater floor since [1982]; significant crater floor thrusting had last been observed, over a smaller area, in May 1985.

Information Contacts: D. Swanson, D. Dzurisin, J. Sutton, and Patrick Pringle, CVO; C. Jonientz-Trisler, University of Washington.

October 1986 (SEAN 11:10) Cite this Report

Strong seismicity and tilt with October extrusion

In late October, Mt. St. Helens completed its second dome-building episode of 1986, about 5 months after the previous episode (SEAN 11:04 and 11:05). After 3 weeks of increasing precursory activity, the new lobe began to emerge from a fissure near the center of the dome during the night of 21-22 October (SEAN 11:09).

The new lobe had added 10 m to the height of the dome by the morning of 22 October, and 20 meters more by the next day, when the [lobe's] dimensions were estimated at 200-300 m by 200 m by 40-45 m thick. Extrusion was slow enough that wooden marker posts on top of the dome remained erect even when carried for tens of meters. Time-lapse film showed the new lava pushing upward through a N-S fissure, rising vertically, and collapsing to the sides in sheet-like masses. When geologists were next able to enter the crater, on 3 November, extrusion had ended and the dome had subsided 10 m. With earlier dome-building episodes, more typical values for post-extrusion subsidence have been 3-4 m. The new lobe appeared morphologically and petrographically similar to the May 1986 and September 1984 lobes, which were disrupted and partially buried by this extrusion episode (figures 34 and 35). Fractures 30 m deep and 30 m wide extended almost completely through the September lobe.

Figure 34. Drawing by Bobbie Myers of the primary features of the composite lava dome at Mt. St. Helens as of 28 October.

Figure 35. Oblique airphoto by Lyn Topinka on 28 October, looking N, showing the lava dome at Mt. St. Helens, the crater breach, Spirit Lake (middle distance), and Mt. Rainier (background).

Thrust faults on the crater floor SW of the dome disrupted debris along a 400-500-m sector. From horizontal displacement measurements on the W side of the dome, just N of the faults, Don Swanson estimates that fault offset was 7 m. Tear faults, some showing strike-slip movement, bounded the thrust blocks.

The October dome-building episode released much more seismic energy than that of May 1986. Most earthquake magnitudes were near or slightly below 3. The high energy release seemed to result from the large number of surface events during the breakup of the oversteepened May and September lobes, and the development of thrust faults in the crater floor.

Tiltmeters recorded consistent outward co-seismic tilting on all four flanks of the dome. The strongest occurred on the W flank, where 11° of outward tilt was recorded before the instrument was destroyed at 0400 on 22 October. Just under 1° of tilt (15,000 µrads) was recorded on the N flank. The S and E flanks tilted outward 4,500 and 4,000 µrads, then several hundred µrads inward after extrusion of the lobe. The N flank did not show a similar relaxation. Total horizontal displacement along EDM lines reached 5-10 m on the SW flank (before that target was destroyed on 22 October) and 7 m on the W flank.

SO₂ emissions peaked at 675 ± 50 t/d on 22 October, up from 550 ± 50 t/d on 21 October. Emission rates decreased to 260 ± 50 t/d on 23 October and 160 ± 15 t/d by 24 October.

Stationary magnetometers on the dome's SE flank and base recorded no change in field intensity during the event. In the past, handheld magnetometer data had indicated that the dome magnetized at the rate of a few gammas/day after extrusion of a new lobe.

Information Contacts: D. Swanson, D. Dzurisin, E. Endo, and P. Pringle, CVO; C. Jonientz-Trisler, University of Washington.

November 1986 (SEAN 11:11) Cite this Report

Seismicity and SO₂ emissions at background

Since the dome-building activity in late October, the volcano has remained quiet. From November through early December, seismicity and SO₂ emissions were at background levels. SO₂ could only be measured on three days: 3 November, 60 plus or minus 20 t/d; 2 December, 33 plus or minus 5 t/d; and 9 December, 80 plus or minus 15 t/d. No significant deformation was recorded.

Information Contacts: D. Swanson and J. Sutton, CVO; C. Jonientz-Trisler, University of Washington.

December 1986 (SEAN 11:12) Cite this Report

Activity at background level

Tilt, horizontal deformation, seismicity, and gas emissions remained at background levels during December and early January.

Information Contacts: D. Swanson, CVO.

January 1987 (SEAN 12:01) Cite this Report

Background seismicity, deformation, SO₂ emission

Seismicity, rates of ground deformation in the crater, and gas discharge remained at background levels throughout January. SO₂ discharge was 35 plus or minus 10 t/d on 8 January, 30 plus or minus 5 t/d on 15 January, and 15 plus or minus 5 t/d on 21 January. There were no ash emissions or explosions from the lava dome.

Information Contacts: D. Swanson and J.Sutton, CVO.

February 1987 (SEAN 12:02) Cite this Report

Seismicity and deformation at background

Since the last dome-building event in October, seismicity and deformation have remained at background levels. Fourteen surface-type events (mostly rockfalls from the crater walls) were recorded in January, compared with nine in December. Three low-frequency earthquakes were recorded, up from one in December.

Information Contacts: D. Swanson, CVO; C. Jonientz-Trisler, University of Washington.

August 1989 (SEAN 14:08) Cite this Report

Small earthquake swarms SW of the summit dome

Activity at Mt. St. Helens has generally remained at background levels since a new lobe was added to the composite lava dome in October 1986 (SEAN 11:09 and 11:10). Some small seismic events have continued beneath the crater, but there has been no significant deformation and rates of SO₂ emission have remained low.

The first of three small bursts of recent seismicity began at 2227 on 24 August 1989 with an event similar to those associated with earlier gas-and-ash emission episodes, last documented in 1986. The initial event had a coda length of 95 seconds consisting of signals from two or more earthquakes, yielding a coda magnitude of roughly 2.7. Numerous small earthquakes saturated a seismic station (GDN) ~700 m N of the dome's center during the next half-hour, and roughly 300-500 individual events could be recognized on its record in the succeeding 4 hours. Another station (HOA) 5 km to the N was saturated (amplitude >68 mm peak to peak) for 15 seconds. Most of the events were centered SW of the dome along an extension of a NE-SW zone defined by earthquakes of the past 9 months (figure 36). The mean focal depth of the 16 best-located shocks was 1.8 plus or minus 0.18 km (figures 37 and 38). A somewhat smaller emission-like event occurred the next morning at 0918, followed by a period of increased background noise and small earthquakes. However, fewer than two dozen events were recorded during the next several hours. None of the 25 August seismicity saturated instruments; amplitudes reached only 7 mm at HOA station. A third emission-like event occurred 30 August at 1758. Crater seismic stations were saturated for 10-30 seconds, while maximum amplitude at HOA station was 19 mm peak to peak. Small earthquakes again followed the initial event, but fewer than a dozen could be recognized on records from the nearby GDN station. Seismic noise remained above background for ~2 hours.

Figure 36. Epicenters of seismic events at Mt. St. Helens recorded by CVO's PC/AT seismic data acquisition system July-August 1989 left and November 1988-August 1989 right. The late August swarm is the group of shocks about 1 km SW of the dome. Note different scales. Courtesy of E. Endo.

Figure 37. Space-time plot of earthquake foci recorded by the USGS in the Mt. St. Helens area, 30 November 1988-31 August 1989. Courtesy of E. Endo.

Figure 38. S-N and W-E cross sections (along lines A-A' and B-B', figure 36) showing earthquake foci at Mt. St. Helens in July-August 1989 left and November 1988-August 1989 right. No vertical exaggeration. Courtesy of E. Endo.

Fieldwork following the seismicity did not reveal any evidence of associated eruptive activity. Seismologists suggested that the activity may have reflected the movement of volatiles (gas and/or liquid phase) a short distance SW from the conduit system beneath the crater.

Information Contacts: E. Endo, CVO.

October 1989 (SEAN 14:10) Cite this Report

Earthquakes in magma conduit

A marked increase in local seismicity began 19 October and continued for ~10 days. The number of locatable events detected by CVO's PC seismic acquisition system averaged only ~0.7/day through 18 October, then rose to 7/day through the end of the month; 11 shocks were recorded on the most active day, the 29th (figure 39). All of the events were small; the strongest, on 11 October at 0448 and 29 October at 0253, had coda magnitudes of 1.6 and 1.5 respectively. Epicenters were generally in the summit area (figure 40), with focal depths ranging from 1.6 to 11 km (figure 41), involving the entire magma conduit. The earthquakes were similar to A-type events, with well-defined S phases, in contrast to the August seismicity (SEAN 14:08), which was characterized by multiple monochromatic events without S phases. Seismicity declined at the end of October, and remained quiet as of mid-November. No associated changes in surface activity or deformation were detected.

Figure 39. Space-time plot of foci for earthquakes ("A" and "B" quality) at Mt. St. Helens recorded by CVO's PC seismic data acquisition system, 1 September-31 October 1989. Courtesy of E. Endo.

Figure 40. Epicenters of earthquakes at Mt. St. Helens detected by the USGS, 1 September-31 October 1989. Courtesy of E. Endo.

Figure 41. N-S and E-W cross-sections of earthquake hypocenters at Mt. St. Helens, 1 September-31 October 1989. Courtesy of E. Endo.

Similar episodes of increased seismicity have been recorded at least eight times since late 1987. Most have lasted a few days to about a week and have included up to a few dozen small shocks, some of which were usually as deep as 10-12 km. The October activity was the longest and included the largest number of events of the post-1987 seismic episodes.

Information Contacts: E. Endo, CVO; Stephen Malone, Geophysics Program, University of Washington, Seattle, Washington 98195 USA.

November 1989 (SEAN 14:11) Cite this Report

Minor ash emission and shallow seismicity

A small ash emission episode on 6 Decembrt was associated with ~5 hours of shallow seismicity. The activity occurred during poor weather, preventing direct observation of the volcano. The seismicity, dominated by a tremor-like signal that was punctuated by individual shallow (less than or equal to 2 km deep) earthquakes centered under the summit lava dome, was recorded between 1609 and 2122. Continuously recording tiltmeters in the crater showed small offsets during the seismicity, and a strainmeter crossing a crack on the W side of the dome measured 2 cm of contemporaneous extension.

Geologists working in the crater a few days later found a thin layer of new ash. Maximum thickness was 8 mm on the W side of the dome, rapidly thinning to a dusting on snow E and N of the dome. All of the ash appeared to be lithic material, without fresh glass shards or vesiculated magma. No new vent was evident, and geologists assumed that the ash had emerged from existing cracks near the dome's summit. Deformation measurements revealed changes of as much as 12 cm on the W side of the dome, declining to barely above noise level on its S side, since the previous fieldwork on 22 November. Deformation data had not previously shown any changes.

The last eruptive activity at Mt. St. Helens was a dome building episode in October 1986 (SEAN 11:09 and 11:10). At least nine periods of increased seismicity have been documented since late 1987 (figure 42), most recently for 10 days beginning 19 October 1989 (SEAN 14:10). Three brief swarms in late August, more energetic than the December activity but without the extended tremor-like signal, resembled seismicity associated with previous ash emission episodes. However, no eruptive activity was documented at that time (SEAN 14:08).

Figure 42. Space-time plot showing focal depths of earthquakes at Mt. St. Helens, 1 January 1987-12 December 1989. Courtesy of C. Jonientz-Trisler.

Information Contacts: D. Swanson, CVO; C. Jonientz-Trisler and S. Malone, University of Washington.

December 1989 (SEAN 14:12) Cite this Report

Small ash emission; ashfalls to 80 km distance

A brief explosive episode at 0537 on 6 January ejected ballistic tephra and a small amount of ash, and triggered rock avalanches. Strong winds carried the ash E, forming a very thin deposit along a narrow discontinuous band that extended 130 km (to the Toppenish area).

The 6 January vent was located at the apex of an arcuate fracture that climbs the dome's N flank to the vent position ~100 m above the crater floor, roughly one third of the way up the dome. The segment N of the fracture moved nearly 2 m outward and downward during the eruption. The ends of the fracture were ~400 m apart. The explosion had a northerly component, sending large ballistic tephra to the N base of the dome, where the Garden Rock seismometer was damaged. South of the vent, there were no new ballistics and only about a centimeter of fine ash had been deposited nearby. Deposition W of the dome was limited to ~1 mm of mud, and only dirty snow was found E of the dome. Rock-and-snow avalanches had originated from the fracture, leaving a scallop-shaped scar on the dome's N flank and forming two separate lobes at the base of the dome. One lobe was composed of dirty snow overlain by granular lithics. To its E, the larger avalanche lobe appeared to extend no more than 200 m from the base of the dome and contained mostly coarse rockfall material with a little snow. Some of its boulders had rolled in snow. Its maximum thickness was ~50 cm at a site roughly 100 m N of the base of the dome. The largest rock fragments beyond the base of the dome were ~30-40 cm in diameter, but 1-m boulders could be seen in the talus chute on the dome. Large ice crystals in the snow suggested that the avalanches had been warm at the time of emplacement, then had refrozen.

The 6 January tephra was dominated by lithic fragments from the dome, but dark, glassy, vesicular, dacitic material of unknown origin was also found in the ejecta. Although its mineralogy was the same as that of previous dome samples, proportions of mineral components were different, and its glass was pale brown in contrast to the clear glass that has characterized previously extruded dome rocks. The new material resembled hornblende-bearing andesites erupted by Mt. St. Helens between AD 1500 and 1800, but appeared very fresh and showed no signs of hydrothermal alteration.

Seismicity associated with the 6 January episode appeared suddenly, and saturated the station (YEL) ~1 km N of the dome for ~9 minutes (compared to 2 minutes for the [6] December seismicity). The signal was characterized by numerous minor shallow earthquakes and volcanic tremor, lasting a little more than 2.5 hours, with alternating periods of intense and low-level activity. Average tremor amplitudes were similar for the first 20 minutes of the [6] December and 6 January episodes, but the later [6] December tremor was stronger and continued for ~5 hours. Some rockfall signals occurred after both episodes. Seismicity following the 6 January episode was dominated by discrete earthquakes, with about a dozen recorded in the next few hours. After continuous tremor ended on 7 December, at least nine high-frequency, tremor-like, signals (cigar-shaped on the seismogram) lasting 2-20 minutes were recorded, accompanied by only two tiny earthquakes. Two of the cigar-shaped signals, each lasting ~2 minutes, were detected in the hours following the 6 January episode. Similar signals have been recorded during eruptions of Old Faithful Geyser at Yellowstone (Kieffer, 1984), and were thought to represent hydrothermal venting or near-surface movement of fluids at Ruiz volcano.

No pre-eruption deformation was detected, and the first sign of the activity on the tiltmeter nearest the vent was the loss of its signal. A second instrument showed a deflection of ~20 µrads during the eruptive episode. Outward movement of the dome's N flank apparently occurred during the eruption, with a maximum measured value of 1.8 m. No deformation was detected on the outer flanks of the edifice.

Reference: Kieffer, S.W., 1984, Seismicity at Old Faithful Geyser: An Isolated Source of Geothermal Noise and Possible Analogue to Volcanic Seismicity; Journal of Volcanology and Geothermal Research, v. 22, p. 59-86.

Information Contacts: D. Swanson and D. Dzurisin, CVO; C. Jonientz-Trisler, University of Washington.

April 1990 (BGVN 15:04) Cite this Report

Small apparent explosion signal on seismic records

The Mt. St. Helens seismic network recorded an explosion-type seismic signal on 25 April at 0126. The seismicity appeared similar to that associated with minor tephra emissions on [6] December and 6 January (SEAN 14:11 and 14:12), but both amplitude and duration were much smaller on 25 April. No eruption plume was reported and fresh snowfall shortly after the episode obscured any material that may have been deposited. The number of tiny earthquakes (detected only by crater stations) remained elevated for ~18 hours after the activity, then returned to background levels. Periods of increased local seismicity have continued since late 1987 (figure 43 and SEAN 14:08 and 14:10).

Figure 43. Time vs. depth plot of seismicity at Mt. St. Helens, 1 January 1989-17 May 1990. Arrows mark episodes of explosion-type seismicity. Courtesy of the Geophysics Program, University of Washington.

Information Contacts: C. Jonientz-Trisler, University of Washington.

October 1990 (BGVN 15:10) Cite this Report

Explosion from N side of lava dome; ash plume and small mudflow

At 0207 on 5 November, the start of a brief explosive episode and ash emission was signalled by 2 minutes of low-amplitude seismicity, followed by an increase to high-amplitude seismicity and the failure of several sensors on the summit dome. Pilots reported the plume at altitudes of ~7.5-9 km traveling SE at 90-110 km/hr; ash was reported as far as Fossil, Oregon (~200 km SE). Strong seismicity lasted for 6 minutes, then decreased to normal levels over the following 2-3 hours.

Geologists visiting the crater that day found that the explosive activity took place at a vent on the N side of the lava dome. Two seismic stations and a steel tower were destroyed, but others continued to function. Hot dome blocks and finer-grained material blanketed the snow on the crater floor, NW and N of the dome; blocks up to 2 m in diameter were scattered on the lower part of the W crater wall (NW of the dome). Rock avalanches and hot debris from the explosions moved down the N side of the dome and across the crater floor, abrading and melting snow and ice. The resultant small debris flow traveled out of the crater into the North Fork of the Toutle River, where it formed a small mudflow that extended 16-19 km.

Fine tephra was collected from the extreme limit of deposition, but had not yet been analyzed at press time. Small quantities of fresh-appearing glass had been found in tephra emitted on 6 January (SEAN 14:12).

No precursory events to the 5 November activity have been identified, although two distinctive "cigar-shaped" events (closely spaced, small, shallow earthquakes with concurrent tremor) lasting several hours were recorded 25 and 26 October. Similar signals were associated with the 6 January ash emission, and were recorded 24 September, when no ash was emitted. These signals have been identified at Old Faithful Geyser (Yellowstone Caldera, USA), and Ruiz (Colombia), where they are thought to represent hydrothermal venting or near-surface movement of fluids.

The 5 November ash emission was very similar to the previous explosive events on [6] December and 6 January (SEAN 14:11 and 14:12). An event on 25 April produced similar explosion-type seismic signals, but bad weather prevented observations and no ash or eruption plume was reported (BGVN 15:04). Each of the events was short-lived (up to 18 hours) and produced little ash. Although the January episode also caused rock and snow avalanches, the November activity was the first to produce a mudflow in the last two years.

Information Contacts: W. Scott and S. Brantley, CVO; SAB.

November 1990 (BGVN 15:11) Cite this Report

Small explosion from lava dome

A small explosion occurred from the lava dome on 20 December at 1259. The explosion was marked by a small seismic signal that decreased to low levels after several minutes, but continued for several hours. Airplane pilots reported a light gray plume to as much as 7.5 km altitude that was carried SSW by strong winds. A diffuse plume was first evident on satellite images at 1320, moving SSW at ~30 km/hour. By 1600, the plume could no longer be detected on satellite imagery. Light ashfall was reported to 15 km SW of the volcano. No mudflow or water flow event was detected.

Information Contacts: CVO; SAB.

January 1991 (BGVN 16:01) Cite this Report

Explosions, avalanche, and flows from lava dome

After more than 6 weeks of quiet, two episodes of dome activity occurred in early February.

Explosive episode, 5 February. A small explosion from the lava dome occurred on 5 February at 0747 without any apparent premonitory activity. The seismic signal, typical of previous episodes of gas/ash emission, remained strong for ~10 minutes, then continued at lesser amplitude for another 20 minutes. Data from the crater seismic station were lost ~20 minutes after the onset of activity. The eruption plume was initially white, then turned a darker color after ~3 minutes. Ground observations suggested that the eruption cloud reached a maximum of slightly > 4 km above sea level, but airplane pilots reported the top of the cloud at 6-7.5 km altitude, with material drifting ENE at low altitude and ESE at higher elevations by 0800. Images from the GOES satellite first showed a very small plume at 0831. By the next image at 0900, the cloud was clearly detached from the volcano (~15 km to the E) and had a N-S length of ~50 km. The cloud was no longer visible on satellite imagery after 0930.

Deposits in the crater indicated that a fairly large explosion or series of explosions had occurred from the dome, roughly comparable to the 20 December episode, but distinctly smaller than that of 5 November (BGVN 15:10 and 15:11). Most of the tephra was ejected from a cone-shaped upper N flank vent (20-30 m across at its rim) that had been the source of the 5 November episode. A steep wall ~100 m high butressed the S side of the vent, apparently directing ejecta to the N. Ballistic fragments, some as large as 1 m across, formed a very dense deposit extending 1 km NNE, and sparse ballistics were found to 1.5 km from the vent, reaching the lower NE crater wall. A slush flow mixed with volcanic debris, emplaced after the ballistics, formed a long dark ribbon extending N across the crater floor from the dome. The slush flow was followed by a small watery flow. Fine ash found to 0.5 km NW, N, and NE of the dome appears to have been warm upon deposition, but did not melt much snow. Another ash deposit darkened snow on the volcano's outer E flank to 1.5 km from the crater rim. This deposit had a very sharp margin and may have formed from a cold density current. Very light ashfall was reported at Goldendale, ~110 km ESE of the volcano. As in previous episodes, most of the tephra seemed to consist of fragmented dome rock. The vent on the dome's upper E flank that was active in the 20 December episode also ejected a few ballistics on 5 February, and the surrounding area was blackened by ash. This vent, roughly 250-300 m from the N-flank vent, was obscured by steam, but appeared to be roughly 50 m in diameter. Geologists noted that the phreatic eruptive activity is apparently driven by groundwater and may have a geyser-like mechanism.

Avalanche and flow episode, 14 February. Strong seismicity associated with a second episode began on 14 February at 0524. Transmission from the station on the lava dome was lost after < 2 minutes, but vigorous seismicity was detected for ~10 minutes on other nearby instruments; numerous small shallow events then persisted for 3-4 hours before declining to background. In contrast to most previous episodes, the 14 February activity was preceded by an increased number of very shallow earthquakes below the dome. Seismologists noted that these events had low stress drops and were characteristic of cracking. Periods of increased seismicity have previously occurred beneath the dome without an associated eruptive episode.

Field data indicated that the 14 February activity was significantly different from the episode 9 days earlier. A hot rock avalanche, accompanied by little if any explosive activity, generated a lithic pyroclastic flow and a granular snow flow. Where the deposit was essentially snow- and water-free near the dome, it was composed entirely of ash- to block-sized lithic material. The snow flow was apparently inflated and relatively fast-moving. Although no more than 10 cm thick in most places, it left granular snow on top of lithic blocks that were decimeters in size and comprised <50% of the deposit. The flow entered Loowit Channel and continued downslope, rising as much as 5 m on the outsides of bends. As it continued downstream, more of the snow melted, and the flow evolved into a dirty flood that reached Spirit Lake.

The area around the N-flank vent had been significantly modified since 5 February, and the large scarp S of the vent had retreated substantially. No obvious rockfall scar was visible on the dome, but a large bare area on the N flank of the otherwise snow-covered dome apparently fed the granular snow flow. There was no evidence of new ballistic ejecta and no new impact craters were found. A thin blanket of pyroclastic-flow material covered Sugarbowl dome, NNE of the active dome, beyond which were deposits from a surge elutriation cloud. Geologists found no airfall ash beyond the base of the volcano, although there were unconfirmed reports of ash mixed with rain in Wapato, WA, roughly 150 km ENE of the volcano, perhaps transported by very strong winds during the activity. An airplane pilot near the volcano at about 0600 was unable to determine whether a plume was present because of clouds and darkness.

No significant deformation has been detected since the last dome-building episode in 1986, although some apparent gravitational adjustments have been detected. Electronic distance measurements have documented changes in line lengths in the crater, but only after explosive episodes.

Information Contacts: Edward Wolfe, Richard Waitt, D. Dzurisin, and S. Brantley, CVO; C. Jonientz-Trisler, University of Washington; S. Hamerla Harner, SAB.

September 1995 (BGVN 20:09) Cite this Report

Steady increase in seismicity through 1995

No explosions or gas-and-ash emissions occurred from the lava dome between 1 January and 30 September 1995. Seismic activity was still low, but the number of small-magnitude (M

Figure 44. Seismicity at Mount St. Helens, January 1986-September 1995. A high concentration of earthquake activity at

This same zone of seismic activity became active in late 1987, about 2 years before the 1989-91 steam explosions began, and it presumably marks the approximate location of the magma conduit system. Those relatively small explosions hurled dome rocks as large as 30-40 cm in diameter at least 800 m from the dome and produced ash plumes as high as ~6 km above sea level. Detailed study of the 1987-91 seismicity and the 1989-91 explosions suggests that both occurred in response to increased pressure in the conduit system.

One possible cause for the pressure increase is that volcanic gas (primarily water vapor) became concentrated along the conduit system as a consequence of the progressive cooling and crystallization of magma. This increased pressure would likely lead to increased rock fracturing immediately surrounding the conduit system, as well as to intermittent sudden gas release. In addition, downward growth of cracks and fractures in the dome during and immediately after periods of intense precipitation could trigger gas explosions when such fractures intersect pressurized areas; many but not all of the 1989-91 explosions followed periods of heavy rainfall. Another possible cause for the pressure increase is intrusion of new magma into the lower depths of the conduit system. There is no evidence, however, that any magma has approached the surface during 1995. Regardless of the cause, it seems likely that the change in seismicity reflects a renewed increase in pressure along the magma conduit system.

Because the 1989-91 steam explosions were not preceded by any specific short-term warning signs, the similarity of the current seismicity raises concerns that future small dome explosions could occur without additional warning. Experience with the 1989-91 explosions, as well as explosions during the years of dome growth, suggests that they would produce hazards primarily within the crater, to a lesser degree in the stream channels leading from the crater, and to an even smaller degree on the upper flanks of the volcano. These hazards could include the impact of ejected dome rocks and rapidly moving pyroclastic flows sweeping the crater floor. During the 5 February 1991 explosion, a small pyroclastic flow reached the N edge of the crater. Heat from a rock avalanche or pyroclastic flow could also generate a lahar in the crater and in channels leading from the crater. Also, gas explosions could generate dilute but visible ash plumes perhaps as high as 6 km above the volcano and light ashfall as far as ~160 km downwind.

Information Contacts: Dan Dzurisin, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA; Steve Malone, Geophysics Program, University of Washington, Seattle, WA 98195 USA. URL: https://volcanoes.usgs.gov/observatories/cvo/).

December 1995 (BGVN 20:11) Cite this Report

Seismicity decreases without any explosive activity

During October-December there were no explosions or gas-and-ash emissions from the lava dome, and no explosion-like seismicity was detected. Surveys of the lava dome indicated that deformation rates have remained at background levels. No increase in deformation of the dome occurred as a consequence of the recent earthquake activity, but the NW side of the dome continued to move downward very slowly as it has since a series of small explosions between 1989 and 1991. Periods of intense rainfall in November generated several lahars from the crater. All of the lahars were detected by the USGS real-time acoustic-flow network and probably flowed into Spirit Lake. Such lahars are common during intense rainfall following the dry summer months.

The number of small-magnitude (M <1) earthquakes beneath the crater decreased slowly from nearly 100/month in September (BGVN 20:09) to ~25/month in December. Seismicity at the end of December was similar to the first 6 months of 1995. The gradual decrease in seismicity, combined with the lack of small explosions related to the September increase, has lowered the concern of scientists monitoring the volcano. Small dome explosions are still possible, but their likelihood is no greater early in 1995.

Information Contacts: Dan Dzurisin, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Steve Malone, Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/ home.html).

June 1996 (BGVN 21:06) Cite this Report

Dwindling seismicity

No eruptive activity occurred during the first half of 1996, and a trend of declining seismicity since October 1995 continued. Thus far in 1996 monthly earthquake totals have been: January, 14; February, 13; March, 17; April, 16; May, 11; and June, 10 (figure 11). At 1651 on 21 February, an M 2.4 earthquake occurred ~4 km beneath the crater floor; this was followed by four locatable aftershocks and then by ~20 very small seismic events that resembled signals typical of rock or snow avalanches. These later events were shallow, apparently triggered by the M 2.4 earthquake that preceded them. Activity returned to normal within a few hours. Except for that on 21 February, the rest of the earthquakes from January to June did not exceed M 2.0. During the night of 9 June, a large seismic event from the volcano triggered an automated, 24-hour alarm system. The character of the signal suggested that the source was a rockfall from the crater wall. This interpretation was confirmed when a USGS crew working in the crater on 11 June observed a large fresh rockfall deposit that originated from the S crater wall. Rockfalls are a common occurrence in the crater during the summer, and generally do not indicate any increase in volcanic activity.

Figure 45. Plot of focal depth versus time for earthquakes at Mount St. Helens, July 1995-June 1996. Courtesy of the Cascades Volcano Observatory, U.S. Geological Survey.

Information Contacts: Dan Dzurisin, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Steve Malone, Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

May 1998 (BGVN 23:05) Cite this Report

Sudden rise in earthquake activity in May

The level of earthquake activity at Mount St. Helens had been gradually increasing over the past several months and accelerated during May. Rates of activity increased from an average of ~60 well-located events per month last winter to 165 events in May. Most of the recent earthquakes were very small with only three events larger than M 2. The largest earthquake was on 1 May with M 2.2. These earthquakes occurred in two clusters directly beneath the lava dome in the crater. One cluster was in the range of 2-5 km and the other 7-9 km below the dome. Very few events were located in the very shallow region of 0-2 km below the dome. None of the earthquakes were low-frequency volcanic events that typically occur as precursors to major eruptions.

This increased activity seems to be similar to that which occurred in 1995, although the activity of May 1998 was more energetic. The 1995 activity lasted for several months, had a maximum earthquake rate of 95 events per month, and resulted in no volcanic activity. A similar increase in earthquake activity in the St. Helens system occurred in 1989-91. However, at that time there were also a number of very shallow earthquakes accompanied by a series of sudden steam explosions. These explosions were small eruptions of steam and gas that ejected rocks and ash from cracks in the dome. Rocks were thrown up to 1 km from the dome, ash clouds reached altitudes up to 6 km, and a dusting of ash was deposited locally downwind. Some explosions melted snow in the crater and generated small lahars that flowed N onto the Pumice Plain.

Because increased earthquake activity within the deep St. Helens system may reflect increased pressure at depth, it is possible that the current seismicity may eventually lead to renewed volcanic activity. However, it is unlikely to do so without significant additional precursors.

Information Contacts: Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: http://www.geophys.washington.edu/SEIS/PNSN/HELENS/).

June 1998 (BGVN 23:06) Cite this Report

Seismicity increases further; magmatic CO₂ detected

The number of well-located earthquakes at Mount St. Helens had increased from an average of ~60 per month last winter, to 165 in May, to 318 in June. However, the June earthquakes were very small: only 11 events were larger than M 1 and the largest was M 1.8. Hence, the total seismic-energy release for June was about the same as that of May. Earthquakes continued to occur chiefly in two depth clusters directly beneath the lava dome in the crater. One cluster was 2-5 km below the dome, the other in the depth range of 7-9 km. Almost no events were located in the very shallow region (0-2 km below the dome). The Cascades Volcano Observatory began providing daily updates showing plots of the number of events and amount of seismic energy registered. The plots are available through the observatory web site.

In response to the increased seismicity, USGS scientists also increased broad-based monitoring in June. An airborne gas survey revealed the presence of magmatic CO₂. Because it is heavier than air, CO₂ can concentrate in surface depressions in the dome or crater floor, especially under calm conditions, and can pose an asphyxiation hazard. Poorly ventilated cavities, such as caves in the mass of snow and ice that is accumulating behind the dome, could also be hazardous.

A network of surveying targets was established on the lava dome, crater floor, and lower flanks of the volcano to detect any ground movements that might occur in response to changes beneath the volcano. No significant movement occurred at any of the targets on the N flank of the dome between the first measurements on 5 June and a follow-up survey on 29 June. Additional measurements of survey targets will be repeated periodically for the foreseeable future.

The increased number of earthquakes and the release of CO₂ probably reflect replenishment of the magma reservoir, a body whose top is thought to lie ~7 km below the crater. Replenishment could lead to magmatic eruptions but scientists don't know how much replenishment has occurred, or how much is necessary to renew magmatic eruptions. However, such eruptions are unlikely without a significant increase in precursory activity. Owing to the recent unrest, the probability of small steam explosions from the dome, like those that occurred between 1989 and 1991, has increased slightly. Concern will be heightened greatly if shallow seismicity increases.

Information Contacts: William E. Scott, Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

July 1998 (BGVN 23:07) Cite this Report

Earthquakes, but CO₂ flux returns to normal

The rate of earthquake activity, which accelerated markedly from May through mid-July, (BGVN 23:05 and 23:06) returned in August to a level similar to that of last winter. The number of well-located earthquakes in July was 445, compared to 318 in June, but most of the earthquakes that took place during July occurred during the first three weeks of the month. The average rate for the first two weeks of August was only about four well-located earthquakes per day. Several temporary increases in earthquake activity have occurred since the last dome-building eruption in October 1986. This recent episode was the most intense.

Airborne gas surveys revealed that magmatic carbon dioxide (CO₂) decreased since June. However escaping CO₂ was still measurable. The CO₂ was probably being released from magma that entered the magma reservoir during the past few months. The reservoir's top was estimated to be about 7 km below the crater. Because CO₂ is heavier than air, it can concentrate in surface depressions on the dome or crater floor, especially under calm conditions, and pose an asphyxiation hazard. Poorly ventilated cavities, such as caves in the mass of snow and ice behind the dome, could be hazardous.

Information Contacts: Cascades Volcano Observatory, U.S. Geological Survey, 5400 MacArthur Blvd., Vancouver, WA 98661 USA (URL: http://volcanoes.usgs.gov/); Geophysics Program, University of Washington, Seattle, WA 98195 USA (URL: http://www.geophys.washington.edu/SEIS/PNSN/HELENS/).

September 2004 (BGVN 29:09) Cite this Report

New eruption starts suddenly—first dome-growth in 18 years

After a hiatus of over 13 years, following phreatic eruptions during November 1990 to February 1991, a new eruption began in the crater at Mount St. Helens. Some shallow seismicity preceded the eruption, but this came only 9 days prior to the first ash emission. There was no progression of deep earthquakes propagating upward with time during the preceding months. Also apparently absent were other classical monitored parameters (deformation, gas emissions, geochemical or thermal anomalies) that could help foretell of an eruption several weeks or months ahead.

The eruption extruded a two-pyroxene, hornblende dacite of low vesicularity. The initial extrusions and uplift affected the S dome area and adjacent crater floor to the S. The intrusion included uplift and deformation of glacial ice, as well as some melting of ice, forming a small, short-lived bubbling lake (nicknamed 'the Jacuzzi') and a minor lahar out of the crater.

This report discusses the first 16 days of the eruption, from the first sign, shallow earthquakes, which began on 23 September 2004. This report was chiefly put together from reports posted by the Cascades Volcano Observatory (CVO), but also benefitted from personal communications with James Quick, Marianne Guffanti (both at USGS, Reston, Virginia), and James Vallance (USGS-CVO).

Synopsis. A photograph of the scene several weeks before the eruption came from climbers on the S rim. They visited during the last week of August 2004, a time when all still seemed quiet (figure 46). A chronology of events in the first 16 days (table 4) includes the dates and times of the events, the hazard status to people on the ground and to aviation traffic, and some brief comments on the seismicity and volcanism taking place.

Figure 46. Photographs looking N from the crater rim at Mount St. Helens during clear, hot weather in the last week of August 2004. Much of the lower half of the top photo shows the S side of the 1980-1986 dome, the flat area arcing around it called the moat, and parts of the engulfing crater walls. In the upper half of the photo lie (from right to left) Spirit Lake, a large open area called the pumice plain, and some ridges, including the one directly across the pumice plain, Johnson Ridge. Glacial ice covers the moat and portions of the lower dome and lower crater walls. Debris from mass wasting covers parts of the glacier.The bottom photo is the same image with shading added by James Vallance to indicate the approximate locations of the extruding dome and the larger region of uplift. Those events took place between 27 September and the present, one to two months after this photograph, dramatically altering the surface morphology between the dome and the southern crater wall. Photo courtesy of Ivan Savov, taken on the S crater rim ~ 200 m W of the upper end of the Monitor Ridge trail.

Table 4. A simplified chronology of the events at Mt. St. Helens from 23 September to 8 October 2004. Taken from material posted by the Cascades Volcano Observatory.

The CVO website contains a complete discussion of the 3-level hazard-status scheme used there during this reporting interval. In brief form, the status rises from 1 - Notice, to 2 - Advisory, to 3 - Alert. (These are short-hand for Notice of Volcanic Unrest: Alert Level One, Volcano Advisory: Alert Level Two, and Volcano Alert: Alert Level Three.) These respective levels of hazard status correspond to an informal color code, rising from green, to yellow, to red (shown in parentheses on table 1). A type of announcement, the Information Statement, discusses events that are unusual or short-lived, or both. On days 1-3, at the start of this crisis, a specific volcanic hazard was not specified.

Description of activity. At about 0200 on the morning of 23 September 2004, an earthquake swarm began at Mount St. Helens. Through 1700 hours on that date about 200 small (less than M 1) earthquakes had been located at the volcano, and many smaller events were recorded throughout the morning. The earthquakes occurred at shallow depths (less than 1 km) mostly under the lava dome that formed between 1980 and 1986.

Such earthquakes are common for the volcano. But a swarm with this many earthquakes has not been recorded for several years. The most recently case was on 3-4 November 2001. By 25 September 2004 seismicity had declined significantly. However it remained elevated above background. On 26 September the character of the swarm changed to include more than ten larger earthquakes (M 2-2.8), the most in a 24-hour period since the eruption of October 1986. Some of the earthquake types suggested pressurized fluids (water and steam) or perhaps magma. As a consequence, CVO and the University of Washington's Pacific Northwest Seismograph Network released the first Notice of Volcanic Unrest at this site in 18 years. The earthquakes occurred at shallow depths (less than 2 km) below the 1980-1986 lava dome.

On 27 September seismicity increased very slowly throughout the day. All earthquake locations were still shallow and beneath the dome. The largest earthquake recorded in the prior 24 hours was about M 1.5. Preliminary results from a gas flight late in the afternoon of 27 September did not indicate any magmatic gas.

Throughout the day of 28 September seismic activity remained at a fairly constant, but high rate of about two small (less than M 2) earthquakes per minute. All earthquake locations were still shallow and in or below the lava dome. That night seismic activity increased significantly. Throughout the day the seismicity remained elevated at 3-4 events per minute. A number of these events were between M 2 and 3. All earthquakes remained in or below the dome. By 30 September and 1 October the seismicity level had increased slightly, including events as large as M 3.3.

Around noon on 1 October a small 25-minute-long eruption occurred from a vent just S of the lava dome. The vent opened in a portion of the glacier that had become increasingly crevassed and uplifted over the past few days. The eruption sent a steam and minor ash plume to an altitude of about 3 km that drifted SW, accompanied by minor, local ashfall. Seismicity dropped several hours after the eruption, but gradually increased with earthquakes (maximum M 3) occurring 1- to 2-times per minute.

A 40-minute steam-and-ash emission started on 2 October at 0943. Steam clouds carrying minor ash billowed out of the crater to an altitude of 3 to 4 km. The emission occurred during a time of gradually increasing seismicity, which dropped slightly after the emission, but continued to increase gradually through the afternoon. CVO's preliminary reports indicated that despite this increase, the event did not generate diagnostic earthquakes or explosion signals. Scientists inferred that hot rock was pushed up into the glacier, which then melted ice and generated the steam.

An interval of tremor lasting 50 minutes occurred immediately after a small steam emission at 1215 on 2 October. When the tremor stopped, the seismic character changed back to the earlier mode with shallow earthquakes under about maximum magnitude M 3 occurring 1- to 2-times per minute. Another period of smaller steam and ash bursts occurred between 1410 and 1440. Visual observations showed that the area of uplift, which includes part of the glacier and a nearby segment of the S flank of the lava dome, continued to rise. Part of the vent for the steam and ash emissions of the past few days was discovered to be covered by a boiling lake.

A gas flight on 3 October found slightly lower concentrations of carbon dioxide. On the other hand, for the first time hydrogen sulfide was detected. The 4 October gas flight detected carbon dioxide, often in association with hydrogen sulfide and occasional sulphur dioxide.

On 5 October at 0943 a 30-minute-long steam-and-ash emission started, and at 1410 a 10-minute-long steam-and-ash emission began. Ashfall on roads SE of the volcano achieved a maximum thickness of 0.2 mm at 8 km from the source. Neither emissions generated diagnostic earthquakes or explosion signals. As on 4 October, steam and ash emissions were associated with sightings of a bubbling lake. After the 4 October emissions, earthquake energy slowly increased to previous high values.

On 5 October continued uplift included part of the glacier and a nearby segment of the dome's S flank. Cracks opened in the dome. Portions of the cracks reached temperatures of 40-50°C (above ambient temperatures at the dome and glacier's surface, but far below the minimum magmatic temperature of ~ 800°C). Rocks avalanched off the dome, falling into the lake and onto the S crater floor. The dome's N flank appeared thermally stable.

The most vigorous steam and ash emission of the reporting period began on 5 October amid an interval with high seismicity shortly after 0900. Steam clouds billowed from the crater for over an hour, with variable ash content. For the first time, the ash content was sufficient to be detected by National Weather Service Doppler Radar. Steam and ash clouds reached about 3 km high and drifted NNE. A light dusting of ash fell in Morton, Randle, and Packwood, Washington, towns about 50 km from the volcano. Seismicity dropped during the emission and stayed relatively low the next day.

On 6 October the low-level tremor observed following the eruption gradually declined. Brief crater observations from Coldwater Visitor Center noted weak steam emissions. Small lahars from the crater traveled N onto the pumice plain during a rainstorm in the early morning of 6 October. Lahars flowed a short distance toward both Spirit Lake and the North Fork of the Toutle river.

The GPS station on the dome's N flank showed a trend of northward displacement totaling 2 cm over the last three days. This is the same sense of movement recorded by the nearby station that was destroyed by the first steam-and-ash emission on 1 October. In contrast, GPS instruments on the outer flanks of the volcano showed no movement.

Additional analysis based on lidar (LIght Detection And Ranging) and photographs of the intensely uplifting area suggested that the total volume change represented by the deformation between late September and 6 October was about 16-20 million cubic meters. The average rate of change was about 2 million cubic meters per day. If this value represents the rate of intrusion of magma into shallow levels of the dome and underlying crater floor, or both, it was an intrusion rate about twice that measured during dome-building eruptions at Mount St. Helens in the 1980s.

The 7 October report noted that a new steam vent opened overnight to join the two that had been present for several days. Steaming from the vents generated a cloud that rose above the lava dome's S side and extended toward the crater rim. Seismic activity was low to moderate with earthquakes of M 1 to 2 occurring about once per minute. Seismicity increased slightly during 7 October. Earthquakes up to M 1 occurred at a rate of ~ 1-1.5 per minute.

Reports issued on 8 October noted seismic activity continued to be low to moderate. Earthquakes occurred at a rate of 1 to 2 per minute with the largest magnitudes about M 1.5. That day field crews reported that there had not been noticeable additional uplift of the S part of the dome and adjacent glacier in the past 24 hours. Measurements from the recent photographs and lidar showed that the intensely deformed and uplifted area on the S side of the lava dome was then about 400 m (N-S) x 500 m (E-W) with a maximum uplift of about 100-130 m.

Seismicity rose gradually for most of 8 October and leveled off overnight. Earthquakes of M 2.4 occurred about once every two minutes. During 10 October seismicity decreased slightly, to levels similar to those observed during the evening hours of 7 October. Earthquakes of 1.0 M or less continued to occur at a rate of about 1 per minute, but most had magnitudes of M 1.0 or less.

On 12 October seismicity remained low. Small earthquakes (maximum about M 1) continued to occur every 5-10 minutes. Thermal imaging around 12 October of the W part of the uplifting area revealed temperatures of 500-600°C, by far the highest yet reported. They were greatest on a large pinkish-gray fin of rock and in nearby fumaroles and cracks. These observations were consistent with new lava having reached the ground surface. Later sampling of adjacent fresh talus using a metal trash can on a line below a helicopter confirmed it's dacitic composition.

Aviation advisories. The Washington VAAC issued advisories beginning on 29 September. Between then and through 8 October, they issued 23 Volcanic Ash Advisories. Some of these were also 'volcano watches.' Most of these were cautionary in nature, and actual plumes were generally minor, frequently steam dominated rather than heavily ash laden.

Information Contacts: Cascades Volcano Observatory (USGS/CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/); Ivan Savov, Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560-0119, USA; Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/).

October 2004 (BGVN 29:10) Cite this Report

Swelling dome rises ~250 m; minor plumes and few earthquakes

At St. Helens, rapid dome growth and pronounced uplift continued. Although this report covers 9 October-12 November 2004, there are several photos and comments on prior events. Figure 47, for example, contains a satellite image from 5 October. R. Scott Ireland photographically documented the 4 and 5 October eruptions, starting from the smallest plumes and including later wind-blown ash-bearing plumes. Digital copies of Ireland's set will be preserved in the Smithsonian's archives. Much of this report came from information posted by the Cascades Volcano Observatory (CVO).

Figure 47. Image of St. Helens on 5 October 2004 from a Geostationery Operational Environmental Satellite (GOES-10) showing a consistent ash-bearing plume extending NE for ~ 40 km. Courtesy of NOAA.

Figure 48 presents four aerial views into the crater, taken on 8 August and 7, 10, and 14 October. They portray the southern part of the crater containing a broad area of uplift and deformation associated with a more restricted zone of dome emergence. On 7 October the broad area of uplift on the S side of the 1980-86 lava dome stood ~ 400 m (N-S) by ~500 m (E-W), with a maximum uplift of about 100-120 m. For perspective on this growth, CVO's 11 November estimate noted an expanded area of uplift and some parts of the dome rising ~250 m above the glacier.

Figure 48. Four aerial photos depicting the southern portion of St. Helens's crater, an area of rapid uplift and dome emergence, from the S on 8 August and 7 October, and from the E on 10 and 14 October. The photos include an older dome lobe that was recently uplifted (Opus), steam releases, faulting (with upwards displacement towards the center), and the emergence of fresh dome lavas. Courtesy of USGS Cascades Volcano Observatory.

Table 5 summarizes CVO's observations. The terminology of numbered days for this eruption began at Day 1 (23 September), when precursory earthquakes began (BGVN 29:09). In contrast to those initial several weeks, during the current reporting interval seismicity generally remained low, an observation consistent with the slow rise of gas-poor magma. The emerging magma drove uplift of the glacier within the crater but did not yield large explosive discharges and tall plumes.

Table 5. A simplified chronology of the events at St. Helens from 23 September to 12 November 2004. Regarding the Hazard Status column, the colors in parentheses represent an informal aviation hazard status (low to high; green, yellow, orange, and red). Taken from material posted by the USGS.

Thermal images of the exposed dome revealed elevated temperatures there. This confirmed that new lava had reached the surface of the uplift.

Other details. The weather enabled clear views on 10 October. A photo of the scene at dawn showed an orange-colored plume. Field observers noted fresh snow over the crater floor contained a thin SE-directed ash deposit stretching to just beyond the crater rim. A steam plume rose to crater rim level or slightly above all day on 10 October and continued to blow SE. USGS field workers described the plume as "lazy," emphasing the absence of gas thrusts or notably vigorous convection. When the field crew visited the volcano, the plume appeared clean, with no noticeable ash nor blue nor orange haze. The odor of H₂S was noted at the crater's breach, but not elsewhere.

On 14 October observers noted an increase in the deforming and uplifting area on the S side of the 1980-1986 lava dome and the new lobe of lava in the W part of that area. The maximum temperature of 761°C was measured in parts of the new lobe from which ash rich jets rose ten's of meters. Magma extruded onto the surface, forming a new lobe of the lava dome. Instruments detected low levels of H₂S and SO₂, but no CO₂.

Crews collected samples and documented clear dome growth on 20-21 October. The new lava extrusion had horizontal dimensions of ~ 300 x 75 m and a thickness of ~ 70 m. The fin-shaped lava spine had collapsed. The 21 October volume estimate was almost 2 x 10⁶ m³. By 21 October the area of uplift and intense deformation had advanced S, nearing the crater wall. That day, ~ 30 cm of new snow with a light dusting of ash covered much of the uplift, except for the new lava extrusion, which steamed heavily. A vigorous steam plume rose to 3 km. Fluxes of gaseous H₂S, SO₂, and CO₂ were low. Samples of the new dome were scooped up by a container slung on a line beneath a helicopter.

Atmospheric conditions on 27 October and 7 November again gave airborne observers clear views into the crater (figures 49, 50, and 51). The N-looking photo in figure 12 documents how the new dome and area of uplift had achieved substantial size, standing topographically above what was previously the moat to the S of the older dome. In plan view, the margin of the dome complex shifted from a circle to a figure-eight.

Figure 49. An aerial photo looking downward and N-ward into the crater of Mt. St. Helens on 27 October 2004. The old (1980-86) dome is in the background and the new one, steaming, is in the foreground. Note uplifted, fractured ice around the margins of the 2004 intrusion. Some areas of ice and snow have gray color indicative of ashfall. The ridge along the inner crater wall intersects the rim at the approximate point where Ivan Savov stood when taking the photo presented in BGVN 29:09. Courtesy of CVO.

Figure 50. A simplified map of the St. Helens crater, based on the scene on 27 October 2004. More complex maps appeared in early November. Courtesy of CVO.

Figure 51. A photogeologic map depicting the southern end of the crater at St. Helens on 7 November 2004 and serving to identify and interpret recent deposits and features there. The map is centered on the new dome (N towards bottom, see arrow; for approximate scale, photo is ~ 1 km wide). The 1980-86 dome lies largely off the bottom of the photo. Courtesy of CVO.

In addition to photos documenting crater changes, a CVO report on 29 October discussed rapid movement at a new GPS station on the southern part of the new dome (an area of uplifted glacial ice, rock debris, and new lava). The station showed continued southward motion of ~6 m in the previous 36 hours. A station near the summit of the old dome showed continued, slow northward motion.

Analysis of aerial photographs taken on 4 November led to an estimate of the volume of the uplifted area and new lava dome at ~ 20 x 10⁶ m³. This followed other preliminary estimates made for 4 and 13 October of ~5 x 10⁶ m³ and ~12 x 10⁶ m³, respectively. This most recent volume estimate (20 x 10⁶ m³) amounted to more than 25% of the 1980-86 lava dome volume.

On 5 November the SO₂ emission rates remained low. No H₂S was detected and CO₂ emission rates were not measurable. On that day viewers noted that a new mass of dacite had extruded, forming a spine rising ~100 m. Exposed rock faces had temperatures of 400-500°C. The steep new faces on the dome generated small hot rockfalls and avalanches. The finer particulate material rose to about 3 km altitude, a height ~900 m above the crater rim.

A sample of the new dome collected on 4 November established that the new dacite lava contained visible crystals of plagioclase, hornblende, and hypersthene. A comparison of the 1986 and 2004 dacites (table 6) shows that the new lava lacks augite, distinctive reaction rims on hornblende, and large plagioclase with sieve-textured cores.

Table 6. A comparison of the dome dacites extruded at St. Helens in 1986 and 2004. Courtesy of CVO.

On 11 November the dome had reached ~ 250 m in height; it lay within a broad area of deformation that was ~ 600 m in diameter. Within this area, the new lava dome continued to occupy the E-central segment (broadly similar to the situation on figures 13 and 14). In plan view, the new dome stood 400 x 180 m. Regarding its height, the 11 November report noted that the highest point on the new lava dome was ~ 250 m "above the former surface of the glacier that occupied that point in mid-September."

Aviation Advisories. The first sentence of this section in BGVN 29:09 should be corrected to read, "The Washington VAAC issued advisories beginning on 29 September" (not 29 October).

The Washington Volcanic Ash Advisory Center issued one Ash Advisory each day during 9-18 October, noting elevated seismicity but a lack of explosive eruptions and substantial plumes. On 18 October the VAAC mentioned GOES-10 and -12 infrared and multispectral imagery of the volcano but concluded that "...after discussion with authorities at [CVO] we are discontinuing the Watch.... There continues to be low level [activity] ... not posing an [imminent] threat to aviation. A Notice to Aviation within ~9 km and below FL 130 should continue [Note: FL130, Flight Level 130, is the aviation community's shorthand for 13,000 feet; an altitude equivalent to 3,962 m, but typically rounded in the Bulletin to the nearest hundred meters]. If threat conditions rise[,] a Watch will again be issued. The Washington VAAC will continue to monitor the area and if ash is observed or reported a Volcanic Ash Advisory will be issued as soon as possible."

As of 12 November, the last Ash Advisory on St. Helens was issued on 6 November. It was in response to a minor ash emission that day. The emission was too small to detect with available satellite imagery. The local webcamera showed a weak, passively rising plume that barely rose above the crater rim.

Information Contacts: Cascades Volcano Observatory (USGS/CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Road, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); R. Scott Ireland, 1660 NW 101 Way, Plantation, FL 33322, USA (URL: http://rsiphotos.com/); Stephen and Donna O'Meara, Volcano Watch International, PO Box 218, Volcano, HI 96785, USA.

February 2005 (BGVN 30:02) Cite this Report

On 21 February the still-growing dome stood 160 m higher than the 1980's dome

Growth of the new lava dome inside the crater of Mount St. Helens has continued since the last report (BGVN 29:10), accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. During such eruptions, episodic changes in the level of activity can occur over days to months. The eruption can also intensify suddenly or with little warning and produce explosions that may cause hazardous conditions within several kilometers of the crater and farther downwind. The current status is Volcano Advisory (Alert Level 2), and the aviation color code is orange.

A small, short-lived explosive event at St. Helens volcano began at approximately 1725 hours on 8 March 2005. Airplane pilot reports indicated that the resulting steam-and-ash plume reached an altitude of about 11 km above sea level within a few minutes and drifted NE.

Results from analysis of imagery by the U.S. Geological Survey of 21 February 2005 showed that the highest part of the new lava dome stands at an altitude of 2.3 km, 160 m higher than the old lava dome, and only 28 m below Shoestring Notch, a low point on the SE crater rim. Further analysis of recent aerial photos revealed that as of 1 February, the whaleback-shaped dome extrusion was about 470 m long and 150 m wide. The new dome and uplifted welt of crater floor and deformed glacier ice have grown to a combined volume of about 38 million m³, almost one-half the volume of the old lava dome.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/).

June 2005 (BGVN 30:06) Cite this Report

Extrusion of smooth-surfaced dome lavas that later crumbled; explosions

Throughout the period covered by this report, March 2005 to July 2005, growth of the new lava dome inside the crater of St. Helens continued, accompanied by low rates of both seismicity and gas and ash emissions. The hazard status remained at 'Volcano Advisory' (Alert Level 2); aviation color code Orange. Results from a digital elevation model produced from imagery taken on 21 February showed the highest part of the new lava dome was 12 m higher than on 1 February; during that 3 week period the volume of dome and surrounding uplift had increased by 3 million cubic meters. The average rate of growth continued at ~2 m³/s. Figure 52 shows four views of changes to the lava dome during the period of this report. Figure 53 shows the seismicity and the time of the larger recognized explosions.

Figure 52. Four views into St. Helens's crater from different perspectives and dates, focusing on the new dome. A: 15 March 2005, view from NE. The whaleback is close to its maximum length of 500 m. Note that the glacier's heavily crevassed, half-moon shaped, E (left) arm lies squeezed between the growing dome and crater wall. Vent is steaming at lower-right whaleback. B: 3 May 2005, view from N. The whaleback has been breaking apart for several weeks. Note the large slab of smooth gouge-covered surface moving E (left). C: 21 June 2005, view from NW. Note the development of broad talus on W (right) flank of dome. An isolated body of smooth gouge-covered surface to the right of the main spine is emerging from talus. D: 26 July 2005, view from E crater rim. The smooth, gouge-covered spine continues to crumble as a result of M > 3 earthquakes and rockfalls. A large slab of March whaleback is visible at left. Most of the dome surface is now talus and disintegrating older whalebacks. By the end of July, the spine had been reduced to a highly fractured stump. All photos courtesy of USGS CVO.

Figure 53. Magnitude of located earthquakes at Mount St. Helens through 27 July 2005 (Pacific Northwest Seismograph Network). Vertical lines represent the time of moderate explosions. Note periods of earthquakes M > 3 that accompanied dome break-ups in December, April, and July. Courtesy of CVO and the Pacific Northwest Seismograph Network.

During 2-7 March, dome growth accompanied low rates of both seismicity and gas and ash emissions. Parts of the growing lava dome continued to crumble, forming rockfalls and generating small ash clouds that drifted out of the crater. The bulging ice on the deformed E arm of the glacier in the crater continued to move rapidly N at about 1.2 m per day (figure 54).

Figure 54. A view of the growing dome at St. Helens from the Sugar Bowl camera just before the 8 March 2005 explosion. The Sugar Bowl digital camera takes a picture every hour from its housing on the NE flanks. The image data are transmitted to a more accessible spot immediately after the pictures are taken. Courtesy of CVO.

A small explosive event began at approximately 1725 on 8 March. The eruption lasted about 30 minutes with intensity gradually declining throughout; a fine dusting of ash from this event later fell ~ 100 km to NW (in Yakima, and Toppenish, Washington). By 0200 on 9 March, the leading edge of the faint, diffuse plume had reached ~ 300 km to the E (over western Montana). After the explosion scientists found the lava dome intact. They recognized ballistics (up to ~ 1 m in diameter) as far as the N flank of the old lava dome and a lack of them along or beyond the crater rim. The explosion vented from the NNW side of the new lava dome, very near the source of the 1 October 2004 and 16 January 2005 explosions (figure 55).

Figure 55. The 8 March 2005 explosion at St. Helens viewed from the Sugar Bowl camera. This shot was taken at about 1727 hours and 42 seconds on 8 March. Courtesy of CVO.

The explosion on 8 March was one of the largest steam-and-ash emissions to occur since renewed activity began in October 2004. The Cascades Volcano Observatory (CVO) lost radio signals from three monitoring stations in the crater soon after the event started. The event followed a few hours of slightly increased seismicity not then interpreted as precursory. There were no other indications of an imminent change in activity.

After the 8 March explosion, St. Helens only emitted steam, and seismicity dropped to a level similar to that during the several hours prior to the explosion. Gas emissions were very low, essentially unchanged from those measured in late February. The hazard status for the ongoing eruption, 'Volcano Advisory (Alert Level 2),' mentioned the possibility of events like the 8 March explosion occurring without warning. That assessment remained unchanged and the hazard status stayed the same.

Analysis of aerial photographs indicated that as of 10 March the topographic changes in the crater resulting from growth of the new dome and consequent glacier deformation had a combined volume of about 45 million m³. The current eruption contributed new materials amounting to about two-thirds the volume of the old lava dome.

From March 2005 through July 2005, growth of the new lava dome continued. Rates remained low for both seismicity and gas and ash emissions. CVO noted that during such eruptions, episodic changes in the level of activity can occur over days to months. During about 26-27 March, a group of M 2 to M 3 earthquakes occurred beneath the volcano, a level of activity considered normal during dome-emplacing volcanism.

A series of large (M >=3) earthquakes occurred during 3-4 April, in addition to the typical array of smaller events. Observations on 6 April revealed that the smooth whaleback-shaped portion of the growing lava dome was broken by numerous fractures, and the edges had crumbled greatly. Several deep gashes on the E, N, and W sides frequently produced rockfalls and accompanying ash clouds. On 10 April the new dome continued to fracture and spread laterally. As a consequence, the dome's summit dropped by a few tens of meters over 2-3 weeks, leaving isolated high-standing remnants. This broken pattern was apparent in a photograph on 3 May (figure 5B).

Earthquakes steadily decreased in magnitude and number through mid-April. A GPS receiver 200 m N of the new dome crept steadily NNW at ~ 10 cm per day. The combination of the GPS measurements adjacent to the lava dome and the qualitative estimate of lateral spreading suggested that extrusion of new lava continued during April.

On the morning of 28 April there were reports of minor amounts of ashfall in the eastern part of the Portland metropolitan area, ~ 80 km SSW of St. Helens. There was no evidence of a new explosion. CVO scientists determined that large convective storms over the Cascades on 27 April entrained ash generated by the frequent hot rockfalls from the growing lava dome and kept it in suspension to fall out as far away as Portland.

During early May poor weather obscured the volcano. Seismic and ground deformation activity remained unchanged. Through much of the night of 4-5 May, however, VolcanoCam images detected intermittent glow from the new dome. The camera is mounted at the Johnston Ridge Observatory at an elevation of 1,400 m and ~ 6.5 km NNW of the volcano, a spot W of the S part of Spirit Lake. During 11-12 May images from the mouth of the crater showed the new spine of lava at the N end of the dome continuing to grow. Data from seismic and GPS instruments in the crater and on the outer flanks continued to lack significant changes over the past few weeks. Through the end of May, lava extrusion continued at the N end of the new lava dome, while the high spines continued to crumble. Other parts of the lava dome moved at the relatively low velocity of about 30 cm/day or remained stagnant. Table 7 compares the older dome with the new one as of 3 May 2005.

Table 7. A comparison of the old (1980-86) and new (2004-) domes at St. Helens. The new dome (unofficially called "the whaleback") started in October 2004, and the reported data reflects conditions seen until 1 February 2005. Courtesy of CVO.

Around 4 June the rate of motion of a GPS unit on the NE part of the new dome slowed slightly, continuing to creep eastward and northward at a rate of several centimeters per day, but no longer rising vertically. The lava spine, however, continued to grow. Through the end of June 2005, seismic and deformation data continued trends similar to the previous few weeks, with small earthquakes approximately every 5 minutes, little to no movement of the old lava dome, minor movement of the N end of the new lava dome, and continued growth of the lava spine. Observations made on 15 June revealed that the lava spine continued to grow and that temperatures in cracks near the base of the spine were near 700°C. Thermal data from 15 June suggested that much of the W part of the dome was moving upward, as well as southward. During the last week of June, the smooth lava spine continued to grow at a rate of about 1.8-3.7 m per day. Rockfalls from the top of the spine kept its height from increasing by that same rate. Analysis of a digital elevation model made from imagery acquired on 15 June showed that the total volume addition to the crater since September 2004 had reached almost 60 million cubic meters.

On 2 July at 0630 a rockfall from the growing lava dome removed a large piece of the dome's top, producing an ash plume that rose above the crater rim and generating a substantial seismic signal. Persistent smaller rockfalls from the growing lava dome built talus aprons on the W and NE flanks of the dome.

On 12 July, CVO reported that rates of seismicity and ground deformation at Mount St. Helens had declined during the previous two weeks to some of the lowest levels since the eruption began in September 2004. A similar lull occurred in December 2004.

Beginning 15 July and continuing through the end of the month, the growing spine and other high areas of the dome to the south produced numerous large rockfalls, most of which were associated with earthquakes of about M 3 (figure 56). Diffuse ash plumes that rose hundreds of meters above the rim were produced by the larger rockfalls. By the end of July most of the smooth gouge-covered surface of the spine had disintegrated, and the spine was reduced to a highly fractured, but still-extruding, stump surrounded by rapidly growing aprons of rockfall debris.

Figure 56. Rockfall and accompanying ash cloud on 26 July 2005 as viewed from station Brutus on the crater's E rim. Rockfall originated from the steep, fractured top of an inclined spine. Note boulders (light-colored specks against shadow) shooting ahead of ash cloud. Another spine is extruding from ground just behind the lower end of the ash cloud. Courtesy USGS and CVO.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismograph Network (PNSN), Seismology Lab, University of Washington, Department of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/).

December 2005 (BGVN 30:12) Cite this Report

2005 dome building amid comparative seismic, deformational quiet

Throughout the period covered by this report, August 2005 to December 2005, growth of the lava dome inside the crater of Mount St. Helens continued, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. This report came from those posted on the website of the Cascades Volcano Observatory (CVO, part of the U.S. Geological Survey). The hazard status remained at Volcano Advisory (Alert Level 2); aviation color code Orange. During the month of August 2005, growth of the lava dome produced rockfalls, resulting in ash plumes that occasionally rose above the rim (figure 57). A rockfall on 21 August at 2056 generated a bright glow of hot rock and a thick ash plume. The induced atmospheric conditions temporarily affected radio transmissions from instruments in the crater.

Figure 57. Before and after views of a rockfall that removed the upper portions of a spiny ridge on the growing St. Helens dome. View as seen from remote camera on the SW crater rim. Both photos were taken on 21 August 2005. Courtesy of USGS Cascades Volcano Observatory.

A digital elevation model of the active lava dome, which was created from aerial photographs taken on 10 August, showed that the volume had grown to 62 million cubic meters with the average growth during late July and early August at about 2 million cubic meters per second.

During 31 August-6 September 2005, the new lava dome inside the crater of St. Helens continued to grow, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Images of the crater showed continued westward motion of the new lava dome.

On 6 September 2005, dry conditions and rockfalls from the lava dome generated occasional ash plumes that rose above the volcano and rapidly dissipated. During the week of 7-13 September 2005, growth of the lava dome continued and photos showed continued slumping of the central part of the dome and W motion of the presently active area. This movement continued throughout September 2005 and is illustrated in a time-series of images showing the active NW portion of the new lava dome as it continued to move W, butting into the W arm of a glacier, spawning rockfalls. This time-series of images is available on the CVO website.

During 28 September-4 October 2005, growth of the new lava dome inside the crater continued to grow, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Reanalysis of late September time-series photographs of the active part of the new lava dome indicated that points on the dome then moved NW and upward at about 5.5 m per day as extrusion continued.

Images taken on 10 of October showed that the pattern of dome growth established during the previous few months continued. The actively growing portion of the dome moved NW; pushing the W arm of the glacier against the W crater wall, causing the glacier to narrow, thicken, and become increasingly fractured. From the end of October to 21 November there were no significant changes in seismicity or edifice deformation.

During the previous few weeks, a prominent linear feature developed on the disintegrating "whaleback" that grew during the previous spring and summer and was currently located E of the actively growing part of the new lava dome (figure 58).

Figure 58. Linear feature developed at St. Helens on the Spring/Summer 2005 spine. Photo taken on 18 October 2005. Courtesy of USGS CVO.

On 22 of November two notable rockfalls occurred at 1200 and shortly after 1500. Both produced dilute ash clouds that rose a few hundred meters above the crater rim, which are common during lava-dome growth (figure 59).

Figure 59. Two photos of dense local plumes created as a result rockfalls from St. Helens' dome on 22 November 2005. The photo on the top was taken from the NE; the photo on the bottom was taken from the NW. Courtesy of USGS CVO.

The well-established pattern of tiny "drumbeat" earthquakes continued at a rate of one every 1-2 minutes; other monitoring data remained in typical ranges. Despite the continuing procession of earthquakes, the overall seismic energy release was very low compared to that during early phases of the eruption. Small rockfalls continued from the growing lava dome, with larger ones producing ash plumes that were visible above the crater rim.

The volume of the lava dome measured on 24 October was 70 million cubic meters?about 90% of the volume of the 1980-1986 dome. Repeat images taken on 15 December from fixed cameras within the crater and at the crater rim showed the seventh lava spine to emerge during the current activity. It continued to push upward and SW from a source just S of the 1980-186 dome (figure 60).

Figure 60. St. Helens' recent dome complex as seen from the NW. Four different stages of growth are visible in this image. They were given these sequence numbers: 4, 5, 6, and 7 (the latter for the one currently extruding). All occurred as whalebacks or spines in 2005. Photo was taken 8 December 2005 by Willie Scott. Courtesy of USGS CVO.

During 21-27 December 2005, seismicity was marked by the repetitive small earthquakes, occurring every 2-3 minutes, that have come to characterize the past 15 months. Tiltmeters within 500 m of the new lava dome showed minute ground deformation; whereas the volcano's flanks were quiet. At the end of December 2005, St. Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.

Table 8 gives a summary of the growth of the dome since October 2004. It was reported on 26 January 2006 that initial analysis of recent photographs from fixed cameras in the crater showed that the top of the currently active part of the new lava dome is at ~ 2,240 m elevation, which is about 90 m higher than it was in early November 2005. On several occasions during the eruption, parts of the dome have been considerably higher, for instance 2,365 m in July 2005. Those high points have since been lowered by disintegration, but still are higher than the top of the currently active part.

Table 8. St. Helens dome summit elevations and volumes compiled at a series of times during October 2004-January 2006. The dome summit elevations have decreased due to repeated failure of the unstable blocks, spires, and protrusions along the dome summits. Also, the method of assessing dome volumes changed in April 2005 (indicated by the asterisk). Courtesy USGS Cascades Volcano Observatory.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

May 2006 (BGVN 31:05) Cite this Report

Intracrater lava dome continues to grow through at least May 2006

From August to December 2005, the lava dome inside the crater of Mount St. Helens continued to grow, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash (BGVN 30:12). The hazard status was at Volcano Advisory (Alert Level 2); aviation color code Orange.

Based on the online reports of the Cascades Volcano Observatory (CVO) of the U.S. Geological Survey (USGS), this pattern of activity continued in January and February 2006 and suggests that the slow extrusion of dacite onto the crater floor at Mount St. Helens continued. Slight decreases in seismicity occurred on two occasions after larger than normal earthquakes. By mid-January the new dome was noticeably taller and broader than in December. Rockfalls from its summit generated small ash plumes that slowly rose above the crater rim and dissipated as they drifted E.

On 24 January a shallow M 2.7 earthquake triggered a rockfall from the new lava dome, which in turn produced an ash plume that filled the crater before dissipating and drifting N over the pumice plain. Analysis of recent photographs from cameras in the crater showed that the top of the new lava dome was at an elevation of ~ 2,240 m, about 90 m higher than it was in early November 2005.

In February, occasional clear views of the volcano revealed incandescence on the currently growing lava lobe and a few incandescent rockfalls. Comparison of photos taken between 17 December and 7 February showed that the base of the active lobe of the lava dome enlarged by about 100 m. Photographs taken during the week of 5 February showed that the active part of the new lava dome continued to extrude, with points on the surface of the dome moving a couple of meters per day (figure 61).

Figure 61. High-angle view of Mount St. Helens new dome from the NNW, taken on 5 February 2006 by John Pallister. Photograph courtesy of USGS.

Gas measurements made on 15 February suggested that the volcanic-gas flux remained unchanged from recent measurements. Observations made on 17 February revealed that the active NE part of the new lava dome was developing a steeply inclined jagged spine. At its top, temperatures as high as 580°C were measured using a thermal sensor.

Growth of the new lava dome inside the crater of Mount St. Helens continued during March, April, and May 2006, accompanied by low rates of seismicity, low emissions of steam and volcanic gases, and minor production of ash. Small earthquakes occurred every several minutes, punctuated by occasional larger earthquakes. The Global Positioning System (GPS) receiver on the new lava dome showed that lava emerging from the vent was still advancing WNW at about a meter per day. Small rockfalls produced small ash clouds that rose from the dome's NW flank. The eruption of lava into the crater continued, shown by ongoing rockfalls and continuous GPS measurements made on the growing lava lobe.

Analysis of photographs revealed that a slab of rock approximately 50,000 cubic meters in volume was shed from the N margin of the growing spine during 6-7 May. This probably coincided with a large seismic signal recorded on the night of 7 May. Rock-avalanche deposits extended a few hundred meters to the NE. The avalanche was accompanied by an ash cloud. The spine continued to grow during 10-15 May, producing rockfalls that intensified on the evening of 14 May. Incandescence was visible on satellite imagery. On 17 May night-time incandescence from rockfalls was observed.

During 24-25 May, seismicity was at levels typical of the continuing lava-dome extrusion at Mount St. Helens. On 29 May, a M 3.1 earthquake and simultaneous large rockfall occurred. An ash plume produced at 0810 reached an altitude of 4.9 km - 6.1 km according to ground observations and pilot reports (figure 62). One pilot report suggested that the plume reached an altitude of 7.3 km. By 1308, ash from the event was no longer visible on satellite imagery. The rockfall originated primarily from the N side of the growing fin (figures 63 and 64).

Figure 62. At Mt. St. Helens, a view from the Brutus camera at 0914 on 29 May 2006. Vapor with light ash obscures most of the extruding lava spine. The light gray swath in the center of the photograph shows the path of the rock avalanche as it flowed downhill. The dark areas adjacent to the rock-avalanche path shows the ash cloud (finer material) that accompanied the avalanche. Photograph courtesy USGS.

Figure 63. Mount St. Helens crater and dome showing aftermath of rockfall event of 29 May 2006, seen from the N. Taken on 30 May 2006 by Willie Scott and Jim Vallance. Photograph courtesy USGS.

Figure 64. Aerial view showing Mount St. Helens crater and dome as seen from the SW. Spirit Lake can just be seen in the upper right corner. Taken on 30 May 2006 by Willie Scott and Jim Vallance. Photograph courtesy USGS.

During June 2006, seismicity indicated that the lava spine continued to grow inside the crater of Mount St. Helens and occasionally produced minor rockfalls. On 9 June, pilots reported that an ash-and-steam plume, generated after a rockfall following a M 3.2 earthquake, reached an altitude of 4.6 km. According to seismic data, a medium-sized rockfall occurred on 13 June. Incandescence was observed on satellite imagery. A small steam plume from the lava dome and dust from minor rockfalls were visible from the US Forest Service's web camera at the Johnston Ridge Observatory on 25 and 26 June. On 26 June, a pilot reported that dust and ash reached an altitude of ~ 2.4 km and drifted W.

From January through June 2006, St Helens remained at Volcano Advisory (Alert Level 2); aviation color code Orange.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

July 2006 (BGVN 31:07) Cite this Report

Eruption still extrudes dacitic dome lavas without energetic explosions

The current and ongoing eruption of the St. Helens started on 11 October 2004. Extrusion of the growing lava dome has continued in the same quiescent mode exhibited over the past year, and levels of seismicity remained generally low, with low emissions of steam and volcanic gases and minor production of ash. From 1830 hours on 26 October 2004 to 15 August 2006, a total of 13,841 seismic triggers have occurred. Figures 65 and 66 summarize seismicity over the past year. A decade-long time-depth plot clearly shows the start of the current eruption (figure 67).

Figure 65. Epicenters of St. Helens earthquakes between 1 July 2005 and 15 August 2006, a total of 1,768 well-located earthquakes. The circle with an "x" represents events on 15 August 2006, and filled circles represent events since 15 July 2006; open circles represent older events in the past year. Black triangles locate Pacific Northwest Seismograph Network (PNSN) seismic stations. Courtesy of the PNSN.

Figure 66. Plots of the number of well-located events and their main root of strain energy for St. Helens earthquakes between 1 July 2005 and 15 August 2006 describing a total of 1,768 earthquakes. Each point on the strain energy plot's curve represents the sum of energy released by all earthquakes in a 14-day period; energy is computed in 14-day time windows, every 7 days. Courtesy of the PNSN.

Figure 67. Time-depth plot of well-located earthquakes at St. Helens between 1996 and 14 September 2006, a total of 22,485 events. Courtesy of the PNSN.

Pictures and movies taken in August 2006 with the Brutus camera (located on the E rim of the 1980 Mount St. Helens crater) showed continued extrusion of spine 7 on the growing lava dome (figure 68) (photos and movies are also available on the CVO website). Between 4-5 and 7-8 August a segment of the middle part of spine 7 temporarily stopped moving. At 1310 on 5 August a magnitude 3.6 earthquake occurred, and subsequent photographs showed that the "stuck" segment became unstuck. Motion again stopped sometime after 1310 on 7 August and much of 8 August, when a M 3.3 earthquake occurred at 2001 on 8 August. Clouds obscured the volcano from view on 9 August, but parted enough on 10 August to show that once again the segment became unstuck. One explanation by CVO scientists for these observations is that the large earthquakes were caused by parts of the spine sticking and then slipping.

Figure 68. Spine 7 of the growing lava dome of Mount St. Helens taken 3 August 2006. Courtesy of CVO.

Information Contacts: U.S. Geological Survey Cascades Volcano Observatory, Vancouver, WA (URL: https://volcanoes.usgs.gov/observatories/cvo/); The Pacific Northwest Seismograph Network, University of Washington Dept. of Earth and Space Sciences, Box 351310, Seattle, WA (URL: http://www.geophys.washington.edu/SEIS/PNSN/).

December 2006 (BGVN 31:12) Cite this Report

Continued lava-dome growth through 2006

The current and ongoing eruption of the St. Helens started on 11 October 2004. Extrusion of the growing dacitic lava dome has continued in the same quiescent but sustained mode exhibited throughout the first half of 2006 (BGVN 31:07) . Levels of seismicity have remained generally low, with low emissions of steam and volcanic gases and minor production of ash.

From 26 July through 3 October 2006, the lava dome continued to grow and produce small rockfalls accompanied by minor earthquakes. M 3-3.6 earthquakes occurred on 26, 28, and 31 July. Resulting dust plumes rose well above the crater rim. A steam plume was observed rising from the growing lava dome on 13 August. During 16-22 August, based on interpretations of seismic data, spine extrusion from the dome continued in conjunction with small earthquakes and rockfalls. By mid-August 2006 the dome's volume was about 85 million cubic meters growing at an average rate of less than 1 m³/s. The lava dome's height above the 1986-crater floor started at 396 m. On 9 and 10 September, five shallow earthquakes greater than M 2 occurred in association with the growing dome. A period of relatively low seismic activity followed.

From 20 September through 3 October, lava extruded slowly from the vent onto the S crater floor; there was only low seismicity that generated occasional rockfalls as talus sloughed off the flanks of the growing dome. The rate of dome deformation was low. There was no change in rock chemistry, suggesting little to no change in eruptive style. The lack of explosive activity coupled with continuing low number of earthquakes and small quantities of volcanic gas indicate that the risks posed by the hazards are currently relatively low.

During October, lava continued to extrude onto the S crater floor of St. Helens and observations and data from deformation-monitoring instruments showed the dome continued to grow. Low seismicity and slight tilting of the crater floor produced small rockfalls. A small steam plume was visible on 9 October. On 22 October, an M 3.5 earthquake triggered the collapse of material from the largest of the lava-dome spines. The resulting ash plume rose to about 3.2 km and quickly dissipated to the W. On 29 October, a M 3.2 earthquake was accompanied by a rockfall that produced a small plume. The plume filled the crater to just above the rim and quickly dissipated.

Throughout November and December, data from deformation-monitoring instruments showed that during 1-7 November, the lava dome continued to grow. Inclement weather prohibited visual observation during most of the reporting period. On 5 and 6 November, acoustic flow monitors recorded rain-induced debris flows within the crater and in the upper part of the North and South Fork Toutle River valleys. Seismicity continued at low levels, punctuated by M 1.5-2.5, and occasionally larger, earthquakes. On 21 November, views from an aircraft and a crater camera showed that an active spine continued to extrude. On 18 December, a steam plume rose several hundred meters above the rim and was visible from the Portland area, about 80 km away.

Scientists working on the "old part" of the new lava dome found evidence to suggest that the lava dome was essentially solidified within several hundred meters beneath the crater floor. The outer 2-3 m of the lava dome was composed of ground rock that transitions to solid rock with numerous fractures. These findings support the stick-slip model of lava dome extrusion. If the model is correct, it may help explain the origin of many of the million plus small, shallow earthquakes as the result of numerous sub-surface slips that created the ground and fractured rock. Scientists have also noted that for short periods (hours to perhaps a day) part of the growing lava dome appears to stick (no movement detected in photographs) and then restarts again after high-M 2 to low-M 3 earthquakes.

Alert Level terminology. On October 1, the alert-level system for all volcanoes monitored by the USGS was changed to a descriptive system (table 9). In the new system, "Normal" indicates background conditions are stable; this is equivalent to aviation color-code Green. The previous alert levels of Volcanic Unrest (Alert Level 1), Volcano Advisory (Alert Level 2) and Volcano Alert (Alert Level 3) have changed to "Advisory," "Watch," and "Warning," respectively. There is a minor additional change for the aviation color-code definitions in that there is no longer an ash-plume threshold given for either Orange or Red. The ash-plume height threshold of 25,000 ft. or less for aviation warning condition "Orange" is no longer mandatory; condition "Red" was for ash above 25,000 ft. Now the height threshold can be adjusted for each case.

Table 9. Revised nomenclature and descriptions of volcano Alert Levels for all volcanos in the Cascade range. Courtesy USGS-CVO.

Throughout the period covered by this report, the hazard status for St. Helens remained at Volcano Advisory Alert Level (2) "Watch;" aviation color code Orange. The alert-level "Watch" is used for two different situations: (1) heightened or escalating unrest indicating a higher potential that an eruption is likely, but still not certain; or (2) an eruption that poses only limited hazard. Descriptor definition "Watch" fits the current lava-dome eruption at St. Helens.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

August 2007 (BGVN 32:08) Cite this Report

Dome growth continues, seismicity remains low

Lava dome growth at St. Helens (as previously reported in BGVN 31:12) continued through at least September 2007. Seismicity remained at low levels punctuated by M 1.5-2.5, and occasionally larger, earthquakes. Inclement weather inhibited field work and created poor visibility for much of the January-September reporting period.

In general, gas-and-steam plumes from the active lava dome, as well as dust plumes resulting from rockfalls, occasionally rose above the crater rim. A gas plume may have been seen on 3 June, and a weak gas-and-steam plume was visible rising from the lava dome on 12 June.

On 3 April, a GPS unit on an active spine showed W-ward movement at a rate of approximately 30 cm/day. Points on the active part of the dome moved away from the vent at an average rate of approximately 0.45m/day July 2007. That rate is similar to but slightly less than it was a year ago.

Growth of the lava dome and changes in crater morphology over the course of this eruption have been well documented (figures 69 and 70).

Figure 69. A panoramic wide-angle view from St. Helens' crater rim looking N on 27 July 2006. The accompanying sketch describes key features in the photo. For example, the dark rim wrapping around the lower margin of the photo represents rock debris on the snow cornice. Courtesy of Willie Scott, USGS Cascades Volcano Observatory.

Figure 70. Comparison photo taken of Mount St. Helens as seen from Harrys Ridge, 8 km N. These photos were taken 25 years apart in 19 May 1982 and 20 April 2007. Courtesy of Gene Iwatsubo, USGS CVO.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/).

September 2009 (BGVN 34:09) Cite this Report

Eruption ceased in late January 2008; quiet continues in late 2009

The eruptive episode that began with the volcano reawakening in October 2004 (BGVN 29:09) ended in late January or early February 2008. The activity included explosions containing ash that rose up to ~ 3 km above the crater and lava dome growth. Sherrod and others (2008) provide a comprehensive discussion of the 2004-2006 portion of the eruption. This report spans 28 November 2007 through October 2009.

A GPS receiver on the W part of the active spine recorded continued SW advance at a rate of 3-4 mm per day during September through November 2007. During 28 November-4 December 2007, small inflation-deflation events occurred, which the USGS Cascades Volcano Observatory (CVO) interpreted as dome-growth pulses. On 31 December 2007 aerial observers saw a new small, snow-free spine on top of the active lobe.

On 25 January 2008, a steam plume rose from the dome slightly above the crater rim. Though seismicity had persisted at low levels through mid-February 2008, very few earthquakes were recorded after late January. Locatable earthquakes were fewer than one per day, all under M 2.0. Ground tilt measurements showed an overall subsidence in the area of the new dome. A GPS receiver on the previously active spine settled about 2 cm per day on a southward path. During February, the daily ground-tilt events stopped and gas emissions were barely detectable.

Comparison of photographs taken by remote cameras during late January to mid-February 2008 showed no evidence of extrusion. Cynthia Gardner (CVO), in a personal communication, noted that dome growth stopped in late January or early February (January 27 ± 10 days).

During March 2008, the most significant developments were a small, M 2.0 earthquake on 4 March and a very small earthquake swarm on 6 March. The latter started with a roughly M 1.2 event, followed by several smaller tremors over a seven-minute period. No tilt changes were associated with the swarm. On 14 March, the Pacific Northwest Seismic Network recorded four very small earthquakes located near the volcano. There were no tilt changes associated with this activity.

Radar imagery analyzed by Jet Propulsion Laboratory staff during late March 2008 showed that the E and W arms of Crater Glacier were touching, or close to touching, just N of the 1980s lava dome. From 30 May 2008 (figure 71) to 8 July 2008, the W arm of the glacier advanced ~ 20 m. By 8 July, the old and new lava domes in the crater were encircled by ice (figure 72). Further down slope glacier ice descended into the gullies that had been carved by erosion into the Pumice Plain. On 10 July, after nearly 5 months without signs of renewed activity, CVO lowered the Alert Level to Normal and the Aviation Color Code to Green.

Figure 71. Aerial view of the St. Helens crater, as seen from the N. The two arms of the Crater Glacier had by 30 May 2008 fully encircled the dome. USGS photograph by Steve Schilling.

Figure 72. Old and new lava domes (center and upper right respectively) in the St. Helens crater encircled by ice, as seen from the NW. USGS photograph taken on 5 August 2009 by Steve Schilling.

As of October 2009, earthquakes, volcanic gas emissions, and ground deformation had all fallen to levels observed prior to the onset of the eruption.

References. Lahusen, R.G., 2005, Acoustic flow monitor system?user manual: U.S. Geological Survey Open-File Report 02-429, 22 p.

Sherrod, D.R., Scott, W.E., and Stauffer, P.H., eds., 2008, A volcano rekindled: the renewed eruption of Mount St. Helens, 2004-2006: U.S. Geological Survey, Professional Paper 1750, 856 p.

Information Contacts: Cascades Volcano Observatory (CVO), U.S. Geological Survey, 1300 SE Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA (URL: https://volcanoes.usgs.gov/observatories/cvo/); Pacific Northwest Seismic Network, University of Washington, Dept. of Earth and Space Sciences, Box 351310, Seattle, WA 98195-1310, USA (URL: http://www.pnsn.org/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).