Two days visiting the crater lake and fumaroles during mid-2007

Researchers from Michigan Technological University (MTU) and Servicio Nacional de Estudios Territoriales (SNET) visited the crater of Santa Ana on 28 June and 5 July 2007 to measure crater lake and fumarole temperatures, and to carry out routine water sampling.

Crater lake. The crater lake appeared yellowish-green and had a maximum temperature of 57.5°C, measured by a thermocouple at the northern shore. The crater lake was observed to have shifted westward in position since the 1 October 2005 eruption, drowning the main pre-eruption fumarole field to the W and receding from its eastern border (figure 13). A subaqueous hot spring was observed in the center of the lake at the end of a peninsula of exposed sediments (figure 14). The hot spring exhibited episodic pulses of bubbling water about every 5 minutes.

Figure 13. The yellowish-green acid crater lake of Santa Ana volcano as seen when viewed on 28 June 2007 looking towards the N. Photo taken by Anna Colvin.

Figure 14. Hot spring emerging in the acid lake at Santa Ana as seen 5 July 2007. Episodic upwelling of whitish fluid radiated out from the base of the large rock in the center of the photo. View is towards the SW; note geologist for scale. Photo taken by Matt Patrick.

Fumaroles. Crater fumaroles were observed to the W and S of the crater lake, and weak fumaroles were also observed on the upper wall above the flat area and below the SW crater rim. The southern crater fumaroles and the upper fumaroles were measured by thermocouple and radiometer (Extech 42545) (figure 15). Fumaroles to the W were not measured due to limited accessibility.

Figure 15. At Santa Ana, the location of fumarole measurements and the hot spring shown in the previous figure. View is towards the SW. Photo mosaic taken 5 July 2007 by Matt Patrick.

The seven largest southern crater fumaroles were measured along an E-W transect. The lower fumaroles emitted mainly water vapor, though some sulfur crystals and a weak sulfurous smell were present. Lower fumaroles temperatures ranged from 92.0 to 95.2°C, and thermocouple and radiometer measurements agreed very well (to within 3%). The upper fumaroles were diffuse and relatively weak, occurring in loosely consolidated tephra. The upper fumaroles emitted mainly water vapor and lacked sulfur deposits or sulfurous smell. Upper fumaroles temperatures ranged from 70.0 to 79.0°C, and thermocouple and radiometer measurements agreed well (to within 6%).

Information Contacts: Demetrio Escobar and Francisco Montalvo, Servicio Nacional de Estudios Territoriales, SNET, Km. 5 1/2 carretera a Santa Tecla y Calle las Mercedes, contiguo a Parque de Pelota, Edificio SNET, Apartado Postal ##27, Centro de Gobierno, El Salvador 2283-2246 (URL: http://www.snet.gob.sv/); Matthew Patrick and Anna Colvin, Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA.

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

December 1980 (SEAN 05:12) Cite this Report

Plume from inner crater fumaroles

Observations were made during a flight over the country. A moderate plume rose from a bank of fumaroles on the SE wall of the inner crater, very similar to its appearance in November 1978.

Information Contacts: R. Stoiber, S. Williams, R. Naslund, L. Malinconico, and M. Conrad, Dartmouth College.

December 1999 (BGVN 24:12) Cite this Report

Scientists visit the summit crater and sample the acid lake

The acid lake within the summit crater of Santa Ana was extensively sampled by a scientific team using a small boat on 28-29 January 2000. The ambient lake temperature was ~18.9°C with 1.01 pH. A fumarole field on the crater wall adjacent to the lake had a maximum temperature of 523°C. A hot spring vent near the fumarole field feeds warm water (80°C) into the lake. This new lake sampling allows comparisons with data taken in 1992-93.

Santa Ana, El Salvador's highest volcano, is a large stratovolcano immediately W of Coatepeque Caldera. The broad volcano summit is cut by several crescent craters. A series of parasitic vents and cones have formed along a fissure system that extends from near the town of Chalchuapa N of the volcano to the San Marcelino cinder cone on the SE flank.

Information Contacts: Demetrio Escobar and Marisa Orantes, Centro de Investigaciones Geotecnicas, Calle Antigua La Chacra, Costado Oriente de Talleres "El Coro", PP 109 San Salvador, El Salvador; Alain Bernard, BRUGEL, Université Libre de Bruxelles 160/02 50, Ave. Roosevelt, 1050 Brussels, Belgium; Bill Rose, Colleen Riley, and Keith MacPhail, Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA; Uwe Grunewald, Insitut fuer Geowissenschaften, Burgweg 11, 07749 Jena, Germany.

April 2001 (BGVN 26:04) Cite this Report

2000-2001 observations of glowing fumaroles and release of magmatic gas

Santa Ana's summit crater contains an acid lake that in 2000 was ~200 m in diameter with a maximum depth of 27 m; its volume was estimated at ~200,000 m³. The lake water has a composition typical of acid-sulfate-chloride lakes (table 1). Dissolved sulfates yielded delta³⁴S of 16.0, suggesting that a significant sulfur source is magmatic SO₂ gas. Several hot springs with 80°C temperatures lie along the shore of the lake and have compositions close to that of the lake waters. Fumaroles on the W side of the lake had a maximum temperature, recorded in January 2000, of 523°C.

Table 1. Compositions of Santa Ana's crater lake waters for 2000-2001. Samples were collected in 2000 during January (sample designation, SAL), July (SAN1), and August (SAN2), and collected in 2001 during February (SAP). Chemical concentrations are in mg/L. Courtesy of the authors.

The Centro de Investigaciones Geotecnicas (CIG) noted that during May-September 2000 the lake temperature increased from 19 to 30°C. On 15 February 2001, the temperature was 26°C and the pH was 0.8-0.9. Bubbling and increased gas emissions were observed in several areas of the lake. During February 2001, sulfur spherules in the lake caused the water color to change to a shade of milky yellow-brown. During May-September 2000, the composition of the lake was only slightly affected, suggesting that no major changes occurred within the hydrothermal system beneath the lake (table 1).

Beginning in August 2000, observers found the increasingly deleterious effects of acidic vapor and rainfall on vegetation in the area SW and N of the crater. Winds are dominated by NE trades, which generally drive the plume from the volcano over the rim of the crater to the S and W. By December 2000, more severe effects on the flora were reported, and an area of ~8-10 km² located S and W of the crater contained markedly discolored and defoliated vegetation. Brief periods of acidic rainfall were reported at Juayua, 13 km W of the crater. In January 2001, incandescent areas within the fumarolic region W of the crater lake were observed at night and may have been present earlier.

COSPEC measurements of the plume were made using the Guatemalan COSPEC on 8 and 9 February 2001. Tripod-based surveys were made from Cerro Verde (elevation ~2,000 m), 2 km S of the crater. Automobile-based traverses conducted along the Santa Ana - Sonsonate highway (5 km W of Santa Ana) were made on 9 February. These surveys resulted in an average SO₂ flux of 393 tons/day on 8 February and 244 tons/day on 9 February. During measurements of the plume from the Cerro Verde site, periods of noticeable puffing were observed, affecting the COSPEC measurements by a factor greater than 8.

Two new, permanent telemetered seismic stations, located 1.5 km SE of the crater and 5.5 km NW on Cerro Retiro, were installed by USGS/VDAP and CIG personnel in February 2001. During February, these stations did not detect abnormal seismicity beneath the volcano. A portable recorder located at Finca San Blas from about 20 January through 9 February registered little volcano-seismic activity apart from a few small fumarolic emissions in late January.

Interpretation: The changes since the summer of 2000 are apparently due to increased venting of a well-developed hydrothermal system through the lake, hot springs, and fumaroles. This hydrothermal system is venting substantially higher SO₂ (and presumably other gases) in an acidic plume blown by NE trade winds. The lack of seismic activity suggests that the hydrothermal activity increase was not driven by the arrival of new magma beneath the crater.

The SO₂ emission rate was high for a quiescent volcano that lacks an open vent. The SO₂ may evolve from a gas reservoir below the hydrothermal system, trapped by a hydrothermal cap (clay or silica) as a result of long term (centuries or millennia) crystallization of magma below the cap (Giggenbach and others, 1990). The recent increase in degassing may reflect fracturing or leaking of the hydrothermal cap.

Crater hazards. Reports suggest that significant phreatic activity could occur at Santa Ana, including ejected bombs and blocks, even without magmatic movement under the volcano. This eventuality, a potential extension of the pulsating SO₂ emissions, could make conditions unsafe in the crater region. Furthermore, unsubstantiated rumors from local residents and ambiguous geological observations (e.g., abundant dust-possibly settled ash-coating the upper surfaces of leaves of plants in the crater and on the upper slopes) have suggested to some that minor, rare phreatic eruptive events have already occurred.

January 2001 earthquake. At about 1135 on 13 January 2001, an earthquake with a magnitude of 7.6 and a depth of 60 km occurred off the El Salvadoran coastline, its epicenter (at 12.8° N latitude and 88.8°W longitude) lay ~150 km SE of Santa Ana volcano. The earthquake caused extensive damage and destruction throughout much of El Salvador. By 19 January the country was struck by over 660 aftershocks. According to information provided by the National Emergency Committee (COEN) on 17 January 2001, the death toll was put at 681, with 2,615 injured. Approximately 20,000 people moved into over 80 temporary shelters; 90,929 houses sustained damaged, with 24,759 destroyed; and 45,842 people had been evacuated.

News sources based on reports from local residents raised concern that the earthquake was the result of eruptions from Santa Ana volcano. This conclusion was almost certainly in error.

However, during a helicopter overflight on 17 January, James Vallance, an American volcanologist, believed he saw incandescence in the crater. During a subsequent overflight on 18 January, Vallance, Carlos Pullinger, and Demetrio Escobar observed that the incandescence came from glowing cracks in the fumarole field, which, as noted above, had measured temperatures exceeding 500°C. They noted that there was no new lava or magma visible in the crater to indicate recent eruptions. This conclusion was substantiated later by measurements with a portable seismograph, which failed to detect local earthquakes under the volcano.

References. Pullinger, C., 1998, Evolution of the Santa Ana Volcanic complex, El Salvador: Unpublished MS thesis, Michigan Technological University, 152 p.

Larde, J., 1923, El Volcan de Izalco, San Salvador.

Gutierrez, R.E., and Escobar, C.D., 1994, Crisis en la actividad del Volcan de Santa Ana (Ilamatepec), del 22 de Julio al 21 de Agosto 1992: San Salvador, Publicacion Especial, Centro de Investiagiones Geotecnicas, 13 p.

Giggenbach, W.F., Garcia, N., Londono, C., Rodriguez, L., Rojas, N., and Calvache, M., 1990, The chemistry of fumarolic vapor and thermal-spring discharges from the Nevado del Ruiz volcanic-magmatic-hydrothermal system, Colombia: J. Volcanol. Geoth. Res. 42, p. 13-40.

Information Contacts: Demetrio Escobar and Marisa Orantes, Centro de Investigaciones Geotecnicas (CIG), Calle Antigua La Chacra, Costado Oriente de Talleres "El Coro," PP 109 San Salvador, El Salvador; Alain Bernard, Université Libre de Bruxelles 160/02 50, Ave. Roosevelt, 1050 Brussels, Belgium; William L. Rose, Department of Geological Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA; Otoniel Matias and Gustavo Chigna, INSIVUMEH, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala; James Vallance, Department of Civil Engineering and Applied Mechanics, McGill University, Montreal, Quebec H3A 2K6, Canada.

September 2005 (BGVN 30:09) Cite this Report

Sudden eruption on 1 October 2005; thousands evacuated

This report discusses a 1 October 2005 eruption at Santa Ana (also called Ilamatepec) that sent a plume to 14 km altitude and led to initial estimates cited in the press of two deaths (perhaps from landslides), several injuries, and the evacuation of over 2,000 people. Observations of glowing fumaroles and release of magmatic gas during 2000-2001 were previously reported at Santa Ana (BGVN 26:04). Servicio Nacional de Estudios Territoriales (SNET) scientists noticed that between the summer of 2000 and April 2001 there was increased venting of a well-developed hydrothermal system through the crater lake, hot springs, and fumaroles, but these changes were not accompanied by detected seismicity, which was then taken to suggest that the increase in hydrothermal activity was not driven by the arrival of new magma beneath the crater. An ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) image from 3 February 2001 shows the volcano's setting well before the eruption (figure 1).

Figure 1. An ASTER image of Santa Ana from 2001 featured in one of several Earth Observatory reports. N is to the top left of the image and Santa Ana is the large, blunt-topped edifice closest to the left side of the image. In the color version of this image can be seen a tiny blue spot in the center of the inner-most crater?a crater lake (often called the lagoon). Behind Santa Ana is a large (7-km-diameter) lake inside the Coatepeque caldera. In the center is Izalco volcano, with dark-colored historical lava flows. Courtesy of NASA's Earth Observatory.

SNET reported that a sudden eruption at Santa Ana took place around 0820 on 1 October 2005. They estimated that it produced an ash-and-gas plume to a height of ~ 10 km above the volcano. According to the Washington VAAC, ash was visible on satellite imagery at an altitude of ~ 14 km. The US Air Force Weather agency provided images of the plume (figure 2).

Figure 2. Two images of a Santa Ana eruptive plume on 1 October 2005. (top) The plume at 1516 UTC; (bottom) the plume at 1650 UTC. Note that the label 'FL 460,' stands for 'flight level 460,' which is equivalent to an altitude of 46,000 feet or 14 km. Courtesy of the US Air Force Weather Agency.

Ash fell in towns W of the volcano, including in Naranjos, Nahuizalco, Juayúa, Ahuachapán (NW), and La Hachadura (at the border, ~ 40 km W, figure 3). SNET produced a graphic similar to an isopach map that showed near-source thicknesses provisionally to over 10 cm. The 10 cm isopach stretched ~ 5 km W; the 1 mm isopach, ~ 20 km W. The outermost isopach, presumably where measurable ash fell, was not closed; instead it was cut off along the Guatemalan border (~ 40 km to W of Santa Ana) and the caption said that ash would fall into valleys in Guatemala and to the sea. Volcanic blocks up to a meter in diameter fell as far as 2 km S of the volcano's crater. Lahar deposits were seen SE of the volcano. The alert level within a 4-km radius around the volcano's central crater was raised to Red, the highest level.

Figure 3. Graphic from SNET showing ashfall distribution from Santa Ana that appeared in the newspaper, La Prensa Grafica, following the 1 October eruption. N is upwards; Santa Ana lies ~ 40 km E of the Guatemalan border. This clearly transmitted the message that the ashfall was variable and W-directed over parts of El Salvador and neighboring Guatemala. The bottom of the graphic discussed the impact of the ash fall, including damage to specialty coffee farms. Credit: Ricardo Orellana, La Prensa Grafica.

According to news reports, two people were killed by landslides (possibly caused by heavy rain in the area) in the town of Palo Campana, and thousands of residents near the volcano were evacuated. As many as 1,400 hectares of crops were damaged by ash (1 hectare = 10,000 m²). News also mentioned other processes such as a flood of boiling mud and water, and molten rocks, some the size of small automobiles, that will be discussed in later reports. A several-minute-long video from the LPG Television website appears as both a hyperlink and an active file on our website. In addition to numerous interviews with evacuees, it shows a host of features including what appear to be the swaths left by previously inflated mudflows passed down steep-sided valleys.

Prior to the eruption, no significant change in seismicity was observed. On 3 October, after the eruption, seismicity fluctuated and small explosions occasionally occurred. Earthquakes associated with explosions were recorded. In addition, there was a decrease in the amount of SO₂ emitted from the volcano.

Strong degassing had been measured at the volcano since June 2004. An ash emission occurred on 16 June 2005, and a slight increase in seismicity and a significant increase in gas emission were measured from 27 July until at least 30 August. SNET also reported a significant increase in seismic activity at Santa Ana on the night of 27 August. A cluster of 17 volcano-tectonic earthquakes were recorded, with four located S of the volcano. Afterwards, continuous high-frequency tremor was recorded until at least 30 August. Observations made on 29 August revealed incandescent rocks in the fumarole field, effects attributed to hot gases heating the rocks to sufficient temperature to glow. A significant increase in SO₂ emission was recorded, and gas-and-steam plumes rose 500-1,000 m above the volcano's crater. As a safety measure, access to the volcano's crater was restricted.

From 27 July until the eruption on 1 October, seismicity and gas emissions were above normal levels, and Santa Ana was at alert level yellow. During the first week of September, tremor continued to be recorded, and on 2 September a cluster of at least eight small earthquakes occurred, which were not felt by local residents. Gas plumes rose to ~ 500 m above the volcano, and the SO₂ flux was over 1,000 metric tons per day during the first two weeks of September. Satellite imagery from 5 September showed a thermal anomaly.

Microseismicity increased significantly on 12 September. During a visit to the volcano on 8 September, larger areas of incandescence were visible at a field of fumaroles than during a visit on 29 August. Satellite imagery showed a thermal anomaly at the volcano on several days during the second week of September.

During 15-19 September gas plumes rose to ~ 500 m above the volcano, and the SO₂ flux reached a maximum of 3,320 metric tons per day on 16 September. Microseismicity remained at relatively high levels. No significant changes were seen at the volcano's crater when observed on 19 September in comparison to 13 September. Intense degassing continued and the crater lake (lagoon) remained a dark coffee color. Incandescence was visible inside some cracks.

During a visit to the crater on 21 September, observers noted that the lagoon had become greener and small rock landslides occurred in the field of fumaroles. Gas plumes rose to ~ 1 km above the volcano on 26 September.

Following the eruption of 1 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana during 5-11 October. Inclement weather prohibited ground and satellite observations, and sulfur-dioxide (SO₂) measurements during much of the report period. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater. Around 11 October, SO₂ measurements were around 600-700 metric tons per day. The alert level within a 5-km radius around the volcano's central crater remained at Red.

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv); Washington Volcanic Ash Advisory Center (VAAC), NOAA/NESDIS Satellite Analysis Branch (SAB), 5200 Auth Road, Camp Springs, MD 20746, USA; Charles Holliday and Jenifer E. Piatt, U.S. Air Force Weather Agency (AFWA)/XOGM, Offutt Air Force Base, NE 68113, USA; NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/NaturalHazards/); La Prensa Grafica and La Prensa Grafica Television, Final bulevar Santa Elena, frente a embajada de EUA, Antiguo Cuscatlán, La Libertad, San Salvador, El Salvador.

January 2006 (BGVN 31:01) Cite this Report

Post-eruption lahars but seismicity and SO₂ fluxes both often low

Previous comments regarding terminal phases of the 1 October 2005 eruption (BGVN 30:09) included: . "Following the eruption of 1 October, small explosions, degassing, and low-to-moderate seismicity occurred at Santa Ana during 5-11 October . . .. During an aerial inspection of the volcano on 11 October, no changes were observed at the crater."

Carlos Pullinger (Servicio Nacional de Estudios Territoriales, SNET) later noted that "The 1 October eruption only lasted about 1 hour. After that we had some small activity, probably associated [with] degassing on Sunday evening [2 October] and at about the same time the continuous rains produced the first of a series of lahars that affected the communities close to the shore of Coatepeque lake. During the rest of the week it was very difficult to know what was going on because of continuous rains and cloudy conditions."

Pullinger further noted that some eye witnesses said that they had observed a column on 2 October. SNET registered strong and continuous tremor during approximately 1900-2400 (local time) on 2 October. Much of this activity coincided with rain-induced lahars. Over 300 mm of rain fell on the volcano that day. Using both witness reports and seismicity, SNET inferred that on 2 October the volcano possibly generated strong degassing or even geyser-type activity. However, there was no confirmation of ashfall deposits from these or other post-1 October events.The same type of seismicity continued intermittently until 5 October, but with much less intensity than on 2 October. SNET could not tell if there was any volcanic activity related to these events, or if it was mainly lahars. After the 5th continuous tremor was not recorded.

Post-eruption behavior. SNET reported that, in general, following Santa Ana's 1 October 2005 eruption, seismicity was relatively stable and there were generally low-level gas emissions (figure 4).

Figure 4. A graph showing Santa Ana's SO2 flux (vertical bars) and average daily seismic amplitude (RSAM, solid line) during 15 August-31 December 2005. The eruption of 1 October 2005 is indicated with an arrow. Courtesy of SNET.

Storms on 12 October 2005 caused lahars that traveled E towards Lake Coatepeque (see ASTER image of the region in BGVN 30:09). On 22 October, a lahar was reactivated in the Potrero Arriba area, NE of the volcano. During 22-25 October, the volcano was subjected to increased tremor and a slight increase in seismicity associated with gas emissions. On 28 October volcanic activity appeared to increase slightly and sulfur-dioxide emission rates during 28 and 29 October averaged 257 metric tons per day. The Alert Level within a 5-km radius around the volcano's central crater was at Red, the highest level.

During the month of November 2005 seismicity, volcanic activity, and gas emissions all remained for the most part at relatively low levels. There were slight increases on 13, 17, and 26 November; but the 17 November increase was attributed to noise from strong winds. On 26 November only slight changes were noted in the color of the lagoon in the crater's interior, but gas emissions rose to ~ 300 m above the volcano. Small earthquakes occurred during November 2005, inferred to be associated with the fracturing of rocks and gas pulses. Sulfur-dioxide emissions were low during the first part of November, with 100 to 200 metric tons recorded daily, and during the latter part of November, with between 100 and ~ 1,500 metric tons recorded daily.

During December 2005, seismicity was above background levels. Observations of Santa Ana's crater on 28 December revealed that there were continuous emissions of steam and gas from the lagoon and fumaroles located within the crater (figure 5). Gas rose 200-500 m above the crater and drifted SW (figure 6). Small earthquakes occurred, but gas emissions rose to over ~ 2,500 tons per day (figure 4). The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Figure 5. A photo taken from the crater rim at Santa Ana showing steam and gas emissions from both the lagoon and fumaroles located within the crater. Courtesy of SNET.

Figure 6. A photo of Santa Ana showing the 28 December 2005 gas emission that rose 200-500 m above the crater rim. Courtesy of SNET.

From 30 December 2005 to early January 2006, seismic and steam emissions were moderate at Santa Ana. Seismicity was slightly above normal levels with small earthquakes occurring, which were interpreted as being associated with gas pulses. Low-level emissions of steam and gas from the lagoon and fumaroles within the crater remained the same as in December 2005. Gas rose 200-500 m above the crater and drifted SW. The sulfur-dioxide flux ranged between 180 and 1,476 metric tons per day. The Alert Level remained at Red, the highest level, within a 5-km radius around the volcano's summit crater.

Background. Santa Ana, El Salvador's highest volcano, is a massive, 2,381-m-high andesitic-to-basaltic stratovolcano that rises immediately W of Coatepeque caldera. Collapse of the volcano during the late Pleistocene produced a voluminous debris avalanche that swept into the Pacific Ocean, forming the Acajutla Peninsula. Reconstruction of the volcano subsequently filled most of the collapse scarp. The broad summit of the volcano is cut by several crescentic craters, and a series of parasitic vents and cones have formed along a 20-km-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro la Olla cinder cones on the SE flank. Historical activity, largely consisting of small-to-moderate explosive eruptions from both summit and flank vents, has been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 13 km to the E.

Information Contacts: Servicio Nacional de Estudios Territoriales (SNET), Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv/)

April 2007 (BGVN 32:04) Cite this Report

Lahars follow October 2005 eruptions; steam emissions

Our last report (BGVN 31:01) discussed post-eruption lahars following the sudden 1 October 2005 eruption (BGVN 30:09). This report contains two sections. The first section addresses regional processes such as vegetation loss, ash accumulation, and lahars on and beyond the E flank of Santa Ana (also known as Ilamatepec) to the shores of Lake Coatepeque. Those lahars began soon after the 1 October 2005 eruption. The information on these lahars chiefly came from a report (SNET, 2006) authored by El Salvador's Servicio Nacional de Estudios Territoriales (SNET).

The second section addresses monitoring and observations such as extensive steaming and drop in the surface elevation of the lake in the summit crater. Material for this section, primarily found on the SNET website, covers January-April 2006, when activity was fumarolic with no large eruptions. The 1 October 2005 eruption was possibly followed by a second one two days later on 3 October (SNET, 2006). A 3 October eruption was not mentioned in previous Bulletin reports.Carlos Pullinger explained that the evidence for the second eruption was tremor that day, but that could stemmed from other causes such as geysers in the summit crater lake, so the evidence for a 3 October eruption remains equivocal.

E-flank issues. October 2005 volcanism took place coincident with unusually high rains during tropical storm Stan (1-10 October 2005). On the E flank, the October 2005 eruptive episode killed extensive vegetation and left loose ash deposits covering the upper slopes (figure 7).

Figure 7. A November 2005 photo looking southward showing Santa Ana in the foreground, along with denuded, ash-laden vegetation. A wisp of steam escapes the summit crater, a basin hosting an acidic crater lake. Santa Ana's plumes and October 2005 ash deposits, coupled with other factors such as steep slopes, stress to vegetation, the lack of surviving permeable soils, and regional rainfall have led to a rash of new E-flank lahars. Peaks beyond Santa Ana include its satellitic cone Cerro Verde and then Izalco (sharp peak beyond the notch). Photo from SNET (2006).

Based on a rain gauge 5 km W of the crater (national meteorological station Los Naranjos), rainfall in October averages 193 mm; the yearly average is 2,155 mm. In the months prior to October 2006, rainfall at that station remained at normal values, always below 460 mm per month. In contrast, rainfall reached 865 mm during October 2006. During the peak of the storm, 3-6 October 2005, the Los Naranjos rain gauge collected more than 100 mm per day; the highest reading of 320 mm was on 5 October.

The lahars on Santa Ana's E slope consisted of both material from the October 2005 eruption as well as previous deposits. The first lahar seen by local witnesses took place on the night of 2 October 2005. It carried material up to 2 m in diameter. The lahars that produced most of the damage were those that occurred immediately after the eruption and reached a maximum thickness of 1.5 m. Other lahars descended later in the storm, persisting well into 2006.The 2006 rainy season did not generate damaging lahars, just heavy runoff with minor sediment. In all, SNET seismically registered 22 lahar events, all of which were confirmed by local residents. The communities used tractors used to keep the main drainages open and to build levees, which confined the lahars inside main drainage areas. The SNET website mentioned several lahar episodes during 2006. Some of these episodes occurred in May, June, and July 2006.

A large scallop in the topographic margin of Coatepeque caldera results in Planes de la Laguna (an area of ~ 10 km²), which was where lahars eventually deposited (figures 8 and 9). This area of less steeply sloped, and in places comparatively level, ground contains numerous coffee plantations and small settlements. The largest settlement is El Javillal (figure 8, adjacent Lake Coatepeque).

Figure 8. Lahars displayed as trains of heavy dots on a topographic base map of the E-central side of Santa Ana and the adjacent W side of Lake Coatepeque. (N is towards the top; light grid-lines are 1 km apart, so the distance from the summit on the W to the large lake on the E is ~ 6.5 km.) In general, the lahars descended from W to E. Coatepeque is a 7 x 10 km caldera and the series of dashed lines across the map indicate the caldera's steep-sided topographic margin in. Several caldera domes are labeled, including Cerro Pacho and Cerro Afate. Note the lahar entering the settlement adjacent Lake Coatepeque ("Caserío El Javillal"). From SNET (2006).

Figure 9. An E-W topographic profile with Santa Ana on the W across to the E side of Lake Coatepeque on the E. Dashed lines indicate the location of Coatepeque's caldera wall. From SNET (2006).

The upslope areas contained numerous channels carrying lahars (figure 8). Several kilometers into the caldera the channels merge as they cross the less steeply sloped Planes de Laguna. The channels eventually grow into two primary channels, La Mina on the S and El Javillal on the N (figure 10). The La Mina channel led directly towards the Cerro Pacho dome, where the lahars proceeded to branch into multiple routes (A, B, C, and D) before entering El Javillal (figure 11).

Figure 10. Annotated aerial photo at unknown date showing part of Coatepeque's Planes de Laguna, W of Santa Ana, taken looking roughly S. The view illustrates lahars in and around El Javillal.The lahars entered the area along two drainages (Quebradas La Mina and El Javillal), both flowing from right to left (arrows). Adjacent to the domes and settlements, the flow patterns become quite complex (as indicated by flow directions A, B, C, and D). Lake Coatepeque appears at the upper left. The steep caldera wall lies along the photo's margin from the upper center to right corner. The large circular dome is Cerro Pacho; the smaller dome to the right is Cerro Guacamayero. Photo from SNET (2006).

Figure 11. Photos showing October 2005 lahar deposits from Santa Ana in El Javillal. Deposits included lava blocks of differing sizes, and a mixture of soil, tree parts, mud, and water. Photos from SNET (2006).

Given the lack of soils and the state of vegetation, lahars were viewed as a potential ongoing hazard. To control lahars, SNET (2006) proposed excavating two channels from the vicinity of the domes to Lake Coatepeque, to carry sediment farther towards the lake. The proposed artificial channels are 2 m deep, with sides that slope at 45° outwards, and with a flat floor 5 m across. One proposed channel follows the S margin of the Cerro Pacho dome, the other follows a path similar to arrow A on figure 10.

Pullinger noted that the jocote de corona crop harvest was not affected because it came out just after the eruption. However, coffee was damaged wherever ash fell. Lahars did not directly hurt coffee plantations, but access roads were damaged and labor for harvesting was minimal, after much of the population had fled.

Monitoring. Moderate seismic activity and steam emissions continued during 2006. During 2006, seismicity was slightly above normal levels. Small earthquakes were interpreted as being associated with gas pulses.

Degassing continued in January 2006 with sporadic gas-and-steam emissions which rose approximately 200 m before dispersing. The SO₂ flux ranged between 163 and 1,578 metric tons/day.

On 2 February, there was an increase in seismicity, possibly related to an earthquake on the coast of Guatemala. From 1-7 February the SO₂ flux averaged 2,000 metric tons per day. A drop in the water level of the steaming, green-colored acidic lake in the summit crater revealed a local topographic high in the lake's center, which took the form of an irregular island (figure 12).

Figure 12. Photo showing the crater lake at Santa Ana volcano. The decrease in the water level has revealed an island of rocks and sediments that was previously covered by the crater lake. Photo taken on 17 February 2006 and provided courtesy of SNET.

Intense bubbling and fumarole activity during 27 February-23 March disturbed the lake's surface and made it difficult to assess the level of the water. During April, instability in the crater led to periodic landslides. One significant landslide deposited material in the SW section of the beach of the crater lake.

Reference. Servicio Nacional de Estudios Territoriales (SNET), 2006, Flujos de escombros en la Ladera Oriente del Volcán Ilamatepec, Departamento de Santa Ana: Perfil de Obras de Mitigacion, Enero de 2006, 12 p.

Information Contacts: Carlos Pullinger, Servicio Nacional de Estudios Territoriales (SNET), Alameda Roosevelt y 55 Avenida Norte, Edificio Torre El Salvador, Quinta Planta, San Salvador, El Salvador (URL: http://www.snet.gob.sv).

July 2007 (BGVN 32:07) Cite this Report

Two days visiting the crater lake and fumaroles during mid-2007

Researchers from Michigan Technological University (MTU) and Servicio Nacional de Estudios Territoriales (SNET) visited the crater of Santa Ana on 28 June and 5 July 2007 to measure crater lake and fumarole temperatures, and to carry out routine water sampling.

Crater lake. The crater lake appeared yellowish-green and had a maximum temperature of 57.5°C, measured by a thermocouple at the northern shore. The crater lake was observed to have shifted westward in position since the 1 October 2005 eruption, drowning the main pre-eruption fumarole field to the W and receding from its eastern border (figure 13). A subaqueous hot spring was observed in the center of the lake at the end of a peninsula of exposed sediments (figure 14). The hot spring exhibited episodic pulses of bubbling water about every 5 minutes.

Figure 13. The yellowish-green acid crater lake of Santa Ana volcano as seen when viewed on 28 June 2007 looking towards the N. Photo taken by Anna Colvin.

Figure 14. Hot spring emerging in the acid lake at Santa Ana as seen 5 July 2007. Episodic upwelling of whitish fluid radiated out from the base of the large rock in the center of the photo. View is towards the SW; note geologist for scale. Photo taken by Matt Patrick.

Fumaroles. Crater fumaroles were observed to the W and S of the crater lake, and weak fumaroles were also observed on the upper wall above the flat area and below the SW crater rim. The southern crater fumaroles and the upper fumaroles were measured by thermocouple and radiometer (Extech 42545) (figure 15). Fumaroles to the W were not measured due to limited accessibility.

Figure 15. At Santa Ana, the location of fumarole measurements and the hot spring shown in the previous figure. View is towards the SW. Photo mosaic taken 5 July 2007 by Matt Patrick.

The seven largest southern crater fumaroles were measured along an E-W transect. The lower fumaroles emitted mainly water vapor, though some sulfur crystals and a weak sulfurous smell were present. Lower fumaroles temperatures ranged from 92.0 to 95.2°C, and thermocouple and radiometer measurements agreed very well (to within 3%). The upper fumaroles were diffuse and relatively weak, occurring in loosely consolidated tephra. The upper fumaroles emitted mainly water vapor and lacked sulfur deposits or sulfurous smell. Upper fumaroles temperatures ranged from 70.0 to 79.0°C, and thermocouple and radiometer measurements agreed well (to within 6%).

Information Contacts: Demetrio Escobar and Francisco Montalvo, Servicio Nacional de Estudios Territoriales, SNET, Km. 5 1/2 carretera a Santa Tecla y Calle las Mercedes, contiguo a Parque de Pelota, Edificio SNET, Apartado Postal ##27, Centro de Gobierno, El Salvador 2283-2246 (URL: http://www.snet.gob.sv/); Matthew Patrick and Anna Colvin, Dept. of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA.