Small eruption on 21 November 2012; subsequent mild degassing and seismicity

A previous report on Tongariro covered the 6-7 August 2012 eruption and subsequent return to quiet (BGVN 37:07). Aside from a short-lived eruptive episode on 21 November 2012, GeoNet Data Centre reported only occasional mild degassing and small earthquakes occurred through August 2013. No further eruptive activity was reported through late 2014.

Activity during 2012. Degassing continued after the August 2012 eruption. Gas flux was relatively low on 29 August; sulfur dioxide emissions were 150 tonnes/day and carbon dioxide flux was 420 tonnes/day. Gas flights on 13 and 18 September revealed sulfur dioxide levels of 980 and 540 tonnes/day, respectively, and carbon dioxide flux of 1,340 tonnes/day on 18 September. The report noted that although the levels were lower than the 2,100 tonnes/day of sulfur dioxide and 3,900 tons of carbon dioxide measured on 9 August, shortly after the eruption, they had increased and were considered significant.

A field team visited the craters on 13 September and noted that Upper Te Maari crater has been widened and deepened by the 6 August eruption. They installed a new webcam pointed directly at the new Te Maari vents.

GeoNet researchers visited the Upper Te Maari Craters on 30 September to sample fumaroles, conduct a carbon dioxide soil gas survey, collect ejecta from the August eruption, and photograph the area (figures 9 and 10). The average carbon dioxide soil gas flux was lower than on 27 July; 24 sites had increased fluxes while 20 had decreased. The estimated soil gas emissions had decreased overall from about 5.8 to 2.5 metric tonnes per day.

Figure 9. View of Te Maari craters at Tongariro looking S on 30 September 2012. Courtesy of GeoNet.

Figure 10. View of Upper Te Maari Crater looking E, 30 September 2012. Courtesy of GeoNet.

On 12 October, GeoNet reported that gas plumes drifting downwind had been detected a hundred kilometers or more away. During the previous two weeks an odor attributed to venting was noticed in Manawatu (112 km S) and Hawke's Bay (120 km ESE). On 30 October the SO₂ flux was 154 tonnes per day and the carbon dioxide flux was 477 tonnes per day. Several teams of scientists visited the Te Maari Craters on 5 November to service portable seismometers (complementing four permanent installations), sample gas vents, and collect samples of ejecta. The GeoNet report noted that not many earthquakes had been recorded recently, and that the hottest gas vent was 235° C while the others ranged from 95-104°.

A small eruption at the Te Maari Craters occurred at 1325 on 21 November (figure 11), without precursory events, prompting GeoNet to raise the Volcanic Alert Level to 2 and the Aviation Colour Code to Red. A report at 1730 noted that the eruption appeared to be over; the Aviation Colour Code was lowered to Orange, signaling that ash was no longer being emitted.

Figure 11. An ash plume rose from the Te Maari Crater at Tongariro at 1330 local time, shortly after the 21 November 2012 eruption. Courtesy of Institute of Geological and Nuclear Sciences, Ltd.

The eruption occurred from the same area as the eruption on 6 August and lasted less than five minutes, although local seismic activity lasted about 15 minutes. GNS staff and hikers saw the eruption; an ash plume rose 3-4 km above the Upper Te Maari crater and produced ashfall across part of State Highway 46 and NE towards Turangi (21 km NE). Two small pyroclastic density currents were produced at the base of the column, to the W and N of the crater, and traveled a few hundred meters downslope. Later that afternoon gas-and-steam plumes drifted SE. On 22 November a sulfur gas odor was reported downwind in Manawatu (S) and Hawke's Bay (115 km ESE). A substantial amount of gas was emitted during 22-23 November. The Aviation Colour Code was lowered to Yellow on 23 November due to the absence of emitted ash. On 26 November GeoNet noted that no further volcanic activity had occurred since the eruption, gas flux had decreased, and seismic activity remained low.

Reports during 2013 and 2014. GeoNet reported on 14 February 2013 that no eruptive activity had been detected since the 21 November 2012 explosion. Steam-and-gas plumes rose from the Te Maari Craters, and had been unusually strong during recent weeks possibly due to weather conditions. On 25 March GeoNet reported continued quiescence, but with continuing steam-and-gas plumes. The Aviation Color Code was lowered to Green (second lowest on a 4 four-color scale) and the Volcanic Alert Level remained at 1 (on a scale of 0-5).

On 6 August 2013 small earthquakes were detected beneath Tongariro. These events were recorded only by a few seismic stations and were too small to be precisely located. GNS Science noted that the earthquakes could simply have been part of the background unrest typical of most active volcanoes, but they were of interest because there had been so few since November 2012 and potentially could have signaled changes inside the volcano.

The amounts of CO₂ and sulfur gases emitted from Tongariro had remained at low levels since the start of 2013 and were about half the amount produced after the November 2012 explosion. These conditions, and the small number and small size of recent earthquakes, were not sufficient to alter the unrest status of the volcano; the Volcanic Alert remained at Level 1; the Aviation Color Code remained Green.

On 23 December 2013 GeoNet reported that the volcano continued to remain quiet. A few small earthquakes were recorded on the northern flanks at a rate of 2-3 per month. Gas measurements showed SO₂ fluxes of about 10-15 tonnes/day, also a low level. Volcanologists also sampled the fumaroles on Te Maari that have been active since the 2012 eruptions; the main fumarole, which often provides strong steam plumes visible from Taupo, was emitting gases at over 400 °C.

A 10 February 2014 survey of carbon dioxide (CO₂) emissions near Red Crater (~4 km SSW from Te Maari) noted little change since a 2009 survey. In 2009 there was a calculated flux of 47 tonnes per day from the survey area. The result from the February 2014 survey indicated a flux of 39 tonnes per day; the difference was not considered significant. The temperature just below the ground surface was also similar to 2009 values.

Information Contacts: GeoNet, a collaboration between the Earthquake Commission and GNS Science (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Center, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/).

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

February 1974 (CSLP 19-74)

Steam and ash eruption in late January

Card 1790 (05 February 1974) Steam and ash eruption in late January

"Ngāuruhoe volcano went into eruption on 22 January 1974. In the evening it was erupting red-hot blocks to a height of several hundred feet and at times down the outer flanks. Aerial inspections on 23 and 24 January, while the eruption of steam and ash continued with varied but moderate intensity, showed no lava to be visible in the crater within at least 100 m of the rim. During this stage of the eruption, blue flames of burning sulphur were seen at a fumarole about 200 m down the N flank.

"Ash eruption continued, interspersed by occasional violent explosions which reached maximum intensity on 26 and 27 January, when a number of explosive eruptions momentarily enveloped the entire summit in ash and sent large blocks cascading down the sides. A number of hot avalanches were observed. By 28 January the frequency of the eruptions was decreasing.

"The activity is continuing but on a reduced scale. The volcano has been intermittently active on a minor scale over the past year but not very impressively. The present activity is more in the nature of a major eruption."

Information Contacts: J. Healy, New Zealand Geological Survey.

April 1974 (CSLP 19-74)

Two vents active within the crater during overflight on 29 March

Card 1839 (10 April 1974) Two vents active within the crater during overflight on 29 March

A scientific party of four plus two pilots left Paraparaumu on the morning of 29 March 1974 by Aero Commander to survey the White Island/Tongariro Volcanoes. The following is the report on the Ngāuruhoe observation:

"The aircraft made several passes over the eastern side of Ngāuruhoe from 1000 to 1015. There was a vigorous light grey ash column rising to approximately 4,000 m, and drifting WNW. Fine ash was falling from the ash cloud and there was a thin covering of recently deposited ash over the whole of the western side of Tongariro. Red Crater had lost its characteristic color and was a uniform light grey. Banks of cumulus cloud had built up on the NW of the mountain probably due to the presence of ash within the cloud. Elsewhere there was no cloud. Haze existed over the area between Ngāuruhoe and Lake Taupo, probably also due to dust particles from the eruption. There appeared to be two vents within Ngāuruhoe crater. Ash was being erupted from a northerly vent and white steam from a southerly vent. It was not possible to look into the crater to estimate the depth of the lava column. The volcano was in a Vulcanian phase of eruption with widespread distribution of fine ash. No blocks were observed to be ejected during the duration of the observations."

Information Contacts: J.W. Cole, Victoria University of Wellington, New Zealand.

March 1975 (CSLP 19-75)

Strong explosive ash eruptions during 12-19 February

Card 2119 (12 March 1975) Strong explosive ash eruptions during 12-19 February

Ngāuruhoe volcano erupted ash intermittently between 12 and 19 February in the first major activity since March 1974. Bad weather restricted observations, but several explosive ash eruptions occurred. An aerial inspection on the morning of 19 February revealed no obvious change in crater topography, and no new large ejecta on the summit. No ash emission was occurring. At about 1300 on 19 February (NZ Daylight Time) Ngāuruhoe suddenly commenced the most voluminous pyroclastic eruption since the 1954 lava flows. A coherent, strongly convoluting eruption column rose to 11,000 m above sea level (8,700 m above the summit), spreading out into an ash cloud reaching 14,600 m above sea level. Continual ash emission was accompanied by a series of visible air shock waves, loud explosions, and large pyroclastic avalanches down the slopes of the 600-m-high cone. Incandescent ejecta was clearly visible in the eruption column. Walking parties within 2 km of the crater were pelted with scoria, and one group was nearly over-run by hot avalanches which passed close to the walking tracks. This eruption was accompanied by vigorous volcanic tremor and lasted until about 1420 on 19 February, when ash emission ceased. Ash was blown 160 km to the NNW to fall on Hamilton City.

A series of violently explosive eruptions followed periods of quiescence later in the afternoon. Explosions were accompanied by large visible air shock waves, closely followed by ejection of very large blocks (many up to 20 m above the crater. Extremely loud and very sharp reports were heard close to the volcano, loud rumblings being heard at least 70 km distant. Very dense eruption slugs expanded violently above the crater to envelop the summit and collapse into large pyroclastic avalanches which flowed rapidly down the cone. Impacting blocks raised large dust clouds down the cone, and set fire to tussock grass on surrounding areas. Smaller blocks (up to 1 m across) were thrown 2 km to the N, forming 2-m-diameter craters near walking tracks on Mt. Tongariro. Sharp volcanic earthquakes accompanied each explosion, which were preceded by periods of low seismic activity. Incandescent ejecta was observed during the night of 19-20 February as activity waned.

Bad weather restricted observations on 20-21 February, but steam explosions and small mud flows were observed. A helicopter landing on the summit was made on 22 February. The summit was covered by large blocks of lava which had been plastic on impact, the largest measuring 27 m long. Part of the western crater wall had collapsed into the vent. The crater was no deeper than after March 1974, due to infilling by collapse of vent walls, and possibly upward movement of a lava plug. No liquid lava was apparent in the vent. Strong gas emission (including SO₂) prevented clear observations and measurements within the crater, which appeared to be between 50 and 100 m deep. A further explosive eruption was reported on 23 February.

Information Contacts: B.W. Marshall, University of Auckland, New Zealand.

July 1977 (Ref 1977)

Eruption columns rise 900 m

The NZGS reports that cigar-shaped eruption columns rose to 900 m above the crater on 4 July 1977 at 0800 and 1030.

Information Contacts: New Zealand Geological Survey.

May 1983 (SEAN 08:05) Cite this Report

Earthquake swarms

The following seismic data, from Balsillie and Latter (1985), replaces the first two sentences of the original report, which dealt only with 16 May seismicity. [Three swarms of A-type earthquakes were recorded during the first half of 1983. The first and smallest included 14 events to M 1.8 on 16-19 February. About 20 A-type and 4 B-type (including the largest, at magnitude 2.4) shocks occurred 1-7 April, plus two episodes of possible volcanic tremor (for 7 hours on 2 April and 10 hours on 4 April). The largest swarm began gradually on 7 May, peaked 14-16 May (maximum magnitude 2.1), and ended about 29 May, accompanied by 4 more episodes of possible volcanic tremor. During the strongest activity, 15-20 events of M >1.0 were recorded daily.]

On 10 May, NZGS personnel measured temperatures of the hot gases issuing from the crater bottom. Rockfalls from the overhanging E crater wall and growing talus fans had reduced the degassing area. Temperatures of 320-350°C were measured 1-2 m from the accessible edge of the hot area. These reflected a nearly steady decrease: 620° (1 July 1978), 520° (24 February 1979), 478° (25 June 1981), 418° (21 January 1982), 458° (10 June 1982), [349° (4 May 1983) and 105° (7 February 1985]. Ngāuruhoe last erupted 12-23 February 1975, when strong explosive activity sent eruption plumes to 10 km and pyroclastic flows moved down the flanks (Nairn and Self, 1978).

Reference. Nairn, I.A., and Self, S., 1978, Explosive eruptions and pyroclastic avalanches from Ngāuruhoe in February 1975: JVGR, v. 3, p. 39-60.

Further Reference. Balsillie, F.H., and Latter, J.H., 1985, Volcano-seismic activity at Ngāuruhoe during 1983: New Zealand Volcanological Record, no. 13, p. 66-71.

Information Contacts: W. Giggenbach, DSIR, Wellington.

May 1992 (BGVN 17:05) Cite this Report

Fumarole temperatures and gas chemistry unchanged from 1989; no significant deformation or seismicity

Fumarole temperatures (93.9 & 94.3°C) and preliminary gas chromatograph data collected on 7 April were unchanged since the previous fieldwork in March 1989. No significant deformation was evident. Seismicity has remained relatively low.

Information Contacts: P. Otway, DSIR Geology & Geophysics, Wairakei.

July 1993 (BGVN 18:07) Cite this Report

Fumarole temperatures decrease

The following ... is based on fieldwork in late March and early May 1993. "Activity in the crater of Ngāuruhoe has declined further over the past year and is at an all time low since records were first kept. Fumarole temperatures have decreased to 90°C after remaining stable at 94°C (boiling point for altitude) since 1986. No volcanic deformation was identified by surveys to points on the crater rim and at the base of the mountain."

Information Contacts: B. Christenson, I. Nairn, P. Otway, and B. Scott, IGNS, Wairakei.

May 1994 (BGVN 19:05) Cite this Report

Fumarole temperatures continue to decline; no deformation

Annual fieldwork was carried out on 30 March and 29 April 1994. Maximum fumarole temperatures had fallen to 78°C by the end of April. ... There was insufficient fumarole discharge for adequate sampling, and temperatures and pressures were at the lowest levels ever recorded. Except for minor landslide debris, no significant changes were noted in the Ngāuruhoe crater.

Tilt leveling surveys were carried out at the Tama Lakes (1.7 km SSW) and Mangatepopo (1.8 km NNW) locations on 30 March. Apparent tilt recorded at Tama Lakes during the previous 11 months represented 4 µrad of inflation, but was within the range of random fluctuations recorded since installation in 1978. At Mangatepopo approximately 14 µrad of tilt towards Ngāuruhoe (deflation) was recorded over the same period. This is ~2-3x the past noise level resulting from normal survey errors and seasonal movements. The most likely explanation, based on earlier experiences, is that two benchmarks near a walking trail have settled.

Repairs were made to the three highest crater rim stations on 30 March and two new stations were installed; two old stations are scheduled for removal after the 1995 survey. All six rim sites were surveyed for horizontal deformation on 29 April. Measurements were made by EDM and theodolite from 2 km N on Tongariro volcano. Relative movement vectors for the 1992-94 period at three stations were well within the normal noise range. Instabilities noted at the other sites resulted from various surface movements. Overall, there was no indication of recent volcanic deformation.

Geological mapping of the crater, N flank, and SW flank accomplished during these visits is part of the ongoing mapping project of the Tongariro complex.

Information Contacts: P. Otway, IGNS Wairakei.

May 1995 (BGVN 20:05) Cite this Report

No fumarolic activity in Ngauruhoe crater or deformation

Extremely low activity levels were found during an annual crater inspection and deformation survey on 11 May in the crater and at the base of Ngāuruhoe. Crater fumaroles failed to discharge gases: the first complete absence of activity ever previously reported there. However, the NE rim of the outer crater was steaming vigorously as in the past. Neither horizontal nor vertical deformation were of sufficient magnitude to suggest volcanic significance. The current level of crater activity is probably the lowest in recorded history.

Ngāuruhoe is the highest and most recent of more than a dozen composite cones that comprise the large Tongariro volcanic massif N of Ruapehu. Ten years ago, in February 1975, Ngāuruhoe produced its last eruption, an event that generated 10-km-high plumes and pyroclastic flows. In 1983 several seismic swarms were recorded.

Information Contacts: P.M. Otway, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.

December 2006 (BGVN 31:12) Cite this Report

Elevated seismicity at Ngauruhoe during May-October 2006

Ngāuruhoe is the youngest and highest volcanic cone (figure 1) of the Tongariro volcanic complex on the North Island of New Zealand. According to New Zealand GeoNet Project volcanologists, the number of small (less than magnitude 2), low-frequency earthquakes near Ngāuruhoe recorded by seismometers increased from less than five per day at the beginning of May 2006 to more than 20/day by the end of May. Typically, only a few earthquakes of any type are recorded in the vicinity of Ngāuruhoe each year. In 1983, 1991, and 1994 there were clusters of similar earthquakes recorded near Ngāuruhoe, but there have been very few recorded since then. Due to the increased seismicity, the Scientific Alert Level was raised to Alert Level 1 (some signs of unrest) on 6 June. Earthquakes of this type are commonly interpreted as being related to the movement of magma and/or volcanic gases.

Figure 1. Snow covered Mt. Ngāuruhoe as seen on 28 July 2006. Photo credit to University of Auckland Snowsports Club.

Earthquakes peaked in early June at about 50/day and then declined to about 10-20/day by the 14th, with the largest about magnitude 1. Seismic activity has remained elevated through the middle of December 2006. Initial observations suggested that hypocenters were 1-4 km deep, slightly N or E of the summit. By mid-June volcanologists had installed three additional seismographs around the base of Ngāuruhoe, including one that could be monitored in real-time. Between 14 June and 3 July the number of volcanic earthquakes recorded near Ngāuruhoe has varied between approximately 20 and 40 per day. Using data from the additional seismographs, volcanologists were able to refine the location of the earthquakes to within about 1 km of the surface beneath the N flank; the largest events were approximately magnitude 1. Elevated seismicity continued at up to 30 events/day through October 2006.

As of the last GeoNet report on 1 November, no other signs of unrest had been recorded. Multiple measurements showed that temperatures and volcanic gas concentrations have not changed since the increased seismicity began in May, and were similar to measurements made in 2003. Carbon-dioxide release through the soil (from degassing magma) is also similar to measurements in 2003. The maximum fumarole temperature near the summit is about 85°C. Reports of steaming in the summit area were investigated, but because no new features were seen that could have caused emissions, the sightings were attributed to clouds rather than volcanic activity.

Information Contacts: New Zealand GeoNet Project, a project sponsored by the New Zealand Government through these agencies: Earthquake Commission (E.C.), Geological and Nuclear Sciences (GNS), and Foundation for Research, Science and Technology (FAST) (URL: https://www.geonet.org.nz/); University of Auckland Snowsports Club, University of Auckland, New Zealand (URL: http://www.uasc.co.nz/).

January 2009 (BGVN 34:01) Cite this Report

High seismicity in January 2008; declined to background by mid-year

Our most recent report on Tongariro (BGVN31:12) discussed elevated seismicity during May-October 2006. The M < 2 long-period seismic event occurred near Ngāuruhoe, the youngest cone of the Tongariro volcanic complex.

Between 1 November 2006 and January 2008, elevated, low-level volcanic earthquakes continued at Ngāuruhoe. The number of events per day typically ranged between 5 and 30. Then, on 6 January 2008, the number of events per day began to increase, and by 9-10 January the number had shot up to 80 per day, before decreasing slightly. The larger events ranged between M 1.2 and 1.5.

In response to these changes, volcanologists from GeoNet visited on 17 January 2008 and measured gas concentrations, temperatures, and soil gas flux at the summit area of Ngāuruhoe. The resulting data were similar to measurements made in 2006-2007. The maximum fumarole temperature near the summit remained about 86°C. No other signs of unrest were found. The data suggested that the earthquakes were occurring within about 1 km of the surface beneath the N flank.

GeoNet noted that the number of volcanic earthquakes since mid-2008 has declined to background levels. Regular measurements of volcanic gas levels and the temperature of the summit gas vent have showed no changes over the previous two and a half years. Consequently, on 2 December 2008, the Alert Level was lowered from 1 to 0 (typical background activity). No thermal anomalies have been measured by MODIS/MODVOLC satellites (HIGP Hot Spots System) in the at least the past 5 years.

Information Contacts: New Zealand GeoNet Project, a collaboration between the Earthquake Commission and GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.geonet.org.nz/); Hawai'i Institute of Geophysics and Planetology (HIGP) Hot Spots System, University of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

July 2012 (BGVN 37:07) Cite this Report

Seismicity preceded phreatic explosion; associated rainfall-fed lahar

Elevated seismicity in July 2012 preceded a phreatic eruption at Tongariro on 6 August. The eruption ejected blocks of old lava from the crater area, and triggered a debris flow down a drainage on a flank of the volcano. Six days later, heavy rainfall remobilized some of the debris flow and generated a small flood/lahar that crossed a state highway. This report summarizes GeoNet alert bulletins and Taupo Civil Defense postings concerning the phreatic explosion and associated events (through 17 August 2012).

Precursory seismicity. GeoNet reported elevated numbers of volcanic earthquakes (M < 2.5) beginning on 13 July (figure 2). Seismicity then declined until 18 July, when volcanic earthquakes returned, increasing in magnitude and abundance through 20 July. The earthquakes were clustered between Emerald Lake and the SE shore of Lake Rotoaira at 2-7 km depth; a subset of the earthquakes were tightly clustered between Blue Lake and Te Maari (Te Mari) Craters within the same depth range (figures 2 and 3). As a result, the Volcano Alert Level was raised from 0 to 1 (on a scale from 0-5) and the Avation Colour Code was raised from Green to Yellow (on a four color scale; Green-Yellow-Orange-Red) on 20 July.

Figure 2. Seismicity at Tongariro during 11 July-6 August 2012, leading up to a phreatic eruption at 2350 on 6 August. Shown are the number of seismic events per day (107 in total, top panel), the depth at which each event occurred (and associated error bars, middle panel), and the magnitude of each event (bottom panel). Courtesy of GeoNet.

Figure 3. Mapped epicenters of seismic events recorded at Tongariro during 25 May-23 August 2012. A phreatic eruption occurred late on 6 August (see figure 2). During the precursory activity (late July and early August), seismicity concentrated between Emerald Lake (E) and Lake Rotoaira (R), with a cluster between Blue Lake (B) and Te Maari Craters (T). Courtesy of GeoNet.

By 23 July, GeoNet had deployed four portable seismometers and had sampled springs and fumaroles. They reported that provisional analyses of gas samples indicated a marked increase in volcanic gases above typical mixtures of hydrothermal and volcanic gas signatures (see subsection "Ash and gas analyses" for detail). By 31 July, GeoNet had also installed a GPS instrument to monitor any deformation.

Phreatic eruption. At 2352 on 6 August, a phreatic eruption occurred from a vent located within the Te Maari Craters area. An explosion generated seismic signals that lasted a few minutes, followed by a series of discrete small earthquakes during the next few tens of minutes. Within 35 minutes, GeoNet posted an Alert Bulletin announcing that ashfall had been reported; the Volcano Alert Level was raised to 2 and the Aviation Colour Code was raised to Red. Taupo Civil Defense responded by closing State Highways 1 and 46 (to the E and N of Tongariro, respectively).

Approximately one hour after the eruption, the Cooperative Institute for Meteorological Satellite Studies (CIMSS) observed in satellite imagery that an ash plume was drifting more than 50 km E from Tongariro (figure 4). They also reported that airports had cancelled flights at Gisborne (210 km ENE), Rotorua (120 km NNE), Taupo (60 km NE), and Palmerston North (135 km S).

Figure 4. A Visible Infrared Imaging Radiometer Suite (VIIRS) image acquired by the Suomi National Polar-orbiting Partnership (NPP) satellite at 1252 UTC on 6 August 2012 (0052 NZST on 7 August) showing an ash plume drifting more than 50 km E from Tongariro, after the phreatic explosion ~1 hour earlier. The windward end of the plume remains over the volcano, implying ongoing emissions. In the main image, the North Island of New Zealand is outlined in teal, and Rotorua is labeled to the N of the plume. In both images, the ash plume is labeled in yellow, and Tongariro is labeled in red, just W of the plume. Courtesy of CIMSS.

GeoNet reported that no volcanic tremor occurred before or after the event, and the Aviation Colour Code was reduced to Orange ~12 hours later (about 1200 on 7 August), and reduced again the next day (1500 on 8 August).

GeoNet conducted an observation flight on 8 August and photographed a variety of features discussed and illustrated in more detail below. They included: (1) a new vent area residing in a small crater, and associated steaming fissures, (2) a debris flow, and (3) impact craters.

The new vent(s) are located in the Upper Te Maari Craters area (figure 5a); low clouds prevented scientists from viewing areas higher than the lowest parts of Upper Te Maari Crater. Photographs of the area revealed a nearby steaming eruptive fissure, and more intense steaming in areas of ground that had been steaming prior to the eruption (figure 5b).

Figure 5. Photographs taken after Tongariro's 6 August 2012 phreatic eruption showing the Upper Te Maari vent area. In (A) the view is looking SE towards Upper Te Maari crater, with areas of fumaroles and a new vent on the outer edge of the crater (indicated by the arrow). (B) is looking NW showing vents along a fissure around the S side of Upper Te Maari crater. Upper Te Maari crater is indicated in each photo with the '*' symbol. Both photographs were taken during a morning observation flight on 8 August 2012. Courtesy of GeoNet.

A debris flow generated by the phreatic eruption comprised rock and soil debris that blocked a stream valley draining NW from the Te Maari Craters area (figure 6). GeoNet reported water ponding around the edges, and ash that had been remobilized into slurry flows. GeoNet noted that areas of the debris flow (especially in the upper sections) had eroded into the substrate (figure 6a).

Figure 6. Upper (A) and lower (B) portions of the debris flow generated by the 6 August 2012 phreatic eruption at Tongariro. The debris flow extended down a stream valley draining NW from the Te Maari Craters (steaming areas visible in the top-most portion of (A)). White asterisks in both photos indicate regions where the debris flow eroded into the substrate. White arrows in (B) indicate areas of newly ponded water at the edges of the flow. Courtesy of GeoNet.

The explosion ejected blocks of lava up to 2 km from the Te Maari Craters area, leaving impact craters in vegetation and ground surfaces (figure 7). All blocks were angular, and none were steaming or surrounded by burnt vegetation; GeoNet thus concluded that the blocks comprised old (non-juvenile) lava(s) ejected from the vent area.

Figure 7. Blocks of lava ejected up to 2 km from the Te Maari Craters area during the phreatic explosion on 6 August 2012 left impact craters in vegetation and ground surfaces (circled). Courtesy of GeoNet.

Ash and gas analyses. Textural analyses indicated that the ash emitted during the 6 August explosion contained little-to-no fresh (juvenile) lava, suggesting that the eruption was primarily steam driven (phreatic). GeoNet also reported that analysis of the fluorine content of the ash indicated that, except in the immediate vicinity of the volcano, there were little health or agricultural concerns.

For 9 August, GeoNet reported emissions of 2,100 tons/day of SO₂, 3,900 tons/day of CO₂, and 364 tons/day of H₂S from vents. Sulfur (H₂S) smells were reported in downwind locations during 11-12 August, and further reports were filed from the Manawatu region on 15 August. GeoNet attributed the sulfur odors to "passive degassing of magma beneath the surface of Tongariro."

Heavy rains spawn minor lahar. In concert with the 11 August release of a new, updated hazard map of Tongariro (figure 8), GeoNet warned motorists not to stop their vehicles along Highway 46 (N of Tongariro) due to hazards in that area. Following heavy rains the next morning, a minor flood/lahar crossed State Highway 46 near the S tip of Lake Rotoaira (at a location ~6 km W of Rangipo). According to the New Zealand Herald, a driver described 13-cm-deep mud crossing the road at 0830 that day. Scientists at GNS Science stressed that the lahar was not a direct result of an eruptive process, and a resident reported that the area was commonly washed out during heavy rains.

Figure 8. The updated hazard map released by GeoNet on 11 August 2012, which includes the 6 August vents seen in Tongariro's Te Maari Craters area (dashed black circle). As seen in the legend, the map indicates the locations prone to pyroclastic flows and mudflows (shaded yellow) and an area within a 3 km radius of the summit that is prone to gas hazards and flying rocks (shaded orange). Where the two zones overlap, the map is shaded red. Red dots indicate eruptive vents active in the last 27,000 years. Orange triangular "hut" icons indicate nearby mountain refuges. The larger poster conveys extensive hazard information. Note disclaimer (upper left) and wide range of agency collaboration (lower right) in map production. The highest peak of the complex is at Ngāuruhoe (near the SW corner of the map). As stated in the comment in the SW corner of the map, "Tongariro and Ngāuruhoe are parts of one active volcanic system." Courtesy of GeoNet.

Ten days after the phreatic explosion, GeoNet reduced the Volcano Alert Level to 1, stating that minor eruptive activity, required for Volcanic Alert Level 2, had ceased. The Aviation Colour Code remained at Yellow as of 24 August 2012.

Information Contacts: GeoNet, a collaboration between theEarthquake Commission and GNS Science (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Center, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/); Earthquake Commission (EQC), PO Box 790, Wellington, New Zealand (URL: http://www.eqc.govt.nz/); The Cooperative Institute for Meteorological Satellite Studes (CIMSS), a collaboration between theUniversity of Wisonsin-Madison, theNational Oceanic and Atmospheric Administration, and theNational Aeronautics and Space Administration, Space Science and Engineering Center, 1225 W. Dayton St., Madison, WI 53706 (URL: http://cimss.ssec.wisc.edu/); University of Wisconsin-Madison (UW-Madison) (URL: http://www.wisc.edu/); National Oceanic and Atmospheric Administration (NOAA) (URL: http://www.noaa.gov/about-noaa.html); National Aeronautics and Space Administration (NASA) (URL: http://www.nasa.gov/); New Zealand Herald (URL: http://www.nzherald.co.nz/).

July 2014 (BGVN 39:07) Cite this Report

Small eruption on 21 November 2012; subsequent mild degassing and seismicity

A previous report on Tongariro covered the 6-7 August 2012 eruption and subsequent return to quiet (BGVN 37:07). Aside from a short-lived eruptive episode on 21 November 2012, GeoNet Data Centre reported only occasional mild degassing and small earthquakes occurred through August 2013. No further eruptive activity was reported through late 2014.

Activity during 2012. Degassing continued after the August 2012 eruption. Gas flux was relatively low on 29 August; sulfur dioxide emissions were 150 tonnes/day and carbon dioxide flux was 420 tonnes/day. Gas flights on 13 and 18 September revealed sulfur dioxide levels of 980 and 540 tonnes/day, respectively, and carbon dioxide flux of 1,340 tonnes/day on 18 September. The report noted that although the levels were lower than the 2,100 tonnes/day of sulfur dioxide and 3,900 tons of carbon dioxide measured on 9 August, shortly after the eruption, they had increased and were considered significant.

A field team visited the craters on 13 September and noted that Upper Te Maari crater has been widened and deepened by the 6 August eruption. They installed a new webcam pointed directly at the new Te Maari vents.

GeoNet researchers visited the Upper Te Maari Craters on 30 September to sample fumaroles, conduct a carbon dioxide soil gas survey, collect ejecta from the August eruption, and photograph the area (figures 9 and 10). The average carbon dioxide soil gas flux was lower than on 27 July; 24 sites had increased fluxes while 20 had decreased. The estimated soil gas emissions had decreased overall from about 5.8 to 2.5 metric tonnes per day.

Figure 9. View of Te Maari craters at Tongariro looking S on 30 September 2012. Courtesy of GeoNet.

Figure 10. View of Upper Te Maari Crater looking E, 30 September 2012. Courtesy of GeoNet.

On 12 October, GeoNet reported that gas plumes drifting downwind had been detected a hundred kilometers or more away. During the previous two weeks an odor attributed to venting was noticed in Manawatu (112 km S) and Hawke's Bay (120 km ESE). On 30 October the SO₂ flux was 154 tonnes per day and the carbon dioxide flux was 477 tonnes per day. Several teams of scientists visited the Te Maari Craters on 5 November to service portable seismometers (complementing four permanent installations), sample gas vents, and collect samples of ejecta. The GeoNet report noted that not many earthquakes had been recorded recently, and that the hottest gas vent was 235° C while the others ranged from 95-104°.

A small eruption at the Te Maari Craters occurred at 1325 on 21 November (figure 11), without precursory events, prompting GeoNet to raise the Volcanic Alert Level to 2 and the Aviation Colour Code to Red. A report at 1730 noted that the eruption appeared to be over; the Aviation Colour Code was lowered to Orange, signaling that ash was no longer being emitted.

Figure 11. An ash plume rose from the Te Maari Crater at Tongariro at 1330 local time, shortly after the 21 November 2012 eruption. Courtesy of Institute of Geological and Nuclear Sciences, Ltd.

The eruption occurred from the same area as the eruption on 6 August and lasted less than five minutes, although local seismic activity lasted about 15 minutes. GNS staff and hikers saw the eruption; an ash plume rose 3-4 km above the Upper Te Maari crater and produced ashfall across part of State Highway 46 and NE towards Turangi (21 km NE). Two small pyroclastic density currents were produced at the base of the column, to the W and N of the crater, and traveled a few hundred meters downslope. Later that afternoon gas-and-steam plumes drifted SE. On 22 November a sulfur gas odor was reported downwind in Manawatu (S) and Hawke's Bay (115 km ESE). A substantial amount of gas was emitted during 22-23 November. The Aviation Colour Code was lowered to Yellow on 23 November due to the absence of emitted ash. On 26 November GeoNet noted that no further volcanic activity had occurred since the eruption, gas flux had decreased, and seismic activity remained low.

Reports during 2013 and 2014. GeoNet reported on 14 February 2013 that no eruptive activity had been detected since the 21 November 2012 explosion. Steam-and-gas plumes rose from the Te Maari Craters, and had been unusually strong during recent weeks possibly due to weather conditions. On 25 March GeoNet reported continued quiescence, but with continuing steam-and-gas plumes. The Aviation Color Code was lowered to Green (second lowest on a 4 four-color scale) and the Volcanic Alert Level remained at 1 (on a scale of 0-5).

On 6 August 2013 small earthquakes were detected beneath Tongariro. These events were recorded only by a few seismic stations and were too small to be precisely located. GNS Science noted that the earthquakes could simply have been part of the background unrest typical of most active volcanoes, but they were of interest because there had been so few since November 2012 and potentially could have signaled changes inside the volcano.

The amounts of CO₂ and sulfur gases emitted from Tongariro had remained at low levels since the start of 2013 and were about half the amount produced after the November 2012 explosion. These conditions, and the small number and small size of recent earthquakes, were not sufficient to alter the unrest status of the volcano; the Volcanic Alert remained at Level 1; the Aviation Color Code remained Green.

On 23 December 2013 GeoNet reported that the volcano continued to remain quiet. A few small earthquakes were recorded on the northern flanks at a rate of 2-3 per month. Gas measurements showed SO₂ fluxes of about 10-15 tonnes/day, also a low level. Volcanologists also sampled the fumaroles on Te Maari that have been active since the 2012 eruptions; the main fumarole, which often provides strong steam plumes visible from Taupo, was emitting gases at over 400 °C.

A 10 February 2014 survey of carbon dioxide (CO₂) emissions near Red Crater (~4 km SSW from Te Maari) noted little change since a 2009 survey. In 2009 there was a calculated flux of 47 tonnes per day from the survey area. The result from the February 2014 survey indicated a flux of 39 tonnes per day; the difference was not considered significant. The temperature just below the ground surface was also similar to 2009 values.

Information Contacts: GeoNet, a collaboration between the Earthquake Commission and GNS Science (URL: http://www.geonet.org.nz/); GNS Science, Wairakei Research Center, Private Bag 2000, Taupo 3352, New Zealand (URL: http://www.gns.cri.nz/).