Aoba

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  • 15.4°S
  • 167.83°E

  • 1496 m
    4907 ft

  • 257030
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Most Recent Weekly Report: 6 March-12 March 2013


According to observations by the Vanuatu Meteorology and Geohazards Department, a report from 6 March stated that the minor activity at Aoba that began in December 2012 was likely continuing. Satellite images acquired on 3 and 26 February detected substantial sulfur dioxide emissions. The Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


Most Recent Bulletin Report: May 2013 (BGVN 38:05)


Minor activity likely continuing into early 2013

In our May 2011 Bulletin we reported that there was increased degassing at Aoba (also known as Ambae) starting December 2009 through at least April 2010. This report summarizes notices pereiodically posted by the Vanuatu Geohazards Observatory (VGO) and covers the time interval from 4 June 2011 through 26 February 2013. The Vanautu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4.)

Observations on 4 June 2011 revealed that small explosions had been occurring from the crater lake and were accompanied by local ashfall around the crater. Some villagers in the N and W parts of the island had observed the explosions.

Based on analysis of data collected by the Vanuatu Meteorology and Geohazards Department (VMGD), the Vanuatu Geohazards Observatory reported that a small series of explosions from Aoba occurred on 10 July 2011. On July 11, VGO noted that there had been recent increases in activity from Ambae and that local earthquakes were volcanic. Satellite images collected by the Ozone Monitoring Instrument showed sulfur dioxide emissions. Photos showed that the volcano was quiet on 12 July 2011, although ongoing earthquakes were detected.

According to the VGO, Ambanga villagers reported that minor activity at Aoba began in December 2012. The OMI instrument detected strong gas emissions on 18 and 25 January 2013; the emissions continued at a lower level through 7 February. Field observations by the Geohazards team during 30 January-2 February 2013 confirmed that activity had significantly changed. Data retrieved from a monitoring station also confirmed ongoing activity. Satellite images acquired on 3 and 26 February 2013 detected substantial sulfur dioxide emissions.

No MODVOLC Thermal Alerts were issued in the previous year ending 16 July 2013.

Information Contacts: Vanuatu Geohazards Observatory (URL: www.geohazards.gov); and Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC 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://hotspot.higp.hawaii.edu).

Index of Weekly Reports


2013: February | March
2011: July
2010: May
2006: January | February | May
2005: November | December

Weekly Reports


6 March-12 March 2013

According to observations by the Vanuatu Meteorology and Geohazards Department, a report from 6 March stated that the minor activity at Aoba that began in December 2012 was likely continuing. Satellite images acquired on 3 and 26 February detected substantial sulfur dioxide emissions. The Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


6 February-12 February 2013

According to the Vanuatu Geohazards Observatory, Ambanga villagers reported that minor activity at Aoba began in December 2012. The OMI instrument detected strong gas emissions on 18 and 25 January; the emissions continued at a lower level through 7 February. Field observations by the Geohazards team during 30 January-2 February confirmed that activity had significantly changed. Data retrieved from a monitoring station also confirmed ongoing activity. The Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


20 July-26 July 2011

Based on analysis of data collected by the Vanuatu Meteorology and Geohazards Department (VMGD), the Vanuatu Geohazards Observatory reported that a small series of explosions from Aoba occurred on 10 July. Photos showed that the volcano was quiet on 12 July, although ongoing earthquakes were detected. On 18 July the Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


6 July-12 July 2011

On 11 July the Vanuatu Geohazards Observatory noted that there had been recent increases in activity from Aoba and that local earthquakes were volcanic. Satellite images collected by the Ozone Monitoring Instrument showed sulfur dioxide emissions. Observations on 4 June revealed that small explosions had been occurring from the crater lake and were accompanied by local ashfall around the crater. Some villagers in the N and W parts of the island had observed the explosions. The Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


5 May-11 May 2010

On 11 May the Vanuatu Geohazards Observatory noted that there had been recent increases in activity from Aoba, starting with reports that local villagers saw a plume over the island in December 2009. Fluctuating gas emissions seen in satellite imagery were also noted at that time. Satellite imagery on 11 April revealed that sulfur dioxide emissions increased to a rate of more than 3,000 tons/day. Scientists flew over Aoba and confirmed increased gas emissions. They also noted two fumarolic zones in the SE part of Lake Manaro that were surrounded by discolored water. The Vanuatu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4).

Source: Vanuatu Geohazards Observatory


24 May-30 May 2006

According to news reports, on 28 May aerial observations by scientists from the Department of Geology and Mines revealed that Lake Voui of Aoba volcano had changed from blue to red in color. Aoba remains at an Alert level 2, which means the crater area is restricted.

Sources: Commision of Volcanic Lakes (CVL); The Age News


15 February-21 February 2006

A news article reported on 16 February that the 5,000 people who evacuated their homes after increased activity began at Aoba on 27 November 2005 returned home after Department of Geology and Mines officials reduced the threat level from 2 to 1.

Source: Radio New Zealand International News


4 January-10 January 2006

According to a news report on 4 January, all of the 3,000 people displaced by the eruption of Aoba returned home. The eruption began on 27 November 2005.

Source: Radio New Zealand International News


7 December-13 December 2005

During 6-10 December, small-scale volcanic activity that began at Aoba (also locally called Mt. Manaro) on 27 November continued from active vents within Lake Vui, the summit crater lake. Molten material entered the crater lake and reacted with water, producing small explosive eruptions and a plume of steam and gas that rose to a height of 3.9-4.5 km (12,800-14,800 ft) a.s.l. The eruption built a cone around the active vents, enclosing them on three sides and forming an island ~200 m wide and 50-60 m high in the lake. There were two active vents; one emitted water, rocks, and mud, and the other emitted steam and gas. As of 10 December, the eruption had little effect outside of the crater lake (minor ashfall occurred only during the first 3 days after the eruption). During the report period, volcanic tremor was recorded at the volcano and a moderate sulfur-dioxide flux was measured (~2,000 tons per day). There was no evidence of ground uplift or cracking near the lake, suggesting that there was no large volume of magma close to the surface.

Sources: Vanuatu Geohazards Observatory; New Zealand GeoNet Project


30 November-6 December 2005

On 27 November the Aoba volcano (also locally called Mt. Manaro) erupted on Ambae Island in Vanuatu. There have been no casualties reported, but volcanic ash has blanketed houses and food crops. There are concerns that the ash may affect the respiratory systems of local residents and contaminate water sources. The government of Vanuatu has declared the island a disaster zone, and by 6 December 5,000 residents in at least 15 communities in high-risk areas had relocated to safe areas. White steam billowing to 1,500 m above the summit and 2,000 tons of ash per day falling on the island have been reported. The level of Lake Voui, one of the lakes in the summit crater, is now only 150 m below the rim, raising the possibility of floods or lahars if large volumes of lake water are ejected. A small cone is also growing within the crater lake.

Sources: International Federation of Red Cross and Red Crescent Societies (IFRC); Port Vila Press; Fairfax New Zealand Limited; News.com.au - News Limited


Index of Bulletin Reports


Reports are organized chronologically and indexed below by Month/Year (Publication Volume:Number), and include a one-line summary. Click on the index link or scroll down to read the reports.

07/1991 (BGVN 16:07) Caldera lake bubbling; burned vegetation

01/1995 (BGVN 20:01) Volcanic seismicity felt during 1-7 December

02/1995 (BGVN 20:02) Increased steam emissions and seismicity in early March; evacuation preparations made

08/1995 (BGVN 20:08) Crater lake exhibits convection cells and steaming as level drops

10/1998 (BGVN 23:10) Monitoring and water chemistry at Voui crater lake

08/2000 (BGVN 25:08) Increase in temperature and acidity at Lake Voui during April-August 2000

01/2001 (BGVN 26:01) Sustained elevation of Lake Voui's temperature indicates increased heat transfer

11/2005 (BGVN 30:11) New eruption begins on 27 November 2005 and builds cone in crater lake

12/2005 (BGVN 30:12) Landscape changes resulting from November 2005 eruption

01/2006 (BGVN 31:01) Crater-lake photos and satellite temperatures data show ongoing activity

05/2006 (BGVN 31:05) During May-June 2006, Lake Voui's water rapidly turns from blue to red

12/2006 (BGVN 31:12) Acidic gas emissions destroy vegetation; islet lake breached

05/2011 (BGVN 36:05) Increased degassing starting December 2009

05/2013 (BGVN 38:05) Minor activity likely continuing into early 2013




Bulletin Reports

All information contained in these reports is preliminary and subject to change.


07/1991 (BGVN 16:07) Caldera lake bubbling; burned vegetation

"Three anomalous 'boiling' areas with large bubbles and burned vegetation were observed at Lake Vui on 13 July, by P. Fogarty (Chief Pilot of VANAIR). This was the first time he had observed such a phenomenon, and he noted that the vegetation had still been green in May. An aerial survey of the two summit calderas was carried out (during a VANAIR flight) on 24 July. At that time, no strong degassing was visible, but 3 areas of discolored water (each several tens of meters in diameter) were noticeable in the crater lake. Burned vegetation was observed up to the crater rim, 120 m above the water. On 26 July, microseismicity in the caldera was very weak and without any volcanic characteristics.

"Although continuous weak solfataric activity occurs beneath Lake Vui (Warden, 1970), an anomalously strong SO2 degassing is believed to have occurred between May and July. This event was unnoticed by island residents, but since Aoba has been quiet for 300 years, vigilance for this kind of phenomenon must be improved. The existence of a summit caldera lake, numerous lahar deposits, and thick layers of ash (vesiculated and accretionary lapilli) demonstrate the hazards that would accompany renewed activity. Thus, as a precaution, a seismological station was installed in July on the SW flank of the volcano.

Reference. Warden, A.J., 1970, Evolution of Aoba caldera volcano, New Hebrides: BV, v. 34, p. 107-140.

Information Contacts: C. Robin and M. Monzier, ORSTOM, Nouméa, New Caledonia; M. Lardy and C. Douglas, ORSTOM, Vanuatu; C. Mortimer, Dept. of Geology, Mines, and Rural Water Supply, Vanuatu; J. Eissen, ORSTOM, France.
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01/1995 (BGVN 20:01) Volcanic seismicity felt during 1-7 December

Unusual seismicity was felt by island residents during 1-7 December 1994, with a maximum of seven small-to-medium events on the 5th. These volcanic events were of high-frequency and lacked individualized phases. At the suggestion of ORSTOM, the National Disaster Management Office (NDO) organized a helicopter reconnaissance on 7 December to inspect the volcano for evidence of possible eruptive activity. Activity at the Lake Vui crater and the fumarolic area on the shore of Lake Manoro was similar to that observed during previous aerial observations on 24 July 1991 and September 1993. At Lake Voui, small areas of hot and gaseous water were evident and the rainforest was completely burned around the crater. No large bubbles like those noted on 13 July 1991 (10 m in diameter) were observed (BGVN 16:07). An automated seismic alert station, with satellite transmission to Port Vila, will be installed near Lake Voui.

Information Contacts: M. Monzier, ORSTOM and Vanuatu Department of Geology, Mines and Water Resources, Vanuatu.
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02/1995 (BGVN 20:02) Increased steam emissions and seismicity in early March; evacuation preparations made

The following report, prepared on 17 March, is from volcanologists of the Institut Francais de Recherché Scientifique pour le Developpement en Cooperation, Office de la Recherché Scientifique et Technique Outre-Mer (ORSTOM), in Vanuatu and Ecuador.

Geological setting. Aoba is the largest basaltic shield volcano in the New Hebrides arc, with the base ~3,000 m below sea level, the summit ~1,500 m asl, and a volume of ~2,500 km3 (Eggins, 1993; Gorton, 1977; Robin and others, 1993). This rainforest-covered island lies in front of the d'Entrecasteaux collision zone, between the N and S Aoba Basins along an ~N50°E fracture transverse to the arc (figure 1; see Greene and others, 1994, for more information). Two concentric summit calderas, the largest 5 km in diameter (figure 2), enclose the central crater containing the 2-km-diameter Lake Voui (Vui) (figure 3). Numerous secondary craters and cones lie along the N50°E fracture, out to the extremities of the island, where previous magma-seawater interactions have produced several maars.

Figure 1. Bathymetric map of central Vanuatu showing the trench and direction of relative movement (arrows), Aoba, and other active volcanoes. Bathymetry is in kilometers. Modified from Greene and others, 1994.
Figure 2. Topographic map of Aoba (Ambae) Island, central Vanuatu. Areas of Recent phreatic explosion cones, spatter and scoria cones, and minor lava flows are approximated from a 1979 geologic map by the New Hebrides Geological Survey (!;100,000). Large dashed circles are 5- and 10-km radius lines. Topographic base map courtesy of C. Robin, ORSTOM.
Figure 3. Photograph of the summit of Aoba Island looking approximately NW. Two concentric calderas enclose the main central crater, which contains the 2-km-diameter Lake Voui (white). A black lake in the E part of the caldera, Lake Manaro, is in the foreground. The photograph was taken by a U.S. pilot during World War II, provided courtesy of C. Robin, ORSTOM.

Eruptive history. Lake Voui and the Manaro Ngoro summit explosion craters and cones formed ~420 years ago. The Ndui Ndui lava flows issued from the N50°E fissure ~300 years ago and reached the NW coast (Warden, 1970). Possible eruption-related lahars (or only secondary mudflows following heavy rains?) annihilated villages on the SE flanks of the island ~120 years ago, producing several casualties. An eruption possibly occurred in 1914 with ashfalls (?) and lahars (12 casualties). . . .

Robin and Monzier (1993, 1994) consider Aoba the most potentially dangerous volcano of the Vanuatu archipelago because of the wide distribution of very young deposits related to strong explosive eruptions. They also cite thick lahar deposits, the presence of Lake Voui, long repose periods (~300-400 years , Warden, 1970), strong degassing at the lake in 1991, and a population of ~3,500 within 10 km of the crater.

Activity in December 1994. Unusual seismicity was felt . . . during 1-7 December 1994 (BGVN 20:01). Records from ORSTOM seismic stations on Santo (70 km W) and Efate (260 km SSE) islands showed that peak activity lasted 24 hours with 13 events, the largest M 4.6 (Regnier, 1995). Crustal hypocenters were located under the S submarine base of the volcano. On 7 December, helicopter reconnaissance showed small areas of rising hot gaseous water at Lake Voui, similar to July 1991 and September 1993, but the rainforest appeared completely burned for up to several hundred meters around the crater. Despite the end of the seismic crisis, ORSTOM emphasized to the NDO the need to remain circumspect of the volcano. In mid-December, according to Robin and Monzier (1994), the following advice was given to NDO: "In the case of a resumption of volcanic activity in the summit area, it will be wise to evacuate, in a first phase, the population of coastal villages of the central part of the island (in a 10 km radius area surrounding Lake Voui) towards the less hazardous NE and SW extremities of the island. If the eruption occurs near these extremities, or spreads along fractures from central vents towards these extremities, then it might be necessary to evacuate part of the population to Santo or Maewo-Pentecost."

Activity in March 1995. According to a VANAIR pilot report on 1 March, Lake Voui was calm with gas emissions from numerous locations. The following day, the lake was steaming all over, bubbling up in the center, and its surface was rough; the pilot also reported black sediment ejections. Early on the morning of 3 March, people on Santo Island observed a gas plume rising 2-3 km above Lake Voui. Simultaneously, crustal seismicity similar to that in December 1994 was recorded.

On 4-6 March, ORSTOM geophysicists (M. Lardy and D. Charley) recorded strong continuous tremor at Ndui Ndui, ~9 km NW from the main crater. This tremor had a monochromatic signal with a 1.4 Hz mean frequency, several hours duration, and an amplitude of 3-4x background. Local observers were trained to watch the activity and the collaboration with VANAIR pilots was reinforced. As usual during the tropical summer, the top of the volcano was covered by thick clouds and rarely visible. However, on 5 March a gas plume was still visible above Lake Voui.

An island resident who stayed several days in the summit area during early March described lake levels and reported that soft mud had been blown all over the shores. On 4 and 6 March the surface of Lake Voui was at least 5.4 m higher than normal. However, on 9 March the lake was hot and steaming, and was ~4.8 m below the normal level, a change of ~10 m within 3 days. Tremor activity remained constant between 9 and 13 March, but with significantly less intensity than during 4-6 March. In addition, shallow, local micro-seismicity was noted since 11 March. During an aerial survey on 13 March, the entire lake was steaming and a strong sulfur smell had been reported around the summit area.

If activity increases in the central crater, magma-water interactions could produce falls of ash, dense lapilli, and accretionary lapilli, as well as pyroclastic flows, base surges and lahars. Lava flows may also erupt from flank fissures, N50°E or other orientations. The ORSTOM seismological team in Vanuatu will be reinforced on 17 March by the arrival of a new seismologist, and 5-7 portable seismic stations will be deployed around the island as soon as possible to improve the focal locations and delineate possible areas of attenuation. Also, a new permanent seismic station will be installed on Aoba. Daily contact is maintained between ORSTOM scientists in Vanuatu and Ecuador; the latter are prepared to move to Vanuatu if necessary.

Evacuation preparations. On 8 March, after discussions between ORSTOM geophysicists in Vanuatu and volcanologists now based in Ecuador, the following advice was given to the Vanuatu Government: ". . .The size of the gas plume observed above Lake Voui crater on March 3, 1995 probably means that magma is now rising within the volcano . . . . Thus, Aoba volcano is now dangerous and it seems necessary to envisage the evacuation of the population of coastal villages located in a 10 km radius area surrounding Lake Voui towards the less hazardous NE and SW extremities of the island . . . ."

Following this advice, Aoba Island was placed on alert and preparations for evacuations were begun. On 9 March, aircraft within a 4-km radius of Aoba up to 2.2 km altitude (7,500 feet) were restricted to scheduled flights and those approved by civil aviation or disaster office authorities. Correcting previous statements that evacuations had already started, the UNDHA reported on 17 March that villages within 10 km of the crater had been identified as threatened, and those within a 5-km radius had been placed on stand-by for immediate evacuation. Evacuation centers were identified, and all available government and several private ships were positioned to assist in a possible evacuation.

References. Eggins, S., 1993, Origin and differenciation of picritic arc magmas, Ambae (Aoba), Vanuatu: Contributions to Mineralogy and Petrology, v. 114, p. 79-100.

Gorton, M.P., 1977, The geochemistry and origin of quaternary volcanism in the New Hebrides: Geochimica et Cosmochimica Acta, v. 41, p. 1257-1270.

Greene, H.G., Collot, J.-Y., Stokking, L.B., and others, 1994, Proceedings of the Ocean Drilling Program, Scientific Results, 134: College Station, TX (Ocean Drilling Program).

Regnier, M., 1995, Rapport préliminaire sur la crise sismique d'Aoba de décembre 1994: Rapport ORSTOM, Port-Vila, 4 p.

Robin, C., and Monzier, M., 1993, Volcanic hazards in Vanuatu: Disaster Management Workshop by National Disaster Management Office, Republic of Vanuatu, 24-28 May 1993, Port-Vila, 8 p.

Robin, C., and Monzier, M., 1994, Volcanic hazards in Vanuatu: ORSTOM and Dept. of Geology, Mines and Water Resources of the Vanuatu Government report, 15 p.

Robin, C., Monzier, M., Crawford, A.J., and Eggins, S.M., 1993, The geology, volcanology, petrology-geochemistry, and tectonic evolution of the New Hébrides island arc, Vanuatu: IAVCEI Canberra 1993, Excursion guide, Record 1993 / 59, Australian Geological Survey Organisation, 86 p.

Warden, A.J., 1970, Evolution of Aoba caldera volcano, New Hebrides: BV, v. 34, no. 1, p. 107-140.

Information Contacts: C. Robin and M. Monzier (geologists) ORSTOM, Quito, Ecuador; M. Lardy (geophysicist), M. Regnier, J-P. Metaxian, R. Decourt (seismologists), and D. Charley (technical assistant), ORSTOM, Vanuatu; M. Ruiz (seismologist), Instituto Geofísico, Escuela Politécnica Nacional, Quito, Ecuador; J-P. Eissen (geologist), ORSTOM, France; BOM, Australia; UNDHA.
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08/1995 (BGVN 20:08) Crater lake exhibits convection cells and steaming as level drops

A pyroclastic explosion on the morning of 3 March 1995 generated a vapor-and-ash column ~3 km high (BGVN 20:02). Preliminary analysis of the resulting deposit did not reveal any juvenile material. On the morning of 5 March, a vapor plume rose ~500 m. It is possible that vapor plumes were emitted over several days, but were not observed at other times because of the thick clouds that usually hide the summit area. The center of activity on 3 March was between two small islands in Lake Voui (figures 4 and 5). Because of poor weather conditions, ORSTOM scientists were unable to observe the lake at close range until 13 March. Aerial photos taken on 20 March (figure 6) show the thermal contrast between Lake Manaro Lakua, formed by the accumulation of water in a low-lying area of the caldera, and Lake Voui, which fills the active crater. Convection cells, ~300-400 m in diameter, could be discerned within Lake Voui.

Figure 4. Sketch of the Aoba summit area, 3 March 1995, showing a very thick dense plume rising from Lake Voui. Based on images taken during an overflight by Vanair pilot Capt. Norman Samson; courtesy of ORSTOM.
Figure 5. Map of the Aoba summit area (after an IGN map) showing the lakes and landing site of the helicopter on 27 June 1995. Courtesy of ORSTOM.
Figure 6. Photograph of the Aoba summit looking approximately SE, showing the steaming Lake Voui in the active crater (foreground) and Lake Manaro Lakua (background), on 20 March 1995. Convection cells ~300-400 m in diameter can be seen in Lake Voui. Courtesy of ORSTOM.

A drop in the level of Lake Voui that began on 6 March (BGVN 20:02) was visible in photographs taken on 20 March. During another overflight on 6 April, the level of the crater lake had dropped by ~2 m. By the time of a 27 June landing on the NW island in Lake Voui (figure 5), the lake level had dropped ~5 m below the maximum, as determined by recent vegetation. Water temperatures measured around the most accessible parts of the island averaged 38-40°C, with highs of 63-67°C. The strongly acidic (pH 2.3) emerald-green lake was mostly obscured by clouds, but vapor emissions were visible between the island and the NW edge of the crater. A small island seen on 6 April in the N part of the lake had enlarged noticeably because of the drop in water level. The topography of the islands is steep towards the center of the lake and gentle towards crater edge. All of the trees on the island were dead, but other vegetation was beginning to reappear. Some blocks of dried mud (40-50 cm in diameter) ejected during the phreatic explosion at the beginning of March were still visible. Sulfur deposits were noted, and gas bubbles were coming from numerous fissures at the edge of the island.

A bathymetric survey of Lake Voui has never been done, but ORSTOM estimates that it has a volume of 50 million cubic meters. Although activity has declined in recent months, ORSTOM will maintain the current low-level alert status until approximately the end of November.

Information Contacts: M. Lardy, D. Douglas, P. Wiart, and K. Kalkaua, Centre ORSTOM, Port Vila, Vanuatu, and Bureau des Desastres Nationaux, P.M.B. 014, Port Vila, Vanuatu; M. Regnier and S. Temakon, ORSTOM et Departement des Mines et de la Geologie et des Ressources en Eaux, Port Vila, Vanuatu; Chief N. Tahi, Village de Nambangahake (Ndui-Ndui) Aoba, Vanuatu; C. Robin and M. Monzier, Centre ORSTOM, Quito, Ecuador J-P.Eissen, Centre ORSTOM de Brest, France; J-P. Metaxian, Universite de Savoie.
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10/1998 (BGVN 23:10) Monitoring and water chemistry at Voui crater lake

Following the 1995 phreatic explosion at Lake Voui (BGVN 20:02 and 20:08) a bathymetric survey of the crater lake was carried out. The 1996 survey confirmed the location of activity that had first been observed in 1992 on a SPOT satellite image. Monitoring of Lake Voui has continued through November 1998.

The average temperature over the whole 1 x 2 km surface of the lake (figures 7 and 8) stayed at ~30°C during November 1996-November 1998, due in part to constant streams of gas that issued from the main vent. As a comparison, in June 1995, three months after the phreatic explosion, the surface temperature was 45°C.

Figure 7. Schematic map of the summit area of Aoba volcano. Monitoring equipment includes: (1) a hydrophone at a depth of 10 m; (2) temperature sensors at a depth of 7 m; (3) power supply, electronics, and ARGOS satellite transmitter station; and, (4) a terrestrial data station measuring seismicity, heat flow, and rainfall. Courtesy Centre ORSTOM, Vanuatu.
Figure 8. Photograph of Aoba showing Lake Voui. Water discoloration marks the zone of activity. The power and transmitter station is located on the islet at the center. Lake Lakua is in the right background. Courtesy Centre ORSTOM, Vanuatu.

The ten major compounds dissolved in the lake's water have changed in concentration with time (table 1), but the samples, taken at the surface and at depths of 15-50 m, were consistent throughout the lake at any one time.

Table 1. Synopsis of the physical and chemical analysis of the waters of Voui lake derived from samples taken during 1995-98. Chemical constituents and ratios are given in mg/L. Courtesy Centre ORSTOM, Vanuatu.

    Date         pH    Conductivity (mS)   Temp.(°C)

    27 Jun 95    2.2       19.5               40
    01 Dec 95    2.3       18.9               35
    01 May 96    2.0       21.4               35
    25 Nov 96    1.5       28.8               30
    17 Jun 97    1.1       33.2               30
    30 Nov 97    1.3       36.9               30
    19 Jul 98    1.4       34.4               30

    Date         Cl     SO4    SO4/Cl   Mg     Mg/Cl

    27 Jun 95    3240   8560    2.6    1910    0.589
    01 Dec 95    2700   8350    3.1    1840    0.681
    01 May 96    2560   9900    3.9    2190    0.858
    25 Nov 96    2530   9510    3.8    2140    0.848
    17 Jun 97    2410  13130    5.4    2100    0.872
    30 Nov 97    2280  15260    6.7    2150    0.942
    19 Jul 98    2100  18010    8.6    1802    0.859

    Date         Ca      Na      K     Fe     Mn    Al

    27 Jun 95    288    1030    440    425    74    75
    01 Dec 95    193    1030    317    253    65    39
    01 May 96    230    1110    307    274    69    41
    25 Nov 96    174     810    219    246    64    --
    17 Jun 97    160     690    161    252    56    62
    30 Nov 97    130     520    113    304    54    60
    19 Jul 98     42     521     97    287    50    77

The average volume of the lake was estimated at 50 x 106 m3, but the level varied significantly. A drop of 275 cm in surface elevation was observed between June 1997 and October 1998. Rainfall varied between 500 and 600 cm/year in the summit area.

Monitoring was conducted twice per year, complemented by seismic recordings taken from a station set up in the dry lake bed of Ngoro. This system is similar to that used on Tanna Island, Vanuatu (BGVN 21:08). The range of monitoring equipment in place on Aoba since 1996 was extended in October 1998 by the installation of an acoustic recording station (0.1-150 KHz) and a device for continuous measurement of lake-water temperature. The data are relayed through an ARGOS satellite transmitter. Identical stations have been set up on Kelut in Indonesia and at Lake Taal in the Philippines.

Information Contacts: Michel Lardy, Inès Rodriguez, Douglas Charley, and Pascal Gineste, Centre ORSTOM, P.O.Box 76, Port-Vila, Vanuatu (Email: lardy@vanuatu.orstom.fr); Michel Halbwachs, and Jacques Grangeon, Université de Savoie, Campus Scientifique, F3376, Le Bourget du Lac, Cédex France (Email: michel.halbwachs@univ-savoie.fr); Janette Tabbagh, Centre de Téléobservation Informatisée des volcans, CNRS-CRG, Garchy, France (Email: j.tabbagh@garchy.cnrs.fr).
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08/2000 (BGVN 25:08) Increase in temperature and acidity at Lake Voui during April-August 2000

Since phreatic eruptions occurred at Voui crater lake in March 1995 (BGVN 20:02 and 20:08) the lake has been closely monitored. No reports of activity were received after October 1998 (BGVN 23:10) until Lake Voui's temperature and acidity increased above normal levels during April through August 2000. Charlie Douglas and Sandrine Wallez reported that in mid-April 2000 the temperature at Lake Voui was ~27°C, but by August it had increased to 35.8 °C (figure 9), which was the highest temperature recorded since they began monitoring the lake in 1998. They also reported that the water's acidity increased. Water analysis conducted on 15 June indicated that the increases were the result of an injection of fumarolic gases into the lake, perhaps related to ascent of new magma.

Figure 9. Water temperature changes in Lake Voui at Aoba during October 1998 to July 2000. Courtesy of Michel Lardy and Michel Halbwachs.

Information Contacts: Stromboli On-line, maintained by Jürg Alean and Roberto Carniel (URL: http://stromboli.net); Charlie Douglas and Sandrine Wallez, Geohazard Mitigation Section, Department of Geology, Mines, and Water Resources of Vanuatu (URL: http://www.sidsnet.org/pacific/sopac/members/vu.html); Michel Lardy and Michel Halbwachs, Institut de recherche pour le développement (IRD), P.O. Box 76, Port Vila, Vanuatu (Email: ird@vanuatu.com.vu).
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01/2001 (BGVN 26:01) Sustained elevation of Lake Voui's temperature indicates increased heat transfer

Voui crater lake's temperature and hydro-acoustic signals are measured continuously by an automated station that transmits in real time via satellite (BGVN 23:10). Recent measurements revealed heavy activity under the lake during March-June 2000 (BGVN 25:08), when the estimated 50 x 106 m3 volume of water rapidly increased in temperature by more than 7°C (figure 10).

Figure 10. At Aoba volcano, recorded variations in crater lake water temperature (top) and relative amplitude of ultrasonic signals (bottom) during 20 December 1999-18 June 2000. During the time intervals delineated by dashed boxes, rapid temperature increases correlated with larger amplitude ultrasonic signals. Courtesy of Michel Lardy.

The increase was accompanied by acoustic signals covering a wide range of frequencies (figure 10, bottom). Those in the audible band (> 100 Hz) were thought to be associated with the emission of gas bubbles and an increase in submarine fumarolic activity. Those in the ultrasound band (30-190 kHz) could stem from fluids circulating within the hydrothermal zone beneath the lake (figure 11).

Figure 11. Schematic long-axis cross-section of Aoba's Lake Voui indicating both volcanic processes (inferred magma chamber, groundwater convection, heat input, degassing, etc.), monitoring instruments, and communication links. After a cross-section composed by Opigez éric and provided courtesy of Michel Lardy.

A consistent first-order rise in water temperatures persisted through December 2000 (figure 12). Despite seasonal variations in air temperature and the cooling effect of heavy tropical rainfall (~5 m/yr), Lake Voui's temperature remained stable at ~36°C as of January 2001. The preceding rise and sustained high temperature indicate continued heat transfer from the bottom of the lake. The effect appears more substantial than the heating seen between 1996 and 1999, when water temperature averaged ~30°C.

Figure 12. Aoba's Lake Voui air and water temperatures during October 1998-January 2001. Courtesy of Michel Lardy.

Information Contacts: Michel Lardy, Institut de Recherche pour le Développement (IRD), Centre d"Ile de France 93143 Bondy Cédex, France (Email: lardy@bondy.ird.fr); Michel Halbwachs, Université de Savoie, BP1104, F 73376 Le Bourget du Lac Cédex, France (Email: michel.halbwachs@univ-savoie.fr); Jeanne Tabbagh, Université Pierre et Marie Curie, Départment de géophysique appliquée, 75252 Paris Cédex O5, France (Email: tabbagh@ccr.jussieu.fr); Douglas Charley, Department of Geology, Mines, and Water Resources, PMB01, Port-Vila, Vanuatu, Oceania (Email: charley@vanuatu.com.vu).
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11/2005 (BGVN 30:11) New eruption begins on 27 November 2005 and builds cone in crater lake

A new eruption began on 27 November 2005 when vapor plumes and ash columns were observed originating from Lake Voui, a crater lake at the summit of Aoba (figure 13). The volcano is also referred to locally as Manaro or Lombenben. Prior to this activity, the most recent reported volcanism consisted of phreatic explosions from the lake during March 1995 (BGVN 20:01, 20:02, and 20:08). Bathymetry conducted by ORSTOM in 1996 showed that the vent feeding gases and magma into Lake Voui had a depth of about 150 m and a diameter of about 50 m. The volume of water in the lake (1 x 2 km) totals some 40 million cubic meters, with a mean pH of 1.8. Lake Voui and the Manaro Ngoro summit explosion craters and cones formed ~ 420 years ago (figure 14). Lake Manaro was formed by the accumulation of water in a low-lying area of the Manaro summit caldera.

Figure 13. Map showing the location of volcanoes, including Aoba, in Vanuatu. Open triangles indicate submarine volcanoes. Modified from a map by IRD.
Figure 14. Digital image of Aoba created by combing shading and color coding of topographic height. The shading indicates direction of the slopes; NW slopes appear bright, while SE slopes appear dark. Color coding shows height, with green at the lower elevations, rising through yellow and tan, to white at the highest elevations. The flattened-looking summit shows that the newest crater is actually nested within older, larger craters. Elevation data used in this image were acquired by the Shuttle Radar Topography Mission (SRTM) aboard the Space Shuttle Endeavour, launched on 11 February 2000. Annotations added by Smithsonian editors. Courtesy of NASA.

Starting on 3 December a team of volcanologists from the Vanuatu Department of Geology, Mines, and Water Resources (DGMWR), the French Institut de recherche pour le développement (IRD), the New Zealand Institute of Geological & Nuclear Sciences (GNS), and New Zealand's Massey University began collaborating on observations and monitoring. The amplitude of tremor recorded by DGMWR instruments from 30 November to 3 December was lower than during the March 1995 activity.

Scientists who visited the lake on 4 and 5 December (figures 15 and 16) observed a similar style of eruptive activity on both days, but some individual explosions appeared larger on the 5th. It was not possible to reach the lake to collect a water sample. There appeared to be two active vents, side by side, in the lake. One was producing eruptions of mud, rocks, and water, and the other appeared to be the source of the large continuous steam plume rising above the crater; the plume did not contain ash. There were no reports of ash falling on the island since the start of the eruptions the previous week. The team estimated that the cone being built in the lake, at an estimated height of more than 20 m on the 4th, was about 70% complete around the active vents, and grew 5-10% higher between 4 and 5 December. Continuous tremor was recorded during this time, and the level of eruptive and seismic activity seemed to be fairly stable.

Figure 15. Photograph showing a telephoto view of an explosive eruption from Lake Voui at Aoba, 4 December 2005. View is approximately towards the east from the crater rim. Courtesy of Philipson Bani, IRD.
Figure 16. Photograph showing an explosive eruption from Lake Voui at Aoba on 4 December 2005. View is approximately toward the E from the crater rim. A large steam plume can be seen rising above the darker zone containing pyroclastic material. Three small islands formed prior to this eruption can also be distinguished, with the active vent area closest to the western-most island. Courtesy of Philipson Bani, IRD.

Cloud cover and rain prevented a visit to the lake on 6 and 7 December. Earthquake recorders from the GNS were installed at the Provincial Centre at Saratamata, the Longana Peoples Centre (Lovonda village), and at Tahamamavi ("place of warm sea") (figure 17). On 7 December, a final recorder from the IRD was installed near Nduidui on the SW side of the island. Over 6-7 December continuous moderate-level volcanic tremor was recorded, with no significant change in its level; there was no other significant seismic activity.

Figure 17. Hazard map of Aoba, showing risk areas, infrastructure, and settlements. See Cronin and others (2004) for additional details. Map produced by the United Nations, OCHA-ROAP Information Management Unit, 2 December 2005.

On 8 December, the group noted that small-scale eruptions continued in Lake Voui, building a volcanic cone in the lake and producing a tall (2.4-3.0 km) steam-and-gas plume. Afternoon observations showed the cone growing taller and surrounding three sides of the active vents. However, the cone was not complete on its E side, allowing lake water to react with the rising magma. Though the resulting explosions became further apart and slightly larger, the total energy involved appeared similar to 4-5 December. There continued to be two active vents, one producing the small explosions, and the second the steam and gas emissions. Seismic recorders continued to record volcanic tremor, but very few local earthquakes. No volcanic ash was present in the plume. The eruption had no immediate effect beyond Lake Voui. The Volcanic Alert Level remained at Level 2. The level of seismic activity seemed to be stable. No other significant seismic activity was recorded.

While departing by air on the evening of 8 December, the group clearly saw the active vents (figure 18). The cone had grown to the W, joining and partly burying one of the old islands. All eruptions occurred from inside the cone. The largest individual eruptions threw material 150-200 m above the lake. There was also a gas-and-steam vent present within the cone, W of the other vent. The level of the lake appeared unchanged.

Figure 18. Aerial photograph showing a steam plume rising from Lake Voui at Aoba, 8 December 2005. Courtesy of Forces Armées en Nouvelle Calédonie (FANC).

On 10 December, the small-scale volcanic eruption continued from active vents within the summit crater lake (Lake Voui). Molten material entered the crater lake and reacted with the water to produce small explosive eruptions and a plume of steam and gas. The eruption built a cone around the active vents, enclosing them on three sides, forming an island about 200 m across and 50-60 m high. There were two vents, one erupting water, rocks and mud, and the other producing a tall column of steam and gas. The eruption had little effect outside the crater lake (minor ashfall occurred only in the first three days of the eruption). Five days of seismic recordings show a moderate level of seismic activity (mostly volcanic tremor).No change was noted in the level of Lake Voui, and there was also no evidence of ground uplift or fractures near the lake.

Sulfur dioxide measurements. SO2 data collected using a DOAS spectrometer on the Islander planes of Unity Air Lines (3 December) and Air Vanuatu (5 December). On 3 December the flux was 32.6-33.6 kg/s (~ 2,900 metric tons/day). By 5 December the flux had decreased about 25%, to 24.7-26.4 kg/s (~ 2,300 metric tons/day). SO2 was clearly detected by the OMI (ozone monitoring instrument) sensor on the NASA Aura satellite (figure 19). One measurement of the volcanic gas output on 10 December showed a moderate level of sulfur dioxide (SO2) gas (about 2,000 t/d) from the active vents.

Figure 19. SO2 data from the Ozone Monitoring Instrument (OMI) on the Aura satellite, 5 December 2005. Courtesy of NASA, the KNMI MOI Science Team, and Simon Carn, University of Maryland-Baltimore County.

Lake temperatures. A monitoring station for continuous measurements of water temperature at Lake Voui was installed in October 1998. The station used a satellite ARGOS transmission system and recorded the last heating episode of 2001 (figure 20), but failed after three years due to the harsh acid environment. ASTER thermal infrared images can also be used for monitoring lake surface temperatures, and Aoba has a freshwater lake (Manaro Lakua) which can be used to remove the seasonal/diurnal variations in atmospheric temperatures. Unfortunately, the top of the volcano is frequently covered by clouds and few ASTER images are exploitable. The most recent ASTER image clearly showing both lakes was collected on 9 July 2005. Difference in temperatures between lake Voui and Lakua was 4.0°C, slightly above background values during 2002-2003. Maximum background temperatures measured with ASTER during the September 2002-October 2005 were at 26.3°C. The last ASTER images before the eruption, on 5 October 2005, showed no unusual temperatures at Lake Voui.

Figure 20. Temperature data from Lake Voui at Aoba, October 1998-December 2005, from in-situ measurements, ASTER satellite imagery, and MODIS satellite data. Delta T represents the thermal anomaly calculated as the temperature differences between the two lakes. The figure includes the first post-eruption ASTER data (24 December 2005). ARGOS data from Michel Halbwachs (Université de Savoie) and Michel Lardy (IRD). Courtesy of Alain Bernard.

MODIS satellites have a more frequent coverage than ASTER but their spatial resolution is only 1 km. The surface area of Lake Voui (2.1 km2) is too small for an accurate measurement of lake temperature, but MODIS can detect rough temperature changes or an increased thermal anomaly. The MODIS pixel footprint is about 1 km along track and 2 km across track, so the measured temperatures are a mixed signal corresponding to the lake and some signal from the adjacent tropical forest (much colder than the lake at night at this elevation). MODIS SST imagery showed a strong thermal anomaly on 21 November 2005 (figure 20). Approximate lake temperatures, likely a minimum, were 30.4°C on 20 November and 29.5°C (Terra)/ 31.4°C (Aqua) on 21 November. On 25 November the temperature jumped to about 42°C.

Reference. Cronin, S.J., Gaylord, D.R., Charley, D., Alloway, B.V., Wallez, S., and Esau, J.W., 2004, Participatory methods of incorporating scientific with traditional knowledge for volcanic hazard management on Ambae Island, Vanuatu: Bulletin of Volcanology, v. 66, p. 652-668. (URL: http://www.proventionconsortium.org/files/tools_CRA/CS/Vanuatu.pdf)

Information Contacts: Esline Garaebiti, Douglas Charley, Morris Harrison, and Sandrine Wallez, Department of Geology, Mines, and Water Resources (DGMWR), Port-Vila, Vanuatu (Email:esline@vanuatu.com.vu); Michel Lardy, Philipson Bani, Jean-Lambert Join, and Claude Robin, Institut de recherche pour le développement (IRD), BP A5, 98 848 Nouméa CEDEX, New Caledonia (URL: http://www.mpl.ird.fr/suds-en-ligne/fr/volcan/vanu_eng/aoba1.htm, Email: lardy@noumea.ird.nc, bani@noumea.ird.nc, join@noumea.ird.nc, crobin@cec.uchile.cl); Brad Scott and Steve Sherburn, Institute of Geological & Nuclear Sciences (GNS), Wairakei Research Center, Taupo, New Zealand (Email: b.scott@gns.cri.nz, s.sherburn@gns.cri.nz); Shane Cronin, Institute of Natural Resources, Massey University, Palmerston, New Zealand (Email: s.cronin@massey.ac.nz, k.nemeth@massey.ac.nz); Alain Bernard, IAVCEI Commission on Volcanic Lakes, Université Libre de Bruxelles, Brussels, Belgium (URL: http://www.ulb.ac.be/sciences/cvl/aoba/Ambae1.html); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); United Nations, Office for the Coordination of Humanitarian Affairs (OCHA), Regional Office for Asia and the Pacific.
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12/2005 (BGVN 30:12) Landscape changes resulting from November 2005 eruption

As previously reported (BGVN 30:11), a new eruption of Aoba began on 27 November 2005 when vapor plumes and ash columns were observed originating from Lake Voui, a crater lake at the summit. Activity continued into early January, building a large cinder cone in the west-central part of Lake Voui (figure 21). The new cone also contained its own crater lake.

Figure 21. A N-looking view of Aoba's Lake Voui taken 9 January 2006. The new island is composed of a cinder cone and the cone's crater appears to host a new, steaming internal lake. This photograph was taken from a fixed-wing aircraft by Job Eassau during his trip to Pentecote Island. Courtesy of Esline Garaebiti, Department of Geology, Mines and Water Resources, Port Vila, Vanuatu.

An image taken by ASTER's visible, near infra-red (VNIR) telescope on 24 December 2005 (UTC) showed the two larger caldera lakes, and steam escaping from an island in the center of Lake Voui (figure 22). The VNIR telescope has a resolution of ~ 15 m and operates in the spectral range 0.52-0.86 ?m.

Figure 22. ASTER VNIR image of Aoba taken at the stated UTC time and date (N is upwards), as it continued to emit a vapor plume from Lake Voui. The 27 November 2005 eruption built a substantial new island (dark area venting steam in the center of lake Voui). This island is almost circular in shape with a mean diameter of 525 m. Lake Manaro Lakua sits in the E caldera and is partly obscured by cloud or shadow. Courtesy of NASA and Alain Bernard (IAVCEI Commission on Volcanic Lakes and Université Libre de Bruxelles).

During September through December 2005, infrared satellite data provided by Moderate Resolution Imaging Spectroradiometer (MODIS) and processed by the MODVOLC Hot-Spot algorithm at the Hawaii Institute of Geophysics and Planetology (HIGP) only observed a single-pixel thermal anomaly. It occurred at 0110 local time on 26 November 2005 ( the image was acquired at 1410 UTC on 25 November 2005). That was 1 day prior to reports of the eruption from ground-based observers, although the ground-based reports could easily have been delayed so it is not clear that the MODVOLC thermal anomaly was actually prior to ground based observations.

Matt Patrick noted that the anomaly is nicely centered in the caldera and is almost certainly volcanic ? no other anomalies occurred on the island in the previous 5 years.

Information Contacts: Alain Bernard, IAVCEI Commission on Volcanic Lakes, Université Libre de Bruxelles, Brussels, Belgium (URL: http://www.ulb.ac.be/sciences/cvl/aoba/Ambae1.html); NASA Earth Observatory (URL: http://earthobservatory.nasa.gov/); Esline Garaebiti, Department of Geology, Mines and Water Resources, Port Vila, Vanuatu; Matt Patrick, University of Hawaii, Hawaii Institute of Geophysics and Planetology (HIGP) Thermal Alerts Team, 2525 Correa Road, Honolulu, HI 96822 (URL: http://www.modis.higp.hawaii.edu, Email: patrick@higp.hawaii.edu).
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01/2006 (BGVN 31:01) Crater-lake photos and satellite temperatures data show ongoing activity

As previously reported, a new eruption at Aoba began 27 November 2005 in one of the crater lakes (Lake Voui). The eruption formed a cinder cone in the lake (figures 23 and 24) that contained a crater with a small hot lake (BGVN 30:11 and 30:12).

Figure 23. A view of Aoba's Lake Voui on 18 January 2006, showing the new island and its steaming internal lake. Courtesy Alain Bernard.
Figure 24. Steam rising from the lake on the island in the middle of Aoba's Lake Voui, 18 January 2006. Courtesy Alain Bernard.

On 31 January a high, dark ash plume caused ashfall in the S part of the island. Small eruptions continued in February.

Alain Bernard recently processed a 26 January 2006 nighttime ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) image. Figure 25 shows the ASTER product called AST_04 (TIR?thermal infrared radiometer, 8.12-11.65 ?m wavelengths?band 10) unprocessed image of Aoba with Lakes Voui and Lakua. The TIR bands, with a spatial resolution of 90 m, give the ability to detect small thermal anomalies (a few degrees C), perform thermal mapping, and monitor temporal variations in the lake surface temperature. As shown in figure 26, Lake Voui's temperature in early January 2006 dropped by ~ 10°C to a mean of 25.4°C (down from 35.7°C one month earlier). Temperature differences between Voui and Lakua dropped to 4.3°C, reaching almost to the background levels observed in July 2005 (see plot "Temperature data from Lake Voui at Aoba, October 1998-December 2005 . . ."; BGVN 30:11). There is still a strong thermal anomaly of 46.1°C inside the new island (figure 13).

Figure 25. A thermal image of Aoba's lakes Voui and Manaro Lakua (to the W and E, respectively) for 26 January 2006 at 1124 UTC (2224 local). The image results from the ASTER On-Demand L2 Brightness Temperature at the Sensor. This AST_04 product is the brightness temperature data as recorded by the satellite, not the temperature of the target at the ground level. To retrieve the actual surface temperature, one needs to correct for atmospheric effects (absorption of water vapor, etc.) that significantly alter the spectral radiance during the travel from the ground to the satellite. A new method for this correction, developed by Alain Bernard and called AST_SW (SW stands for "split window"), is explained on his ("multispectral") website. Courtesy of Alain Bernard.
Figure 26. A plot of computed temperatures from 1 October 2005 to 1 February 2006 for Aoba's Lake Voui. The two different symbols distinguish processed MODIS and ASTER thermal data. A similar plot for an earlier period appeared in BGVN 30:11. Courtesy of Alain Bernard.

As of 11 February 2006 at 1011 hours (10 February 2006 at 2311 UTC), Alain Bernard reported that Lakes Voui and Lakua temperatures were, respectively, 27.2°C and 23.2°C (delta T = 4°C). The maximum temperature for the mud pool was ~ 57°C.

Information Contacts: Alain Bernard, IAVCEI Commission on Volcanic Lakes, Université Libre de Bruxelles (ULB), CP160/02, avenue F.D. Roosevelt 50, Brussels, Belgium (Email: abernard@ulb.ac.be, URL: http://www.ulb.ac.be/sciences/cvl/aoba/Ambae1.html; http://www.ulb.ac.be/sciences/cvl/multispectral/multispectral2.htm); Esline Garaebiti, Department of Geology, Mines, and Water Resources (DGMWR), Port-Vila, Vanuatu (Email:esline@vanuatu.com.vu).
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05/2006 (BGVN 31:05) During May-June 2006, Lake Voui's water rapidly turns from blue to red

Alain Bernard reported that Lake Voui in Aoba-Ambae volcano (BGVN 31:01) was undergoing a spectacular change in its color?the previously aqua-colored lake was turning red (figure 27).

Figure 27. Lake Voui at Aoba as seen from the air on 28 May (top) and 3 June 2006 (bottom). Images courtesy of Esline Garaebiti (top) and Philippe Métois (bottom).

Images of a pale reddish Lake Voui were obtained by Esline Garaebiti, who flew over the volcano 28 May 2006. Philippe Métois, who flew over on 3 June 2006, photographed a blood-red lake. These photos were are posted on the CVL website along with recent ASTER temperature data. This color change was tentatively attributed to a rapid shift in the lake water's redox state. The change might be linked to the ratio of SO2/H2S in the hydrothermal fluids.

Information Contacts: Alain Bernard, IAVCEI Commission on Volcanic Lakes (CVL), Université Libre de Bruxelles (ULB), CP160/02, avenue F.D. Roosevelt 50, Brussels, Belgium (Email: abernard@ulb.ac.be, URL: http://www.ulb.ac.be/sciences/cvl/aoba/Ambae1.html; http://www.ulb.ac.be/sciences/cvl/multispectral/multispectral2.htm); Esline Garaebiti, Department of Geology, Mines, and Water Resources (DGMWR), Port-Vila, Vanuatu (Email:esline@vanuatu.com.vu); Philippe Métois, World of Wonders (URL: http://www.worldofwondersvu.com/, Email: worldofwondersvu@hotmail.com).
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12/2006 (BGVN 31:12) Acidic gas emissions destroy vegetation; islet lake breached

The Aura/OMI satellite detected elevated SO2 concentrations above Aoba volcano during July and August 2006. Comparison of MODIS imagery between 3 June and 31 August 2006 (figure 28) revealed the effects of emissions on vegetation around the crater. The conditions in the field were investigated by a scientific team from Institut de Recherche pour le Développement (IRD). They concluded that a significant area of the summit (30 to 40 km2) was burned by acid gas emissions.

Figure 28. Satellite moderate resolution imaging spectroradiometer (MODIS) views of Aoba volcano summit lakes (Voui and Lakua). (a) A view taken at 1010 on 3 June 2006, before the last phase of gas emissions. (b) A view taken at 1005 on 31 August 2006 shows a large (15-20 km2) gray area around the lakes where the reflectance from vegetation has significantly decreased. Courtesy of Alain Bernard.

When IRD scientists conducted a visit to Aoba in late November 2006 vegetation surrounding the crater lake had been recently defoliated (figure 29), with trees completely burned and dead, due to plumes of acidic gas and aerosols during June-August 2006. They also concluded that heavy rainfalls since September 2006 diluted the acidity of plumes. Occasional green spots seen during the November visit were where new growths of ferns and tree ferns had become established. The acid effects were more extensive than previously seen since the early 1990s. This new behavior may reflect increased degassing from the source vent inside the ring-shaped tephra (or tuff) cone.

Figure 29. Aerial view of the vegetation downwind of Lake Voui at Aoba as of 25 November 2006. The scene was one of dead, completely defoliated trees. Courtesy of Michel Lardy, IRD.

On 25 November 2006 an IRD team measured an SO2 flux of 3,000 tons/day. This value coincided with the measurement provided by the ozone monitoring instrument (OMI on the EOS Aura satellite). The value represented a marked reduction in SO2 degassing compared to that measured on 10 June 2006.

The team noted that the main lake in the crater, Lake Voui, was still a red color, an effect due to oxidation of the iron in its large mass of water (BGVN 31:05). Within that larger lake resides the ring-shaped island, which largely formed during the late 2005-early 2006 eruptions (BGVN 31:01). The island's form had been that of an unbroken ring, but by the time of their 25 November visit, the preceding month's heavy rains had eroded the smaller islands wall, allowing water in the two lakes to easily mix (figure 30). The W shore of Lake Voui has also been eroded, and fumaroles were observed in the lake. The breach in the tephra ring coincided with gas emissions ceasing.

Figure 30. Aerial view of Lake Voui at Aoba as of 25 November 2006. The vent is now open to the lake and plume degassing stopped. Image courtesy of Michel Lardy, IRD.

The IRD team implemented the first permanent real-time temperature monitoring during their visit. Due to the heavy rainfall since June 2006 and the lowered levels of evaporation associated with the lowered average lake temperature (~ 25°C on 25 November 2006), the lake level remained high. In addition, the average level of Lake Voui is higher due to volcanic material (ash, scoria) deposited between December 2005 and January 2006, and it should continue to fluctuate seasonally, as in the past.

Information Contacts: Michel Lardy, Institut de Recherche pour le Développement (IRD), BP A 5 98 848 Noumea Cedex, New Caledonia (Email: Michel.Lardy@noumea.ird.nc, URL: http://www.ird.nc/actualities/volcanology/); Department Geology Mines and Water Resources (DGMWR), Geohazard Section, PMB 01 Port-Vila, Republic of Vanuatu (Email : observatoire@vanuatu.com.vu); Alain Bernard, Universite Libre de Bruxelles, Brussels, Belgium (URL: http://www.ulb.ac.be/sciences/cvl/aoba/Ambae1.html).
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05/2011 (BGVN 36:05) Increased degassing starting December 2009

Our last Bulletin report (BGVN 31:12) on Aoba (Ambae) described the destruction of vegetation by acidic gas emissions and the breach of the islet lake during 2006. This report discusses comparative quiescence into late 2009 when degassing escalated (substantial gas plumes were seen) and the hazard status rose. The volcano has remained quiet into mid-2011.

The Vanuatu region lies ~2,200 km N off the New Zealand coast and ~2,100 km NE off the coast of Australia (figure 31). A 1999 census suggested ~9,400 people resided on Ambae. Cronin and others (2004) describe the residents as "dispersed amongst more than 276 small extended family settlements and villages (Wallez 2000). Settlements are mostly restricted to the lower island slopes within 4 km of the coast. The highest population densities occur at the NE and SE ends of the island."

Figure 31. (A) Major islands of the Republic of Vanuatu. (B) Aoba (Ambae) Island, showing locations of settlements, main stream channels and roads. Cronin and others (2004) discuss the communities of Lolovange and Lolowai (ellipses). Taken from Cronin and others (2004) after work by Wallez (2000).

The Vanuatu Geohazards Observatory (VGO) noted increases in activity from Aoba (Ambae) starting in December 2009.This began when local villagers near the volcano reported seeing a plume over the island. In December 2009 the Vanuatu Volcanic Alert Level (VVAL) was raised to Level 1. The scale ranges from 0 to 4: 0 represents normal low-level activity and 4 represents a large eruption and island wide danger. The reported source of activity is a recent cone located in the crater lake, Voui (BGVN 30:11 and 30:12). The cone is known as Manaro Lakua (Manaro Voui).

The VGO went on to note that "An expatriate pilot based on Gaua, also witnessed a plume on Ambae on Tuesday 6th April on his way back to Gaua from Santo. Aerial pictures that were taken by two Geohazards staff on 11 April 2010 also confirmed gas emissions that were more concentrated than normal . . . [which] reaffirms the [Ozone Monitoring Instrument or OMI] satellite image of gas emissions above. Another observation made on Ambae is the presence of sulphur-hydromagmatic activity on the SE part of the second crater of Ambae enclosing Manaro Lakua indicated by what seemed like two fumarolic zones . . .. There was also some discoloration of the water in Manaro Lakua near the 'fumaroles' with some areas near the shore [colored] brown, and some areas [colored] pale blue—a sign of the incorporation of sulphur dioxide. It was also reported that while flying above the area, strong sulphur dioxide gas could be smelt even at 5,000 feet [~1.5 km altitude] on 11 April."

The VGO also noted that the OMI satellite pictures depicted fluctuating gas emissions during this period. The image for 11 April 2010 indicated elevated SO2 and gave the integrated concentration-pathlength as 15 kilotons. On this day, VGO had noted SO2 fluxes over 3,000 tons/day.

References. Cronin, SJ, Gaylord, DR, Charley, D., Alloway, BV, Wallez, S, and Esau, JW, 2004, Participatory methods of incorporating scientific with traditional knowledge for volcanic hazard management on Ambae Island, Vanuatu, Bulletin of Volcanology, v. 66, pp.652-668, Springer-Verlag.

Wallez S, 2000, Socio-economic survey of the impact of the volcanic hazards for Ambae Island: geo-hazards mitigation program section. Department of Geology, Mines and Water Resources, Port Vila, Vanuatu. p 39.

Information Contacts: Vanuatu Geohazards Observatory (VGO) (URL: http://www.geohazards.gov.vu/); Ozone Monitoring Instrument (OMI), Sulfur Dioxide Group), Joint Center for Earth Systems Technology, University of Maryland Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: http://so2.umbc.edu/omi/).
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05/2013 (BGVN 38:05) Minor activity likely continuing into early 2013

In our May 2011 Bulletin we reported that there was increased degassing at Aoba (also known as Ambae) starting December 2009 through at least April 2010. This report summarizes notices pereiodically posted by the Vanuatu Geohazards Observatory (VGO) and covers the time interval from 4 June 2011 through 26 February 2013. The Vanautu Volcano Alert Level (VVAL) remained at 1 (on a scale of 0-4.)

Observations on 4 June 2011 revealed that small explosions had been occurring from the crater lake and were accompanied by local ashfall around the crater. Some villagers in the N and W parts of the island had observed the explosions.

Based on analysis of data collected by the Vanuatu Meteorology and Geohazards Department (VMGD), the Vanuatu Geohazards Observatory reported that a small series of explosions from Aoba occurred on 10 July 2011. On July 11, VGO noted that there had been recent increases in activity from Ambae and that local earthquakes were volcanic. Satellite images collected by the Ozone Monitoring Instrument showed sulfur dioxide emissions. Photos showed that the volcano was quiet on 12 July 2011, although ongoing earthquakes were detected.

According to the VGO, Ambanga villagers reported that minor activity at Aoba began in December 2012. The OMI instrument detected strong gas emissions on 18 and 25 January 2013; the emissions continued at a lower level through 7 February. Field observations by the Geohazards team during 30 January-2 February 2013 confirmed that activity had significantly changed. Data retrieved from a monitoring station also confirmed ongoing activity. Satellite images acquired on 3 and 26 February 2013 detected substantial sulfur dioxide emissions.

No MODVOLC Thermal Alerts were issued in the previous year ending 16 July 2013.

Information Contacts: Vanuatu Geohazards Observatory (URL: www.geohazards.gov); and Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC 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://hotspot.higp.hawaii.edu).
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Aoba, also known as Ambae, is a massive 2500 cu km basaltic shield volcano that is the most voluminous volcano of the New Hebrides archipelago. A pronounced NE-SW-trending rift zone dotted with scoria cones gives the 16 x 38 km island an elongated form. A broad pyroclastic cone containing three crater lakes is located at the summit of the Hawaiian-style shield volcano within the youngest of at least two nested calderas, the largest of which is 6 km in diameter. Post-caldera explosive eruptions formed the summit craters of Lake Voui (also spelled Vui) and Lake Manaro Ngoru about 360 years ago. A tuff cone was constructed within Lake Voui about 60 years later. The latest known flank eruption, about 300 years ago, destroyed the population of the Nduindui area near the western coast.

Summary of Holocene eruption dates and Volcanic Explosivity Indices (VEI).

Start Date Stop Date Eruption Certainty VEI Evidence Activity Area or Unit
2011 Jun 4 2011 Jul 10 Confirmed   Historical Observations Lake Manaro
2005 Nov 27 2006 Feb Confirmed 2 Historical Observations Lake Voui
1995 Mar 2 1995 Mar 3 (?) Confirmed 2 Historical Observations Lake Voui
[ 1915 ± 1 years ] [ Unknown ] Uncertain    
1870 (?) Unknown Confirmed 2 Historical Observations SE side Lake Manaro Lakua; Lake Voui?
1670 (?) Unknown Confirmed 2 Anthropology Lake Voui and upper west flank
1530 (?) Unknown Confirmed   Radiocarbon (uncorrected) Lakes Voui and Manaro Ngoru

This compilation of synonyms and subsidiary features may not be comprehensive. Features are organized into four major categories: Cones, Craters, Domes, and Thermal Features. Synonyms of features appear indented below the primary name. In some cases additional feature type, elevation, or location details are provided.


Synonyms

Ambae | Aboa | Oba | Omba | Leper's Island

Cones

Feature Name Feature Type Elevation Latitude Longitude
Alaembakeo, Vuti Cone 15° 24' 0" S 167° 44' 0" E
Anagona Cone 15° 18' 0" S 167° 58' 0" E
Kerekere, Vuti Cone 756 m 15° 25' 0" S 167° 45' 0" E
Lakwa, Vuti Cone 590 m 15° 25' 0" S 167° 44' 0" E
Lolwai Tuff cone 15° 17' 0" S 167° 58' 0" E
Lombenben Shield volcano 1496 m 15° 24' 0" S 167° 50' 0" E
Namui, Vusi Cone 15° 19' 0" S 167° 56' 0" E
Ngwala Rock
    Devil Rock
Tuff cone 15° 27' 0" S 167° 40' 0" E
One, Vusi Cone 662 m 15° 21' 0" S 167° 54' 0" E
Talao, Vuti Cone 15° 28' 0" S 167° 49' 0" E
Tambunarakwe, Viti Cone 15° 27' 0" S 167° 41' 0" E
Tangwangi, Vuti Cone 15° 28' 0" S 167° 47' 0" E
Tapunikandi, Vuti Cone 594 m 15° 26' 0" S 167° 44' 0" E
Tasovi, Vusi Cone 15° 22' 0" S 167° 53' 0" E
Tokolotai, Viti Cone 15° 26' 0" S 167° 43' 0" E
Vuitavoa, Vusi Cone 15° 20' 0" S 167° 55' 0" E
Vuvungana, Vuti Cone 15° 28' 0" S 167° 51' 0" E

Craters

Feature Name Feature Type Elevation Latitude Longitude
Manaro Ngoru Crater 1391 m 15° 24' 0" S 167° 49' 0" E
Voui, Lake
    Vui, Lake
Crater 1340 m 15° 24' 0" S 167° 50' 0" E
An aerial overflight during World War II shows the summit of Aoba volcano from the SE. A broad pyroclastic cone containing three crater lakes is located at the summit of the Hawaiian-style shield volcano within the youngest of at least two nested calderas. The largest of these is 6 km in diameter. Post-caldera explosive eruptions formed the summit crater of Lake Voui (the light-colored lake at left-center) about 360 years ago. Lake Manaro Lakua at the lower right (also known as Manaro Ngoru) is trapped against the eastern caldera wall.

Photo by U.S. Air Force (courtesy of Claude Robin, ORSTOM; Bull. Global Volcanism Network, 1995).
The summit of Aoba volcano is seen looking approximately SE, with the steaming Lake Voui (foreground) and Lake Manaro Lakua (also known as Manaro Ngoru) on March 20, 1995, following an eruption on March 2. Convection cells ~300-400 m in diameter can be seen in Lake Voui. On March 2 bubbling and black sediment ejection was observed, and on the 3rd a 3-km-high steam-and-ash column ejected lake sediments. The eruption took place near the center of the lake, between the two small islands. No eruptive activity was observed after March 3.

Photo courtesy of ORSTOM, 1995 (published in Global Volcanism Program Bulletin).
A telephoto view looking approximately east from the crater rim on December 4, 2005 shows an explosive eruption from Lake Voui, on Aoba volcano. This explosive eruption began in Lake Voui on November 27 and resulted in formation of a new cinder cone in the lake.

Photo by Philipson Bani, 2005 (Institut Recherche Développement, IRD).
An explosive eruption from Lake Voui at Aoba on December 4, 2005 is seen looking approximately east from the crater rim. A large steam plume can be seen rising above the darker zone containing pyroclastic material. Three small islands formed prior to this eruption can also be distinguished, with the active vent area closest to the westernmost island.

Photo by Philipson Bani, 2005 (Institut Recherche Développement, IRD).
An ASTER image of Aoba taken on December 24, 2005 shows a vapor plume from Lake Voui and a new island formed during an eruption that began on November 27. The eruption built the substantial new island with its own crater lake seen venting steam near the center of Lake Voui. This island is almost circular in shape with a mean diameter at this time of 525 m. Lake Manaro Lakua at the right sits in the east caldera and is partly obscured by cloud or shadow.

NASA ASTER image, 2005 (courtesy of Alain Bernard, Université Libre de Bruxelles).
Steam rises from a crater lake inside the new island formed during an eruption beginning on November 27. This January 9, 2006 view taken from a fixed-wing aircraft on a flight to Pentecoste Island, looks toward the north. The new island is composed of a cinder cone, and the cone's crater appears to host a new, steaming internal lake. The new cinder cone was more than 500 m wide at this time.

Photo by Job Eassau, 2006 (courtesy of Esline Garaebiti, Dept of Geology, Mines, and Water Resources, Port Vila).
Two lakes of variable color lie with the summit caldera of Aoba (Ambae) volcano, as seen in this aerial view from the west. The light blue lake in the foreground is Lake Voui (or Vui), which is enclosed in the crater of a tuff ring in the summit caldera. The dark blue Lake Manaro Lakua lies between the tuff ring and the eastern caldera wall. Aoba, the most voluminous volcano of the New Hebrides archipelago, forms a 16 x 38 km elongated island. Pentecost Island appears in the background.

Photo by Karoly Nemeth, 2005 (Massey University).

The following references have all been used during the compilation of data for this volcano, it is not a comprehensive bibliography. Discussion of another volcano or eruption (sometimes far from the one that is the subject of the manuscript) may produce a citation that is not at all apparent from the title.

Bani P, Oppenheimer C, Varekamp J C, Quinou T, Lardy M, Carn S, 2009. Remarkable geochemical changes and degassing at Voui crater lake, Ambae volcano, Vanuatu. J Volc Geotherm Res, 188: 347-357.

Cronin S J, Gaylord D R, Charley D, Alloway B V, Wallez S, Esau J W, 2004. Participatory methods of incorporating scientific with traditional knowledge for volcanic hazard management on Ambae Island, Vanuatu. Bull Volc, 66: 652-668.

Fisher N H, 1957. Melanesia. Catalog of Active Volcanoes of the World and Solfatara Fields, Rome: IAVCEI, 5: 1-105.

Macfarlane A, Carney J N, Crawford A J, Greene H G, 1988. Vanuatu--A review of the onshore geology. In: Greene H G and Wong F L (eds) {Geology and Offshore Resources of Pacific Island Arcs--Vanuatu Region}, Circum-Pacific Council Energy Min Resour Earth Sci Ser, 8: 45-91.

Nemeth K, Cronin S J, Charley D, Harrison M, Garae E, 2006. Exploding lakes in Vanuatu -- "Surtseyan-style" eruptions witnessed on Ambae Island. Episodes, 29: 87-92.

New Hebrides Geological Survey, 1979. Geology of Aoba and Maewo. New Hebrides Geol Surv, 1:100,000 geol map sheet 5.

Robin C, Monzier M, Crawford A J, Eggins S M, 1993. The geology, volcanology, petrology-geochemistry, and tectonic evolution of the New Hebrides island arc, Vanuatu. IAVCEI Canberra 1993 excursion guide, Aust Geol Surv Org, Rec 1993/59, 86 p.

Smithsonian Institution-GVN, 1990-. [Monthly event reports]. Bull Global Volc Network, v 15-33.

Warden A J, 1970. Evolution of Aoba Caldera volcano, New Hebrides. Bull Volc, 34: 107-140.

Volcano Types

Shield
Caldera
Pyroclastic cone(s)
Tuff ring(s)

Tectonic Setting

Subduction zone
Intermediate crust (15-25 km)

Rock Types

Major
Basalt / Picro-Basalt

Population

Within 5 km
Within 10 km
Within 30 km
Within 100 km
73
1,279
4,326
73,308

Affiliated Databases

Large Eruptions of Aoba Information about large Quaternary eruptions (VEI >= 4) is cataloged in the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database of the Volcano Global Risk Identification and Analysis Project (VOGRIPA).
WOVOdat WOVOdat is a database of volcanic unrest; instrumentally and visually recorded changes in seismicity, ground deformation, gas emission, and other parameters from their normal baselines. It is sponsored by the World Organization of Volcano Observatories (WOVO) and presently hosted at the Earth Observatory of Singapore.
EarthChem EarthChem develops and maintains databases, software, and services that support the preservation, discovery, access and analysis of geochemical data, and facilitate their integration with the broad array of other available earth science parameters. EarthChem is operated by a joint team of disciplinary scientists, data scientists, data managers and information technology developers who are part of the NSF-funded data facility Integrated Earth Data Applications (IEDA). IEDA is a collaborative effort of EarthChem and the Marine Geoscience Data System (MGDS).
Smithsonian Collections Search the Smithsonian's NMNH Department of Mineral Sciences collections database. Go to the "Search Rocks and Ores" tab and use the Volcano Name drop-down to find samples.