The November 2005 eruption of Aoba volcano, Vanuatu

Last update December 31, 2007

Lake voui is turning red (see also at the bottom of this page).


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MODIS view (250m resolution with Red = Band 1, Green = Band 2 and Blue = Band 1 ) of summit lakes (Voui and Lakua). The August image shows a large (15-20km2) grey area around the lakes where the reflectance from vegetation (Band 2) has significantly decreased. This spectacular change is most probably related to an increase in the degassing from the new cinder cone. The emission of a plume of acid gases and aerosols burned the vegetation downwind. The Aura/OMI satellite detected elevated SO2 concentrations above Aoba volcano during July and August 2006 as reported by Simon Carn, Joint Center for Earth Systems Technology (JCET), University of Maryland Baltimore County (UMBC).


Aerial view of the vegetation downwind lake Voui as of November 25, 2006 and showing a complete defoliation of trees as the result of the emission of a plume of acid gases and aerosols during June to August 2006. Green spots are Tree Fern back to life. Heavy rainfalls since September have diluted the acidity from the volcano. The opening of the vent stopped also the gas emission (see at the bottom of this page). Image courtesy of Michel Lardy IRD, Nouméa. Full report at


The lake Voui and the 2005 eruption



The 27 November 2005 eruption of Ambae-Aoba volcano built a new island (cinder cone) within Lake Voui (see GVN bulletin 30/11). The eruption was small in magnitude but the temperature of lake Voui showed very rapid heating and cooling. The first thermal anomaly was detected by MODIS satellite on November 19, 2005.

As of 26 January 2006, lake Voui returned back to near-background temperature. See at the bottom of this page.


The volcanic lakes of Gaua and Ambae volcanoes are visible on this MODIS image (October 3, 2005).


Two volcanic lakes are present at the top of this large basaltic shield volcano. The dark one (on the right) is Lake Lakua a freshwater lake and the blue one is Lake Voui.
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Voui is a voluminous acidic lake. The maximum depth reported is 150 meters and its volume is estimated at 50 millions m3. Lake surface area is 2.12km2. The lake composition is typical of acid sulfate chloride waters with a pH = 1.6. Sediments or chemical precipitates (silica, gypsum) can be observed on satellite images when the convective activity within the lake is sufficiently strong like in this Landsat image of 1999.



The last significant hydrothermal activity was reported in early 1995 when a short phreatic explosion occurred within the lake following few days of seismic unrest (see Smithsonian Institution (USA) GVN Bulletin 02/1995 (BGVN:20:02). Nice convection cells within Lake Voui can be seen on this picture.


Evolution of lake temperatures since 1998 and satellite data:


A monitoring station for continuous measurements of in-situ temperatures was installed in October 1998. The station used a satellite ARGOS transmission system and recorded the last heating episode of 2001. The station failed after 3 years due to the harsh acid environment. Argos data courtesy of Michel Halbwachs (Université de Savoie) and Michel Lardy (IRD). See IRD web site for a complete background information on Vanuatu volcanoes.
ASTER thermal infrared images can be used for monitoring lake surface temperatures. Aoba volcano offers the same situation as Taal volcano in the Philippines with a freshwater lake (Lakua) which can be used to remove the seasonal/diurnal variations in atmospheric temperatures. DT represents the thermal anomaly calculated as the temperature differences between the two lakes. Unfortunately, the top of the volcano is frequently covered by clouds and only few ASTER images are exploitable. The last ASTER images before the eruption was recorded on October 5, 2005 and shows only Lake Voui and no unusual temperatures. The method used and calibration of ASTER satellite is described here.
MODIS satellite data show an increase in lake temperatures well before the eruption of November 27. More on MODIS data can be seen on the next page.



Most recent ASTER images
The last ASTER image showing clearly the two lakes was collected on July 9, 2005. Difference in temperatures between lake Voui and Lakua was 4.0 °C, sligthly above the background values recorded during 2002-2003.



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ASTER VNIR (simulated natural colors) of Lake Voui and thermal anomaly(center image).

The discolored (gray) area visible in 2001 is the consequence of acid degassing. The vegetation was burned by acid gases or aerosols.


MODIS satellite shows increase in lake temperature on November 19, 2005 and possibly at an earlier date.

See data here.


Post-eruption images
The 27 November 2005 eruption built a new island which is visible on this ASTER image. The island (black area in the center of lake Voui) is still emitting a vapor plume. The island is almost circular in shape with a mean diameter of 525 meters. Lake Lakua is partly masked by clouds.


ASTER thermal image processed with SW algorithm and a slight pixel interpolation. Maximum lakeVoui temperature is 38.7°C (mean: 35.7°C). The hot spot with temperature as high as 44°C is located within the crater (red line) of the new cinder cone (black line) and could reflect the temperature of an inner crater lake. Lake Lakua temperature is similar to ambient: 23.8°C. DT between Voui and Lakua = 11.9°C.

The new cinder cone and its crater filled with a new lake (?).

ãPhotograph from Job EASSAU during his trip to Pentecote Island on 9th January 2006.

Courtesy of Esline Garaebiti. Department of Geology, Mines and Water Resources. Port Vila, Vanuatu.


January 18, 2006 view of the crater of the new island with a boiling mud pool.

Courtesy of Esline Garaebiti, Dept. of Geology Mines and Water Resources, Port Vila, Vanuatu.


2006 ASTER images


January 26, 2006


AST_04 (TIR band 10) unprocessed image of Aoba showing lakes Voui and Lakua. A small vapor plume seems to be rising from the new crater.


AST_04 data processed with SW algorithm and no pixel interpolation. Lake Voui temperature dropped by about 10 degrees to a mean of 25.4°C (down from 35.7°C one month earlier). Differences in temperatures between Voui and Lakua dropped also to DT = 4.3 °C, reaching almost the background level observed in July 2005. There is still a strong thermal anomaly (T= 46.1°C) inside the new island. Contrary to what is reported in the Bulletin of the Global Volcanism Network (BGVN 31:1, January 2006), the temperature data of this image are believed to be close to the actual surface temperature since these data are corrected with the split-window method.


February 10, 2006


February 10, 2006 at 23.11 UTC. ASTER VNIR simulated natural color image. Both lakes are visible in the middle of the clouds. Voui and Lakua temperatures are respectively: 27.2 °C and 23.2 °C. DT = 4.0 °C. Mud pool maximum temperature is around 57°C.


April 7, 2006


April 7, 2006 at 11:31 UTC. AST_04 data processed with SW algorithm and pixel interpolation. Both lakes Voui and Lakua show almost equivalent temperatures. These are respectively : 24.9°C (Voui) and 23.2°C (Lakua) with a DT = 1.7 °C. Mud pool maximum temperature is unchanged and around 56°C. The decrease in temperature for lake Voui could result from a nearly complete closing of the new vent where the main discharge of hot fluids from the hydrothermal system is restricted.


May 2006: Colour of lake Voui is turning red.
View of lake voui on May 28, 2006. Image courtesy of Esline Garaebiti from the Dept. Geology, Mines and Water Resources in Port Vila, Vanuatu. This colour change is suggesting a rapid evolution in the redox state of the lake waters that could be linked to an evolution of the SO2/H2S in the hydrothermal fluids.
June 3 , 2006 at 11:25 UTC. AST_04 data processed with SW algorithm and pixel interpolation. Both lakes Voui and Lakua show almost equivalent temperatures. These are respectively : 23.0°C (maximum: 24.8°C) for Voui and 21.1°C (Lakua) with a DT = 1.9 °C. These ASTER_SW data are remarkably close to field measurements done by Philipson Bani (IRD, Nouméa) between June 12-15. Phil measured in Voui: 23°C (East side)- 25°C (North side) and for Lakua: 19.5°C. Mud pool thermal anomaly (new island) : 52.9°C. Thermal power dissipated by lake Voui shows no significant evolution compared to April 7, 2006.


Aerial view of lake Voui as of November 25, 2006. The vent is now open to the lake and plume degassing stopped. Image courtesy of Michel Lardy IRD, Nouméa. Full report at:




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The new island is now largely eroded. Some remnants of the island with a crescent shape are still visible on the VNIR image. The TIR (Split-window) image on the right shows a hot spot (red) with 33.5°C located at the vent and a thermal plume (yellow) with 31-32 °C drifting to the North. The rest of the lake is at a temperature of 28-29°C. Lakua lake (visible at the bottom right on the VNIR image) is at a temperature of 21-22°C. Temperatures difference between Voui and Lakua (DT) = 7 °C. Thermal anomalies outside the lake area are bare soil heated by the sun.


12 October 2007 (night scene). AST_L1B data processed with SW algorithm and pixel interpolation. Mean temperature of lake Voui is slightly increasing from 28-29°C (January 2007) to 31.4°C. Hot spot located on the right of the cold area (island) is at 38.5°C. Temperatures difference between Voui and Lakua (DT) = 10.7 °C.
4 November 2007 (night scene). AST_L1B data processed with SW algorithm and no pixel interpolation. Mean temperature of lake Voui is 32.5°C and maximum: 34.7°C. Temperatures difference between Voui and Lakua (DT) = 10.4 °C.
ASTER VNIR image of December 14 showing the remnants of the new island still present in the middle of lake Voui.


Evolution of lake temperatures since the last eruption. During the past 6 months, temperatures of lake Voui were above 30°C. With these temperatures, the thermal power dissipated by the lake surface is close to 500MW. Delta T is the difference in temperature between Voui and Lakua lakes.



ASTER and MODIS satellite data courtesy of NASA, USA.

ASTER data are distributed by the Land Processes Distributed Active Archive Center (LP DAAC), located at the U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS)

Satellite images processing and page editor: Alain Bernard.


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Université Libre de Bruxelles. Last modification: 31 December 2007.

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