Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 8, August 1995

Aoba (Vanuatu)  Crater lake exhibits convection cells and steaming
       as level drops

Aoba (Ambae)
Ambae Island, Vanuatu
15.40 S, 167.83 E; summit elev. 1,496 m

A pyroclastic explosion on the morning of 3 March 1995 generated a
vapor-and-ash column ~3 km high (Bulletin v. 20, no. 2).
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 5 and
6). 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 7) 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.

A drop in the level of Lake Voui that began on 6 March (Bulletin v.
20, no. 2) 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 6), 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 deg C, with highs of 63-67 deg 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, BP 76, Port Vila, Vanuatu, and Bureau des
Desastres Nationaux, P.M.B. 014, Port Vila, Vanuatu (Email:
lardy@vanuatu.orstom.fr); M. Regnier and S. Temakon, ORSTOM et
Departement des Mines et de la Geologie et des Ressources en Eaux,
P.M. Box 007, Port Vila, Vanuatu (Email:
regnier@vanuatu.orstom.fr); Chief N. Tahi, Village de Nambangahake
(Ndui-Ndui) Aoba, Vanuatu; C. Robin and M. Monzier, Centre ORSTOM,
Apartado 17-11-6596, Quito, Ecuador (Email: robin@orstom.ec); J-P.
Eissen, Centre ORSTOM de Brest, BP 70, 29280 Plouzane, France
(Email: eissen@orstom.orstom.fr); J-P. Metaxian, Universite de
Savoie, 73376 Le Bourget du Lac Cedex.

Figure 5. 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 6. 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 7. 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.