Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 2, February 1995
Aoba (Vanuatu)  Increased steam emissions and seismicity in early
       March; evacuation preparations made

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

The following report, prepared on 17 March, is from volcanologists
of the Institut Francais de Recherche Scientifique pour le
Developpement en Cooperation, Office de la Recherche 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 above sea level, and a volume of ~2,500 km^3
(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 ~N50E
fracture transverse to the arc (figure 5; see Greene and others,
1994, for more information). Two concentric summit calderas, the
largest 5 km in diameter (figure 6), enclose the central crater
containing the 2-km-diameter Lake Voui (Vui) (figure 7). Numerous
secondary craters and cones lie along the N50E fracture, out to the
extremities of the island, where previous magma-seawater
interactions have produced several maars.

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 N50E 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).

On 13 July 1991, a VANAIR pilot observed three anomalous "boiling"
areas at Lake Voui (Bulletin v. 16, no. 7) and burned vegetation
that had been green in May 1991. On 24 July 1991, an aerial survey
revealed three areas of discolored water in Lake Voui. Anomalously
strong degassing between May and July had apparently burned
vegetation up to the crater rim, 120 m above the water. This event,
unnoticed by island residents, marked the end of a long quiescence.

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 by Aoba
inhabitants during 1-7 December 1994 (Bulletin v. 20, no. 1).
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 National Disaster
Management Office (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, N50E 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 United Nations
Department of Humanitarian Affairs 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 preliminaire sur la crise sismique
d'Aoba de decembre 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 Hebrides 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: Bulletin Volcanologique, v. 34, no. 1, p. 107-140.

Information Contacts: C. Robin and M. Monzier (geologists) ORSTOM
and Department of Geology, Mines and Water Resources of the Vanuatu
Government, currently at ORSTOM, AP 17-11-6596, Quito, Ecuador
(Email: and; M. Lardy
(geophysicist), M. Regnier, J-P. Metaxian, R. Decourt
(seismologists), and D. Charley (technical assistant), ORSTOM, BP
76, Port-Vila, Vanuatu (Email:; Mario Ruiz
(seismologist), Instituto Geofisico, Escuela Politecnica Nacional,
Quito, Ecuador; J-P. Eissen (geologist), ORSTOM, Centre de Brest,
B.P. 70, 29280 Plouzane, France (Email:;
Bureau of Meteorology, Australia (see Dukono); UN Department of
Humanitarian Affairs, Palais des Nations, CH-1211 Geneva 10,

Figure 5. Bathymetric map of central Vanuatu showing the trench and
direction of relative movement (arrows), Aoba, and other active
volcanoes. Bathymetry in kilometers. Modified from Greene and
others, 1994.

Figure 6. 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 (1:100,000). Large dashed
circles are 5- and 10-km radius lines. Topographic base map
courtesy of C. Robin, ORSTOM.

Figure 7. 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. Photograph was taken by a U.S. pilot during World War
II, courtesy of C. Robin, ORSTOM.