The island of Ambrym corresponds to one of the voluminous active volcanoes of Vanuatu (500 km3).
It is an imposing stratovolcano of 35X50 Km which rises to 1800 m above the surrounding seabed.
This elongated E-O island is located at the central segment of the Vanuatu arc, in the zone of influence of the Entrecasteaux ridge, probably with a transverse fracture oriented N100 °.
This mainly basaltic volcano has the typical Hawaiian shield volcano shape.
The main cone is truncated, at about 800 m altitude, by a caldera.
This collapse structure, about 12 km in diameter, has an escarpment from a few tens to more than 450 m high and could be linked to a recent paroxysmal eruption, recorded by a large cone of pyroclastic products covering the slopes of the l Building around the edge of the caldera, and resulting from a cataclysmic explosion that would have occurred about 2000 years ago.
The post-caldera activity was mainly developed on the two active cones, Marum (1270 m) and Benbow (1160 m), characterized by permanent lava lakes.
Ambrym, the most active volcano in the archipelago, has had a very large number of historic eruptions since 1774. The last major eruptions occurred in 1986 and 1988-1989 and resulted in lava flows intra-caldera .
Lava lakes of varying volumes but present almost permanently in the craters of Marum and Benbow periodically drain in the form of basaltic flows on the floor of the caldera. The magma is also ejected as ashes, lapillis and slag, which fall partly in the caldera, and partly on the NO flank of the island.
Gravimetric geophysical studies revealed a thinning of the crust from west to east in Ambrym, from 25 to 19 km thick.
Magnetic studies have revealed the presence of several superficial magmatic chambers located under the caldera and under the eastern part of the island.
More recently, a seismological study of the very long-term tremor made it possible to identify two magmatic sources active under the caldera: one (most active) under the zone from which the 1986 andesitic lava flow was emitted To the east of the caldera and the other to the west under the Marum cones (basaltic casting of 1988) and the Benbow.
The estimated depths of these magmatic bodies are between 3.5 and 3.7 km in relation to the caldera. The presence of two magmatic chambers under the caldera seems to be in agreement with the bimodal volcanism observed between 1986 and 1989 and with the complex internal structure of the volcano.
Ph. Bani et al. (2012) A. Beaumais et al. (2013).