A visit to the summit caldera on 8-9 July did not permit an approach to the lava lakes in the Benbow and Marum craters due to poor weather. An overflight on the night of 20 July permitted observations of surface bubbling in Marum's lava lake. Two other overflights, on 21 and 22 July, allowed observation of activity in both lakes for several minutes. During these observations, the surface of the Benbow lake was fairly calm. However, Marum's lava lake, ~100 m in diameter, exhibited occasional explosions that threw glowing magma fragments some meters above the surface; bubbling was clearly visible from the airplane.

Geological Summary. Ambrym, a large basaltic volcano with a 12-km-wide caldera, is one of the most active volcanoes of the New Hebrides Arc. A thick, almost exclusively pyroclastic sequence, initially dacitic then basaltic, overlies lava flows of a pre-caldera shield volcano. The caldera was formed during a major Plinian eruption with dacitic pyroclastic flows about 1,900 years ago. Post-caldera eruptions, primarily from Marum and Benbow cones, have partially filled the caldera floor and produced lava flows that ponded on the floor or overflowed through gaps in the caldera rim. Post-caldera eruptions have also formed a series of scoria cones and maars along a fissure system oriented ENE-WSW. Eruptions have apparently occurred almost yearly during historical time from cones within the caldera or from flank vents. However, from 1850 to 1950, reporting was mostly limited to extra-caldera eruptions that would have affected local populations.

Information Contacts: Henry Gaudru, C. Pittet, C. Bopp, and G. Borel, Société Volcanologique Européenne, C.P. 1, 1211 Genève 17, Switzerland (URL: http://www.sveurop.org/); Michel Lardy, Centre ORSTOM, B.P. 76, Port Vila, Vanuatu.