The Vanuatu arc was visited by an ORSTOM mission 5-18 September. The following is modified from their report in the LAVE Bulletin.

Thick puffs of ash rose several hundred meters, and scattered blocks were ejected, from a vent 200 m below the rim of Niri Tamo, which formed adjacent to Mbuelesu crater in 1989 (14:10). The approach to the crater was sprinkled with blocks 10 cm to 1 m in diameter. One block, 2 m in diameter, was located near a possible new crater (Niri Taten) that was S of Mbuelesu, near the source of the 1988 lava flows and [~500 m] from Niri Tamo. A zone of intense degassing, with temperatures of at least 625°C, occurred within Niri Taten. Mbuelesu appeared more elongate to the NE than represented on 1989 maps and no longer contained a lava lake. One vent, sounding like a reactor, violently emitted ash, gas, lava blocks, and fragments. The plume rose vertically at 30 m/s, and projectiles frequently landed beyond the rim of the crater. Benbow crater emitted a strong bluish plume, suggesting a significant SO₂ content.

Further Reference. Eissen, J.P., Monzier, M., Robin, C., Picard, C., and Douglas, C., 1990, Report on the volcanological field work on Ambrym and Tanna Islands (Vanuatu) from 2 to 25 September 1990: Rapport Missions Sci Terre Geologie-Geophysique – ORSTOM (Noumea), no. 22, p. 1-22.

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: M. Lardy, ORSTOM, New Caledonia; B. Marty, CNRS, France; LAVE.