The following report discusses observations of Marum and Benbow craters (in the central and WSW portions of Ambrym's caldera, respectively). The observations were made in September-October 1999 and August-October 2000. Our previous Ambrym report discussed aerial observations made in late February 2000 (BGVN 25:04). Marum and Benbow host long-standing lava lakes with active surfaces (sometimes molten and sometimes chilled).

1999 Marum observations. On 24 September 1999 the lava lake was once again present at more or less the normal site. It measured ~60 m in diameter and underwent significant degassing accompanied by turbulent waves and the escape of incandescent fragments. On several occasions, observers witnessed large collapses on the periphery as well as rapid and sudden variations in the lake's surface level. At the foot of the SE face, perhaps three explosive vents ejected plumes of ash and cauliflower-shaped discharges of steam at irregular intervals varying from 8 to 30 minutes. At night, observers distinguished incandescence along concentric faults on the lava lake.

On 22 October 1999 observers camped on the crater's edge, on the ash-covered floor found to its ESE. They noted that the main lava lake had grown since September, and it displayed more violent, regular degassing. Its surface was continuously disrupted by waves and escaping incandescent fragments that rained all the way down the active terrace. Observers saw a second small elongate lake and reported that its surface too was sometimes very agitated. At night, several incandescent faults were seen on the bottom of the terrace. These spread open and then closed, indicating that the entire zone had a thin, partially solidified crust.

On 23 October bad weather prevented visual observations, but at night observers saw intense red glow, felt tremors, and heard rumblings. On 24 October, at sunrise, the meteorological conditions were excellent, but volcanic gas obscured the crater.

1999 Benbow observations. On 23 September 1999, observers looking toward Benbow crater from the sea saw a large column of ash and gas rising about 1,200 m above the crater. On two occasions at night, visitors saw brief instances of weak incandescence in the plume's interior.

A month later, on 23 October 1999, despite unstable weather conditions, observers reached summit crests and saw gas occupying the crater's interior. They saw the first terrace only for a brief moment and then only partially, making it impossible to say whether the lava lake was again molten at the surface. The characteristic rumblings that accompanied the regular degassing on the lake's surface were perceptible but weak, as if molten material lay beneath a thin chilled surface.

2000 Marum observations. During the nights of 2 and 3 August 2000 a lava lake was clearly visible (about 100 x 60 m in size). Regular and sustained degassing agitated the surface with big waves. Showers of incandescent fragments rose to heights of ~50-70 m.

On the night of 16 August a tropical depression crossed the region affecting an area extending at least as far as the Banks Islands ~200 km N. Heavy rains fell on the Marum plateau. Thick "smoke" rose from the crater, impeding visual observations.

On the night of 30 September-1 October, incessant rain again fell on the Marum plateau. Very poor visibility in the crater stopped for a few minutes around 0100, enabling observers to confirm the absence of the lava lake's exposed molten surface. At that time, only two small and closely spaced circular vents emitted incandescent gases.

2000 Benbow observations. Observers descended to the first terrace level on 3 August 2000, crossing along the crests that encircled the central shaft. At this time, there was no exposed molten lava on the lake's surface. Still, violent and continuous explosions fed a darkly colored, dense ash plume. These outbursts came from a vent situated in the deepest part of the central opening. The excursion failed to get around the NE vent on the terrace level N1 due to a zone of mass wasting that left a scar ~160 m deep. On 17 August, torrential rains and excessively violent winds impeded attempts to approach Benbow.

On 1 September, people descending to terrace N1 felt sub-continuous tremor but found comparatively little gas. What gas there was looked blue in color and hung over the central opening. The lava lake was once again present (but difficult to see due to impeded access caused by the above-mentioned scar). Degassing accompanied by rumblings and strong detonations sent incandescent lava fragments to heights of ~100 m.

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: G. de St. Cyr, c/o Bulletin of the Geneva Society of Volcanology, C.P. 6423, CH-1211, Geneva 6, Switzerland.