Ambrym Island was investigated by John Seach and Perry Judd during a climb into the caldera 1-8 January 1999. A lava lake in Benbow cone was present during 1-3 January but was covered by deposits from an avalanche that occurred overnight 4-5 January. Fumarolic and Strombolian activity was observed at other craters.

Activity at Benbow. Benbow crater was climbed from the S, after which observers lowered themselves using ropes 200 m down from the crater rim to a point where they could observe the crater interior. In the center of the crater, an active lava lake was seen 220 m below the observation point. The lava lake was ~40 m in diameter and constantly in motion. Large explosions caused lava fountains that reached 100 m high. Bombs glowed for up to one minute in daylight and radiated great heat. Bombs could be heard landing on the side of the pit where they caused glowing avalanches. At night a strong glow from the lava lake was visible in the sky over Benbow.

Elsewhere inside Benbow crater, Pele's hair covered the ground and fumaroles were active on the NE crater wall. Acid rain burned eyes and skin. Heavy rainfall caused many waterfalls to form inside the crater rim and a shallow brown pond formed on the floor of the first level.

During 4-5 January violent Strombolian explosions could be heard almost hourly. Each series of explosions lasted 5-10 minutes and produced dark ash columns above the crater. At some time during these explosions an avalanche on the W side of the lava lake crater completely covered the lava lake. No night glow was visible above the crater after the night of 5 January.

On 6 January Benbow crater was entered again. The wall collapse that covered the lava lake was confirmed visually. In the location of the former lava lake was a depression of rubble with two small, glowing vents nearby. The entire crater was clear of magmatic gases. Three violent Strombolian eruptions were viewed from the crater rim in the afternoon. Bombs were thrown 300 m into the air and dark ash clouds were emitted.

Activity at Niri Mbwelesu Taten. This small collapse pit continuously emitted white, brown, and blue vapors. Red deposits covered the crater walls. A small amount of yellow deposits covered the S wall. Fumarole temperatures were 66 to 69°C at a point 40 m SE of the pit. On 6-7 January numerous deep, loud degassings were heard from a distance of 4 km.

Activity at Niri Mbwelesu. Pungent, sulfurous-smelling white vapor was emitted from this crater. Periods of good visibility enabled views 200 m down from the crater rim, but the bottom could not be seen. Rockfalls were heard inside the crater.

Activity at Mbwelesu. Excellent visibility to the bottom of this crater enabled detailed observations of the lava lake. Night observations were also obtained. The lava lake was in constant motion and splashing lava out over the sides of the pit. The lake was at a lower level than during observations made three months earlier (BGVN 23:09). Large explosions sent lava fountains up to 100 m in height and threw lava onto the sides of the pit causing glowing avalanches. During one night observation a 20 x 5 m section of the crater wall broke off and fell into the lava lake. The 60-m-wide lake radiated heat that could be felt from the viewing area 380 m away. North of the lava lake was a circular vent 20 m in diameter that glowed brilliantly from magma inside and huffed out burning gasses every 20 seconds. Foul gas, smelling of rotten fish, was emitted from the crater. South of the lava lake was an elongated vent (40 x 10 m) that spattered lava every 5-10 seconds and sent showers of glowing orange lava spray 150 m high.

On the S side of Mbwelesu, fumarole temperatures averaged 43°C at 10 m from the crater edge. On the SE side, 40 m from the crater edge, fumaroles measured 57°C. On 4 January ashfall occurred on the S side of the caldera.

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: John Seach, P.O. Box 16, Chatsworth Island, NSW, 2469, Australia.