Eruptive activity continued at Ambrym in late 1999 and through January 2000. A volcanic ash advisory regarding this volcano was issued to aviators on 1 November 1999 reporting "smoke and ash" rising to ~1,500 m altitude. Similar notices were issued on 5 and 6 November. [Aviation reports on 9-10 December] described an ash cloud up to 2,700 m altitude.

John Search and Geoff Mackley investigated Ambrym caldera during a 19-28 January 2000 climb. Lava lakes had disappeared from both Benbow and Mbwelesu craters and a new vent had opened inside the previously inactive 1953 crater. A series of earthquakes were registered around Ambrym Island on 27 November 1999. The largest of these was magnitude 7.1. The earthquakes were followed by a month of reduced activity during which there were no reported observations of lava lakes. Landslides were visible in the caldera and ground cracking visible at Benbow, Mbwelesu, and Niri Mbwelesu craters.

Activity at Benbow Crater. Four vents were active inside Benbow. On 19 January a white plume tinged with blue and yellow rose 1,000 m above the crater rim. Twin plumes were visible the next day rising from the S end of the crater at 15 m/s and from the N end of the crater, where they were tinged with brown. Each time the crater was climbed from the S on 22, 23, and 24 January the pit was full of vapor and no sounds were heard. On 25 January the observers lowered themselves into Benbow using 200 m of rope. The floor of the first level was covered with fine brown ash and a shallow brown pond was present in the SW end of the crater. The inner crater was climbed and observations made from its rim. Below the observers was a ledge 120-140 m down covered with ash and containing a 10-m circular vent emitting white vapor. The main vent was 50 m farther down and 40 m in diameter. This was the vent that contained the lava lake in January 1999 (BGVN 24:02). No lava was observed inside this vent and it made no sound. At 1300 a large roar from the vent was followed by brown ash emission. At the NE end of the inner crater was a plume emission from an unseen vent.

The N end of Benbow crater (on the first level) contained another vent that could not be directly observed but regularly emitted light brown ash. On 26 January a loud continuous 30-second degassing heard from the N vent was followed by brown ash emission and rain of small cinders on observers at the S crater edge. From the central pit the vapor was rising at 5 m/s. During the late afternoon two visible atmospheric perturbations were observed above the main vent. The first followed a loud degassing sound and rose at 40 m/s to a height of 200 m above the vent. Rockfalls were also heard during the afternoon. During the night of 26 January twin skyglows of fluctuating intensity were visible above Benbow followed by a large brown ash emission that rose 1,400 m above the crater in 3 minutes.

Activity at Niri Mbwelesu Taten. On both 19 and 20 January light brown or red/brown ash was emitted from the collapse pit and rose 200-250 m. On 21 January a brown pond of water 150 m NE of the pit was bubbling from both fixed and random locations. Active fumaroles were present on a ledge 60 m down. There were large cracks on the SE side and evidence of wall collapse since August 1999. Ash fell on observers in the area N of the pit. On 23 January larger ash emissions occurred about every hour.

On 24 January the collapse pit was entered using ropes. Fumaroles on the ledge 60 m down averaged 64°C. The pit bottom was 120-140 m below the ledge covered in brown ash. Small clouds of ash were emitted occasionally from two large fissures. Bubbles of hot blue vapors, 6 m in diameter, rose past the observer. Continual degassing sounds were heard in the pit, like the sound of waves crashing on the beach. On 26 January from 0600 to 1100 dark gray ash clouds were continually being emitted from the pit. Plumes rose at 8 m/s to a height of 200 m above the pit, filling the caldera in all directions. During the afternoon the pit returned to a low level of activity. On 27 January a continuous emission of brown ash occurred all day to a height of 800 m above the pit.

Activity at Niri Mbwelesu. On 20 January white vapor tinged with blue was constantly emitted to 600 m above crater. During the evening a very intense pulsating night glow was visible. The glow would brighten (sometimes flicker), then rapidly drop to a lower level of illumination. The bright/dim cycle would repeat every 10-15 seconds. On 21 January in the afternoon degassing was heard from the crater rim and during the evening clouds were illuminated 250 m above the crater. Observers on the crater edge felt hot vapor. When the crater was climbed on the evening of 25 January a clearing of the vapor enabled the bottom to be seen 280 m down. A 40-m-diameter vent was visible emitting bright yellow burning gas, radiant heat was felt on the faces of observers, and moderate degassing was heard.

Activity at Mbwelesu. Observations were made of Mbwelesu crater on 21 January. The two lava lakes observed in August 1999 had disappeared (BGVN 24:08). A brown pond surrounded by fumaroles was in the Vent B location, with large amounts of ash and rock to the SE. The sill on the SE edge of the crater had large craters and several large sections (over 10 m) that had broken off and fallen into the crater. The fumarole field 40 m SE of the crater rim had a temperature of 72.7°C. Heavy rains caused waterfalls and rockfalls inside the crater. The crater was otherwise quiet with some vapor emissions from many fumaroles on the floor. Fumaroles were also present in the location of the former lava lake at Vent C.

Activity in the 1953 Crater. The 1953 crater contained two levels. The higher (W half) contained a brown pond. The lower (E half) had developed a deep smoking vent. This was in the location of the green pond observed in August 1999.

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, PO Box 16, Chatsworth Island, N.S.W. 2469, Australia; Wellington Volcanic Ash Advisory Center (VAAC), MetService, PO Box 722, Wellington, New Zealand (URL: http://www.metservice.co.nz/).