Eruption ends; details of activity given

"The eruption that began under the caldera lake (figure 1) in early November was effectively over by early December. By late November there was no longer any seismic evidence of eruptive activity. However, mild hydrothermal activity was continuing as indicated by the occasional appearance of patches of discoloured water at the lake surface. The temporary observation post on the NE rim of the caldera was closed on 3 December. There have been no more reports from Long Island, so it is presumed that the eruption is over."

Figure 1. Map of Long Island showing the caldera lake (Lake Wisdom), Motmot Island, and the November 1993 eruption site. Courtesy of RVO.

The following report from RVO provides additional details of the visit . . . on 11 November (BGVN 18:11).

"A temperature survey around Motmot (a cone in the S part of the caldera) revealed that temperatures were much lower than normal; the highest reading was ~45°C. Previously, temperatures at springs and in the beach sand around Motmot were up to ~90°C. It was not possible to carry out a complete temperature survey because of erosion and landslides on the flanks of Motmot and a rise in lake surface level. A GPS survey was also conducted on Motmot, with the survey marks used as photogrammetric targets for the purpose of producing a map of Motmot.

"During an aerial inspection of the caldera lake the source of the eruption was identified. Near the centre of the lake was a fresh sub-circular patch of grey-brown water. The central part of the patch of discoloured water was being frequently disturbed by underwater explosions, and there were several sites of non-explosive upwelling fluids and fine solid particles. The frequent underwater explosions were producing visible shock waves, seen as flashes of white light. The larger explosions broke the lake surface and ejected sprays of water and ash a few tens of metres above the lake surface. The active vents, oriented along an E-W linear vent system that may have been ~500 m long, were located at water depths of 300-350 m.

"A temporary observation post equipped with a portable seismograph was established at the NE rim of the caldera, about 200 m above the lake level, on 13 November. Activity similar to that observed and recorded on 11 November continued until the 16th and then began to decline. After 17 November the explosions were not strong enough to break the surface of the lake, and generated only upwelling cells of fluids and fine solid particles. Audible explosive activity was not detected after 21 November. At the end of November the seismicity was so weak that it could not confidently be distinguished from normal background microseismicity. However, mild underwater activity was continuing as plumes of discoloured water continued to be formed spasmodically. Sometimes the lake had a uniform (normal) blue-green colour.

"The time of commencement of the eruption is not precisely known. However, direct visual observations of water and ash ejections above the lake surface, from a new site remote from Motmot, were made by a local government official on 5 November. These details did not reach the Rabaul Volcano Observatory until 24 November. Another indication of the time of onset of the eruption comes from the seismic records generated at the permanent seismic station at Karkar Island, ~160 km NW of Long Island. Unusual strong, continuous, harmonic tremor, attributed to the eruption at Long Island, began to be recorded at about 1100 on 3 November. It is likely that the sound effects of the underwater explosions alerted the local government official at Long Island to hike in from the coast to the caldera rim to investigate on 5 November.

"The previous eruptive activity at Long Island in the period 1973-75 was centered at Motmot. Short bursts of Strombolian activity erected a number of small spatter and cinder cones, and produced a small lava flow. Temperature surveys indicated minor changes in the plumbing system of springs around Motmot until the mid-1980's. A progressive weakening of thermal activity around Motmot was recorded from late 1985."

Information Contacts: C. McKee and R. Stewart, RVO.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

May 1973 (CSLP 58-73)

Small eruption within the crater lake on 18 April

Card 1626 (08 May 1973) Small eruption within the crater lake on 18 April

The Central Volcanological Observatory reported the following . . . by cable. "A small eruption has occurred on Long Island. The first reports received on 1 May indicate that the eruption started on 18 April. An aerial inspection was made on 2 May and there appears to be no immediate danger to the 700 inhabitants. The eruption center is a small island in the caldera lake ejecting steam and ash at 5-50 second intervals 100-150 m high. Investigations are continuing."

Information Contacts: R.A. Davies, Central Volcanological Observatory, Rabaul, Papua New Guinea.

January 1979 (SEAN 04:01) Cite this Report

New fumarolic activity

"A report was received on 26 January of a supposed eruption of Motmot cone about mid-January. A flypast by a volcanologist on 15 January had revealed nothing unusual. However, aerial inspections on 29 January indicated considerable new vapour emission, including one powerful fumarole that was jetting white vapour and possibly water at the W shoreline of Motmot Island (350 m in diameter). Vapour from this jet obscured details nearby, but no clear evidence of recent eruptive activity was noted. No new craters or cones were present, and no new ejecta could be recognised on the island. Pending further information, it is assumed that only vapour activity has occurred so far."

Information Contacts: R. Cooke, RVO.

February 1979 (SEAN 04:02) Cite this Report

Fumarolic activity, but no eruption

"An aerial inspection of Long Island on 23 February confirmed that no eruption had taken place in January. Only small volumes of vapour were seen emanating from the summit craters and from a fumarole at the W shoreline. This fumarole is in a different position from the powerful one seen on 29 January."

Information Contacts: C. McKee, RVO.

September 1990 (BGVN 15:09) Cite this Report

Low-level activity at thermal areas

"An aerial inspection on 3 September indicated that thermal areas on Motmot . . . remained at a low level of activity. Vegetation was spreading in some areas, notably in the crater of the 1974 cone, and no fresh sulphur deposits were seen.

Information Contacts: I. Itikarai and C. McKee, RVO.

November 1993 (BGVN 18:11) Cite this Report

Eruption in early November, first since 1976

"A new eruption began . . . in early November. The first report was received on 8 November but contained few details of activity. A team from RVO overflew Long Island on 9 November and found that almost the entire body of water in the caldera lake (Lake Wisdom) had changed colour, from blue-green to orange-brown, but there was no visible eruptive activity. The source of the discoloured water could not be discerned, although it appeared to be remote from the Motmot cone in the S part of the caldera, the site of the most recent eruptions (mid-1970's). There was no sign of a recent eruption at Motmot itself.

"Volcanologists returned . . . on 11 November to carry out ground-based investigations at Motmot. While setting up a seismograph, frequent earthquakes were felt, and muffled thudding noises were heard. The noises were more noticeable on the N to NE side of Motmot, suggesting that the source of the activity might be offshore in that direction. The seismogram consisted of continuous harmonic and irregular tremor with variable amplitude corresponding with the sound effects."

Information Contacts: I. Itikarai, and C. McKee, RVO.

December 1993 (BGVN 18:12) Cite this Report

Eruption ends; details of activity given

"The eruption that began under the caldera lake (figure 1) in early November was effectively over by early December. By late November there was no longer any seismic evidence of eruptive activity. However, mild hydrothermal activity was continuing as indicated by the occasional appearance of patches of discoloured water at the lake surface. The temporary observation post on the NE rim of the caldera was closed on 3 December. There have been no more reports from Long Island, so it is presumed that the eruption is over."

Figure 1. Map of Long Island showing the caldera lake (Lake Wisdom), Motmot Island, and the November 1993 eruption site. Courtesy of RVO.

The following report from RVO provides additional details of the visit . . . on 11 November (BGVN 18:11).

"A temperature survey around Motmot (a cone in the S part of the caldera) revealed that temperatures were much lower than normal; the highest reading was ~45°C. Previously, temperatures at springs and in the beach sand around Motmot were up to ~90°C. It was not possible to carry out a complete temperature survey because of erosion and landslides on the flanks of Motmot and a rise in lake surface level. A GPS survey was also conducted on Motmot, with the survey marks used as photogrammetric targets for the purpose of producing a map of Motmot.

"During an aerial inspection of the caldera lake the source of the eruption was identified. Near the centre of the lake was a fresh sub-circular patch of grey-brown water. The central part of the patch of discoloured water was being frequently disturbed by underwater explosions, and there were several sites of non-explosive upwelling fluids and fine solid particles. The frequent underwater explosions were producing visible shock waves, seen as flashes of white light. The larger explosions broke the lake surface and ejected sprays of water and ash a few tens of metres above the lake surface. The active vents, oriented along an E-W linear vent system that may have been ~500 m long, were located at water depths of 300-350 m.

"A temporary observation post equipped with a portable seismograph was established at the NE rim of the caldera, about 200 m above the lake level, on 13 November. Activity similar to that observed and recorded on 11 November continued until the 16th and then began to decline. After 17 November the explosions were not strong enough to break the surface of the lake, and generated only upwelling cells of fluids and fine solid particles. Audible explosive activity was not detected after 21 November. At the end of November the seismicity was so weak that it could not confidently be distinguished from normal background microseismicity. However, mild underwater activity was continuing as plumes of discoloured water continued to be formed spasmodically. Sometimes the lake had a uniform (normal) blue-green colour.

"The time of commencement of the eruption is not precisely known. However, direct visual observations of water and ash ejections above the lake surface, from a new site remote from Motmot, were made by a local government official on 5 November. These details did not reach the Rabaul Volcano Observatory until 24 November. Another indication of the time of onset of the eruption comes from the seismic records generated at the permanent seismic station at Karkar Island, ~160 km NW of Long Island. Unusual strong, continuous, harmonic tremor, attributed to the eruption at Long Island, began to be recorded at about 1100 on 3 November. It is likely that the sound effects of the underwater explosions alerted the local government official at Long Island to hike in from the coast to the caldera rim to investigate on 5 November.

"The previous eruptive activity at Long Island in the period 1973-75 was centered at Motmot. Short bursts of Strombolian activity erected a number of small spatter and cinder cones, and produced a small lava flow. Temperature surveys indicated minor changes in the plumbing system of springs around Motmot until the mid-1980's. A progressive weakening of thermal activity around Motmot was recorded from late 1985."

Information Contacts: C. McKee and R. Stewart, RVO.