Geologic Background:
Long Island, between Karkar and New Britain, is
composed of the Mount Reamur and Cerisy Peak
stratovolcanoes at the N and S ends of the island. The
broad profile of hexagonal-shaped Long Island is
dominated by two steep-sided stratovolcanoes, Mount
Reaumur in the north and Cerisy Peak in the south.
Collapse of the volcanic complex produced a large 10 x
12.5 km caldera, now filled by Lake Wisdom. Caldera
formation occurred during at least three major explosive
eruptions, about 16,000, 4,000, and 300 years ago (around
1660). The latter was one of the largest historical
eruptions in Papua New Guinea and deposited tephra across
the New Guinea highlands, prompting legends of a "Time of
Darkness." Post-caldera eruptions have constructed a
small cone, Motmot Island, in the south-central part of
Lake Wisdom.
Historic Activity:
- A major eruption around 1660, as large as Krakatau's
1883 event, is documented largely by oral legends and
extensive ash deposits in the New Guinea Highlands.
- Moderate explosive eruptions have occurred during the
20th century from vents at and near Motmot Island.
- Motmot (a cone that forms an island in the caldera
lake) erupted intermittently in 1973 and 1974 producing
several small cones and 3 small lava flows.
- A small new crater was formed in the interval between
aerial observations on Sept. 5, 1975 and April 29,
1976.
Recent Activity:
- The first eruption since 1976, occurred in early Nov.
1993 and was effectively over by early December. During
an aerial inspection of the caldera lake the source of
the eruption was identified. Near the centre of the lake
was a fresh patch of gray-brown water. The central part
of the patch was being frequently disturbed by underwater
explosions, and there were several sites of non-explosive
upwelling fluids and fine solid particles. The active
vents, oriented along an E-W linear vent system that may
have been about 500 m long, were located at water depths
of 300-350 m.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 4, No.1 and 2)
- Global Volcanism Network (V. 15, No. 9 to V. 18, No.
12).
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