Halamahera is the largest of the Maluku islands in Indonesia; the term Halamahera Islands is used to imply Halamahera Island plus the smaller islands around it, particularly the string of islands running parallel to the main island's west coast.
The Halmahera Islands.
The northernmost of these islands is Ternate, a volcanic island that forms the tip of a vast, submerged volcano, Gamalama. Gamalama is a stratovolcano (a cone-shaped volcano made up of layers of ash and lava, the sort of volcano you see in a Hollywood movie), with three cones in a north-south line across the island, with the youngest in the north and the oldest in the south. Gamalama is a very active volcano, with several eruptions a decade; since 1990 it has erupted in June 1991, May 1993, January-October 1994, July-October 2003, August 2007 and May 2008. Almost all eruptions are from the summit cones, though eruptions from fissures on the flanks have been recorded, in 1763, 1770, 1775 and 1962-3.
1788 illustration of an eruption on Gamalama, by George Bernard.
Having been quiet since 2008, on 1 December 2011 a white plume was seen reaching 150 m above Gamalama. The summit was obscured by cloud on the 2nd and 3rd, but when this cleared the plume was still present. The Indonesian Pusat Vulkanologi Dan Mitigasi Bencani Geologi (Centre for Volcanology and Geological Hazard Mitigation) recorded a number of small Earth tremors on the island during this period. On the 5th and 6th a plume of ash was recorded rising 2.4-3.0 km above the island and drifting southeast by the Darwin Volcanic Ash Advisory Centre. This plume has persisted till the time of writing (21 December 2011) reaching as high as 6.1 km, though it has often been obscured by heavy cloud in the area.
Over a hundred kilometers to the northeast lies Dukono, on the northwestern limb of Halmahera Island. Dukono has been in more-or-less constant eruption since 1933. It has a complex structure, with multiple overlapping craters and numerous peaks. Explosive eruptions and lava-flows are a regular feature of Dukono. The most recent explosive eruptions took place in August this year, since when a plume of ash has been more-or-less constant over the area, reaching as high as 3 km above the volcano.
An ash cloud over Dukono in 1991.
There are 13 other volcanoes in the Halmahera Islands. Of these Tarakan, Tobaru, Todoku-Ranu, Jailolo, Hiri, Tidore, Mare, Moti, Tigalalu, Amasing and Bibinoi have not erupted in historical times. Ibu last erupted in 2009, Gamkonora in 2007 and Makian in 1988.
The volcanos of the Halmahera Islands.
The Halmahera Islands arc a volcanic arc; formed where one tectonic plate is being subducted beneath another, with the underlying plate being melted by the heat of the Earth's interior, and lighter minerals bubbling up through the overlying plate to form volcanoes. However the Halmahera Islands are unique in that they lie on a double subduction zone. The underlying plate, a northeaster extension of the Molucca Sea Plate, is being overridden form the Philippine Plate from the east and the Eurasian Plate from the west. The Halmahera volcanoes are located where the Philippine Plate is overriding the Molucca Sea Plate; to the west the Sangihe Islands lie where the Molucca Sea Plate is being overridden by the Eurasian Plate.
The disappearing Molucca Sea Plate is giving rise to two island arcs, as it is subducted from two directions.
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