Karkar is a volcanic island about 30 km north of Papua New Guinea. The whole island is a single giant stratovolcano (cone-shaped volcano), rising 1839 m from the sea. The summit is made up of two concentric craters. The outer, older crater is thought to have last been active around 9000 years ago. The inner crater, which has formed since the last eruption of the outer crater, is still occasionally active.
Map of Karkar Island, which clearly shows it's volcanic nature. Karkar is roughly 25 × 19 km.
On 1 February 2012 the Darwin Volcanic Ash Advisory Centre reported an ash cloud rising 7.6-10.7 km above the island. Such ash clouds seem to be a moderately common event on Karkar, the most recent having been in February 2010, and the one prior to that in November 2009. These ash plumes are often associated with mass-tree deaths on the heavily forested island. The last full eruption on the island was in 1978-81, when two volcanologists, Robin Cooke and Elias Ravian, both of the Rabaul Volcanological Observatory, were killed in a summit explosion. This eruptive cycle lasted 15 months, and was marked by continuous ash clouds and fumaroles (gas emissions), explosions and matter being thrown from the crater.
Karkar Island is located on the South Bismarck Plate, sightly to the north of is boundary with the Woodlark Plate (the nearby coast of Papua is on the Woodlark Plate). This is a complex plate margin; to the east the Solomon Sea Plate is being subducted beneath the South Bismarck Plate as it does so it is melted by the heat of the Earth's interior, with some of the melted material rising up through the overlying South Bismark Plate as magma and erupting at the surface to form the volcanoes of New Britain. On the eastern end of the margin between the Woodlark Plate and the South Bismark the situation is the same, with the Woodlark Plate sinking beneath the South Bismarck, but, slightly to the southwest of Karkar, this turns to a transform margin, with the plates simply sliding past one-another, and further to the west the margin becomes divergent, with the two plates drawing apart and new land being created between them.
See also Series of Earthquakes shakes Vanuatu, Earthquake in Vanuatu, Magnitude 6.8 Earthquake hits Bam Island, Earthquake in the Kermadec Islands triggers tsunami alert and Volcanoes on Sciency Thoughts YouTube.
