Sunday 22 July 2012

Eruption on Pagan Island.

Pagan Island is located in the Northern Mariana Islands (a US territory in the west Pacific), roughly 500 km north of Guam. It is made up of the exposed tips of two submarine stratovolcanoes (cone-shaped volcanoes made up of layers of ash and lava), connected by a narrow isthmus of land. The northern of these two volcanoes is the largest and most active, reaching 570 m above sea-level; almost all recorded eruptions on the island have come from this peak. The southern cone is smaller, at 548 m, and has only been recorded to erupt twice, in 1864 and 1929. This southerly volcano has a more complex structure than the northern, with five craters strung out in a line from northeast to southwest.

Simple map of Pagan Island. Smithsonian Global Volcanism Program.

On 10 July 2012 the Washington Volcanic Ash Advisory Center reported a cloud of volcanic ash rising 1.5 km above Pagan Island, which drifted roughly 50 km to the northwest. Some ashfall was also reported on the island. Plumes of gas and steam are extremely frequent on Pagan, and small ash eruptions once or twice a year, though this is the largest such event since August 2010.

The largest eruption recorded on the island happened in 1981, when a mushroom cloud reached 13.5 km above sea-level and 53 people had to be evacuated from the island. The island was evacuated again in 1993.

Terra Satellite image of a steam plume from North Pagan on 7 May 2012. NASA/GSFC.

The volcanic Mariana Islands are located on the eastern margin of the Philippine Plate, close to its margin with the Pacific Plate. The Pacific Plate is being subducted beneath the Philippine Plate along the Mariana Trench. As it sinks into the Earth it is heated by a combination of the heat of the planet's interior and friction with the overlying Philippine Plate. This causes it to partially melt, producing liquid magma which then rises up to fuel the volcanoes of the Mariana Arc.

Simplified diagram of the subduction of the Pacific Plate beneath the Philippine plate, and how this fuels the volcanos of the Mariana Islands. NOAA Ocean Explorer.

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