Thursday, 6 September 2012

Seismic activity on Little Sitkin.

Little Sitkin is a circular volcanic island roughly six kilometers in diameter in the Rat Islands, in the Western Aleutians. The island contains to volcanic calderas, one within the other. The outer caldera is probably Pleistocene in age, and forms a 4.8 km diameter, with two summits on the rim, a 1188 m summit in the northeast and a 597 m summit in the south. The inner caldera is roughly 2.7 × 4.0 km, and shares much of the southern and eastern rim of the outer crater. 

Satellite image of Little Sitkin taken on 23 October 2002. Ocean Dots/NASA/Johnson Space Center.

On 29 August 2012 the Alaska Volcano Observatory, which maintains a remote monitored four-station seismic network on the island, detected the first of a series of seismic swarms which rose in frequency until 31 August, since when the level of activity has declined but not ceased. Such swarms of earthquakes have important implications on volcanoes, as they may indicated the movement of fresh magma into chambers beneath the volcano, which may in turn indicate the volcano is about to erupt.

Little Siskin is last thought to have erupted in about 1900, based upon analysis of dacite (lava) flows on the south and west of the island; though this is far from precise, the eruption could have happened anywhere between 1870 and 1930. The last eruption to have been observed lasted from 1828 to 1830. An eruption was also observed in 1776.

The Aleutian Islands are a volcanic arc (or more accurately a series of volcanic arcs) on the North American Plate to the north of its boundary with the Pacific Plate. The Pacific Plate is being subducted beneath the North American Plate along the Aleutian Trench, to the south of the islands. As the Pacific Plate sinks into the Earth it is partially melted by a combination of the friction with the North American Plate and the heat of the planet's interior. Some of the melted material then rises through the overlying North American Plate as magma, fueling the volcanoes of the Aleutian Arc.

The subdution of the Pacific Plate (yellow) beneath the North American Plate (red) along the Aleutian Trench. Alaska Volcano Observatory.


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