Eruptive Activity on Great Sisken Island.

The Alaska Volcano Observatory reported a vapour plume rising 300 m about the summit of Great Sitkin, a volcanic island in the Andreanof Islands, part of the Aleutian Island chain, on Sunday 19 November 2017. This is the first observed eruptive activity on the volcano since February 1974, though it is not entirely unexpected, as there has been a steady increase in seismic activity beneath the island since July 2016. Seismic activity beneath volcanoes can be significant, as they are often caused by the arrival of fresh magma, which may indicate that a volcano is about to undergo an eruptive episode. A second small plume was spotted over the island by a satellite on Tuesday 21 November.

 Small plume over Great Siskin Island on 19 November 2017. Alaska Volcano Observatory.

  

The volcanoes of the Alaskan Peninsula and Aleutian Islands are fed by magma rising from the Pacific Plate, which is being subducted beneath the North American Plate to the south along the Aleutian Trench. As the subducting plate sinks into the Earth it is subjected to enormous heat and pressure, causing more volatile minerals to melt. These then rise through the overlying North American plate as magma, fuelling the Alaskan volcanoes.

 How the subduction of the Pacific Plate beneath the North American Plate fuels the volcanoes of Alaska. Alaska Volcano Observatory.

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Magnitude 7.0 Earthquake in the Aleutian Islands.

The United States Geological Survey recorded a Magnitude 7.0 Earthquake at a depth of 33.5 km to the south of the Andreanof Islands in the southern Aleutians at 4.25 am local time (4.25 pm GMT) on Friday 30 August 2013. This is a large quake and could potentially have been very dangerous in a more settled area, though this was in a very remote location, and there are no reports of any damage or casualties, though the quake was felt on Atka Island.

The approximate location of the 30 August 2013 Andreanof Islands Earthquake. Google Maps.

Alaska lies on the North American Plate, with the Pacific Plate underlying the ocean to the south. The Aleutian Trench runs along much of the south coast of Alaska and the Aleutian Islands, with the Pacific Plate being subducted beneath this and passing under Alaska as it sinks into the Earth. This is not a smooth process, and the plates frequently stick together, then break apart as the pressure builds up, causing Earthquakes.

Witness accounts of quakes can help geologists to understand these events and the rock structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) you can report it to the USGS here.

See also Eruption on Mount Veniaminof, Further eruptions on Mount Pavlof, Volcanic activity on Mount Veniaminof, Eruption on Mount Pavlof and Eruption on Mount Cleveland.

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