Sunday, 8 July 2012

Seismic activity on Mount Spurr, Alaska.

Mount Spurr is the highest peak in the Aleutain Arc, the chain of volcanoes that runs through the  Aleutain Islands then onto the Alaskan mainland to the north of Cook Inlet. The current peak rises 3374 m above sea-level, but this is located within the rim of a larger, more ancient caldera. This 5 km wide crater is thought to have collapsed in the late Pleistocene or early Holocene (10-12 000 years ago), spreading debris 25 km to the southeast, including blocks of up to 100 m in diameter. Mount Spurr has undergone two significant recorded eruptions, on in 1953 and one in 1992. On both occasions ashfalls reached Anchorage, 130 km to the east. More recently the volcano underwent a significant period of tectonic activity in 2004-6, though this did not produce any significant eruptive activity.

Steam issuing from Crater Peak, the active vent on Mount Spurr, in September 1992. Behind the vent part of the ancient crater forms the highest point on the volcano. The debris flow to the left of the vent formed during the collapse of that crater, about 10 000 years ago. USGS.

On 25 June 2012 the Alaska Volcano Observatory recorded an increase in seismic activity on Mount Spurr, lasting about 45 minutes. This comprised several small Earthquakes reaching magnitude 1 on the Richter Scale, not in themselves dangerous or alarming (a human observer would be highly unlikely to notice such a small quake, but instruments are more sensitive), but even very minor quakes on a volcano can signify magma movements beneath the summit. However there has been no repeat of this activity, so it is not thought on this occasion that the movements were the precursor to an eruption. It is now thought possible that these tremors may have been the result of the sudden movement of a large body of water possibly a glacial flood outburst on the southern flank of the volcano (i.e. a sudden flood caused by the collapse of a glacier trapping a lake). 

The Aleutain Arc is a chain of volcanoes running from the mid North Pacific to the coast of Alaska, then onshore along the west shore of Cook Inlet. They follow the path of the Aleutain Trench, along which the Pacific Plate is being subducted beneath the North American Plate. As the Pacific Plate sinks into the Earth, a combination of the friction caused by its passage under the North American Plate and the heat of the planets interior causes it to partially melt. Some of this melted material then rises through the overlying North American Plate as liquid magma, fueling the volcanoes of the Aleutian Arc.

Diagrammatic representation of the subduction of the Pacific Plate (yellow) beneath the North American Plate (red). Alaska Volcano Observatory.


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