The US National Weather Service has issued a warning to aviation following an eruption on Mount Shishaldin, a 2870 m stratovolcano (cone-shaped volcano made up of layers of ash and lava), located on Unimak Island in the Aleutian Island Chain, on Friday 14 July 2023. The volcano began erupting on Tuesday 11 July, but did not produce any major distuprion until slightly before 1.10 am local time onl Friday, when a large explosive eruption produced an ash cloud which rose to over 12 000 m above sealevel. A second eruption at about 7.10 am produced a smaller ash cloud, which on this occasion rose to about 4500 m.
Volcanic ash is extremely hazardous to aircraft in a number of ways. At its most obvious it is opaque, both visually and to radar. Then it is abrasive, ash particles physically scour aircraft, damaging components and frosting windows. However, the ash is most dangerous when it is sucked into jet engines, here the high temperatures can melt the tiny silica particles, forming volcanic glass which then clogs engine. When this happens the only hope the aircraft has is to dive sharply, in the hope that cold air passing through the engine during the descent will cause the glass to shatter, allowing the engine to be restarted. Obviously, this is a procedure that pilots try to avoid having to perform.
Mount Shishaldin is the westernmost of three volcanoes located in an east-west line across Unimak Island. It is the largest volcano in the Aleutian Islands, and considered to be coincided to be the most symmetrical volcano on Earth, with contour lines forming near perfect circles, although the north and south slopes are steeper than the east and west slopes. The current cone structure of Mount Shishaldin is thought to be less than 10 000 years old, and is built on the remnants of an older cone, which is exposed to the west and northeast sides. To the northwest of the main volcano is an area of massive lava flows, with 24 smaller cones, thought to be parasitic on the main volcano.
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.
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