User:Ian.podmore/Mount Shishaldin

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Shishaldin Volcano, or Mount Shishaldin (/ʃɪˈʃældən/), is one of six active volcanoes on Unimak Island in the eastern Aleutian Islands of Alaska. It is the highest mountain peak of the Aleutian Islands, rising to a height of 9,373 ft (2,857 m) above sea level. Shishaldin's magma supply is generated via flux melting above the Aleutian Trench, where the Pacific Plate subducts beneath the North American Plate. Due to its remote location and frequently inclement weather, the Alaska Volcano Observatory (AVO) monitors the volcano remotely via satellites and a seismic network deployed in 1997. Shishaldin is one of the most active volcanoes in the Aleutian Islands, with 40 confirmed eruptions in the last 11,700 years. Notably, Shishaldin produced a sub-Plinian (VEI 3) eruption in 1999.

Geography and Geology

Shishaldin is located on Unimak Island, the easternmost of the Aleutian Islands. The nearest settlement to the volcano is False Pass (population ~100) at 45 km away. The view of Shishaldin from False Pass is obscured by the Round Top and Isanotski volcanoes, preventing direct observations of eruptions in many instances. The upper 6,600 ft (2,000 m) is almost entirely covered by glacial snow and ice. In all, Shishaldin's glacial shield covers about 35 sq mi (91 km²). The Shishaldin cone is less than 10,000 years old and is constructed atop the remnants of the ancestral volcano. The modern cone is highly symmetrical, with near-perfect circular topographic contours above 6,500 ft (2,000 m). The Shishaldin edifice contains about 300 km³ (72 cu mi) of material. A very steady steam plume rises from its small summit crater which is about 500 ft (150 m) across and slightly breached along the north rim. The ancestral cone is exposed on the west and northeast sides of the volcano between 1,500-1,800 m elevation and hosts rougher topography than the modern cone. The west and northwest sides of the volcano are home to over 50 flank vents. These flank vents consist mostly of cinder cones with some maars and tuff cones; most vents produced explosive eruptions, however some cinder cones are associated with lava flows from effusive eruptions. In appearance, the volcano can vary from nearly completely white to almost all black, depending on the occurrence of ashfall deposits.

Shishaldin is one of many volcanic centers located along the 4,000 km Aleutian Arc, which stretches from Kamchatka, Russia, to mainland Alaska, United States. The Aleutian Arc is formed where the Pacific Plate subducts beneath the North American Plate. Subduction is orthogonal in the eastern 2,500 km of the arc, giving rise to active volcanism, and becomes increasingly oblique westward until the plate interface becomes strike-slip near Buldir Island and volcanism ceases. Magma generation occurs via flux melting, where the dehydration of hydrous minerals in subducted oceanic crust inputs water into and lowers the melting temperature of the peridotite mantle wedge above the subducted slab. Average magma composition in the Aleutian arc is typically basaltic to basaltic andesite. In addition to active volcanism, the Aleutian Arc hosts significant seismicity, with roughly 70 M_W 7.0+ earthquakes since 1900. The rupture zone of the 1957 M_W 9.1 Great Alaska Earthquake extended to the western side of Unimak Island (where Shishaldin is located). In addition, the 1946 M_W 8.6 megathrust earthquake, associated with a significant tsunami, ruptured offshore of Unimak Island.

Eruptive History

Written record of eruptive activity began in 1775 by Russian explorers and traders in the Aleutians. There have been 40 confirmed Holocene eruptions ranging from 0-3 on the Volcano Explosivity Index (VEI), the majority of which were VEI 2 eruptions. Shishaldin produces basaltic eruptions, typically Strombolian in style, and frequently produces ash clouds which rise tens of kilometers into the atmosphere. Lava flows are occasionally produced by the summit crater.

Because Shishaldin is rarely visible due to its remote location and inclement weather, Alaska Volcano Observatory (AVO) relies on remote sensing to detect eruptive activity. In 1997, AVO installed a network of six seismometers and one pressure sensor on the volcano. AVO also uses satellites sensitive to thermal anomalies and volcanic plumes to monitor Shishaldin for eruptive activity.

1999 Eruption

Shishaldin displayed thermal signals of unrest detected by satellite on February 9, 1999. Throughout February and March, remotely detected thermal and seismic signals exceeded background levels. Strombolian activity was confirmed on 17 April via overflight. Shishaldin produced a sub-Plinian (VEI 3) eruption on 19 April, ejecting basaltic tephra in two separate plumes which rose 9 and 16 km into the atmosphere. The plumes deposited ash and tephra on the southern slopes of the volcano and into the Pacific Ocean, and several lahars were documented on the north flanks. Intense Strombolian activity persisted for ~3 hours after the large explosive events. Several days of intermittent Strombolian activity followed, ash plumes again depositing material on the southern slopes of the volcano. The largest thermal anomaly and most significant seismic activity of the 1999 eruptive sequence were detected on 22-23 April; although there were no direct observations of the activity, the seismic and thermal signals closely resembled the intense Strombolian activity of 19 April. Activity then declined, although intermittent Strombolian activity continued for several months afterward.

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In 1967, Shishaldin Volcano was designated as a National Natural Landmark - the Shishaldin Volcano National Natural Landmark - by the National Park Service. The landmark is 62,714 acres (25,379 ha) and a plaque was erected near the coast.