Shishaldin Volcano, or Mount Shishaldin (), 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 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 satellite and a seismic network deployed in 1997. Notably, Shishaldin produced a sub-Plinian (VEI 3) eruption in 1999.
Name
The Aleuts named the volcano Sisquk or Sisagux, meaning "mountain which points the way when I am lost." The spelling Shishaldin comes from the Russian version, Шишалдина, of the Aleut name. 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<sub>W</sub> 7.0+ earthquakes since 1900. The rupture zone of the 1957 M<sub>W</sub> 7.1 earthquake extended to the western side of Unimak Island (where Shishaldin is located). In addition, the 1946 M<sub>W</sub> 8.6 megathrust earthquake, associated with a significant tsunami, ruptured offshore of Unimak Island. During this period of non-eruptive seismic activity, it has been puffing steam, with puffs also occurring about every 1–2 minutes. There were reports in 2004 of small quantities of ash being emitted with the steam. In 2014, a low-level effusive eruption cycle started which lasted into 2016.
2019 eruption
A new period of activity started in July 2019 with incandescence observed in the summit crater during a time of increased seismic activity. On July 23, an active lava lake and minor spattering within the summit crater was observed. A new lava effusion event began on October 13 advancing over the next several weeks. The summit cone partially collapsed on November 25, producing a pyroclastic flow down the northwest side of the volcano and a new lava flow. On December 12, a short-lived explosion from Shishaldin expelled an ash cloud to . During late December, eruptive activity continued with lava flows and low-level explosive activity at the summit. On January 3, 2020, seismicity led to an ash cloud eruption that reached as high as . Another large ash cloud was emitted on January 19, 2020. This event ended shortly afterwards with an abrupt end in seismicity. The lava flows emitted during the eruptive events have cut deep channels in the snow and ice mantle of the volcano's north slope.
2023 eruption
After more than two years of dormancy, another eruptive cycle began and on 14 July 2023, the volcano emitted ash plumes up to high after several explosions at the summit. Fourteen significant explosive events occurred at Shishaldin between 12 July and 3 November 2023. For approximately a week following the November 3 explosive event, activity consisted of frequent small explosions, steam and gas emissions, and collapse events around the summit crater rim. By January 2024, only minor steam emissions and small events deep in the summit crater continued. The Alaska Volcano Observatory declared this eruptive cycle ended by late August 2024. As a result of the collapses in the summit during this eruption, the previously symmetrical summit crater now has a deep east–west trending notch.
Climbing
The first recorded ascent of Shishaldin was in 1932, by G. Peterson and two companions. Given the straightforward nature of the climbing (Alaska Grade 1, snow up to 40 degree slope), it is possible that an earlier ascent occurred, either by native Aleuts, Russians, or other visitors.