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Major eruption at Shishaldin volcano, ash to 12 km (40 000 feet) a.s.l., Alaska

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A major eruption is ongoing at Shishaldin volcano in Alaska, with satellite imagery suggesting the top of the ash cloud was approaching 12 km (40 000 feet) above sea level just before 08:00 UTC on July 23, 2023. As a result, the Aviation Color Code was again raised to Red.

The level of unrest has increased at Shishaldin Volcano over the past 6 hours, the Alaska Volcano Observatory (AVO) reported at 00:53 UTC on July 23.1

“A steady increase in seismic tremor and intermittent infrasound signals consistent with small explosions are ongoing,” AVO said, adding that despite cloud cover obscuring the volcano, elevated surface temperatures consistent with lava erupting at the summit are evident in the latest satellite data.

At 07:30 UTC today, an ash cloud from the volcano was reaching 9 km (30 000 feet) a.s.l., as observed in satellite data and reported by pilots, AVO said.

In response, the aviation Color Code was raised to Red and the Volcano Alert Level to Warning. The National Weather Service has issued a SIGMET for this ash cloud, and a Special Weather Statement has been issued for possible trace ash on False Pass.

“Pilot reports at 07:27 UTC confirmed height to 9 km (30 000 feet) a.sl. but satellite imagery now suggests the top of the ash cloud is approaching 12 km (40 000) feet,” the Anchorage VAAC reported in the Volcanic Ash Advisory issued at 07:50 UTC.2

Based on previous eruption cycles, significant ash emissions are likely to continue for an hour or more. Pyroclastic and mudflows are likely on the immediate flanks of the volcano, AVO said.

The last three significant ash events during the current eruption resulted in ash clouds up to 12 km (40 000 feet) above sea level, similar to other historical eruptions. These events can occur with little warning.

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Shishaldin volcano at 07:56 UTC on July 23, 2023 (mid-infrared). Credit: NOAA-18/AVHRR, AVO/USGS

The symmetrically-shaped, ice-covered Shishaldin is the tallest and one of the most active volcanoes in the Aleutian Islands, situated on the westernmost part of three large stratovolcanoes in the eastern segment of Unimak Island.

The volcano was built upon an older, glacially sculpted structure, with its composition being predominantly basaltic. Remains of a prehistoric volcano are visible on the western and northeastern sides at 1 500 – 1 800 m (4 921 – 5 906 feet) altitude.

There are more than twenty pyroclastic cones on its northwestern slope, which is covered by extensive aa lava flows. Regular explosive activity, mostly consisting of Strombolian ash eruptions from the small crater at the summit, sometimes generating lava flows, has been documented since the 1700s.3

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Shishaldin volcano on July 18, 2023 (SWIR). Image credit: Copernicus EU/Sentinel-2, EO Browser, The Watchers

References:

1 VONA – Shishaldin – AVO – 00:53 UTC, July 23, 2023

2 VAA – Shishaldin – Anchorage VAAC – 07:50 UTC, July 23, 2023

3 Shishaldin – Geological summary – GVP

Featured image: Shishaldin volcano on July 18, 2023 (SWIR). Image credit: Copernicus EU/Sentinel-2, EO Browser, The Watchers

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