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Reactivation of dormant Edgecumbe volcano, Alaska

mount edgecombe viewed from sitka

Researchers reporting in AGU’s Geophysical Research Letters show that the seismic swarm detected near Mt. Edgecumbe volcano in Southeast Alaska, U.S. in April 2022 was caused by magmatic activity taking place under this transform fault volcano previously considered dormant.

  • As Edgecumbe is located near the major Queen-Charlotte Fairweather fault that separates the Pacific and North American plates, similar earthquakes were previously assumed to be tectonic
  • While the eruption is not imminent, the activity at Edgecumbe presents a rare opportunity for observation of the reactivation of the dormant volcanic system
  • The last eruption at this volcano took place in 2080 BCE ± 90 years

“Since magmatic activity at volcanoes is often accompanied by ground deformation, we analyzed available satellite radar data going back to the fall of 2014,” the authors noted.1

The analysis revealed ongoing crustal motion toward the satellite of up to 7.1 cm (2.8 inches) per year starting in August 2018.

Modeling of the deformation suggests that magma sourced from a gently dipping tabular body at a depth of 20 km (12.4 miles) is pooling at about 10 km (6.2 miles) depth.

“We believe that we are observing magma rising through malleable crust into an existing magmatic system and that the observed earthquakes are created as the overlying rock adjusts to the increased magmatic pressure. The observed activity is rare, especially in similar tectonic settings, and presents an opportunity to better understand the reactivation of dormant volcanoes.”

Uplift at Edgecumbe volcano
Main map: Kruzof Island with Mount Edgecumbe and Crater Ridge labeled. Cumulative displacement measured in the line of sight (LOS) of Sentinel 1 radar satellites from Nov 2014-Dec 2021 (positive LOS displacement means uplift and horizontal motion toward the satellite). All deformation is given with observations at the reference pixel removed (chosen away from volcano to only capture tectonic and glacial isostatic adjustment / uplift), resulting in predominantly volcanic motion shown. Red indicates up to 27 cm (10.6 inches) of inflation. Timeseries insets show deformation over time at locations numbered 1-4 on the main map. Vertical line shows onset of inflation in Aug 2018. We observe linear inflation of up to 8.7 cm (3.4 inches) /yr since then. Figure by Yitian Cheng and Ronni Grapenthin, AVO/UAFGI

A retrospective analysis of earthquake data in the area of Mount Edgecumbe showed that a small number of earthquakes started occurring under the volcano in 2020.2

The recent earthquake activity that started on April 11 was unusual in having a greater number of events, however.

The earthquakes detected under the volcano since 2020 are all M3.0 or smaller.

Note that only the largest of the earthquakes can be located by regional seismic networks; hundreds of very small additional events have been detected, but not located.

edgecumbe volcano earthquakes 1990 - 2022
Map showing earthquake epicenters near Sitka Alaska from 1990 through 2022 using data products generated by the Alaska Earthquake Center and obtained via the ANSS Combined Catalog. Epicenters before 2010 are shown as black circles and from 2010 to the present are color-coded by date (see legend for timeline). Inset shows epicenters at Kruzof Island for the same time period. The lower panel in the inset shows earthquake focal depth versus time from 2010 to the present with blue symbols for from 2020 through 2021 and red symbols for 2021 to the present. Figure by Ronni Grapenthin, AVO/UAFGI

In May 2022, the Mount Edgecumbe volcanic field (MEVF) was classed as “historically active” by the standards of the Alaska Volcano Observatory (AVO).3

AVO defines an “active” volcano as a volcanic center that has had a recent eruption or a period of intense deformation, seismic activity, and/or fumarolic activity that likely reflects the accumulation of magma in the crust below the volcano. Note that this definition does not require an actual eruption and instead uses eruptions as well as measurements of magma beneath a volcano.

AVO uses the term “historical” to mean within the past 300 years and the beginning of written records in Alaska.

Mount Edgecumbe volcano on April 13 2022 map
Mount Edgecumbe volcano on April 13, 2022. Credit: Copernicus EU/Sentinel-2, TW

Geological summary

The Pleistocene-to-Holocene Edgecumbe volcanic field covers about 260 km2 (100 miles2) of Kruzov Island west of Sitka in the SE panhandle of Alaska.

The basaltic-to-dacitic field is dominated by the large composite cones of Mount Edgecumbe, Crater Ridge, and Shell Mountain, and has an unusual tectonic setting only 16 km (10 miles) E of the Queen Charlotte-Fairweather transform fault separating the North American and Pacific plates.

Mount Edgecumbe is a stratovolcano with a well-defined crater and is the largest volcano in the field.

Crater Ridge is truncated by a 1.6 km (1 mile) wide, 240 m (787 feet) deep caldera. These and other vents are oriented along a SW-NE line.

Volcanic activity originated about 600 000 years ago along fissures cutting Kruzof Island.

A series of major silicic explosive eruptions took place about 9 000 – 13 000 radiocarbon years ago.

The latest dated eruptions were phreatomagmatic explosions during the mid-Holocene, and all postglacial activity has been pyroclastic. Reports of historical eruptions of Mount Edgecumbe are unsubstantiated.4

References:

1 Return From Dormancy: Rapid Inflation and Seismic Unrest Driven by Transcrustal Magma Transfer at Mt. Edgecumbe (L’úx Shaa) Volcano, Alaska – AGU Geophysical Research Letters – October 2022 – DOI: 10.1029/2022GL099464

2 EDGECUMBE VOLCANO – ALASKA VOLCANO OBSERVATORY INFORMATION STATEMENT U.S. Geological Survey – April 22, 2022

3 Mount Edgecumbe volcanic field changes from “dormant” to “active” — what does that mean? – Alaska Volcano Observatory – May 9, 2022

4 Mount Edgecumbe – Geological summary – GVP

Featured image: Mount Edgecumbe, viewed from Sitka, Alaska. Photo courtesy of Duncan Marriott

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