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U.S. Geological Survey Scientific Investigations Map 3145

Geologic Map of Mount Gareloi, Gareloi Island, Alaska

By Michelle L. Coombs, Robert G. McGimsey, and Brandon L. Browne

Thumbnail of and link to map sheet PDF (5.7 MB)Introduction

Gareloi Island (lat 51.7° N., long 178.8° W.) is located in the Delarof Islands group of the Aleutian Islands, approximately 2,000 km west-southwest of Anchorage and 150 km west of Adak, the westernmost town in Alaska. This small (~8- x 10-km diam), uninhabited island is constructed exclusively of eruptive products from Mount Gareloi volcano, a Pleistocene and Holocene, steep-sided, composite stratocone that rises 1,573 m (5,161 ft) above sea level (asl). Mount Gareloi has been one of the most active Aleutian volcanoes since its discovery by the Bering expedition in the 1740s, though, because of its remote location, detailed observations of eruptive activity have been scant.

As part of an effort to both monitor and study all historically active volcanoes in Alaska, the Alaska Volcano Observatory (AVO) undertook a field program at Mount Gareloi in the summer of 2003. During a month-long period, seismic networks were installed at Mount Gareloi and the neighboring Tanaga volcanic cluster. During this time, we undertook the first geologic field study of the volcano since Robert Coats visited Gareloi Island for four days in 1946. Understanding the geology of this relatively small island is important from a hazards perspective, because Mount Gareloi lies beneath a heavily trafficked air route between North America and Asia and has frequently erupted airborne ash since 1760. At least two landslides from the island have deposited debris on the sea floor; thus, landslide-generated tsunamis are also a potential hazard. Since seismic instruments were installed in 2003, they have detected small but consistent seismic signals from beneath Mount Gareloi's edifice, suggesting an active hydrothermal system. Mount Gareloi is also important from the standpoint of understanding subduction-related volcanism, because it lies in the western portion of the volcanically active arc, where subduction is oblique to the arc front. Understanding the compositional evolution of Mount Gareloi fills a spatial gap in along-arc studies.

The digital database contains the geologic map information used to publish the map. The database was constructed in order to produce a geologic map as a basis for understanding volcanic processes and hazards of Mount Gareloi. All the geologic map information used to publish Scientific Investigations Map 3145 is contained in this digital database. The database includes feature classes that depict geologic map units and the contacts between them, geologic structures, and sample locations. The feature dataset GarGeology contains the feature classes GarGeoLines, GarGeoPoints, GarGeoPolys, GarStructure, GarStations, GarGeoPolysAnno, and All_samplesAnno. Additionally, the geodatabase includes a nonspatial table of geochemistry data, GarGeochem.

First posted May 16, 2012

Updated January 9, 2015

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  • This report is also available in print from:

    USGS Information Services, Box 25286,
    Federal Center, Denver, CO 80225
    telephone: 888 ASK-USGS; e-mail:

For additional information contact:
Alaska Volcano Observatory staff
4210 University Drive
Anchorage, AK 99508
Alaska Volcano Observatory

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Suggested citation:

Coombs, M.L., McGimsey, R.G., and Browne, B.L., 2012, Geologic map of Mount Gareloi, Gareloi Island, Alaska: U.S. Geological Survey Scientific Investigations Map 3145, pamphlet 18 p., 1 sheet, scale: 1:24,000, available at

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