Umnak, Aleutian Islands

Umnak is the third largest island of the Aleutian Islands at 1,793 km² in area — only Unimak and Unalaska are larger. Umnak is located towards the western end of the Fox Islands group, 6 km across the Umnak Pass from the southern end of Unalaska (Unalaska is partially visible at the top-right corner of the above image).

The island has an overall length of 117 km with east to west dimensions averaging 30 km. There are two distinct lobes to the outline of Umnak, formed where Inanudak Bay cuts deeply into the central regions of the west coast; the northern and southern lobes are connected to one another via a relatively narrow isthmus of 5 km in width.

The northern half of Umnak is formed largely from the imposing, 35 km wide, bulk of the Okmok Volcano. The broad summit of Okmok contains a 9.5 km caldera formation with a generally flat central basin with an average elevation of 370 m above sea level. The rim of the caldera rises to a height of 1,073 m. The remains of an older caldera formation lie outside of the Okmok caldera as evidenced by the presence of two arcuate ridge formations on the northern and eastern sides of the younger caldera. A number of smaller satellite cones and lava domes dot the broad flanks of Okmok, including the 1,253 m tall Tulik Volcano on the southeastern slopes and Mt Idak on the northeast.

The southern half of Okmok contains the steep-sided, symmetrical, stratovolcano of Mount Vsevidof. Topped by a 1.2 km wide crater, Vsevidof rises to a height of 2,149 m making it the highest point on Umnak. 10 km immediately to the east from Mt Vsevidof lies the heavily eroded Mt Recheshnoi, rising to 1,984 m above sea level. Extensive glaciation has eroded the once symmetrical outline of Recheshnoi into a series of sharp ridges and steep-sided valleys that descend to the north and south from a central, east to west trending, ridge of 4 km in length.

The Geyser Bight and Hot Springs Cove regions (both features are embayments of the larger Inanudak Bay) contain some of Alaska's most active geothermal areas, where thermal springs, fumaroles and the only known geysers within the state of Alaska are found. The geothermal activity of this region is fueled by magmatic activity associated with Mt Recheshnoi.

image: earth sciences and image analysis laboratory, nasa johnson space center

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