Fisher Volcano | John Seach


Unimak Island, Aleutian Islands, Alaska

54.65 N, 164.43 W
summit elevation 1094 m

Fisher caldera is located on western end of Unimak Island, Alaska. It is the largest of 12 Holocene calderas in the Aleutian volcanic arc, with a diameter of 18 km x 11 km.

Fisher volcano is located in a region of high convergence rate between the Pacific and North American plates. This correlates with the region of greatest volcanic
activity. In contrast to nearby volcanoes, Fisher has shown little historic activity.

Eruptions at Fisher volcano can be divided into three periods - Pleistocene, the caldera-forming eruption, and Holocene.

Pleistocene eruptions
Eruptions at Fisher volcano began about 600,000 years ago. Eruptive deposits are mainly stratocones and sediments that form much of the caldera walls.

Caldera-forming eruption
The caldera-forming eruption occurred 9372 years ago. The eruption produced a dacitic pumice fall, a dacitic pyroclastic flow to the south, and a mixed composition pyroclastic flow to the north.

Holocene eruptions
Post-caldera eruptions includes the formation and partial collapse of Turquoise Cone, and the development of other mono- and polygenetic vents both inside and outside the caldera. Recent eruptions at Mt. Finch may have deposited a thin (0.5 cm) layer of scoriaceous ash inside the caldera.

Current Activity
Current activity at Fisher caldera is hydrothermal, with springs south of Turquoise Cone up to 44 8C, compared to temperatures of the larger intracaldera lakes of 5 deg C.

Turquoise Cone, Mt. Finch, and the Eastern Mound occur along a line parallel to the major axis of the caldera, suggesting a linear zone of weakness.

Fisher Volcano Eruptions

1830, 1826-27, 1795