Magma type and crustal structure in the Aleutian arc

The Aleutian volcanic arc, at the northern extremity of the Pacific Ocean, contains about 40 active volcanoes, and extends about 1580 mi. from Mt. Spurr on the northeast, to Buldir Island on the west. Structurally the arc is even longer, merging with the Alaska Range at one end, and abutting against the Kamchatka Peninsula at the other.

At the eastern end, the volcanoes rest on a foundation, the exposed part of which is mostly Mesozoic rocks, with minor amounts of Paleozoic sedimentary and volcanic rocks, intruded by later Mesozoic batholiths. West of Pavlof Volcano on the Alaska Peninsula, however, Paleozoic or Mesozoic sedimentary rocks have been identified at but two localities.

Most of the exposed area of the Aleutian Islands consists of volcanic rocks and volcanogenic sedimentary rocks of pro-Quaternary age, in part Miocene to Oligocene, in part perhaps older; these have been intruded by small stocks of rocks ranging in composition from gabbro to granodiorite and albite granite. The stratified rocks have been slightly deformed into open folds, with axes, locally at least, trending parallel to the main are. Some younger undeformed Tertiary volcanic rocks rest on these. In original composition, the Tertiary rocks range from basalt to dacite, but many are albitized.

A belt of Quaternary volcanoes lies north of the main structural axis in the Aleutian Islands proper. These Quaternary volcanoes are made up of rocks ranging in composition from olivine basalt to rhyolite, and are predominantly andesite, with pyroxene andesite most abundant but including less abundant hornblende andesite in some volcanoes. The compositions and trends of differentiation are very similar from Katmai to Buldir Island. The eastern volcanoes rest on a crust of continental type, but geophysical evidence suggests that sialic material is lacking beneath most of the western volcanoes. Hypotheses involving assimilation of sial as an essential source for andesites therefore seem inadequate.

A reasonable hypothesis for the origin of the volcanic rocks of the Aleutian arc is as follows: Eugeosynclinal sediments and basaltic volcanics were carried down to depths of at least 100 km along a major thrust that dips northward at an angle of roughly 30 degrees beneath the arc and is represented by a zone of earthquake foci. Water and material of granitic composition were sweated out of these materials and were added to a molten fraction of basaltic composition that was interstitial to peridotite of the mantle. This magma rose in the block overlying the thrust zone, probably along tensional fractures or tear faults, and was concentrated at moderate depth in magma chambers. There differentiation of the water-rich magma under constant or increasing partial pressure of oxygen produced the observed variety of volcanic rocks.