Reconnaissance geology of some western Aleutian Islands, Alaska

Abstract

The general features of the western Aleutian Islands, from Kasatochi to Attu, were studied in 1946 in connection with investigations of northern Adak Island, northern Kanaga Island, and other Aleutian islands. The geology of the western Aleutian Islands is complex. Its deducible history begins in the Paleozoic (?) era, when a land made up of metamorphic rocks, intruded by granite, occupied the region. Successive additions of basaltic volcanic rocks and sediments of volcanic derivation were made during Paleozoic(?) and early Tertiary time, producing a highlands south of the present islands from about the longitude of western Tanaga Island to that of eastern Amchitka Island. Submergence of most of the land resulted in the carving of wide wave-cut benches that later emerged from the water. In late Tertiary time, glaciers accumulated in the highland and moved northward over the uplifted benches. In very late Tertiary and in Quaternary time many basaltic shield volcanoes and composite cones were built on the wave-cut and locally glaciated remnants of the older volcanoes. The new volcanoes were alined on a great arc along the northern margin of the present belt of islands. The eruptions that produced many of these volcanoes began at a time when sea level was higher, relative to the land, than it is now; marine sediments were deposited offshore and are now exposed above sea level. A period of downfaulting followed, and most of the ancient highlands in the southern part of the belt disappeared beneath the Pacific Ocean. Many of the islands were again beveled by marine erosion. Later, the islands reemerged in about their present positions. In Quaternary time, new glaciers developed in the higher parts of the islands, except on the very active volcanoes and on some isolated peaks. Some of the volcanoes became extinct before and during the most recent period of glaciation, but many are still active. Mild volcanic activity, such as eruptions of ash, can be expected to occur frequently in the northern part of the island belt; violent activity can be expected to occur infrequently. INTRODUCTION In the summer of 1946 several of the Aleutian islands west of Atka Island were mapped geologically. In addition to areas mapped in detail on Adak and Kanaga Islands, a number of the islands from Great Sitkin to eastern Attu were examined briefly on the ground, and others from Kasatochi westward were studied from the air. The purpose of this reconnaissance investigation was to choose areas for later, more detailed work, and to acquire general knowledge of the