Violent geologic events formed Adak Island: erupting volcanoes and tectonic subduction (movement of the oceanic crust toward and beneath the continental crust). Advancing and receding glaciers, frequent rainfall, and high winds sculpted the island's dramatic hills, valleys, cliffs, and floodplains.
Adak is located about half way out the Aleutian chain and is adjacent to and overlying a zone at the boundary of the Pacific and the North American tectonic plates. The Pacific plate moves under the edge of the other. Earthquakes continue to shake Adak.- USFWS