North America

In the early Jurassic period, 200 million years ago, the northernmost edge of North America tore away from the continent and began rotating counterclockwise. This part of the continent came to rest to the northwest of North America, forming the original piece of Alaska-its northernmost mountains, the Brooks Range. In the late Cretaceous period, the farthest part of this landmass from North America struck the edge of Siberia, and became the Chukotsk Peninsula. The remaining landmass of Alaska joined North America bit by bit, in the form of exotic terrains. The Aleutians, a classic island arc, formed in the Tertiary period. The about 40 active volcanos of the Aleutians have erupted numerous times in the twentieth century, including several eruptions in the last decade from Mt. Augustine, Pavlov, Shishaldin, and Mt. Redoubt.