During the Mesozoic Era, the Age of Dinosaurs, the oceanic Farallon Plate stretched along the west side of the North and South American Plates; at that time, the west coast of North America was at the longitude of central Utah. By the mid Jurassic, about 160 million years ago, the Atlantic Ocean began to open and the American Plates were pushed westward, forcing the Farallon Plate to subduct beneath their western edge.
This subduction of oceanic plate triggered the formation of the Andes in South America and, over time, added a large number of exotic terrains and island arcs to the west coast of North America, building up the Great Basin, California, the Pacific Northwest, British Columbia and Alaska. The Cascades of Washington and Oregon began to form about 36 million years ago as this process continued.
Today, most of the Farallon Plate has disappeared beneath the Americas but three remants persist: the Juan de Fuca Plate along the Pacific Northwest, the Cocos Plate along the west edge of Central America and the Nazca Plate along the west coast of South America. All three are still subducting and volcanism will persist in these areas until either the "Farallon fuel" is used up or the Atlantic stops expanding.
