Plate Tectonics of the Western United States Plate Tectonics of the Western United States Plate tectonic map of the Western United States and nearby Pacific Ocean. Volcanoes shown by red triangles.
Arrows show direction of plate movement.
Simplified cross-section of the subduction zone that produces the volcanoes of the Cascades. The volcanoes of the western United States are the result of subduction of the Juan de Fuca and Gorga plates beneath the edge of the North American Plate. The North American Plate is made of continental crust and lithospheric mantle. Water from the oceanic plate helps to melt the hot asthenospheric mantle beneath the North American Plate. Magma rises through the plate to supply volcanoes.