Geology of the Rocky Mountains Japan, Indonesia and the Aleutians; others are fragments of oceanic crust obducted onto the continental margin while others represent small isolated mid-oceanic islands. Magma generated above the subducting slab rose into the North American continental crust about inland. Great arc-shaped volcanic mountain ranges, known as the Sierran Arc, grew as lava and ash spewed out of dozens of individual volcanoes. Beneath the surface, great masses of molten rock were injected and hardened in place. For 270 million years, the effects of plate collisions were focused very near the edge of the North American plate boundary, far to the west of
Geology of the United States west they went, the older the lavas. A probable explanation is that a hot spot, an extremely hot plume of deep mantle material, is rising to the surface beneath the Columbia Plateau Province. We know that beneath Hawaii and Iceland, a temperature instability develops (for reasons not yet well understood) at the boundary between the core and mantle. The concentrated heat triggers a plume hundreds of kilometers in diameter that ascends directly through to the surface of the Earth. When the hot plume arrives at the base of the lithosphere, some of the lighter rock of the lithosphere rapidly melts.