When one piece of the earth’s crust--a tectonic plate--is forced against another, one rides
up above the other. The plate that is pushed below, in a process called subduction, is often
pushed deep enough into the mantle that the rock melts. The molten rock is lighter than
the rock of the mantle, and forms intrusions into the rock above. When these intrusions
reach the surface and erupt, they form volcanoes.
The Ancestral Rocky Mountains produced vast quantities of volcanic ash that covered the
dinosaurs of Jurassic time, and produced the colorful Morrison Formation.