Why are there so many volcanoes in the Pacific Northwest?
Basically the west coast plate is moving into the Pacific plate, although there are actually a couple plates involved. The magma pools are formed deep underground by the plates grinding against each other. Once enough pressure builds up in the magma, it comes to the surface through fissures and flows out and eventually forms the volcanoes. Many are in the Pacific Northwest due to the contact points of the plates there and the frictional heating involved.
