Today, the calm beauty of Crater Lake obscures the violent forces that formed it. Crater Lake lies inside the top of an ancient volcano known as Mount Mazama. This dormant volcano is just one in a chain of huge cones that extends along the crest of the Cascade Range from Lassen Peak in California to Mount Garibaldi near Vancouver, British Columbia. The earth's surface, seemingly solid, is actually broken up into many huge plates, all floating on top of the Earth's molten interior. As these plates slowly drift, the continents and adjacent sea floor either move apart or push into one another.
Continental crust is thicker than oceanic crust and tends to be less yielding.
When a plate carrying oceanic crust pushed into what is now the northwestern United States, it was forced under the less-yielding continental plate. Tremendous pressures were exerted on the oceanic plate, causing it to deform and even melt. This melted rock is called magma. It is lighter and more fluid than the surrounding rock and tends to rise. Volcanic eruptions eventually bring the magma back onto the surface of the earth where it is then called lava.