The clean, clear, cold lakewater contained no fish until they were introduced by humans from 1888 to 1941. The plateau base of the Cascade Range was built as the Earth's crust folded and uplifted, pushing seas westward. Molten rock pushed toward the surface, creating both violent eruptions and the welling up of lava through enormous cracks. This Cascade Range of volcanoes extends from Canada's Mount Garibaldi to Lassen Peak in northern California. Today, the calm beauty of Crater Lake belies the violent earth forces that formed the lake.
Crater Lake lies inside the top of an ancient volcano known as Mount Mazama.
This dormant volcano is just one in a group of huge cones that extends along the crest of the Cascade Range. This range extends from Lassen Peak in California to Mount Garibaldi near Vancouver, British Columbia. In this process, the continents move over the Earth's surface as large land plates. As they move, the continents and adjacent sea floor either move apart or push into one another. When an oceanic plate pushed into what is now the northwestern United States, it was forced under the less dense continental plate.