Historic Resource Study, Crater Lake National Park, Oregon, 1984
V. Geological and Biological Information on Crater Lake Area
A. Mount Mazama
For millions of years the Crater Lake region was a land of fire, “one of the largest known volcanic fields of the world.”  During that time both large and small volcanoes erupted throughout an area of the Northwest extending from the Rocky Mountains to the Pacific Ocean and embracing portions of Wyoming, Montana, Idaho, Washington, Oregon, and California. The western limit of this range was marked by the Cascades, formed by tremendous flows of molten rock expelled from a series of fissures extending from northern California to central Washington and beyond. Over time, alternate layers of lava and fragmented material were deposited on top of this vast lava plateau, forming the strato volcanoes of the Cascade Range. Roughly speaking, the southern end of volcanic activity is marked by Lassen Peak and, within the United States, its northern edge by Mount Baker. The Cascade Range volcanoes were most active during the Miocene period, when the greater part of this range was formed. Many of the peaks built up during this time, such as Lassen Peak, Mount Shasta, and Mount Mazama, continued their volcanic activity into the glacial period. 
At its zenith the volcanic peak now referred to as Mount Mazama probably reared its icy head 12,000 feet high, one of the Titans of the Cascades alongside Mounts Baker, Rainier, St. Helens, Adams, Hood, and Shasta. Not a perfect cone-shaped peak, but rather an asymmetrical group of overlapping cones, the mountain had been built up over hundreds of thousands of years by repeated flows of molten lava and ash and by cinder debris from sporadic violent eruptions. The final explosion of Mazama has been described as the “crowning event in the volcanic history of the Cascade range.” During the Ice Age, glaciers covered its valley slopes. At least four periods of glaciation occurred during Mazama’s growth. In this glacio-volcanic sequence, vast ice fields were continually formed by heavy precipitation and then destroyed by renewed lava eruptions. Finally there came a time when Mount Mazama was peaceful, possibly for centuries. Although large glaciers still radiated from its summit, dense stands of evergreens had by now taken root on the lower slopes. Numerous streams trickled over flowering banks, and Indians roamed the land in search of game.