Origin of the Mountain.
The slope, which you ascend to view the lake, and the caldera wall rising 500 to 2,000 feet above the water, are remnants of Mount Mazama.
In comparatively recent geologic time, numerous volcanic peaks were formed near the western edge of a vast lava plateau covering parts of Oregon, Washington, Idaho, Montana, Nevada, and California. These are the Cascade Range, of which Mount Mazama was one of the commanding peaks. It was built by successive lava flows with some accumulation of volcanic ash. The cone thus formed was modified by streams and glaciers which carved valleys in its sides and deposited rock debris on its flanks. The layered character and different formations of the mountain are now clearly exposed in numerous places within the caldera wall.