Mount Mazama is one of the major volcanoes of the Cascades Arc. Crater Lake is located within the collapsed caldera of Mount Mazama on the crest of the Cascade Range in southern Oregon about 90 km (55 mi) north of the city of Klamath Falls and about 100 km (60 mi) northeast of Medford. This volcano formed at the intersection of the Cascade chain of volcanoes with the Klamath graben, a north-northwest low-lying basin that is surrounded by tectonic faults, and is bounded to the east by the Basin and Range province. The geology of the area was first described in detail by Diller and Patton (1902) and later by Williams (1942), whose vivid account led to international recognition of Crater Lake as the classic collapse caldera.
Mount Mazama began erupting relatively continuously 420,000 years ago as a complex of overlapping shields and stratovolcanoes, each of which probably was active for up to 70,000 years. The massive volcano erupted violently 7,700 years ago, accompanied by collapse of the entire upper half of the edifice. Prior to its climactic eruption Mount Mazama had a summit elevation of about 3,700 m (12,000 ft). The present high point is Mount Scott at 2,721 m (8929 ft), 3 km (about 2 mi) east of the caldera rim. Today, the caldera rim ranges in elevation from approximately 2,040 m (6,700 ft) to 2,484 m (8,150 ft), with maximum relief of about 600 m (2000 ft) above the surface of Crater Lake, virtually equal to maximum water depth. Surviving flanks of Mount Mazama consist of lava that slopes gently away from the caldera rim, incised by deep glacial valleys that are partially filled with pyroclastic-flow deposits of the climactic eruption. All but the steepest slopes are covered with deposits of the climactic eruption.
In the area surrounding Mount Mazama, regional volcanism has been active throughout at least the last 700,000 years, but continuity of regional activity prior to approximately 200 ka is uncertain. Several regional volcanoes, some of them large shields, were active during the interval from about 200 to 100 ka. There was comparatively little regional volcanism between about 100 to 40 ka. However, after 40,000 years, during the growth of the climactic magma chamber, large amounts of basalt to basaltic andesite lavas were erupted from several vents west of Mazama; this suggests that abundant magma from deep within the earth was moving into the crust around Mount Mazama at this time.