The geologic setting of Crater Lake, Oregon

Bacon, C; Lanphere, M

"Crater Lake partly fills the 8-by-10-km diameter caldera that collapsed during the climactic eruption of Mount Mazama about 6850 yr. B.P. This eruption vented 50 km3 of magma and deposited pumice and ash over a large part of the Pacific Northwest. Mount Mazama was a large composite volcano whose summit elevation apparently was about 3600 m. This volcano was made up of several overlapping shield and stratovolcanoes constructed episodically from al least 420 ka to about 40 ka. Most rocks older than 100 ka are hydrothermally altered and are there for less permeable than younger lava flows and fragmental deposits. Relatively young, permeable materials are below the lake level between Pumice Point and Wineglass, and these may regulate seepage from the lake and limit maximum lake level. Beneath the east half of Mount Mazama is a large field of older rhyodacite lava, itself resting on basaltic andesite on its southwest side and on andesite and dacite on the southeast. The mountain lies at the head of the Klamath graben, a downfaulted basin. Related north-south-trending normal faults cut Mazama lavas on the southwest flanks of the volcano. Postcauldera andesitic lava flows and pyroclastic material formed the central platform, Merriam Cone, and Wizard Island within the caldera shortly after it collapsed. The small rhyodacite dome east of Wizard Island was erupted later, about 4000 yr. B.P. All postcaldera lavas save the top of Wizard Island pile are beneath the lake surface."

The geologic setting of Crater Lake, Oregon

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