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Geologic History
The history of the formation of Crater Lake.
Formation
Crater Lake sits atop Mount Mazama, a Quaternary volcano that is part
of the Western Cascade Range. Mazama began to form a half million years ago.
42,000 years ago it stood at its tallest height at 12,000 feet.
Eruption
Mazama had its most destructive eruption about 7,700 years ago, spewing 12 cubic miles of rhyolite magma in the form of tephra as far north as Alberta, Canada, as far east as Wyoming and as far south as Nevada
and northern California. Pyroclastic flows and lahars descended down around the volcano and a thick layer of tuff formed on the landscape and can still be seen today.
As a result of the eruption, Mazama lost enough material that the weight of the peak of the volcano could not be supported and it collapsed upon itself, creating a caldera.
Wizard Island
Mazama continued to have smaller eruptions, which sealed the caldera floor and created a cinder cone within the caldera which is know called Wizard Island.
Formation of the Lake
Over 700 to 1500 years, rain and snow melt gradually filled the caldera, forming Crater Lake. Today, there is a balance between evaporation and precipitation and the water level in the lake usually fluctuates less than three feet year to year.
Human Activity
In terms of geologic time, Crater Lake is very young. It is believed
that humans likely witnessed the explosion. In fact, a sandal was
found buried in the ash from the eruption. Klamath tribes have legends of how Crater Lake came to be.
