The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Origin of the Caldera
How Much of Mount Mazama has Disappeared?
The area of the caldera enclosed by the rim is approximately 27 square miles. Its original depth is a matter for conjecture. Parts of the floor of the Krakatau caldera have settled more than 200 feet since 1883. Conceivably, the floor of Crater Lake subsided several hundreds of feet as the residual magma in the reservoir contracted by crystallization.
Assuming that the depth of the caldera from the rim varied between 1500 and 3500 feet, and allowing for the configuration of the bottom, the volume of the original depression must have been of the order of 12 cubic miles.
To this must be added the volume of the lost portion which rose above the caldera rim. If a peak 12,000 feet high existed immediately before the final eruptions, the additional volume to be accounted for would be about 8 cubic miles, less the volume of the summit crater or craters and the volume of the glacial cirques on the sides. The volcano may already have been reduced in height, however, by collapse at the time of the pumice eruptions which preceded the dacite flows from the Northern Arc of Vents. If the summit was lowered at that time, the amount of reduction cannot have been large. Even supposing that the top had been lowered as much as 2000 feet, the volume of the cone would not have been diminished by more than a cubic mile.
We conclude, therefore, as Diller did on other grounds, that approximately 17 cubic miles of Mount Mazama disappeared to form the present caldera. Only three explanations are possible: first, the top of the volcano was removed by explosion; second, it collapsed; third, these processes were combined.