The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Climax: Culminating Explosions of Pumice and Scoria

Total Volume of Material Erupted During the Culminating Activity of Mount Mazama

It remains to consider how much magma was evacuated from the reservoir beneath the volcano, and how much old rock was torn from the sides of the conduits and from the top of the mountain during the climactic activity.

The following table summarizes the results obtained.

The total volume of material laid down is thus between 9.75 and 11.75 cubic miles. Of this amount, between 1.65 and 2.62 cubic miles consists of crystals and between 1.10 and 1.84 cubic miles of old rock.

Using approximate figures for the sake of convenience, we may say that 11 cubic miles of ejecta were deposited, including about 2 cubic miles of crystals and 1.5 cubic miles of old rock fragments. The remaining 7.5 cubic miles are composed of frothy pumiceous and scoriaceous glass.

Exactly how much liquid magma is represented by the glass is uncertain. The vesicularity of the pumice varies considerably; allowance must be made for the pores between the fragments; and there is no means of determining how vesicular the magma itself was prior to eruption. Certainly, if all the pumice and scoria were remelted to bubble-free liquid, its volume would be reduced by much more than half. Hence, it seems safe to say that at most 3 cubic miles of actual liquid were emptied from the reservoir beneath the volcano.

The final estimates are consequently as follows:

Volume of liquid magma erupted……….At most 3 cu. miles

Volume of crystals erupted……………..Approx. 2 cu. miles

Volume of lithic fragments………………Approx. 1.5 cu. miles

   Total…………………………………….Approx. 6.5 cu. miles

Vast as this total is, it is still much less than half the volume of Mount Mazama that has disappeared. How then did the caldera originate?

 

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