The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Age of Crater Lake
HAVING concluded that the caldera was formed principally by engulfment of the top of Mount Mazama, we are next confronted with the question, How long ago did this take place? Although the answer cannot be exact, it seems worth while to inquire concerning the evidence.
I. Just before the beginning of the culminating eruptions, the glaciers on the south side of the volcano descended to elevations between 6250 and 6500 feet. These glaciers occupied the upper stretches of Munson, Sun, and Kerr valleys and were between 3 and 4 miles long. Where they cross the present rim of Crater Lake they were probably not more than 150 feet thick. Everywhere else on the mountain the ice had retreated above the caldera rim. The Union Peak volcano either was completely bare or at most supported very small patches of ice under the shadow of the summit crags. Yet at the time of their greatest extent, many glaciers in this region were more than 10 miles long and more than 1000 feet thick. Therefore the rigorous climate of the Pleistocene must have ended long before the top of Mount Mazama disappeared.
If the volcanic peak existed today, glaciers would descend almost as low as the present caldera rim. In deep canyons the ice might even descend to still lower levels. In other words, if the top of the volcano were restored, the glaciers would not be very different from those which mantled the upper part of the cone just before its demolition.
2. A second criterion of age, though no more precise, is the degree to which the deposits of pumice and scoria have been entrenched by stream erosion. Since the glowing avalanches choked the canyons around Crater Lake, streams have cut narrow, steep-walled troughs to a maximum depth of 300 feet, to an average depth between 100 and 200 feet, and to a width of approximately 600 feet. In a few places streams have cut through the pumice and scoria to the moraines and lavas below, but thus far they have made little impression on these older deposits. Considering that where the pumice and scoria are massive and columnar they fall easy prey to undermining and that where they are incoherent they are just as readily removed, the small size of the troughs signifies a recent origin. Furthermore, when the caldera was formed the heads of the deepest troughs were still occupied by remnant tongues of ice, and for some time thereafter melt waters must have provided a plentiful source of power. In addition, the gradient of the streams was steep; when erosion began, Annie Creek fell mo feet per mile, and the gradients of the other streams were not much smaller. If the average rate of downcutting was as little as an inch a year, and in the beginning it must have been very much more, it would have required only 3600 years to carve even the deepest canyon.
3. When the caldera was formed, the Klamath Lakes and Klamath Marsh did not contain much more water than they do today. At most their levels were a few feet higher.
4. The walls of Crater Lake have suffered little erosion considering their steepness and the nature of the rocks involved. All who have witnessed the havoc caused by a single summer storm will agree that imposing slides of talus might well be formed in a few thousand years. Besides, most of the large slides may have been produced when the top of the volcano was engulfed.