Geological History of Crater Lake National Park

The glaciation and structure of the rim clearly establish the former existence of Mount Mazama, but there may well be doubt as to its exact form and size. Judging from the fact that Mount Shasta and the rim of Crater Lake have the same diameter at an altitude of 8,000 feet, and that their lavas are similar, it may with some reason be inferred that Mount Mazama and Mount Shasta were nearly of equal height. The slopes of Mount Shasta may be somewhat steeper than those of the rim of Crater Lake at an equal altitude, but the glaciation of the rim is such as to require a large peak for its source. A restoration of Mount Mazama based on a photograph of the rim of Crater Lake as seen from the south west is shown in figure 20.

FIG. 20—RESTORATION OF MOUNT MAZAMA.
Vertical and horizontal scales the same.
Diagram furnished by Smithsonian Institution.

FIG. 21—CALDERA OF CRATER LAKE LEFT BY THE ENGULFMENT OF MOUNT MAZAMA.

In figure 22 is given a section of Crater Lake and its rim, with the probable outline of Mount Mazama, and in figure 23 is given a profile and surface sketch of a cross section of the natural park through Crater Lake. Wonderful as the lake, encircled by cliffs, may be, it serves but to conceal in part the greatest wonder—that is, the enormous pit or caldera which is half filled by the lake. The caldera is 4,000 feet deep. An impressive illustration of it is seen in figure 21 which was prepared from a photograph of a model of Crater Lake now in the United States National Museum. The water surface is represented by glass, so that one may see through to the bottom and get the full impression of the depth of this tremendous hole in the ground. It extends from the top of the rim, which is the very summit of the Cascade Range, halfway down to the sea level, and nearly a square mile of its bottom is below the level of Upper Klamath Lake at the eastern foot of the range. The volume of the caldera is nearly a dozen cubic miles, and if we add the volume of the lost Mount Mazama that amount would be increased by at least one-half. How was it possible to remove so large a mass and in process develop so great a depression?

FIG. 22—SECTION OF CRATER LAKE AND ITS RIM, WITH THE PROBABLE OUTLINE OF MOUNT MAZAMA, STRUCTURAL DETAILS GENERALIZED.

FIG. 23—PROFILE SKETCH OF CRATER LAKE NATIONAL PARK.
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