33 Sun Creek Dacite Flow

The Geology and Petrography of Crater Lake National Park, 1902






The canyon of Sun Creek below the falls has upon the west a number of cliffs and terraces, due to successive flows of dacite, exposing a total thickness of about 600 feet. At 6,300 feet east of Crater Peak the cliffs begin. The first is 200 feet, made by a solid flow of dacite with conspicuous structural features. The next terrace, at 5,900 feet, exposes a decidedly perlitic and spherulitic rhyolite banded with small lithophysae, and forms a bluff 75 feet in height.

There are bands of perlitic and spherulitic grains alternating with others which show neither structure. At 5,700 feet is a plain of fine material, filling the valley to a width of three-fourths of a mile. In the soft material Sun Creek has cut a canyon 200 feet deep. Upon the eastern side in the canyon wall dacitic rocks (123, 124) appear, containing cavities lined with minute crystals of tridymite. The layer of pumice covers nearly everything beyond the canyon, so that the nature of the underlying rock is to a considerable extent a matter of doubt. On the West Fork of Sun Creek, just above where it begins to cut a canyon in the material filling the valley, masses of dacite (125, 126) appear, and if not in place have not been moved far, for the rocks immediately to the north are andesite. On the spur between the West Fork and the main stream of Sand Creek a well-marked dacite (127) occurs, with conspicuous fluidal banding and aligned cavities sparkling with tridymite; it appears to form the whole ridge excepting the small capping of andesite. A short distance farther north a strong spring issues from the contact between the dacite and the overlying andesite.

The Sun Creek dacite is the oldest about Crater Lake. It is evidently older than the andesite which lies upon it at an altitude of 6,600 feet. This is considerably above the level of the lake, and if the rhyolite came from Mount Mazama it should appear on the inner slope of the rim under Dutton Cliff. From a boat by the shore the cliff was examined to see if the dacite flow could be recognized, but it was not found.

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