35 The Southern Dacite Flows and Domes

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

The Earlier Dacite Eruptions


The Southern Dacite Flows and Domes

Probably the first dacites erupted by Mount Mazama were those forming Grayback Ridge and those making up the southern end of Vidae Ridge. After eruption, they were almost completely buried by glaciers which choked and overflowed the canyons of Sun, Sand, and Annie creeks. The lavas therefore date back to a time preceding the maximum glaciation of Mount Mazama. On the other hand, the younger flows of dacite erupted from the Northern Arc of Vents belong to a much later period when the great valley glaciers had dwindled to small tongues extending only a short distance beyond the present rim of Crater Lake.

The dacites of Vidae and Grayback ridges can be followed up the slopes of Mount Mazama to an elevation of slightly more than 6500 feet. At Tututni and Maklaks passes they disappear. Unfortunately, their relation to the neighboring andesites at those points is masked by glacial drift. They can never have extended farther up the sides of the volcano, however, for if they had they would be exposed on the sides of Dutton Ridge and Vidae Ridge. Neither there nor on the walls of the caldera is there any trace of them. Accordingly, they must have escaped in part from fissures close to Tututni and Maklaks passes.

Southward from the probable vents, the thickness of the dacites increases rapidly. A mile away, both the Grayback and Vidae lavas reach a thickness of 500 to 600 feet. The Grayback flows continue to thicken until, close to the southeast corner of the park, their thickness approximates 1000 feet.