The Union Peak Volcano

The southwest corner of the park is occupied by a dissected volcanic cone that culminates in the sharp pinnacle of Union Peak at an elevation of 7698 feet. (See plate 4, figure I.) Even though the flanks of the volcano have been deeply excavated by glaciers, it is easy to imagine the original form, for the attitude of the lavas is clearly displayed on the radiating ridges that separate the cirques.

The volcano became extinct either before Mount Mazama began to develop or shortly thereafter. Much later, when the central pent had ceased to erupt, and probably when Mazama had almost reached full height, a fissure opened far down the eastern slope of the Union Peak volcano, and several cinder cones, including the Goose Nest, were built along it. The recentness of these cones and their arrangement along a line directed toward the center of Crater Lake suggest that they are related in origin to the cone of Mazama rather than to Union Peak.

One may see Union Peak from any high vantage point on the rim of Crater Lake and, by turning in the opposite direction, may see Mount Thielsen. The observer must be struck at once by the remarkable similarity of their forms. Closer inspection strengthens this impression. Indeed, so similar are the history and internal structure of these two volcanoes, the largest neighbors of Mount Mazama, that a description of one almost exactly fits the other. Briefly, each is a broad, heavily glaciated lava shield surmounted by a pyroclastic cone which occupies the central crater and is intruded by one or more large plugs forming the topmost crags. Judging by the degree to which the volcanoes have been glaciated, they ended their activity at approximately the same time. Possibly both had already reached maturity before the onset of Pleistocene glaciation.

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