46 The Parasitic Scoria Cones of Mount Mazama

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

The Parasitic Scoria Cones of Mount Mazama


THE growth of parasitic cones and viscous domes on the flanks of a composite volcano is usually a sign of old age. Mount Mazama had probably attained its maximum elevation about the close of the Pleistocene period. Shortly thereafter, when the glaciers on its flanks had dwindled to small proportions, numerous scoria or cinder cones began to form near its base and on the surrounding flats. A few scoria cones had been formed long before, when glaciers still filled the canyons and covered the intervening divides, but by far the majority show no signs of having suffered from glaciation and their forms are consequently well preserved.

Within the limits of the park there are no less than thirteen of these parasitic cones, of which only the two small ones on the north side of Castle Creek have been modified by glacial action. Of the other eleven, three lie on or just beyond the northern slopes of Mount Mazama, namely, Red Cone, Desert Cone, and Bald Crater; one, Forgotten Crater, lies on the western slope, not far from the caldera rim; the others lie on the southern slopes or on the adjacent flanks of the Union Peak volcano.

Outside the park there are eleven other cinder cones. Though these lie beyond the base of Mount Mazama, they are of approximately the same age as the eleven cones just mentioned and were probably fed from the same magma chamber (map, plate 2).

The dacite flows of Llao Rock, Grouse Hill, Cleetwood Cove, Redcloud, and Cloudcap and the dacite domes at the eastern base of Mount Mazama also belong to this period of activity. In other words, after Mount Mazama had reached full height by the outpouring of flows of andesite, the closing stages of its history were marked by the eruption of magmas of opposite types, namely, basalt and basaltic andesite in the form of cinder or scoria cones and dacite in the form of viscous flows, pumice, and steep-sided domes. A similar sequence of events has been recognized on other Cascade volcanoes. Indeed, eruption of highly differentiated magma seems to characterize the decadence of many andesitic volcanoes the world over. The parasitic cones now to be described show so many features in common that a brief discussion of each will suffice. All were formed primarily by eruptions of gas-rich magma as scoriaceous bombs and lapilli. A few also erupted short flows of lava.


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