43 Development of Mount Mazama

The Geology and Petrography of Crater Lake National Park, 1902




Scott Peak is only a large cone adnate to Mount Mazama. It belongs to the same center and holds essentially the same relation to it as Shastina does to Shasta. The slopes of Mount Mazama reach to the plains at its eastern base, and it is one of the largest mountains of the Cascade Range.

The beginnings of Mount Mazama are now deeply buried beneath the lavas of the range, including those displayed on the lower slopes of the great caldera beneath the water of Crater Lake. The earliest lavas now visible are those of the southern and western lake border, and when they were erupted the volcano was normally active, sending out with its streams of lava large contributions of fragmental material to make the heavy conglomerates of the older portion of the rim. The many succeeding flows of andesite and layers of conglomerate built up the mountain slope to the crest of the rim upon the southern and western side, and Scott Peak, too, had attained its full development when the peripheral vents of basalt opened and by a series of eruptions built up the surrounding country with adnate cones upon the outer slope of the rim of the lake. Then followed the large eruptions of dacite, forming Llao Rock and the northern crest of the rim to Cloud Cap. These flows occurred during the period of glaciation of Mount Mazama, and streams of lava alternated with streams of ice, a combination which doubtless gave rise to extensive floods upon the slopes, and filled the valleys below with volcanic debris from the mountain. In connection with the eruption of these viscous lavas (dacites) there were great explosive eruptions of pumice, which was spread for 20 miles or more across the adjacent country. The explosive activity of Mount Mazama culminated in the eruption of the peculiar dark pumice, rich in hornblende, which followed the outflow of the tuffaceous dacite.

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