42 The Dacite Flow of Llao Rock

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

The Northern Arc of Vents


The Dacite Flow of Llao Rock

The thickest sheet of lava on the walls of Crater Lake forms the imposing cliff known as Llao Rock. In striking contrast with the underlying flows of andesite, which are between 20 and 100 feet thick, the dacite of Llao Rock reaches a maximum thickness of no less than 1200 feet. It was erupted into a hummocky glacial valley, approximately U-shaped in cross section, about 1/2 mile wide and from 500 to 600 feet deep. Not only did the dacite fill this valley, but it accumulated as a broad, domical pile above it and spread across the valley rims. Naturally, where the lava overflowed the valley the thickness was much less than over the center. On the west bank, the thickness is at most 400 feet; on the opposite bank, it is only about half as much. The total width of the flow is slightly more than 1 1/4 miles, and its volume approximates 1/4 cubic mile.

Though the flow is exceptionally thick, it spread little more than a mile beyond the rim of the caldera. This was partly because the slope of the glacial valley was gentle, but mainly because of the highly viscous nature of the flow.

Source of the flow. Beneath the lowest part of the Llao dacite on the caldera wall are two thin dikes. Neither dike can be seen to connect with the lava, and it seems unlikely that, being only 5 and 7 feet thick respectively, they could have served to erupt 1/4 cubic mile of viscous dacite. For this reason, some have suggested that the lava was erupted from a vent higher up the slopes of Mount Mazama. This theory, however, is also untenable.

Only by scrambling up the cliffs to the base of Llao Rock can the true relations be observed. There is then no doubt that the buttress which projects lakeward at the bottom of the cliffs represents the actual filling of the conduit. In other words, the Llao lava escaped from a vertical-sided vent on the bottom of a glacial valley, several thousand feet below the former summit of Mount Mazama. The photograph (plate 12, figure I) and the section (figure 10) show the projecting buttress and feeder in relation to the main body of the flow.