The great cliff of Llao Rock is composed of rhyodacite lava (PErh; 70.5%–72% SiO2) that was emplaced about 170,000–120,000 years ago. Llao Rock dominates the northwestern caldera wall and is in striking contrast to the underlying andesite flows. By contrast, the rhyodacite of Llao Rock has a maximum thickness of no less than 370 m (1,200 ft). It was erupted into a hummocky glacial valley, approximately U-shaped in cross section, about 0.8 km (0.5 mi) wide and between 150 and 180 m (500 and 600 ft) deep. The lava filled this valley, as well as accumulated as a broad, dome-like pile above it. Although the flow is exceptionally thick and the eruption apparently had ample material to vent, the lava only spread about 2 km (1 mi) beyond the rim of the caldera. This was partly because of the gentle slope of the glacial valley, but mainly because of the highly viscous nature of the flow.

Credits and Sources:

"Crater Lake National Park Geologic Resources Inventory Report," National Park Service, http://www.nature.nps.gov/geology/inventory/publications/reports/crla_gri_rpt_view.pdf (accessed November 5, 2015).