Thick lava flow of the past: Llao Rock

Llao Rock

Llao Rock half-dome perched on the rim of Crater Lake

Llao Rock is a rhyodacite-composed half-dome on the rim of lava-wall-encircled Crater Lake. Where is the other half? When the caldera-forming explosion of Mount Mazama, Crater Lake's precursor, happened about 7,700 years ago, it dropped into the caldera. The original “complete Llao Rock” formed during precaldera volcanism. As an on-site panel explains, it formed when a lava flow filled an explosion crater on the north slope of Mount Mazama.

The massive gray cliff of Llao Rock can be seen across the lake from the Sinnott Memorial Overlook at Rim Village. Llao Rock is named for the Chief of the Below World—an invisible spirit of American Indians. In Chapter 29 with the title “The Battle of Llao and Skell” in the book “Agents of Chaos”, Stephen L. Harris compares geomythology and modern geoscience within Crater Lake's historical context.
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Around Crater Lake
Crater Lake National Park: Geologic Resources Inventory Report
Crater Lake Institute: The Dacite Flow of Llao Rock
Sinnott Memorial Overlook
Rim Village Visitor Center and Book Store
Crater Lake Lodge