English: Rhyodacite pumice & volcanic ash in the Holocene of Orgeon, USA.

Crater Lake is a large, deep, freshwater lake in the Cascade Range of America's Pacific Northwest. It formed about 7,700 years ago when an ancient volcano called Mt. Mazama had an enormous explosive ash eruption. The event involved an estimated 50 cubic kilometers of magma being erupted. This was followed by collapse of the mountain, leaving a large depression which later filled with water. Large holes or depressions formed when a volcano destroys itself or collapses are called calderas. Crater Lake Caldera in Oregon is a world-class example of this type of volcanic feature.

Shown here is an unconsolidated volcanic deposit at the caldera rim. This material is a mix of rhyodacite ash and small pumice fragments deposited during an explosive eruption in the early Holocene. It may be from a pre-caldera forming eruption or the caldera-forming eruption itself (unit cp or unit rhp of Bacon, 2008)

Locality: rim of Crater Lake Caldera, just south of Grouse Hill picnic area, Crater Lake National Park, southwestern Oregon, USA

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Reference cited:

Bacon (2008) - Geologic map of Mount Mazama and Crater Lake Caldera, Oregon. United State Geological Survey Scientific Investigations Map 2832 [accompanying pamphlet].