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Paleofumarole pinnacles (Pinnacle Valley, Crater Lake National Park, Oregon, USA) 1 | by James St. John
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Paleofumarole pinnacles (Pinnacle Valley, Crater Lake National Park, Oregon, USA) 1

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 eruption 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.


Seen here are the Pinnacles - differentially eroded pillars composed of volcanic ash and larger pyroclasts. The volcanic debris was deposited during Mazama's caldera-forming eruption (= an ultraplinian eruption). The light-colored lower unit erupted early - it is composed of rhyodacite ash and pumice. It ultimately came from magma near the top of the subsurface magma chamber. The darker-colored upper unit erupted later - it is composed of basaltic andesite ash and scoria. It came from magma nearer the bottom of the magma chamber, because it was more dense.


The deposit is part of the Wineglass Welded Tuff, a unit representing ash flows from collapse of the ultraplinian eruptive column.


The pinnacles themselves are sites of paleofumarole conduits. Fumaroles are hot steam vents. The subsurface plumbing was preferentially cemented and has resisted erosion compared with the less-lithified surrounding materials. The result is prominent, pointed columns.


Stratigraphy: Wineglass Welded Tuff, lower Holocene, ~7700 years old


Locality: Pinnacles, along Wheeler Creek, Pinnacle Valley, southeast of Crater Lake Caldera, Crater Lake National Park, southwestern Oregon, USA


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Taken on August 4, 2012