File:Columnar-jointed trachybasalt (Postpile Flow, Upper Pleistocene, 82 ka; Devils Postpile National Monument, eastern California, USA) 81.jpg

English: Trachybasalt in the Pleistocene of California, USA.

Famous localities for seeing excellent columnar jointing include Giants Causeway (Ireland), Devils Tower (Wyoming, USA), and Devils Postpile (California, USA). Columnar jointing forms as a lava flow cools and contracts, resulting in the development of shrinkage cracks. As shrinkage cracks grow, they branch at ~120º angles (as seen in plan view). Crack networks merge with other networks to form columns having a polygonal cross-section shape. Most columns are hexagonal or pentagonal in shape. A few are 3-sided, 4-sided, or 7-sided.

Devils Postpile is a trachybasalt (or basaltic trachyandesite) lava flow with well-developed columnar jointing. Erosion has toppled many of the columns into a large pile at the base of the flow. The flow represents part of the activity of the Long Valley Volcano, which is now a large caldera in the eastern Sierra Nevada Mountains of California. The Devils Postpile lava flow erupted outside the southwestern margin of the Long Valley Caldera.

Stratigraphy: Postpile Flow, Upper Pleistocene, 82 ka

Locality: Devils Postpile National Monument, west of town of Mammoth Lakes, eastern California, USA

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Info. synthesized from:

Huber et al. (2001) - The Story of Devils Postpile, a Land of Volcanic Fire, Glacial Ice and an Ancient River, Updated from the Original Edition.

Bailey (2004) - United States Geological Survey Professional Paper 1692.

Mahood et al. (2010) - Geological Society of America Bulletin 122: 396-407.