Columnar Basalt, Devil's Postpile California A natural volcan | Geology of the world and the Environment

Columnar Basalt, Devil's Postpile California

A natural volcanic formation, columnar basalt has a seemingly man-made appearance. The (mostly) hexagonal columns form naturally as thick lava rapidly cools, contracting and creating cracks in the surface of the new rock. These unusual geological formations can be seen across the globe.

Location

Devils Postpile National Monument is a U.S. National Monument located near Mammoth Mountain in Eastern California. The monument protects Devils Postpile, an unusual rock formation of columnar basalt. It encompasses 798 acres (323 ha) and includes two main attractions: the Devils Postpile formation and Rainbow Falls, a waterfall on the Middle Fork of the San Joaquin River.

Geology of Devils Postpile National Monument

Devils Postpile National Monument contains rare geologic features—polygonal columns of Quaternary basalt, the “posts” for which the monument is named. In addition to the basalt columns, Quaternary (2.6 million years ago to the present) extrusive volcanic rocks and Cretaceous (145.5 to 65.5 million years ago) intrusive igneous rocks form the landscape of Devils Postpile National Monument. These rocks range in composition from granodiorite to leucogranites (granites rich in potassium feldspar and aluminum).

The column formation is summarized as follows (Huber and Eckhardt 2002):

- Surface cracks developed when the tension caused by the shrinkage of cooling was greater than the strength of the lava.

- As cracks reached about 25 cm (10 in), they branched to form a Y-shape with equal angles of approximately 120 degrees around all sides, an angle that provides the greatest stress relief.

- Each new crack branched again when it reached the critical length, and together with other similar cracks formed an irregular polygonal pattern.

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