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Long Valley Caldera Field Guide - Devils Postpile

A spectacular display of a columnar-jointed basalt flow.

Geologic Summary:

Devils Postpile National Monument is renowned for its spectacular display of columnar-jointed basalt (a common formation of multiple columns of rock). Although smaller, it rivals the columnar-jointing displays at Fingal's Cave in Scotland and Giant's Causeway in Ireland. An average of 55% of the columns have classical six-sided (hexagonal) shapes, which is a higher proportion than other columnar-jointed basalt exposures around the world. Several factors are thought to have caused this example of columnar-jointing including uniform magma composition, small crystal size, and slow cooling history in a river canyon.

Around 80,000 years ago basaltic lavas partially filled the San Joaquin valley. The eruption occurred about the same time as similar flows were erupting around the area that was to become Mammoth Mountain. The composition of the magma was approximately homogenous (uniform in composition) and the cooled rock contains small-sized mineral crystals. Scientists believe the lava flowed up against a barrier causing it to pond and form a 400–foot deep pool. Once ponded, the lava cooled towards the center from the bottom where it touched the cooler granite bedrock and from the top where it was in contact with the air. As the rock cooled and shrank, cracks formed at the surface and base. As the cracks grew, they branched forming 120° angles, like those seen in other areas of nature such as mud cracks.


The Postpile flow has since been eroded by glaciation, as is shown by the finely polished and striated (grooved) outcrops along the trail at the top of the Postpile and by the down-cutting of the San Joaquin River gorge. Many of the columns have fallen since the last glaciation due to earthquakes and freeze and thaw. Freeze and thaw is the process by which water, in the cracks of a rock, expands as it freezes in the winter and then shrinks or drains away as it thaws in the summer. Over time, small amounts of water can enlarge cracks and cause the rocks to eventually break. The column fragments form a talus pile at the base of the outcrop.


References

Huber and Exkhardt, 2002, The Story of Devils Postpile A Land of Volcanic Fire, Glacial Ice and an Ancient River, Sequoia Natural History Association.

Field Stop Location: Devils Postpile

Quadrangle: Mammoth Mountain and Crystal Crag, California 7.5 minute topographic quadrangles
Coordinates: about 37°37.512' N, 119°05.094' W
Approximate Elevation: 7,648 ft (2331 m)

Directions to Devils Postpile:

Entry fee required. Daytime access by shuttle only, information at (760) 873-2427) or on NPS Devils Postpile website.

Directions from Mammoth Lakes exit U.S. 395 and CA-203 Go this distance
1. Start at Mammoth Lakes exit from U.S 395 and head west on CA-203 W/Minaret Rd toward Sawmill Rd. and Mammoth Mountain Ski Lodge. Continue to follow CA-203 W. Go 3.7 miles
2. Turn right onto Minaret Road [Start here if you are already in Mammoth Lakes]. Go 4.2 miles
3. Park in the Mammoth Mountain Main Lodge parking lot and purchase shuttle/entry tickets to Devils Postpile from the lodge. At the Postpile bus stop, exit the bus and go to the visitors center; follow the footpath 0.3 mi to the Postpile.