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Devils Postpile National Park Geologic Story

Rocks of Devils Postpile Formed

Although the rocks of the Devils Postpile formed during the ice age, the Postpile owes its origin to geologic processes initiated earlier. These rocks are but one manifestation of widespread volcanic activity in this part of the Sierra Nevada that began more than three million years ago and has continued to the present. Perhaps born of the same underlying cause as the volcanism, contemporaneous large-scale fault movements created the steep eastern front of the Sierra Nevada; volcanic activity was greatest along or adjacent to the eastern escarpment. Long Valley, from the headwaters of Owens River to Lake Crowley, is a giant volcanic caldera. Mammoth Mountain- a massive volcanic dome- has grown on the caldera margin.

Less than 100,000 years ago, basalt lava, which was to become the Devils Postpile, erupted in the already glaciated valley of the Middle Fork of the San Joaquin River. The age of volcanic rocks can be estimated by study of the radioactive decay of elements in the rocks. Previous estimates for the age of the Postpile basalt, ranging from about 600,000 years to nearly a million years, are now thought to be seriously in error. Although an exact age for the Postpile flow still is not known, we believe that an age of less than 100,000 years, based on radiometric age determinations on rocks thought to be correlative, is more reasonable.

The Postpile lava reached the surface in the vicinity of the Upper Soda Springs Campground at the north end of Pumice Flat (see map on page 14). Across the San Joaquin River from the campground, rust-colored basalt cinders- products of explosive eruptions- rest upon granite bedrock and are probably the remains of a volcanic cinder cone, similar to the Red Cones southeast of the Monument. Two basalt dikes- tabular bodies of solidified lava- can be seen cutting upward through the cinder pile; when molten, these dikes may have been feedervents for lava flows. We do not know how extensive the lava outpourings were, because most of the basalt has been removed by combined glacial and stream erosion, but we can piece together some minimum figures from the scattered outcrops that remain. Lava filled the valley from side to side for a distance of at least three miles, from Pumice Flat south to the Devils Postpile, and in the vicinity of the Postpile the lava was probably more than 400 feet deep.

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Material in this site is adapted from a pamphlet, Devils Postpile Story, by N. King Huber, USGS, and Wymond W. Eckhardt, NPS. It is published by Sequoia Natural History Association, Sequoia Natural History Association, HCR-89, PO Box 10, Three Rivers, CA 93271-9792, Telephone (559) 565-3759.

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