Devils Postpile National Park Geologic Story |
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Genesis of the PostsThe lava flow that became the Devils Postpile was very fluid and remained molten almost throughout while it pooled in the valley of the San Joaquin River. Because its top was exposed to air and its bottom to underlying cooler granite bedrock, the flow cooled to solid rock inward from these surfaces. The flow shrank, too, because virtually all liquids occupy less space when frozen, or solidified.
As the solidifying lava shrank, tensional stresses developed in response. Those stresses operating in a vertical direction simply caused the mass to settle in response to gravity, but those oriented in horizontal directions were relieved only by cracking of the solidifying rock. Columnar joints or cracks are formed as a lava flow cools and shrinks under certain conditions that allow the flow to crack into long vertical columns ideally tending toward a hexagonal cross section. Uniform cooling conditions tend to promote the development of columnar joints, but the degree of homogeneity of the solidifying lava is probably more important. It can be shown mathematically that the surface of a homogeneous medium should be divided by a crack system defining regular hexagons when it is subjected to uniform shrinkage because a hexagonal system provides the greatest relief with the fewest cracks. Regular hexagons are rare, because in natural lava flows, cooling stresses in rocks are never completely uniform, and therefore the columns are generally bounded by curved cracks forming irregular-shaped polygons with variable numbers of sides. Cooling conditions are never ideal at the surface of a flow, but as cooling, solidification, and cracking proceed from the surface of a flow into its interior, a point may be reached where the shrink age forces may be uniform enough for irregular jointing to give way to the formation of columnar joints. In higher parts of the Devils Postpile flow, remnants of which may still be seen in the general area, jointing is irregular or poorly columnar. In lower parts, where cooling was more uniform, nearly perfect columns formed. These are now exposed by erosion as the Devils Postpile. We see in the Postpile only the lower part of the lava flow. Columns have an average diameter of 2 feet and a maximum diameter of 31/2 feet; some are 60 feet long. The columns have various numbers of sides, as indicated by a sample of 200 posts: 4 sides, 2%; 5 sides, 37%, 6 sides, 55%, 7 sides, 5%. A few columns have only three sides. |
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Continue to The Postpile is Exposed, See the Contents or Return to the Beginning |
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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