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USGS Geology in the Parks

Devils Postpile National Park Geologic Story

Genesis of the Posts

The 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.

Death Valley Dunes
  1. 1. Surface cracks started when the tension caused by the shrinkage of cooling was greater than the strength of the lava.
  2. 2. As cracks reached about 10 inches in length, they branched, thus forming angles of approximately 120* which provided the greatest stress relief.
  3. 3. Each new crack branched again when it obtained the critical length and together with other similar cracks formed an irregular polygonal pattern. Ideal conditions, existing within the cooling lava, allowed surface cracks to deepen which resulted in the formation of the long columns.

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.

Continue to The Postpile is Exposed, See the Contents or Return to the Beginning



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