Conchoidal Fracturing in Sandstone
August 07, 2023
Photographer: Stan Celestian
Summary Author: Stan Celestian
While visiting Nevada's Valley of Fire State Park I came across this outcrop on the west side of the park (left photo). The rock is Jurassic age Aztec Sandstone. It’s an aeolian deposit that once represented an extensive dune field, existing across present day California, Nevada and Arizona and parts of Utah and New Mexico. This area was similar to today's Sahara Desert but 180 million years ago.
After its formation this area of sandstone experienced an uplift with accompanied stresses and faulting. The fracturing of the sandstone was rapid, almost explosive and the energy stored in the rock was suddenly released. In this case it produced a massive conchoidal fracture pattern as seen in the middle photo. The right-most photo shows a telephoto view. Clearly visible are the radiating waves of the fracture created by the tensional release of the compressional forces within the sandstone.
A closer inspection of the image shows the cross bedding of this dune deposit. These more or less horizonal lines were formed as the dune was forming by the movement of the sand grains falling into position on the face of the dune. This view is like a slice through a "petrified" dune that allows a look at its internal structure. However, the major point here is that sandstone doesn’t normally display conchoidal fracturing, especially one so dominated by cross bedding structures.
This conchoidal fracture then represents a very large force exerted over a thick portion of the Aztec Sandstone. Thick enough that the cross bedding was irrelevant to the movement of the force producing the shock wave. Additionally, the shock wave was imposed quickly, not by an impact, but rather by the sudden release of pressure as along a fault line during an earthquake. Photos taken on May 5, 2023.
Valley of Fire State Park, Nevada Coordinates: 36.4757, 114.5384
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