Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a shock with pressures in the range of 2-30 GPa.
Shatter cones have a distinctively conical shape that radiates from the top (apex) of the cones repeating cone-on-cone in large and small scales in the same sample. Sometimes they're more of a spoon shape on the side of a larger cone. In finer-grained rocks such as limestone, they form an easy to recognize "horsetail" pattern with thin grooves (stria). Coarser grained rocks tend to yield less well developed shatter cones, which may be difficult to distinguish from other geological formations such as slickensides. Geologists have various theories of what causes shatter cones to form, including compression by the wave as it passes through the rock or tension as the rocks rebound after the pressure subsides. The result is large and small branching fractures throughout the rocks.
Shatter cones can range in size from microscopic to several meters. A very large example of more than 10 meters in length is known from the Slate Islands impact structure, Canada. The azimuths of the cones's axes typically radiate outwards from the point of impact, with the cones pointing upwards and toward the center of the impact crater. Though the orientation of the rocks may be changed by post-cratering geological processes at the site.
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