Shock metamorphism & shatter cones

Above: A shatter cone in dolomite from the Kentland, Indiana impact structure (Specimen from Lawrence University.)

Meteorite impacts are pretty amazing things. An impact crater forms instantaneously on a human time scale, with the shock wave from the impact propagation through the target rock in the following stages: contact and compression, crater excavation, and crater modification.

Some rocks and minerals are deformed by shock-metamorphism that is caused by the high pressures and temperatures; certain shock-metamorphic features can be used to unambiguously identify an impact crater, as there are no other natural processes capable of producing such effects. During the contact and compression stage, shock waves will travel faster than the speed of sound through the medium and pressures will reach in excess of those in the upper mantle.

Shatter  cones are the only shock-metamorphism indicators visible at the outcrop scale and form at pressures from around ~1-10 GPa. There are other shock-metamorphic features such as planar fractures or planar deformation features in quartz grains that are visible on the microscopic scale, but if you find a shatter cone, you know for sure that you’ve got an impact crater on your hands.

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