Undulatory extinction
Undulatory extinction is a wavy, sweeping pattern of darkness across a crystal under crossed polars, caused by mild shock.
Under a polarising microscope with crossed polars, an undeformed crystal goes completely dark, or extinguishes, at certain angles as the stage turns. If the crystal has been bent or strained, different parts go dark at slightly different angles, and a shadow seems to sweep across it. This is undulatory extinction.
In meteorites, undulatory extinction in olivine is the defining feature of shock stage S2, the lowest level of shock above unshocked S1. It records peak pressures of roughly 5 to 10 gigapascals. At higher shock stages it appears together with planar fractures and later with mosaicism.
Undulatory extinction is also common in Earth rocks that have been deformed by tectonic forces, so it is not unique to impacts. In meteorites, however, it is part of a well-calibrated scale. Petrologists check many olivine grains, because the shock effects vary across a sample, and assign the stage recorded by most of them.
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