Shock stage
The shock stage is a grade, usually S1 to S6, describing how strongly a meteorite was shocked by impacts on its parent body.
Collisions in space send shock waves through rock, and minerals respond in recognisable ways as the pressure rises. In 1991 the researchers Dieter Stöffler, Klaus Keil and Edward Scott published a scale for ordinary chondrites based on these effects, seen mainly in olivine and plagioclase under a polarising microscope. A revised version followed in 2018.
In outline: S1 is unshocked, with crystals that go dark (extinguish) sharply as the microscope stage rotates. S2 shows wavy, undulatory extinction. S3 adds planar fractures. S4 shows weak mosaicism, where a crystal breaks into slightly misaligned domains. S5 has strong mosaicism, and plagioclase has turned into the glass maskelynite. S6 includes local melting and high-pressure minerals such as ringwoodite. Each stage corresponds to a range of peak pressures, from below about 5 GPa to many tens of GPa.
The shock stage appears in full classifications alongside the group, type and weathering grade, for example "L6, S4, W1". Tenham, an L6 chondrite that fell in Australia in 1879, is a classic heavily shocked example with ringwoodite-bearing veins. Darkened rock and black shock veins on a cut face hint at a high stage.
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