Shocked quartz
Quartz whose crystal structure carries microscopic damage that only the extreme pressures of an impact can produce.
Quartz is common and tough, which makes it a perfect witness to impact. When a shock wave of roughly 10 gigapascals or more passes through it, extremely thin layers of the crystal are converted to glass along particular crystal planes. These features are called planar deformation features. At higher pressures quartz can turn wholly to glass, or partly change into dense forms of silica, coesite and stishovite. Shocked quartz is one of the most accepted lines of evidence that a structure is an impact crater, because volcanoes and mountain building do not reach those pressures at the speed required.
It played a major part in two discoveries. At Barringer Crater in Arizona, the finding of coesite in 1960 helped confirm an impact origin. And grains of shocked quartz found worldwide in the thin clay layer at the K–Pg boundary, 66 million years old, supported the idea that a huge impact ended the age of dinosaurs, later linked to the Chicxulub crater.
Shocked quartz cannot be identified by eye. Geologists cut rock into thin sections and examine them under a polarising microscope, looking for several sets of fine, straight, parallel lines crossing each grain.
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