Shatter cone
A cone-shaped rock fracture with fanning, horsetail-like grooves, formed by the shock wave of a meteorite impact.
Shatter cones are striking fracture surfaces shaped like cones or partial cones, decorated with grooves that branch outwards from the tip like a horse's tail. Their size ranges from about a centimetre to several metres. They form as the impact shock wave passes through rock at pressures of roughly a few gigapascals up to about 30, tens of thousands of times atmospheric pressure, though exactly how they form is still researched.
They are the only diagnostic shock feature visible to the naked eye, so they are often the first evidence that a structure is an impact site. Ordinary geological processes such as volcanoes or earthquakes do not make them. In undisturbed rock the tips tend to point towards the centre of the impact, which helps in reconstructing deeply eroded structures. Many large ancient craters, including Vredefort in South Africa and Sudbury in Canada, were confirmed partly through shatter cones.
They form best in fine-grained rock: superb examples come from the limestone of the Steinheim Basin in Germany. Recognise them by the repeating, nested cones with branching striations; similar-looking concretions and cone-in-cone structures lack the branching pattern. Many impact sites are protected, so collecting needs permission and must follow local law.
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