What is a shatter cone?
A shatter cone is a cone-shaped fracture in rock with radiating grooves, formed by the shock wave of a meteorite impact.
Shatter cones are distinctive features found in rocks at impact craters. They are cone-shaped fractures, with surfaces marked by fine, branching grooves that radiate from the tip, a bit like horsetails or the feathers of a shuttlecock. They range from less than a centimetre to several metres long.
How they form. When an asteroid or comet strikes, a shock wave spreads outwards through the rock at enormous pressure. Shatter cones form at pressures of roughly 2 to 30 gigapascals. Exactly how is still debated, but they are thought to result from the interaction of the shock wave with small irregularities in the rock.
Pointing the way. In many impact structures, shatter cones tend to point towards the centre of impact, or upwards, as they were before later tilting. This helps geologists reconstruct the original structure.
Diagnostic. Shatter cones are widely accepted as proof of an impact, because no other natural process produces them. They were first described at the Steinheim crater in Germany in the early twentieth century and have since been found at many confirmed structures, including Vredefort in South Africa, Sudbury in Canada, Gosses Bluff in Australia and Kentland in the United States.
In different rocks. They form best in fine-grained rocks such as limestone and quartzite, but can occur in many rock types.
For collectors. Shatter cones are collected as specimens, though they are not meteorites. Collecting at protected sites is often forbidden.
Shatter cones are one of the most visible pieces of evidence of Earth's impact history.