Hypervelocity impact
A collision so fast that the object hits faster than sound can travel through rock, making the impact behave like an explosion.
In an ordinary collision a thrown stone simply dents what it hits. In a hypervelocity impact the impactor arrives faster than shock waves can carry the force away through the rock, typically above a few kilometres per second. Asteroids striking Earth usually travel at around 15–25 kilometres per second. At those speeds the energy is so great that the impactor and part of the target are crushed, melted and partly vaporised in a split second, and the resulting crater is many times wider than the object that made it.
Because the shock spreads out from the point of contact much like an explosion, most impact craters are round even when the object came in at a slant. Only very shallow angles give oval craters.
Barringer Crater (Meteor Crater) in Arizona shows the result. About 50,000 years ago an iron asteroid perhaps a few tens of metres wide excavated a bowl roughly 1.2 kilometres across. Most of the impactor was destroyed, leaving only fragments, known as the Canyon Diablo meteorites. Tell-tale signs of hypervelocity impact include shatter cones, shocked quartz, high-pressure minerals such as coesite, and impact melt.
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