Impact melt
Rock that was melted by the heat and pressure of an impact, then cooled to form glass or crystalline rock.
When a projectile hits at many kilometres per second, the rock right beneath it is shocked to such high pressure that, once the pressure releases, it is left molten or even vaporised. This impact melt may cool rapidly into glass, or, in large volumes, crystallise slowly like volcanic rock. It forms veins and dykes in the crater floor, coats fragments in breccias and, in large craters, pools into melt sheets that can be hundreds of metres thick.
Impact melt is valuable because the impact itself resets its radioactive clocks, so dating the melt dates the event. Lunar impact melts collected by the Apollo missions are central to working out the Moon's history of bombardment. Melt can also contain chemical traces of the projectile.
The Sudbury structure in Ontario, Canada, about 1.85 billion years old, contains a vast body of impact-melted rock, the Sudbury Igneous Complex, linked to some of the world's richest nickel and copper ore deposits. At Manicouagan in Quebec, a melt sheet around 214 million years old lies within the ring of a large reservoir. In meteorites, thin black shock veins of melt record collisions in space. Melt rocks are recognised by glassy or fine-grained textures enclosing partly digested rock fragments.
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