Impact glass
Glass made when the heat and pressure of an impact melt rock so fast that it cools before crystals can grow.
When an asteroid or comet hits the ground at many kilometres per second, part of the target rock is melted or even vaporised in an instant. If that melt cools quickly, its atoms freeze in a disordered jumble instead of forming crystals, producing impact glass. It can appear as veins and blobs inside crater rocks, as bubbly pieces scattered around small craters, or as splash-form glasses thrown far away; the airborne, widely scattered kind is called a tektite.
Impact glass is valuable to geologists because it can be dated, often by argon–argon dating, giving the age of the crater. It may also trap traces of the impactor itself, such as extra nickel or iridium.
A famous example is Libyan Desert Glass, a pale yellow, almost pure silica glass found across a patch of the Sahara in western Egypt and dated to about 29 million years ago. Another is Darwin glass from Tasmania. Clues for recognition include frothy bubbles, flow banding, swirls of unmelted mineral grains and sometimes tiny metal spheres. Many volcanic glasses and industrial slags look similar, so laboratory work is needed to confirm an impact origin.
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