How do scientists prove a crater was made by an impact?
They look for evidence that only an impact can produce, such as shatter cones, shocked quartz, high-pressure minerals, impact melt and meteorite fragments or chemical traces.
Many circular features on Earth look like craters but turn out to be volcanoes, sinkholes or other formations. To confirm an impact crater, scientists look for specific evidence.
Shatter cones. These are cone-shaped fractures in rock, with fine radiating lines, ranging from centimetres to metres long. They form when a shock wave passes through rock. They are found at many impact structures, including Vredefort in South Africa and Sudbury in Canada, and are considered diagnostic.
Shocked quartz. Under a microscope, quartz grains from impact sites show sets of very thin, parallel planes called planar deformation features. These form only at pressures above about 10 gigapascals, far beyond those produced by volcanoes or earthquakes.
High-pressure minerals. Coesite and stishovite, high-pressure forms of silica, and reidite, a high-pressure form of zircon, indicate shock pressures typical of impacts. Shocked diamonds can form from graphite.
Impact melt and breccias. Rocks melted by impact, often containing fragments of target rock, and breccias with shocked fragments, such as suevite, are characteristic.
Meteorite traces. Fragments of the impactor, such as iron meteorite pieces at Meteor Crater, or chemical traces such as unusual amounts of iridium, nickel and chromium, or distinctive osmium and chromium isotopes, point to extraterrestrial material.
Geophysics. Gravity and magnetic surveys reveal buried structures, but alone are not proof.
About 200 impact structures on Earth have been confirmed using these criteria.