Impact crater

Impact craters are depressions formed on the surfaces of solid astronomical bodies by the hypervelocity collision of smaller objects such as asteroids, comets or meteoroids. Unlike volcanic craters, which originate from internal explosions or collapses, impact craters typically feature raised rims and floors that lie below the surrounding terrain. Although usually circular, they may appear elliptical or irregular when modified by local geological processes. Their sizes range from microscopic pits on lunar samples to vast multiring basins that dominate planetary landscapes.
Impact craters are among the most common landforms on many bodies of the Solar System, including the Moon, Mercury, Callisto, Ganymede and numerous smaller moons and asteroids. On worlds where erosion, tectonics and volcanic activity are active—such as Earth, Venus, Io, Europa, Titan and Triton—craters are less abundant or are heavily altered over time. In such cases, the terms impact structure or astrobleme are used when original topography has been largely destroyed. Early geological literature sometimes described these features as “cryptoexplosion” or “cryptovolcanic” structures before their impact origins were understood.

Cratering Rates and Preservation

The heavily cratered surfaces of Mercury and the Moon preserve evidence of the Late Heavy Bombardment, a period roughly 3.9 billion years ago when large numbers of impacts affected the inner Solar System. Earth’s cratering rate is much lower today but remains significant; on average, one to three craters large enough to be preserved form every million years. Variations in the impact rate occur due to collisions in the asteroid belt that generate fragment families capable of entering the inner Solar System.
Because Earth’s surface is continually reshaped by erosion, sedimentation, volcanism and plate tectonics, only about 190 terrestrial impact craters have been identified. These range from small, recent features to large structures more than two billion years old, though most preserved craters are younger than 500 million years. Craters are most often found in stable continental regions where rocks are less affected by destructive geological processes. Very few undersea craters are known because of rapid modification of the ocean floor and subduction into the mantle.

Scientific Recognition

The modern understanding of impact cratering has developed gradually. Daniel M. Barringer argued as early as 1903 that Arizona’s Meteor Crater was the result of a meteorite impact, although many geologists at the time favoured a volcanic explanation. In the 1920s Walter H. Bucher studied several sites later recognised as impact structures but believed explosive volcanic forces were responsible. By the 1930s John D. Boon and Claude C. Albritton reinterpreted these features as impact-related.
On the Moon, Grove Karl Gilbert proposed in 1893 that lunar craters were formed by impacts, an idea revived and elaborated by Ralph Baldwin in 1949. Around 1960 Gene Shoemaker advanced the modern impact paradigm. Comparing Meteor Crater with nuclear test sites in Nevada, he recognised the diagnostic similarity of their structures and investigated the mechanics of hypervelocity impacts for his doctoral work. In 1960 Shoemaker and Edward C. T. Chao discovered coesite, a high-pressure form of silica, at Meteor Crater, providing conclusive evidence of shock metamorphism. They later identified similar features at the Nördlinger Ries in Germany, confirming its impact origin.
Armed with these criteria, researchers such as Carlyle S. Beals and Wolf von Engelhardt undertook systematic searches for terrestrial craters, identifying dozens by 1970. The Apollo missions offered additional support by showing that the Moon’s surface is dominated by impact structures and that Earth must have experienced a similar bombardment even if its geological activity has erased much of the evidence.

Formation Process

Impact cratering occurs at velocities far beyond those found in everyday collisions, often exceeding the speed of sound in the impacted material. Even the slowest cosmic impacts on Earth, discounting atmospheric braking, reach approximately 11 km/s—the planet’s escape velocity—while the fastest can exceed 70 km/s. Atmospheric drag slows small meteoroids, but larger bodies retain much of their cosmic velocity.
Cratering involves a sequence of high-energy processes:

1. Initial contact and compressionThe impactor transfers kinetic energy to the target, generating powerful shock waves that compress both materials to extremely high pressures and temperatures. Melting and vaporisation occur instantly.
2. ExcavationThe over-pressurised region rebounds and expands, creating a transient cavity. Material is launched outward as ejecta, producing rays, blankets and secondary craters.
3. ModificationGravity and rock mechanics modify the transient cavity. Small impacts produce simple bowl-shaped craters, while larger ones form central peaks, terraces and multiring basins as the crust rebounds and collapses.