Impact crater
A depression formed by hypervelocity impact of a smaller object.
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An impact crater forms when a smaller object slams into a solid astronomical body at extremely high speed, leaving a depression on the surface. Unlike volcanic craters, which are created by explosions or internal collapse, these craters usually have raised edges and a floor that sits lower than the surrounding ground.
Most impact craters are round, but landslides or other events can make them elliptical or irregular in shape. Their sizes vary wildly: some are microscopic, like those found on lunar rocks brought back by the Apollo Program, while others are simple bowl-shaped pits or enormous, complex basins with multiple rings. A well-known small example on Earth is Meteor Crater.
Quick Facts
- Earth's known impact craters
- ~190 confirmed
- Largest confirmed on earth
- Vredefort crater, South Africa (~300 km (186 mi) diameter, 2 billion years old)
Facts from the source article.
Lore & Background
Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede, and most small moons and asteroids. On other planets and moons that experience more active surface geological processes, such as Earth, Venus, Europa, Io, Titan, and Triton, visible impact craters are less common because they become eroded, buried, or transformed by tectonic and volcanic processes over time. Where such processes have destroyed most of the original crater topography, the terms impact structure or astrobleme are more commonly used.
In early literature, before the significance of impact cratering was widely recognised, the terms cryptoexplosion or cryptovolcanic structure were often used to describe what are now recognised as impact-related features on Earth. The cratering records of very old surfaces, such as Mercury, the Moon, and the southern highlands of Mars, record a period of intense early bombardment in the inner Solar System around 3.9 billion years ago. The rate of crater production on Earth has since been considerably lower, but it is appreciable nonetheless.
Earth experiences, on average, about one impact large enough to produce a 20 km crater every 1–2 million years. This indicates that there should be far more relatively young craters on the planet than have been discovered so far. The cratering rate in the inner solar system fluctuates as a consequence of collisions in the asteroid belt that create a family of fragments that are often sent cascading into the inner solar system. Formed in a collision 80 million years ago, the Baptistina family of asteroids is thought to have caused a large spike in the impact rate.
The rate of impact cratering in the outer Solar System could be different from the inner Solar System. Although Earth's active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified. These range in diameter from a few tens of meters up to about 300 km, and they range in age from recent times (e.g. the Sikhote-Alin craters in Russia whose creation was witnessed in 1947) to more than two billion years, though most are less than 500 million years old because geological processes tend to obliterate older craters.
Reader's Guide
Impact cratering involves high velocity collisions between solid objects, typically much greater than the speed of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the slowing effects of travel through the atmosphere, the lowest impact velocity with an object from space is equal to the gravitational escape velocity of about 11 km/s. The fastest impacts occur at about 72 km/s in the 'worst case' scenario in which an object in a retrograde near-parabolic orbit hits Earth.
The median impact velocity on Earth is about 20 km/s. However, the slowing effects of travel through the atmosphere rapidly decelerate any potential impactor, especially in the lowest 12 kilometres where 90% of the Earth's atmospheric mass lies. Meteors of up to 7,000 kg lose all their cosmic velocity due to atmospheric drag at a certain altitude (retardation point), and start to accelerate again due to Earth's gravity until the body reaches its terminal velocity of 0.09 to 0.16 km/s. The larger the meteoroid (i.e. asteroids and comets) the more of its initial cosmic velocity it preserves.
While an object of 9,000 kg maintains about 6% of its original velocity, one of 900,000 kg already preserves about 70%. Extremely large bodies (about 100,000 tonnes) are not slowed by the atmosphere at all, and impact with their initial cosmic velocity if no prior disintegration occurs. Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density. Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater.
Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement. Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives. Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters. This describes impacts on solid surfaces.
Frequently Asked Questions
Where do you find the most impact craters?
They dominate the surfaces of the Moon, Mercury, Callisto, Ganymede, and most small moons and asteroids throughout the Solar System. Earth also has them, though erosion and plate tectonics erase many over time.
How large can an impact crater get?
They span an enormous range, from tiny microscopic pits to vast multi-ringed basins roughly 300 kilometers across on Earth. The scale depends on the size, speed, and angle of the impacting body.
What exactly causes an impact crater to form?
A smaller object—such as a meteorite, comet fragment, or asteroid—strikes a solid surface at hypervelocity, releasing enough energy to excavate a depression and fling material outward. The resulting shape is usually circular, though landslides or oblique angles can make it elliptical or irregular.
What's a well-known impact crater on Earth?
Meteor Crater in Arizona is one of the best-preserved and most studied impact structures on our planet. It gives the public a tangible, visible sense of what these ancient collisions look like.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Impact crater (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
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