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An impact crater is an approximately circular
depression Depression may refer to: Mental health * Depression (mood), a state of low mood and aversion to activity * Mood disorders characterized by depression are commonly referred to as simply ''depression'', including: ** Dysthymia ** Major depressive ...
in the surface of a
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
,
moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

moon
, or other solid body in the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

Solar System
or elsewhere, formed by the
hypervelocity Hypervelocity is very high velocity The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed In e ...
impact Impact may refer to: * Impact (mechanics), a high force or shock (mechanics) over a short time period * Impact, Texas, a town in Taylor County, Texas, US Science and technology * Impact crater, a meteor crater caused by an impact event * Impact e ...

impact
of a smaller body. In contrast to
volcanic crater A volcanic crater is an approximately circular depression in the ground caused by volcanic activity. It is typically a bowl-shaped feature within which contains either multiple vents or a singular vent. During volcanic eruptions, molten magma an ...
s, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex,
multi-ringed impact basins A multi-ringed basin (also a multi-ring impact basin) is not a simple bowl-shaped crater, or a Peak ring (crater), peak ring crater, but one containing multiple concentric Topography, topographic rings; a multi-ringed basin could be described as ...
.
Meteor Crater Meteor Crater is a meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet o ...

Meteor Crater
is a well-known example of a small impact crater on Earth. Impact craters are the dominant geographic features on many solid Solar System objects including the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
,
Mercury Mercury usually refers to: * Mercury (planet) Mercury is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman g ...

Mercury
, Callisto,
Ganymede
Ganymede
and most small moons and
asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a telescope and was not observ ...

asteroid
s. On other planets and moons that experience more active surface geological processes, such as
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
,
Venus Venus is the second planet from the Sun. It is named after the Venus (mythology), Roman goddess of love and beauty. As List of brightest natural objects in the sky, the brightest natural object in Earth's night sky after the Moon, Venus can ...

Venus
,
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
,
Europa Europa may refer to: Places *Europe Europe is a continent A continent is one of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven regions are commonly regard ...
, Io and
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictional ...
, visible impact craters are less common because they become
eroded In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

eroded
, buried or transformed by
tectonics Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building A mountain is an elevated portion of the Earth's crust, gen ...
over time. Where such processes have destroyed most of the original crater topography, the terms
impact structure 250px, Impact crater structure An impact structure is a generally circular or craterlike geologic structure of deformed bedrock or sediment produced by impact on a planetary surface, whatever the stage of erosion of the structure. In contrast, an im ...
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 from one to three impacts large enough to produce a crater about once every million years on average. 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 The Baptistina family ( FIN: FIN tbl#403, 403) is an asteroid family of more than 2500 members that was probably produced by the breakup of an asteroid across 80 million years ago following an impact with a smaller body. The two largest presumed ...
of asteroids is thought to have caused a large spike in the impact rate. Note that 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, abou
190
terrestrial impact craters have been identified. These range in diameter from a few tens of meters up to about , 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. They are also selectively found in the stable interior regions of continents. Few undersea craters have been discovered because of the difficulty of surveying the sea floor, the rapid rate of change of the ocean bottom, and the
subduction of the ocean floor
subduction of the ocean floor
into Earth's interior by processes of
plate tectonics Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. L ...
. Impact craters are not to be confused with landforms that may appear similar, including
calderas
calderas
,
sinkholes A sinkhole, also known as a cenote, sink, sink-hole, swallet, swallow hole, or doline (the different terms for sinkholes are often used interchangeably), is a depression or hole in the ground caused by some form of collapse of the surface lay ...
, glacial cirques,
ring dike A ring dike or ring dyke is an intrusive igneous Igneous rock (derived from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in th ...
s, salt domes, and others.


History

Daniel M. Barringer, a mining engineer, was convinced already in 1903 that the crater he owned,
Meteor Crater Meteor Crater is a meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet o ...

Meteor Crater
, was of cosmic origin. Yet most geologists at the time assumed it formed as the result of a volcanic steam eruption. In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States. He concluded they had been created by some great explosive event, but believed that this force was probably
volcanic A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet ar ...

volcanic
in origin. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded that the craters that he studied were probably formed by impacts.
Grove Karl Gilbert Grove Karl Gilbert (May 6, 1843 – May 1, 1918), known by the abbreviated name ''G. K. Gilbert'' in academic literature, was an American geologist. Biography Gilbert was born in Rochester, New York and graduated from the University of Roches ...

Grove Karl Gilbert
suggested in 1893 that the Moon's craters were formed by large asteroid impacts. Ralph Baldwin in 1949 wrote that the Moon's craters were mostly of impact origin. Around 1960,
Gene Shoemaker Eugene Merle (Gene) Shoemaker (April 28, 1928 – July 18, 1997) was an American geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as ...
revived the idea. According to , Gene "saw the craters on the Moon as logical impact sites that were formed not gradually, in , but explosively, in seconds." For his
Ph.D. A Doctor of Philosophy (PhD, Ph.D., or DPhil; or ''doctor philosophiae'') is the most common at the highest academic level awarded following a course of study. PhDs are awarded for programs across the whole breadth of academic fields. Be ...
degree at
Princeton Princeton University is a private Private or privates may refer to: Music * "In Private "In Private" was the third single in a row to be a charting success for United Kingdom, British singer Dusty Springfield, after an absence of nearly two ...

Princeton
(1960), under the guidance of
Harry Hammond Hess Harry Hammond Hess (May 24, 1906 – August 25, 1969) was an American geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes the Earth and other terrestrial planets, as well as the processes that ...
, Shoemaker studied the impact dynamics of
Barringer Meteor Crater Meteor Crater is a meteorite impact crater approximately east of Flagstaff, Arizona, Flagstaff and west of Winslow, Arizona, Winslow in the northern Arizona desert of the United States. The site had several earlier names, and fragments of th ...

Barringer Meteor Crater
. Shoemaker noted Meteor Crater had the same form and structure as two
explosion crater An explosion crater is a type of crater formed when material is ejected from the surface of the ground by an explosive event at or immediately above or below the surface. 400px, Stylised cross-section of a crater formed by a below-ground explosion ...
s created from
atomic bomb A nuclear weapon (also known as an atom bomb, atomic bomb, nuclear bomb or nuclear warhead, and colloquially as an A-bomb or nuke) is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fiss ...
tests at the
Nevada Test Site The Nevada National Security Site (N2S2 or NNSS), known as the Nevada Test Site (NTS) until August 23, 2010, is a United States Department of Energy The United States Department of Energy (DOE) is a cabinet Cabinet or The Cabinet may refer ...
, notably Jangle U in 1951 and Teapot Ess in 1955. In 1960, Edward C. T. Chao and Shoemaker identified
coesite Coesite is a form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to ...

coesite
(a form of
silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to oxygen. An oxide () is a chemical compound that con ...
) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within
suevite Suevite is a Rock (geology), rock consisting partly of melted material, typically forming a breccia containing glass and crystal or Lithic fragment (geology), lithic fragments, formed during an Impact crater, impact event. It forms part of a g ...

suevite
at
Nördlinger Ries The Nördlinger Ries is an impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Solar System or elsewhere, formed by the hy ...
, proving its impact origin. Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the
Dominion Astrophysical Observatory The Dominion Astrophysical Observatory, located on Observatory Hill, in Saanich, British Columbia, was completed in 1918 by the Canadian government. The Dominion Architect responsible for the building was Edgar Lewis Horwood. The main instrume ...
in
Victoria, British Columbia Victoria is the capital city of the Canadian province The provinces and territories of Canada are sub-national divisions within the geographical areas of Canada Canada is a country in the northern part of North America. Its Provinc ...
, Canada and Wolf von Engelhardt of the
University of Tübingen The University of Tübingen, officially the Eberhard Karl University of Tübingen (german: Eberhard Karls Universität Tübingen; la, Universitas Eberhardina Carolina), is a public university, public research university located in the city of Tüb ...
in Germany began a methodical search for impact craters. By 1970, they had tentatively identified more than 50. Although their work was controversial, the American
Apollo Apollo, grc, Ἀπόλλωνος, ''Apóllōnos'', label=genitive , ; , grc-dor, Ἀπέλλων, ''Apéllōn'', ; grc, Ἀπείλων, ''Apeílōn'', label=Arcadocypriot Greek, ; grc-aeo, Ἄπλουν, ''Áploun'', la, Apollō, ...

Apollo
Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the
Moon The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia (continent), Australia), it is the largest natural satellite in the Solar System relative to the size of its plane ...

Moon
. Because the processes of erosion on the Moon are minimal, craters persist. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters.


Crater formation

Impact cratering involves high velocity collisions between solid objects, typically much greater than the
speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. At , the speed of sound in air is about , or one kilometre in or one mile in . It depends s ...
in those objects. Such hyper-velocity impacts produce physical effects such as
melting Melting, or fusion, is a physical process that results in the phase transition In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiari ...

melting
and
vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomenon. ...

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 #REDIRECT Escape velocity#REDIRECT Escape velocity In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matt ...
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 In statistics Statistics is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data Data (; ) are individual facts, statistics, or items of information, often numeric. In a m ...

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. Meteorites 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 Terminal velocity is the maximum velocity (speed) attainable by an object as it falls through a fluid In physics, a fluid is a substance that continually Deformation (mechanics), deforms (flows) under an applied shear stress, or external fo ...

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 wave of an attached shock on a sharp-nosed supersonic F/A-18F Super Hornet in transonic flight Flight or flying is the process by which an object (physics), object motion (physics), moves through a space without contacting any planetary surfac ...
s in solid materials, and both impactor and the material impacted are rapidly
compressed Compression may refer to: Physical science *Compression (physics), size reduction due to forces *Compression member, a structural element such as a column *Compressibility, susceptibility to compression *Gas compression *Compression ratio, of a co ...
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 An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion An explosion is a rapid expansion in volume associated with an extremely vigorous outward release ...
than by mechanical displacement. Indeed, the
energy density In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular suc ...

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
explosion An explosion is a rapid expansion in volume Volume is a expressing the of enclosed by a . For example, the space that a substance (, , , or ) or occupies or contains. Volume is often quantified numerically using the , the . The volum ...

explosion
s, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p. This describes impacts on solid surfaces. Impacts on porous surfaces, such as that of Hyperion, may produce internal compression without ejecta, punching a hole in the surface without filling in nearby craters. This may explain the 'sponge-like' appearance of that moon. It is convenient to divide the impact process conceptually into three distinct stages: (1) initial contact and compression, (2) excavation, (3) modification and collapse. In practice, there is overlap between the three processes with, for example, the excavation of the crater continuing in some regions while modification and collapse is already underway in others.


Contact and compression

In the absence of
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...

atmosphere
, the impact process begins when the impactor first touches the target surface. This contact the target and decelerates the impactor. Because the impactor is moving so rapidly, the rear of the object moves a significant distance during the short-but-finite time taken for the deceleration to propagate across the impactor. As a result, the impactor is compressed, its density rises, and the
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

pressure
within it increases dramatically. Peak pressures in large impacts exceed 1 T Pa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions. In physical terms, a shock wave originates from the point of contact. As this shock wave expands, it decelerates and compresses the impactor, and it accelerates and compresses the target. Stress levels within the shock wave far exceed the strength of solid materials; consequently, both the impactor and the target close to the impact site are irreversibly damaged. Many crystalline minerals can be transformed into higher-density phases by shock waves; for example, the common mineral quartz can be transformed into the higher-pressure forms
coesite Coesite is a form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to ...

coesite
and
stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titani ...

stishovite
. Many other shock-related changes take place within both impactor and target as the shock wave passes through, and some of these changes can be used as diagnostic tools to determine whether particular geological features were produced by impact cratering. As the shock wave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the shock wave raises the temperature of the material. In all but the smallest impacts this increase in temperature is sufficient to melt the impactor, and in larger impacts to vaporize most of it and to melt large volumes of the target. As well as being heated, the target near the impact is accelerated by the shock wave, and it continues moving away from the impact behind the decaying shock wave.


Excavation

Contact, compression, decompression, and the passage of the shock wave all occur within a few tenths of a second for a large impact. The subsequent excavation of the crater occurs more slowly, and during this stage the flow of material is largely subsonic. During excavation, the crater grows as the accelerated target material moves away from the point of impact. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity that continues to grow, eventually producing a
paraboloid In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position of ...

paraboloid
(bowl-shaped) crater in which the centre has been pushed down, a significant volume of material has been ejected, and a topographically elevated crater rim has been pushed up. When this cavity has reached its maximum size, it is called the transient cavity. The depth of the transient cavity is typically a quarter to a third of its diameter.
Ejecta Ejecta (from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the R ...
thrown out of the crater do not include material excavated from the full depth of the transient cavity; typically the depth of maximum excavation is only about a third of the total depth. As a result, about one third of the volume of the transient crater is formed by the ejection of material, and the remaining two thirds is formed by the displacement of material downwards, outwards and upwards, to form the elevated rim. For impacts into highly porous materials, a significant crater volume may also be formed by the permanent compaction of the pore space. Such compaction craters may be important on many asteroids, comets and small moons. In large impacts, as well as material displaced and ejected to form the crater, significant volumes of target material may be melted and vaporized together with the original impactor. Some of this impact melt rock may be ejected, but most of it remains within the transient crater, initially forming a layer of impact melt coating the interior of the transient cavity. In contrast, the hot dense vaporized material expands rapidly out of the growing cavity, carrying some solid and molten material within it as it does so. As this hot vapor cloud expands, it rises and cools much like the archetypal mushroom cloud generated by large nuclear explosions. In large impacts, the expanding vapor cloud may rise to many times the scale height of the atmosphere, effectively expanding into free space. Most material ejected from the crater is deposited within a few crater radii, but a small fraction may travel large distances at high velocity, and in large impacts it may exceed
escape velocity #REDIRECT Escape velocity#REDIRECT Escape velocity In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matt ...
and leave the impacted planet or moon entirely. The majority of the fastest material is ejected from close to the center of impact, and the slowest material is ejected close to the rim at low velocities to form an overturned coherent flap of ejecta immediately outside the rim. As ejecta escapes from the growing crater, it forms an expanding curtain in the shape of an inverted cone. The trajectory of individual particles within the curtain is thought to be largely ballistic. Small volumes of un-melted and relatively un-shocked material may be
spall Spall are fragments of a material that are broken off a larger solid Solid is one of the four fundamental states of matter (the others being liquid A liquid is a nearly incompressible fluid In physics, a fluid is a substance t ...

spall
ed at very high relative velocities from the surface of the target and from the rear of the impactor. Spalling provides a potential mechanism whereby material may be ejected into inter-planetary space largely undamaged, and whereby small volumes of the impactor may be preserved undamaged even in large impacts. Small volumes of high-speed material may also be generated early in the impact by jetting. This occurs when two surfaces converge rapidly and obliquely at a small angle, and high-temperature highly shocked material is expelled from the convergence zone with velocities that may be several times larger than the impact velocity.


Modification and collapse

In most circumstances, the transient cavity is not stable and collapses under gravity. In small craters, less than about 4 km diameter on Earth, there is some limited collapse of the crater rim coupled with debris sliding down the crater walls and drainage of impact melts into the deeper cavity. The resultant structure is called a simple crater, and it remains bowl-shaped and superficially similar to the transient crater. In simple craters, the original excavation cavity is overlain by a lens of collapse
breccia Breccia ( or ) is a rock composed of broken fragments of minerals In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid ...

breccia
, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat. Above a certain threshold size, which varies with planetary gravity, the collapse and modification of the transient cavity is much more extensive, and the resulting structure is called a
complex crater 250px, Impact crater structure Complex craters are a type of large impact crater An impact crater is an approximately circular depression (geology), depression in the surface of a planet, natural satellite, moon, or other solid body in the Sol ...
. The collapse of the transient cavity is driven by gravity, and involves both the uplift of the central region and the inward collapse of the rim. The central uplift is not the result of ''elastic rebound'', which is a process in which a material with elastic strength attempts to return to its original geometry; rather the collapse is a process in which a material with little or no strength attempts to return to a state of . Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and
terraced walls A terraced wall, also a terrace wall, or a terraced retaining wall is a wall that is divided into sections (Terrace (earthworks), terraces) over a slope. Such designs are useful when building on a steep Grade (slope), grade. Terraced walls may be ...
. At the largest sizes, one or more exterior or interior rings may appear, and the structure may be labeled an ''impact basin'' rather than an impact crater. Complex-crater morphology on rocky planets appears to follow a regular sequence with increasing size: small complex craters with a central topographic peak are called ''central peak craters'', for example
TychoTycho is a masculine given name, a latinization of Greek Τύχων, from the name of Tyche ( grc-gre, Τύχη, link=no), the Greek goddess of fortune or luck. The Russian form of the name is ''Tikhon'' (Тихон). People Given name * Tycho Br ...
; intermediate-sized craters, in which the central peak is replaced by a ring of peaks, are called ''peak-ring craters'', for example Schrödinger; and the largest craters contain multiple concentric topographic rings, and are called ''multi-ringed basins'', for example . On icy (as opposed to rocky) bodies, other morphological forms appear that may have central pits rather than central peaks, and at the largest sizes may contain many concentric rings.
Valhalla In Norse mythology Norse or Scandinavian mythology is the body of myths Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the traditions common to that cultu ...
on Callisto is an example of this type.


Identifying impact craters

Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. Impact craters produce melted rocks as well, but usually in smaller volumes with different characteristics. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as
shatter cone (type locality), Germany. Image:Shattercone_Wengenhausen.JPG, Typical shatter cone from Ries impact crater, Germany. Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or Undergr ...
s, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however. Impacts produce distinctive shock-metamorphic effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include: * A layer of shattered or "
breccia Breccia ( or ) is a rock composed of broken fragments of minerals In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid ...

breccia
ted" rock under the floor of the crater. This layer is called a "breccia lens". *
Shatter cone (type locality), Germany. Image:Shattercone_Wengenhausen.JPG, Typical shatter cone from Ries impact crater, Germany. Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or Undergr ...
s, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks. * High-temperature rock types, including laminated and welded blocks of sand,
spherulite , Arizona. Image:Rhyolite pmg ss 2006.jpg, Thin section, Photomicrograph of rhyolite showing spherulitic texture (brown, between grey to white crystals). In petrology, spherulites () are small, rounded bodies that commonly occur in Glass, vitre ...

spherulite
s and
tektite Tektites (from Greek , "molten") are gravel-sized bodies composed of black, green, brown, or gray natural glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, te ...
s, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: scientific literature has reported that some "shock" features, such as small shatter cones, which are often associated only with impact events, have been found also in terrestrial volcanic ejecta. * Microscopic pressure deformations of minerals. These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or
stishovite Stishovite is an extremely hard, dense tetragonal form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titani ...

stishovite
and
coesite Coesite is a form ( polymorph) of silicon dioxide Silicon dioxide, also known as silica, is an oxide of rutile. Ti(IV) centers are grey; oxygen centers are red. Notice that oxygen forms three bonds to titanium and titanium forms six bonds to ...

coesite
, varieties of
shocked quartz Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz is deformed along planes inside the crystal. These pla ...
. * Buried craters, such as the , can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.


Economic importance of impacts

On Earth impact craters have resulted in useful minerals. Some of the ores produced from impact related effects on Earth include ores of
iron Iron () is a chemical element In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behav ...

iron
,
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
,
gold Gold is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elemen ...

gold
,
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
, and
nickel Nickel is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

nickel
. It is estimated that the value of materials mined from impact structures is five billion dollars/year just for North America.Grieve, R., V. Masaitis. 1994. The Economic Potential of Terrestrial Impact Craters. International Geology Review: 36, 105–151. The eventual usefulness of impact craters depends on several factors especially the nature of the materials that were impacted and when the materials were affected. In some cases the deposits were already in place and the impact brought them to the surface. These are called “progenetic economic deposits.” Others were created during the actual impact. The great energy involved caused melting. Useful minerals formed as a result of this energy are classified as “syngenetic deposits.” The third type, called “epigenetic deposits,” is caused by the creation of a basin from the impact. Many of the minerals that our modern lives depend on are associated with impacts in the past. The Vredeford Dome in the center of the
Witwatersrand Basin The Witwatersrand () (locally the Rand or, less commonly, the Reef) is a , north-facing scarp in South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. With over Demographics of So ...
is the largest goldfield in the world which has supplied about 40% of all the gold ever mined in an impact structure (though the gold did not come from the bolide). The asteroid that struck the region was wide. The
Sudbury Basin The Sudbury Basin (), also known as Sudbury Structure or the Sudbury Nickel Irruptive, is a major geology, geological structure in Ontario, Canada. It is the third-largest known impact crater or astrobleme on Earth, as well as one of the oldest ...
was caused by an impacting body over in diameter. This basin is famous for its deposits of
nickel Nickel is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

nickel
,
copper Copper is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

copper
, and
Platinum Group Elements The platinum-group metals (abbreviated as the PGMs; alternatively, the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs)) are six Noble metal, noble, Precious metal, precious meta ...
. An impact was involved in making the Carswell structure in Saskatchewan, Canada; it contains
uranium Uranium is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical elem ...

uranium
deposits. Hydrocarbons are common around impact structures. Fifty percent of impact structures in North America in hydrocarbon-bearing sedimentary basins contain oil/gas fields.


Martian craters

Because of the many missions studying
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
since the 1960s, there is good coverage of its surface which contains Martian craters, large numbers of craters. Many of the List of craters on Mars, craters on Mars differ from those on the Moon and other moons since Mars contains ice under the ground, especially in the higher latitudes. Some of the types of craters that have special shapes due to impact into ice-rich ground are pedestal craters, rampart craters, expanded craters, and LARLE craters.


Lists of craters


Impact craters on Earth

On Earth, the recognition of impact craters is a branch of geology, and is related to planetary geology in the study of other worlds. Out of many proposed craters, relatively few are confirmed. The following twenty are a sample of articles of confirmed and well-documented impact sites. See the Earth Impact Database, a website concerned with 190 () scientifically-confirmed impact craters on Earth.


Some extraterrestrial craters

* Caloris Basin (Mercury) * Hellas Basin (Mars) * Herschel (Mimantean crater), Herschel crater (Mimas) * Mare Orientale (Moon) * Petrarch crater (Mercury) * South Pole – Aitken basin (Moon)


Largest named craters in the Solar System

# North Polar Basin (Mars), North Polar Basin/Borealis Basin (disputed) – Mars – Diameter: 10,600 km # South Pole-Aitken basin – Moon – Diameter: 2,500 km # Hellas Basin – Mars – Diameter: 2,100 km # Caloris Basin – Mercury – Diameter: 1,550 km # Mare Imbrium, Imbrium Basin – Moon – Diameter: 1,100 km # Isidis Planitia – Mars – Diameter: 1,100 km # Mare Tranquilitatis – Moon – Diameter: 870 km # Argyre Planitia – Mars – Diameter: 800 km # Rembrandt (crater), Rembrandt – Mercury – Diameter: 715 km # Mare Serenitatis, Serenitatis Basin – Moon – Diameter: 700 km # Mare Nubium – Moon – Diameter: 700 km # Beethoven (crater), Beethoven – Mercury – Diameter: 625 km #
Valhalla In Norse mythology Norse or Scandinavian mythology is the body of myths Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the traditions common to that cultu ...
– Callisto – Diameter: 600 km, with rings to 4,000 km diameter # Hertzsprung (crater), Hertzsprung – Moon – Diameter: 590 km # Turgis (crater), Turgis – Iapetus – Diameter: 580 km # Apollo (crater), Apollo – Moon – Diameter: 540 km # Engelier (crater), Engelier – Iapetus – Diameter: 504 km # Mamaldi (crater), Mamaldi – Rhea – Diameter: 480 km # Huygens (crater), Huygens – Mars – Diameter: 470 km # Schiaparelli (Martian crater), Schiaparelli – Mars – Diameter: 470 km # Rheasilvia – 4 Vesta – Diameter: 460 km # Gerin (crater), Gerin – Iapetus – Diameter: 445 km # Odysseus (crater), Odysseus – Tethys – Diameter: 445 km # Korolev (lunar crater), Korolev – Moon – Diameter: 430 km # Falsaron (crater), Falsaron – Iapetus – Diameter: 424 km # Dostoevskij (crater), Dostoevskij – Mercury – Diameter: 400 km # Menrva (crater), Menrva – Titan – Diameter: 392 km # Tolstoj (crater), Tolstoj – Mercury – Diameter: 390 km # Goethe (crater), Goethe – Mercury – Diameter: 380 km # Malprimis (crater), Malprimis – Iapetus – Diameter: 377 km # Tirawa (crater), Tirawa – Rhea – Diameter: 360 km # Mare Orientale, Orientale Basin – Moon – Diameter: 350 km, with rings to 930 km diameter # Evander (crater), Evander – Dione – Diameter: 350 km # Epigeus (crater), Epigeus – Ganymede – Diameter: 343 km # Gertrude (crater), Gertrude – Titania – Diameter: 326 km # Telemus (crater), Telemus – Tethys – Diameter: 320 km # Asgard (crater), Asgard – Callisto – Diameter: 300 km, with rings to 1,400 km diameter # Vredefort crater – Earth – Diameter: 300 km # Kerwan (crater), Kerwan – Ceres – Diameter: 284 km # Powehiwehi (crater), Powehiwehi – Rhea – Diameter: 271 km There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars. Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus.


See also

* * * * * * * * * * * * * * * , 1998 book from Lunar and Planetary Institute – comprehensive reference on impact crater science


References


Bibliography

* * * * *


Further reading

*


External links

*
The Geological Survey of Canada Crater database, 172 impact structuresAerial Explorations of Terrestrial Meteorite CratersImpact Meteor Crater Viewer
Google Maps Page with Locations of Meteor Craters around the world
Lunar and Planetary Institute slidshow: contains pictures
{{DEFAULTSORT:Impact Crater Impact craters, * Impact geology, Crater Lunar science Depressions (geology) Articles containing video clips Planetary geology