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The prominent impact crater Tycho on the Moon. NASA photo.

In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body. In most common usage, the term is used for the approximately circular depression in the surface of a planet, moon or other solid body in the Solar System, formed by the hyper-velocity impact of a smaller body with the surface. This is in contrast to the pit crater which results from an internal collapse. Impact craters typically have raised rims, and they range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is perhaps the best-known example of a small impact crater on the Earth.

Impact craters provide the dominant landforms 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, Mars, Europa, Io and Titan, visible impact craters are less common because they become eroded, buried or transformed by tectonics 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.

In the early Solar System, rates of impact cratering were much higher than today. The large multi-ringed impact basins, with diameters of hundreds of kilometers or more, retained for example on Mercury and the Moon, record a period of intense early bombardment in the inner Solar System that ended about 3.8 billion years ago. Since that time, the rate of crater production on Earth has been considerably lower, but it is appreciable nonetheless; Earth experiences from one to three impacts large enough to produce a 20 km diameter 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.

Although the Earth’s active surface processes quickly destroy the impact record, about 170 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 were witnessed in 1947) to more than two billion years, though most are less than 200 million years old because geological processes tend to obliterate older craters. They are also selectively found in the stable interior regions of continents. Few under sea 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 into the Earth's interior by processes of plate tectonics.

Impact craters are not to be confused with other landforms that in some cases appear similar, including calderas and ring dikes.

Contents

History

Eugene Shoemaker, pioneer impact crater researcher, here at a stereoscopic microscope used for asteroid discovery

Daniel Barringer (1860-1929) was one of the first to identify an impact crater, Meteor Crater in Arizona; to crater specialists the site is referred to as Barringer Crater in his honor. Initially Barringer's ideas were not widely accepted, and even when the origin of Meteor Crater was finally acknowledged, the wider implications for impact cratering as a significant geological process on Earth were not.

In the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the USA. He concluded they had been created by some great explosive event, but believed that this force was probably 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.

The concept of impact cratering remained more or less speculative until the 1960s. At this time a number of researchers, most notably Eugene M. Shoemaker, (co-discoverer of the comet Shoemaker-Levy 9), conducted detailed studies of a number of craters and recognized clear evidence that they had been created by impacts, specifically identifying the shock-metamorphic effects uniquely associated with impact events, of which the most familiar is shocked quartz.

Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory in Victoria, British Columbia, Canada and Wolf von Engelhardt of the University of Tübingen 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 Moon landings, which were in progress at the time, provided supportive evidence by recognizing the rate of impact cratering on the Moon. Processes of erosion on the Moon are minimal and so craters persist almost indefinitely. 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 movie.ogg
A laboratory simulation of an impact event and crater formation

Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity 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 more than 70 km/s, calculated by summing the escape velocity from Earth, the escape velocity from the Sun at the Earth's orbit, and the motion of the Earth around the Sun. The median impact velocity on Earth is about 20 to 25 km/s.

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.

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.

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Contact and compression

In the absence of atmosphere, the impact process begins when the impactor first touches the target surface. This contact accelerates 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 within it increases dramatically. Peak pressures in large impacts exceed 1 TPa to reach values more usually found deep in the interiors of planets, or generated artificially in nuclear explosions.

In physical terms, a supersonic shock wave initiates 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 and 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 sub-sonic. During excavation, the crater grows as the accelerated target material moves away from the impact point. The target's motion is initially downwards and outwards, but it becomes outwards and upwards. The flow initially produces an approximately hemispherical cavity. The cavity continues to grow, eventually producing a 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.

Herschel Crater on Saturn's moon Mimas

The depth of the transient cavity is typically a quarter to a third of its diameter. Ejecta thrown out of the crater does 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 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 spalled 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

Weathering may change the aspect of a crater drastically. This mound on Mars' north pole may be the result of an impact crater that was buried by sediment and subsequently re-exposed by erosion.

In most circumstances, the transient cavity is not stable: it 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, ejecta and melt rock, and a portion of the central crater floor may sometimes be flat.

Multi-ringed impact basin Valhalla on Jupiter's moon Callisto

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. 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 gravitational equilibrium.

Complex craters have uplifted centers, and they have typically broad flat shallow crater floors, and terraced walls. 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 Tycho; 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 Orientale. On icy as opposed to rocky bodies, other morphological forms appear which may have central pits rather than central peaks, and at the largest sizes may contain very many concentric rings – Valhalla on Callisto is the type example of the latter.

Identifying impact craters

Impact crater structure
Shocked polymictic breccia from the Azuara impact structure, Spain.

Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides 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. An exception is that impact craters on Venus often have associated flows of melted material.

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, 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:

Close-up of shatter cones developed in fine grained dolomite from the Wells Creek crater, USA.
  • A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".
  • Shatter cones, 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, spherulites and tektites, 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: it is reported in the scientific literature that some "shock" features, such as small shatter cones, which are often reported as being associated only with impact events, have been found 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 and coesite, varieties of shocked quartz.

Lunar crater categorization

In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorization of lunar impact craters. They used a sampling of craters that were relatively unmodified by subsequent impacts, then grouped the results into five broad categories. These successfully accounted for about 99% of all lunar impact craters.

The LPC Crater Types were as follows:

  • ALC — small, cup-shaped craters with a diameter of about 10 km or less, and no central floor. The archetype for this category is 'Albategnius C'.
  • BIO — similar to an ALC, but with small, flat floors. Typical diameter is about 15 km. The lunar crater archetype is Biot.
  • SOS — the interior floor is wide and flat, with no central peak. The inner walls are not terraced. The diameter is normally in the range of 15–25 km. The archetype is Sosigenes.
  • TRI — these complex craters are large enough so that their inner walls have slumped to the floor. They can range in size from 15–50 km in diameter. The archetype crater is Triesnecker.
  • TYC — these are larger than 50 km, with terraced inner walls and relatively flat floors. They frequently have large central peak formations. Tycho is the archetype for this class.

Beyond a couple of hundred kilometers diameter, the central peak of the TYC class disappear and they are classed as basins.

Lists of craters

Impact craters on Earth

On Earth, the recognition of impact craters is a branch of geology, as opposed to astronomy on other worlds. Out of many proposed craters, relatively few are confirmed. The following are a sample of articles of confirmed and well-documented impact sites.

See the Earth Impact Database,[1] a website concerned with over 170 scientifically-confirmed impact craters on Earth.

Some extraterrestrial craters

Largest named craters in the Solar System

  1. South Pole-Aitken basin - Moon - Diameter: 2,500 km
  2. Hellas Basin - Mars - Diameter: 2,100 km
  3. Caloris Basin - Mercury - Diameter: 1,550 km
  4. Mare Imbrium - Moon - Diameter: 1,100 km
  5. Isidis Planitia - Mars - Diameter: 1,100 km
  6. Mare Tranquilitatis - Moon - Diameter: 870 km
  7. Argyre Planitia - Mars - Diameter: 800 km
  8. Rembrandt – Mercury – Diameter: 715 km
  9. Mare Serenitatis - Moon - Diameter: 700 km
  10. Mare Nubium - Moon - Diameter: 700 km
  11. Beethoven - Mercury - Diameter: 625 km
  12. Valhalla - Callisto - Diameter: 600 km, with rings to 4,000 km diameter
  13. Hertzsprung - Moon - Diameter: 590 km
  14. Turgis - Iapetus - Diameter: 580 km
  15. Apollo - Moon - Diameter: 540 km
  16. Huygens - Mars - Diameter: 470 km
  17. Schiaparelli - Mars - Diameter: 470 km
  18. Menrva - Titan - Diameter: 440 km
  19. Korolev - Moon - Diameter: 430 km
  20. Dostoevskij - Mercury - Diameter: 400 km
  21. Odysseus - Tethys - Diameter: 400 km
  22. Tolstoj - Mercury - Diameter: 390 km
  23. Goethe - Mercury - Diameter: 380 km
  24. Tirawa - Rhea - Diameter: 360 km
  25. Mare Orientale - Moon - Diameter: 350 km, with rings to 930 km diameter
  26. Epigeus - Ganymede - Diameter: 340 km
  27. Gertrude - Titania - Diameter: 320 km
  28. Asgard - Callisto - Diameter: 300 km, with rings to 1,400 km diameter
  29. Vredefort crater - Earth - Diameter: 300 km
  30. Mead - Venus - Diameter: 270 km

There are approximately twelve more impact craters/basins larger than 300 km on the Moon, five on Mercury, and four on Mars.[2] Large basins, some unnamed but mostly smaller than 300 km, can also be found on Saturn's moons Dione, Rhea and Iapetus.

See also

References

  • Charles A. Wood and Leif Andersson, New Morphometric Data for Fresh Lunar Craters, 1978, Proceedings 9th Lunar and Planet. Sci. Conf.
  • Bond, J. W., "The development of central peaks in lunar craters", Moon and the Planets, vol. 25, December 1981.
  • Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p.
  • Baier, J., Die Auswurfprodukte des Ries-Impakts, Deutschland, in Documenta Naturae, Vol. 162, 2007. ISBN 978-3-86544-162-1

Further reading

Mark, Kathleen (1987). Meteorite Craters. Tucson: University of Arizona Press. ISBN 0816509026.  

External links


Simple English

. NASA photo.]]

File:Craterstructure.gif
Impact crater structure
File:Antarctica Map Wilkes L
Position of supposed Wilkes Land crater
File:Azuara-impact-structure-Shocked
Shocked breccia from the Azuara impact structure, Spain.
from the Wells Creek crater, USA.]]

Impact craters are formed by meteorites or comets striking the Earth (or Moon, or planet). There is now a system for recording and assessing craters.[1]

Contents

Largest craters on Earth

The five largest confirmed impact structures are:

  1. Vredefort, South Africa: 300km diameter, 2023 million years ago.
  2. Sudbury, Ontario, Canada: 250km diameter, 1850 million years ago.
  3. Chicxulub, Mexico: 170km diameter, 65 million years ago.
  4. Popigai, Russia: 100km diameter, 35.7 million years ago.
  5. Manicougan, Quebec, Canada: 100km diameter, 214 million years ago.

There are some other suggested impact structures which are larger. It is a feature of the Earth that climate, weathering and plate tectonics removes most of the older features and events. In comparison, the Moon retains a nearly complete record of its past impact events. The implication is that the Earth once suffered a similar bombardment in its early history. This period is known as the Late Heavy Bombardment, because it occurred after the Earth and Moon formed.

Largest unconfirmed craters

Any of these may or may not be meteorite craters:

  1. Massive Australian Precambrian/Cambrian Impact Structure, Northern Territory, Australia: multiple rings 500km – 2000km, 545 million years ago at the Neoproterozoic/Cambrian boundary.
  2. Shiva, Indian Ocean, west of India: ~600km length, 400km width, 65 million years ago. Obviously this is of high interest, because of the date.
  3. Ross, Antarctica: 550km, Oligocene
  4. Cheko, Siberia, Russia: ~500 km diameter, ~100 million years ago.
  5. Bedout, Indian Ocean near Western Australia: 200km, 250 million years ago.
  6. Wilkes Land, Antarctica: 485km, ~~ 250 million years ago?

Identifying impact craters

The distinctive mark of an impact crater is the presence of rock which has undergone shock-metamorphic effects, shattered or melted rocks, and crystal deformations. Examples:

  • A layer of shattered rock under the floor of the crater. This layer is called a 'breccia lens'.[2]
  • Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.[3][4][5][6]
  • High-temperature rock types, including laminated and welded blocks of sand, tektites, or glassy spatters of molten rock. They may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt.
  • 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 varieties of shocked quartz.

References

  1. Earth Impact Database: [1]
  2. Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix,[2]
  3. Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions. They are evidence that the rock has been subjected to a high-pressure shock.
  4. French B.M. (1998). Traces of catastrophe. Lunar and Planetary Institute. http://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html. Retrieved 2007-05-20. 
  5. Sagy A.; Fineberg J.; Reches Z. (2004). "Shatter cones: branched, rapid fractures formed by shock impact". Journal of Geophysical Research 109: B10209. doi:10.1029/2004JB003016. http://web.cocc.edu/breynolds/classes/UO_Geol_353/shatter%20cones.pdf. 
  6. French, Bevan M. (2005). "Stalking the wily shatter cone: a critical guide for impact-crater hunters" (pdf). Impacts in the Field (Impact Field Studies Group) 2 (Winter): s 3–10. http://web.eps.utk.edu/~faculty/ifsg_files/newsletter/Winter_2005.pdf 


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