(Redirected from Astrobleme)
An 'impact crater' is an 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. 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 landform on many 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 as they become
eroded, buried and transformed by
tectonics over time. Where such processes have destroyed most of the original crater topography, the term 'impact structure' is more commonly used. In early literature, before the significance of impact cratering was widely recognised, the terms 'astrobleme' and 'crypto-volcanic' were used to describe impact-related features on Earth.
In the early Solar System, rates of impact cratering were very much higher than today. The large multi-ringed impact basins, with diameters of 100's km 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 an impact large enough to produce a 20 km diameter crater about once every million years on average. 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 metres up to about 300 km, and they range in age from about two thousand to about two billion years.
History
Daniel Barringer (1860-1929) was one of the first to identify a geological structure as an impact crater, the Barringer
Meteorite Crater (or the "
Meteor Crater") in
Arizona, but at the time his ideas were not widely accepted, and when they were, there was no recognition of the fact that Earth impacts are common in geological terms.
In the 1920s, the American geologist
Walter H. Bucher studied a number of craters in the US. He concluded they had been created by some great explosive event, but believed they were the result of some massive volcanic eruption. However, in 1936, the geologists
John D. Boon and
Claude C. Albritton Jr. revisited Bucher's studies and concluded the craters he studied were probably formed by impacts.
The issue remained more or less speculative until the 1960s. A number of researchers, most notably
Eugene M. Shoemaker, conducted detailed studies of the craters that provided clear evidence that they had been created by impacts, identifying the shock-metamorphic effects uniquely associated with impacts, 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, (
Victoria, British Columbia,
Canada), and
Wolf von Engelhardt of the
University of Tübingen in
Germany began a methodical search for "impact structures". By 1970, they had tentatively identified more than 50.
Their work remained controversial, but the American
Apollo Moon landings, which were in progress at the time, provided evidence of 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.
The age of known impact craters on the Earth ranges from about a thousand (e.g. the
Haviland crater in Kansas) to almost two billion years, though few older than 200 million years have been found, as geological processes tend to obliterate older ones. They are also selectively found in the
stable interior regions of continents. Few underwater 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.
Current estimates of the rate of cratering on the Earth suggest that from one to three craters with a width greater than 20 kilometers are created every million years. This indicates that there are far more relatively young craters on the planet than have been discovered so far.
Crater formation
Impact cratering involves collisions between solid objects at high speeds; typically the velocity of impact is higher than the
velocity of sound in those objects. Such hyper-velocity impacts produce physical effects, including
melting and
vaporization, that are quite different from those that occur in familiar sub-sonic collisions. On Earth, ignoring the effects of travel through the atmosphere, the lowest velocity at which impact on the surface can occur is the gravitational
escape velocity of about 11 km/s. The fastest impacts occur at more than 70 km/s which represents the sum of 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 in the region 20 to 25 km/s.
Impacts at these high speeds produce
shockwaves in solid materials, and both the impactor and the material impacted, are rapidly
compressed to high density. Following this 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 the material in most impacts is many times higher than that in the highest 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.
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 shockwave initiates from the point of contact. As this shockwave expands, it decelerates and compresses the impactor, and it accelerates and compresses the target. Stress levels within the shockwave far exceeds 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 shockwaves, 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 shockwave 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 shockwave decays, the shocked region decompresses towards more usual pressures and densities. The damage produced by the shockwave raises the temperature of the material, and 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 shockwave, and it remains moving away from the impact behind the decaying shockwave.
Excavation
Contact, compression, decompression, and the passage of the shockwave 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 moves away from the impact point. The motion is initially downwards and outwards, and with time this evolves to 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 centre 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
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 crater 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
Schrodinger; 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
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:
★ 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.
Craters can also be created from underground
nuclear explosions. One of the most crater-pocked sites on the planet is the
Nevada Test Site, where a number of craters were purposely made during its years as a center for
nuclear testing (see, for example,
Operation Plowshare).
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 d. The diameter is normally in the range of 15-25 km. The archetype is
Sosigenes crater.
★ ''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 d inner walls and relatively flat floors. They frequently have large central peak formations.
Tycho crater 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
Notable impact craters on Earth
Main articles: List of impact craters on Earth
★ Aorounga Crater (Chad)
★ Barringer Crater, aka Meteor Crater (Arizona, US)
★ Bosumtwi crater (Ghana)
★ Chesapeake Bay impact crater (Virginia, US)
★ Chicxulub, Extinction Event Crater (Mexico)
★ Clearwater Lakes (Quebec, Canada)
★ Connolly Basin crater (Western Australia)
★ Deep Bay crater (Saskatchewan, Canada)
★ Gosses Bluff crater (Australia)
★ Haughton impact crater (Nunavut, Canada)
★ Kaali crater (Estonia)
★ Kara-Kul crater (Tajikistan)
★ Kebira crater (Libya/Egypt)
★ Lonar crater (India)
★ Mahuika crater (New Zealand)
★ Manicouagan Reservoir (Quebec, Canada)
★ Manson crater (Iowa, US)
★ Mistastin crater (Labrador, Canada)
|
★ Morokweng crater (South Africa)
★ Nördlinger Ries (Germany)
★ Panther Mountain (New York, US)
★ Popigai crater, (Siberia)
★ Rio Cuarto craters (Argentina)
★ Rochechouart crater (France)
★ Roter Kamm crater (Namibia)
★ Shoemaker crater (Western Australia)
★ Shunak crater (Kazakhstan)
★ The Siljan Ring (Sweden)
★ Silverpit crater (North Sea off the United Kingdom)
★ Sudbury Basin (Ontario, Canada)
★ Vredefort crater (South Africa)
★ Weaubleau-Osceola impact structure (Missouri, US)
★ Wilkes Land crater (Antarctica)
★ Wolfe Creek crater (Western Australia)
★ Woodleigh crater (Western Australia)
★ Yarrabubba crater (Western Australia)
|
See the
Earth Impact Database,
[1] a website concerned with over 170 identified impact craters on the Earth.
Some extraterrestrial craters
★
Caloris Basin (Mercury)
★
Hellas Basin (Mars)
★
Mare Orientale (Moon)
★
Petrarch crater (Mercury)
★
Skinakas Basin (Mercury)
★
South Pole-Aitken basin (Moon)
★
Herschel crater (Mimas)
Largest named craters in the Solar System
#
South Pole-Aitken basin - Moon - Diameter: 2,500 km
#
Hellas Basin - Mars - Diameter: 2,100 km
# "
Skinakas Basin" - Mercury - Diameter: ~1,600 km
#
Caloris Basin - Mercury - Diameter: 1,350 km
#
Mare Imbrium - Moon - Diameter: 1,100 km
#
Isidis Planitia - Mars - Diameter: 1,100 km
#
Mare Tranquilitatis - Moon - Diameter: 870 km
#
Argyre Planitia - Mars - Diameter: 800 km
#
Mare Serenitatis - Moon - Diameter: 700 km
#
Mare Nubium - Moon - Diameter: 700 km
#
Beethoven - Mercury - Diameter: 625 km
#
Valhalla - Callisto - Diameter: 600 km, with rings to 4,000 km diameter
#
Hertzsprung - Moon - Diameter: 590 km
#
Apollo - Moon - Diameter: 540 km
#
Huygens - Mars - Diameter: 470 km
#
Schiaparelli - Mars - Diameter: 470 km
#
Menrva - Titan - Diameter: 440 km
#
Korolev - Moon - Diameter: 430 km
#
Dostievskij - Mercury - Diameter: 400 km
#
Odysseus - Tethys - Diameter: 400 km
#
Tolstoj - Mercury - Diameter: 390 km
#
Goethe - Mercury - Diameter: 380 km
#
Mare Orientale - Moon - Diameter: 350 km, with rings to 930 km diameter
#
Epigeus - Ganymede - Diameter: 340 km
#
Gertrude - Titania - Diameter: 320 km
#
Asgard - Callisto - Diameter: 300 km, with rings to 1,400 km diameter
#
Vredefort crater - Earth - Diameter: 300 km
#
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
★
Caldera
★
Cretaceous-Tertiary extinction event
★
Impact event
★
Nemesis
★
Rampart crater
★
Ray system
References
1. http://www.unb.ca/passc/ImpactDatabase/essay.html
2. http://planetarynames.wr.usgs.gov/
★ 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, Dec. 1981.
★ Melosh, H.J., 1989, Impact cratering: A geologic process: New York, Oxford University Press, 245 p.
External links
★
Photographs of terrestrial impact craters.
★
a study of a South Carolina crater
★
The Geological Survey of Canada Crater database, 172 impact structures
★
A recent news report about tektites
★
Aerial Explorations of Terrestrial Meteorite Craters
★
Google Earth Placemarker based on the Geological Survey of Canada Crater database (KML)
★
All 172 confirmed meteor impact sites on earth, viewable in Google Earth (Largest, Most recent, Per continent, Including size indicator)
★
Impact sites, with individual bibliographies
★
Solarviews: Terrestrial Impact Craters
★
Earth Impact Database
★
Giant Crater Found: Tied to Worst Mass Extinction Ever
★
WA Geological Survey Meteorite Impacts in Western Australia
★
Terrestrial Impact Craters
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