impact crater

Astron., Geol.

crater (def. 2).

[1890-95]

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▪ landform

Introduction

      any feature on the continents that has been created by the impact of cosmic bodies (meteorites, asteroids (asteroid), or comets (comet)) on the Earth's surface. In fact, the collision of a Mars-sized body with the Earth early in the history of the solar system may have caused the formation of the Moon and the continents themselves, or rather the ocean/continent dichotomy of the Earth's outer layers.

      Impact craters differ from one another principally in terms of size, progressing from small, simple cup-shaped depressions originally with raised rims, through hollows having a central hill (or peak), to those with more complicated central peaks and basins, and finally to multi-ringed basins up to 1,000 kilometres in diameter. The surfaces of the Moon, Mercury, and certain regions of Mars and Venus, as well as most of the satellites of the solar system, are dominated by impact craters. They are much rarer on the Earth, which has about 100 ranging in size from a few tens of metres to 160 kilometres. These have been identified with varying degrees of certainty. The reason for the relative scarcity of impact craters on the Earth is that the processes that formed its surface—largely those of weathering, erosion, and mountain building—have undoubtedly removed the majority of the older structures, such as are observed on the surfaces of the other planets mentioned above.

      There is abundant evidence that impact cratering was not only an important surface process in planetary history but that large impact events produced effects that were crustal in scale. The formation of multi-ring basins on the early Moon, for example, is just as important a process in defining the tectonic framework of that body as plate-tectonic phenomena are on the Earth. Evidence from several planets indicates that the effects of very large-scale impacts go beyond the simple formation of an impact structure and serve to localize increased internal geologic activity over an extended period of geologic time. Although no longer occurring with the same frequency and magnitude as during the early solar system, large-scale impact events have continued to affect the local geology of the inner planets.

      The Moon travels through essentially the same orbital space as the Earth. Since the Moon has not been subjected to the surficial geomorphic processes and tectonic activity that prevail on Earth, the lunar surface serves as an invaluable repository of information about the rate and effects of large body impacts that are likely to have occurred on the Earth. Telescopic observations, orbiting remote-sensing spacecraft, and the Apollo manned explorations of the Moon conducted by the United States during the early 1970s have provided the necessary data to establish surface impact rates. These are quite comparable to rates derived independently from astronomical studies for fluxes of asteroidal and cometary objects in near-Earth space. (For details pertaining to this and related questions, see asteroid, comet, and meteorite.)

      Some important conclusions for the Earth follow. Asteroidal objects as large as 20 kilometres in diameter probably have struck the planet during the last few billion years, and bodies measuring 10 kilometres across apparently may collide with it every 50,000,000 to 100,000,000 years. Cometary nuclei of similar size may have nearly comparable rates of collision. A 10-kilometre stony object with a density of about three grams per cubic centimetre (0.12 pound per cubic inch) striking the terrestrial surface at a velocity of 25 kilometres per second (15.5 miles per second) would have kinetic energy in excess of 100,000,000 megatons—far greater than that contained in the world's total nuclear arsenal. On land, the impact of such an object would produce a transient crater 60 to 70 kilometres in diameter; the subsequent collapse of the basin walls would result in a final crater having a diameter of possibly 100 to 125 kilometres. Moreover, an impact of this kind would probably exert a planetwide influence on biological evolution because it could trigger mass extinctions of entire species, as perhaps in the case of the dinosaurs 65,000,000 years ago.

General characteristics of impact craters on the terrestrial surface

      In general, the larger the structure, the longer some of it is preserved but also the less certain the interpretation as an impact structure. Young craters have circular topographic highs around a cup-shaped depression. Older structures may completely lack discernible crater rims, yet they may still retain some degree of circularity, have distinctive internal deformation features, or contain remnants of original rock fragments (breccia) that formed during the impact event. Radial, circular, and annular drainage patterns are common.

      Older, more eroded examples of craters are called astroblemes (astrobleme), or more commonly cryptoexplosion structures. The latter designation is a modification of the term cryptovolcanic structure, which is no longer used because investigators have found no evidence for volcanism but much evidence for impact at many such sites. Thus, in a broader sense, an impact crater can be defined as any structure now at or near the surface that contains evidence of a shock-producing impact in which the disturbance by deformation and fragmentation of rock or soil is generally circular and which fades out rapidly in intensity of deformation upon reaching diameter-to-depth ratios of about three to one.

      Complex impact structures can be characterized by the presence or absence of peaks and rings, with subdivision into central peak craters, central peak basins, peak ring basins, and multi-ring basins. In general, the sequence from simple structures to multi-ring basins corresponds to increasing diameter or impact energy. The transition diameter of the morphological change from simple to complex form varies between planets probably because of differences in gravity. Most known and suspected terrestrial impact structures more than about three kilometres in diameter contain central uplifts, whereas the smaller ones do not. On the Moon, where gravity is one-sixth that on Earth, craters with central peaks are usually more than 15 to 20 kilometres in diameter. The central peaks make up roughly 10 percent of the width of the total circular deformed zone. Although there is evidence that planetary gravity exerts some control over the value of the transition diameter, it is apparent that parameters such as target rock characteristics also have an effect. This is evident on the Earth where the transition from simple to complex form occurs at four to five kilometres in crystalline rocks but at two to three kilometres in sedimentary rocks. Most of what is known about these various types is based on studies of the lunar surface.

C. Ronald Seeger

Formation of impact craters

Mechanics of the cratering process

 The cratering mechanics of simple craters are fairly well understood (Figure 1—>). On impact, the bulk of the kinetic energy of the projectile is transferred to the planetary surface, where a shock wave is formed. A shock wave is a transient pulse that moves at velocities higher than those of elastic (seismic or sonic) waves in the same material or medium and that produces an almost instantaneous rise in pressure behind its advancing front. Ahead of this pressure rise, the medium is not yet affected in any way. Behind it, the medium is immediately compressed, leading to a decrease in volume (increase in density) until decompression, or rarefaction, waves allow progressive relaxation. Pressure magnitudes in shock waves exceed the dynamic elastic limits of the transmitting materials. For rocks, this limit falls between 20 and 100 kilobars (1 kilobar equals 1,000 bars, or approximately 14,700 pounds per square inch). By comparison, nuclear explosions and impacts generate pressure waves whose initial amplitudes can reach megabars (1,000,000 bars) in the materials undergoing shock.

      An important property of shock waves involves a possible change of phase upon encountering a free surface or another medium of different density and wave propagating characteristics. An initial compression wave will thus be converted to a rarefaction wave, which, if reflected over the previous wave path, can “unload” the state of compression or place the medium under tension (i.e., cause it to stretch or tear). As a shock wave diverges spherically from a point source, its expansion into an ever larger volume also reduces its magnitude, so that the pressure at the moving front continually decreases with increasing radial distance from its source.

Stages of formation

 The sequence of events involved in the formation of a simple impact crater (shown schematically in Figure 1—>) is as follows. Immediately after an incoming object (e.g., a meteorite) strikes the ground, shock waves are imparted both to the rocks and to the object itself. The shock front in the rocks outruns the penetrating meteorite and forms an expanding cavity. The rocks behind the front are strongly compressed and generally are set in radial motion outward from the region of penetration. Depending on the properties of the rocks present, varying fractions of this target material are vaporized, melted, crushed, fragmented, or fractured. Ultimately, different segments of the volume of rock actually excavated are mixed together and dispersed. The initially intense shock attenuates as it diverges outward. The net effect is that the degree of shock damage attributable to compression diminishes with increasing radial distance from the line of penetration. Only a small proportion of the total volume excavated undergoes strong to intense shock pressures, and many of these rocks experience little or no permanent damage.

      The actual excavation process requires rarefaction waves. These waves form as the advancing compression waves (longitudinal wave) become isolated, or “detached,” from the excavation flow and move onward in the target as a fast-moving shock wave. The shock pressure declines to zero behind the front, but the particle velocity does not. This soon sets up rarefactions that effectively place the rock medium under tension, resulting in a general disruption of the component units. Because these now fragmented pieces were already in motion, many retain enough momentum to carry them forward and out of the developing crater at low angles as ejecta. Fragments that move out of the crater at high angles fall back as crater-filling breccia deposits. Some of the rock material is carried downward without ever leaving the crater to form concentrations of fragmental debris along its base. Beneath these rocks lies a rupture zone consisting of broken and displaced rock that underlies the breccia fill. Compression and rarefaction waves become too weak to efficiently excavate material below the upper boundary of the rupture zone.

      Within the rupture zone below the true crater floor and within its walls, the rocks may be folded, faulted, and intricately deformed. Especially in the rim, which marks the outer limit of surface excavation, layered units may be folded over completely to form structures that are similar in complexity to some of the great nappes (folded rock masses moved by thrust faulting) of the Alps. The overturning results when adjacent units below the uplifting free surface are peeled back as they move in close succession. Massive rocks do not fold back but tend to undergo uplift along fractures. All crater-related deformation, however, decreases abruptly away from the junction between the breccia and rupture zone, so beyond this interface the transition from shocked or disturbed rock to ordinary, unmodified country rock is rapid.

      During the initial compression stage, materials behind the shock front experience pressures so high (many hundreds of kilobars) that their dynamic compressive strengths are greatly exceeded. Under these conditions rocks behave much like fluids.

      As the spherical shock front advances outward in larger impacts, the compressed shell of material behind it mixes with the meteorite and ejects steadily increasing amounts at the onset of the excavation stage. The complex geometric patterns of particle movements that follow progressive relaxation by rarefaction waves result in the lateral deflection of flowing materials from their earlier radial paths. Below the line of impact, particle movements still proceed mainly downward, but they deviate increasingly sideward until a tangential flow from the crater at low angles dominates the region of the expanding rim. Ejection and crater growth represent a continuous, orderly process involving a steady flow of materials along flow lines. Cratering ceases when stresses become too weak to break up materials.

Variations in structure

 In its original state, a simple crater is characterized by a well-defined rim, a bowl-shaped cross section, extensive fallback deposits that thicken toward the centre, and small to moderate amounts (1 to 2 percent) of impact melt concentrated along the central base (Figure 1—>). Its initial diameter to depth ratio is about five to one. When the crater diameter is greater than four to five kilometres, the rupture zone beneath the central base tends to uplift along shear lines.

 Large complex craters have broad, dome-like uplifts, which are sometimes preserved in topographic expression as central peaks. Rocks within this uplifted portion may be strongly shocked and intricately contorted. The mechanism for uplift is related to the size of the impact, properties of the materials, and planetary gravity. It results from the plasticlike rebound of the materials upward under the centre of the impact (Figure 2—>, uplift stage). In layered materials the uplift normally contains stratigraphic units from depths below the general level of the crater base, which indicates the real upward movement of rocks.

      Some large craters with central peaks may have been caused by comet impact rather than meteorite impact. When the nucleus of a low-density comet strikes the ground, the dominant energy release remains near the surface. As a result, lateral excavation is more effective than in the case of penetrating iron or stony meteorites.

      Generally, complex craters have their breccia-melt deposits in an annular depression between the rim and central peak.

      The passage of the transient pressure waves through the rocks and their constituent minerals induces distinctive effects of very high pressures and temperatures and rapid strain rates that alter their composition and properties. Such effects are described as shock metamorphism. They include the formation of high-pressure polymorphs (different crystalline forms of the same composition) that normally are not stable in the Earth's upper crust; fragmented, fractured, and granulated rock masses; unusual intracrystalline markings; solid-state and selective melting; and brief vesiculation (bubbling) caused by pressures ranging up to 500–750 kilobars (comparable to pressures at depths of 1,400–1,800 kilometres within the Earth). The effective temperatures may rise above 2,000° C (3,600° F), and the strain rates are millions of times faster than those operating during mountain building. For these reasons, shock pressures can be determined from rock samples, and the original size of ancient, eroded craters can be estimated. At 20–30 kilobars, cleavage planes in mica start to bend and kink as though folded like layered strata. At higher pressures, more individual mica crystals begin to become distorted, and the intensity of deformation increases in all flakes thus affected. When the shock pressures in the passing waves exceed values of 50–75 kilobars (the dynamic elastic limit of many granitic rocks), changes unknown even in equivalent rock types buried deep within the crust begin to appear.

      Another such change is the development of shatter cones. These are peculiar external fracture surfaces in which closely-spaced “grooves” seemingly radiate outward from the apex of a cone somewhat like the hair of a horse's tail. Many such cones usually cluster together. Most have apexes that point in a common direction, which, when separate cone-bearing layers are rotated to their original position by photographic analysis, converge toward a single origin that appropriately coincides with the explosion centre of the initial impact. In 1947 Robert S. Dietz (Dietz, Robert S.) of the United States proposed that these features were indicative of impact—i.e., that they originated from shock waves. Subsequent research has proved this hypothesis to be correct.

      In quartz grains, sets of lamellae-like planes form within crystals over a pressure range from about 100 to 400 kilobars. These planar features presumably result from the crushing or slipping of crystal portions along preferred planes of weakness as the grains are compressed rapidly into much smaller volumes. In the higher pressure ranges over which the quartz planar features are produced, high-pressure forms of silica called coesite and stishovite, develop. Other shock-induced polymorphic transformations, as, for example, diamond from other forms of carbon, have been discovered in impact structures.

      At still higher pressures (450–600 kilobars), the effect is one of rapidly rising temperature from the conversion of mechanical to thermal energy that leads first to incipient melting of individual grains and then eventually to general melting and variable mixing of the whole rock material.

      The net result is the formation of a crater and the transformation of some of the local rocks into impact lithologies: impact melt rocks, shocked breccias (fragmented rocks), and deformed rocks. Evidence from rock samples returned from the lunar highlands, 90 percent of which are of impact origin, suggests that impact lithologies were important geologic constituents of the early crusts of all the inner planets of the solar system.

Nicholas M. Short C. Ronald Seeger

Population of terrestrial impact craters

      It has been estimated that over the past billion years the land surface of the Earth has been subjected to about 130,000 impacts that have produced craters one kilometre or more in diameter. For the reasons mentioned above, there simply remains no record of most of these impacts. Approximately 100 major craters have been identified worldwide. Numerous small craters have been detected as well. Some are concentrated in clusters and are so small that they cannot be considered landforms. Various small craters also have been associated with specific meteorites.

      Terrestrial impact craters range in age from Precambrian to Recent, but the record is heavily biased toward the Recent (see above). Examples of simple bowl-shaped craters, complex central peak structures, central peak basins, and peak-ring basins have been found on the Earth. Examples of multi-ring basins are rare, however. This is a result of the relatively young surface of the Earth and the sharp decrease in the number of impacting bodies in recent geologic time, as compared with the period when large multi-ring basins were formed on the Moon. Based on the lunar record scaled to terrestrial conditions, the Earth, which is more than 4,000,000,000 years old, may have had 25 to 50 basins larger than 1,000 kilometres in diameter. It is possible that the Sudbury crater in Ontario, Canada, and the Vredefort crater in South Africa, both of which measure about 140 kilometres across, originally had a multi-ring form. However, both are Precambrian in age and are highly degraded or modified.

      Possibly the best candidates for a well-preserved, multi-ring structure on Earth are the Popigai crater in northern Siberia, which measures roughly 100 kilometres in diameter and apparently has three rings in the crystalline rocks (rock) of its floor; or the Acraman crater in South Australia, which seems to have an inner depressed area about 30 kilometres in diameter, an intermediate depression or ring approximately 90 kilometres in diameter, and possibly an outer ring about 160 kilometres in diameter. Also, investigators have located what appears to be a large crater with central peaks in Wilkes Land, Antarctica. This depression is more than 848 metres deep and 243 kilometres across.

C. Ronald Seeger

Additional Reading

Two general books on the subject are Ronald Greeley, Planetary Landscapes (1985); and Bruce Murray, Michael C. Malin, and Ronald Greeley, Earthlike Planets: Surfaces of Mercury, Venus, Earth, Moon, Mars (1981). Collected papers from scientific meetings include Bevan N. French and Nicholas M. Short (eds.), Shock Metamorphism of Natural Materials (1968); and Leon T. Silver and Peter H. Schultz (eds.), Geological Implications of Impacts of Large Asteroids and Comets on the Earth (1982), in which see especially Richard A.F. Grieve, “The Record of Impact on Earth: Implications for a Major Cretaceous/Tertiary Impact Event,” pp. 25–37. The physical processes of cratering are described in H.J. Melosh, Impact Cratering: A Geologic Process (1989). Other helpful sources include: Glen A. Izett, The Cretaceous/Tertiary Boundary Interval, Raton Basin, Colorado and New Mexico and its Content of Shock Metamorphosed Minerals (1990); Virgil L. Sharpton and Peter D. Ward (eds.), Global Catastrophes in Earth History: An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality (1990); U.S. Geological Survey, This Dynamic Planet: World Map of Volcanoes, Earthquakes, Impact Craters, and Plate Tectonics (1994); Alex Bevan and Ken McNamara, Australia's Meteorite Craters (1993); and L.J. Pesonen and H. Henkel (eds.), Terrestrial Impact Craters and Craterform Structures with a Special Focus on Ferroscandia (1992).C. Ronald Seeger

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