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Introduction

A meteorite is defined as any extraterrestrial solid mass that reaches the Earth's surface (Farris Lapidus, 1990). The term strictly extends from the nano scale e.g. dust particles, through to the macro scale e.g. 100s of metres in diameter (Zanda and Rotaru, 2001). However, the term meteorite is generally reserved for the larger particles that reach the Earth's surface.

A meteorite crater is the expression left on the Earth's surface following the impact of a meteorite. The formation of a meteorite impact crater can be divided into three main stages: 1) contact and compression, 2) excavation and 3) modification (Kenkman, 2002). Impact craters are categorised into three different types: simple, complex and multiring basins, which are generated following the collapse of an unstable transient, or initial, crater (Melosh, 1989). In general it is the size of the crater that determines its morphology, but there are some environmental factors that can also be of influence e.g. if the impact is sub-aerial or sub-marine. If the impact occurs in deep water (deeper than 1km) then the craters tend to be concentric and often lack melt sheets and rim walls but have deposits and radial gulleys formed by resurge of the sea (Ormö and Lindström, 2000). The size of the crater is intimately related to the size, speed and angle at which the meteorite collides with the Earth (Melosh, 1989).

Simple Craters

Simple craters (Figure 1 - top) are the smallest of the three crater types and range from 0 - 20km in diameter.

Figure 1: Differences in formation of a simple crater and a complex crater. The central peak of the complex crater is formed as a result of uplift of material stratigraphically beneath the crater, which rebounds in response to compression caused by the impact. From Melosh (1989)

They are characterised by a simple bowl shape similar to that of the transient crater suggesting minor gravitational collapse following impact. Simple craters generally have depth / diameter ratios of between 1/5 (0.2) and 1/3 (0.33) (Melosh, 1989). One example of a simple crater is the Barringer Crater, Arizona (Figure 2), which is 1.186km in diameter today.

Figure 2: The Barringer crater formed 50,000 years ago following the impact of a meteorite. It is a simple crater 1.186km in diameter. From Earth Impact Database (2003)

Complex Craters

The formation of complex craters (Figure 1 - bottom) arises as a result of large-scale gravitational collapse of the transient crater (O'Keefe and Ahrens, 1999) and, as such, complex craters are commonly larger than simple craters. The transition from simple to complex craters on Earth can occur in craters as small as 5km in diameter (Kenkman, 2002). Complex craters are characterised by the presence of such features as central peaks, terraced walls and flat floors (Melosh, 1989). The most complicated complex crater structure is a peak ring crater (Figure 3) whereby a series of rings develop within the original crater rim. This occurs in the larger complex craters where the central peak collapses and creates a peak ring before the motion stops (Melosh, 1989).

Figure 3: Formation of complex and peak ring craters. Peak ring craters are the more complex form and occur as larger (greater than 50km) complex craters. From Melosh (1989)

The morphological study of complex craters from eroded remnants exposed at the surface suggests that their formation is just an extension of the simple crater formation. However, geologic investigation of the central peak highlights that they are composed of deformed and fractured rocks, which originally underlay the transient crater (Melosh, 1989). Structural studies of terrestrial craters suggest that the modification from the bowl-shaped transient crater to the complex form occurs as a result of complete gravity driven collapse of an initially deep transient crater. The collapse is achieved principally by uplift of the rocks underlying the crater's centre as a result of the release of a pressure overburden, with the rock units near the rim slumping downwards and inwards (Melosh, 1989).

Multiring Basins

Multiring basins are large circular structures with not just one rim but an additional concentric raised ring or rings and a system of radial furrows. In order to be classified as a multiring basin and not a peak ring crater the basin must posses at least two asymmetric scarped rings, one of which may be the crater rim (Melosh, 1989). Multiring basins are much larger structures (100s to 1000s km in diameter) than either simple or complex craters. At present no multiring basins have been formally recognised on Earth and as such the study of multiring basins is based principally on examples seen on the Moon, Callisto and Ganymede (Figure 4) (Melosh, 1989).

Figure 4: The Valhalla basin on Callisto an example of a multiring basin with an internal deformed region and a number of outward facing concentric ring scarps. From Melosh (1989)

The formation of multiring basins is more complicated than just gravitational collapse and a number of different hypotheses have been developed in explanation. These hypotheses include; the volcanic modification hypothesis (Hartmann and Yale, 1968), the megaterrace hypothesis (Head 1974) and the nested crater hypothesis (Hodges and Wilhelms, 1978). However, it is the ring tectonic theory (Melosh and McKinnon, 1978) that is the widely accepted hypothesis. The ring tectonic theory suggests that in layered media in which the strength decreases with increasing depth, one or more ring fractures arise outside the rim of the original crater (Figure 5) (Melosh and McKinnon, 1978). This suggests that for the formation of multiring basins to occur there must be a high brittle-ductile thickness ratio in the impacted material i.e. where thick crust exists over a deeper ductile layer (Allemand and Thomas, 1999).

Figure 5: The widely accepted ring tectonic theory of multiring basin formation. Drawings (a) through to (d) illustrate the effect of decreasing lithosphere thickness. From Melosh (1989)

Conclusion

A meteorite is defined as any extraterrestrial solid mass that reaches the Earth's surface and is generally reserved for the larger particles that reach the Earth's surface. A meteorite crater is the expression left on the Earth's surface following the impact of a meteorite. The formation of a meteorite impact crater can be divided into three main stages: 1) contact and compression, 2) excavation and 3) modification. The structure of a crater is related to the size, although environmental factos do influence this. The size of the crater is related to the size and the speed and angle of impact of a meteorite. There are three main types of crater; simple, complex and multiringed.

References

Allemand, P. and Thomas, P., 1999. Small-scale models of Multiring basins. Journal of Geophysical Research 104 No. E7, pp 16,501-16,514.

Earth Impact Database (2003): www.unb.ca/passc/ImpactDatabase/index.html

Farris Lapidus, D., 1990. Dictionary of geology. London and Glasgow, Collins, pp 349-350. Buy it

Kenkmann, T., 2002. Folding within seconds. Geology30, No. 3, pp 231-234.

Melosh, H. J., 1989. Impact cratering: A geologic process New York, Oxford University Press.

O'Keefe, J. D. and Ahrens, T. J., 1999. Complex craters: Relationships of stratigraphy and rings to impact conditions. Journal of Geophysical Research, 104, no. E11, pp 27,091-27,104.

Ormö, J. and Lindström, M., 2000. When a cosmic impact strikes the sea bed. Geological Magazine 137, no. 1, pp 67-80.

Zanda, B., and Rotaru, M., 2001. Meteorites: Their Impact on Science and History Cambridge University Press, pp 31-39. Buy it