The time necessary to the formation of an impact crater, depending on the size of the impact event, is of the order of a few seconds to some minutes (e.g., Melosh, 1989; Ivanov and Artemieva, 2002); however it can take up to few hours for the proximal ejecta to be completely deposited. Target rocks are deformed and fractured, partially melted and vaporized as a result of a catastrophic burst of energy. Asteroid and comets collided the Earth at hypervelocity, between about 10 to 72 km/s (e.g., Melosh, 1989). Small bodies of a few meters across normally disintegrate in the atmosphere and only in rare cases, a fragment survives and reaches the ground at low speed (not more than a few hundreds m/s). Such a rock, which may excavate a small hole, rarely much wider than the rock itself, is called a meteorite. Larger asteroidal or cometary bodies are not significantly decelerated in the atmosphere and hit the Earth’s surface at cosmic velocity. As a result of this collision, an impact crater is formed. The formation of an impact crater (i.e., cratering process) is a continuous process which is generally subdivided into three main stages (for a review, see, e.g., Gault et al., 1968; Grieve, 1987; Melosh, 1989; French, 1998): the contact and compression, excavation and modification stages (Figure 4).

Figure 4. Cross-section diagrams showing the different stages of formation of an impact structure. At small diameters (i.e. diameter <2–4 km), a simple impact crater forms, when for diameters >2–4 km, the initial transient crater is unstable and a complex impact crater forms. The first stage, so–called contact/compression stage, as well as the starting excavation stage is depicted on a unique series of cross-section for both, simple and complex impact craters, as these stages of formation are almost identical in the two cases. See text for discussion on the different stages of formation (modified after French, 1998).

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