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). |