The excavation stage, which immediately follows the contact and compression stage, is dominated by complex interactions between the outward-directed shock waves and the downward-directed rarefaction waves with the target, leading to the formation of the so-called transient crater or transient cavity. The excavation starts when tensional stress exceeds the mechanical strength of the target rocks. Rocks are fractured and shattered, moving in various directions, mainly upwards and out in the upper excavated zone and mainly down and outwards in the lower zone of the transient cavity. The opening of the transient cavity stops when shock and rarefaction waves are not energetic enough to eject material beyond the cavity rim. The transient crater, typically 20-30 times larger than the projectile diameter (e.g., French, 1998), shows an uplifted rim. During the excavation stage, the material in the excavated zone is ejected beyond the transient cavity rim, forming impact ejecta. The material in the displaced zone remains within the transient cavity, forming crater fill impactites (Figure 4). The entire excavation stage takes seconds to minutes to be completed, depending upon the crater size. About 6 seconds are necessary to excavate a 1-km-diameter crater, while a 200-km-diameter crater is excavated in about one minute and thirty seconds (Melosh, 1989; French, 1998).
The modification stage begins soon as the transient cavity reaches its maximum dimension. The magnitude of modifications is mainly function of the gravity of the impacted planet (or asteroid) and it depends also of the crater size. Major readjustments of the transient cavity occur for the formation of complex craters, while the bowl-shaped transient crater is quasi not modified in the case of simple craters. Two competing processes act during the modification stage; downward-directed gravitational collapse of the inner rim and uplift of the transient crater floor. The initially steep walls of the transient crater collapsed under gravitational forces forming characteristic terraces. Concerning the development of the central uplift, it occurs by displacements along faults as a brittle component in the case of moderately sized impact structures (such as for Bosumtwi; 10.5-km-diameter), whereas in the case of larger impact structures central uplifts involve fluidization and/or large differential movements of target blocks.
The modification stage takes typically only a few minutes to be completed, even though further readjustments of the crater, can take years or even millions of years, as a result of sedimentation within the crater, seismic readjustments, and/or erosion processes. In addition, rock-forming minerals are modified by impact-associated hydrothermal activity and weathering (e.g., Naumov, 2005).
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