Transient crater
The short-lived cavity excavated during an impact before collapse and modification produce the final crater.
Crater formation is divided into three stages: contact and compression, as the projectile hits and a shock wave forms; excavation, as the expanding shock and following rarefaction set target material moving outwards and upwards; and modification, as gravity reshapes the cavity. The transient crater is the cavity at the end of excavation. It is roughly bowl-shaped, with a depth around one-third to one-quarter of its diameter, and only part of it has been excavated: material from the upper zone, down to roughly one-tenth of the transient diameter, is ejected, while deeper material is displaced downwards.
In simple craters, modification is minor; wall material slides down to form a breccia lens and the final rim diameter grows only slightly. In complex craters, the transient cavity collapses entirely: the floor rebounds into a central peak or peak ring and the walls fail as terraces, so the final crater is much wider and shallower than the transient one.
Reconstructing transient dimensions is crucial for scaling laws linking projectile size and energy to crater size. For Chicxulub, with a final diameter of about 180 kilometres, models constrained by drilling of its peak ring suggest a transient cavity of roughly 100 kilometres. Clues include the radial distribution of shock levels and the depth of excavated rocks in ejecta.
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