Peak ring (crater)

A peak ring crater is a type of complex crater, which is different from a multi-ringed basin or central-peak crater. A central peak is not seen; instead, a roughly circular ring or plateau, possibly discontinuous, surrounds the crater's center, with the crater rim still farther out from the center.
Formation.
The rings form by different processes, and inner rings may not be formed by the same processes as outer rings.
Peak rings have long been viewed to form in the stage subsequent to central peak formation in craters. The central peaks of craters are believed to originate from hydrodynamic flow of material lifted by inward-collapsing crater walls, while impact-shattered rock debris is briefly turned to fluid by strong vibrations that develop during crater formation. The peak-ring structure of Chicxulub crater was probably formed as inward-collapsing material impacted the over-steepened central peak, to form a hydraulic jump at the location where the peak ring was located.
Other theories have been formulated. Perhaps, in the case of Chicxulub crater, an over-high central peak collapsed into the peak ring.
Chicxulub is Earth's only crater to have an intact peak ring structure.
More ideas are available.