Impact craters on the moon and Mercury have been partially filled by volcanic deposits and material eroded from the crater rims. Volcanism on both planetary bodies was initiated by impacts which blasted away material from the crust and thus reduced the pressure on underlying hot mantle rocks. Rocks melt at lower temperatures when pressure is reduced, so the removal of pressure by the impact event caused the mantle rocks to partially melt.

Melted rocks are typically more buoyant than surrounding solid rocks, so magma was able to rise through cracks in the crust and erupt as volcanic products onto the surface of impact craters.

Water and air play significant roles in the weathering of rocks on Earth. Neither the Moon nor Mercury has atmosphere or water; yet weathering still occurs at a very slow pace. "Space weathering" occurs when both tiny meteorites and particles emitted by the sun collide with planetary bodies. The rims of craters on the moon and Mercury jut out above the surrounding plains, and are thus more vulnerable to impacts from the tiny particles. Breakdown of crater rims by chemical reactions also occurs, albeit at a very slow pace. By these two processes, the crater rims are broken up over time and their material is deposited in craters.

Impact craters on the moon and Mercury do not get filled in with time. They may look filled in because the rock after being hit by the impactor turns to liquid and then re-freezes.