Page "Impact crater" Paragraph 13

Impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density.

Following initial compression, the high-density, over-compressed region rapidly depressurizes, exploding violently, to set in train the sequence of events that produces the impact crater.

Impact-crater formation is therefore more closely analogous to cratering by high explosives than by mechanical displacement.

Indeed, the energy density of some material involved in the formation of impact craters is many times higher than that generated by high explosives.

Since craters are caused by explosions, they are nearly always circular – only very low-angle impacts cause significantly elliptical craters.