Underground nuclear tests create great caverns beneath the surface, which later may cave in under the weight of the surface material. A similar effect occurs with meteoroid impacts. The crater is not created by the mechanics of impact. The projectile penetrates deep below the surface, the kinetic energy of the impactor converts it to a plasma which may either explode, ejecting material from the crater, or create a collapsing subterranean cavern similar to the underground nuke I described above.

This is why impact craters are always circular, never elliptical, even though the impactor may have originally struck the crust at a grazing angle.