The explosion is what carves out the crater so almost all craters are round (otherwise the great majority would be oblong in shape).
Most of the impact basins---craters measured in hundreds of kilometers---were made during this time.
It is noteworthy that about the time the heavy bombardment ended, life took hold.
The last stage of that "sweeping up", called the occurred from about 4.1 to 3.8 billion years ago.
Impacts as large as the one that led to the demise of the dinosaurs in much more recent history were happening about once a month.
The craters on all of the moons except Io, Mercury, and most of the ones on Mars are from impacts.
The kinetic energy of the impacting meteorite or asteroid is converted into heat, sound, and mechanical energy---the projectile explodes on impact.
Terrestrial planets have hard surfaces that can be re-shaped by several different processes: impact cratering, volcanism, erosion, and tectonics.
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Volcanism is any eruption of molten lava onto the surface.