Why are there not many craters left on earth from billions of years ago?
Very little of the present oceanic crust is more than 200 million years old; any impact crater that was present on the seafloor before that time has long since vanished. On the continents, impact craters are subject to the processes of the rock cycle in the same manner as are mountains. Eventually, they are weathered and eroded away, leaving little, if any, visible evidence.
