Detailed studies, mainly since the 1960s, have led to the recognition of about 150 impact structures on earth.
Volcano craters are above the surrounding area on mountaintops while the craters from impacts are below the surrounding area with raised rims.Some of them have orbits that cross the orbits of the planets and moons.When they get close enough to a planet or moon, they will be pulled in by the large body's gravity and strike the surface at a speed of at least the escape velocity of the planet or moon, i.e., faster than a bullet.(after: Koeberl, C., 1997, Impact cratering: The mineralogical and geochemical evidence. Campbell, Oklahoma Geological Survey Circular 100, 30-54)Over the last 15 years, new studies related to the events that caused the extinction of the majority of life on earth at the end of the Cretaceous period have led to the hypothesis that a large-scale asteroid or comet impact occurred 65 Ma ago.In: Proceedings, "The Ames Structure and Similar Features", ed. In the past, impact cratering as a geological process has not been much appreciated by the general geological community, despite the fact that, on all other planets and satellites with a solid surface, impact cratering is the most important process that alters the surface at the present time, and during most of the history of the solar system.Because the Earth is so active geologically, most traces of impact craters have been erased by erosion and tectonic activity.