Impact cratering on Earth

Approximately 160 known terrestrial impact structures are distributed worldwide, with concentrations in geologically stable areas. The basic types are simple' craters, up to 4 km in diameter, with uplifted and overturned rimrocks surrounding a bowl-shaped depression, and complex' structures, generally over 3 km in diameter, with a central uplifted peak and/or ring, an annular trough, and a slumped rim. Morphology is used only to identify possible impact structures. The impact origin of smaller craters can often be determined by the presence of fragments of the impactor. At large craters, however, the impact commonly melts or completely vapourizes the impactor and verification is based on shock metamorphism. The formation of impact craters as small as 20 km reduces sunlight and disrupts temperature in a manner similar to a nuclear winter. A large impact event at the Cretaceous Tertiary (K T) boundary is the most likely cause of impactor mass extinctions that occurred at that time.