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Catastrophism is the assumption that geological features are shaped by catastrophic events, such as collision of planets or a worldwide flood. It is contrasted with uniformitarianism. The debate between catastrophism and uniformitarianism raged at the beginning of the 19th century, with Georges Cuvier the proponent of the former, and Charles Lyell of the latter. It ended with the acceptance of uniformitarianism after Lyell published his book Principles of Geology in 1830, bringing decisive evidence for uniformitarianism.
Cuvier was an old-earth catastrophist; there are still some of those today, particularly the followers of Immanuel Velikovsky, but most modern catastrophists are young-earth creationists, holding to the global flood as the mechanism of geology and the fossil record. Young-earth creationists are vehemently opposed to uniformitarianism, and claim that Lyell's saying "the present is the key to the past" is an erroneous assumption. They often mistake "uniformitarianism" for "tranquility", although uniformitarian geology does not have to be tranquil (for example, the shaping of the Himalayan mountain range when the Indian subcontinent collided into Asia was anything but tranquil).
However, the last half-century has seen the recognition of forms of catastrophic change, like the recognition of continent-scale glacial-dam-break floods (jï¿½kulhlaups) in the Pleistocene. In the 1920's, J Harlen Bretz proposed that the Columbia River valley had been carved by such floods. However, other geologists were completely skeptical about the occurrence of such floods until they got a chance to see that valley at first hand. Since then, however, evidence of similar giant floods has been recognized in the Altai Mountains -- and on Mars.
More dramatically, the occurrence of impact cratering has become well-established over the last half-century; such craters can often be recognized with the help of the "shock metamorphism" they produce in rocks -- some minerals will recrystallize under the high pressures of impact. Over 200 such craters are recognized on Earth, though most of them are very eroded and not nearly as apparent as the Great Barringer Meteor Crater of Arizona. Elsewhere in the Solar System, however, impact craters are usually much better preserved, as is readily apparent on the Moon.
Impacting objects are suspected to have caused several mass extinctions, notably the Cretaceous-Tertiary (K-T) extinction, where the iridium at that stratigraphic boundary had been contributed by the impactor that produced the Chicxulub crater. However, the chain of events from impact to extinctions continues to remain uncertain.
Perhaps the biggest of all occurred very early in the Earth's history. The current favorite theory of the origin of the Moon involves a "Big Whack", an impact by a Mars-sized object that put some rocky material into orbit around the Earth. This material then condensed and formed the Moon.