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Catastrophism

earth impact history structure

Catastrophism is the doctrine that Earth's history has been dominated by cataclysmic events rather than gradual processes acting over long periods of time. For example, a catastrophist might conclude that the Rocky Mountains were created in a single rapid event such as a great earthquake rather than by imperceptibly slow uplift and erosion.

Catastrophism developed in the seventeenth and eighteenth centuries. A prominent British theologian, Bishop James Ussher (1581–1656) added together the ages of people in the Bible and calculated that Earth must have been created in 4004 B.C. His calculation implied that all of the features of Earth's surface must be less than 6,000 years old and were therefore, formed as the result of violent upheavals or catastrophes. Current research, in contrast, suggests that the earth is about 4.5 billion years old. Baron Georges Cuvier (1769–1832), a French anatomist, tried to reconcile the fossil record with Biblical history. Cuvier stated that different groups of fossil organisms were created and then became extinct as the result of geologic catastrophes, the last of which was the great flood described in the Bible. Each catastrophe, according to Cuvier, killed the fossilized organisms and deposited the sediment that solidified into the rock surrounding the fossils.

A new concept, uniformitarianism, grew from the work of the Scottish geologist James Hutton (1726–1797) and eventually replaced catastrophism. Uniformitarianism is the doctrine that geologic processes operate at the same rates and with the same intensity now as they did in the past. Hutton suggested that Earth had a very long history that could be understood in terms of currently observable processes such as the weathering of rocks and erosion of sediment. Sandstone, for example, was formed by the same kinds of physical processes that form modern sandy beaches or deserts. Therefore, catastrophic events were not needed to explain Earth's history. Geologists often summarize the idea of uniformitarianism with the phrase, "The present is the key to the past."

The concept of catastrophism has been revived with the discovery of large meteorite impact structures and evidence of mass extinctions in the fossil record. The most notable of these events was the asteroid impact marking the boundary between the Cretaceous and Tertiary Periods about 65 million years ago, which coincides with the extinction of the dinosaurs. The resulting Chicxulub impact structure, located in the Gulf of Mexico near the Yucatan Peninsula, is approximately 120 mi (180 mi) wide and 1 mi (1.6 km) deep. It is thought that the crater was formed by the catastrophic impact of an asteroid 4–9 mi (6–15 km) in diameter. The impact produced a thin layer of clay that contains elements rare on Earth but abundant in meteorites and minerals that can only be formed under very high pressure. Soot within the clay also suggests that the impact triggered extensive wildfires, which may have acted with sulfate minerals pulverized in the impact, to slow photosynthesis and cause global cooling to occur. Clouds of dust may have darkened the atmosphere for weeks or months. Other large impact structures on Earth include a 100 mi (120 km) wide structure in Acraman, Australia, a 120 mi (200 km) wide structure near Sudbury, Ontario, Canada, and an 50 mi (85 km) wide structure in Chesapeake Bay, Virginia and Maryland. Meteor Crater, Arizona, is an example of a relatively small impact structure.

The recognition that Earth's history has been punctuated by rare but catastrophic events such as asteroid impacts has led most geologists to abandon strict uniformitarianism in favor of a doctrine known as actualism. Actualism states that the laws of nature do not change with time and much of Earth's history can be explained in terms of currently observable processes. It acknowledges, however, that rates of geologic change are not constant over long periods of time and there have been some catastrophic geologic events that are far beyond the range of human experience.

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