Earth Impacts as a Threat to Civilization /english/ (WinWord)

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Earth Impacts as a Threat to Civilization.
University of Minnesota.
Natalya S. Myers
September 24,2002
In recent years we get more & more facts showing that the present appearance of our planet was formed not only by slow evolutionary processes, like wind erosion, but also by gigantic natural cataclysms. Geologists find evidence in sedimentary rock that extremely powerful impacts repeatedly occurred in the geological history of the Earth.
Since the late Paleozoic period (i.e. for the last 250 million years) there are noticeable gaps in the evolution of living organisms. Paleontologists established that around 247, 220 and 65 million years ago about 95% of all life on Earth perished. Most recently, for example, the gigantic dinosaurs died out. It is obvious that such extinctions of terrestrial living organisms are the complex result of many processes. Sharp climate variations, ice formations, fluctuation of the level of oceans, decreases of concentration of oxygen in the waters of seas and oceans, and different extraterrestrial circumstances all played a part. However, to name the most important reason, was extremely difficult to discover.
Scientists assumed that at that time the Earth underwent bombardment by large asteroids. Space and aerial photography, executed in slanting solar illumination, revealed likely asteroid impact sites. Studies at the sites confirmed the Earth's encounters with celestial bodies.
Impact with an asteroid about 10 kilometers in diameter is thought to have caused mass death of dinosaurs. If the impact had arrived on the land, it would have sharply cooled, but if over the surface of ocean or sea, then water vapor would have produced a greenhouse effect and begun to warm the Earth everywhere. The Earth may also have witnessed hot nitric acid rains, whose action on the environment and the animal kingdom was catastrophic. Such a large impact could occur on Earth every hundred million years. Impacts of a smaller size are more frequent a

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