Dinosaurs disappeared abruptly from the fossil record sixty five million years ago, within a time frame of less than two million years. their extinction marked the end of the Mesozoic era. Some animal groups however survived this mass extinction. there are many explanations to this tragedy which includes volcanoes and infectious diseases, the most widely accepted theory assigns the blame to an asteroid that hit the earth then. Physicist Luis W. Alvarez and his associates discovered that the usually rare element iridium was abundant in many parts of the world in a thin layer of sediment that marked the end of the crustaceous period. iridium is rare on earth but common in meteorites.
Alvarez and his colleagues have proposed that if a large meteorite ten kilometers in diameter struck the surface of the earth sixty five million years ago, a dense cloud cloud would have been thrown up. the cloud would have been rich in iridium, and as its particles settled, the iridium would have been incorporated into the layers of sedimentary rock being deposited at the time. By darkening the world, the cloud would have greatly slowed or temporarily halted photosynthesis and driven many kinds of organisms to extinction.
The Alvarez hypothesis has become widely accepted among biologists, although it remains controversial among some. these holdouts argue that it is not certain that dinosaurs became extinct suddenly, as they would have by a meteorite collision. the issue was largely settled in the minds of most scientists when an impact crater was discovered in the sea just off the coast of Yucatan Peninsula. For hundreds of kilometers in all directions are signs of the meteors’ impact , including quartz crystals with shock patterns that could only have been produced by an enormous impact, they are produced, for example, as a by-product of nuclear tests. large amounts of of soot were deposited in rock worldwide at the time of the extinction, indicating very widespread burning. at what date does radiodating place the Yucatan Peninsula impact, its around sixty five million years ago.