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Alvarez hypothesis
The Alvarez hypothesis posits that the mass extinction of the dinosaurs and many other living things was caused by the impact of a large asteroid on the Earth sixty-five million years ago, called the Cretaceous–Paleogene extinction event. Evidence indicates that the asteroid fell in the Yucatán Peninsula, at Chicxulub, Mexico. The hypothesis is named after the father-and-son team of scientists Luis and Walter Alvarez, who first suggested it in 1980. In March 2010 an international panel of scientists endorsed the asteroid hypothesis, specifically the Chicxulub impact, as being the cause of the extinction. A team of 41 scientists reviewed 20 years of scientific literature and in so doing also ruled out other theories such as massive volcanism. They had determined that a space rock hurtled into earth at Chicxulub. The rock's size could be approximately the size of Martian moon Deimos (mean radius 6.2 km); the collision would have released the same energy as , over a billion times the energy of the atomic bombs dropped on Hiroshima and Nagasaki.
==History==
In 1980, a team of researchers led by Nobel prize-winning physicist Luis Alvarez, his son geologist Walter Alvarez and chemists Frank Asaro and Helen Vaughn Michel discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary (Cretaceous–Tertiary boundary or K–T boundary) contain a concentration of iridium hundreds of times greater than normal. Iridium is extremely rare in the Earth's crust because it is very dense and has the affinity for iron that characterizes the siderophile elements (see Goldschmidt classification), and therefore most of it sank into the Earth's core while the earth was still molten. The Alvarez team suggested that an asteroid struck the earth at the time of the Cretaceous–Paleogene boundary.
Geologists Allan O. Kelly and Frank Dachille in a 1953 publication analyzed geological evidence from around the earth and concluded that one or more giant asteroids impacted the earth, causing an angular shift in the earth's axis, global floods, fire, atmospheric occlusion and causing extinction of the dinosaurs.〔http://impact.arc.nasa.gov/news_detail.cfm?ID=72〕 There were other earlier speculations on the possibility of an impact event, but no evidence had been uncovered at that time.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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