Starting from the Cambrian (540 mln years ago), our planet survived five large-scale catastrophes that led to mass extinction of living organisms. These catastrophes took place in the late Ordovician (438 mln years), Devonian (370 mln years), Permian (251 mln years), Triassic (200 mln years) and Cretaceous (65 mln years). In the last cataclysm almost 75 percent of all terrestrial and marine organisms died, including dinosaurs* that had appeared on the Earth 230 mln years ago and lived during subsequent 170 mln years, i.e. in the Mesozoic era. Why did they die out? One more attempt to answer this question was made in the article by Andrei Grachev, Dr. Sc. (Geol. & Min.), chief research assistant of the RAS Institute of Physics of the Earth named after O. Schmidt, published in Zemlya i Vselennaya (The Earth and Universe) magazine.
In 1980, American physicist Luis Alvarez (Nobel Prize Winner, 1968) recommended his son Walter, a qualified geologist, to apply a newly developed neutron activation method to study elements of the platinum group discovered in the clays of Gubbio Slit in Italy. It has turned out that in the 2 cm thick layer on the edge of Cretaceous and Palaeogene deposits, the concentration of iridium is much higher than it was recorded for the terrestrial rocks before. Scientists have established that this phenomenon is typical of meteorites; on the basis of this discovery the Alvarez family concluded that the mass extinction of biota and the fall of a big cosmic body (asteroid) were interrelated. This hypothesis was also supported by discovery of the meteorite crater Chikhulub, the largest on the Earth, on Yucatan Peninsula (Mexico) that had appeared 65 mln years ago, i.e., coincides with the time
* See: Yu. Avsyuk et. al., "Did Dinosaurs Die Out Suddenly?", Science in Russia, No. 3, 2002; V. Alifanov, A. Averyanov, "Time of Dinosaurs", Science in Russia, No. 5, 2003.–Ed.
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