One of the mysteries of the history of the Earth is the layer of clay that was deposited around the entire globe approximately 65 million years ago. The layer marks the K-T boundary the end of the Cretaceous (K) and beginning of the Tertiary (T) periods. It is best known as the time when not only the dinosaurs but nearly half of all life forms became extinct.
Chemical evidence in this layer of clay preserved from 65 million years ago in Caravaca, Spain, indicates an asteroid or comet struck the Earth, possibly at around 170 times the speed of sound, causing a disaster resulting in the extinction of half of all life forms, including the dinosaurs.
At the beginning of the last decade, Nobel Laureate Luis Alvarez and his team members discovered a 9 ppb (parts per billion) abundance of the element iridium (Ir) while studying 1-cm-thick samples at the K-T boundary layer. The fact that the high level of iridium coincided exactly with the classic end of the Cretaceous-Tertiary mass extinction event led them to propose a theory linking these two observations. They proposed that an asteroid between 6 and 14 km in diameter struck the Earth, and the impact lofted enormous amounts of pulverized Earth materials high into the Earth s atmosphere. They speculated that this dust-sized, impact ejecta caused an environmental catastrophe.
Under a microscope, these quartz grains, called tectites, show lines that are characteristic of high shock and are found only with meteorite impacts or atomic explosions.
Additional research by other scientists suggests that if the extraterrestrial object was an asteroid, it most likely impacted the Earth at a velocity of 50 times the speed of sound and measured 15 km in diameter. Because asteroids of this size are very few in number in our solar system, the object could also have been a comet, most likely moving even faster, possibly 170 times the speed of sound but measuring only 10 km in diameter.
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