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PHYS 0106 (Spring, 2001) Earth History Timeline |
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| Phanerozoic Eon: Mesozoic Era: Cretaceous Period | |||
Timeline Links Phanerozoic Neoproterozoic Mesoproterozoic Paleoproterozoic Archean Other Links Historical Geology Page Physical Geology Page Mike's Home Page |
Writer/Researcher: John Cloutier Editor: Nancy Sacchetti Web Page Builder: John Cloutier 
Sixty-five million years ago, 70% of all species that were then living on Earth
disappeared within a very short period of time in what is called the Cretaceous-Tertiary
mass extinction. Among the species that disappeared were the last of the dinosaurs. The
cause of this has many theories, but one seems to be more accepted.
In 1980, Luis Alverez found a very high concentration of iridium in a clay layer
that was laid down at the time of K-T extinction. Iridium is very rare on Earth. This
led Alverez to believe that an asteroid about 10 km in diameter struck the Earth during
the K-T period with enough iridium to account for the worldwide clay layer.
The spot where the asteroid was believed to have hit was the Yucatan region of Mexico. In that area they found a 180-kilometer structure centered on the present coastline buried beneath several hundred meters of sediment deposits called Chicxulub. The Chicxulub structure has been dated back to 65 million years ago.
The asteroid struck the pennisula at a 30% degree angle and it came from the southwest. This caused a huge amount of debris to fly 5,400 miles to the west, landing in the middle of the Pacific Ocean. The debris was found in 1995 when Frank Kyte, who was a geochemist from UCLA, was examining sediment cores that were taken from the K-T boundary. This boundary contained high levels of iridium and shocked quartz, which is quartz that has experienced extremely high pressures. The shocked quartz indicates a high speed collsion.
Almost all of the asteroid should have been vaporized on impact, but very small fragments should have survived. The high levels of iridium are good evidence of an extraterrestial hit. This is the only way that that meteorite particles could have landed in the middle of the Pacific. Although the debris amount is small, it is still thousands of times bigger than the mud sized particles that surrounds it. The debris would have been too big to be carried there by the wind. The only way for particles of this size and composition to land in the sediment of the Pacific is to have fallen from the sky.
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