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The asteroid extinction theory is also known as the K-T asteroid theory and, occasionally, the Alvarez Asteroid Impact Theory. All of these theories vary slightly, but they all center around an impressive event that suddenly destroyed most of the life on Earth around 65 million years ago.
The asteroid extinction theory holds that many of the dinosaurs went extinct long before the catastrophic mass extinction at the end of the Cretaceous period, 65 million years ago. Background extinctions and many minor extinctions accounted for the disappearance of most of the dinosaur species. The latter part of the period saw some heavy tectonic shifting and volcanic activity. The super continents had all separated or were in the process of separating. Many mountain ranges were formed and sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. As the Earth aged these climate changes had caused many species to die out and others to emerge.
Another form of the asteroid extinction theory is the K-T (Cretaceous-Tertiary) theory. It is associated with a geological signature known as the K-T boundary, usually a thin band of sedimentation found in various parts of the world. K is the traditional abbreviation for the Cretaceous Period derived from the German name Kreidezeit, and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). Non-avian dinosaur fossils are only found below the K–T boundary, indicating that dinosaurs became extinct immediately before, or during the event. A very small number of dinosaur fossils have been found above the K–T boundary, but they have been explained as reworked, that is, fossils that have been eroded from their original locations then preserved in later sediment layers.
The third form of the asteroid extinction theory is the Alvarez Asteroid Impact Theory. The asteroid-impact theory was first proposed in detail in 1978, by a team led by American geologist Walter Alvarez and physicist Luis Alvarez. The Alvarez team analyzed sediment collected in the 1970s from the K-T layer near the town of Gubbio, Italy. The samples showed a high concentration of the element iridium, a substance rare on Earth but relatively abundant in asteroids. Other samples of K-T boundary strata from around the world were also analyzed; excess iridium was found in these samples as well. Using the average thickness of the sediment as a guide, they calculated that a meteorite about 10 km in diameter would be required to spread that much iridium over the whole Earth.
Although each theory uses a different method to prove its facts they all point to one catastrophe that killed a majority of the dinosaurs around 65 million years ago and they all seem to agree that it happened in the Yucatan Peninsula in Mexico. What is now known as the Chicxulub Crater was the epicenter of the event.
There are several article on the Chicxulub Crater and the Yucatan Event here on Universe Today. Check out the website at Science. jrank.com for another opinion on the Alvarez Asteroid Impact Theory and Astronomy Cast has a great episode about asteroids as bad neighbors.