Because dinosaurs were not the only organisms to become extinct the scientific theory of what caused this massive disappearance should have certain guidelines. "Clearly any successful extinction theory must explain not only why the dinosaurs became extinct; it must also explain what happened to the marine ecosystems, that so many lineages of plants and animals were wiped out. It must also explain why so many organisms were able to survive the catastrophe. It is estimated that somewhere in the range of 20% to 25% of all species extant at the beginning of the Late Cretaceous were extinct by its close" (4).
At the end of the Cretaceous Period over half the number of organisms disappeared leaving traces of evidences of what caused this extinction. Luis and Walter Alveraz were studying the K-T boundary in Gubbio, Italy where they found an unusual layer of clay at the boundary point which contained around thirty times the normal level of the rare element iridium (3),(5). The two most common source of iridium are from outer space in the form of cosmic dust, which is constantly showering the planets and the Earth's core when there are eruptions of certain types of volcano (3).This evidence alluded to two different theories, The Impact Theory and the Volcanic Theory.
The volcanic theory is somewhat similar to the impact theory and is also heavily supported by a number of scientists. As mentioned before the K/T boundary was discovered to have high contents of iridium. The basaltic lava eruptions of the Deccan Traps of western India was mentioned as a possible source of the iridium found in the K/T boundary "iridium spike" (1). "It has been proposed that the out gassing, combined with possible explosive episodes, could have been sufficient of itself to have caused the same kind of a "Nuclear Winter" scenario as proposed for the giant meteor impact" (1). Other areas in the Pacific Basin show a fairly high level of volcanic eruptions (1). This theory would further explain the possibility of how some organisms survived and the irregular distribution of iridium peaks in some areas.
The impact theory or the possibility that an asteroid or comet destroyed over half of the species on earth has been a strongly supported theory of why the dinosaurs became extinct. The impact of the asteroid or comet caused a chain of events beginning with the flying object becoming intensely hot and heading towards earth (1). This asteroid is believed to impact the ocean "ejecting massive amounts of dust and water into the atmosphere" (1). Everything within the radius of 500 kilometers would have been instantly destroyed (3).Many of the groups of organisms that were hit hardest by the K/T extinction lived in the ocean (4).Many enormous tidal waves would be started causing even more damage as well as start a chain reaction of earthquakes and volcanic activity (3).The dust particles will envelop the earth covering the earth into total darkness making vegetation to dematerialize away, leave little food for the animals, and create a cold environment (2).Temperatures may have fluctuated below freezing point to high temperatures and photosynthesis may have ceased (2).At the same time large fires would have been started by the intense shock wave later creating global fires that would release large amounts of carbon dioxide increasing temperatures and cause chemical reactions forming acid rain (3).The impact may also have released sulfur dioxide, which may help created acid rain with a low pH causing an environment where some organism may not have been able to survive (2).Evidence of acid rain was found in a number of rock samples (1).
One of the greatest discoveries that show great support for the impact theory is a crater called the Chicxulub(1),(2), (3),(5). Alan Hildebrand found a ring structure 180 kilometers in diameter in the Yucatan Peninsula. The Chicxulub is also believed to be 65 million years old. Alan Hildebrand wrote further explaining that the K/T boundary had two layers (2).The upper layer, the Fireball layer, was about 3mm thick and represented 1500 cubic kilometers of debris deposited globally with not variation in thickness. The Lower layer, the ejecta layer, was about 2mm thick (2).The Fireball layer contains certain traces of elements in approximately the same proportion as some meteorites (2).Furthermore, Tektites and micro tektites, black glassy objects produced at the time of the impact, have been found in the ejecta layer of the K-T boundary (2).
Evidences that further support the impact theory is the presence of soot within the clay layer. The presence of soot usually implies that a large global fire occurred as a result of temperature changes from an impact (3),(5). Quartz crystals were also found physically altered within the clay layer (3),(5). "This alteration only occurs under conditions of extreme temperatures and pressure" (3).These evidences also show that a great climatic change has taken place.
Most of the evidence that was discovered points toward the impact theory. This theory explains most of the changes that occur towards the end of the Cretaceous period. Though the volcanic eruption explain some evidence that are not supported by the Impact theory, the Impact theory clearly explains the events that occurred during the period of dinosaur downfall and is more strongly supported by the evidence that was found. Temperature data shows that there was a temperature drop and later a temperature rise. The 65 million year old crater that exist on the Yucatan Peninsula shows that an asteroid did indeed collided onto Earth. Iridium, which is a common component of asteroids, was found in sediments of the Cretaceous layer. These evidences as well as other information strongly support the theory that an asteroid impacted the earth 65 million years ago and destroyed a large number of species including the massive sized dinosaurs.
2)"Dinosaurs: Death from Space"
3)"Dinosaur Extinction Page"
4) "Dinosaur Extinction"
5) "What Killed the Dinosaurs"
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