�PAGE � �PAGE �1� Elizabeth Hoover
Research Essay
May 7, 2013
The Causes of the Dinosaurs' Extinction
About sixty five million year ago something happened to change the Earth. At the end of the Cretaceous-Tertiary Period seventy-five to eighty percent of all animal species, including dinosaurs, became extinct. This event is known as the K-T mass extinction because it occurred at the boundary between the Cretaceous (K) and Tertiary (T) time periods. Although there are numerous theories and explanations as to what happened to the dinosaurs, the asteroid theory presented by physicist Luis W. Alvarez in 1981 is today the most commonly accepted. However there are a number of other factors that may have contributed to their extinction.
The Asteroid Theory, proposed by physicist Luis W. Alvarez, his geologist son Walter Alvarez and two colleagues at the Lawrence Berkeley laboratory, Frank Asaro and Helen Michel ("Dinosaur" 19), stated that an asteroid four to nine miles wide (10km) made impact with the Earth; this caused dense clouds of dust, ash, and steam to be thrown up into the stratosphere where they cut off the sunlight causing the earth to become dark and cooling it to dangerous levels.
Once the Earth became dark and cold the process of photosynthesis stopped, which removed the basic food for marine and terrestrial life. As greenhouse gases became more dominant, temperatures rose well above what they were before the impact. Causing what was left of the dinosaurs who had survived the cold temperatures to become extinct.
The Alverez team found evidence in a sixty-five-million-year-old clay sedimentary layer near the K-T boundary in Gubbio, Italy. It gave weight to their theory that the asteroid was the cause of the extinction based on the high levels of the element iridium in the K-T layer. The sedimentary layer...