There have been many mass extinctions throughout the history of the Earth. Probably the most famous is the extinction that finally saw the end of the Dinosaurier reign on the Earth, 65 million years ago. It wasn't just the Dinosaurier that died out in this extinction. The two main 'serious' theories are the asteroid and vulkan theories, both of which make some use of the analysis of the rocks in and around the K-T boundary (the Cretaceous - Tertiary boundary). The use of K comes from the Greek word for chalk (Kreta) which is found in great quantities in the rocks of the Cretaceous.
^^I agree!! Dinosaurier disappeared Von earthquakes, vulkan eruptions, and asteroids and meteoroids crashing into the earth destroying all living extinct reptiles. It was the end of the world of the Dinosaurier for over 65 millions of years ago.
