The Mass Extinction of Dinosaurs

The dinosaurs had died off gradually through the late Cretaceous, The mass extinction of the dinosaurs occur 65 millions year ago which is the end of the Cretaceous period.

"A vast explosion caused by a comet or asteriod hitting the earth 65 millions years ago, is supposed to have upset the Earth's sufficiently to promote a spate of extinctions."

Scientists have used several evidences in supporting this theory. A crater 300 km (187 mi) in diameter was formed in Yucatán, Mexico, approximately at the time this mass extinction occurred. The element iridium was found in the Cretaceous-Tertiary boundary around the impact craters, this metal is common in meteorites but rare on earth. Alan Hildebrand and William Boynton from the University of Arizona has carried out investigation on the "ejecta" layers resulted from the impact and they believed Haiti was quite close to the centre of the impact.

This impact is believed to have the following effect on the Earth:

~ clouds of smoke and dust that scattered had lead to global cooling.

~fires lit by hot fragments would have swept through forests - lot of soot in the K-T boundary layer.

~ water vapour and carbon dioxide from the burning boosted the greenhouse effect.

~ further effect by the impact was the increase in radiation and the change in climate.

However, this theory along with other explanations to the mass extinction, had not fully explained why the dinosaurs died off while many species of the smaller animals were not affected.