Mass extinctions through geological time

A mass extinction is when there is a massive decrease in the number of different species, over a relatively short period of time, perhaps spanning several thousand or a few million years. For any one species, extinction is catastrophic. the normal process of extinctions occurs continually, generating a regular change of all the species living on earth, called background extinction. Sometimes, however,extinction rates rise suddenly for a relatively short time - as a mass extinction event.

The most famous of the mass extinctions is the Cretaceous - Tertiary mass extinction- because it is when the dinosaurs became extinct. This was the most recent large-scale mass extinction and has been well documented. Mass extinction events are not rare and some environmentalists and biologists believe that we are in the middle of another major mass extinction event, fuelled by mans effect on the environment.

5 major mass extinctions

• The Ordovician-Silurian boundary 
• an event towards the end of the Devonian
• Triassic-Jurassic boundary 
• Cretaceous-Tertiary boundary 
• Permian- Triassic boundary