According to the plate tectonic theory, the earths crust is made up of 8 main plates, as well as a number of smaller ones. These plates float on top of the asthenosphere and move around quite regularly. This movement causes interaction between the plates which is thought to bring about seismic and volcanic events at its margins. Therefore the plate tectonics theory is a key concept in understanding the distribution of seismic and volcanic events.

Plate tectonic theory was first suggested by Alfred Wegener in 1912. He believed that a single continent called Pangea existed around 300 million years ago before splitting it began to split because of continental drift, into two continents and continuing to split to form the continents we see today. His theory was supported by both geological and biological evidence, such as the bulge of South America seeming to fit into the indent of West Africa. He believed this suggested that they were once attached and had been separated through continental drift. Later evidence was also found as fossil remains of the reptile mesosaurus were found in both South America and Southern Africa. It is unlikely that the same reptile could have developed in both area or that it could have migrated across the Atlantic especially due to the fact that the reptile was believed to be a poor swimmer.
This continental drift leads to seismic and volcanic events and is very important in understanding their distribution.

Although Wegeners ideas seemed simple, they were later supported by the discovery of the Mid-Atlantic ridge. This ridge led to the discovery of sea-floor spreading and the understanding that the earths crust is being both destroyed and created, leading us to believe in the existence of plates and plate boundaries. Oceanic ridges are created when two plates diverge and magma rises between them from the asthenosphere, creating the