Plate tectonics

The Earths Tectonic plates

At the centre of the Earth there is the core surrounded by a large mass of molten rock called the mantle. At the surface there is a thin crust 'floating' on the mantle below. There are two main types of crust - oceanic crust which is denser and about 5km thick and continental crust which is lighter but about 30km thick.

The Earth's crust is not one continuous layer but is made up of seven large tectonic plates and many smaller ones. The Earth's crust is unstable because the plates are moving in response to rising hot currents called convectional currents within the mantle. The movement of the plates has greatest impact at the plate boundaries, where two tectonic plates meet. The centres of the plates, away from the boundaries, tend to be stable.

Compressional plate boundaries

Plates that move together form a compressional (destructive) plate boundary. For example, the Nazca plate is made of Oceanic crust which is denser than the continental crust of the South American plate. The Nazca plate is forced to sink below into the mantle where it melts 700km below the surface in the subduction zone. Energy builds up in the subduction zone - at certain times this may be released as an earthquake. The molten rock called magma may rise upwards, causing volcanic eruptions and leading to the creation of composite volcanoes (cone). The lighter continental crust stays at the surface but becomes crumpled into fold mountains. The Andes are the fold mountains that have formed along the west coast of South America.

Tensional plate boundaries

Some plates, like the North American and the Eurasian plates, are moving in opposite directions, away from eachother. This is called a tensional (constructive) plate boundary. This type of movement mostly happens under the oceans. As the plates move apart the gap is filled by magma rising up from the mantle below. This rising magma creates shield volcanoes which, if they become high enough, form volcanic islands, such as Iceland and the Hawaiin Islands. The edges of the plates also buckle to form ridges like the Mid-Atlantic Ridge.

At the San Adreas fault in California, the North American plate and the Pacific plate are sliding past each other. They are moving in the same direction but the North American plate is moving slightly faster. Pressure builds up along the fault until one plate jerks past the other causing an earthquake. The movement also causes the land to become ridged and crumpled, this type of plate boundary is known as a passive (conservative) boundary.