Plate tectonics

The plate tectonic theory gives a coherant explanation for many of the major global geological events such as earthquakes, volcanoes, faults, folds and cold mountains. It brings together a number of earlier hypotheses, which attempt to explain the distribution of the continents and oceans

The uppermost layer of the Earth is divided into a number of sections, which are constantly in motion relative to each other, carried by moving material beneath:

• the sections are the rigid lithosphere plates
• the moving material is the plastic asthenosphere, which is partially melted and acts as a rheid - a solid that can flow

The plate boundaries or margins between the lithospheric plates are zones of geological activity along with the relative motion is taken up. By comparison, areas within the plates are geologically inactive (aseismic)

• new crust is formed along the MOR and destroyed along subduction zones, because plates are moving apart or coming together
• where plates meet there will be displacement and therefore earthquakes, which suggests a way of marking out the boundaries between them
• the interior parts of the plates will not be involved in any interactions and will be aseismic

Most plates are a combination of oceanic and continental crust, so the term 'oceanic' or 'continental' applies to that part of the plate at a plate margin

The Eurasian plate that we live on starts at the Mid-Atlantic Ridge where it is oceanic, then extends as a continental plate across Europe and Asia

Oceanic plate

Thickness of lithosphere: about 5km under the MOR, thickening to 200km away from the ridge

Density (g/cm3): 2.9

Age: 0-200Ma

Average composition: Lithospheric mantle (peridotite of the upper mantle) capped by basaltic oceanic crust with a thin layer of sediments on top

Examples: Pacific, Cocos and Nazca plates

Continental plate

Thickness of lithosphere: up to 300km thick under major moutain ranges; about 75km under the continental platform

Density (g/cm3): 2.7

Age: 0-3960Ma

Average composition: lithospheric mantle (peridotite of the upper mantle) capped by granitic crust, which is deformed and contains a mix of igneous, mentamorphic and sedimentary rocks

Examples: Arabian Plate is almost entirely capped by continental crust

Most other plates carry continental crust