Plate Tectonics

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The Moving Earth

A Unifying Theory - Plate Tectonics

The plate tectonic theory gives a coherent explanation for many of the major global geological events such as earthquakes, volcanoes, faults, folds and fold mountains. It brings togther a number of earlie hypotheses, which attempt to explain the distribution of the continents and the 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 materal 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 which the relative motion is taken up. By comparison, areas within the plates are geologically inactive.

Identifying Plate Margins

New crust is formed along the MOR and destroyed along subduction zones, because plates are moving apart or coming togther. 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.

The evidence to help mark plate boundaries:

earthquake zones outline lithospheric plates

changes in heat flow

chains of volcanoes

trenches and rift valleys

changes in gravity

fold mountain belts

Types of Plates

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 starts at the Mid-Atlantic Ridge where it is oceanic, then extends as a continental plate across Europe and Asia.

Oceanic Plate:

about 5km under MOR thickening to 200km away from the ridge

2.9g/cm3

0-200 Ma

lithospheric mantle (peridotite of the upper mantle) capped by basaltic oceanic crust with a thin layer of sediments at the top

Pacific, Cocos and Nazca plates

Continental Plate:

up to 300km thick under major mountain ranges, about 75km under continental platform

2.7g/cm3

0-3960 Ma

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

Arabian Plate is almost entirely capped by continental crust, most other plates carry continental crust

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