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An important feature that sets the Earth apart from its planetary
siblings is the process of plate tectonics, whereby rafts of
rigid rock 'floating' on a sea of more mobile rock move slowly over
the surface, powered by the dynamo of convection currents in the
liquid metal of the Earth's outer core. Why is this process important for life? Plate tectonics is a vital
part of the global
cycle that maintains the carbon balance between atmosphere,
hydrosphere and lithosphere. The phenomenon of plate tectonics, also known as "continental
drift", derives directly from the internal structure of the Earth:
a molten outer metallic core surrounding a solid inner metallic core,
overlain by the mantle, a layer of partially molten silicate rocks,
above which is the buoyant but rigid lithosphere. The upper part of the lithosphere, the crust, is thinner
below the oceans (~ 7 km thick) than it is below the continents
(generally ~ 35 km thick, but can be up to 80 km in places). The lithosphere is not a continuous layer of material, but is made
up of individual pieces, or plates, that fit together like the pieces
of a giant jigsaw puzzle. The plates are bodies of slowly moving rock
floating on top of the mantle. The plates are in constant motion,
colliding with each other in some parts of the globe, or sliding past
each other, or moving apart. The plate boundaries (see the diagram) are characterised by
different types of tectonic activity, depending on whether new crust
is being formed or old crust destroyed. Where two oceanic plates
collide, or an oceanic plate collides with a continental plate, there
is a subduction zone, where the lithosphere is dragged down
into the mantle and melted. A typical example is along the line of the
Andes in South America, where the eastwards moving oceanic Nazca plate
is being pulled below the westwards moving South American continental
plate, at a rate of ~ 9cm per year. Subduction zones are marked by arcs of volcanic activity. At
collision zones between continents, rocks are uplifted into mountain
chains. For example, the Himalayas have formed where the Indian plate
moves northwards against the Eurasian plate at a rate of ~ 5cm per
year. Where plates move past each other, transform faults develop,
associated with which are some of the world's major earthquake zones,
e.g., the San Andreas fault in California, where the Pacific plate is
moving eastwards against the north-north westwards moving North
American plate at a rate of ~ 1 cm per year. Spreading centres are regions where plates are moving apart, where
new ocean crust is forming (e.g., the mid-Atlantic ridge, where the
North American plate moves away from the Eurasian plate at a rate of ~
4 cm per year). Spreading centres are usually characterised by chains
of volcanoes, hydrothermal vents and hot springs. The Earth is delicately poised between snowball and
greenhouse, and the key to this balance is the fate of carbon dioxide
(CO2). The balance is maintained by the process of
plate tectonics, removing CO2 at subduction margins and
producing CO2 at volcanoes and hot springs. As far as we know, the Earth is the only planet in the Solar
System to exhibit plate tectonics, although there has been a
suggestion that a limited type of tectonic activity occurred early in
Mars' history. Allied to the significance of plate tectonics in terms of the
carbon cycle are the importance of mountain building and the
competing process of erosion. At collision boundaries, rocks
are uplifted into mountain chains. Gradual erosion of the mountains
is countered by continued uplift. If plate tectonics were to cease, then so would this mountain
building activity. Erosion would then, eventually, abrade down the
mountains. Sediments removed by erosion, carried into the oceans by
rivers and streams would eventually lead to a rise in sea level. If
the process continued, it is calculated that eventually a global ocean
would cover the Earth completely, resulting in catastrophic extinction of all
land-based species.
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Last updated June 28, 2001