Plate Tectonics

Plate Tectonics

The earth's crust consists of a number of moving pieces or plates, that are always colliding or pulling apart. The Lithosphere consists of nine large plates and twelve smaller ones. The continents are imbedded in continental plates; the oceanic plates make up much of the sea floor. The study of Tectonic plates - called plate tectonics - helps to explain continental drift, the spreading of the sea floor, volcanic eruptions and how mountains are formed. The force that causes the movement of the tectonic plates may be the slow churning of the mantle beneath them. Mantle rock is constantly moved upwards to the surface by the high temperatures below and then sinks by cooling. This cycle takes millions of years.

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Continental drift
The continents have slowly drifted across the surface of the Earth over millions of years.

Image by:
Dave Donkin,
Dorling Kindersley

Continental drift:
The drift of the plates across the surface of the earth has been going on over millions of years, which still changes the outward appearance of the earth. When you look at the map of the world, you see how well the east coast of North and South America fits into the west coast of Europe and Africa. Over millions of years these continents have slowly drifted apart. (continental drift).

Diverging plates:
Where plates pull apart, hot molten rock (fluid magma) emerges as lava and so new matter is added to the plates. In this way new oceanic plates are formed. The place where this happens is known as a mid-ocean ridge. Mid-ocean ridges are rarely more than about 4,920 ft.(1,500 m) high, but they may snake along the ocean bed for thousands of miles. Beneath each of the world's great oceans there is a mid-ocean ridge. An example is the Mid-Atlantic Ridge in the Atlantic Ocean, which stretches from the North Pole to the South Pole. Mid-ocean ridges are areas of much volcanic and earthquake activity.

Converging plates:
In many places the huge plates of the earth's surface are slowly moving together with unimaginable force. Sometimes the edge of one plate is gradually destroyed by the force of collision, sometimes the impact simply crimps the plates' edges, thereby creating great mountain ranges.
When one tectonic plate bends beneath the other, it is called subduction. Most of the time this happens because a dense oceanic plate collides with a lighter continental plate. You can see this along the Pacific coast of South-America. The oceanic plate dips beneath into the Asthenosphere. Through the heat of the Asthenosphere the subducted plate melts. At the surface an ocean trench is created, followed by an arc of islands. In this area also volcanic activities and earthquakes occur.

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The Growing Atlantic
Iceland sits on top of the Mid-Atlantic Ridge. This illustration shows what may be happening along the ridge as the North American and Eurasian plates gradually pull apart. The ridge - viewed here from the south - continues beyond the northern tip of Iceland.

Image by:
Colin Rose,
Dorling Kindersley

When continental plates collide, one of the plates splits up into two layers: a lower layer of dense mantle rock and an upper layer of lighter crustal rock. As the mantle layer subducts, the upper layer is peeled off and crumples up against the other plate, thus forming mountain ranges, like the Alps. These are called crumpled mountains.

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