About Plate Tectonics

The original debate about continental drift was only a prelude to a more radical idea; plate tectonics. The basic theory is that the Earth's crust is made up of a series of rigid plates which float on a soft layer of the mantle and are moved about by convection currents in the Earth's interior. These plates converge and diverge along margins marked by earthquakes, volcanoes and other seismic activity. Plates diverge from mid-ocean ridges where molten lava pushes upwards and forces the plates apart at a rate of up to 30 mm a year.

Converging plates form either a trench, where the oceanic plate sinks below the lighter continental rock, or mountain ranges where two continents collide. This explains the paradox that while there have always been oceans none of the present oceans contain sediments more than 150 million years old.

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