Tectonic plates

The earth's surface is divided into slabs called tectonic plates. Each plate is a fragment of the Earth's rigid outer layer, or lithosphere ( see the lithosphere ).

There are 16 large plates and several smaller ones. Plates are approximately 100 km thick but can vary in thickness from 8 km to 200 km.

The biggest plate is the Pacific plate, which underlies, the whole of the Pacific Ocean. The Pacific Ocean represents half of the world's ocean area.

Tectonic plates are moving all the time - by about 10 cm a year. Over hundreds of millions of years they move vast distances. Some have moved halfway round the globe. The continents are embedded in the tops of the plates, so as the plates move the continents move with them.

The Pacific plate is the only large plate with no part of a continent situated on it. It represents more than one - third of the Earth's surface area.

The movement of tectonic plates accounts for many things, including the pattern of volcanic and earthquake activity around the world.

There are three kinds od boundary between plates:

• convergent
• divergent
• transform

Tectonic plates are probably driven by convection currents of molten rock that circulate within the Earth's mantle.