Plates and Convection Currents

Plates and Types of Crust
The earth's crust is divided up into a series of slabs of crusts known as plates. These plates consist of two different types of crust: continental crust and oceanic crust. There are important differences between the two types. Oceanic crust is constantly being created and destroyed, it is therefore younger than continental crust and its higher density means that it can be subducted and destroyed. As continental crust is ligher with a lower average density, it is permanent and cannot sink. It is also much older and thicker, reaching up to 70km under mountains. In terms of rock type, continental crust is mainly granite, whereas oceanic crust is mainly made up of basalt.

Plate Movements and Convection Currents
The earth's tectonic plates are in motion, moving like giant 'rafts' on top of the semi-molten mantle below. However this movement is slow and rates vary from less than 2.5cm /yr to over 15cm/yr.

The movement of the earth's crustal plates is believed to be due to convection currents which occur in the semi-molten mantle. These convection currents are created by heat from within the earth - much of which is generated by radioactive decay in the core.



So how do convection currents cause plate movements? As semi-molten rock in the mantle is heated it becomes less dense than its surroundings and rises. As it reaches the crust above, it spreads out carrying the plates above with it. As the semi-molten rock then cools, it gradually sinks back down to be re-heated. (see diagram above)

Follow up links:
The Earth's Crust in Motion - an excellent page from St Vincent's College - exploring the nature of the crust and its movement due to convection currents - well worth a read!
A simple animation of convection and the resulting movement of plates
A clear and detailed animation of convection currents in the mantle
An animation of convection in the mantle (Exploring Earth)
The Mantle A good overview of the characteristics and influence of the mantle in tectonics

Key terms check:
Tectonic plate - individual slab of the earth's crust
Convection Current - transfer of heat throughout the mantle resulting in the rising and falling motion of semi-molten rock
Mantle - the area between the crust and the earth's core the majority of which is in a semi-molten state