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

Plate Tectonics: An Introduction

Continental Drift and the Structure of the Earth

Image courtesy of USGS

Our next unit is the study of plate tectonics. In this unit we will be studying the forces of nature which have shaped our planet including the processes behind natural hazards such as earthquakes and volcanoes. We will also be considering the impact that such hazards have on people across the world. It is believed that out continents have not always been in their present configuration and that over millions of years our continents have changed their position (see animation). This theory is known as continental drift.

Millions of years ago there was one supercontinent called Pangea. Over time this has split into smaller continents which have gradually moved into the positions in which they exist today. There are various pieces of evidence for this including the apparent jigsaw fit between the east coast of South America and the west coast of Africa.

In order to understand how this is possible we need to consider the structure of the earth.

The earth is up to 6,000km in radius from the inner core to the surface. It is made up of four main layers. The surface layer is known as the crust. This is the relatively thin layer on which we live and it consists of solid rock. The crust 'floats' on top of the mantle. The mantle has very high temperatures resulting in rock being in a 'molten' state. This 'molten' rock is known as magma and is able to move. At the centre of the earth is the core. This is divided into the outer and the inner core. The outer core is partly molten whilst the inner core is solid, this is due to the extreme temperature and pressures which exist here, with temperatures reaching up to 5,000 oC.

Follow up Links:
Some useful and clear animations showing the movement of the continents over millions of years in the process of continental drift can be found here:

- Continental Drift

- The 'fit of the continents' (evidence for continents having moved)

- Sea floor spreading showing the break up of Pangea

Check out this excellent site for more information about the structure of the earth and its layers. Further detail on the structure can be found here.

Key Terms Check:

Continental Drift - the theory that our continents have changed their postion over time
Crust - the outer layer of the earth (up to 75km thick)
Mantle - the middle and thickest layer of the earth part of which is semi-molten in nature
Outer Core - the outer layer of the core is semi-molten
Inner Core - central part of the earth which is solid due to extreme temperature and pressure
Magma - molten rock in the mantle