The Earth’s layers

Before the development of plate tectonic theory, earth scientists divided the interior of the Earth into three layers; the crust, the mantle and the core.

The Core
Made up of dense rocks containing iron and nickel alloys and is divided into a solid inner core and a molten outer one, with a tempeature of over 5,000 degrees Celsuis.

The Mantle
This is made up of molten and semi-molten rock containing lighter elements such as silicon and oxygen.

The Crust
This is even lighter because of the elements that are present, the most abundant being silicon, oxygen, aluminium, potassium and soidium. The crust varies in thickness – beneath the oceans it is only 6-10km thick but below the continents this increases to 30-40km. Under the highest mountain ranges the crust can be up to 70km thick.

The crust and upper mantle can be further subdivided into two more sections:
Lithosphere – this consists of the crust and the rigid upper section of the mantle and is approximately 80-90 km thick. It is divided into seven very large plates and a number of smaller ones.
Asthenosphere – this is below the lithosphere and is semi-molten and the plates float and move on this.
Map of major plate boundaries
There are two different types of crust
Oceanic crust
Thickness: 6-10km
Age: less than 200 million years ago
Density: very dense
Composition: mainly basalt; silicon, magnesium and oxygen
Forms constantly
Can be destroyed
Continental curst
Thickness: 30-70km
Age: Over 1,500 million years
Density: not very dense
Composition: mainly granite, silicon, aluminium, oxygen
Is not formed
Unlikely to be destroyed
Why do the plates move?
Hotspots around the core of the Earth generate thermal convection currents within the asthenopshere, which cause magma to rise towards the crust and then spread before cooling and sinking. This circulation of magma is the vehicle upon whch the crustal plates move. The crust can be thought of as ‘floating’ on the more dense material of the asthenosphere. this is a continuous process with new crust being formed along the line of constructive boundaries between plates (where plates move apart from each other) and older crust being destroyed at destructive boundaries (where plates are moving towards each other.