GCSE Geography | The Structure of the Earth (Tectonic Hazards 1)

The Earth is divided up into a series of layers - the crust, the mantle and the core. These layers have different chemical compositions and mechanical properties (particularly in terms of solidity).

The thinnest layer is the crust - this is the outermost layer. There are two types of crust - continental and oceanic - which vary in thickness, density and composition.

Continental crust is between 20 and 200km thick and is less dense than oceanic crust. It is mainly made up of granite and cannot be destroyed. It is much older than oceanic crust - up to 3.8 billion years old.

Oceanic crust is between 5 and 10km thick. It is made up of basalt and is very dense. Oceanic crust sinks when it meets continental crust at a destructive plate margin (subduction) and will be melted to form magma. It is much younger than continental crust - less than 200 million years old.

The crust and upper (rigid) part of the mantle form the lithosphere. The lithosphere is broken into fragments, called tectonic plates, which move very slowly over the upper mantle. It is around 100km on average and is solid and brittle - this means that it breaks under pressure, so is where earthquakes occur.

There are two types of lithosphere - oceanic, which is topped by oceanic crust - this is dense and subducts when two plates collide; and continental, which is topped by continental crust - this is less dense and too light to subduct, therefore gets pushed up when two plates collide.

Below the lithosphere, but still in the mantle, you will find the asthenosphere - the denser, weaker layer which lies between about 100-400km below the surface of the Earth. The lithosphere and asthenosphere have the same chemical composition, but have different mechanical properties. The mantle is made of solid rock, but the asthenosphere is subject to much higher temperatures (around 1300°C) and pressure meaning that rocks soften, becoming semi-molten and ductile, meaning it can flow. It will not melt entirely due to the high pressure.*

Below this is the lower mantle - which is hotter and denser, and much less ductile than the upper mantle.

The core is at the centre - it is under a huge amount of pressure and is extremely hot - the inner core is solid, whereas the outer core is liquid.

*Thanks to Alistair Hamill for helping to clarify this.