Final dates! Join the tutor2u subject teams in London for a day of exam technique and revision at the cinema. Learn more

Study Notes

The Earth’s Internal Structure - Explained

AS, A-Level
AQA, Edexcel, OCR, Eduqas, WJEC

Last updated 22 Mar 2021

The Earth is made up of 3 main sections: the core, mantle and crust.

The core

At the centre of the earth lies the core. This is split into two sections: the inner and outer core.

The inner core is solid and is composed of iron and nickel. It is approximately 1,221 km thick and has a density of 12.8g/cm3 at the top of the section and 13.1g/cm3 at its deepest point.

The outer core is composed of liquid iron and nickel. It is approximately 2,259 km thick. Its density is 9.9g/cm3 at the top and 12.2g/cm3 at the bottom where it changes into the inner core.

The mantle:

The mantle is the thickest layer, being approximately 2,800 km thick. It makes up the bulk of the planet and lies between the core and the crust. Its density is 3.4g/cm3 at the top and 5.6g/cm3 at the bottom where it changes into the outer core. It is a solid layer but acts like a viscous liquid due to temperatures being close to the melting point of key minerals in this layer.

All of this mantle rock is comprised of a variety of oxides. Their atomic elements include oxygen, silicon and magnesium.

The crust:

This is the thinnest layer and forms the outer shell on which life exists. It varies in thickness from just 1km in some places to more than 80 kilometres in others.

There are two types of crust: oceanic and continental.

Oceanic crust forms the bed of the world’s oceans and is thinner and younger as it is constantly renewed when it is subducted into the mantle and reappears as new crustal material at subterranean constructive margins, such as the Mid-Atlantic Ridge. It is, on average, 6km thick.

Continental crust, on the other hand, is older and much thicker (on average 36 km thick) and yet, despite its thickness, it is less dense material than that which forms oceanic crust. This means it floats on the mantle with more buoyancy and it is the denser oceanic crust that descends into the mantle when the two types of crustal material collide at destructive margins.

Radioactive decay in the core releases considerable heat and this moves through the mantle in the form of rising convection currents. As the currents approach the crust they spread out horizontally, cool and sink back down into the mantle. This heat conveyor belt within the mantle is responsible for the movement of the crustal plates that make up the Earth’s surface as they are dragged across the earth by the dominant convection currents.

© 2002-2024 Tutor2u Limited. Company Reg no: 04489574. VAT reg no 816865400.