The 4 Key Layers of the Earth

Our Earth is one of eight planets that orbit a dwarf yellow star that we call the Sun. Some of the planets are made mainly from gas but ours is one that is classed as a rocky planet.

When talking about the Earth, you will often come across references to various different layers – the crust, the core and the mantle. But what are they?

Layers of the Earth.

[image: Kelvinsong]

The Core

The core accounts for two of the layers of the Earth. Lying at the centre of our planet, the core is the hottest part and the most difficult to study. It is made mainly from the metals iron and nickel; it is very dense and temperature estimates vary from about 3500 to almost 6000 degrees celsius. There are many theories about what the core is like and what is happening there, but there is still a lot of uncertainty.

The Inner Core

The inner core is believed to be solid and composed primarily of iron with nickel as the second most abundant element. The density suggests that there are also some heavier metals such as gold and platinum present. As the Earth cools, it is thought that the inner core is probably getting larger, but at only a very slow rate.

The Outer Core

Unlike the inner core, the outer core is still liquid and convection currents in the outer core are believed to be the cause of the Earth’s magnetic field but the mechanism is poorly understood. The magnetic field is essential to the existence of life on earth as it protects the surface from harmful particles arriving from the Sun.

The Mantle

The mantle is the thick layer of hot rock that lies above the core. Different regions of the mantle have been discovered – the inner and outer mantle and the asthenosphere. Temperatures are very high but high pressures keep the rocks in solid form. The mantle is approximately 3000km in thickness.

The Asthenosphere is the very top of the mantle and is believed to be responsible for plate tectonics. Even though the rocks of the mantle are solid, including the asthenosphere, their high temperatures mean that they can slowly flow. As the rocks of the asthenosphere flow, friction between them and the layer above – the crust, is thought to move the tectonic plates. The mantle accounts for a little less than 70% of the Earth’s mass.

The Crust

This is the solid and coolest of the rocky layers of the Earth. It is the layer with which we are most familiar as it is the part on which we live and the easiest to study. It is cracked into large ‘plates’ some of which carry the continents and other land masses and some of which form the beds of the oceans. Where the plates meet you get earthquakes and volcanoes. The crust has a maximum thickness of about 70km (under the Himalaya and the Andes mountain ranges. It is at its thinnest near the mid ocean ridges. The junction of the crust and the mantle is called the ‘Moho’ (short for Mohorovičić discontinuity). The crust contains less than half a percent of the mass of the Earth

Depth of the Moho, layers of the Earth.

Map showing the depth of the Moho [image: AllenMcC]

How do Scientists Study the Layers of the Earth?

Layers of the Earth - using earthquake waves.

How earthquake waves reveal the internal structure of the Earth [image: USGS]

The quick answer to this is that they use earthquake waves that travel to their detectors from the other side of the world. When an earthquake happens, the different types of wave spread out through the whole of the Earth. Depending on where they are detected, they can have travelled through the crust or the core and mantle as well. By comparing what happens to these waves and how long they take to arrive, scientists can work out what has happened to the waves on their journey. It is a difficult branch of science and their understanding of the structure of the Earth changes all the time. New theories are formed, old ones are abandoned.

Taught science for 16 years at a secondary school in the East Midlands.