What is the structure of the Earth?

Edexcel B GCSE Geography > Hazardous Earth > What is the structure of the Earth?

What is the structure of the Earth?

The Earth is composed of several layers, each with distinct properties. You can imagine the planet like a layered ball: a thin outer crust, a thick rocky mantle, and a hot metal core at the centre.

The Crust

The crust is the Earth’s thin, outermost layer. Together with the upper mantle, it forms the lithosphere, a rigid, brittle layer about 100 km thick.

The lithosphere is not one solid shell. Instead, it is split into huge pieces called tectonic plates. Each plate is composed of a crust sitting on top of the upper mantle. Where plates are beneath the oceans, they form oceanic lithosphere topped by oceanic crust. Where they underlie land, they form continental lithosphere topped by continental crust.

There are two main types of crust:

Continental crust – forms the continents. It is thicker (around 30–50 km deep), less dense and mostly made of granite-type rocks.
Oceanic crust – found beneath the oceans. It is thinner (around 6–8 km), denser and mainly made of basalt.

Both types of crust form the huge slabs we call tectonic plates.

The Mantle

The asthenosphere is the part of the Earth’s mantle located below the lithosphere, extending from approximately 100 km to about 400 km in depth. The lithosphere and asthenosphere are made of similar materials, but they behave very differently. The lithosphere is rigid and brittle, while the asthenosphere is solid but ductile (plastic-like), meaning it can flow slowly over long periods of time.

Temperature increases with depth in the asthenosphere and reaches at least 1300°C. This is hot enough to melt peridotite (the main rock in the mantle) at the surface, but the immense pressure at these depths stops it from melting. Instead, it stays solid but flexible enough to move. Below this, the lower mantle remains ductile but becomes even denser as pressure continues to increase.

The Core

At the centre is the core, made of iron and nickel.

It has two parts:

Outer core – a liquid layer that surrounds the very centre of the Earth. The movement of this liquid metal creates the Earth’s magnetic field.
Inner core – a solid ball of metal. Even though it is extremely hot (over 5,000°C), the huge pressure keeps it solid.

How do we know what’s inside the Earth?

Humans have only drilled a short way into the crust, so scientists rely on indirect evidence:

Seismic waves from earthquakes change speed and direction when they pass through different materials. This helps scientists map each layer.
Meteorites are made of the same materials thought to exist inside Earth’s core.
Satellites and computer models help estimate temperatures, density and movement inside the planet.

Summary

The Earth’s Layers
The Earth is made up of three main layers: the crust, mantle, and core, each with different properties and compositions.
The Crust
The outermost layer of the Earth. It is thin and solid, with continental crust (thicker and less dense) and oceanic crust (thinner and denser).
The Mantle
A thick layer of hot, solid rock beneath the crust. It makes up most of the Earth’s volume and slowly moves, driving tectonic activity.
The Asthenosphere and Lithosphere
The lithosphere includes the crust and upper mantle, forming tectonic plates. Beneath it, the asthenosphere is semi-molten and allows these plates to move.
The Core
The core is divided into a liquid outer core and a solid inner core, both made mainly of iron and nickel. Movement in the outer core creates the Earth’s magnetic field.
Evidence for Structure
Scientists study seismic waves from earthquakes and meteorite composition to understand what lies deep inside the Earth.