What is happening inside this 12,700 km diameter ball we call the Earth? Some of the Earth’s internal architecture is an 'educated guess' (mainly from seismology), because the longest drill we have has only penetrated 12.6 km – and there’s 6,300 km between us and the centre of the Earth.

Despite these limitations, we know that the centre of the Earth is very hot. We only have to drill a small way to observe temperature rises of some 25°C per kilometre, so at some point, everything might be so hot that we would strike molten rock like the lava that flows from volcanoes.

However, common sense tells us that this rapid rise in temperature we see from surface drilling could not continue indefinitely or the centre of the Earth would be hotter than the Sun.

The currently accepted theory of the Earth’s structure is that it has a core (a solid inner core and a liquid outer core), a viscous mantle (divided into an upper mantle and mantle) and a silicate crust.

We believe that gravitational attraction and melting has nicely sorted the composition of the Earth so that dense materials have been pulled to the core, leaving lighter elements on the surface to form the crust.

The core

Recording the amount of time it takes refracted and reflected seismic waves generated by earthquakes to travel through different layers of the Earth has told us a lot about the type and location of things below the crust. For example, S-waves cannot pass through the Earth’s core at all, while the changes in seismic velocity between layers cause some waves to refract (bend like light passing between two media, such as air and water) and other waves to reflect like light from a mirror.

Based on studies of dense meteorites (parts of other planets), we can reasonably assume that the Earth’s solid core is composed mainly of iron (around 80%), with small amounts of nickel. The liquid outer core is composed of iron mixed with nickel and trace amounts of lighter elements. At present, we believe that convection in the liquid outer core combined with the rotation of the Earth causes the Earth’s magnetic field.

Nature of Science

Because scientists can only directly observe a small portion of the Earth's structure, they have pieced together indirect evidence from fields such as seismology, astronomy and other fields of geophysics to postulate on the nature of the Earth's internal architecture. By gathering the evidence, they have formulated a model comprised of the crust, upper mantle, mantle and outer and inner core. This model explains many of the phenomena we observe at the Earth's surface.

The mantle

The mantle, divided into two parts of differing viscosity, is composed of silicate rocks that are rich in iron and magnesium compared to the crust. High temperatures within the mantle cause the solid material to behave in a viscous manner, rather like bitumen used in roading – it can be shattered with a hammer but, over time, can flow like a liquid. Convection of the mantle creates the movement of the tectonic plates of the crust and uppermost mantle. It is these plate movements that cause earthquakes.

Contrary to what one might expect, the upper mantle behaves in a more liquid-like manner than the mantle below. This is caused by the tremendous increase in pressure the deeper you go and by chemical changes within the mantle.

The crust

The crust is composed mainly of basalt and granite and, with the uppermost part of the upper mantle, is broken into tectonic plates. The crust is cooler and more rigid than the deeper layers. The thickness of the crust varies considerably. Beneath continents, it is about 30 km. Mountains have a corresponding root within the Earth, sometimes increasing the thickness of the crust to over 70 km. Oceanic crust is thinner than continents crust, usually 5-10 km thick, and is mostly composed of basalt.

The last word should probably go to Monty Python:

Just remember that you're standing on a planet that's evolving
and revolving at 900 miles an hour,
that’s orbiting at 19 miles a second, so it’s reckoned,
around the Sun that is the source of all our power.

The Sun, and you and me, and all the stars that we can see
are moving at a million miles a day
in an outer spiral arm, at 40,000 miles an hour,
of a galaxy we call the Milky Way 

From The Galaxy Song by Monty Python

Useful links

The Kola Superdeep Borehole was the result of a scientific drilling project of the former USSR, which attempted to drill as deep as possible into the Earth's crust.

Detailed information on the structure of the Earth.

An interactive look at Earth structure.


    Published 21 July 2007