At the very centre is the dense core, containing large amounts of iron and sulphur. Scientists divide it into the inner core, which is solid, and the outer core, which is liquid. The next layer out is the mantle. It is fairly dense, but not a lot about its chemical make-up is known, except that it contains large amounts of igneous rocks like peridotite.

Life flourishes only on, and inside of, the crust, which is composed of huge "tectonic plates". The plates are giant slabs of rock and earth which sit on top of the mantle. In some places, such as Iceland, you can see the "fault lines" where two plates meet.

The relief of the crust is determined mostly by how the plates move around and crash into each other. Mountains form when two plates push together, and volcanoes often form where one slides under another. Volcanic and earthquake activity is most common around these fault lines. They can be seen in many parts of the world, including Iceland and North America, and even under the sea.

How do we know all this?

The structure of the Earth has a geophysical and a chemical aspect, each of which is partly revealed by evidence.

PHYSICAL STRUCTURE
The planet Earth is thought to comprise of a series of layers. The relatively thin outermost layer, the lithosphere, is immediately available for study, being just below our feet. It is made up of a “jigsaw puzzle” of continental plates, as evidenced directly by plate boundaries which have been discovered and mapped. Various pieces of evidence suggest that the plates move around over time, such as interlocking coastlines and glacial striations occurring on either either side of oceans.

Their movement is accommodated by the underlying asthenosphere, which meets the lithosphere at a depth of between 5 and 100 km. The boundary (discontinuity) between the two is named after Mohorivnic, or Moho for short, who discovered it.

The asthenosphere forms the outer part of the mantle, which is thought to solid, whereas the underlying outer core is thought to be liquid. This is inferred from the behaviour of the seismic waves produced by earthquakes. There are three varieties of seismic waves – surface (L) waves, P waves and S waves. S waves, being transverse, rely on the rigidity of the medium through which they pass, which means that liquids are impervious to them. They are never detected more than 103º round from the focus because they cannot pass through the outer core. This suggests that the outer core is liquid and that it begins at a depth of about 2900km. This boundary is called the Gutenburg discontinuity.

P waves, being longitudinal and so made up of a series of compressions and rarefactions, rely on the incompressibility of the medium and therefore increase with depth (and pressure). They are, however, refracted towards the normal as they pass through the Gutenburg discontinuity, re-emerging between 142 and 142 degrees either side of the focus. The area between 103º and 142º in which no seismic waves are detected is the shadow zone.

Further analysis of seismic wave arrivals indicates another discontinuity at depth of 5,155km beyond which the rock appears to be solid again, presumably due to the immense pressure.

CHEMICAL STRUCTURE
The chemical composition of the Earth can be inferred from several sources of evidence.

The rocky plates of the lithosphere are composed of both oceanic and continental crust. These are chemically different – the continental crust is largely composed of granodiorite whereas the oceanic crust is mainly basaltic. This is revealed by direct analysis of the rocks that make up both. In the case of the oceanic crust, cores are extracted for analysis by the Ocean Drilling Programme.

Direct evidence for the composition of the mantle is provided by volcanoes, which eject molten material originating from below the crust. These reveal high concentrations of iron, magnesium and silicate minerals like olivine. Deeper in the mantle, minerals assume different forms because of the increased pressure.

It has been shown that the earth is considerably warmer than it would be had it cooled from non-radioactive rocks. This suggests that radioactive minerals are present which release the otherwise unaccounted for thermal energy that drives plate tectonics. The concentration of radioactive material in the mantle does not seem to be very high, but because it is so immense (accounting for 80% by volume and 70% by mass of the earth) even a low concentration, if distributed throughout, would produce enough energy.

Laboratory testing and computer modeling are valuable ways of testing theories about the structure and composition of the Earth. Simulations of seismic waves through a variety of materials suggest that the outer core is composed of sulphur and an alloy of iron and nickel. They also suggests that the inner core is mostly iron and nickel, and this is backed up by evidence from meteorites.

Chemical analysis of meteorites, which formed at the same time and from the same source material as planet Earth, gives some indication of the compounds making up the earth. Meteorites tend to be high in heavy metals such as iron and nickel, which ties in with other evidence about the composition of the earth. Three types of meteorite, iron, stoney and iron-stoney. Each correspond in their composition to layers of the earth: the core, the core-mantle and the crust respectively.

 See also: Strata