Structure of the Earth
Almost 5 billion years ago, the Earth is formed out of debris around the solar protoplanetary disk. With time, these gases cooled
and condensed to form a solid body, spinning on a vertical axis. As a result, the most dense material became concentrated at the centre. This effect can be illustrated from calculations of the average density of the Earth (5500 kg/m2) from astronomic observations. Rocks at the surface have lower densities, ranging from 2500 to 3500 kg/m2, suggesting that, indeed, rocks towards the centre must have a higher than average density.
Earth interior structure
The structure of the interior of the Earth has been investigated by refraction seismology. Shock waves from earthquakes and nuclear explosions can be detected at various points around the globe, and their arrival times used to determine the depths of major density changes within the Earth.
These studies suggest that the Earth's interior can be divided into 3 main layers: the core, mantle and crust. The broad chemical composition of these layers is determined by comparison with rocky meteorites that have landed on the Earth's surface.
The core is made of a complex iron-nickel alloy and consists of an inner and outer layer. The outer core is liquid and its movement is responsible for the Earth's magnetic field.
The mantle is a very thick layer and occupies almost 70% of the Earth's volume. It is made of silicate minerals and, although solid, can undergo plastic deformation over very long time periods (so-called mantle convection currents).
The Earth's crust varies in thickness from 6 to 90 km, with an average of 35 km. The crust is therefore very thin relative to the average radius of the planet (6370 km). There are important differences between oceanic crust, which floors the ocean basins, and continental crust beneath the continents and continental shelves.
The oceanic crust is thin (6-12 km) compared to the continental crust (30-90 km) and also has a more homogeneous composition. Seismic investigation of oceanic crust has shown that the deep ocean basins are floored by rocks which consist only of basalts and gabbros (coarse-grained equivalent of basalt).
In contrast, continental crust has a more varied composition. The lower crust consists mainly of diorites (a type of igneous rock). The upper crust is more granitic with some metamorphic rocks, and is covered by a layer of sedimentary rocks which can be several kilometers thick.