Temperature gradient and density

Temperature gradient and density

It is well known from deep mining operations that temperature increases downwards at an average rate of 300C per km. This rate is higher near a source of heat such as an active volcanic centre, and is also affected by the thermal conductivity of the rocks at a particular locality. Assuming for the moment that the temperature gradient continues at the average rate, calculation shows that at a depth of some 30 km the temperature would be such that most known rocks would begin to melt. The high pressure prevailing at that depth and the ability of crustal rocks to conduct heat away to the surface of the Earth result in the rock material there remaining in a relatively solid condition; but there will be a depth at which it becomes essentially a viscous fluid and this defines the base of the lithosphere (Greek: Iithos = stone).

The mean mass density of the Earth, which is found from its size and motion around the Sun, is 5.527 gem"3. This is greater than the density of most rocks found at the surface, which rarely exceeds 3; sedimentary rocks average 2.3, and the abundant igneous rock granite about 2.7. In order to bring the mean density to 5.5 there must therefore be denser material at lower levels within the Earth. This has been confirmed from the study of the elastic waves generated by earthquakes, in particular from research into the way in which earthquake waves are bent (by diffraction at certain boundaries) as they pass through the Earth: our knowledge of the Earth's interior comes mainly from such studies.

These have shown that our planet has a core of heavy material with a density of about 8. Two metals iron and nickel have densities a little below and above 8 respectively, and the core is believed to be a mixture of these composed mainly of iron. Surrounding this heavy core is the region known as the mantle and overlying that is the crust, which is itself composite. In continental areas the average thickness of the crust is about 30 km: in the oceans it is 10km. The mantle has a range of density intermediate between that of the crust and the core, as indicated in the figure. In order to discuss further the evidence from seismic work for this earth structure we turn to the subject of earthquakes.