Sources of Information about the Interior

The Earth’s radius is roughly 6,378 km. No human can physically reach the centre, make observations there, or collect samples from such depths. So how do scientists claim to know what lies beneath our feet? The honest answer is that most of our knowledge about the interior is built on estimates and inferences. But a part of it does come from direct observation and analysis of actual materials.

Direct Sources

The most easily available solid earth material is surface rock, or the rocks we pull out from mining areas. The deepest gold mines in South Africa go down about 3 to 4 km. Going beyond that depth is simply not possible because it becomes extremely hot. Mining, however, is not the only way to reach deeper layers. Scientists have launched major projects to penetrate further into the crust and explore its conditions. Two such efforts are the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project. The deepest drill so far, at Kola in the Arctic Ocean, has reached a depth of 12 km. These deep drilling projects have yielded a large volume of information through the analysis of materials collected at different depths.

Volcanic eruptions form another direct source. When molten material, or magma, is thrown onto the Earth’s surface during an eruption, it becomes available for laboratory analysis. The catch is that it is difficult to determine the exact depth from which that magma originated.

Indirect Sources

Beyond direct sampling, scientists analyse the properties of matter to infer what lies inside. Mining activity has shown that temperature and pressure both increase as we move from the surface towards the interior. It is also known that the density of material increases with depth. Since the rate of change of these characteristics can be measured, and the total thickness of the Earth is known, scientists have estimated the values of temperature, pressure, and density at different depths. The details of these characteristics for each layer are discussed later in the chapter.

Meteors that reach the Earth provide another indirect source. It must be noted that the material from meteors is not from the Earth’s interior. However, meteors are solid bodies formed from materials the same as, or similar to, those of our planet, and the structure observed in them resembles that of the Earth. So they serve as a useful source of information about the interior.

The other indirect sources are gravitation, magnetic field, and seismic activity. …