Direct Sources

The most direct way to study the Earth's interior is to examine material we can actually touch. Surface rocks and rocks brought up from mining operations are the easiest solid Earth materials to access. But mining has a hard limit: gold mines in South Africa, among the deepest in the world, go down only about 3 to 4 kilometres. Beyond that depth, the heat becomes so intense that further digging is simply not possible.

Because mining cannot go deeper, scientists have launched major projects to drill into the crust and explore its conditions directly. Two of the most significant international efforts are the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project. The deepest drill so far, at Kola near the Arctic Ocean, has reached about 12 kilometres. These deep drilling projects, along with many others, have yielded a large volume of information by analysing the materials collected at different depths.

Volcanic eruptions provide another direct source of information. When molten material, or magma, is thrown onto the Earth's surface during an eruption, it becomes available for laboratory analysis. However, there is a serious limitation: it is difficult to ascertain the exact depth from which such magma originates. So while volcanic rocks tell us a great deal about the mantle's composition, they do not tell us precisely where in the mantle they came from. …