How we know the earth's interior

The earth's surface is shaped by exogenic (outside) and endogenic (inside) processes. To understand earthquakes or tsunamis, we must know the interior. But the earth's radius is about 6,378 km, and no one can reach its centre, so most of our knowledge comes from estimates and inferences, with some direct observation.

Direct sources:

• Surface rocks and rocks from mines. Gold mines in South Africa go 3-4 km deep; deeper, it is too hot.
• Deep drilling projects, such as the Deep Ocean Drilling Project and the Integrated Ocean Drilling Project. The deepest, at Kola (in Russia's far north, near the Arctic Ocean), reached about 12 km.
• Volcanic eruptions bring magma to the surface for laboratory study, though we cannot tell how deep it came from.

Indirect sources:

• Mining shows that temperature, pressure and density increase with depth. Knowing the rate of change and the earth's thickness, scientists estimate their values at each depth.
• Meteors: their material does not come from inside the earth, but they formed from the same or similar material as our planet, so they are a useful clue.
• Gravity (g) is greater near the poles and less at the equator, because the equator is farther from the centre. It also varies with the mass of material below. The difference between measured and expected gravity is a gravity anomaly; anomalies show how mass is spread in the crust.
• Magnetic surveys show the spread of magnetic materials in the crust.
• Seismic waves (earthquake waves) give the most complete picture of the layered interior.

Example (new): a doctor cannot look inside a patient without surgery, so she uses an X-ray, a pulse reading and a blood test, each giving part of the picture. Scientists read the earth's interior the same way, from gravity, magnetism, meteorites and, above all, earthquake waves.