Indirect Sources

The surface of the Earth can be studied directly, but the interior is out of reach. So scientists turn to indirect sources — ways of reading the hidden interior through its effects on things we can observe. The logic is simple: matter has properties, and those properties change in predictable ways with depth. By analysing those changes, we can infer what lies beneath.

The most basic indirect evidence comes from mining. As we go deeper into the Earth, temperature and pressure both rise steadily with distance from the surface. The density of the material also increases with depth. Because these changes follow a measurable rate, scientists can calculate the rate of change and, knowing the total thickness of the Earth, estimate the temperature, pressure, and density at any given depth. The specific values for each layer are discussed later in the chapter.

Another indirect source is meteors. When meteors reach the Earth, the material available for analysis is not actually from the Earth's interior — but it is still valuable. Meteors are solid bodies formed from the same kind of material as our planet, and their structure closely resembles the Earth's. So studying them gives scientists a working idea of what the Earth's interior might be made of.

Three more indirect sources complete the picture: gravitation, magnetic field, and seismic activity.

Gravity is not uniform across the Earth's surface. The gravitational force (g) is greater near the poles and less at the equator, because the distance from the centre is greater at the equator than at the poles. But gravity values also change according to the mass of material beneath a location. The uneven distribution of mass within the Earth influences the reading. When the measured gravity at a place differs from the expected value, that difference is called a gravity anomaly. These anomalies reveal how mass is distributed in the crust of the Earth. …