5. Gravity and the figure of the Earth

‘Gravity and the figure of the Earth’ discusses the measurement of gravity and its variation at the Earth’s surface and with depth. Gravity is about 0.5 per cent stronger at the poles than at the equator and it first increases with depth until the core–mantle boundary and then sinks to zero at the Earth’s centre. Using satellites to carry out geodetic and gravimetric observations has revolutionized geodesy, creating a powerful geophysical tool for observing and measuring dynamic processes on the Earth. The various measurement techniques employed fall in two categories: precise location of a position on the Earth (such as GPS) and accurate determination of the geoid and gravitational field. Bouguer and free-air gravity anomalies and isostasy are explained.

2. Planet Earth

Two important physical laws determine the behaviour of the Earth as a planet and the relationship between the Sun and its planets: the law of conservation of energy and the law of conservation of angular momentum. ‘Planet Earth’ explains these laws along with the ‘Big Bang’ theory that describes the formation of the solar system: the Sun; the eight planets divided into the inner, terrestrial planets (Mercury, Venus, the Earth, and Mars) and the outer, giant planets (Jupiter, Saturn, Uranus, and Neptune); and the Trans-Neptunian objects that lie beyond Neptune. Kepler’s laws of planetary motion, the Chandler wobble, the effects of the Moon and Jupiter on the Earth’s rotation, and the Milankovitch cycles of climatic variation are also discussed.

1. What is geophysics?

Geophysics is a field of earth sciences that uses the methods of physics to investigate the complex physical properties of the Earth and the natural processes that have determined and continue to govern its evolution. ‘What is geophysics?’ explains how geophysical investigations cover a wide range of research fields—including planetary gravitational and magnetic fields and seismology—extending from surface changes that can be observed from Earth-orbiting satellites to complex behaviour in the Earth’s deep interior. The timescale of processes occurring in the Earth also has a very broad range, from earthquakes lasting a few seconds to the motions of tectonic plates that take place over tens of millions of years.

4. Seismicity—the restless Earth

Hundreds of thousands of earthquakes occur worldwide each year, but most of them go unnoticed. Only a few are very destructive. Most earthquakes have a tectonic origin and happen in well-defined, relatively narrow seismic zones. ‘Seismicity—the restless Earth’ first describes the elastic rebound model that explains how an earthquake occurs. There are two measures of the size of an earthquake: its magnitude and intensity. Magnitude is a measure of the energy released by the earthquake classified by the Richter scale, while intensity is a qualitative measure based on observed effects using a twelve-part scale. Maps of the locations of earthquake epicentres show that these are concentrated in narrow seismically active zones. Earthquake monitoring and prediction are discussed.

8. Afterthoughts

Geophysics has made many important advances towards understanding the behaviour and properties of planet Earth. Geophysical research continues to make discoveries and developments that benefit society. Many problems presented by the Earth—such as the prediction of both the time and location of an earthquake—have been researched by geophysicists for decades, but remain unsolved because of the complex nature of their causes. ‘Afterthoughts’ explains that earth scientists cannot control the natural events that occur, but can only observe and try to understand them. The equipment for attaining these goals improves continually, in a remarkably rapid and encompassing manner. Land-based methods are now augmented in several fields by remarkable observations from space.

7. The Earth’s magnetic field

The Earth is surrounded by a magnetic field, which originates inside its molten core, and which for centuries has helped travellers to navigate safely across uncharted regions. The magnetic field protects life on the Earth by acting as a shield against harmful radiation from space, especially from the Sun. ‘The Earth’s magnetic field’ explains that the magnetic field at the Earth’s surface is dominantly that of an inclined dipole. The Sun’s deforming effect on the magnetic field outside the Earth is described, as are the magnetic fields of other planets. The magnetism of rocks forms the basis of palaeomagnetism, which explains how plate tectonics displaced the continents and produced oceanic magnetic anomalies whenever the geomagnetic field reversed polarity.

6. The Earth’s heat

The Earth’s internal heat is its greatest source of energy. It powers global geological processes such as plate tectonics and the generation of the geomagnetic field. ‘The Earth’s heat’ explains that the internal heat arises from two sources: the decay of radioactive isotopes in crustal rocks and the mantle, and primordial heat left over from the planet’s fiery formation. The internal heat has to find its way out of the Earth. The three basic forms of heat transfer are radiation, conduction, and convection. Heat is also transferred in compositional and phase transitions. Heat transport by conduction is most important in solid regions of the Earth, while thermal convection occurs in the viscoelastic mantle and molten outer core.

3. Seismology and the Earth’s internal structure

Seismology is the most powerful geophysical tool for understanding the structure of the Earth. It is concerned with how the Earth vibrates. Physically, seismic behaviour depends on the relationship between stress and strain in the Earth. ‘Seismology and the Earth’s internal structure’ explains compressional and shear elastic deformation and the four types of seismic waves caused by earthquakes: P-waves and S-waves that travel through the body of the Earth, and Rayleigh and Love waves that spread out at and near the Earth’s surface. It describes the reflection, refraction, and diffraction of body waves and how their observation and measurement by seismometers can be used to understand the internal structure of core, mantle, and crust.