Geophysics

"Historically geophysics has been the study of the classical linear equations of physics, Laplace's equation for gravity and magnetics, the wave equation for propagating seismic waves, and the heat equation for thermal problems."

"Most observers of the geophysical scene, those involved with and in particular, are aware of the seminal contributions of rock magnetism and its sister discipline, paleomagnetism, which some have called “applied rock magnetism.” That was in the heady days of the 1960s. As a discipline, however esoteric, rock magnetism goes further back to the 19th century when the Swedish prospectors for iron ores and Italian and German paleomagnetists in search of the history of the geomagnetic field were using rock magnetism as an exciting new tool. The first book on rock magnetism, Der Gesteinsmagnetismus by H. Haalck, was published in 1942, more than a decade before 's Rock Magnetism, cited by Dunlop and Özdemir as the earliest work on the subject. There have been more recent volumes on the subject, including one that I coauthored with Frank Stacey. However, I am not exaggerating when I say that Rock Magnetism: Fundamentals and Frontiers leaves the others in the dust."

"Rock magnetism is the term commonly applied to the study of the magnetic properties of rocks and minerals, how these properties depend on factors such as grain size and shape, temperature and pressure, and the origin and characteristics of the different types of . Although all minerals possess some magnetic properties, even if only paramagnetic or diamagnetic, the term ‘magnetic minerals’ is used here only for those minerals which are capable of carrying remanent magnetism."

"(1907) was intrigued by the fact that iron and other ‘soft’ ferromagnetic materials with small permanent magnetization become strongly magnetized when exposed to quite weak magnetic fields. Weiss proposed that external fields play a minor role compared to a hypothesized internal ‘molecular field’ which aligns the magnetic moments of individual atoms, producing a spontaneous magnetization, Ms. In the absence of any external field, the magnetic moments of regions with different directions of Ms (now called domains) cancel almost perfectly, but even a small applied field will either rotate domains or enlarge some at the expense of others. Weiss’ theory is the starting point for modern ideas about ferromagnetism ... and ferromagnetic domains ..."

"In rock magnetism our interest in very low temperature properties is restricted to two effects, the processes of demagnetization which accompany phase changes in magnetite and hematite, and the low-temperature evidence for cation distribution between non-equivalent lattice sites in ferrites. However, s themselves are important. They provide the mechanism whereby thermal excitation affects the magnetic properties of ferromagnetic materials ... The structure of spin waves is closely related to that of domain walls ..."

"... You can imagine the Earth as an enormous battery whose magnetic field is formed by the movement of molten metal (caused by the Earth's rotation) around the solid metal inner core. At present the positive terminal is at the , but in times past the orientation has switched so that the positive terminal is at the . Over the past 76 million years the Earth's magnetic field has reversed 171 times. No-one knows exactly why this happens, but at present the magnetic field is weakening a little each decade. Many researchers think that this presages another shift. Just how long before the reversal happens, however, is unknown."

"Since the development of simple compasses, the Earth's magnetic field has been an important aid for navigation, enabling travelers to find their way in uncharted regions. It became an essential tool in the exploration of the planet and facilitated the great voyages of discovery in the late 15th century that established links between the continents. It is one of the central properties of the Earth and has been studied with purpose-built equipment on land, at sea, and in the past half-century from satellites. The magnetic field extends far into space around the planet, where it interacts with the energetic particles from the Sun and interstellar space that impact on the Earth. It produces sideways forces on the rapidly moving, electrically charged particles and deflects them around the planet. In this way, the magnetic field acts as a protective shield against harmful radiation and enables life to exist on the planet."