Geophysics

Geophysics is the physics of the Earth and its environment in space. Its subjects include the shape of the Earth, its gravitational and magnetic fields, the dynamics of the Earth as a whole and of its component parts, the Earth's internal structure, composition and tectonics, the generation of magmas, volcanism and rock formation, the hydrological cycle including snow and ice, all aspects of the oceans, the atmosphere, ionosphere, magnetosphere and solar-terrestrial relations, and analogous problems associated with the Moon and other planets.

Geophysics is also applied to societal needs, such as mineral resources, mitigation of natural hazards and environmental protection. Geophysical survey data are used to analyze potential petroleum reservoirs and mineral deposits, to locate groundwater, to locate archaeological finds, to find the thicknesses of glaciers and soils, and for environmental remediation.

Ancient and classical eras

The magnetic compass existed in China back as far as the fourth century BC. It was used as much for feng shui as for navigation on land. It was not until good steel needles could be forged that compasses were used for navigation at sea; before that, they could not retain their magnetism for long. The first mention of a compass in Europe was in 1190.

In circa 240 BC, Erastothenes of Cyrene deduced that the Earth was round and measured the circumference of the Earth, using trigonometry and the angle of the Sun at more than one latitude in Egypt. He developed a system of latitude and longitude and measured the tilt of the Earth's axis.

Perhaps the earliest contribution to seismology was the invention of a seismoscope by the prolific inventor Zhang Heng in 132 CE. This instrument was designed to drop a bronze ball from the mouth of a dragon into the mouth of a toad. By looking at which of eight toads had the ball, one could determine the direction of the earthquake. It was 1571 years before the first design for a seismoscope was published in Europe, by Jean de la Hautefeuille. It was never built.

Beginnings of modern science

One of the publications that marked the beginning of modern science was William Gilbert's De Magnete (1600), a report of a series of meticulous experiments in magnetism. Gilbert deduced that compasses point north because the Earth itself is magnetic.

In 1687 Isaac Newton published his Principia, which not only laid the foundations for classical mechanics and gravitation but also explained a variety of geophysical phenomena such as the tides and the precession of the equinox.

The first seismometer, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes in 1844.

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Geophysics is a highly interdisciplinary subject and geophysicists contribute to every area of the Earth sciences. To provide a clearer idea of what constitutes geophysics, this section describes phenomena that are studied in physics and how they relate to the Earth and its surroundings.

Gravity

The gravitational attraction of the Moon and Sun give rise to two high tides and two low tides a day. Gravitational forces cause rocks to press down on deeper rocks, making them increase in density as the depth increases. Measurements of gravitational acceleration and gravitational potential at the Earth's surface and above it can provide information on mineral deposits and the dynamics of plates (see also gravity anomaly and gravimetry). A particular geopotential surface called the geoid is one definition of the shape of the Earth: it would be the global mean sea level if the oceans were in equilibrium and could be extended through the continents (such as with very narrow canals).

Heat flow

The Earth is cooling, and the resulting heat flow generates the Earth's magnetic field through the geodynamo and plate tectonics through mantle convection. The main sources of heat are the primordial heat and radioactivity, although there are also contributions from phase transitions. Heat is mostly carried to the surface by thermal convection, although there are two thermal boundary layers - the core-mantle boundary and the lithosphere - in which heat is transported by conduction. Some heat is carried up from the bottom of the mantle by mantle plumes. The heat flow at the Earth's surface is about 4.2 × 10¹³ W , and it is a potential source of geothermal energy.

Vibrations

Vibrations of the Earth can take the form of seismic waves, which travel through the Earth's interior or along its surface, and normal modes or free oscillations. Seismic waves that are caused by localized sources such as earthquakes or explosions can be received at a distance by seismographs and the observed motion can provide information on the source as well as the Earth in between. Seismic reflections can provide information on near-surface structure while seismic refraction can be used to infer the deep structure of the Earth. The locations of earthquakes provide information on plate tectonics and mantle convection.

Earthquakes pose a risk to humans. Understanding their mechanisms (which depend on the type of earthquake, e.g., intraplate or deep focus can lead to better assessments of earthquake risk and improvements in earthquake engineering.

Radioactivity

Radioactive decay, in addition to being the main source of heat in the Earth (see geotherm), is an invaluable tool for geochronology. Unstable isotopes decay at predictable rates, and the decay rates of different isotopes cover several orders of magnitude, so radioactive decay can be used to accurately date both recent events and events in past geologic eras.

Electricity

Although we mainly notice electricity during thunderstorms, there is always a downward electric field near the surface that averages 120 V m-1. Relative to the solid Earth, the atmosphere has a net positive charge due to bombardment by cosmic rays. A current of about 1800 A flows in the global circuit. It flows downward from the ionosphere over most of the Earth and back upwards through thunderstorms. The flow is manifested by lightning below the clouds and sprites above.

A variety of electric methods are used in geophysical survey. Some measure spontaneous potential a potential that arises in the ground because of man-made or natural disturbances. Telluric currents flow in Earth and the Oceans. They have two causes: electromagnetic induction by the time-varying, external-origin geomagnetic field and motion of conducting bodies (such as seawater) across the Earth's permanent magnetic field. The distribution of telluric current density can be used to detect variations in electrical resistivity of underground structures. Geophysicists can also provide the electric current themselves (see induced polarization and electrical resistivity tomography).

Electromagnetic waves

Electromagnetic waves occur in the ionosphere and magnetosphere as well as the Earth's outer core. they have a variety of others. dawn chorus is caused by high-energy electrons that get caught in the Van Allen radiation belt. Whistlers are produced by lightning strikes. Hiss may be generated by both. Electromagnetic waves may also be generated by earthquakes (see seismo-electromagnetics).

In the Earth's outer core, electric currents in the highly conductive liquid iron create magnetic fields by magnetic induction (see geodynamo). Alfvén waves are magnetohydrodynamic waves in the magnetosphere or the Earth's core. In the core, they probably have little observable effect on the geomagnetic field, but slower waves such as magnetic Rossby waves may be one source of secular variation.

Electromagnetic methods that are used for geophysical survey include transient electromagnetics and magnetotellurics.

Magnetism

The Earth's magnetic field protects the Earth from the deadly Solar wind and has long been used for navigation. It originates in the fluid motions of the Earth's core (see geodynamo). The magnetic field in the upper atmosphere gives rise to the auroras.

The Earth's field is roughly like a tilted dipole, but it changes over time (a phenomenon called secular variation). Mostly the geomagnetic pole stays near the geographic pole, but at random intervals averaging a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals are recorded in rocks (see natural remanent magnetization) and their signature can be seen in striped magnetic anomalies on the seafloor. These stripes provide quantitative information on seafloor spreading, a part of plate tectonics. In addition, the magnetization in rocks can be used to measure the motion of continents (see paleomagnetism).

Fluid dynamics

Fluid motions occur in the magnetosphere, atmosphere, ocean, mantle and core. Even the mantle, though it has an enormous viscosity, flows like a fluid over long time intervals (see geodynamics). This flow is reflected in phenomena such as isostasy and post-glacial rebound. The mantle flow drives plate tectonics and the flow in the Earth's core drives the geodynamo.

Geophysical fluid dynamics is a primary tool in physical oceanography and meteorology. The rotation of the Earth has profound effects on the Earth's fluid dynamics, often due to the Coriolis effect. In the atmosphere it gives rise to large-scale patterns like Rossby waves and determines the basic circulation patterns of storms. In the ocean they drive large-scale circulation patterns as well as Kelvin waves and Ekman spirals at the ocean surface. In the Earth's core, the circulation of the molten iron is structured by Taylor columns.

Waves and other phenomena in the magnetosphere can be modeled using magnetohydrodynamics.

Condensed matter physics

The physical properties of minerals must be understood to infer the composition of the Earths' interior from seismology, the geothermal gradient and other sources of information. Mineral physicists study the elastic properties of minerals as well as their high-pressure phase diagrams, melting points and equations of state at high pressure. Studies of creep determine how rocks that are brittle at the surface can flow deep down. These properties determine the rheology that determines the geodynamics.

Water is a very complex substance and its unique properties are essential for life. Its physical properties shape the hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation. Some of the precipitated water becomes groundwater, and groundwater flow includes phenomena such as percolation, while the conductivity of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as salinity have a large effect on its motion in the oceans.

Water is a very complex substance and its unique properties are essential for life. Its physical properties shape the hydrosphere and are an essential part of the water cycle and climate. Its thermodynamic properties determine evaporation and the thermal gradient in the atmosphere. The many types of precipitation involve a complex mixture of processes such as coalescence, supercooling and supersaturation. Some of the precipitated water becomes groundwater, and groundwater flow includes phenomena such as percolation, while the conductivity of water makes electrical and electromagnetic methods useful for tracking groundwater flow. Physical properties of water such as salinity have a large effect on its motion in the oceans.

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Size and form of the Earth The Earth is roughly spherical, but it bulges towards the Equator, so it is roughly in the shape of an ellipsoid (see Earth ellipsoid). This bulge is due to its rotation and is nearly consistent with an Earth in hydrostatic equilibrium. The detailed shape of the Earth, however, is also affected by the distribution of continents and ocean basins, and to some extent by the dynamics of the plates.

Evidence from seismology, heat flow at the surface, and mineral physics is combined with the Earth's mass and moment of inertia to infer models of the Earth's interior - its composition, density, temperature, pressure. The Earth's mass is M = 5.975 × 1024 kg and its mean radius is R = 6371 km , so its mean specific gravity is < ρ > = 5.515. This is substantially higher than the typical specific gravity (2.7–3.3) of rocks at the surface. Its moment of inertia is 0.33 M R², whereas it would be 0.4 M R² if the earth was a sphere of constant density. Both lines of evidence point to a concentration of mass near the center. However, the density of the rock will increase with depth because of the increasing pressure. To determine how large this effect is, the Adams–Williamson equation is used to determine how density increases with pressure. The conclusion is that pressure alone cannot account for the increase in density. Instead, we know that the Earth's core is composed of an alloy of iron and other minerals.

Reconstructions of seismic waves in the deep interior of the Earth show that there are no S-waves in the outer core. This indicates that the outer core is liquid, because liquids cannot support shear. The outer core is liquid, and the motion of this highly conductive fluid generates the Earth's field (see geodynamo). The inner core, however, is solid because of the enormous pressure.

Reconstruction of seismic reflections in the deep interior indicate some major discontinuities in seismic velocities that demarcate the major zones of the Earth: inner core, outer core, mantle, lithosphere and crust. The mantle itself is divided into the upper mantle, transition zone, lower mantle and D′′ layer. Between the crust and the mantle is the Mohorovičić discontinuity.

The seismic model of the Earth does not by itself determine the composition of the layers. For a complete model of the Earth, mineral physics is needed to interpret seismic velocities in terms of composition. The mineral properties are temperature-dependent, so the geotherm must also be determined. This requires physical theory for thermal conduction and convection and the heat contribution of radioactive elements. The main model for the radial structure of the interior of the Earth is the Preliminary Reference Earth Model (PREM). Some parts of this model have been updated by recent findings in mineral physics (see post-perovskite) and supplemented by seismic tomography. The mantle is mainly composed of silicates, and the boundaries between layers of the mantle are probably due to phase transitions.

The mantle acts as a solid for seismic waves, but under high pressures and temperatures it deforms so that over millions of years it acts like a liquid. This makes plate tectonics possible. Geodynamics is the study of the fluid flow in the mantle and core.

The magnetosphere

If a planet's magnetic field is sufficiently strong, its interaction with the solar wind forms a magnetosphere around a planet. Early space probes discovered the gross dimensions of the terrestrial magnetic field, which extends about 10 Earth radiii towards the Sun. The solar wind, a stream of charged particles, streams out and around the terrestrial magnetic field, and continues behind the magnetic tail, hundreds of Earth radii downstream. Inside the magnetosphere, there are relatively dense regions of solar wind particles, the Van Allen radiation belts.