Age of the sea floor. Much of the dating information comes from magnetic anomalies.
Although geophysics was only recognized as a separate discipline in the 19th century, its origins date back to ancient times. The first magnetic compasses were made from lodestones
, while more modern magnetic compasses played an important role in the history of navigation. The first seismic instrument was built in 132 AD. Isaac Newton
applied his theory of mechanics to the tides and the precession of the equinox
; and instruments were developed to measure the Earth's shape, density and gravity field, as well as the components of the water cycle. In the 20th century, geophysical methods were developed for remote exploration of the solid Earth and the ocean, and geophysics played an essential role in the development of the theory of plate tectonics.
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. In Geophysics, principles of Physics are applied to study the "Interior" of the Earth. Depending on the problem under study, one has to decide which method should be applied. e.g. for ground water surveys, Electrical method is helpful. For mineral deposits, one can adopt Gravity and/or Magnetic surveys. For Oil & Natural Gas, one has to carry out Gravity, Magnetic surveys to get rough idea about structure of rock formations. If the desired structure is existing, for detailed study of rock formations, one has to carry out Seismic and/or Magneto-telluric surveys.
A map of deviations in gravity from a perfectly smooth, idealized Earth.
The gravitational pull of the Moon and Sun give rise to two high tides and two low tides every lunar day, or every 24 hours and 50 minutes. Therefore, there is a gap of 12 hours and 25 minutes between every high tide and between every low tide.
Gravitational forces make rocks press down on deeper rocks, increasing their density as the depth increases.
Measurements of gravitational acceleration
and gravitational potential
at the Earth's surface and above it can be used to look for mineral deposits (see gravity anomaly
The surface gravitational field provides information on the dynamics of tectonic plates
. The geopotential
surface called the geoid
is one definition of the shape of the Earth. The geoid 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).
Illustration of the deformations of a block by body waves and surface waves (see seismic wave
are vibrations that travel through the Earth's interior or along its surface. The entire Earth can also oscillate in forms that are called normal modes
or free oscillations of the Earth
. Ground motions from waves or normal modes are measured using seismographs
. If the waves come from a localized source such as an earthquake or explosion, measurements at more than one location can be used to locate the source. The locations of earthquakes provide information on plate tectonics and mantle convection.
Recording of seismic waves from controlled sources provide information on the region that the waves travel through. If the density or composition of the rock changes, waves are reflected. Reflections recorded using Reflection Seismology
can provide a wealth of information on the structure of the earth up to several kilometers deep and are used to increase our understanding of the geology as well as to explore for oil and gas.
Changes in the travel direction, called refraction
, can be used to infer the deep structure of the Earth
Although we mainly notice electricity during thunderstorms
, there is always a downward electric field near the surface that averages 120 volts
Relative to the solid Earth, the atmosphere has a net positive charge due to bombardment by cosmic rays
. A current of about 1800 amperes
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
In the highly conductive liquid iron of the outer core, magnetic fields are generated by electric currents through electromagnetic induction. Alfvén waves
waves in the magnetosphere or the Earth's core. In the core, they probably have little observable effect on the Earth's magnetic field, but slower waves such as magnetic Rossby waves
may be one source of geomagnetic secular variation
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 outer core.
The magnetic field in the upper atmosphere gives rise to the auroras
Earth's dipole axis (pink line) is tilted away from the rotational axis (blue line).
The Earth's field is roughly like a tilted dipole
, but it changes over time (a phenomenon called geomagnetic secular variation). Mostly the geomagnetic pole
stays near the geographic pole
, but at random intervals averaging 440,000 to a million years or so, the polarity of the Earth's field reverses. These geomagnetic reversals
, analyzed within a Geomagnetic Polarity Time Scale
, contain 184 polarity intervals in the last 83 million years, with change in frequency over time, with the most recent brief complete reversal of the Laschamp event
occurring 41,000 years ago during the last glacial period
. Geologists observed geomagnetic reversal recorded
in volcanic rocks, through magnetostratigraphy correlation
(see natural remanent magnetization
) and their signature can be seen as parallel linear magnetic anomaly stripes on the seafloor. These stripes provide quantitative information on seafloor spreading
, a part of plate tectonics. They are the basis of magnetostratigraphy
, which correlates magnetic reversals with other stratigraphies to construct geologic time scales.
In addition, the magnetization in rocks
can be used to measure the motion of continents.
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
Radiometric mapping using ground and airborne gamma spectrometry
can be used to map the concentration and distribution of radioisotopes near the Earth's surface, which is useful for mapping lithology and alteration.
Geophysical fluid dynamics is a primary tool in physical oceanography
. 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
The physical properties of minerals must be understood to infer the composition of the Earth's interior from seismology
, the geothermal gradient
and other sources of information. Mineral physicists study the elastic
properties of minerals; their high-pressure phase diagrams
, melting points and equations of state
at high pressure; and the rheological properties
of rocks, or their ability to flow. Deformation of rocks by creep
make flow possible, although over short times the rocks are brittle. The viscosity
of rocks is affected by temperature and pressure, and in turn determines the rates at which tectonic plates move.
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
. 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
Some 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.
Regions of the Earth
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.
Structure of the interior
Seismic velocities and boundaries in the interior of the Earth
sampled by seismic waves.
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. For example, the Earth's mean specific gravity
(5.515) is far higher than the typical specific gravity of rocks at the surface (2.7–3.3), implying that the deeper material is denser. This is also implied by its low moment of inertia
( 0.33 M R2
, compared to 0.4 M R2
for a sphere of constant density). However, some of the density increase is compression under the enormous pressures inside the Earth. The effect of pressure can be calculated using the Adams–Williamson equation
. 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. Earth's 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 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 consistent with 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
Schematic of Earth's magnetosphere. The solar wind
flows from left to right.
If a planet's magnetic field
is strong enough, its interaction with the solar wind forms a magnetosphere. Early space probes
mapped out the gross dimensions of the Earth's magnetic field, which extends about 10 Earth radii
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 called the Van Allen radiation belts.
Gravity measurements became part of geodesy because they were needed to related measurements at the surface of the Earth to the reference coordinate system. Gravity measurements on land can be made using gravimeters
deployed either on the surface or in helicopter flyovers. Since the 1960s, the Earth's gravity field has been measured by analyzing the motion of satellites. Sea level can also be measured by satellites using radar altimetry
, contributing to a more accurate geoid
In 2002, NASA
launched the Gravity Recovery and Climate Experiment
(GRACE), wherein two twin satellites map variations in Earth's gravity field by making measurements of the distance between the two satellites using GPS and a microwave ranging system. Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.
Satellites and space probes
Satellites in space have made it possible to collect data from not only the visible light region, but in other areas of the electromagnetic spectrum
. The planets can be characterized by their force fields: gravity and their magnetic fields, which are studied through geophysics and space physics.
Geophysics emerged as a separate discipline only in the 19th century, from the intersection of physical geography
, meteorology, and physics.
However, many geophysical phenomena – such as the Earth's magnetic field and earthquakes – have been investigated since the ancient era
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 long enough to be useful. The first mention of a compass in Europe was in 1190 AD.
Perhaps the earliest contribution to seismology was the invention of a seismoscope
by the prolific inventor Zhang Heng
in 132 AD.
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.
The first seismometer
, an instrument capable of keeping a continuous record of seismic activity, was built by James Forbes
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