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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.
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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 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. 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 possible.
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. Geophysical measurements are generally at a particular time and place. Accurate measurements of position, along with earth deformation and gravity, are the province of geodesy.
While geodesy and geophysics are separate fields, the two are so closely connected that many scientific organizations such as the American Geophysical Union , the Canadian Geophysical Union and the International Union of Geodesy and Geophysics encompass both. Absolute positions are most frequently determined using the global positioning system GPS. A three-dimensional position is calculated using messages from four or more visible satellites and referred to the Geodetic Reference System.
An alternative, optical astronomy , combines astronomical coordinates and the local gravity vector to get geodetic coordinates. This method only provides the position in two coordinates and is more difficult to use than GPS. However, it is useful for measuring motions of the Earth such as nutation and Chandler wobble. Relative positions of two or more points can be determined using very-long-baseline interferometry.
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 s, 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. Gravity variations detected by GRACE include those caused by changes in ocean currents; runoff and ground water depletion; melting ice sheets and glaciers.
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. Measuring the changes in acceleration experienced by spacecraft as they orbit has allowed fine details of the gravity fields of the planets to be mapped. For example, in the s, the gravity field disturbances above lunar maria were measured through lunar orbiters , which led to the discovery of concentrations of mass, mascons , beneath the Imbrium , Serenitatis , Crisium , Nectaris and Humorum basins.
Geophysics emerged as a separate discipline only in the 19th century, from the intersection of physical geography , geology , astronomy , meteorology, and physics. 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 AD.
Geodynamo Conductivity Limits
In circa BC, Eratosthenes 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. Perhaps the earliest contribution to seismology was the invention of a seismoscope by the prolific inventor Zhang Heng in AD. By looking at which of eight toads had the ball, one could determine the direction of the earthquake. It was years before the first design for a seismoscope was published in Europe, by Jean de la Hautefeuille. It was never built.
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One of the publications that marked the beginning of modern science was William Gilbert 's De Magnete , a report of a series of meticulous experiments in magnetism. Gilbert deduced that compasses point north because the Earth itself is magnetic. In 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 From Wikipedia, the free encyclopedia.
For the journal, see Geophysics journal. Main article: Gravity of Earth.
Further information: Physical geodesy and Gravimetry. Main article: Geothermal gradient. Main article: Seismology. Further information: Earth's magnetic field , Aeromagnetic survey , and Paleomagnetism. Further information: Radiometric dating. Main article: Geophysical fluid dynamics. Further information: Mineral physics. Main article: Figure of the Earth.
Earth's Core and Lower Mantle | NHBS Academic & Professional Books
Main article: Structure of the Earth. Main article: Magnetosphere. Main article: Geodesy. Main article: History of geophysics. Earth sciences portal Geophysics portal Physics portal. April Geochemistry, Geophysics, Geosystems. Bibcode : GGG National Aeronautics and Space Administration. Retrieved 13 November Introduction to seismology 2nd ed. Cambridge: Cambridge University Press. Geoscience Australia.
Commonwealth of Australia. Retrieved 23 June Natural Resource Management. Archived from the original on 21 March Acta Geodaetica et Geophysica Hungarica. American Geophysical Union About AGU.