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وبلاگ بزرگ مقالات زمین شناسی - ژئوفیزیک - Geophysics

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ژئوفیزیک - Geophysics

Geophysics

 The study of the Earth and its relations to the rest of the solar system using the principles and practices of physics. Geophysics is considered by some to be a branch of geology, by others a branch of physics. It is distinguished from geology by its use of instruments to make direct and indirect measurements of the phenomena being studied in contrast to the more direct observations of geology, and by its concern with other members of the solar system.  See also: Earth; Geology; Solar system

Subdivisions of Geophysics

Geophysics is divided into a variety of subdisciplines. These can be grouped according to the portion of the Earth with which each is concerned, although there is much overlap. Solid-earth geophysics is concerned with the Earth's interior; oceanography and hydrology with the aqueous parts; meteorology with the lower atmosphere; and aeronomy with the upper atmosphere. Because students of the rest of the solar system use methods similar to those employed by geophysicists to study the Earth, geophysics has grown to encompass studies of the other planets, the Sun, and the space in which these bodies and the Earth move.

Solid-earth geophysics

 This discipline is subdivided, according to the methods used to study the Earth, into the sciences of geodesy, geologic thermometry, seismology, and tectonophysics.

Geodesy

Geodesy is the science of the shape and size of the Earth. Estimates of its diameter were made at least as early as the second century B.C. A basic gridwork of carefully located points on the Earth's surface was started early in the seventeenth century, and today forms the basis of all surveys and maps of the Earth's surface.  See also: Geodesy

From the dimensions of the Earth and its gravity, its mass can be calculated. The distribution of mass in the interior is determined from the variation of gravity from place to place on and above the surface and from calculations involving the moment of inertia and the velocities of seismic waves in the Earth's interior. Precise measurements of the orbits of satellites map the gravitational field in the space surrounding the Earth.  See also: Earth, gravity field of

 Geothermal studies

Heat is escaping continually from the Earth's interior by conduction and by volcanic processes. Measurements of the variations in the rate of upward heat flow provide evidence of the processes active in the Earth's interior. Studies of the thermal conductivity of rocks and of the rate of heat generation by disintegration of radioactive trace elements in the rocks make it possible to estimate the temperature distribution in the Earth's interior. The history of temperature variations in the interior can be modeled by using different assumptions of starting conditions and comparing the results with the limited knowledge of present conditions to test ideas on the evolution of the Earth.  See also: Earth, heat flow in; Geologic thermometry

Volcanic processes supplement the heat loss and are an important area of study because of their danger to human activities and because they provide information on how ore deposits form. Studies of volcanic processes on the ocean floor have revealed an environment for living creatures whose energy sources are not derived from sunlight, and may yield clues to the early evolution of life on Earth.  See also: Deep-sea fauna; Prebiotic organic synthesis; Volcanology

 Seismology

This is the science of earthquakes and other ground vibrations. Seismological studies may lead to the development of techniques for reliable earthquake prediction. In addition, earthquake studies may provide better understanding of the nature of the resultant motions, so that buildings and other structures can be designed to withstand their vibrations. By using earthquake vibrations, the structure of the Earth's interior and the patterns of deformation of the Earth can be mapped.  See also: Earthquake; Seismology

Study of the times of passage of seismic waves through the Earth's interior is the principal source of information on the distribution of different types of rocks. Studies of the waves' rate of absorption and coefficients of reflection give information on the physical properties of different layers of the Earth. From the motions recorded at places on the surface, it is possible to locate where within the Earth each earthquake occurs, details of the mechanism of generation of the waves, and the amount of energy released. This provides clues as to the processes of change occurring in the Earth's interior which lead to short- and long-range deformation of the surface.

Because large explosions produce seismic waves which are recorded at great distances from the source, much effort has been given to seismic research as a means of monitoring compliance with a nuclear test ban treaty.

Tectonophysics

Also called geodynamics, this is the science of the deformation of rocks. It consists of tectonics, the study of the broader structural features of the Earth and their origins, as in mountain building; and rock mechanics, the measurement of the strength and related physical properties of rocks.  See also: Continents, evolution of; Geodynamics; Orogeny; Plate tectonics; Rock mechanics

Because the increase of temperature and pressure with depth in the Earth profoundly changes the physical behavior of rocks, a proper understanding of processes in the Earth's interior is possible only when based on the results of laboratory experiments carried out under extreme conditions. Pressures at depths greater than a few tens of miles can be duplicated only under very transient conditions, and usually not at the high temperatures which are prevalent. The atomic structure of minerals goes through phase changes at high pressures which result in the occurrence at great depths of mineral varieties different from those encountered at the surface, with resultant uncertainty as to composition of the Earth's deepest layers. The Earth's interior is likely to remain incompletely explored for a long time in spite of the best efforts of geophysicists.  See also: Earth interior; High-pressure mineral synthesis

 Hydrospheric geophysics

Water is the compound that makes the Earth unique among large bodies in the universe. It occurs as a gas, a liquid, a solid, and as a component (often a trace element) in rocks. That part of the Earth's water which occurs as a liquid and as ice, largely free to move relatively easily from place to place, is called the hydrosphere. Specialized branches of geophysics have evolved to study water in this region.  See also: Hydrosphere

Oceanography

Scientific study of the oceans is concerned with the shape and structure of the ocean basins, the physical and chemical properties of seawater, ocean currents, waves and tides, thermodynamics of the ocean, and the relations of these to the organisms which live in the sea. Knowledge of the ocean is important because it is a source of food, much of the world's commerce travels across its surface, and its heat balance is a major factor affecting weather worldwide.  See also: Heat balance, terrestrial atmospheric; Marine geology; Oceanography

 Hydrology

Fresh water in lakes, in streams, and in the pores of near-surface rocks is the concern of hydrology. Every organism requires water to live. The distribution and purity of the water supply are consequently important. Surface water also carries away much of society's wastes, both sanitary and industrial. Hydrology is concerned with the chemical, physical, and biological processes by which these wastes are changed and removed as water moves through and over the ground toward the oceans. The constant exchange of water between the Earth's interior, the hydrosphere, and the atmosphere is an important aspect of hydrology. The availability of pure water for personal, agricultural, and industrial uses is so essential that pollution control is one of the most rapidly growing scientific and technological problems of modern society.  See also: Ground-water hydrology; Hydrology; Water pollution

 Glaciology

An appreciable fraction of Earth's water occurs in the form of snow and ice. If all of the water frozen in glaciers were to melt, the ocean surface would rise several hundred feet, changing coastlines substantially. Glaciology, which is often considered a branch of hydrology, is the scientific study of the distribution and movement of this frozen water, how it accumulates and melts, and what it does to the underlying rocks as it flows over them.  See also: Glaciology; Terrestrial water

 Meteorology

 Meteorology is the study of the composition and movements of the mass of air known as the atmosphere; of the interaction of the atmosphere with living organisms, the hydrosphere, and the solid earth; and of the flow of energy within the atmosphere and to and from the space beyond. An important goal of meteorology is to predict changes in atmospheric conditions, the weather and climate, from observations of current and past conditions and theoretical calculations based on models of how the atmosphere behaves. For this purpose, temperature, pressure, and the moisture conditions in the atmosphere are measured continuously or periodically at a large number of places at and above the Earth's surface, reported to data-processing centers, and used to make regular weather predictions. These are widely distributed by government agencies, private companies, and the mass media, and are used to guide individual and organizational activities.  See also: Agricultural meteorology; Climate modification; Industrial meteorology

Short-term weather predictions attempt to describe conditions to be expected within a few hours or days; but attempts are made at predicting long-term trends for weeks, months, or even years ahead as well. Data gathered by satellites orbiting the Earth play an important role in making predictions and understanding atmospheric processes. It has become the responsibility of government to warn the public of upcoming extreme weather.  See also: Atmosphere; Mesometeorology; Meteorology; Satellite meteorology; Weather forecasting and prediction; Weather modification

 Aeronomy

There is no distinct upper boundary to the atmosphere, which becomes progressively less dense with elevation, eventually merging with the Sun's extended atmosphere through which Earth moves. However, the properties of the atmosphere undergo a radical change at an elevation of about 60 mi (100 km), where the atmosphere becomes so highly ionized that its electrical properties become important in controlling its behavior. Aeronomy is the science of this upper part of the atmosphere. Solar radiations are mainly responsible for this ionization, although cosmic rays play a role. Molecules are dissociated into their component atoms, and selective diffusion results in an upward concentration of hydrogen, which may be lost continuously from the Earth. Free electrons as well as ions are present. The electrical particles move, forming currents which induce fluctuations of the Earth's magnetic field.  See also: Cosmic rays; Ionosphere; Upper-atmosphere dynamics

The impact of charged particles from the Sun can be so violent as to produce magnetic storms which disrupt radio communication. It also produces the aurora in high latitudes. Farther out, moving charged particles carried by the Earth in its motion around the Sun form a layer called the magnetosphere, whose shape is distorted by the flow of material radiated from the Sun. The charged layers of the atmosphere protect the Earth from the strongest of the Sun's radiations. Knowledge of the nature and variability of these radiations and their effects on the Earth and its inhabitants has been accumulating rapidly only in the last few decades, and much remains to be discovered. Exploration of the upper atmosphere and the outlying space became possible only with development of rocket probes and placing of artificial satellites in orbits around the Earth. As these researches into the outer fringes of the Earth's atmosphere have progressed, they have overlapped astronomical studies of the Sun to such a degree that solar science has become largely accepted as a part of geophysics.  See also: Aeronomy; Aurora; Magnetosphere; Solar wind; Sun

 Planetology

 Until the exploration of space using rockets, the only information available about planets and natural satellites was obtained by use of astronomical telescopes. With the advent of rocket probes, the same diversity of measurements made on the Earth could be carried out on each of the other bodies. Because the methods of measurement used are similar to those used by geophysicists to examine the Earth, studies of the other planets and their moons have become a branch of geophysics known as planetology.  See also: Moon; Planet; Planetary physics

 Overlapping Fields

 In addition to the regional subdivisions of geophysics defined above, there are other overlapping distinct specialties.

Geomagnetism

Geomagnetism is concerned with a detailed description of the Earth's magnetic field and its changes and with the magnetic properties of rocks. Because the Earth's magnetic field is used as a guide for navigation and to locate north in surveying, knowledge of its current patterns and changes both at the Earth's surface and in the surrounding space is important. The magnetism of rocks is also useful in delineating the history of changes in the Earth, providing the principal line of evidence showing how the present patterns of continents and oceans have evolved from very different arrangements in the past. Variations in magnetic field strength are also useful in mapping variations in the distribution of rocks of different compositions.

Changes in the magnetic field induce electric currents in the Earth's interior and in the atmosphere. Because magnetism and electricity are so closely linked, geoelectricity is generally considered a part of geomagnetism. Patterns of electric currents can be used to map the variations in the electrical conductivity of buried rocks, providing evidence of the Earth's internal structure.  See also: Geoelectricity; Geomagnetism; Rock magnetism

 Geochronology

This field deals with the dating events in the Earth's history. The principal technique is based on radioactive disintegrations: the proportion of daughter to parent elements in a mineral or rock is a measure of the age of the material. Other methods depend on the redshift of the spectra of distant stars, the rate of recession of the Moon, annual variations in the growth rates of plants and animals including fossils, and the rates of erosion and sedimentation.  See also: Dating methods; Geochronometry

 Geocosmogony

 This is the study of the origin of the Earth. The many hypotheses fall into two groups: those which postulate that the Earth is primarily an aggregate of once smaller particles, and those which claim that it is essentially a fragment of a larger body. Current speculation favors the former theory. Geocosmogony is initimately linked with the origin of the solar system and the Milky Way Galaxy. Many lines of evidence suggest that the formation of the Earth was a typical minor event in the evolution of the Milky Way or of the universe as a whole, occurring 5–10 × 109 years ago.  See also: Cosmochemistry; Milky Way Galaxy

 Exploration and prospecting

 Geophysical techniques are widely used not only to study the general structure of the Earth but also to prospect for petroleum, mineral deposits, and ground water and to map the sites of highways, dams, and other structures. Seismic methods are the most widely used, but electrical, electromagnetic, gravity, magnetic, and radioactivity surveying methods are also well developed. Many types of geophysical surveys can be made by lowering measuring apparatus into boreholes.  See also: Geophysical exploration; Prospecting; Well logging

 

Bibliography

  •  J. De Bremaecker, Geophysics: The Earth's Interior, 1985
  • Ali Fazeli = egeology.blogfa.com
  • C. M. R. Fowler, The Solid Earth: An Introduction to Global Geophysics, 2d ed., 2004
  • Ali Fazeli = egeology.blogfa.com
  • M. N. Hill et al., The Sea, vols. 1–8, 1962–1983
  • Ali Fazeli = egeology.blogfa.com
  • P. Kearey, M. Brooks, and I. Hill, An Introduction to Geophysical Exploration, 3d ed., 2002
  • Ali Fazeli = egeology.blogfa.com
  • F. K. Lutgens and E. J. Tarbuck, The Atmosphere: An Introduction to Meteorology, 10th ed., 2006
  • Ali Fazeli = egeology.blogfa.com
  • W. M. Kaula, An Introduction to Planetary Physics, 1976
  • Ali Fazeli = egeology.blogfa.com
  • P. V. Sharma, Geophysical Methods in Geology, 2d ed., 1986
  • Ali Fazeli = egeology.blogfa.com

 

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