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Earth, one of the planets in the solar system, the third in distance from the sun and the fifth largest of the planets in diameter. The mean distance of the earth from the sun is 149,503,000 km (92,897,000 mi). It is the only planet known to support life, although some of the other planets have atmospheres and contain water.

The earth is not a perfect sphere but is slightly pear-shaped. Calculations based on perturbations in the orbits of artificial satellites reveal that the earth is an imperfect sphere, because the equator bulges, or is distended, 21 km (13 mi); the North Pole bulges 10 m (33 ft); and the South Pole is depressed about 31 m (about 100 ft).

Motion

In common with the entire solar system, the earth is moving through space at the rate of approximately 20.1 km/sec or 72,360 km/hr (approximately 12.5 mi/sec or 45,000 mph) toward the constellation of Hercules. The Milky Way galaxy as a whole, however, is moving toward the constellation Leo at about 600 km/sec (about 375 mi/sec). The earth and its satellite, the moon, also move together in an elliptical orbit about the sun. The eccentricity of the orbit is slight, so that the orbit is virtually a circle. The approximate length of the earth's orbit is 938,900,000 km (583,400,000 mi), and the earth travels along it at a velocity of about 106,000 km/hr (about 66,000 mph). The earth rotates on its axis once every 23 hr 56 min 4.1 sec (based on the solar year). A point on the equator therefore rotates at a rate of a little more than 1600 km/hr (about 1000 mph), and a point on the earth at the latitude of Portland, Oregon (45� North), rotates at about 1073 km/hr (about 667 mph).

In addition to these primary motions, three other components of the total motion of the earth exist: the precession of the equinoxes (see Ecliptic), nutation (periodic variation in the inclination of the earth's axis caused by the gravitational pulls of the sun and moon), and variation of latitude (see Latitude and Longitude).

Composition

The earth consists of five parts: the first, the atmosphere, is gaseous; the second, the hydrosphere, is liquid; the third, fourth, and fifth, the lithosphere, mantle, and core, are largely solid. The atmosphere is the gaseous envelope that surrounds the solid body of the planet. Although it has a thickness of more than 1100 km (more than 700 mi), about half its mass is concentrated in the lower 5.6 km (3.5 mi). The lithosphere, consisting mainly of the cold, rigid, rocky crust of the earth, extends to depths of 100 km (60 mi). The hydrosphere is the layer of water that, in the form of the oceans, covers approximately 70.8% of the surface of the earth. The mantle and core are the heavy interior of the earth, making up most of the earth's mass.

The hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters. The average depth of the oceans is 3794 m (12,447 ft), more than five times the average height of the continents. The mass of the oceans is approximately 1,350,000,000,000,000,000 (1.35 � 1018) metric tons, or about 1/4400 of the total mass of the earth.

The rocks of the lithosphere have an average density of 2.7 and are almost entirely made up of 11 elements, which together account for about 99.5% of its mass. The most abundant is oxygen (about 46.60% of the total), followed by silicon (about 27.72%), aluminum (8.13%), iron (5.0%), calcium (3.63%), sodium (2.83%), potassium (2.59%), magnesium (2.09%) and titanium, hydrogen, and phosphorus (totaling less than 1%). In addition, 11 other elements are present in trace amounts of from 0.1 to 0.02%. These elements, in order of abundance, are carbon, manganese, sulfur, barium, chlorine, chromium, fluorine, zirconium, nickel, strontium, and vanadium. The elements are present in the lithosphere almost entirely in the form of compounds rather than in their free state. These compounds exist almost entirely in the crystalline state, so they are, by definition, minerals.

The lithosphere comprises two shells—the crust and upper mantle—that are divided into a dozen or so rigid tectonic plates (see Plate Tectonics). The crust itself is divided in two. The sialic or upper crust, of which the continents consist, is made up of igneous and sedimentary rocks whose average chemical composition is similar to that of granite and whose density is about 2.7. The simatic or lower crust, which forms the floors of the ocean basins, is made of darker, heavier igneous rocks such as gabbro and basalt, with an average density of about 3.

The lithosphere also includes the upper mantle. Rocks at these depths have a density of about 3.3. The upper mantle is separated from the crust above by a seismic discontinuity, called the Moho, and from the lower mantle below by a zone of weakness known as the asthenosphere. Shearing of the plastic, partially molten rocks of the asthenosphere, 100 km (60 mi) thick, enables the continents to drift across the earth's surface and oceans to open and close.

The dense, heavy interior of the earth is divided into a thick shell, the mantle, surrounding an innermost sphere, the core. The mantle extends from the base of the crust to a depth of about 2900 km (1800 mi). Except for the zone known as the asthenosphere, it is solid, and its density, increasing with depth, ranges from 3.3 to 6. The upper mantle is composed of iron and magnesium silicates, as typified by the mineral olivine. The lower part may consist of a mixture of oxides of magnesium, silicon, and iron.

Seismological research has shown that the core has an outer shell about 2225 km (1380 mi) thick with an average density of 10. This shell is probably rigid, and studies show that its outer surface has depressions and peaks, the latter forming where warm material rises. In contrast, the inner core, which has a radius of about 1275 km (795 mi), is solid. Both core layers are thought to consist largely of iron, with a small percentage of nickel and other elements. Temperatures in the inner core may be as high as 6650�C (12,000�F), and the average density is estimated to be 13.

Internal Heat Flow

Intense heat from the inner core is continually radiated outward, through the several concentric shells that form the solid portion of the planet. The source of this heat is thought to be energy released by the radioactive decay of uranium and other radioactive elements. Convection currents within the mantle transfer most of this heat energy from deep within the earth to the surface and are the driving force behind continental drift. Convective flow supplies hot, molten rock to the worldwide system of midocean ridges (see Ocean and Oceanography) and feeds the lava that erupts from volcanoes on land.

Age and Origin of the Earth

Radiometric dating has enabled scientists to arrive at an estimate of 4.65+ billion years for the age of the earth (see Dating Methods). Although the oldest earth rocks dated this way are not quite 4 billion years old, meteorites, which correlate geologically with the earth's core, give dates of about 4.5 billion years, and crystallization of the core and meteorites is considered to have occurred at the same time, some 150 million years after the earth and solar system first formed (see Solar System: Theories of Origin).

After originally condensing, by gravitational attraction of cosmic dust and gas, the earth would have been almost homogeneous and relatively cool. But continued contraction of these materials caused them to heat, as did the radioactivity of some of the heavier elements. In the next stage of its formation, as the earth became hotter, it began melting under the influence of gravity. This caused the differentiation into crust, mantle, and core, with the lighter silicates moving up and outward to form the mantle and crust and the heavier elements, mainly iron and nickel, sinking downward toward the center of the earth to form the core. Meanwhile, by volcanic eruption, light, volatile gases and vapors continually escaped from the mantle and crust. Some of these, mainly carbon dioxide and nitrogen, were held by the earth's gravity and formed the primitive atmosphere, while water vapor condensed to form the world's first oceans.

Terrestrial Magnetism

The phenomenon of terrestrial magnetism results from the fact that the entire earth behaves as an enormous magnet. The English physician and natural philosopher William Gilbert was the first to demonstrate this similarity in about 1600, although the effects of terrestrial magnetism had been utilized much earlier in primitive compasses.

Magnetic Poles

The magnetic poles of the earth do not correspond with the geographic poles of its axis. The north magnetic pole is presently located off the western coast of Bathurst Island, in the Canadian Northwest Territories, almost 1290 km (almost 800 mi) northwest of Hudson Bay. The south magnetic pole is presently situated at the edge of the Antarctic continent in Ad�lie Land about 1930 km (about 1200 mi) northeast of Little America.

The position of the magnetic poles is not constant and shows an appreciable change from year to year. Variations in the magnetic field of the earth include secular variation, the change in the direction of the field caused by the shifting of the poles. This is a periodic variation that repeats itself after 960 years. A smaller annual variation also exists, as does a diurnal, or daily, variation that can be detected only by sensitive instruments.

Dynamo Theory

Measurements of the secular variation show that the entire magnetic field has a tendency to drift westward at the rate of 19 to 24 km (12 to 15 mi) per year. Clearly the magnetism of the earth is the result of a dynamic rather than a passive condition, which would be the case if the iron core of the earth were solid and passively magnetized. Iron does not retain a permanent magnetism at temperatures above 540� C (1000� F), however, and the temperature at the center of the earth may be as high as 6650� C (12,000� F). The dynamo theory suggests that the iron core is liquid (except at the very center of the earth where the pressure solidifies the core), and that convection currents within the liquid core behave like the individual wires in a dynamo, thus setting up a gigantic magnetic field. The solid inner core rotates more slowly than the outer core, thus accounting for the secular westward drift. The irregular surface of the outer core may help to account for some of the more irregular changes in the field.

Field Intensity

The study of the intensity of the earth's magnetic field is valuable from the points of view of pure science and of engineering, and also for geological prospecting for mineral and energy resources. Intensity measurements are made with instruments called magnetometers, which determine the total intensity of the field and the intensities in the horizontal and vertical directions. The intensity of the magnetic field of the earth varies in different places on its surface. In the temperate zones it amounts to about 0.6 oersted, of which 0.2 oersted is in a horizontal direction.

Paleomagnetism

Studies of ancient volcanic rocks show that as they cooled, they "froze" with their minerals oriented in the magnetic field existing at that time. Worldwide measurements of such mineral deposits show that through geological time the orientation of the magnetic field has shifted with respect to the continents. The north magnetic pole 500 million years ago, for example, lay south of Hawaii, and for the next 300 million years the magnetic equator lay across the United States. To account for this, geologists believe that the outer crust of the earth has gradually shifted around, even though the axis on which the earth spins has remained the same. If this were the case, the climatic belts would have remained the same, but the continents would have drifted slowly through different "paleolatitudes."

Magnetic Reversals

Recent studies of remanent (residual) magnetism in rocks and of magnetic anomalies on the floors of the oceans have shown that the magnetic field of the earth has reversed its polarity at least 170 times in the past 100 million years. Knowledge of these reversals, which can be dated from radioactive isotopes in the rocks, has had a great influence on theories of continental drift and the spreading of ocean floors.

Terrestrial Electricity

Three electrical systems generated in the earth and in the atmosphere by natural geophysical processes are known. One of them is in the atmosphere, and one is within the earth, flowing parallel to the surface of the earth. The third, which transfers an electric charge continuously between the atmosphere and the earth, flows vertically. See Electricity.

Atmospheric electricity, except for that associated with charges within a cloud and lightning, results from the ionization of the atmosphere by solar radiation and from the movement of clouds of ions carried by atmospheric tides (see Ion; Ionization). Atmospheric tides result from the gravitational attraction of the sun and the moon on the earth's atmosphere (see Gravitation; Tide), and, like the oceanic tides, they rise and fall daily. The ionization, and consequently the electrical conductivity, of the atmosphere close to the surface of the earth is low, but it increases rapidly with increasing altitude. Between 40 and 400 km (25 to 250 mi) above the earth, the ionosphere forms an almost perfectly conducting spherical shell. The shell reflects radio signals back to earth and absorbs electromagnetic radiations approaching the earth from space. The ionization of the atmosphere varies greatly, not only with altitude but with the time of day and the latitude.

Earth Currents

Earth currents constitute a worldwide system of eight loops of electric current rather evenly distributed on both sides of the equator, plus a series of smaller loops near the poles. Although it has been contended that this system is induced entirely by the daily changes in atmospheric electricity (and this may be true for short-term variations), it is likely that the origins of the system are more complex. The core of the earth, which consists of molten iron and nickel, is capable of conducting electricity and can be likened to the armature of a huge electric generator. Thermal convection currents in the core are believed to move the molten metal in loop patterns relative to the magnetic field of the earth, producing the system of earth currents that mirror the pattern of convection currents within the core.

The Surface Charge of the Earth

The surface of the earth has a negative charge of electricity. Although the conductivity of air near the earth is small, air is not a perfect insulator, and the negative charge would drain off quickly if it were not being continuously replenished in some way.

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