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This discussion of the Earth's structure is an overview of its geology and the nature of its atmosphere.

Shape

The Earth's shape is that of an oblate spheroid, with an average diameter of approximately 12,742 km (~ 40,000 km / π). The rotation of the Earth causes the equator to bulge out slightly so that the equatorial diameter is 43 km larger than the pole to pole diameter. The largest local deviations in the rocky surface of the Earth are Mount Everest (8,850 m above local sea level) and the Mariana Trench (10,911 m below local sea level). Hence compared to a perfect ellipsoid, the Earth has a tolerance of about one part in about 584, or 0.17%. For comparison, this is less than the 0.22% tolerance allowed in billiard balls. Due to the bulge, the feature farthest from the center of the Earth is actually Mount Chimborazo in Ecuador. The mass of the Earth is approximately 5980 yottagrams (5.98 x 1024 kg).

Structure

The interior of the Earth, like that of the other terrestrial planets, is chemically divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. The liquid outer core gives rise to a weak magnetic field due to the convection of its electrically conductive material.

New material constantly finds its way to the surface through volcanoes and cracks in the ocean floors (see seafloor spreading). Many of the rocks now making up the Earth's crust formed less than 100 million (1) years ago; however the oldest known mineral grains are 4.4 billion (4.4) years old, indicating that the Earth has had a solid crust for at least that long *.

Taken as a whole, the Earth's composition by mass * is:

iron:35.1%
oxygen:28.2%
silicon:17.2%
magnesium:15.9%
nickel:1.6%
calcium:1.6%
aluminium:1.5%
sulfur:0.70%
sodium:0.25%
titanium:0.071%
potassium:0.019 %
gold:?.? %
plutonium:?.? %
uranium:?.? %
thorium:?.? %
other elements:    0.53%

Interior

Interior heat

The interior of the Earth reaches temperatures of 5650 ± 600 kelvins [http://www.carnegieinstitution.org/news_010905.html. This heat is radiated through the crust of the earth at the rate of approximately 6.18 x 10-12 Watts per kilogram. Since the earth contains 5.97 x 1024 kilograms, this enables us to calculate that the Earth, if in thermostatic equilibrium, must be producing energy at the rate of 36.92 x 1012 Watts (36.9 Terawatts). Recent studies have shown that 24 Terawatts of this energy comes from radioactive decay. The remaining 12.9 Terawatts of the planet's internal heat was originally generated during its accretion (see gravitational binding energy), currently expressed by the continued sorting of materials between the mantle and the core. The additional heat from radioactivity also has a half life of about 1 billion years, with the result that the Earth currently has less than 6.2% of its original radioactive energy, which continues to be generated by the decay of radioactive elements such as uranium, thorium, and potassium. The heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun. 90% of this energy is radiated through mid ocean ridges and the force of sea floor spreading, with the remaining 10% being carried away from the core by mantle plumes.

Structure

Earth's composition (by depth below surface):
  • 0 to 60 km - Lithosphere (locally varies 5-200 km)
    • 0 to 35 km - Crust (locally varies 5-70 km)
    • 35 to 60 km - Uppermost part of mantle
  • 35 to 2890 km - Mantle
  • 2890 to 5100 km - Outer core
  • 5100 to 6378 km - Inner core

Inner core

In the Herndon view of the inner core, a fully crystallized inner core of nickel silicide would have precisely the mass measured for the inner core. It is not necessary to postulate a growing inner core, because, if the core of the Earth is like the alloy portion of certain highly reduced enstatite chondrites, major proportions of uranium and, presumably, thorium will exist within the Earth's core; high-temperature precipitation and gravitationally driven accumulation will inevitably lead to a fissionable mass. The geochemical and geophysical basis for uranium and thorium occurring in the Earth's core precipitating and accumulating at the planet's center is an emerging subject of discussion.

At the pressures that prevail in the Earth's core, density is a function almost exclusively of atomic mass and atomic number. Uranium, thorium, and other actinides, being high-temperature precipitates and the densest substances, by the action of gravity, would tend to concentrate, possibly scavenged by other precipitates, ultimately forming a fissionable, critical mass. The same mechanism for concentrating the actinides (i.e., gravitational separation by density at high pressure) should cause the lighter fission products to separate from the heavier actinides, thus helping to maintain a nuclear-reactor-critical configuration.

The frequent, but irregular, variability in intensity and direction of the Earth's magnetic field may be understandable from a fissionogenic energy-production standpoint---a consequence of fission-product accumulation with concomitant nuclear fuel dilution and the subsequent gravitationally driven fission product separation with nuclear fuel reconcentration. Unlike other globally significant energy sources, nuclear reactor output can be variable or intermittent, depending on changes in composition and/or the position of fuel, moderators, and neutron absorbers. For example, one might imagine instances in which the rate of production of fission products exceeds their rate of removal by gravitationally driven diffusion. In such an instance, the power output of the geo-reactor would decrease and the reactor might eventually shut down, thereby diminishing and ultimately shutting down the Earth's magnetic field. As the fission products diffuse out of the reactor region to a region of lower density and the actinide fuel diffuses inward, the reactor restarts. As the reactor increases in power, the geomagnetic field reestablishes itself, either in the same direction or in the reverse direction.Ref.

Core

The average density of the Earth is 5515 kg/m3, making it the densest planet in the Solar system. Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within the core of the Earth. Further evidence for the high density core comes from the study of seismology. In its earliest stages, about 4.5 billion (4.5) years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust. As a result, the core is largely composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials).

Seismic measurements show that the core is divided into two parts, a solid inner core with a radius of ~1220 km and a liquid outer core extending beyond it to a radius of ~3480 km. The solid inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel. Some have argued that the inner core may be in the form of a single iron crystal *. The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements. It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation (see: Coriolis effect), gives rise to the Earth's magnetic field through a process described by the dynamo theory. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core.

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet. In August 2005 a team of geophysicists announced in the journal Science that, according to their estimates, the earth's inner core rotates approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface *.

While the scientifically mainstream explanation for these temperatures gradients is that the heat is simply left over from the planet's initial formation (see above), A theory espoused by J. Marvin Herndon states that fast breeder nuclear reactor type reactions occur in the core of the earth.

Mantle

Earth's mantle extends to a depth of 2890 km. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales. Convection of the mantle is expressed at the surface through the motions of tectonic plates. The melting point and viscosity of a substance depends on the pressure it is under. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 1021 and 1024 Pa·s, depending on depth, *. In comparison, the viscosity of water is approximately 10-3 Pa·s and that of pitch 107 Pa·s. Thus, the mantle flows very slowly.

Why is the inner core solid, the outer core liquid, and the mantle solid/plastic? The answer depends both on the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and on the increase in temperature and pressure as one moves deeper into the Earth. At the surface both nickel-iron alloys and silicates are sufficiently cool to be solid. In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity. In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle. The metallic nickel-iron outer core is liquid despite the enormous pressure as it has a melting point that is lower than the mantle silicates. The inner core is solid due to the overwhelming pressure found at the center of the planet.

Crust

The crust ranges from 5 to 70 km in depth. The thin parts are oceanic crust composed of dense (mafic) iron magnesium silicate rocks and underlie the ocean basins. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks. The crust-mantle boundary occurs as two physically different events. First, there is a discontinuity in the seismic velocity, which is known as the Mohorovičić discontinuity or Moho. The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted into the continental crust and preserved as ophiolite sequences.

Hydrosphere

Earth is the only planet in our solar system whose surface is known to have liquid water. Water covers 71% of Earth's surface (97% of it being sea water and 3% fresh water *); the surface is divided into five oceans and seven continents. Earth's solar orbit, vulcanism, gravity, greenhouse effect, magnetic field and oxygen-rich atmosphere seem to combine to make Earth a water planet.

Earth is actually beyond the outer edge of the orbits which would be warm enough to form liquid water. Without some form of a greenhouse effect, Earth's water would freeze. Paleontological evidence indicates that at one point after blue-green bacteria (Cyanobacteria) had colonized the oceans, the greenhouse effect failed, and Earth's oceans may have completely frozen over for 10 to 100 million years in what is called a snowball Earth event.

On other planets, such as Venus, gaseous water is destroyed (cracked) by solar ultraviolet radiation, and the hydrogen is ionized and blown away by the solar wind. This effect is slow, but inexorable. This is one hypothesis explaining why Venus has no water. Without hydrogen, the oxygen interacts with the surface and is bound up in solid minerals.

In the Earth's atmosphere, a tenuous layer of ozone within the stratosphere absorbs most of this energetic ultraviolet radiation high in the atmosphere, reducing the cracking effect. The ozone, too, can only be produced in an atmosphere with a large amount of free diatomic oxygen, and so also is dependent on the biosphere (plants). The magnetosphere also shields the ionosphere from direct scouring by the solar wind.

Finally, vulcanism continuously emits water vapor from the interior. Earth's plate tectonics recycle carbon and water as limestone rocks are subducted into the mantle and volcanically released as gaseous carbon dioxide and steam. It is estimated that the minerals in the mantle may contain as much as 10 times the water as in all of the current oceans, though most of this trapped water will never be released.

The total mass of the hydrosphere is about 1.4 kg, ca. 0.023% of the Earth's total mass.

Atmosphere

Earth has a relatively thick atmosphere composed of 78% nitrogen, 21% oxygen, and 1% argon, plus traces of other gases including carbon dioxide and water vapor. The atmosphere acts as a buffer between Earth and the Sun. The Earth's atmospheric composition is unstable, and is maintained by the biosphere. The large amount of free diatomic oxygen is maintained through solar energy by the Earth's plants, and, without the plants supplying it, the oxygen in the atmosphere will over geological timescales combine with material from the surface of the Earth. Free oxygen in the atmosphere is a signature of life.

The layers, troposphere, stratosphere, mesosphere, thermosphere, and the exosphere, vary around the globe and in response to seasonal changes.

The total mass of the atmosphere is about 5.1 kg, ca. 0.9 ppm of the Earth's total mass.

Historical development and alternative conceptions

In 1692 Edmund Halley (in a paper printed in Philosophical Transactions of Royal Society of London) put forth the idea of Earth consisting of a hollow shell about 500 miles thick, with two inner concentric shells around an innermost core, corresponding to the diameters of the planets Venus, Mars, and Mercury respectively. Halley's construct was a method of accounting for the (flawed) values of the relative density of the Earth and the Moon that had been given by Sir Isaac Newton, in Principia (1687).“Sir Isaac Newton has demonstrated the Moon to be more solid than our Earth, as 9 to 5" Halley remarked; "why may we not then suppose four ninths of our globe to be cavity?” (quoted Kollerstrom 1992).

In 1818, John Cleves Symmes, Jr. suggested that the Earth consisted of a hollow shell about 800 miles (1,300 km) thick, with openings about 1400 miles (2,300 km) across at both poles with 4 inner shells each open at the poles. Jules Verne, in Journey to the Center of the Earth imagined vast interior caverns, and William Reed, in Phantom of the Poles (1906) imagined a hollow earth.

Some Christian writers resisted the idea of a spherical Earth on theological grounds, without gaining widespread acceptance. The Flat Earth Society, previously presided by Charles K. Johnson, in the USA work hard to keep the concept alive, and have claimed a few thousand followers *. Some Christians in England and the United States tried to revive flat earth thinking in the 19th century.

The fact, that Kola Superdeep Borehole, drilled in the USSR, by 1989 reached the depth of 12,262 meters and became the deepest hole ever made by humans, resulted in speculations. Namely, an urban legend "well to hell hoax", that first appeared in English through a 1989 dispatch created by the Trinity Broadcasting Network: according to the hoax, often reported as fact by some Christian sites *, scientific drilling at the Kola Peninsula had broken through to Hell.

See also

References

  • Herndon, J. Marvin (1994) Planetary and Protostellar Nuclear Fission: Implications for Planetary Change, Stellar Ignition and Dark Matter Proceedings: Mathematical and Physical Sciences, Vol. 445, No. 1924 (May 9, 1994) , pp. 453-461
  • Herndon, J. Marvin (1996) Substructure of the inner core of the Earth Vol. 93, Issue 2, 646-648, January 23, 1996, PNAS
  • Hollenbach, D. F. ,dagger and J. M. HerndonDagger (2001) Deep-Earth reactor: Nuclear fission, helium, and the geomagnetic field Published online before print September 18, 2001, 10.1073/pnas.201393998, September 25, 2001, vol. 98, no. 20, PNAS
  • Lehmann, I. (1936) Inner Earth, Bur. Cent. Seismol. Int. 14, 3-31
  • Schneider, David (Oct 1996) A Spinning Crystal Ball, Scientific American

Geology Earth

stavba Zeme | Struktura Zemlje

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Structure of the Earth".

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