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seismol

Structure of the Earth

The interior of Earth, similar to the other terrestrial planets, is chemically divided into layers. 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. Many of the rocks now making up 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 Earth has had a solid crust for at least that long. Most of Earth's structure is theoretical, being based on extrapolations of physical evidence which has come from the first few kilometres of Earth's surface, samples brought to the surface from deeper depths by volcanic activity, and analysis of seismic waves that pass through it.

Assumptions

The force exerted by Earth's gravity can be used to calculate its mass, and by estimating the volume of the planet, its average density can be calculated. Astronomy can also calculate Earth's mass from its orbit and effects on nearby planetary bodies. Observation of rocks, bodies of water and the atmosphere allow estimation of the mass, volume and density of rocks to a certain depth, the remaining mass must be in the deeper layers.

Structure

The structure of Earth can be visualised in two ways: chemically or by material properties. Chemically, Earth can be divided into the crust, mantle, outer core, and inner core. By comparing material strength, the layering of Earth is categorized as lithosphere, asthenosphere, upper mantle, lower mantle, outer core, and the inner core. The geologic component layers of Earth are at the following depths below the surface:

Depth Layer
Kilometres Miles
0–60 0–37 Lithosphere (locally varies between 5 and 200 km)
0–35 0–22 Crust (locally varies between 5 and 70 km)
35–60 22–37 … Uppermost part of mantle
35–2890 22–1790 Mantle
100–200 62–125 Asthenosphere
35–660 22–410 … Upper mantle
660–2890 410–1790 … Lower mantle
2890–5150 1790–3160 Outer core
5150–6360 3160–3954 Inner core

The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in the other layers. The changes in the seismic velocity between the different layers causes refraction owing to Snell's law. Reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror.

Core

The average density of 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 Earth's core. Further evidence for the high density core comes from the study of seismology.

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 ~3400 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.

In early stages of Earth's formation 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 (see also the iron catastrophe), while less-dense materials would have migrated to the crust. The core is thus believed to largely be 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). 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.

Recent speculation suggests that the innermost part of the core is enriched in gold, platinum and other iron-loving (siderophile) elements.

The matter that Earth is composed of is connected in fundamental ways to the matter of certain chondrite meteorites, and to the matter of the outer portion of the Sun . There is good reason to believe that Earth is, in the main, like a chondrite meteorite. Beginning as early as 1940, scientists, including Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth, while totally ignoring another, albeit less abundant type, called enstatite chondrites. The principal difference between the two meteorite types is that enstatite chondrites formed under circumstances of extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.

Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to the Earth's magnetic field. 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, Earth's inner core rotates approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface.

The current scientific explanation for the Earth's temperature gradient is a combination of the heat left over from the planet's initial formation, the decay of radioactive elements, and the freezing of the inner core.

Mantle

Earth's mantle extends to a depth of 2890 km, making it the largest layer of Earth. 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.

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, like basalt. The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks, like granite. 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.

Historical development of 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 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?” This theory of the "Hollow earth" is now understood to be totally inexact, although a feature of Conspiracy Theories.

It has been also suggested that Earth's core consists of metallic hydrogen.

See also

Notes

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

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