Structure of the Earth


Structure of the Earth
Earth cutaway from core to exosphere. Left picture is not to scale.

The interior structure of the Earth, similar to the outer, is layered. These layers can be defined by either their chemical or their rheological properties. 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. Scientific understanding of Earth's internal structure is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of the gravity field of the Earth, and experiments with crystalline solids at pressures and temperatures characteristic of the Earth's deep interior.

Contents

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. Astronomers can also calculate Earth's mass from its orbit and effects on nearby planetary bodies. Observations of rocks, bodies of water and atmosphere allow estimation of the mass, volume and density of rocks to a certain depth, so the remaining mass must be in the deeper layers.

Structure

Earth's radial density distribution according to the preliminary reference earth model (PREM).
Earth's gravity according to the preliminary reference earth model (PREM). Comparison to approximations using constant and linear density for Earth's interior.

The structure of Earth can be defined in two ways: by mechanical properties such as rheology, or chemically. Mechanically, it can be divided into lithosphere, asthenosphere, mesosphere, outer core, and the inner core. The interior of the earth is divided into 5 important layers. Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth[1] 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
35–2,890 22–1,790 Mantle
100–200 62–125 … Asthenosphere
35–660 22–410 … Upper mesosphere (upper mantle)
660–2,890 410–1,790 … Lower mesosphere (lower mantle)
2,890–5,150 1,790–3,160 Outer core
5,150–6,360 3,160–3,954 Inner core
Mapping the interior of the Earth with earthquake waves.

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 other layers. The changes in seismic velocity between 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 5,515 kg/m3. Since the average density of surface material is only around 3,000 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 ~1,220 km[2] and a liquid outer core extending beyond it to a radius of ~3,400 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 (4.5×109) 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.[3][4]

On August 30, 2011, Professor Kei Hirose, professor of high-pressure mineral physics and petrology at the Tokyo Institute of Technology, became the first person to recreate conditions found at the earth's core under laboratory conditions, subjecting a sample of iron nickel alloy to the same type of pressure by gripping it in a vice between 2 diamond tips, and then heating the sample to approximately 4000 Kelvins with a laser. The sample was observed with x-rays, and strongly supported the theory that the earth's inner core was made of giant crystals running north to south.[5]

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 siderophile elements.[6]

The matter that comprises Earth is connected in fundamental ways to matter of certain chondrite meteorites, and to matter of outer portion of the Sun.[7][8] 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 Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilize the magnetic field generated by the liquid outer core. The average magnetic field strength in the Earth's outer core is estimated to be 25 Gauss, 50 times stronger than the magnetic field at the surface.[9][10]

Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet.[11] 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.[12][13]

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

Mantle

Schematic view of the interior of Earth. 1. continental crust - 2. oceanic crust - 3. upper mantle - 4. lower mantle - 5. outer core - 6. inner core - A: Mohorovičić discontinuity - B: Gutenberg Discontinuity - C: Lehmann discontinuity

Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of the 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.[14] In comparison, the viscosity of water is approximately 10−3 Pa·s and that of pitch is 107 Pa·s.

Crust

The crust ranges from 5–70 km in depth and is the outermost layer. The thin parts are the oceanic crust, which underlie the ocean basins (5–10 km) and are composed of dense (mafic) iron magnesium silicate rocks, like basalt.The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks, like granite. The rocks of the crust fall into two major categories - sial and sima (Suess,1831–1914). As the main mineral constituents of the continental mass are silica and aluminium, it is thus called sial (si-silica, 65–75% and al-aluminium). The oceanic crust mainly consists of silica and magnesium. it is therefore called sima (si-silica and ma-magnesium). It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere. 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, in oceanic crust, 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 onto the continental crust and preserved as ophiolite sequences.

Many rocks now making up Earth's crust formed less than 100 million (1×108) years ago; however, the oldest known mineral grains are 4.4 billion (4.4×109) years old, indicating that Earth has had a solid crust for at least that long.[15]

Historical development of alternative conceptions

Edmond Halley's hypothesis.

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.[16] 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?"[16]

See also

DirkvdM rocks.jpg Earth sciences portal

Notes

  1. ^ T. H. Jordan, "Structural Geology of the Earth's Interior", Proceedings of the National Academy of Science, 1979, Sept., 76(9): 4192–4200.
  2. ^ Monnereau, Marc; Calvet, Marie; Margerin, Ludovic; Souriau, Annie (May 21, 2010), "Lopsided Growth of Earth's Inner Core", Science 328 (5981): 1014–1017, doi:10.1126/science.1186212 
  3. ^ Cohen, Ronald; Stixrude, Lars. "Crystal at the Center of the Earth". http://www.psc.edu/science/Cohen_Stix/cohen_stix.html. Retrieved 2007-02-05. 
  4. ^ Lars Stixrude and R. E. Cohen, "High-Pressure Elasticity of Iron and Anisotropy of Earth's Inner Core", Science 31 March 1995: Vol. 267. no. 5206, pp. 1972 - 1975 DOI: 10.1126/science.267.5206.1972
  5. ^ BBC News, "What is at the centre of the Earth?
  6. ^ Wootton, Anne (September 2006) "Earth's Inner Fort Knox" Discover 27(9): p.18;
  7. ^ Herndon, J. M., The chemical composition of the interior shells of the Earth. Proc. R. Soc. Lond, 1980, A372, 149-154.
  8. ^ Herndon, J. M., Scientific basis of knowledge on Earth's composition. Curr.Sci., 2005, 88(7), 1034-1037.
  9. ^ http://www.science20.com/news_articles/first_measurement_magnetic_field_inside_earths_core
  10. ^ http://www.nature.com/nature/journal/v468/n7326/full/nature09643.html
  11. ^ Chang, Kenneth (2005-08-25). "Earth's Core Spins Faster Than the Rest of the Planet". The New York Times. http://www.nytimes.com/2005/08/25/science/25cnd-core.html. Retrieved 2010-05-24. 
  12. ^ Kerr, Richard A. (26 August 2005) "Earth's Inner Core Is Running a Tad Faster Than the Rest of the Planet" Science 309(5739): p.1313;
  13. ^ Chang, Kenneth (26 August 2005) "Scientists Say Earth's Center Rotates Faster Than Surface" The New York Times Sec. A, Col. 1, p.13;
  14. ^ http://www2.uni-jena.de/chemie/geowiss/geodyn/poster2.html
  15. ^ Spaceflight Now | Breaking News | Oldest rock shows Earth was a hospitable young planet
  16. ^ a b N. Kollerstrom, 1992. "The hollow world of Edmond Halley" from Journal for History of Astronomy 23, 185-192

References

  • Kruglinski, Susan. "Journey to the Center of the Earth." Discover.
  • Lehmann, I. (1936) Inner Earth, Bur. Cent. Seismol. Int. 14, 3-31
  • Schneider, David (October 1996) A Spinning Crystal Ball, Scientific American
  • Wegener, Alfred (1915) "The Origin of Continents and Oceans"

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