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گوشته زمین-Earth mantle

Earth mantle

At the high pressures and temperatures found in the Earth's mantle, rock responds to stress by slow, creeping flow. The mantle, although solid, is a dynamic system. Convection is driven by density variations primarily caused by temperature variations and possibly also by compositional variations. Hot, relatively low-density material wells up to the surface, and cold, higher-density material plunges into the interior. This phenomenon influences the Earth's surface processes and drives the motion of the tectonic plates.

A long-standing question in mantle convection concerns the nature of this flow and whether the mantle has a uniform bulk composition or is stratified at some depth. All the evidence regarding the structure and dynamics of the mantle is obtained indirectly by observations made at the surface. The most important source of information comprises observations of seismic velocity variations, which are affected by temperature and composition. Over the past several decades, the accumulation of more and better data from global seismic networks has made it possible for seismologists to generate clearer views of the structure of the mantle on global and regional scales. Additional information comes from other types of observations, including measurements of the composition of volcanic rocks derived from the Earth, the fluxes of noble gases from the mantle, the surface heat flow, the heat production of mantle materials, and the Earth's shape and gravity field (which are related to internal density variations). Forward modeling of convection on computers and by laboratory analogs provides additional insight into the processes occurring in the mantle.

 

Seismic velocity variations

 

The velocity of seismic waves traveling through the Earth varies with the composition, temperature, and phase of the material that the wave is traveling through. The overall radial structure of the Earth is well known from these seismic velocities. The crust and mantle are composed primarily of solid oxide crystals, while the core is composed of iron alloy and is molten from the core-mantle boundary to a depth of 5150 km (3193 mi) [see illus.]. Several internal interfaces have been identified within the mantle. One such interface is the mantle transition zone at a depth of 670 km (415 mi). At shallower depths, the mantle consists largely of the oxide crystals olivine, pyroxene, and garnet; below this depth, pressures and temperatures are such that perovskites dominate.

 

 

Schematic cross section of the Earth. Note the subducted slab and possible layering in the mantle. Seismic waves from earthquakes travel through the Earth and are recorded by seismometers, revealing the structure of the interior of the planet.

 

 

 

 

 

 

Mapping out the regions in which seismic velocity diverges from its average value at a given depth provides a means for inferring the structure caused by mantle convection and for searching for compositional layering in the mantle. Seismological observations of compressional-wave (P) velocity and shear-wave (S) velocity typically show anomalously fast velocities associated with subducted slabs. These high velocities are likely due to colder temperatures in the slabs, which were cooled at the surface of the Earth before plunging into the interior. As slabs reach the mantle transition zone, the slabs appear to deform, thicken, and bend. Seismic images show that deformed subducted slabs penetrate through the mantle transition zone and into the deep mantle to at least 1700 km (1054 mi) depth. Seismic signals from the slabs break up in the lowermost 1000 km (620 mi) of the mantle, suggesting that slabs may not extend all the way to the core-mantle boundary.

At very great depths, there is evidence that lateral variations in composition play a role in seismic velocity. The lowermost mantle exhibits large-amplitude lateral variations in S- and P-wave speed, indicating a mix of different compositions and variations in temperature. In some regions, S-wave speed is not positively correlated with P-wave speed as would be expected if the velocity variations were due to temperature alone. Regions of anomalous composition also appear to scatter high-frequency seismic waves in the lower mantle.

After very large earthquakes, such as the magnitude-8.4 Bolivia earthquake of 1994, long-period oscillations are recorded which are sensitive to long-wavelength structure in the Earth. The periods of free oscillations in the Earth depend on the seismic velocity and the Earth's density as a function of depth. Comparing density and seismic velocity data obtained from free oscillations to models that assume a uniform composition for the mantle suggests that the lower mantle is slightly (up to 2%) more dense than it would be if it had the same composition as the upper mantle. The free-oscillation data indicate that material that is intrinsically dense but hot and therefore somewhat buoyant may be welling up beneath Africa and the Pacific.

 

Heat production and surface heat flow

 

Mantle convection cools the interior of the Earth. Heat generated by radioactive decay of potassium, uranium, and thorium, and heat left from the formation and early differentiation of the Earth, are lost by cooling of the plates at the surface. Heat currently flows out of the Earth at a rate of 44 terawatts. Some of this (6 TW) is generated by radioactive decay within the crust. The remainder is provided by a combination of heat generation within the mantle and core (which is then transferred out of the Earth by mantle convection), or by cooling of the planet.

The exact proportions of current heat generation to cooling are not known and vary over time, but an approximate heat “budget” can be estimated. It is thought that the Earth contains heat-producing elements in approximately the same proportions as the class of meteorites known as chondrites. A chondritic Earth would have a total heat production and loss of 31 TW, with the remaining 13 TW provided by cooling at an average rate of 65°C (117°F) per billion years. The rate of cooling is expected to have been higher in the Archean, and 3 billion years ago heat production was approximately twice as high as the current rate.

 

Heat-producing elements

 

The concentration of heat-producing elements in the upper mantle is estimated from the composition of mid-ocean ridge basalts (MORB), which form by partial melting and melt extraction at the spreading centers between divergent tectonic plates. As diverging plate boundaries migrate randomly over the Earth's surface, the MORB source provides a reasonable approximation of average upper mantle. The source region of MORB is quite uniform in composition, and is depleted in heat production by a factor of 5 to 10 relative to the value that would be expected in the mantle of a chondritic Earth. Thus if the entire mantle has the same composition as the MORB source, it would currently produce only 2–6 TW, comparable to the amount currently produced in the much smaller volume of crust. The remainder of the current heat flow from the Earth would have to originate from cooling of the planet, at an exceptionally high rate of 175°C (315°F) per billion years. Extrapolating this rate of heat loss back to the early Earth would produce mantle temperatures that are not compatible with the composition of magmas produced during the Archean. Thus the heat flow data for the Earth predict that there is an additional reservoir that contains a higher concentration of heat-producing elements than the MORB source region.

 

Outgassing of noble gases

 

The process of melting that creates MORB and oceanic island basalts also outgasses noble gases such as helium and argon into the atmosphere. Like the heat budget, the argon budget can be used to infer the composition of the mantle. The isotope argon-40 is produced as a product of radioactive decay of potassium-40. Only about half of the radiogenic argon that must have been produced in the Earth resides in the atmosphere; the rest remains stored in the interior of the planet, most likely within the mantle, although some may reside in the crust. If the composition of the mantle is like that of the MORB source region, it would not be able to account for the missing argon. However, a layer deep within the mantle could hold the extra radiogenic argon through billions of years of Earth's history.

 

Computer and analog models of convection

 

Further insight into the dynamical processes within the mantle arises from numerical simulations of thermo-chemical convection on the computer and from analog models in the laboratory. Advances in computer power, especially the advent of parallel computers, are beginning to make it possible to realistically simulate these processes. Numerical simulations of thermochemical convection have been carried out to try to understand how intrinsically dense but hot material would behave in the deep mantle. These models indicate that intrinsically dense material would tend to pile up under mantle upwellings and be deflected downward beneath subducted slabs. The intrinsic high density is balanced to some extent by the reduction in density due to high temperatures in the deep mantle. Thus any compositional interface in the deep mantle is likely to exhibit substantial topography. Some recent data indicate a large zone of anomalously slow seismic velocity beneath Africa; this has been interpreted as a large mantle upwelling which may be associated with ongoing volcanic activity in the Afar rift. Anomalously hot mantle beneath Africa would explain the elevated topography of this continent. Ongoing seismic experiments and numerical and laboratory models will make it possible to produce more accurate images of the deep mantle structure within the next few years.

 See also: Earth; Earth, convection in; Earth, heat flow in; Mid-Oceanic Ridge; Seismology; Subduction zones

Louise Kellogg

 

Bibliography

 

 

  • A. W. Hofmann, Mantle geochemistry: The message from oceanic volcanism, Nature, 385:219–229, 1997
  • R. Jeanloz and T. Lay, The coremantle boundary, Sci. Amer., 268:48–55, May 1993
  • L. H. Kellogg, B. H. Hager, and R. Van der Hilst, Compositional stratification in the deep mantle, Science, 283:1881–1884, 1999
  • R. D. Van der Hilst, S. Widiyantoro, and E. R. Engdahl, Evidence for deep mantle circulation from global tomography, Nature, 386:578–584, 1997
  •  Alifazeli=egeology.blogfa.com

 

Additional Readings

 

 

  • L. Breger and B. Romanowicz, Three-dimensional structure at the base of the mantle beneath the central Pacific, Science, 282:718–720, 1998
  • U. R. Christensen, The influence of trench migration on slab penetration into the lower mantle, Earth Planet. Sci. Lett., 140:27–39, 1996
  • A. M. Dziewonski, Global seismic tomography of the mantle, Rev. Geophys., 33:419–423, 1995
  • Ishii and Tromp, Normal-mode and free-air gravity constraints on lateral variations in velocity and density of Earth's mantle, Science, 285:1231–1236, 1999
  • Y. LeStunff, C. W. Wicks, and B. Romanowicz, P′ P′ precursors under Africa: Evidence for mid-mantle reflectors, Science, 270:74–77, 1995
  • C. Lithgow-Bertelloni and P. G. Silver, Dynamic topography, plate driving forces and the African superswell, Nature, 395:269–272, 1998
  • P. J. Tackley, Three-dimensional simulation of mantle convection with a terhmo-chemical basal boundary layer: D′′?, in M. Gurnis et al. (eds.), The Core-Mantle Boundary Region, American Geophysical Union, Geodynamics Series, vol. 28, 1998
  • L. X. Wen and D. L. Anderson, Layered mantle convection: A model for geoid and topography, Earth Planet. Sci. Lett., 146:367–377, 1997
  • Earth Reference Models, including the Geochemical Earth Reference Model
  • Pages about mantel convection and plate tectonics by an Australian Scientist
  • Page about the Earth's interior by a Scientist at the University of Nevada
  • Information about how earthquakes are used to study the Earth, from the IRIS consortium
  • This Dynamic Earth, a page on plate tectonics from the USGS stratification of the Earth's lower mantle  
  • Alifazeli=egeology.blogfa.com

 

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