asthenosphere
   HOME

TheInfoList



OR:

The asthenosphere () is the mechanically weak and
ductile Ductility is a List of materials properties, mechanical property commonly described as a material's amenability to Drawing (manufacturing), drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a materia ...
region of the upper mantle of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large list of largest lakes and seas in the Solar System, volumes of water can be found throughout the Solar System, only water distributi ...
. It lies below the
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not well defined. The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock) contributes to its mechanical weakness. More extensive decompression melting of the asthenosphere takes place where it wells upwards, and this is the most important source of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
on Earth. It is the source of
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
(MORB) and of some magmas that erupted above
subduction Subduction is a geological process in which the oceanic lithosphere is Geochemical cycle, recycled into the Earth's mantle at convergent boundary, convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less d ...
zones or in regions of continental rifting.


Characteristics

The asthenosphere is a part of the upper mantle just below the
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
that is involved in plate tectonic movement and isostatic adjustments. It is composed of
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
, a rock containing mostly the minerals
olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...
and
pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming Silicate minerals#Inosilicates, inosilicate minerals found in many Igneous rock, igneous and metamorphic rock, metamorphic rock (geology), rocks. Pyroxenes have t ...
. The lithosphere-asthenosphere boundary is conventionally taken at the isotherm. Below this temperature (closer to the surface) the mantle behaves rigidly; above this temperature (deeper below the surface) it acts in a
ductile Ductility is a List of materials properties, mechanical property commonly described as a material's amenability to Drawing (manufacturing), drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a materia ...
fashion. The asthenosphere is where the mantle rock most closely approaches its melting point, and a small amount of melt is likely present in this layer.
Seismic wave A seismic wave is a wave of Sound, acoustic energy that travels through the Earth. It can result from an earthquake, types of volcanic eruptions, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produ ...
s pass relatively slowly through the asthenosphere compared to the overlying lithospheric mantle. Thus, it has been called the '' low-velocity zone'' (LVZ), although the two are not strictly the same; the lower boundary of the LVZ lies at a depth of , whereas the base of the asthenosphere lies at a depth of about . The LVZ also has a high seismic attenuation (seismic waves moving through the asthenosphere lose energy) and significant anisotropy (shear waves polarized vertically have a lower velocity than shear waves polarized horizontally). The discovery of the LVZ alerted seismologists to the existence of the asthenosphere and gave some information about its physical properties, as the speed of seismic waves decreases with decreasing rigidity. This decrease in seismic wave velocity from the lithosphere to the asthenosphere could be caused by the presence of a very small percentage of melt in the asthenosphere, though since the asthenosphere transmits S waves, it cannot be fully melted. In the
ocean The ocean (also the sea or the world ocean) is the body of Saline water, salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Water distribution on Earth, Earth's water. An ocean can also refer to any of the ...
ic mantle, the transition from the lithosphere to the asthenosphere (the LAB) is shallower than for the continental mantle (about 60 km in some old oceanic regions) with a sharp and large velocity drop (5–10%). At the
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
s, the LAB rises to within a few kilometers of the ocean floor. The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about. Due to the
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
conditions in the asthenosphere, rock becomes
ductile Ductility is a List of materials properties, mechanical property commonly described as a material's amenability to Drawing (manufacturing), drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a materia ...
, moving at rates of deformation measured in cm/yr over lineal distances eventually measuring thousands of kilometers. In this way, it flows like a
convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoya ...
current, radiating heat outward from the Earth's interior. Above the asthenosphere, at the same rate of deformation, rock behaves elastically and, being brittle, can break, causing faults. The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling isostatic equilibrium and allowing the movement of
tectonic plates Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
.


Boundaries

The asthenosphere extends from an upper boundary at approximately below the surface to a lower boundary at a depth of approximately .


Lithosphere-asthenosphere boundary

The lithosphere-asthenosphere boundary (LAB) is relatively sharp and likely coincides with the onset of partial melting or a change in composition or anisotropy. Various definitions of the boundary reflect various aspects of the boundary region. In addition to the mechanical boundary defined by seismic data, which reflects the transition from rigid lithosphere to ductile asthenosphere, these include a thermal boundary layer, above which heat is transported by thermal conduction and below which heat is conduction largely by
convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoya ...
; a rheological boundary, where the viscosity drops below about 1021 Pa-s; and a chemical boundary layer, above which the mantle rock is depleted in volatiles and enriched in magnesium relative to the rock below.


Lower boundary of asthenosphere

The lower boundary of the asthenosphere is less well defined, but has been placed at the base of the upper mantle. This boundary is neither seismically sharp nor well understood but is approximately coincident with the complex 670 km discontinuity. This discontinuity is generally linked to the transition from mantle rock containing ringwoodite to mantle rock containing bridgmanite and periclase.


Origin

The mechanical properties of the asthenosphere are widely attributed to partial melting of the rock. It is likely that a small amount of melt is present through much of the asthenosphere, where it is stabilized by the traces of volatiles (water and carbon dioxide) present in the mantle rock. However, the likely amount of melt, not more than about 0.1% of the rock, seems inadequate to fully explain the existence of the asthenosphere. This is not enough melt to fully wet grain boundaries in the rock, and the effects of melt on the mechanical properties of the rock are not expected to be significant if the grain boundaries are not fully wetted. The sharp lithosphere-asthenosphere boundary is also difficult to explain by partial melting alone. It is possible that melt accumulates at the top of the asthenosphere, where it is trapped by the impermeable rock of the lithosphere. Another possibility is that the asthenosphere is a zone of minimum water solubility in mantle minerals, so that more water is available to form greater quantities of melt. Another possible mechanism for producing mechanical weakness is grain boundary sliding, where grains slide slightly past each other under stress, lubricated by the traces of volatiles present. Numerical models of mantle convection in which the viscosity is dependent both on temperature and strain rate reliably produce an oceanic asthenosphere, suggesting that strain-rate weakening is a significant contributing mechanism.


Magma generation

Decompression melting of asthenospheric rock creeping towards the surface is the most important source of
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
on Earth. Most of this erupts at
mid-ocean ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
s to form the distinctive mid-ocean ridge
basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
(MORB) of the ocean crust. Magmas are also generated by decompressional melting of the asthenosphere above
subduction zones Subduction is a geological process in which the oceanic lithosphere is Geochemical cycle, recycled into the Earth's mantle at convergent boundary, convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less d ...
and in areas of continental rifting. Decompression melting in upwelling asthenosphere likely begins at a depth as great as , where the small amounts of volatiles in the mantle rock (about 100 ppm of water and 60 ppm of
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetraval ...
) assist in melting not more than about 0.1% of the rock. At a depth of about , dry melting conditions are reached and melting increases substantially. This dehydrates the remaining solid rock and is likely the origin of the chemically depleted lithosphere.


See also

*


References


Bibliography

* *


External links


San Diego State University, "The Earth's internal heat energy and interior structure"
{{Authority control Structure of the Earth Plate tectonics