Asthenosphere

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Inner structure of the earth
Layers according to chemical composition:
1 earth crust, 2 earth mantle, 3 earth core
( 3a outer earth core, 3b inner earth core)
Layers according to mechanical properties:
4 lithosphere, 5 asthenosphere,
6 outer earth core, 7 inner earth core
The outer layers of the earth in connection with plate tectonics (orange: asthenosphere)

The asthenosphere ( Greek  ἀσθενής asthenés “powerless, weak”), also called the Low Velocity Zone in geophysics , is the second outermost layer of the rheological model of the Earth's interior and joins below the lithosphere . It begins, depending on the thickness of the lithosphere, at a depth of 60 to 210 km and extends down to a depth of 300 to 410 km. To simplify matters, their mechanical behavior can be described as ductile ( plastically deformable ). The comparatively rigid and less dense lithospheric plates move on the asthenosphere .

properties

In terms of mineral composition, the asthenosphere does not differ from the rock in the upper mantle . The main ingredients are olivine , orthopyroxene , spinel and garnet . The mean density is 3300 kg / m³.

The far more significant difference between the asthenosphere and the layers on top and below it is their mechanical properties. To spread the seismic wave from within the Asthenosphäre slower than in the immediately adjacent layers. The English designation low velocity zone (LVZ) refers to this . Depending on the definition, the LVZ is synonymous with the asthenosphere, or it only covers the upper 100 km. The mean speed of the P waves drops from 8.3 km / s below the Mohorovičić discontinuity to less than 8 km / s below the lithosphere-asthenosphere boundary (LAB), only to increase again with increasing depth. Ultimately, it rises to 13.6 km / s up to the Earth's core. Shear waves (S waves) are dampened by the partially melted rock.

The dynamic viscosity of the asthenosphere can be estimated from postglacial uplifts . It lies between 10 20  Pascal seconds (Pa · s) under mid-ocean ridges , subduction zones and other seismically very active regions and 10 24  Pa · s under cratons . The viscosity of the crust is  significantly higher at more than 10 25 Pa · s. For comparison: the viscosity of glass is approx. 10 17  Pa · s.

The flowability of the asthenosphere depends on the solidus temperature , the content of rock water and the geothermal gradient. The solidus temperature is the pressure (and thus the depth) dependent temperature at which the rock begins to melt. If this temperature is exceeded in the asthenosphere, 1–5% of the rock can partially melt ( magma ). The formation of the melt is mainly caused by the instability of water-containing silicate minerals at this depth. Even in small amounts, water causes a significant reduction in the solidus temperature. It comes from orthopyroxene and to a lesser extent from olivine. In the presence of aluminum, orthopyroxene can bind larger amounts of water, which it releases again at a depth of 100 to 150 km when the pressure increases. The viscosity also depends on the geothermal gradient and thus also on the age of the plate above. The older the plate, the lower the gradient and the higher the viscosity.

Asthenosphere of the moon

On the Earth's moon , the plastic asthenosphere begins at a depth of 800–1000 km, around half the lunar radius. This is also where most of the moonquakes have their origin.

This depth results from the measured heat flow of the Apollo missions and from the strong damping of the seismic shear waves. At what depth the lunar core begins, however, is still uncertain.

Individual evidence

  1. See Duden online: Asthenia
  2. a b Attreyee Ghosh, William E. Holt: Plate Motions and stress from Global Dynamic Models . In: Science . 335, No. 6070, 2012, pp. 838-843. doi : 10.1126 / science.1214209 .
  3. G. Kaufmann, K. Lambeck: Glacial isostatic adjustment and the radial viscosity profile from inverse modeling . In: Journal of Geophysical Research . 107, No. B11, 2002, p. 2280. bibcode : 2002JGRB..107.2280K . doi : 10.1029 / 2001JB000941 . PDF