Asthenosphere

The asthenosphere (Greek a sthenos " without strength " ), called in geophysics and Low Velocity Zone, 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 depths between 60-210 km and extends down to a depth of 300-410 km. Your mechanical behavior can be described simply as ductile ( malleable ). On the asthenosphere move the elastic and less dense lithospheric plates.

Properties

From the mineral composition ago, the asthenosphere is not different from the rock of the upper mantle. The main constituents are olivine, orthopyroxene, spinel and garnet. The average density is 3300 kg / m³.

The far more significant difference between the asthenosphere and her on - and subjacent layers in their mechanical properties. So wide seismic waves. Within the asthenosphere slowly than in the immediately adjacent layers Then the English name refers low velocity zone ( LVZ ). Depending on the definition, the LVZ is synonymous with the asthenosphere, or providing only the upper 100 km. The average speed of P waves fall of 8.3 km / s below the Mohorovičić discontinuity at less than 8 km / s below the lithosphere - asthenosphere boundary ( LAB ) from to increase again with increasing depth (up to the Earth's core it rises ultimately to 13.6 km / s at ). Shear waves (S- waves) are attenuated by partially molten rock.

The dynamic viscosity of the asthenosphere can be estimated from postglacial uplift. It lies between 1020 Pascal seconds (Pa · s ) at mid-ocean ridges, subduction zones and other seismically very active regions and 1024 Pa · s under cratons. The viscosity of the crust with more than 1025 Pa.s significantly greater. For comparison, the viscosity of glass is approximately 1017 Pa · s

The flowability of the athenosphere depends on the solidus temperature, the water content of the rock and the geothermal gradient. The solidus temperature is that of the pressure (and thereby the depth) dependent temperature at which the rock begins to melt. Is the asthenosphere this temperature is exceeded, the rock may be 1-5% partial melt ( magma ). The formation of the melt is mainly caused by the instability of hydrous silicate minerals at this depth. Water causes even in small amounts, a significant decrease in the solidus temperature. It dates from the orthopyroxene and to a lesser extent, of the olivine. In the presence of aluminum can orthopyroxene bind large amounts of water, which gives it again with increasing pressure at a depth of 100 to 150 km. The viscosity also depends more on geothermal gradient, and thus the age of the overlying plate. The older a plate, the lower the gradient, a higher viscosity will result.

Asthenosphere of the Moon

When Earth's moon, the plastic asthenosphere begins at 800-1000 km depth, ie approximately at half moon radius. Here also the most moonquakes originate.

This depth is obtained from the measured heat flow of the Apollo missions and from the strong attenuation of seismic shear waves. Whereas the lunar core begins, is still uncertain.