asthenosphäre

The asthenosphere is a highly viscous, mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, which is the rigid outer part of the Earth, consisting of the crust and upper mantle. The asthenosphere extends from the base of the lithosphere to a depth of about 700 kilometers (430 miles) below the Earth's surface. Its upper boundary is defined by the seismic low-velocity zone, where seismic waves travel more slowly, indicating a change in rock properties.

The asthenosphere is characterized by temperatures high enough that the rock is close to its melting point. This high temperature, combined with immense pressure, causes the rock in the asthenosphere to behave in a ductile manner, meaning it can flow very slowly over geological timescales. This flow is crucial for plate tectonics, as it allows the rigid lithospheric plates above to move. Convection currents within the asthenosphere are the primary driving force behind this plate movement, carrying heat from the Earth's interior towards the surface.

The asthenosphere is not a liquid layer, but rather a solid that can deform plastically. Its properties are inferred from seismic wave velocities, which decrease in this region. The composition of the asthenosphere is generally thought to be similar to that of the lithospheric mantle, primarily peridotite, but with a higher temperature and a greater degree of partial melting. This partial melting, even if only a small percentage, significantly reduces the rock's strength and allows for ductile flow.