Asthenosphere

The asthenosphere is the portion of Earth's mantle that lies beneath the lithosphere and behaves as a mechanically weak, ductile region. It extends from roughly 100 kilometers (about 60 miles) below the surface to several hundred kilometers depth, though the exact base varies with tectonic setting. In many areas the lithosphere above it is thinner, so the asthenosphere begins at shallower depths, while under thick continental lithosphere it lies deeper.

The defining characteristics of the asthenosphere are high temperatures and pressures that bring rocks toward their melting point, producing a partially molten, malleable layer. Seismically, it shows lower shear-wave velocities and notable anisotropy, reflecting its plastic nature and mantle composition, which is similar to upper-m mantle peridotite. It is not a distinct, abrupt boundary but a gradational zone that becomes progressively weaker with depth.

Role and significance in geodynamics include enabling plate tectonics. The asthenosphere provides a lubricating layer on which the rigid lithospheric plates can move and interact. Mantle convection and the partial melt in this region contribute to the decoupling of the plates from deeper mantle flow, facilitating motion, plume upwellings, and isostatic adjustments. The concept is supported by seismic imaging, heat-flow data, and geochemical studies, which together portray the asthenosphere as a soft, dynamic part of the upper mantle.