Asthenosphere

Earth's outer shell is commonly divided by mechanical behaviour into the rigid lithosphere and the weaker asthenosphere below it. The lithosphere includes the crust and the cool, stiff uppermost mantle; it is broken into tectonic plates. The asthenosphere is part of the upper mantle, not a separate chemical layer. Its name derives from a Greek word meaning weak. The boundary between lithosphere and asthenosphere is not a single universally sharp surface: its inferred depth depends on the physical property measured and on local conditions.

Asthenospheric material is predominantly solid silicate rock, rather than a global underground ocean of magma. At the high temperatures and pressures found at depth, mantle minerals deform ductilely and creep over long periods. Small amounts of melt or water may influence the material's strength in some settings, but the details and extent vary and remain active research topics. The distinction between a solid that flows over geological time and a liquid is essential to understanding the layer.

The asthenosphere is inferred from several types of geophysical evidence, including seismic-wave speeds, deformation of Earth's surface and models of how the mantle responds to changing loads. Seismic waves travel more slowly in some upper-mantle zones, often called low-velocity zones, which can reflect temperature, composition, fluids or partial melt. A low-velocity zone is associated with the asthenosphere in many places but is not automatically identical to the whole asthenosphere: the latter is also defined by its mechanical weakness, and its lower boundary is difficult to locate precisely.

Tectonic plates move as strong lithospheric slabs over the more deformable mantle beneath them. The asthenosphere allows this relative motion without requiring plates to slide over a perfectly frictionless surface. It also participates in mantle circulation and heat transfer. Plate motion is driven by interacting forces, including the sinking pull of cold, dense slabs and forces associated with ridges and mantle flow; it is an oversimplification to attribute all plate movement to a single asthenospheric conveyor current.