Asthenosphere

The asthenosphere (from Ancient Greek ἀσθενός (asthenós) 'without strength') is the mechanically weak[1] and ductile region of the upper mantle of Earth.

More extensive decompression melting of the asthenosphere takes place where it wells upwards, and this is the most important source of magma on Earth.

It is the source of mid-ocean ridge basalt (MORB) and of some magmas that erupt above subduction zones or in regions of continental rifting.

The asthenosphere is a part of the upper mantle just below the lithosphere that is involved in plate tectonic movement and isostatic adjustments.

[4] Seismic waves pass relatively slowly through the asthenosphere[5] compared to the overlying lithospheric mantle.

The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about.

The rigid lithosphere is thought to "float" or move about on the slowly flowing asthenosphere, enabling isostatic equilibrium[12] and allowing the movement of tectonic plates.

[9] The lithosphere-asthenosphere boundary (LAB[15][7]) is relatively sharp and likely coincides with the onset of partial melting or a change in composition or anisotropy.

[10] Weakening below oceanic plates is partly caused by their motion itself, thanks to the non-linear dislocation creep mechanism.