The asthenosphere (frae Greek asthenēs 'weak' + sphere) is the heichly viscous, mechanically waik an ductilely-defurmin region o the upper mantle o the Yird. It lies belaw the lithosphere, at depths atween ~80 an ~200 km (~ 50 an 124 mile) ablo the surface. The lithosphere-asthenosphere boondary is uisually referred tae as LAB. Asthenosphere is generally solit altho some its regions coud be meltit (e.g. belaw mid-ocean ridge). The lawer boondary o the asthenophere is nae well defined. The thickness o the asthenosphere depends mainly on the temperatur. For some regions asthenosphere coud extend as deep as 700 km (430 mi). It is considered the soorce region o mid-ocean ridge basalt (MORB).