Definition - What does Asthenosphere mean?
Asthenosphere is a region on the earth's upper mantle that is highly ductile and viscous. The region also has weak mechanical properties. It is located just beneath the lithosphere (crust and upper most solid mantle) and its depth ranges from 80 to 200 kilometers from the earth's surface. The boundary between the lithosphere and asthenosphere is called LAB. Due to the high temperature and pressure in the region, rocks are ductile.
Petropedia explains Asthenosphere
Asthenosphere forms a part of the upper mantle which lies just below the lithosphere and is involved in the plate tectonic movement and isostatic adjustments. This region has a low resistance to plastic flow and is much hotter and contains more fluid as compared to lithosphere. The boundary temperature in this region is conventionally taken as 1300 degree Celsius isotherm. At this temperature, the mantle becomes highly rigid and behaves in ductile fashion. Also the seismic waves pass at slow speeds from this region.
The area that falls under asthenosphere is mostly solid; however, some areas could be in a molten state.