What Is the Asthenosphere? The asthenosphere is part of the upper mantle located below the crust of the earth. The asthenosphere boundary is primarily defined by temperature, and it starts when the crust heats up to 1300° C. At this temperature the crust begins to melt and move more as a liquid. The asthenosphere is important in plate tectonics, as convection currents slowly move the tectonic plates that lie above.