The asthenosphere is made up of mafic and ultramafic rocks. It has the same material as the upper lithosphere, but the material in the asthenosphere is in a plastic, viscous state and moves around much more readily than the material in the lithosphere.
Mafic rocks are rocks that are made from silicates or ferromagnesian minerals. These minerals include pyroxene, olivine and amphibole. The rocks they produce include volcanic rocks like basalt, breccia, gabbro and diabase.
Ultramafic rocks are simply rocks that are very high in mafic minerals and low in silicates. These types of rocks include peridotite, lamproite and kimberlite, from which diamonds are mined.
The asthenosphere is a low, weak zone in the earth's upper mantle. The temperatures in the asthenosphere are high, though lower than they are in the inner core of the planet. However, the pressure is considerably lower. This is what allows the materials in the asthenosphere to exist in a plastic state and allows the lithosphere to float on top of it. This happens even though the materials in the lithosphere are relatively rigid.
The asthenosphere is about 62 miles thick and doesn't have well-defined upper or lower boundaries. Its boundaries depend on the state of the earth's mantle overall.