Answer:
The uppermost portion of the mantle makes up the bottom portion of the lithosphere. On average, it is 30 kilometers thick, but its thickness depends on the age of that portion of the lithosphere and temperature and pressure conditions. The mantle consists largely of heavy ultramafic rock like olivine.
Step-by-step explanation:
The mantle is made up of molten rock called magma. This magma circulates in currents determined by the cooling and sinking of heavier minerals and the heating and rising of lighter minerals. All but the very uppermost portion of the mantle is part of the asthenosphere, which refers to the liquid zone of the inner Earth.