The Earth's lower mantle is the largest part of the mantle at 1,400 miles thick, and is made of dense rock so hot it exists almost in liquid form. The lower mantle, or lower mesosphere, is one of the three main layers that make up the Earth, along with the upper mantle and the Earth's core.
The lower mantle lies between the upper mantle and the Earth's core. The entire mantle is 1,802 miles thick with the lower mantle being 1,400 miles of that, starting at the depth of 400 miles and extending to the depth of 1,800 miles. It is made up of dense rock that is so hot it exists in a not-quite-solid form with temperatures ranging from 5,500 to 3,300 degrees Kelvin. The average density in the lower mantle is 5.5 tons per cubic meter.
The lower mantle is made of up magnesium-bearing silicates, iron-bearing silicates, olivine, pyroxene, garnet peridotite and water. The lower the mantle is, the more fluid the rock that composes it. This fluidity, along with convection, resistance friction and electric currents, causes seismic activity. The parts of the lower mantle farthest from the core are less fluid and more rigid and are the beginning of geologic faults.