The transition zone is the part of Earth's mantle that is located between the lower and the upper mantle, most strictly between the seismic-discontinuity depths of about 410 to 660 kilometres, but more broadly defined as the zone encompassing those discontinuities, i.e., between about 300 and 850 kilometres depth. Earth's solid, rocky mantle, including the mantle transition zone, consists primarily of peridotite, an ultramafic igneous rock.