The transition zone is part of the Earth’s mantle, and is located between the lower mantle and the upper mantle, between a depth of 410 and 660 km. The Earth’s mantle, including the transition zone, consists primarily of peridotite, a course grained, ultramafic, igneous rock.
The mantle was divided into the upper mantle, transition zone, and lower mantle as a result of sudden seismic-velocity discontinuities at depths of 410 and 660 km. This occurs as a result of rearrangement of atoms in olivine (which constitutes a large portion of peridotite) at a depth of 410 km, to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 660 km, atoms rearrange yet again to form an even denser crystal structure.
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