The mantle can be divided into the upper mantle (above 410 km depth), transition zone (between 410 km and 660 km), and the lower mantle (below 660 km). Much of the mantle consists of olivine and its high-pressure polymorphs. At the top of the transition zone, it undergoes a phase transition to wadsleyite, and at about 520 km depth, wadsleyite transforms into ringwoodite, which has the spinel structure.