Topography of the transition zone seismic discontinuities

Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. Ther modynamic principles govern phase transformations, making them sensitive to changes in the mantle's ambient conditions through the thermodynamic respo nse: Changes in temperature or composition shift the transformation to a di fferent pressure, creating topography on a level discontinuity. With this u se as an exploratory tool for the mantle in mind, we review the basic seism ological and phase equilibrium concepts underlying the detection and interp retation of the seismic wave arrivals associated with the transition zone d iscontinuities. Reviewing the evidence for and against the phase transition model, we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the trans ition zone.