Evidence for a ubiquitous seismic discontinuity at the base of the mantle

A sharp discontinuity at the base of Earth's mantle has been suggested from seismic waveform studies; the observed travel time and amplitude variation s have been interpreted as changes in the depth of a spatially intermittent discontinuity. Most of the observed variations in travel times and the spa tial intermittance of the seismic triplication can be reproduced by a ubiqu itous first-order discontinuity superimposed on global seismic velocity str ucture derived from tomography. The observations can be modeled by a solid- solid phase transition that has a 200-kilometer elevation above the core-ma ntle boundary under adiabatic temperatures and a Clapeyron slope of about 6 megapascal per kelvin.