Abstract
For decades researchers have debated whether the mantle, the vast layer of viscous rock between Earth9s molten iron core and the outer shell of tectonic plates, is neatly divided at a depth of 660 kilometers into two layers that never mix, or whether it churns from top to bottom over the eons. Now in this issue of Science (beginning on p. 1881), seismologists and modelers offer a new model that incorporates elements of both. In this model Earth9s radiogenic heat--abetted by plunging tectonic plates--causes the bottom mantle layer to vary markedly in thickness, bulging upward in some places and squeezing close to the mantle floor in others, but a very deep rock layer, from 1700 kilometers or so down to the base of the mantle at 2900 kilometers, remains intact.
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