| Science -- Kerr 283 (5409): 1826 (966 words) |
 | For decades researchers have debated whether the mantle is more like a giant layer cake, neatly divided at a depth of 660 kilometers into two layers that never mix, or a boiling pot of water, churning from top to bottom over the eons. |
| 1881), seismologists and modelers offer a new model that incorporates elements of each of the old ones and might best be described with a third metaphor--that of a lava lamp on low. |
| 1881) with a bottom layer 4% denser than the overlying rock. |
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