| The Lava-Lamp Earth | |
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The Lava Lamp™ refuses to die. Back in the mid-20th century, Lava Lamps were party toys that were amusing if you were in a certain mood. Now not only have the originals become collectibles, but it turns out that the Earth may be one, sort of.
More precisely, it's the Earth's mantle that seems to behave that way, but the mantle—the white-hot, stony part between the iron core and the thin layer of crust that we live on—accounts for two-thirds of Earth's mass. So to a first approximation, as they say in science, the mantle is Earth. And a set of papers in the 20 March 1999 Science laid out a new lava-lamp theory of how the mantle behaves.
The working model of the mantle for the last couple decades is a first approximation too—that there's an upper mantle and a lower mantle, with a boundary between them about a quarter of the way down, at 660 kilometers depth. In the upper mantle, the crust slips down from above as part of plate tectonics, and the upper mantle likewise rises up making fresh crust at the mid-ocean ridges, and so the mantle mixes. In this process the upper mantle loses water, carbon, oxygen, aluminum and other elements to the atmosphere and the rocks of the crust, leaving itself depleted in those elements. (More details are in "Plate tectonics in a nutshell.")
The boundary at 660 km is where the pressure crunches the mineral olivine into a denser crystal form, and the working model says that this change in density tends to prevent mixing below that depth. The lower mantle has a "primitive," undepleted composition and mostly keeps to itself, except where hotspots punch a little of it up to the surface. That's the theory.
Einstein said that a theory should be as simple as possible, but no simpler. And the working model is well known to be too simple even for a first approximation.
The more pictures we make of the deep mantle, from seismic tomography, the less that boundary at 660 km looks like a good barrier. Images seem to show plumes of hot material pushing up through it from below and pieces of crust sinking down through it too. One of the papers in Science describes what looks like one of those crustal fragments, sitting 1500 km down. Yet geochemical evidence from the world's volcanoes still points to a two-part mantle, with a depleted part and undepleted part that don't mix. However, there's plenty of argument about all of these points.
The model from the Science papers sets the boundary lower, more than twice as deep as before. And it's harder to see in the seismic images because the boundary is very irregular, not at all like the crisp 660-km line. The mantle is more like the two liquids in the Lava Lamp, or like a ruined fondue with all the cheese on the bottom, or like . . . well, look at it yourself and see what it reminds you of.
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Louise Kellogg, a numerical geodynamicist at the University of California Davis, ran a computer simulation of this kind of two-layer mantle model along with two guys at MIT, Brad Hager and Rob van der Hilst. Their Science paper suggests that the two layers stay separate for billions of years. And the third paper, by van der Hilst and one of his grad students, reports that indeed the sinking crust seems to hit a barrier down around 1600 km depth, and the rock below appears to be different from what overlies it.
This new picture of the mantle might be the start of a better first approximation, or it might be nothing, undermined by too many false assumptions. Deep-Earth studies are notoriously poorly constrained, and it takes the insights of many different specialists to avoid mistakes. But the lava-lamp model is evidence that mantle studies continue to be active, like the mantle itself.
Louise Kellogg's school also wrote up this story and added some interesting biographical detail about her.
