ANN ARBOR—Strange things happen in the lower reaches of our planet’s mantle, that plastic-like layer between Earth’s crust and core that flows under pressure, lifting or lowering features on the surface. Geologists have been intrigued by observations that some seismic waves travel faster than others in particular patches of the lowermost mantle, but they haven’t known exactly why that happens. New work by researchers at the University of Michigan and Yale University, published in the March 21 issue of Nature, helps explain the phenomenon and offers new insights into Earth’s inner workings.
“Different mechanisms can cause certain seismic waves to move faster than others,” says lead author Allen McNamara, a Ph.D. student in the U-M Department of Geological Sciences. “By understanding which mechanisms are occurring, we can learn something about how the mantle is flowing.”
In general, the speed of seismic waves (waves of energy that shake the ground during earthquakes) depends upon the density and strength of the rock through which the waves are traveling. There are two ways to account for why some waves might move faster than others through the same patch of mantle. One explanation is that the patch is made up of layers of materials with different chemical compositions. The other possible mechanism involves the alignment of mineral grains in the material through which the seismic waves are passing. Both of these mechanisms can cause a certain type of seismic wave (the shear wave) to split into two components, one faster than the other.
The researchers used experiments and mathematical modeling to sort out which mechanism is operating in the hot, plastic lower mantle—the region that extends from 670 to 2,900 kilometers below Earth’s surface. High pressure in this region causes the formation of minerals unlike those in the upper mantle, the area between Earth’s crust and the lower mantle. McNamara and colleagues found that mineral grain alignment, not chemical heterogeneity, causes the unusual seismic wave behavior in the lower mantle.
“The intriguing thing is that, within the lower mantle, we only see these patches at the core-mantle boundary,” in places where slabs of Earth’s crust and upper mantle once were pushed down and now are colliding with the core-mantle boundary, says Peter van Keken, U-M assistant professor of geological sciences. “It’s clear that there’s a lot going on at the core-mantle boundary, so it’s likely an important place to understand the thermal and chemical evolution of the Earth and to increase our overall understanding about how the solid Earth works. This study provides just one glimpse at what’s happening there.”
The work also is important in demonstrating how the study of seismic waves can provide a window into Earth’s inner workings, says van Keken. “Seismology and geodynamics have been integrated quite successfully in studies of the upper mantle, where much clearer observations and the better illumination of interior structure leads to a more robust understanding of physical processes,” he says. “But this is really the first step toward using that approach in the lower mantle.”
