A sixth of the way to the center of the Earth, things get goopy. Using variations in the planet’s gravitational tug, geophysicists have discovered that the viscosity of Earth’s mantle rapidly increases about 1,000 kilometers below ground.
The increasingly viscous rock acts like geologic molasses, slowing down anything trying to push through it. That includes sinking tectonic plates and the rising plumes of hot rock that fuel volcanoes, geophysicist Maxwell Rudolph of Portland State University in Oregon and colleagues report in the Dec. 11 Science. While the origins of the viscosity increase remain unknown, its discovery should help geologists better understand the flow of heat and rock through the planet’s interior.
The depth of the viscosity jump may even be a more suitable dividing line between the upper and lower mantle than the 660-kilometer-deep region currently used, suggests MIT geophysicist Robert van der Hilst, who was not involved in the study. “In terms of structure and dynamics, 1,000 kilometers could be more important,” he says.
Earth’s mantle extends from roughly 7 to 35 kilometers down to 2,900 kilometers beneath the ground and makes up about 84 percent of the planet’s total volume. Monitoring the mantle is important for understanding the mechanisms that drive plate tectonics and volcanism. But it’s hard to study because even the deepest drill sites do not descend far enough to even scratch the top of the mantle.
Geophysicists indirectly peek at the mantle by measuring how fast the ground rebounds after being compressed by gargantuan glaciers and by listening for the ricochets of earthquakes that rattle through the planet’s interior. Those techniques, however, offer an incomplete view of the mantle.
Rudolph’s team employed a relatively new technique that relies on variations in Earth’s gravitational tug. Rising and falling material in the mantle can cause dips and rises on the surface. Near Hawaii, for instance, mantle upwelling causes the ground to bulge roughly 1.3 kilometers in places. The downward force felt over one of these rises will be stronger than over a dip.
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How strongly the mantle alters the local gravitational pull depends on both the viscosity and density of the mantle material. Seismic waves traveling through Earth’s interior partially reflect off boundaries where the mantle becomes denser. That information allowed the researchers to separate the two effects.
Using knowledge of Earth’s composition and gravity measurements collected by NASA’s GRACE satellites (SN: 1/4/03, p. 6), the researchers created a profile of Earth’s interior. A computer program systematically simulated hundreds of thousands of different configurations of Earth’s innards, hunting for setups consistent with the GRACE gravity data. Most of the simulations suggested that the viscosity of the mantle increases 10- to 150-fold around 1,000 kilometers down — below the 660-kilometer-deep boundary.
The 1,000-kilometer depth corresponds to a region where many sinking tectonic plates stall, a separate research team reports online December 11 in Science Advances. The researchers used reflecting seismic waves to track the motions of several sinking tectonic plates around the world.
While some previous studies proposed such a rapid viscosity increase at that depth, the two new papers provide a clear confirmation that the viscosity increase exists, says geodynamicist Lijun Liu of the University of Illinois at Urbana-Champaign, who was not involved with either study.
Scientists have put forward several explanations for the viscosity increase, Rudolph says. For instance, certain minerals may become stronger at greater depths, thickening the mantle. “It’s going to be a lot of fun figuring out what’s responsible for this jump in viscosity,” Rudolph says.