Why is there a mysterious boundary deep in the Earth's mantle?

Two separate teams of scientists propose explanations for a suspected boundary marking a property change in our planet's mantle.

Courtesy of Nicholas Schmerr/Vedran Lekic/UMD
Sinking slabs of ocean crust and rising plumes of hot rock in Earth's mantle are observed to behave differently below one megameter (1,000 kilometers) depth. Two explanations for this behavior were published on Dec. 11, 2015. At left, Rudolph et al. (Science, 2015) propose a viscosity increase (dark blue) below the megameter boundary. At right, Ballmer et al. (Science Advances, 2015) propose a density increase due to accumulated ocean crust (dark squiggles) below the boundary.

About 1,000 kilometers, or 621 miles, below the Earth's surface, something strange is happening.

Partway through the Earth's mantle, the layer between the crust and the outer core, there seems to be a boundary. Slabs of the Earth's lithosphere, the rocky layer just under the surface, are dragged down into the Earth by subduction only to be stalled at this depth. Also, as hot rock upwellings push up through the lower mantle, the material is deflected sideways around the same depths.

This phenomenon puzzled scientists so much that two separate research teams set out to figure out what's going on down there.

One group of scientists proposed that a change in viscosity, how sluggishly a material flows, is responsible. The other team blames a change in composition, and therefore density, at the suspected boundary.

Both teams found a shift in the property they studied, about 1 megameter, that is, 1,000 kilometers, or 621 miles, below the Earth's surface. One suggests that the mantle beyond that depth is more viscous, flowing more slowly, than the shallower parts of the mantle. The other suggests a shift in rock composition around the apparent boundary meaning the lower mantle is more dense.

In either explanation, a subducting tectonic plate would have trouble sinking any deeper into the Earth past that property change. Likewise upwelling hot rock would also react to the change.

Although separate explanations for the same phenomenon, "The two explanations aren't incompatible," geologist Maxwell Rudolph, lead author of the paper on viscosity published Friday in the journal Science, tells The Christian Science Monitor in an interview. "A change in composition could certainly be accompanied by a change in viscosity."

The paper that focuses on a composition and density change deep in the mantle was published Friday in the journal Science Advances.

"The existence of the megameter boundary has been suspected and inferred for a while," geologist and co-author of the Science paper Vedran Lekic said in a news release. "These papers are the first published attempts at a detailed explanation and it's possible that both explanations are correct."

Both research teams used computer models to test their hypotheses.

"Earth's mantle is solid rock," Dr. Rudolph says. So how could a tectonic plate be moving through this section of the planet? "On very long timescales, timescales of tens of millions of years, it can deform just like these fluids that we encounter in every day life," he explains. So "as a tectonic plate goes back into the mantle, the mantle is flowing around the plate."

Determining basic physical properties, like density and viscosity, of the mantle can help scientists learn more about the processes deep in our planet.

"Learning about the anatomy of the mantle tells us more about how the deep interior of Earth works," geologist and co-author of the Science Advances paper Nicholas Schmerr said in the news release.

"The dynamics deep within Earth's interior do have important consequences for the long-term evolution of the planet," Rudolph says. "Not just for the deep interior of the planet, but for all of the other systems."

"This work can tell us a lot about where Earth has been and where it is going, in terms of heat and tectonics," Dr. Schmerr said. "When we look around our solar system, we see lots of planets at various stages of evolution. But Earth is unique, so learning what is going on deep inside its mantle is very important."

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