Newspaper article The Christian Science Monitor

Why Is There a Mysterious Boundary Deep in the Earth's Mantle?

Newspaper article The Christian Science Monitor

Why Is There a Mysterious Boundary Deep in the Earth's Mantle?

Article excerpt

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. …

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