'Essentially, We Have Discovered a New Layer in the Earth'

An ultra-hard layer of rock is causing a geologic traffic jam nearly a thousand miles beneath our feet.

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Mar 23 2015, 4:40pm

​Image: Shutterstock

Nine hundred and thirty miles underground, Earth's got a bit of a traffic problem. Sinking tectonic plates reach this depth, and they get stuck. Tectonic pile-ups in the lower mantle may trigger deep and powerful earthquakes. It's sorta like a clogged pipe, only the consequences are mind-bogglingly greater.

Now, it'd seem we've found the culprit. By squeezing minerals between diamond anvils, a study team led by geologist Lowell Miyagi and mineral physicist Hauke Marquardt has uncovered an ultra-hard layer of rock within Earth's lower mantle, one which appears to be backing up the whole tectonic conveyor belt. Rocks at 930 miles down are 300 times stiffer than they are half that depth. For a more salient comparison, they're a thousand billion billion times stiffer than peanut butter. The discovery appears today in Nature Geoscience.

"The Earth has many layers," said Miyagi in a statement. "Essentially, we have discovered a new layer in the Earth. This layer isn't defined by the minerals present, but by the strength of these minerals."

What's more, the tectonic traffic jam caused by this stiff layer may explain mysterious earthquakes occurring deep inside our planet. The ultra-hard layer could also account for strange magmas spewing out of seafloor volcanoes across the world. And finally, just as city traffic produces heat, this geologic pile-up could also mean that the Earth's interior is a lot hotter than we expected.

A slab of tectonic plate sinking through the upper mantle, through the boundary between the upper and lower mantle 410 miles deep, then stalling and pooling at 930 miles down. Image: Lowell Miyagi, University of Utah

Geologists have spent over a century peeling back our planet's layers and trying to piece together the processes that keep the little heat engine we call home chugging. We know that the Earth consists of several main layers, including a thin crust four to 50 miles deep, a much thicker and hotter mantle extending to a depth of 1,800 miles, and a molten, iron-rich core. The crust, and some of the upper mantle, comprise the tectonic plates that shuffle around our planet's surface. As these plates have lifted up, subducted downwards, stretched apart, and crashed together over geologic time, the face of our planet has evolved dramatically.

But broad brushstrokes aside, many aspects of our planet's inner workings are a mystery. For instance, seismologists were befuddled for years over the observation that many sinking tectonic slabs, including those beneath Indonesia and South America's Pacific coast, appear to pool at about 930 miles down.

"This observation has puzzled seismologists for quite some time, but in the last year, there is new consensus that most slabs pool," Miyagi said.

Because of the hellish temperatures and pressures, we obviously can't travel to Earth's mantle to study it directly. "We can't get down there, so we have to do experiments to see how these minerals behave under a wide range of conditions, and use that to simulate the behavior of the Earth," Miyagi said.

To solve the mystery of pooling tectonic plates, Miyagi and Marquardt took hair-thin samples of ferropericlase—one of the dominant minerals in Earth's lower mantle—and squeezed them to pressures nearly a million times greater than atmospheric using small, gem-quality diamond anvils.

Crushing thousands of samples of minerals in this manner, the researchers noticed a striking pattern. Ferropericlase's viscosity begins to increase at pressures equivalent to 410 miles deep—the upper-lower mantle boundary—and continues to increase until maximum pressures are reached, at 930 miles down.

"The result was exciting," Miyagi said. "This viscosity increase is likely to cause subducting slabs to get stuck—at least temporarily—at about 930 miles underground."

"We know a lot more about the surface of Mars than we do Earth's interior"

Through their experiments, the researchers also determined that just below the 930-mile-deep zone of high viscosity, slabs sink more easily as the lower mantle becomes less stiff. Like a stretch of open highway before a traffic jam, this region of easier motion could accelerate the geologic pile-up that occurs beneath it.

The discovery of a new layer of ultra-hard rock deep inside the Earth could help solve other geologic mysteries, including the occurrence of deep, underground earthquakes. The finding could also help explain why geologists see different types of magma spewing out of our planet's interior in different parts of the world. Magma that erupts along mid-ocean ridges—the rising end of the tectonic conveyor belt—has a chemical signature of younger, more well-mixed tectonic plates. But the magma found on island volcanoes like Hawaii, which was created by a hotspot of partially molten rock, hails from an older, deeper source.

Another implication of the new layer is that Earth's interior may be even hotter than we imagined. "If you decrease the ability of the rock in the mantle to mix, it's also harder for heat to get out of the Earth," Miyagi explained. He estimates that at 930 miles down, temperatures average a searing 3,900 degrees Fahrenheit.

So, sorry sci-fi fans, but we probably won't be tunneling any gravity trains, a la Total Recall reboot, through our planet's center any time soon. Still, it's fascinating to gain these additional insights into Earth's hot, dark interior, a world that remains more alien than many alien planets.

"We know a lot more about the surface of Mars than we do Earth's interior," Miyagi said.

But as we continue to peel back its layers, we're starting to appreciate just how complex the behemoth heat engine that keeps our planet alive actually is.