Loosening up under pressure

Despite what you may remember from the diagrams in your old Earth science textbook, there are few distinct boundaries underground. Instead, the earth’s depths tend to be jumbled – more like a goulash than a layer cake. But there is one place, about 35 kilometers, or 21.7 miles down, where there is a sharp separation. Its abruptness has baffled scientists for more than a century.

It’s called the Moho – or the Mohorovičić discontinuity, named for the Croatian seismologist who discovered it in 1909 – and it marks the divide between Earth’s crust and mantle. Now a Florida State University geologist named Mainak Mookherjee has come up with a surprising explanation for the Moho: a quirky property of a common mineral called feldspar.

Found in 60 percent of the rocks in the Earth’s outermost, crustal layer, feldspar has some unexpected properties. When most materials are subjected to extreme pressure, they unsurprisingly become more compact. However, Mookherjee recently published research in Nature demonstrating that the opposite happens to feldspar. The more force that’s exerted on the pale-colored crystal, the softer it becomes. That is, until you reach the pressure found at roughly the same depth at which the crust ends and the next layer of the Earth, the mantle, begins.

At this depth, the softened feldspar suddenly decomposes into highly dense quartz, Mookherjee’s research shows. It is that transition, he says, that explains the abrupt shift from the relatively soft, rocky, crust to the dense, compact mantle on which it floats.

His idea is shaking up the field. “I think the calculations are great and exciting,” says Oliver Jagoutz, a geologist at the Massachusetts Institute of Technology who studies the origins of the earth’s crust. “But they might not account for every kind of Moho,” he says.

Jagoutz focuses his research on a different explanation for the origins of the Moho — a natural flow of the materials that make up the crust and mantle. Jagoutz’s work, which focuses on the crust around the edges of islands to try to explain the origins of continental crust, suggests that the Moho stems from denser, cooler materials collapsing lower into the Earth while hot, less-dense materials arc upwards, sort of like a gigantic terrestrial lava lamp.

“I’m not sure how [Mookherjee] will explain the arcs,” says Jagoutz.

Mookherjee did not comment for this story, but his paper suggests that his explanation for the Moho applies best to the thicker segments of crust found under continents, not the thinner crust found underneath the ocean floor — the latter is well-explained by Jagoutz’s arcs.

Theories are all we have because no one has ever managed to drill all the way down to the Moho and take in situ samples. But we’ve known it exists ever since Andrija Mohorovičić noticed that seismic waves change speed based on how deep into the earth they propagate. He discovered that these mechanical waves, given off by earthquakes, travel along two paths — one that stays within the less dense crust and another that passes through a more compact layer, now known as the mantle. These two waves, he found, would reach the same destination at different times based on the density of the materials they passed through. By tracking the relative speeds of these mechanical waves, Mohorovičić could measure the density of each layer.

So far, every attempt to drill through the entirety of the Earth’s crust and observe the Moho firsthand, such as the 1960s expedition dubbed “Mohole,” has failed due to funding shortages and the difficulties of digging so far down into the Earth. Though an ongoing expedition in the Indian Ocean may finally reach the Earth’s mantle within the next few years, the inability to reach the Moho means that all data must be collected secondhand. This is why Mookherjee’s research relies primarily on computer simulations. It was these simulations that revealed how feldspar becomes softer when subjected to six to eight billion Pascals of pressure — that’s up to 80,000 times the atmospheric pressure at sea level.

The softening of feldspar, a common form of which is the sodium-rich mineral albite, is based on how the geometric structure of albite’s atoms responds to pressure.  Mookherjee found that most of the albite remains rigid, but one of its four tetrahedral structures bends when subjected to extreme force in a specific direction, causing an increase in the flexibility and softness of the albite feldspar.

However, once albite feldspar is subjected to pressures corresponding to the depth at which the continental crust ends and the mantle begins, about 35 kilometers down, the increasingly-soft feldspar suddenly shifts to a mixture of jadeite and quartz, Mookherjee found.  Both minerals are far denser than albite feldspar, so this transformation may explain why there is a sharp separation, rather than a gradual increase in density, between crust and mantle.

While Mookherjee’s simulations are based on experimental data collected by others, the inability to go explore the Moho and take samples prevents him from verifying his conclusions — and has prompted some skepticism.

“I’m dubious,” says Warren Hamilton, a geophysicist at the Colorado School of Mines, who criticizes the lack of new, empirical evidence behind Mookherjee’s work. “The rare exposures of the continental Moho give no hint of such a transformation.” These exposed sections of the boundary, rubble pushed from beneath the ocean floor to the surface by tectonic activity, don’t support Mookherjee’s findings, though his work focused on the Moho elsewhere, under the continents.

MIT’s Jagoutz is less concerned with Mookherjee’s methods, arguing that the reliance on computer simulations is a reasonable way to conduct geophysical research. “We never have basic observations in earth science that say, ‘Oh, that’s how it is.’ We are left with indirect observations,” Jagoutz says. “All these simulations and computations are on the edge of what we can do.”

“Moho is often a messy and arbitrary concept forced on ambiguous seismic data,” counters Hamilton, acknowledging how tricky it can be to substantiate explanations for geological activity.

Hamilton did agree that Mookherjee’s findings are plausible enough to be applicable to some Moho locations. Jagoutz agrees, “Just because a finding doesn’t work in all situations doesn’t mean it’s wrong,” he says.