Scientist Uncovers Earth’s Mysterious Layer

Laboratory measurements of a high-pressure mineral believed to exist deep within the Earth show that the mineral may not, as geophysicists hoped, have the right properties to explain a mysterious layer lying just above the planet’s core.

A team of scientists, led by Sébastien Merkel of the University of California-Berkeley, now at CNRS/the University of Science of Technology of Lille, France, made the first laboratory study of the deformation properties of a high-pressure silicate mineral named post-perovskite. The work appears in the June 22 issue of the scientific journal Science.

The team included Allen McNamara of ASU’s School of Earth and Space Exploration, part of the College of Liberal Arts and Sciences. McNamara, a geophysicist, modeled the stresses the mineral typically would undergo as convection currents deep in Earth’s mantle cause it to rise and sink. Also on the team were Atsushi Kubo and Thomas Duffy, Princeton University ; Sergio Speziale, Lowell Miyagi and Hans-Rudolf Wenk, University of California-Berkeley; and Yue Meng, HPCAT, Carnegie Institution of Washington, Argonne , Ill.

“This the first time the deformation properties of this mineral have been studied at lower mantle temperatures and pressures,” McNamara says. “The goal was to observe where the weak planes are in its crystal structure and how they are oriented.”

The results of the combined laboratory tests and computer models, he says, show that post-perovskite doesn’t fit what is known about conditions in the lowermost mantle.

Earth’s mantle is a layer that extends from the bottom of the crust, about 25 miles down, to the planet’s core, 1,800 miles deep. Scientists divide the mantle into two layers separated by a wide transition zone centered around a depth of about 300 miles. The lower mantle lies below that zone.

Most of Earth’s lower mantle is made of a magnesium silicate mineral called perovskite. In 2004, earth scientists discovered that under the conditions of the lower mantle, perovskite can change into a high-pressure form, which they dubbed post-perovskite. Since its discovery, post-perovskite has been geophysicists’ favorite candidate to explain the composition of a mysterious layer that forms the bottom of Earth’s lower mantle.

Known to earth scientists as D” (dee-double-prime), this layer averages 120 miles thick and lies directly above Earth’s core. D” was named in 1949 by seismologist Keith Bullen, who found the layer from the way earthquake waves travel through the planet’s interior. But the nature of D” has eluded scientists since Bullen’s discovery.

“Our team found that while post-perovskite has some properties that fit what’s known about D”, our laboratory measurements and computer models show that post-perovskite doesn’t fit one particular essential property,” McNamara says.

That property is seismic anisotropy, he says, referring to the fact that earthquake waves passing through D” become distorted in a characteristic way.

“Down in the D” layer, the horizontal part of earthquake waves travel faster than the vertical parts,” McNamara says. “But in our laboratory measurements and models, post-perovskite produces an opposite effect on the waves. This appears to be a basic contradiction.”

McNamara notes that the laboratory measurements, made by team members at Princeton University and at Berkeley, were extremely difficult. They involved crushing tiny samples of perovskite on a diamond anvil until they changed into post-perovskite. The scientists then shot X-rays through the samples to identify the mineral crystals’ internal structure.

This information was used by other team members at the University of California-Berkeley to model how these crystals would deform as the mantle flows. The deformation results let the scientists predict how the crystals would affect seismic waves passing through them.

McNamara’s work modeled the slow churn of the mantle, in which convection currents in the rock rise and fall about as fast as fingernails grow – roughly an inch a year. He calculated stresses, pressures and temperatures to draw a detailed picture of where post-perovskite would be found. This let him profile the structure of the D” layer.

“All these computations have been in two dimensions,” he says. “Our next step is to go to three-dimensional modeling.”

So does their work rule out post-perovskite to explain the D” layer?

“Not completely,” McNamara says. “We’ve begun to study this newly found mineral in the laboratory, but the work isn’t yet over. It’s possible that post-perovskite does exist in the lowermost mantle, and another mineral is causing the seismic anisotropy we see there.”

Geophysicists Detect Molten Rock Layer Deep Below American Southwest

A sheet of molten rock roughly 10 miles thick spreads underneath much of the American Southwest, some 250 miles below Tucson. From the surface, you can’t see it, smell it or feel it.

But Arizona geophysicists Daniel Toffelmier and James Tyburczy detected the molten layer with a comparatively new and overlooked technique for exploring deep within Earth that uses magnetic eruptions on the sun.

Toffelmier, a hydrogeologist with Hargis + Associates Inc. in Mesa, graduated from ASU’s School of Earth and Space Exploration in 2006 with a master’s degree in geological sciences. Tyburczy, a professor of geoscience in the school, was Toffelmier’s thesis adviser. Their findings, which grew out of Toffelmier’s thesis, are presented in the June 21 issue of the scientific journal Nature.

“We had two goals in this research,” Tyburczy says. “We wanted to test a hypothesis about what happens to rock in Earth’s mantle when it rises to a particular depth – and we also wanted to test a computer modeling technique for studying the deep Earth.

“Finding that sheet of melt-rock tells us we we’re on the right track.”

Deep squeeze

In 2003, two Yale University geoscientists published a hypothesis about the composition and physical state of rocks in the Earth’s mantle. They proposed that mantle rock rising through a depth of 410 kilometers (about 250 miles) would give up any water mixed into its crystal structure, and the rock then would melt.

“This idea is interesting and fairly controversial among geophysicists,” Tyburczy says. “So Dan and I thought we’d test it.”

Geophysicists often study the planet’s structure using earthquake waves, which are good at detecting changes in rock density. For example, seismic waves show that Earth’s density abruptly alters at particular depths. The biggest change, or discontinuity, comes at the core-mantle boundary, about 2,900 kilometers (1,800 miles) deep. Another lies at a depth of 660 kilometers (410 miles), while the third most-prominent discontinuity occurs 410 kilometers (250 miles) down.

But seismic waves don’t tell scientists much about rocks’ chemical makeup, or about minor elements they contain, or their various mineral phases. Scientists need a different method to study mantle rocks that change composition as they shed water at 410 kilometers’ depth and become partly molten in the process.

A geophysical survey technique sensitive to these factors is called magnetotellurics, or geomagnetic depth sounding.

“Basically, this method measures changes in rocks’ electrical conductivity at different depths,” Toffelmier says.

Calibrated by laboratory work, magnetotelluric methods permit scientists to estimate the composition of rocks they won’t ever be able to hold in their hands.

“Rocks are semiconductors,” Tyburczy says. “And rocks with more hydrogen embedded in their structure conduct better, as do rocks that are partially molten.”

A common source for hydrogen is water, which can lodge throughout a mineral’s crystal structure.

But how to measure the conductivity of rocks buried hundreds of miles underfoot? The answer lies 93 million miles away, on the surface of the sun.


The sun emits a continuous flow of charged atomic particles called the solar wind. This varies in strength as activity on the sun rises and falls. When gusts of particles reach Earth, they induce changes in the planet’s magnetosphere, causing in turn weak, but measurable electrical currents to flow through terrestrial rocks deep inside Earth.

Toffelmier and Tyburczy used electromagnetic field data collected by others for five regions of Earth: the American Southwest, northern Canada, the French Alps, a regionally averaged Europe and the northern Pacific Ocean. Only these few data sets contained information gathered over a long-enough period to be useful in the computer modeling.

“The long-period waves tell you about deep events and features, while short-period ones resolve shallower features,” Tyburczy says.

He says to think of it like an inverted cone extending down into Earth. The deeper you go, the wider the area that’s sampled – and the coarser the resolution.

The modeling approach Toffelmier and Tyburczy used was to start with an initial guess as to rock composition at different depths, run the model, compare the results to the actual field data, and then alter the run’s starting point. As they worked, they found that only the data for the southwestern United States showed signs of a water-bearing melt layer at the 410-kilometer (250-mile) depth.

“Without a melt zone at that depth, we can’t match the field observations,” Toffelmier says.

But, adds Tyburczy, “when we added a highly conductive melt zone, five to 30 kilometers (three to 20 miles) thick, we got a much better fit.”

The extent of the melt sheet is unknown, however, because the data set is limited in area. There’s little chance that any molten rock from it would erupt at the surface, the researchers say.

Seismic surveys show the 410-kilometer discontinuity is global in scope. But Toffelmier and Tyburczy’s work shows that melting at the 410-kilometer depth is patchy at best, and far from global. So the Yale hypothesis remains only partly confirmed.

So what’s next?

“Our modeling has been only in one dimension,” Tyburczy says. “We need to start looking in two and three dimensions. We also need to understand better how rocks and minerals change at the incredible pressures deep inside the Earth.”

“We’ve seen only the tip of the iceberg,” Toffelmier says.