Study May Solve Age-old Mystery of Missing Chemicals From Earth’s Mantle

Observations about the early formation of Earth may answer an age-old question about why the planet’s mantle is missing some of the matter that should be present, according to UBC geophysicist John Hernlund.

Earth is made from chondrite, very primitive rocks of meteorites that date from the earliest time of the solar system before the Earth was formed. However, scientists have been puzzled why the composition of Earth’s mantle and core differed from that of chondrite.

Hernlund’s findings suggest that an ancient magma ocean swirled beneath the Earth’s surface and would account for the discrepancy.

“As the thick melted rock cooled and crystallized, the solids that resulted had a different composition than the melt,” explains Hernlund, a post-doctoral fellow at UBC Earth and Ocean Sciences.

“The melt held onto some of the elements. This would be where the missing elements of chondrite are stored.”

He says this layer of molten rock would have been around 1,000 km thick and 2,900 km beneath the surface.”

Published in today’s edition of the journal Nature, Hernlund’s study explores the melting and crystallization processes that have controlled the composition of the Earth’s interior over geological time. Co-authors are Stéphane Labrosse, Ecole Normale Superieure de Lyon and Nicolas Coltice, Université de Lyon.

The centre of Earth is a fiery core of melted heavy metals, mostly iron. This represents 30 per cent while the remaining 70 per cent is the outer mantle of solid rock.

Traditional views hold that a shallow ocean of melted rock (magma) existed 1,000 km below the Earth’s surface, but it was short lived and gone by 10 million years after the formation of Earth.

In contrast, Hernlund’s evolutionary model predicts that during Earth’s hotter past shortly after its formation 4.5 billion years ago, at least one-third of the mantle closest to the core was also melted.

The partially molten patches now observed at the base of the Earth’s mantle could be the remnants of such a deep magma ocean, says Hernlund.

Findings may reveal secret to interaction between Earth’s core and mantle

The boundaries between grains of rock could be a pathway for metals to move between Earth's core and mantle. - Photo Credit: Leslie Hayden/Rensselaer Polytechnic Institute
The boundaries between grains of rock could be a pathway for metals to move between Earth’s core and mantle. – Photo Credit: Leslie Hayden/Rensselaer Polytechnic Institute

Leslie Hayden’s research into deep Earth interactions has led to some important findings, particularly for someone so new to the field, and the scientific world is paying attention. Hayden, a graduate student at Rensselaer Polytechnic Institute, is first author on a paper to be published in the scientific journal Nature. The findings will be published in the Nov 29, 2007 edition of the journal.

Hayden performed her research under the guidance of Bruce Watson, Institute Professor of Science at Rensselaer.

Hayden used some powerful equipment and creative techniques to uncover a potential pathway for metals to move between the core and mantle of the Earth. “Core-mantle interactions are a hotly debated topic,” Hayden said. “Some scientists believe that there is no chemical interaction at all between the Earth’s molten metal core and solid silicate mantle. Others believe they see signs of such interaction, but no mechanism or pathway has been found that could deliver metal atoms over distances of more than a few meters. ”

Hayden’s experiments may have uncovered such a pathway. If true, the findings could have broad implications on how geologists understand the deep Earth. They could also one day provide important information on how valuable elemental resources like gold and platinum are deposited.

Hayden and Watson developed an experiment that simulated the interface between the core of the Earth and the mantle. The highly pressurized core consists mostly of iron and nickel and is also believed to contain other “iron-loving” elements like gold and platinum. The mantle is comprised of silicate rocks rich in magnesium. For the experiments they placed a rock that is representative of the material found in the Earth’s mantle in between what they refer to as a source and a sink layer. The source layer was one of the metals found in the core, such as gold, platinum, copper, and other lesser known metals like ruthenium and tungsten. For each metal, the miniaturized core/mantle boundary was then heated to extreme temperatures and pressures to represent conditions in the deep Earth. Following the experiments, each source metal was found in the sink, proving that the metals could in fact find a pathway through the mantle rock that is believed to be impenetrable by some scientists.

Hayden and Watson hypothesize that the metal atoms move along the surfaces formed between adjacent grains of the mantle rock. Like a sugar cube, mantle rocks are comprised of individual crystals squeezed tightly together into a larger structure. The atoms of the core metals are too large to diffuse through the structured arrangement of atoms that make up an individual crystal or grain of rock. But, the boundary between each grain is less crowded with atoms, according to the researchers, and could be a fast pathway for metals to migrate between the mantle and core.

“[In our experiments], some of the metals moved through grain boundaries at surprisingly fast rates – about as fast as sodium ions move through water,” Hayden said. “This shows that metals can in fact travel over great distances through mantle materials. Over geologic time, this diffusion of metals could have a significant impact on their distribution in the Earth.” Their experiments revealed that some elements could move up to 100 kilometers through the Earth’s mantle in a billion years.

The findings have implications for the field, but also for broader economic reasons, Hayden explains. If these metals are able to move out of core and into the mantle as their findings suggest, they would enter the geologic upwelling of mantle convection and could be gradually moved toward Earth’s surface, potentially leading to valuable deposits. “As we learn more about the movement of precious and base metals through the Earth, we could at some point find out how they are deposited, where, and why,” she said.

The research was funded by the National Science Foundation (NSF).

Hayden is from Marlton, New Jersey. She plans to become a post-doctoral researcher at UCLA upon completion of her doctorate at Rensselaer, expected in December.

Seismologists See Earth’s Dynamic Interior as Interplay of Temperature, Pressure, Chemistry

Surface topography and bathymetry around South America (top) overlays variable topography in Earth's upper mantle at 410 kilometers and 660 kilometers depth. - Credit: Arizona State University, Nicholas Schmerr/Edward Garnero
Surface topography and bathymetry around South America (top) overlays variable topography in Earth’s upper mantle at 410 kilometers and 660 kilometers depth. – Credit: Arizona State University, Nicholas Schmerr/Edward Garnero

Seismologists have recast their understanding of the inner workings of Earth from a relatively homogeneous environment to one that is highly dynamic and chemically diverse.

The research, conducted by scientists Nicholas Schmerr and Edward Garnero of Arizona State University in Tempe, is published in the October 26 issue of the journal Science.

This view of Earth’s inner workings depicts the inner planet as a living organism where events that happen deep within can affect what happens at its surface, like the rub and slip of tectonic plates and the rumble of volcanoes.

The new research into these inner workings shows that in Earth’s upper mantle (an area that extends down to 660 kilometers), more than temperature and pressure play a role.

“It has long been a goal of seismologists to distinguish between thermal and chemical effects on seismic velocities in Earth’s deep interior,” said Robin Reichlin, program director in the National Science Foundation (NSF)’s division of earth sciences, which funded the research. “This work is a tantalizing step toward solving this important problem, necessary for understanding the internal structure and dynamics of our planet.”

The simplest model of the mantle–the layer of the Earth’s interior just beneath the crust–is that of a convective heat engine. Like a pot of boiling water, the mantle has parts that are hot and welling up, as in the mid-Atlantic rift, and parts that are cooler and sinking, as in subduction zones. There, crust sinks into the Earth, mixing and transforming into different material “phases,” like graphite turning into diamond.

“A great deal of past research on mantle structure has interpreted anomalous seismic observations as due to thermal variations within the mantle,” Schmerr said. “We’re trying to get people to think about how the interior of the Earth can be not just thermally different but also chemically different.”

Schmerr’s and Garnero’s work shows that Earth’s interior possesses an exotic brew of material that goes beyond simply hot and cold convection currents.

To make their observations, Schmerr and Garnero used data from the USArray, which is part of the NSF-funded EarthScope project.

“The USArray is 500 seismometers deployed in a movable grid across the United States,” Schmerr said. “It’s an unheard of density of seismometers.”

Schmerr and Garnero used seismic waves from earthquakes to measure where phase transitions occur in the interior of Earth by looking for where waves reflect off these boundaries.

They studied seismic waves that reflect off the underside of phase transitions halfway between an earthquake and a seismometer. The density and other characteristics of the material they travel through affect how the waves move, and give geologists an idea of the structure of the inner Earth.

Beneath South America, Schmerr’s research found the 410-kilometer phase boundary bending the wrong way. The mantle beneath South America is predicted to be relatively cold due to cold and dense former oceanic crust and the underlying tectonic plate sinking into the planet from the subduction zone along the west coast. In such a region, the 410-kilometer boundary would normally be upwarped, but using energy from far away earthquakes that reflect off the deep boundaries in this study area, Schmerr and Garnero found that the boundary significantly deepened.

They postulate that either hydrogen or iron concentrations are responsible for the observed deflection of the 410 discontinuity.

“This study lets us constrain the temperature and composition to a certain degree, imaging this structure inside the Earth and saying: ‘These are not just thermal effects — there’s also some sort of chemical aspect to it as well,'” Schmerr said.

Life-giving Rocks From A Depth Of 250 Km

If our planet did not have the ability to store oxygen in the deep reaches of its mantle there would probably be no life on its surface. This is the conclusion reached by scientists at the University of Bonn who have subjected the mineral majorite to close laboratory examination. Majorite normally occurs only at a depth of several hundred kilometres under very high pressures and temperatures.

The Bonn researchers have now succeeded in demonstrating that, under these conditions, the mineral stores oxygen and performs an important function as an oxygen reservoir. Near the earth’s surface the structure breaks down, releasing oxygen, which then binds with hydrogen from the earth’s interior to form water. Without this mechanism our “Blue Planet” might well be as dry and inhospitable as Mars.

The proverbial “solid ground” under our feet is actually in constant flux. At the boundaries between the tectonic plates in what are called the subduction zones this seemingly solid ground is drawn down many hundreds of kilometres into the hot interior. As the material descends it takes with it oxygen, which is bound as iron oxide in the earth’s mantle oxygen that derives from the dim distant beginnings of the universe.

Far below the earth’s surface high pressures and temperatures prevail. As the mantle material melts the iron oxide undergoes a chemical metamorphosis in which its oxygen component becomes, in a sense, more reactive. Moreover, it changes its medium of transportation, now being incorporated into the exotic mineral majorite which only occurs at these depths. And, as Professor Dr. Christian Ballhaus from the Mineralogical Institute at the Bonn University explains, “The higher the pressure, the more oxygen can be stored by majorite.”.

Oxygen takes the elevator

We can envisage the majorite as operating like an elevator for oxygen. But this time it moves in the opposite direction: the mineral rises like warm air above a heater. In fact, the experts talk here about “convection”. However, nearing the earth’s surface the pressure in the mantle becomes too weak to maintain the majorite, which then decomposes. “That’s where the stored oxygen is released,” notes Ballhaus, whose team is the first to investigate this mechanism under laboratory conditions. “Near the surface it is made available for all the oxidation reactions that are essential for life on earth.”

In particular, the earth constantly exudes hydrogen, which combines with this oxygen to form water. Without the “oxygen elevator” in its mantle the earth would probably be a barren planet hostile to life. “According to our findings, planets below a certain size hardly have any chance of forming a stable atmosphere with a high water content,” points out Arno Rohrbach, doctoral student in the research team at the Mineralogical Institute. “The pressure in their mantle is just not high enough to store sufficient oxygen in the rock and release it again at the surface.”

Bastion against solar wind

The bigger the planet, the greater is its capacity to store heat; and, correspondingly, the longer-lasting and more intensive is the convection in its crust. Mars, for example, with a diameter of about 7,000 kilometres (the earth’s diameter measures 12,700 km) cooled down long ago to a level at which there is no longer any movement in its mantle. “Its crust has therefore lost the ability to transport oxygen and maintain a lasting water-rich atmosphere,” Professor Ballhaus elucidates.

In other respects, too, the size of a planet is decisive for the formation of an atmosphere. Only if temperatures in a planet’s interior are high enough for it to have a fluid metal core can it develop a magnetic field. The magnetic field operates like a bastion in the face of solar winds. Over time, these winds would otherwise simply blow the atmosphere away.

The findings of the Bonn-based scientists have been published in the journal “Nature” (doi:10.1038/nature06183).

Researchers Reassess Theories on Formation of Earth’s Atmosphere

Scientists propose that argon in our atmosphere came from the weathering of the upper crust and not the melting of the mantle
Scientists propose that argon in our atmosphere came from the weathering of the upper crust and not the melting of the mantle

Geochemists at Rensselaer Polytechnic Institute are challenging commonly held ideas about how gases are expelled from the Earth. Their theory, which is described in the Sept. 20 issue of the journal Nature, could change the way scientists view the formation of Earth’s atmosphere and those of our distant neighbors, Mars and Venus. Their data throw into doubt the timing and mechanism of atmospheric formation on terrestrial plants.

Lead by E. Bruce Watson, Institute Professor of Science at Rensselaer, the team has found strong evidence that argon atoms are tenaciously bound in the minerals of Earth’s mantle and move through these minerals at a much slower rate than previously thought. In fact, they found that even volcanic activity is unlikely to dislodge argon atoms from their resting places within the mantle. This is in direct contrast to widely held theories on how gases moved through early Earth to form our atmosphere and oceans, according to Watson.

Scientists believe that shortly after Earth was formed, it had a glowing surface of molten rock extending down hundreds of miles. As that surface cooled, a rigid crust was produced near the surface and solidified slowly downward to complete the now-solid planet. Some scientists have suggested that Earth lost all of its initial gases, either during the molten stage or as a consequence of a massive collision, and that the catastrophically expelled gases formed our early atmosphere and oceans. Others contend that this early “degassing” was incomplete, and that primordial gases still remain sequestered at great depth to this day. Watson’s new results support this latter theory.

“For the ‘deep-sequestration’ theory to be correct, certain gases would have to avoid escape to the atmosphere in the face of mantle convection and volcanism,” Watson said. “Our data suggest that argon does indeed stay trapped in the mantle even at extremely high temperatures, making it difficult for the Earth to continuously purge itself of argon produced by radioactive decay of potassium.”

Argon and other noble gases are tracer elements for scientists because they are very stable and do not change over time, although certain isotopes accumulate through radioactive decay. Unlike more promiscuous elements such as carbon and oxygen, which are constantly bonding and reacting with other elements, reliable argon and her sister noble gases (helium, neon, krypton, and xenon) remain virtually unchanged through the ages. Its steady personality makes argon an ideal marker for understanding the dynamics of Earth’s interior.

“By measuring the behavior of argon in minerals, we can begin to retrace the formation of Earth’s atmosphere and understand how and if complete degassing has occurred,” Watson explained.

Watson’s team, which includes Rensselaer postdoctoral researcher Jay B. Thomas and research professor Daniele J. Cherniak, developed reams of data in support of their emerging belief that argon resides stably in crystals and migrates slowly. “We realized from our initial results that these ideas might cause a stir,” Watson said. “So we wanted to make sure that we had substantial data supporting our case.”

The team heated magnesium silicate minerals found in Earth’s mantle, which is the region of Earth sandwiched between the upper crust and the central core, in an argon atmosphere. They used high temperature to simulate the intense heat deep within the Earth to see whether and how fast the argon atoms moved into the minerals. Argon was taken up by the minerals in unexpectedly large quantities, but at a slow rate.

“The results show that argon could stay in the mantle even after being exposed to extreme temperatures,” Watson said. “We can no longer assume that a partly melted region of the mantle will be stripped of all argon and, by extension, other noble gases.”

But there is some argon in our atmosphere – slightly less than 1 percent. If it didn’t shoot through the rocky mantle, how did it get into the atmosphere?

“We proposed that argon’s release to the atmosphere is through the weathering of the upper crust and not the melting of the mantle,” Watson said. “The oceanic crust is constantly being weathered by ocean water and the continental crust is rich in potassium, which decays to form argon.”

And what about the primordial argon that was trapped in the Earth billions of years ago? “Some of it is probably still down there,” Watson said.

Because Mars and Venus have mantle materials similar to those found on Earth, the theory could be key for understanding their atmospheres as well.

Watson and his team have already begun to test their theories on other noble gases, and they foresee similar results. “We may need to start reassessing our basic thinking on how the atmosphere and other large-scale systems were formed,” he said.

The research was funded by the National Science Foundation.

Scientist Studies Minnesota’s Rock In Antarctica

An intrusion (the forcible entry of molten rock or magma into or between other rock formations) in Antarctica. Unlike Minnesota, geologists get a perfectly clear view of intrusions in Antarctica.
An intrusion (the forcible entry of molten rock or magma into or between other rock formations) in Antarctica. Unlike Minnesota, geologists get a perfectly clear view of intrusions in Antarctica.

Geologists learn by looking at rocks. Of course, it’s not that simple. Here in Minnesota, the tapestry of mineral-laden geology lies buried under forests, soils and parking lots. This makes Dean Peterson’s job difficult. As one of the economic geologists at the University of Minnesota, Duluth’s Natural Resources Research Institute (NRRI), his job is to understand the state’s geology–where and what types of ore minerals were deposited some 1.1 to 2.7 billion years ago. In Minnesota, geologists figure it out by reading scattered outcroppings and drilling holes. It’s doable, but it’s difficult.

So when Peterson was offered an opportunity to spend a month in Antarctica’s Dry Valleys, he jumped at the chance. Yes, that’s a long way from Minnesota, but surprisingly, the geology is the same. Both areas were focal points of dynamic magmatic systems associated with continental rifting-molten rock flowed up from the earth’s mantle, forming intrusions in the upper crust. The geologic setting was the same.

But the beauty of Antarctica for geologists is the 100 percent exposure of rock. They can look at layer upon ancient layer of deposits, up to 10,000 feet high. In Minnesota, the Duluth Complex, a large, composite of mafic rocks (rich in dark-colored minerals like magnesium and ireon) in northeastern Minnesota, was the hot spot for dynamic magmatic molten movement. It’s where NRRI’s economic geologists go to identify valuable mineral deposits.

Understanding local deposits

“In the Duluth Complex, I study the ‘plumbing’ of the intrusions. That’s the key to finding the higher grade ore deposits,” says Peterson. “So in the Dry Valleys I can actually see how the magma moves up from the earth’s crust, how it crosses certain rock bodies, and where it picks up sulfur to form sulfide minerals. In Antarctica I could see the ‘plumbing’ that I can’t see in Minnesota.”

If that wasn’t exciting enough for Peterson (and it was) he also spent a month with one of the most renowned geologists in the country, Bruce Marsh of Johns Hopkins University.

Did you know?

Antarctica is the coldest, windiest, and harshest continent. The continent is covered in continuous darkness during the austral winter and continuous sunlight in the summer. (The average annual temperature is -56°F at the Amundsen-Scott South Pole Station, the southernmost continually inhabited place on the planet).

Source U.S. Antarctic Program

“Spending time seeing this fabulous geology and learning from Dr. Marsh is really something special,” says Peterson.

Paul Morin, a visualization expert in the geology and geophysics department on the U’s Twin Cities campus, and researchers from Poland and Slippery Rock University in Pennsylvania joined Peterson on the expedition. The trip was funded by a grant from the National Science Foundation.

From Peterson’s travel notebook:

  • Antarctica is not as cold as people might think. Temperatures were, on average, in the 20s to 30s Fahrenheit and sometimes down to 10 at night, but we got used to it right away. After a day we were in shirtsleeves and a windbreaker. The sun is always out and intense.

  • When the wind stops blowing there is utter silence. There is nothing to make a noise. It’s eerie at first, but then I got used to it. The silence really gives you time to think. When we went back to McMurdo (U.S. Field Station) the noise created by 1,100 people living in close quarters was unbelievable.

  • Humans have evolved in humid environments where water vapor in the atmosphere selectively absorbs light–as you look into the distance things get bluer and bluer. We unconsciously perceive distance using the air’s absorption of light. Antarctica is the driest place on earth. The humidity in the Dry Valleys averages about 1 or 2 percent. The air’s dryness adds an additional dimension to an Antarctic experience–light doesn’t change color with distance. Mount Erebus, 120 miles away, will look exactly like it would if you were right next to it. It’s hard to visually calculate any distance.

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.