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Journey to the Center of the Earth

Hundreds of miles under our feet lie the Earth's secrets-and Bruce Watson's passion

By Patrick Kurp

Photograph by Mark McCarty

 

E. Bruce Watson, Institute Professor of Science in the Department of Earth and Environmental Sciences, passed among his students a fist-sized chunk of granite flecked with crystals of pink, white, and black.   

It came from the Llano Uplift in Texas, and may have proved the only bona fide, dug-from-the-Earth rock the students touched all semester. Watson is a geochemist, not a rock hound, the class was Introduction to Geochemistry, and the day’s lecture was devoted to radiometric age-dating.   

Watson was discussing the first scientific attempt to determine the age of the Earth. That came in 1863, when William Thompson (who would later become Lord Kelvin), extrapolating from the temperature differential between mine shafts and the surface, concluded our planet is 98 million years old. A commendable guess, except that he was off by 4.5 billion years, give or take a few million.

“Lord Kelvin really was a great physicist. It’s just that his foray into geology was poorly timed. He was treating the Earth as a simple physical object, almost as though it were something you could control in the laboratory. As we know today, it’s not,” Watson says.   

Like Lord Kelvin, Watson also employs extrapolation, his based on experimental chemistry, to advance our understanding of the Earth. Thanks to his work and that of other earth scientists, we’ve learned in recent decades that the rock beneath our feet is, in fact, a highly complex, dynamic, and still-evolving chemical system. There’s nothing inert about it.   

“There are many popular misconceptions about the Earth. Apart from the outer core, no place in the Earth is molten, for instance. There’s no extensive reservoir of entirely molten material. The Earth is basically solid from the surface, but there are localized regions of partial melting. The Earth’s core is like stainless steel, a mixture of iron and nickel, solid in the innermost reaches,” Watson says.   In 1997, in recognition of his pioneering efforts to understand the Earth’s complicated chemistry, Watson was named to a prestigious seat on the National Academy of Sciences. The NAS was established in 1863 by an act of Congress, and induction remains among the highest honors to be accorded a scientist in the United States.   

“Bruce has done the next important level of research in the field of geochemistry. He’s given us new insights into how fluids move and can be moved through the Earth. He’s done lots of pioneering work. What’s interesting about Bruce is that he’s always creative, he’s not afraid to try new things in the laboratory,” says Timothy L. Grove, professor of geology in the Earth, Atmospheric, and Planetary Sciences Department at the Massachusetts Institute of Technology.   

Another geologist, Professor David Walker of the Department of Earth and Environmental Sciences at Columbia University, describes Watson as a pioneer in the field: “Yes, he is there, usually first, with the most elegant experiment to answer the big question. He has also defined some of those questions in the course of answering them.”

Despite coming down on the “pure” end of the “pure-versus-applied” scientific spectrum, Watson has managed to earn a patent while at Rensselaer. With Minoru Tomozawa, professor of materials science and engineering, he devised a variety of glass resistant to radiation darkening that might prove useful for (among other things) the windows of tanks during a nuclear attack.   

His focus, however, is the Earth’s interior. Though ever-present beneath our feet, it remains a remote, inhospitable region, difficult to study directly. Our planet’s center is about 4,000 miles away, and an especially deep oil well seldom probes further than three miles below the surface. In the ’60s, Russian scientists began boring the deepest hole in the world, and so far they’ve only reached about eight miles.   

To compensate for the inner-earth’s inaccessibility, Watson must resort to an approach known to mathematicians as the inverse method—gathering information about a closed-off region by probing it from the outside. It’s like working backward—deducing a cause from an effect.   

Basically, Watson simulates inner-earth conditions, to a depth of about 100 miles, in his laboratory. With furnaces and hydraulic presses, he can achieve pressures of 50,000 atmospheres and temperatures exceeding 3,000 degrees Fahrenheit. In geochemistry, temperature correlates directly with pressure: The deeper you go, the hotter it gets.   

An initial visit to Watson’s laboratories in the basement of the Science Center is briefly disorienting. His labs, fitted with such seemingly ungeological hardware as metal lathes, grinders, and welding equipment, resemble machine shops. Watson even keeps an old-fashioned wooden toolbox filled with hammers and needle-nosed pliers.   

“This is a hands-on business. We rely on all kinds of tools, and not all of them are high-tech,” Watson says, though his research focuses on five high-pressure devices known as piston-cylinder apparatuses.

On a recent afternoon, one of Watson’s graduate students was running an experiment in a high-pressure machine. The temperature on the digital thermometer hovered around 2,260 degrees Fahrenheit, equivalent to the heat found at a depth of 30 miles below the surface of the Earth (about 15,000 atmospheres). The machine was not hot to the touch, and the only sound it produced was the muted hum of its cooling fan.   

Watson places small quantities of synthetic minerals in minute (measuring, say, 2 millimeters by 4 millimeters), handmade platinum containers that look like miniature cocktail shakers. These are placed in the center of bulky, highly polished metal disks, each weighing about 29 pounds. The core is tungsten-carbide, and each successive layer is forged of a softer steel alloy. Otherwise, the center, under all that heat and pressure, would flow.   

The experiments may last hours or weeks. What’s left after all that heat and pressure may be glass or a crystal aggregate, and it resembles ash.   

“Because of Bruce, we know a lot more about the details of igneous geochemical processes. We know a lot more about the rates at which these processes of partial melting and crystal growth occur. We know a lot more about the role and the mechanisms of the migration of liquids in the crust and mantle,” says Professor Walker of Columbia University.   

Once Watson has the results of his experiments—say, the growth and dissolution rates of a given crystal—he figures out ways to model his data and draw conclusions about processes within the Earth that occur in geological time, on the scale of hundreds of millions of years.   

“Bruce brought trace elements into experimental petrology, and vice versa,” Walker says. “Before Bruce, trace elements were largely the province of analysts, not experimentalists. Bruce brought this work into the laboratory.”   

Watson defends his experimental approach by arguing that fieldwork is difficult if not impossible when it comes to tracing the paths of elements through the inaccessible inner reaches of the Earth.   

“This is a challenge faced by many earth scientists—reaching conclusions about complex systems operating over enormous time periods. I don’t really study rocks, so in the eyes of some colleagues I’m not really a geologist, but I try to be answerable to what real geologists observe in the field. Even I believe that conclusions reached from experiments and models should be viewed with suspicion if they are clearly contradicted by observation of real Earth materials,” he says.   

Watson’s strategy is one among several used by geoscientists to understand the origin, composition, and evolution of the Earth. In Watson’s own department, for instance, Michael Gaffey studies asteroids (and their earthbound offspring, meteorites) to investigate the materials that make up the solar system (including the Earth).   

Other professors, Steven Roecker and Robert McCaffrey, use the tools of seismology and seismic tomography to chart our planet’s inner workings, and also employ data from Global Positioning Satellites to measure the motions of the Earth’s crust. Others use more classical approaches such as map-making, and still others use thermodynamics, isotopic analyses, or mathematical modeling, sometimes in combination.   

“Geoscience is a very large field. If you take in such disciplines as oceanography and meteorology, then you have the still broader field of earth sciences. Obviously, there’s a lot to learn and people are taking many different approaches to learn it,” Watson says. Watson was born and raised on a 200-acre dairy farm in Hollis, a small town outside Nashua in southern New Hampshire, and he describes himself as “a farm boy, through and through.”   

Soft-spoken and still boyish at the age of 49, he doesn’t seem overly impressed with the scientific eminence conferred by NAS membership.   

Watson traces some of his scientific bent to his mother, who still enjoys reading popular science books and subscribes to Science News. More important, however, was farm life itself—working with livestock, watching the weather, paying attention to the rhythms of the seasons.   

“If you’re a farm kid and are observant, nature is right in front of you. Being outdoors, I was naturally attracted to geology. That’s ironic, because virtually all of my work today is in the laboratory,” he says.

Watson spent a year at Williams College, where he began a major in political science, largely because of the times—Vietnam, the civil rights movement. He took two geology classes, however, and in his sophomore year shifted to the University of New Hampshire. That’s where he experienced “a kind of epiphany.”   

As he remembers it, much of the science and math he had already encountered was presented as a “done deal,” as though the intellectual possibilities of chemistry and physics had been exhausted.

“Geology was totally different. There was a simple acknowledgement from many of the professors that they just didn’t know, whereas physics and chemistry were taught as though everything was known and there was nothing left to learn. Strangely, that inspired me,” he says.   

Watson went on to get a bachelor’s degree in geology from the University of New Hampshire and a doctorate in geochemistry from Massachusetts Institute of Technology. He joined the Rensselaer staff in 1977.

Among Watson’s team of researchers is a physicist, Daniele Cherniak, a research associate professor of earth and environmental sciences, who first met him in 1989. Cherniak admires what she calls the “elegance” of the experiments Watson devises.   

“He’s one of those people who look at a very complex system, come up with some elegant experiment that you would never expect, and he ends up revealing all sorts of new information. He’s always open to new ideas and he’s never afraid to experiment. I’m a physicist but he’s welcomed me into his lab. He makes you feel like a colleague and doesn’t try to control everything. He’s very collegial and supportive,” Cherniak says.   

For more than 20 years, Watson has received funding from the National Science Foundation, and now has three federally funded projects under way.   

“I’ve always felt extremely privileged to have this support, because it’s allowed me to pursue, for the most part, my own frivolous interests at taxpayers’ expense,” he says.   

One current project, paid for with a grant from the U.S. Department of Energy’s Division of Basic Energy Science, is titled “Transport Properties of Fluid-Bearing Rocks.” Translated into everyday English, Watson’s research poses this question: At what rate do atoms migrate through fluids deep in the Earth’s crust? The word “fluid” is important, because water even 15 kilometers beneath the surface, under all that heat and pressure, reaches a super-critical state, neither liquid nor gas.   

Such research may seem rather remote from the concerns of the federal government’s energy program. In fact, the feds are interested in whether radioactive and other toxic waste, once it is buried deep in the Earth, will stay put or move around.   

“Pure science is the wellspring of application,” Watson says.   

The NSF backs both of his other federally funded research projects, and both concern the high-temperature behavior of rare-earth minerals that concentrate radioactive elements. For instance, Watson has developed an intense interest in zircon—a silicate of zirconium, number 40 on the periodic chart of elements.   

“Zircon is incredibly special. In your average rock from the continental crust, it’s ubiquitous, even though one of its crystals measures about 100 microns across. They’re almost literally our window into the geological past, because they concentrate radionuclides and are incredibly durable,” Watson says.

Besides the funded work he carries out with colleagues and students, Watson likes to reserve one research project to himself, usually something so idiosyncratic he can’t justify it professionally or financially, but so intellectually intriguing he can’t leave it alone.   

“It’s exploratory, maybe even eccentric stuff no one else thinks of doing. It’s important that I work on crazy ideas,” he says.   

His current “crazy idea” is finding a means of detecting and mapping the minute presence of the lanthanide series elements in synthetic rocks. Those are the rare-earth metals, numbers 54 through 71, in the periodic table of elements, and they take their name from the 54th: lanthanum.   

“Any rock contains the entire periodic table, every element in the universe is in every rock. Where are the rare-earth elements in a rock like this, and how do they move around as the crystals grow?” he asks, lifting from his desk what looks like a chunk of concrete studded with green crystals.   

“I haven’t got a clue where this will go, and it isn’t something I could turn over to students. It's just something I want to explore, in between other things," he says.

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