A breakthrough technology leads to a new understanding of the Earth’s mobile, and malleable, inner core. By Mike Martin
For centuries, explorers and scientists have gradually demolished misconceptions about the size, shape and character of the Earth.
The ancient Babylonians, Egyptians, Greeks and Chinese thought the Earth was flat. Later observers, most notably the 17th-century British astronomer and comet chaser Edmund Halley, suggested our planet was spherical but hollow — a conjecture that led French science fiction writer Jules Verne to famously ponder a journey to its center.
Thanks to a pantheon of more contemporary truth seekers, we now know our planetary home is a round, reasonably solid mass that won’t accommodate center-bound travelers.
Still, misconceptions abound, even among some of the world’s most sophisticated Earth investigators. One of the more persistent involves the idea that the planet’s lithosphere, its rocky outer crust, conducts heat equally well at all temperatures.
This subtle yet significant mischaracterization—one borne of the difficulty in obtaining temperatures deep in the Earth’s crust—has led to inaccurate models of some of geology’s most fundamental processes, among them how mountains are formed, how tectonic plates move, and why volcanoes erupt.
In March, MU geology professors Alan Whittington and Peter Nabelek, along with Washington University geophysicist Anne Hofmeister, published an important corrective in the British journal Nature. Their paper, three pages of densely written explication and calculations, details a new method of measuring what scientists call the “thermal diffusivity and conductivity” of the crust: how well, in other words, rocks hold and transfer heat. The new method allowed the team to show conclusively that current measurement methodologies have crippling limitations.
“The most important aspect of large-scale geothermal systems is transport of heat in circulating fluids like molten rock,” says Whittington. “The thermal conductivity of rock controlled how quickly the Earth solidified after it formed, for example. Our results should help to improve thermal models of these systems.”
Using a form of laser-flash analysis, a technique first applied to geological materials by Hofmeister, the team discovered that the hotter and more molten a rock becomes, the better it insulates. This means that the Earth’s crust conducts and releases heat fairly quickly at lower temperatures. But where the crust is hot, it tends to stay hot; at least much hotter and much longer than geologists previously thought.
Nature podcaster Natasha Gilbert was so impressed she invited Whittington onto her weekly program. Fellow geologists have also weighed in favorably. Jean Braun, noted earth scientist and geodynamics professor at the University of Rennes in Rennes, France, has written that the team’s observations have “far-reaching implications” that should prompt a “reassessment of most quantitative models of Earth’s dynamical behavior.”
Geological history is largely the story of how our once-molten planet has cooled and solidified over its 4.5 billion years of existence.
In the beginning, small planetoid bodies, aptly named planetesimals, collided and coalesced to form the Earth. The impacts created a tremendous reservoir of primordial heat that has been gradually dissipating — along with heat from ongoing radioactive decay — ever since.“
The Earth is thought to have been mostly molten following these impacts,” Whittington says. “A magma ocean began to solidify from the top down. Heat conduction through the solidified lid allowed further cooling and solidification of the interior.”
Because this solidification is not complete, the Earth’s rigid outer crust, that fractured shell of landmass and ocean floor that supports all life, floats atop a red hot, less-than-solid interior consisting of a lower crust, mantle, outer core and inner core. Not all portions of the floating crust are equally buoyant, Nabelek says. “Depends on how hot it is. Heat transport ultimately causes movement of all materials in the Earth. Volcanism and geothermal energy are good examples of this dynamic.”
Understanding heat transport involves parsing the finer points of thermal conductivity and diffusivity, phenomena that are themselves the products of two distinct forces: vibration and light. At lower temperatures, vibrations between atoms are responsible for conducting heat. At higher temperatures, light contributes by radiating heat.
Typically scientists interested in measuring conductivity place electrodes in contact with rock. But this approach incorrectly measures radiation’s contribution, thus tending to overestimate how efficiently heat disperses. This is especially problematic near rocks’ melting point. To compensate, geologists do something uncharacteristically unscientific: they ignore the error, assuming instead that rock conductivity stays pretty much constant, regardless of the temperature.
“By pretending there’s no problem, geologists get measurements that are as much as 30 percent off, and introduce this error into every model, every theory,” says Hofmeister. “It’s absolutely maddening.”
In her lab on the Washington University campus, Hofmeister works to bring sanity to the process.
Analyzing the planet’s crust means testing dozens of rock samples from around the world. Hofmeister handles quartz from Pakistan, granite from Brazil, mica from South Dakota’s Black Hills and rhyolite from the St. Francois Mountains in Missouri.
Each sample starts as a cylindrical core, sliced into a millimeter-thin wafer about half an inch across. During laser-flash analysis, Hofmeister positions the sample in her lab’s furnace and gradually increases its temperature. She then fires a pulse from an infrared laser at the sample — her laser of choice is one typically used in industrial applications such as steel welding — while sensitive instruments record the time it takes the pulse’s heat to travel from one side of the wafer to the other. The result yields an accurate measure of the sample’s thermal conductivity.
The final step involves averaging results across a range of samples, an exercise that ensures the data represent the real-world crust conditions.
“Conventional methods of measuring thermal conductivity put the rock in contact with a heater and thermocouples. But radiant heat transfer between the sample and its contacts reduces accuracy by about 25 percent,” Hofmeister says. “Laser flash analysis involves no contacts, and only exhibits about a 2 percent reduction in accuracy.”
Charted on a simple graph, the numbers are striking. As the temperature of the sample rises, its thermal diffusivity, how well it releases heat, and its thermal conductivity, how well it transports heat, each decline considerably. Whittington says this is true for “every mineral, rock, and glass that we have looked at so far.”
The finding becomes particularly significant in the lower part of the Earth’s crust, where high temperatures reduce thermal conductivity by as much as 50 percent more than conventional methods predict.
“With a reduced crustal conductivity, the lower crust may be hotter than we thought,” says Jean Braun. “This finding has direct implications for our understanding of the early Earth’s differentiation and the distribution of elements in its crust, mantle and core.”
Data from laser-flash analysis, the research team says, could also have a profound effect on plate tectonics, a theory that seeks to describe the process that, quite literally, underlies all geology.
The Earth’s crust is divided into approximately 12 distinct plates that meet at “crustal boundaries,” dynamic transition points where earthquakes are frequent and volcanic eruptions are common. As the crust heats up and melts, it becomes more mobile and malleable.
Such planetary dynamism is especially notable in “orogenic belts” — places where mountains form through vast earthen upheavals. Here crustal melting was previously attributed to extraordinary processes such as high levels of heat-producing radioactivity.
But laser-flash analysis suggests a more ordinary process, strain heating, may be responsible for orogenic belt melting. If that’s the case, the clear implication is that the Earth is even more dynamic, and more prone to catastrophic upheavals, than current models suggest.
Strain heating occurs when material is deformed, heats up, and converts mechanical energy to heat, Whittington says. “Bend a spoon back and forth rapidly and you will find it warms up quite quickly. The same thing happens to rocks being deformed in the middle and lower crust.”
The hypothesis that strain heating can cause crustal melting helps explain another intriguing question. When continents collide, crust melts, says Nabelek. “But we don’t see intrusions of basaltic magma from the mantle that could cause crustal melting. We do see strain heating.”
Braun says many models of mountain formation will “need to be revisited” in view of Whittington and colleagues’ new measurements. These include, he says, “a recent and vigorously debated model for the evolution of large, hot mountain belts, such as the Himalayan–Tibetan system.”
Other questions that may need rethinking include: How much magma is required to produce a volcano? Probably less than previously thought. How far will lava flow before it cools and solidifies? Most likely farther than previously thought. And how long can a dangerous volcano like Mount St. Helens brew and stew? Perhaps a very long time, the scientists suggest. “Volcanic and geothermal systems would now be predicted to last longer,” Nabelek says.
In recognition of the importance of pursuing answers to such questions, Nabelek, Whittington and Hofmeister recently received a $330,000 research grant from the National Science Foundation.
Getting to the bottom of earth-science mysteries falls in large part to a select cohort of professional petrologists, geologists who specialize in the study of how rocks form. Petrologists like Whittington and Nabelek focus chiefly on all three types of rock: igneous, such as granite, that come from molten magma; sedimentary, those comprised of sandstone, shale or limestone; and metamorphic, rocks such as slate or marble that started out as sedimentary or igneous rock but changed under extreme pressure or high temperatures.
Photos of Nabelek’s expeditions show both the global nature of petrological study and Nabelek’s passion for sharing his wide-ranging expertise with students. One shows Nabelek, MU doctoral student Joe Hill, and a third student perched at Mount Rushmore, sitting just below the stony gaze of Thomas Jefferson. Another depicts Tibetan prayer flags snapping in the breeze along the Himalayan ridge where Nabelek and his assistants are developing thermal models of granite generation.
Whittington, for his part, is equally wide-ranging in his dedication to research and education. His Enhancing Thermodynamic Applications and Learning in Petrology, or EnThALPy project (the acronym is a term from thermodynamics that describes heat content) was recently funded as part of a coveted CAREER Award from the National Science Foundation. The $440,000, 5-year grant is one of a limited number of awards given to “early career researchers” who show promise for a lifetime of scientific and educational achievement. (Another recent CAREER recipient, computer scientist Dmitry Korkin, is featured in this month’s Profile.)
According to the award citation, Whittington’s grant will allow him to more fully demonstrate igneous petrology’s “fundamental importance in shaping the planet” while emphasizing the “human scale” of the research. “Millions of people are threatened by potentially active volcanoes. Eruption products include lava, ash and gases such as water and carbon dioxide which in large part drive explosive volcanic activity.”
Thanks to satellites and moon walks, we’re assured that the Earth is a big blue sphere. We can see it and measure it directly. “But we know little about the temperature of the deepest parts of Earth’s continental crust, which can be 30–40 kilometers below the surface,” Braun says. “We can’t measure temperature directly beyond a few kilometers down at the bottom of deep mines or drill holes.”
It’s a long way from crust to core, nearly 4,000 miles, where “current estimates put the temperature at about 3000 degrees Celsius,” agrees Hofmeister. Her lifetime goal, she adds, is to ascertain the truth of that estimate. To do that, however, she must rely on indirect measurements like those she performs in her lab for global extrapolation.
“No one has ever drilled a hole even to the base of the continental crust,” Whittington told the Nature podcasters. “A lot of what we think we know about temperature inside the crust comes from a variety of measurements on how much heat energy is flowing through the crust.”
Indirectly observing the otherwise unobservable may therefore be the greatest value of techniques like laser-flash analysis, which has already taught us that “the lower crust is a good insulator and thus acts as a blanket over the mantle,” Braun says.
In the future, Whittington suggests, it’s indeed time that geologists look beneath the blanket, a subject covered in a paper published in Nature earlier this year. “This paper is focused on the crust,” Whittington says. “I would say that the next big thing is looking at melting in the mantle.”