Putting a new spin on an old technique, Anne M. Hofmeister, Ph.D., research professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, has revolutionized scientists' understanding of heat transport in the Earth's crust, the outermost solid shell of our planet.
Temperature is an important driver of many geological processes, including the generation of magmas (molten rocks) in the deepest parts of the Earth's crust, about 30 to 40 kilometers below the surface. Yet, until recently, temperatures deep inside the Earth's crust were uncertain, mainly because of difficulties associated with measuring thermal conductivity, or how much heat is flowing through the rocks that compose the crust.
In conventional methods of measuring thermal conductivity, measurement errors arise as the temperature of a rock nears its melting point. At such high temperatures, heat is not just transported from atom to atom by vibrations, but also by radiation (light). Since conventional methods cannot separate heat flow carried by vibrations from that associated with radiation, most measurements of how efficiently rocks transport heat at high temperatures have been overestimated. Because of this experimental uncertainty, scientists have assumed rock conductivity to be constant throughout the crust in order to make advances in models describing Earth's geological behavior.
Using an industrial laser that is typically used for steel welding, Hofmeister was able to circumvent the problems that plagued the older methods. Her facility at WUSTL is the first in the world to employ such a laser for geoscience research.
Her technique, laser-flash analysis, provides much more accurate data on heat transport through rocks than conventional methods. In laser-flash analysis, a rock sample is held at a given temperature and then subjected to a laser pulse of heat, allowing Hofmeister to measure the time it takes for the heat to go from one end of the sample to the other. This measurement of thermal diffusivity, or how fast heat flows through matter, is another way to describe the thermal conductivity of a rock. Since measuring heat transport in the crust itself is impossible, Hofmeister used the laser to measure heat transport in individual rock samples at various temperatures and then averaged across samples to represent the dynamics of the crust. In collaboration with researchers from the University of Missouri - Columbia, Peter I. Nabelek, Ph.D., professor of geological sciences, and Alan G. Whittington, Ph.D., assistant professor of geological sciences, Hofmeister applied her findings to explain geological phenomena observed in the environment.
The results, published in Nature on March 19, 2009, suggest that rock conductivity is not constant as was previously assumed, but instead varies strongly with temperature. Hofmeister explains, "Our analysis shows that rocks are more efficient at conducting heat at low temperatures than was previously thought and less efficient at high temperatures. The process of moving heat around really depends on the temperature of the rocks."
Hofmeister and her collaborators found that the conductivity of rocks in the lower crust, where the external temperature is very high, is much lower — by as much as 50 percent — than was predicted by conventional methods. These results also suggest that the lower crust may be much hotter than scientists previously recognized. Since rocks become better insulators and poorer conductors at high temperatures, the lower crust acts like a blanket over the heat-generating mantle, the layer underlying the crust.
The observation that the lower crust is a good thermal insulator has broad implications for scientists' understanding of fundamental geological processes such as magma production.
Hofmeister explains, "The new methods change our understanding of how heat is transported in geological environments. This pertains to where you find magmas, where you cook metamorphic rock, and where lavas form on ocean ridges."
She and her colleagues used the new temperature-dependent data to inform computer models that predict the consequences of burying and heating up rocks during mountain belt formation, as occurs in the present-day Himalayas. While prior models relied upon extraordinary processes such as high levels of radioactivity to explain melting of the crust in the Himalayas, Hofmeister and her collaborators' work suggests that the thermal properties of the rocks themselves might be sufficient to generate magmas.
In particular, they find that the strain heating, or friction, caused by mountain belt formation can trigger crustal melting. Because the lower crust is such a good thermal insulator, strain heating is much faster, more efficient, and more self-perpetuating than previously recognized.
"The melt is more insulating than the rock," explains Hofmeister, "Once you get rocks melting, the thermal diffusivity goes down, which makes it harder to cool the rocks. They stay hot longer and there's the potential for more melting."
According to Hofmeister, the Himalaya situation described in the study is probably not unique. Because heat transport is such an important driver, many models of Earth's geological behavior will need to be revisited in light of Hofmeister and her collaborators' findings.
These advances bring Hofmeister much closer to accomplishing what she describes as her life-long career objective. "The goal for most of my career has been to determine the temperature inside the earth. It's the time dependence, how long it takes heat to flow through rocks, that is going to tell us how hot the interior is," she says.
According to Hofmeister, understanding the temperature of the Earth's interior is the first step towards understanding the thermal evolution of the earth.
Anne Hofmeister | EurekAlert!
Further reports about: > Earth's crust > Himalaya > Magma machine > Radiation > computer model > conventional methods > geological environments > geological processes > geological sciences > heat transport > laser-flash analysis > lavas form on ocean ridges > magmas > measurement errors > metamorphic rock > molten rock > mountain belt formation > strain heating > thermal conductivity > thermal insulator
World’s oldest known oxygen oasis discovered
18.01.2018 | Eberhard Karls Universität Tübingen
A close-up look at an uncommon underwater eruption
11.01.2018 | Woods Hole Oceanographic Institution
On the way to an intelligent laboratory, physicists from Innsbruck and Vienna present an artificial agent that autonomously designs quantum experiments. In initial experiments, the system has independently (re)discovered experimental techniques that are nowadays standard in modern quantum optical laboratories. This shows how machines could play a more creative role in research in the future.
We carry smartphones in our pockets, the streets are dotted with semi-autonomous cars, but in the research laboratory experiments are still being designed by...
What enables electrons to be transferred swiftly, for example during photosynthesis? An interdisciplinary team of researchers has worked out the details of how...
For the first time, scientists have precisely measured the effective electrical charge of a single molecule in solution. This fundamental insight of an SNSF Professor could also pave the way for future medical diagnostics.
Electrical charge is one of the key properties that allows molecules to interact. Life itself depends on this phenomenon: many biological processes involve...
At the JEC World Composite Show in Paris in March 2018, the Fraunhofer Institute for Laser Technology ILT will be focusing on the latest trends and innovations in laser machining of composites. Among other things, researchers at the booth shared with the Aachen Center for Integrative Lightweight Production (AZL) will demonstrate how lasers can be used for joining, structuring, cutting and drilling composite materials.
No other industry has attracted as much public attention to composite materials as the automotive industry, which along with the aerospace industry is a driver...
Scientists at Tokyo Institute of Technology (Tokyo Tech) and Tohoku University have developed high-quality GFO epitaxial films and systematically investigated their ferroelectric and ferromagnetic properties. They also demonstrated the room-temperature magnetocapacitance effects of these GFO thin films.
Multiferroic materials show magnetically driven ferroelectricity. They are attracting increasing attention because of their fascinating properties such as...
08.01.2018 | Event News
11.12.2017 | Event News
08.12.2017 | Event News
19.01.2018 | Materials Sciences
19.01.2018 | Health and Medicine
19.01.2018 | Physics and Astronomy