Recent reports out of Northwestern University that vast amounts of water are not only in the oceans and topmost layers of the solid Earth but also stored deep within the Earth’s mantle hundreds of kilometers down may have rocked mainstream media and the general public to the core, but the announcement didn’t come as news to Texas A&M University researcher Wolfgang Bangerth.
“I think the people in the field knew this for a long time,” said Bangerth, a professor in the Texas A&M Department of Mathematics since 2005 and an expert in computational science and mathematical modeling. “It’s not really a surprise result. The novelty lies in the fact that someone could experimentally show it.”
Such revealing results are made possible by people like Bangerth, a broad-based scientist with proficiency in both mathematics and geophysics who teamed with former Texas A&M colleague Timo Heister a few years ago to write a software program called ASPECT (Advanced Solver for Problems in Earth’s ConvecTion). Their code — the evolving result of a five-year, $800,000 subcontract that runs through 2015 with the National Science Foundation-funded Computational Infrastructure for Geodynamics in California — is used by geodynamics researchers around the world who are working to develop a clearer picture of the Earth’s interior and to accurately describe how material flows in the Earth’s mantle with the help of computerized simulation and mathematical modeling.
Bangerth cites one such international researcher — Juliane Dannberg, a graduate student at the German Research Center for Geosciences in Potsdam, Germany — who is using the ASPECT code to model what happens when hot material is transported from the Earth’s core-mantle boundary upward into the surrounding colder regions. Normally, the enormous pressures inside Earth prevent material from melting, but chemical impurities in the rock — for instance, the presence of small amounts of water — can lead to partial melting that radically alters the rock’s properties.
Dannberg’s resulting video (shown below and also directly viewable here, shows this process, called thermochemical convection, with each frame representing activity during a 500,000-year period to simulate a total of 275 million years of inner-Earth history.
The Northwestern findings suggest that material 400 miles beneath North America — the so-called “transition zone” — is partially molten and that this can only be explained by the presence of water in the rocks there. The researchers say the water may have been driven from the Earth’s surface to these depths by plate tectonics, one of ASPECT’s distinct modeling competencies.
“Recall that in places like the Pacific Northwest or the west coast of Southern America, an oceanic plate is diving under the continent, and that oceanic plate has a kilometer or more of water-saturated sediments on it,” Bangerth said. “Surely some of these sediments will dive with the plate into the depth and take the water with it. So it must be somewhere, and presumably there must also be a way for it to come back to the surface again. So, that there is water is something one can be fairly certain about from theoretical considerations. That one can point to a place where it has a measurable effect is pretty cool, though.”
At the same time, Bangerth notes, the evidence for melting in the transition zone is only indirect — inferred by measuring the speed of earthquake waves as they travel from their sources to detectors at the Earth surface. When it comes to how conclusive that data is, Bangerth says he’s not going to hold his breath.
“It’s so difficult to interpret seismic data that it may or may not be the correct interpretation of what they see,” he added. “I think there are a lot of experiments that will still need to be done before this is settled. There will also need to be many more computer simulations with codes such as ASPECT to rule out other explanations.”
The ASPECT program is based off of deal.II, a software library that Bangerth began developing as an undergraduate student in the 1990s to help researchers harness the power of supercomputers to provide more accurate pictures of complex problems and processes in a host of vital areas, from the environment and energy to healthcare. He and a couple dozen researchers and programming experts worldwide constantly are working in tandem with several hundred international users to refine the software, which he says is purposefully structured as an open-source community project to ensure broad access, maximum flexibility and overall developmental fun.
“We just had a hackathon where 14 people from around the world got together to spend 10 days programming and extending the ASPECT code,” Bangerth said. “It was so productive, resulting in about 500 changes to the program, that we will probably do this every year from now on.”
Bangerth, who has just returned to Texas A&M after teaching a weeklong deal.II course in Seoul, South Korea, will be offering additional ASPECT tutorials this summer in conjunction with geodynamics-research-related events in Santa Barbara, California; Tokyo, Japan; and Postdam, Germany.
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Watch Juliane Dannberg’s video that uses ASPECT code to show thermochemical convection in the Earth’s mantle:
Contact: Shana K. Hutchins, (979) 862-1237 or firstname.lastname@example.org or Dr. Wolfgang Bangerth, email@example.com