Looking for answers – Eric Ferré, professor of geology at Southern Illinois University Carbondale, examines a rock that came from the Earth’s mantle recently. Ferré, along with fellow SIU geology faculty James Conder, associate professor, and Justin Filiberto, assistant professor, are in the midst of a two- to three-year study examining the magnetic properties of the internal structure of the Earth, specifically the upper mantle. (Photo by Steve Buhman)
June 10, 2015
Study challenges old notions about Earth’s mantle
CARBONDALE, Ill. – Professor Eric Ferré has seen the Hollywood movie, “The Core.” It left the geology researcher unimpressed with its realism.
“That’s not going to happen anytime soon,” he said with a chuckle, referring to the plot of the 2003 movie in which a group of explorers drill their way to the Earth’s core in order to save the world.
But that doesn’t mean Ferré and his colleagues at Southern Illinois University Carbondale don’t have a deep and abiding interest in what lies beneath. Deep, deep beneath.
Ferré, along with fellow SIU geology faculty James Conder, associate professor, and Justin Filiberto, assistant professor, are in the midst of a two- to three-year study examining the magnetic properties of the internal structure of the Earth, specifically the upper mantle, and in doing so are challenging notions scientists thought they put to bed nearly 40 years ago.
The mantle, as students learn in basic geology class, is the broad middle structure of the Earth, sandwiched between the planet’s relatively thin crust and its molten core. The mantle is too deep to reach at most places on the Earth. So scientists rely on magnetic and seismological measurements to understand its nature.
But magnetic properties often are temperature-dependent. And much like an old music cassette tape left inside a car on a hot summer day, higher temperatures result in the destruction of magnetic memory. Consequently, the tape sounds terrible and hot and deep minerals won’t exhibit magnetic properties.
Measuring magnetic properties in relation to temperature, therefore, is a key aspect of studying the mantle. It assumes that temperatures at a certain depth – say about 30 kilometers – are high enough that minerals can no longer carry the requisite magnetic characteristics. This depth, where materials reach a temperature known as their Curie Point and lose their magnetic properties, has served as a baseline for understanding the Earth’s unreachable, unseen inner structure.
But the SIU trio, along with collaborators from the University of Notre Dame and the University of Minnesota, are taking a new, more detailed look at the magnetic properties of the mantle, which scientists previously mapped using NASA’s Magsat satellite. Those global images of the Earth’s magnetic field, taken from space and somewhat backed up by measurements on the ground, have served as the scientific standard since their creation in the early 1980s.
The SIU researchers, however, are using a $225,000 grant from the National Science Foundation to investigate and challenge this previous understanding. And the evidence they have found so far is backing up their hypothesis.
The information deduced from magnetic satellites provides vital information on regions of the globe where tectonic plates collide and create abundant seismicity. It also will help supplement an ongoing study by European Space Agency satellites that currently are taking measurements from space.
“Up to now, we thought that mantle rocks, at depths of about 30 kilometers, did not carry any magnetic minerals, an assumption based on measurements made in the 1980s,” Ferré said. “We are now re-evaluating this concept, hoping to provide a far more accurate knowledge base to interpret new magnetic satellite data.”
The team approach to the issue plays to each members’ strengths. Ferré and his students focus on magnetic measurements, while Conder and his students do numerical modeling. Filiberto subjects samples to high temperatures and pressures while observing the changes that take place.
Ferré said the NASA map was an invaluable tool for many years, but scientists began noticing discrepancies between that data gathered from space and more direct observable data collected on the ground.
“By the time you’re with a satellite you are quite far away from the source of the magnetism and the quality of the image degrades. But if you want global coverage, including the oceans, you really have to use a satellite to get it,” he said. “At the time, the conventional wisdom was that anything below 30 kilometers didn’t matter because things that deep were too hot to be magnetic anyway.
“We are challenging this wisdom,” Ferré said. “Some data sets over the years began showing things that did not match up with the old satellite model. There were some significant mismatches, in fact. Lots of places where one would see them. So we thought there was room to reinvestigate.”
The mantle, however, still lies too deep for humans to reach, even with modern drilling techniques developed since the Magsat program. The research team, however, has arrived at a solution in regards to looking first-hand at chunks of the mantle.
“Since we can’t go down to the mantle, we let the mantle come to us,” Ferré said. ”There are volcanoes all over the planet, and some bring up chunks of the mantle.”
These so-called “xenoliths,” or “rocks that don’t belong,” hold a wealth of information about the upper mantle and may be the keys to rewriting what is known about the upper mantle. The SIU team is feasting on a huge variety of such rocks, borrowed from collections all over the world – some of them extremely rare, pristine and somewhat jealously guarded -- and representing regions from the Arctic Circle to all the continents and oceans.
Using different techniques, the researchers can pinpoint the temperature and pressure at which the rocks were formed before being jettisoned from the volcanoes. All these rocks feature peridot, a green crystal sometimes used for making jewelry.
“What’s really unique is we are working with rocks from all over the planet,” Ferré said. “Our students are examining rocks from Russia, Japan, Chile and even the Arctic Circle. We have gained access to private collections, government collections and some collections that are very rare and difficult to gain access to. We knock on doors and beg people to lend us their samples.”
SIU graduate students, supervised by the three researchers, are using electron microscopy and other microanalytical techniques to examine the specimens. The team also is using decompression experiments and numerical modeling to try and understand how the rocks change during their ascent from the upper mantle to the surface, via the volcanic systems. Taken together, the data should help the scientists develop new models of magnetic anomalies in certain geologic regions.
The SIU team has examined about 400 samples so far. Along the way they have seen evidence – in some cases including hand-written notes from the earlier Magsat work – that indicates the science was little rushed back then.
“In some cases the samples didn’t look that fresh, and it was a little flimsy,” Ferré said. “It appeared they were in a hurry because NASA wanted results.”
This time around, however, the researchers have access to far more physical samples from far more locations then scientists back in the 1980s. This can only help the reliability of the ultimate results, Ferré said, and eventually will help an ongoing multisatellite-based mission that is using a new approach to map the Earth’s magnetic field from space.
Launched in 2013, the European Space Agency’s Swarm mission will monitor Earth’s magnetic field for four years, looking as deep as the Earth’s core to the heights of its upper atmosphere. The mission is aimed at providing unprecedented insights and performing precise measurements using an array of three satellites.
While the mission capabilities are much improved from the Magsat days, it is still based in space, Ferré said. Eventually, the satellite measurements will need to be verified by ground work, such as the type he, Conder, Filiberto and their graduate students are performing.
“Our project provides upstream data for that mission,” he said.
Ultimately, the findings will help scientists better understand the Earth’s geological features, such as subduction zones, where one tectonic plate is being pushed under another. Temperature information for such areas tell scientists something about type of movement might happen there – shallow or deep earthquakes, based on the brittleness of the plate, for instance. This can help scientists understand and better predict catastrophic events, such as tsunamis.
“It may tell us more about how active certain zones are, why others are not as active,” Ferré said.
Commercial applications also include finding better ways to locate new diamond mines, which also are dependent on earth temperature.
Ferré said the team has been fortunate in gaining access to so many rare samples. Some of them, for example, were collected by geologists in Russia at great risk to their own lives while they were still quite hot after being ejected from volcanoes there.
“It’s somewhat the art of negotiation,” he said, smiling. “We have been very lucky. We’ve been able to obtain access to things I never thought people would be willing to share with us. Things that are so rare that there are maybe only 10 available in whole world. And we are kind of picky. We only want to use the fresh stuff.”