3-D Global Map Shows Electrical Conductivity in Earth's Mantle

Water conducts electricity extremely well, so it turns out electrical conductivity in parts of Earth's mantle may signal the presence of water.
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Water conducts electricity extremely well, so it turns out electrical conductivity in parts of Earth's mantle may signal the presence of water.

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Oregon State University scientists have mapped Earth's electrical conductivity on a global-scale in 3-D for the first time. Photo: Anna Kelbert

In a study published this week in the journal Nature, researchers described the first global 3-D map of electrical conductivity in the mantle.

What did it show? The areas of high conductivity coincide with subduction zones—places where tectonic plates are being subducted beneath the Earth's crust.

The Oregon State University scientists who performed the research looked at the Earths mantle with electromagnetic induction sounding, a method very sensitive to interconnecting pockets of fluid in rocks and minerals.

"This work is important because it complements global 3-D seismic imaging of Earth's interior, which uses sound waves generated by earthquakes," said Robin Reichlin, program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "Scientists may be able to combine these two methods to tease out a more detailed understanding of variations in Earth's inner composition, water content and temperature."

Subducting plates are comparatively colder than surrounding mantle materials and should be less conductive, geologists have believed. However, the OSU scientists suggest, conductivity in these areas may be enhanced by water drawn downward during the subduction process.

"Many earth scientists thought that tectonic plates are not likely to carry much, if any, water deep into the Earth's mantle," said Adam Schultz, a geologist at OSU and a co-author of the Nature paper. "Our model, however, clearly shows a close association between subduction zones and high conductivity. The simplest explanation is water."

The study provides new insights into the fundamental ways in which our planet works, Schultz says. Despite advances in technology, scientists are still unsure how much water lies beneath the ocean floor and how much of it makes its way into the mantle.

"In fact, we don't really know how much water there is on Earth," said Gary Egbert, an oceanographer at OSU and co-author of the paper. "There is some evidence that there is many times more water below the ocean floor than there is in all the oceans of the world combined. Our results may shed some light on this question."

Anna Kelbert, a post-doctoral researcher at OSU and lead author of the paper, says that the next step is to replicate the experiment with newly available data from both ground observatories and satellites, then conduct further research to better understand the water cycle and how its interaction with deep-Earth minerals works.

Ultimately, the scientists hope to produce a model quantifying how much water may be in the mantle, locked up in its rocks. Their work is also supported by NASA.


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