Geophysicists have found sunken worlds in the Earth’s mantle — the planet’s bulky middle layer — that, according to both earlier imaging and understanding of plate tectonics, simply shouldn’t be there.
When neighboring tectonic plates continue their slow-motion collision, sometimes one subducts, or slides underneath the other, often leaving geological remnants behind. Now, new imaging techniques have revealed similar leftovers far from tectonic boundaries. Geophysicists detected these secret worlds beneath large oceans or under the middle of continents — even though there is no record of subduction in those areas, according to a study in Scientific Reports.
“Apparently, such zones in the Earth's mantle are much more widespread than previously thought,” Thomas Schouten, a graduate student at ETH Zurich and an author of the paper, said in a news release.
Learning About Secret Worlds
The team of geophysicists from Switzerland and the California Institute of Technology found these zones by looking for them in a new way.
Scientists have long recorded seismic waves as they move away from their source, measuring how they are partially absorbed, bounced, or redirected, among other properties. They then calculate backward, to find these waves’ origins. These calculations have always led to tectonic boundaries, where one plate subducts.
But not this time. The findings run against conventional geological wisdom, because the newly discovered areas lie far from any tectonic boundaries and can’t be associated with any known geological activity.
Read More: Drilling Deep: How Far Have We Gone Under Earth's Crust?
A New Mantle Mystery
Instead of clarifying how tectonic movement shapes the mantle, the study adds confusion and mystery to it.
“That's our dilemma. With the new high-resolution model, we can see such anomalies everywhere in the Earth's mantle,” Schouten added in the release. “But we don't know exactly what they are or what material is creating the patterns we have uncovered.”
Andreas Fichtner, the ETH researcher who developed the tool, compared its use to medical imaging. Doctors can see certain things with X-rays. But when they add CT or PET scans, they sometimes find additional information.
Now that these unexpected areas have been identified, the next step is to figure out how and why they were created. There is no shortage of theories.
“It could be either ancient, silica-rich material that has been there since the formation of the mantle about 4 billion years ago and has survived despite the convective movements in the mantle, or zones where iron-rich rocks accumulate as a consequence of these mantle movements over billions of years,” said Schouten.
Finding a solid reason that explains the discovery will likely require new ways of thinking about how different kinds of waves travel through the Earth and what that information says about the mantle’s makeup and activity.
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Scientific Reports. Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies
ETH Zurich. Sunken worlds under the Pacific?
Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.
