Earth's Underworld is Real and Scientists Just Mapped It
Scientists have created an "Atlas of the Underworld" compiling 94 slabs of ancient tectonic plates that now lie deep beneath Earth¿s surface. The atlas spans roughly 300 million years of Earth¿s history. The image above show the Cocos

Scientists have created an ‘Atlas of the Underworld,’ compiling of  oceans and mountains lost to Earth's history

At intersections of tectonic plates worldwide, slabs of ocean crust dive into the mantle, part of the continuous cycle that not only drives the continents’ drift, but also fuels the volcanism that builds up island chains like Japan and mountains like the Andes. The disappearance of these slabs, called subduction, makes it difficult to reconstruct oceans as they existed hundreds of millions of years ago, as well as the mountains flanking them.

“Every day, we’re losing geologic information from the face of the Earth,” says Jonny Wu, a geologist at the University of Houston in Texas. “It’s like losing pieces of broken glass as you’re trying to put it together again.”

But geoscientists have begun to pick up these pieces by peering into the mantle itself, using earthquake waves that pass through Earth’s interior to generate images resembling computerized tomography (CT) scans.

In the past few years, improvements in these tomographic techniques have revealed many of these cold, thick slabs as they free fall in slow motion to their ultimate graveyard—heaps of rock sitting just above Earth’s molten core, 2900 kilometers below.

Now, the complete x-ray of Earth’s interior is coming into focus. Next month, at a meeting of the American Geophysical Union in San Francisco, California, a team of Dutch scientists will announce a catalog of 100 subducted plates, with information about their age, size, and related surface rock records, based on their own tomographic model and cross-checks with other published studies.

“Step by step we went deeper and deeper, older and older,” says Douwe van Hinsbergen, a geologist at Utrecht University in the Netherlands, who led the project along with Utrecht geologists Douwe van der Meer and Wim Spakman.

This “atlas of the underworld,” as they call it, holds the ghosts of past geography. By rewinding the clock and bringing these cataloged slabs back to the surface, scientists can figure out the sizes and locations of ancient oceans.

Moreover, they can locate where the sinking slabs would have triggered melting, releasing blobs of magma that rose into the crust and drove volcanism. That has helped earth scientists pinpoint where ancient mountains rose and later eroded away, their traces visible only in unexplained rock records. “It’s a pretty exciting time to be able to pull all of these pieces together,” says Mathew Domeier, a tectonic modeler at the University of Oslo.

That has only recently become possible, as the underlying technique, mantle tomography, is plagued with uncertainties. It relies on millions of seismic waves received by sensors scattered unevenly around the world. Waves with faster arrival times are assumed to have passed through the colder rock of subducted slabs.

But seismometer coverage is patchy; earthquakes—the sources of the seismic waves—don’t occur everywhere; and the waves get fuzzier as they pass near the core or travel long distances. “Very often for regions that have the most interesting structures, you have the most uncertainty,” says Ved Lekic, a tomographer at the University of Maryland in College Park.

Academic groups around the world use more than 20 models to interpret tomographic data, and their pictures of the mantle and its structures often conflict, says Grace Shephard, a postdoc at the University of Oslo.

In the coming months, she will publish a comparison of 14 different models that will assess which slabs seem most likely to be real. Her results could cast doubt on some of the slabs in the Utrecht atlas. But the image of Earth’s interior is becoming more believable, thanks to improved computing power and such intercomparison projects.

By now the picture of lost plates is precise enough for scientists to try rewinding the clock, reconstructing vanished worlds. In earlier tomography, the plunging slabs looked like blobs in a lava lamp. But as the models have improved, the slabs in the upper mantle have been revealed to be stiff, straight curtains, says John Suppe, who heads the Center for Tectonics and Tomography at the University of Houston.

The images make it clear that as they plunge, the 500-kilometer-thick slabs flex but don’t crumple—and that has made it easier for Suppe and others to unwind them. “We’re finding these plates unfold fairly easily, and they’re not that deformed,” Suppe says.

These slab-driven reconstructions are calling into question plate movements inferred from ancient oceanic crust that was scraped off and preserved on the continents, Suppe says. “Almost everywhere we’ve looked at this,” Suppe says, “what we find in the mantle isn’t exactly what would be predicted.”

The study was published  in Science Advances.

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