Giant, Previously Unknown Structure Discovered Deep Inside Earth

Researchers have detected a large, previously unknown structure—consisting of unusually dense, hot rock—deep inside the Earth below the Pacific Ocean.

The structure, known as an ultralow-velocity zone (ULVZ), is located on the boundary between Earth's superheated, molten core and the solid mantle, lying directly underneath the volcanic Marquesas Islands in French Polynesia in the South Pacific, according to a study published in the journal Science.

The international team from the University of Maryland (UMD), Johns Hopkins University and Tel Aviv University, Israel, also found evidence to suggest that a previously identified ULVZ beneath the Hawaiian Islands at the core-mantle boundary is much larger than previously thought.

ULVZs lie at the bottom of plumes within the Earth, geological features where hot rocky material is thought to rise from the core-mantle boundary to the planet's outer crust, leading to the creation of volcanic islands, such as Hawaii and the Marquesas. In fact, the ULVZ located beneath Hawaii is the largest example known to science.

"There is a theory that if you have a plume of rising hot rock—which produces the hot spot volcanism at the surface that creates ocean island chains like Hawaii and Marquesas—that this rising rock will kind of suck on the melt and pull it up, so the ULVZ ends up being very large in areas where material is going up. However, there are others who think ULVZs just represent regions where mantle is very enriched in iron," Doyeon Kim, lead author of the study from UMD's Department of Geology, told Newsweek.

The scientists were able to detect the structures by analyzing data on seismic waves, which can reveal hidden, subterranean structures as they travel through the planet.

These seismic waves, which are generated by earthquakes, travel thousands of miles below the surface. But as the material that they pass through varies in density, temperature or composition, the waves change speed, bend or scatter, producing echoes that scientists can detect using instruments known as seismometers.

Using this data, researchers can put together a picture of the rock that lies below the surface and estimate its physical properties. In the latest study, the authors used a machine learning algorithm called "the Sequencer" to simultaneously analyze around 7,000 seismic wave recordings—known as seismograms—generated by hundreds of 6.5-plus magnitude earthquakes that struck the Pacific region between 1990 and 2018.

These waves diffracted along the core-mantle boundary, providing a comprehensive view of the deep Earth below the Pacific region.

"Seismic waves travel up to 30 percent slower in ULVZs than through surrounding mantle materials. When seismic waves interact with ULVZs, some of the energy can bounce off, producing loud echoes," Kim said.

"By looking at thousands of core-mantle boundary echoes at once, instead of focusing on a few at a time, as is usually done, we have gotten a totally new perspective," Kim said in a statement. "This is showing us that the core-mantle boundary region has lots of structures that can produce these echoes, and that was something we didn't realize before because we only had a narrow view."

Earth diagram
A diagram of the Earth illustrating the new research. Doyeon Kim/University of Maryland

To their surprise, the researchers found that nearly half of the diffracted waves had been scattered by three-dimensional structures near the core-mantle boundary, casting new light on this region of the Earth underneath the Pacific. They found many structures previously identified, but also a new, ultra-low-velocity zone beneath the Marquesas Islands.

The scale of typical ULVZs found elsewhere are in the order of around 100 kilometers (62 miles) across. But what the team uncovered beneath the Marquesas Islands is an order of magnitude larger than those typical ULVZs—a structure around 1,000 kilometers (620 miles) across. ULVZs of this extraordinary size are called "mega-ULVZs."

"What was known previous to our study is that there are three mega-ULVZs on Earth— beneath Hawaii, Iceland, and Samoa. We observed loud echoes generated by mega-ULVZs whose properties are very different from the surrounding mantle: one beneath Hawaii which turned out to be much larger than previously thought and one beneath Marquesas which is one of the new discoveries we made," Kim told Newsweek.

"We were surprised to find such a big feature beneath the Marquesas Islands that we didn't even know existed before," Vedran Lekic, a co-author of the study from UMD, said in the statement. "This is really exciting, because it shows how the Sequencer algorithm can help us to contextualize seismogram data across the globe in a way we couldn't before."

In addition to the particularly loud echoes from below Hawaii and the Marquesas Islands, indicating the presence of large ULVZs, the team also detected weaker echoes that are widespread, observed from almost everywhere beneath the Pacific.

"We found echoes on about 40 percent of all seismic wave paths," Lekic said. "That was surprising because we were expecting them to be more rare, and what that means is the anomalous structures at the core-mantle boundary are much more widespread than previously thought."

The team say these weaker echoes must indicate more distributed structures, as opposed to localized structures, such as ULVZs.

"We think that the most likely—but not the only—explanation is that these widespread 'pervasive' echoes come from the boundaries of a continent-sized structure called an LLSVP (large low shear velocity province. However, in some locations, this cannot explain the signals, and they instead tell us about much smaller structures being present to produce the echoes, though we cannot precisely determine where," Kim said.

In the study, the scientists focused on echoes produced by a specific class of seismic waves known as shear waves. According to the United States Geological Survey, shear waves move the ground back and forth perpendicular to the direction that the wave is moving.

When looking at just a single seismogram, it is difficult to distinguish echoes generated by diffracted shear waves from random noise. However, looking at many seismograms recorded simultaneously can provide valuable insights into the Earth's interior.

Despite the latest findings, scientists still know relatively little about the composition of ULVZs. However, studies like the latest paper could have implications for our understanding of geological processes, such as plate tectonics, as well as the evolution of our planet.

This article was updated to include additional comments from Doyeon Kim.