For decades, scientists have observed isolated spots near the base of Earth’s mantle where traveling seismic waves slow significantly. Researchers now believe these features may be more widespread than previously thought.
In a study published in Science Advances, scientists mapped these so-called ultralow-velocity zones (ULVZs) in a previously unexamined part of the Southern Hemisphere. New simulations showed they could be the remains of foundered ancient ocean floor.
ULVZs are poorly understood, and their origins are debated. Scientists have proposed two broad theories, though they are not mutually exclusive. One holds that ULVZs form where temperature differences in the deep mantle result in partial melting, which slows seismic waves. Studies have associated ULVZ locations with mantle plumes—the upwelling of material from deep within Earth—such as those below the Hawaiian and Iceland hot spots.
The other theory posits that ULVZs are compositionally different from surrounding material—the result of chemical interactions between the liquid iron core and the silicate mantle or the remnants of subducted material from Earth’s crust.
Probing from the Bottom of the World
Only about 20% of the core-mantle boundary (CMB) had previously been examined for ULVZs, and in the Southern Hemisphere, data were particularly sparse. Using seismic monitors stationed in Antarctica between 2012 and 2015, a group of scientists picked up seismic waves traveling through Earth’s interior from earthquakes happening around the globe. The recordings were serendipitous; the scientists had been studying the structure of the Transantarctic Mountains when they realized the data could help illuminate the mantle.
The researchers analyzed changes in seismic wave speeds to generate high-resolution imagery of the mantle and found ULVZs at the CMB all over the Southern Hemisphere. Their abundance was a surprise, said Samantha Hansen, a geologist at the University of Alabama and lead author of the study.
Previous studies found that ULVZ thickness varied, ranging from about 8 to 20 kilometers. The authors of the new study also observed similar variation in the Southern Hemisphere ULVZs. “We questioned what could cause such a widespread, variable structure along the CMB,” Hansen said.
The researchers analyzed whether subducted material could explain the origins and characteristics of the ULVZs using a global model that simulates the movement of material in Earth’s interior.
At long timescales, the mantle flows like a soft solid, driven by convection. If ULVZs are chemically distinct from the surrounding material, the researchers posited that the features would be pushed laterally along the CMB toward upwelling zones or much bigger features called large low shear velocity provinces. “Anything that’s been on the CMB more than a few tens of millions of years is going to be under an upwelling,” said geophysicist John Hernlund at the Tokyo Institute of Technology, who was not involved in the study.
The results were intriguing because the authors mapped ULVZs away from upwellings, suggesting they have not yet migrated into these areas and are relatively young, Hernlund explained. “The observation of these features away from upwellings provides very good evidence that at least some ULVZs are being created today, and that’s very interesting.”
The model also showed that once oceanic crust subducts, it is dense enough to sink to the bottom of the mantle. It also indicated that plate tectonic movement over roughly the past 200 million years could cause enough oceanic crust to sink into the mantle to cover the CMB.
ULVZ material is pushed around and piled up, creating zones of varying thickness, Hansen said. “The consistency between the seismic findings and the models makes for a very compelling interpretation.”
The results could shed light on how ULVZs affect the movement of heat within Earth. Understanding these dynamics is important because heat in this region has been shown to strongly affect Earth’s magnetic field, Hansen said.
Furthermore, the findings could help scientists understand why hot spots—such as the one beneath Hawaii—and the plumes that feed them form where they do. “Since mantle plumes are largely controlled by the thermal conditions near the CMB, the temperature influence of ULVZs may help dictate where plumes form,” Hansen said.
Hansen and her colleagues plan to broaden the data set to learn more about ULVZs and how they work. Preliminary data from additional seismic monitors in Antarctica were consistent with the results of this study, she added.
—Tim Hornyak (@robotopia), Science Writer