Where the Earth’s outer core joins the mantle, thin structures have been discovered that drastically impede the transmission of seismic waves. These features, known as Ultra Low-Velocity Zones (ULVZs), are made up of layers of different materials, most likely leftover from when the Earth’s outer regions were a magma ocean, according to new research. At least some ULVZs might have formed because of the same event that triggered the Moon’s genesis. Despite Jules Verne’s grandiose ambition, we are still a long way from making an actual visit to the Earth’s core. As a result, the majority of what scientists know about our planet’s internal structure comes from studying seismic waves.
These waves are to geologists what X-rays are to doctors: aberrations in seismic wave patterns may be used to recreate the planet’s innermost workings. This is how we know the Earth’s core is not completely rigid. Flat formations where the core meets the mantle, as detailed in Nature Geoscience, are a fresh contribution to this understanding. In the 1990s, seismic wave measurements revealed vast zones less than 10 kilometers (6.2 miles) high, whose composition delayed waves by up to 50% as they travelled between the mantle and core, or bounced off the border.
Little was known, however, about these zones, which research co-author Dr. Michael Thorne of the University of Utah characterized as “some of the most severe characteristics found anyplace on the globe” in a statement. We do not even know how prevalent ULVZs are, according to Professor Hrvoje Tkali of the Australian National University, because detecting them needs both deep earthquakes large enough for signals to survive distortion and well-placed monitoring gear.
The Tonga-Fiji subduction zone regularly produces the appropriate kind of earthquakes. Fortunately, they are quite deep and entail lateral rather than vertical movement, therefore the area is spared from regular tsunamis, according to Tkali. These quakes, on the other hand, are terrific instruments for investigating the globe. In both Australia and North America, the quakes expose ULVZs midway between the source and a plethora of detectors, in excellent places to investigate as waves bounce off them on their approach to the detectors.
Tkali and co-authors, in particular, went to great lengths to investigate a ULVZ beneath the Coral Sea off the coast of Australia. “The most important discovery is that they are stratified,” Tkali told IFLScience, explaining that each stratum has distinct impacts on seismic waves and hence varied compositions.
“That’s a significant intellectual leap. Internal structure shifts the worldview… They resemble Earth’s sedimentary rocks, and we can use what we know about formation processes to help them.” The researchers believe ULVZs formed in a magma ocean, similar to how sedimentary rocks form in an ocean or lake. Tkali continued, “Heavy elements went down, light elements went up.” There may have been multiple instances during the Earth’s creation when it grew so heated that its outer layers separated in this fashion. Most planetary scientists believe this happened when a Mars-sized asteroid, which ejected material that eventually became the Moon, affected the Earth.
“These zones have churned near to the planet’s core yet mostly stayed intact during billions of years of the Earth’s forming and reshaping,” Tkali said in a statement. “It’s like an egg in a cake that doesn’t get mixed in with the other components and remains as [yolk] and egg white despite frequent mixing all around it.” Tkali has some bad news, as exciting as this finding is. ULVZs, according to some geologists, might explain the mysterious origin of hotspots like those under Hawaii and Iceland. “There is no clear spatial link,” Tkali told IFLScience, “between the two events.”
