Many seismologists and researchers have long believed that Earth possesses a fast-flowing and well-mixed mantle. But that theory may require some revisions according to new findings from researchers at Utrecht University of the Netherlands. Their evidence comes from a pair of submerged, continent-sized “out-thin” geological islands on a “Graveyard” tectonic plate 1,800 feet-below the planet’s surface.
Their study, published Jan. 22 in the journal Naturerelies on the tones generated during large earthquakes that cause the Earth to vibrate like a giant bell. Like one associated university announcement Explains, seismologists study the planet’s interior by analyzing the acoustic signatures of these oscillations. Experts can then also identify anomalies based on whether regions are out of alignment, or if their volume is muted.
More than 25 years ago, Researchers discovered that some of these deep Earth reverberations pointed to the existence of two subterranean “supercontinents” hundreds of miles beneath Africa and the Pacific Ocean. At the time, scientists were unsure whether these formations near the boundary between Earth’s mantle and core were a temporary phenomenon, or whether they had existed there for millions or billions of years. What they knew, however, was what the pair of mysteries encapsulated.
“These two large islands are surrounded by a graveyard of tectonic plates that were transported there by a process called ‘Subduction,'” study co-author and Utrecht University seismologist Arwen Deuss stated in Thursday’s announcement. During subduction, a tectonic plate shifts beneath another, forcing it from the Earth’s surface to almost 1,900 feet.
The two subcontinents, as well as any other areas that produce seismic waves, are known as large low seismic velocity provinces or LLSVPs. One of the main reasons why acoustic delays occur is due to the hotter temperature of an LLSVP compared to the surrounding environment. Duess and collaborators focused on the LLSVPs’ ability to “dampen” seismic waves, referring to the energy loss that occurs as waves travel through the planet. They paid particular attention to where tones went out of tune, as well as how loud or quiet they became during their travels.
“Against our expectations, we found little damping in the LLSVP, which makes the notes there sound very loud,” explained co-author Sujania Talavera-Soza. “But we found a lot of damping on the cold plate, where the notes sounded very soft.”
This is in contrast to the readings collected from the upper mantle which looked as expected – waves dampened by the warm temperatures. Talavera-Soza likened the difference to going for a run in warm or cold weather. When it is hotter, runners tend to slow down and become easier than when the temperature is much cooler.
Colleagues suggested moving beyond examining temperatures to examining the mineral composition of an LSVP, particularly its individual grain sizes. According to Duess, grain size turned out to be “much more important”.
Deuss explained that the Slab Graveyard’s LLSVPs are made of small grains that formed after the minerals recrystallized during each formation’s downward journey to the planet. Smaller grains mean a much larger number, as well as a larger number of the small gaps between them. Any acoustic waves traveling through these formations lose energy as they pass through the many grain boundaries, leading to increased damping. But because the two LLSVPs released very little damping, their grain sizes must be much larger.
Larger grains also imply that these LLSVPs are much older than researchers first assumed—at least 500 million years old, but perhaps even reaching 1 billion years. These mineral grains are also much more rigid, making them able to resist the flow of the Earth’s mantle, called mantle convection.
“After all, the LLSVPs must somehow be able to survive mantle convection,” Talavera-Soza said.
The recent discoveries contradict the descriptions of a very fluid and well-mixed mantle in most geology books. Such a potentially major seismological overhaul has reverberations far beyond LLSVP composition, age, or motion. Understanding how these massive formations grow in size and interact with their environment helps better illuminate Earth’s planetary evolution. It also affects the inner workings of volcanoes and mountains.
“Earth’s mantle is the engine that drives all these phenomena,” Duess said, offering mantle plumes as an example. These large pockets of molten material rise through the Earth from deep within its interior, much like the movement in a lava lamp. Once near the surface, these plumes help cause volcanic eruptions.
‘[W]e think those mantle plumes originate at the edges of the LLSVPs,” Duess said.
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