"Of all Earth’s deep interior features, these are the most fascinating and complex..."
A team of researchers led by the University of Cambridge has obtained an unprecedented “image” of a pocket of rock lying three thousand kilometers beneath the surface of our planet.
There is an enigmatic area of rock, something like a Bermuda triangle for geology, located directly beneath the Islands of Hawaii. The region beneath is known as one of several “ultra-low velocity zones” referred to by experts as such since earthquake waves tend to slow down dramatically when they pass through them.
Not much was known about these spots. Now, research titled “Kilometer-scale structure on the core-mantle boundary near Hawaii” and published in Nature Communications clears up some of the mysteries behind the area, revealing complex internal variability of the pockets in unprecedented detail and exposing a landscape of our planet’s interior and the processes that occur thousands of kilometers beneath our feet.
“Of all Earth’s deep interior features, these are the most fascinating and complex. We’ve now got the first solid evidence to show their internal structure – it’s a real milestone in deep earth seismology,” revealed lead author Zhi Li, a Ph.D. student at Cambridge’s Department of Earth Sciences.
An onion world
Our world is structured like a kind of gigantic onion, composed of several structures that lie within the interior of the planet: the iron-nickel core is at the center, surrounded by a thick layer called the mantle, and on top of that lies the crust. Despite being solid rock, the mantle is hot enough to flow exceptionally slowly. Tectonic plates are moved by these internal convection currents, which fuel volcanic eruptions and drive tectonic plate movement.
The echoes and shadows of seismic waves from earthquakes provide scientists with radar-like images of the depths beneath the surface of Earth. It has, however, been difficult to analyze “images” of structure at the core-mantle boundary, a key region for understanding our planet’s internal heat flow.
In the core-mantle boundary, the researchers were able to spot kilometer-scale structures using the most advanced numerical modeling methods.
Furthermore, researchers observed a 40% reduction in seismic wave speeds beneath Hawaii’s ultralow velocity zone. Based on this evidence, it is reasonable to assume that the zone contains much more iron than the surrounding rocks – and is, therefore, denser and more sluggish.
Using this research, scientists might also learn more about the causes of volcanic chains like those found in the Hawaiian Islands.
It has become apparent recently that there is a correlation between the location of so-called hotspot volcanoes, such as those found in Hawaii and Iceland, and the ultra-low velocity zones at the base of the mantle. Many theories have been proposed in regard to the origin of hotspot volcanoes, but the most popular theory has it that plumes of mantle material bring hot material to the surface from the core-mantle boundary.
Gathering unprecedented evidence
The team is now able to gather rare physical evidence that may point to the root of the plume feeding Hawaii, thanks to newly captured images of the ultra-low velocity zone beneath Hawaii.
Observations of iron-rich rock beneath Hawaii would support these observations on the surface.
More imaging of the core-mantle boundary must be done to determine if all surface hotspots have pockets of dense material at their bases. In addition, using seismic stations to record earthquake waves helps determine where and how the core-mantle boundary can be reached.
Source: Nature Communications (2022), DOI: 10.1038/s41467-022-30502-5
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