A new study led by researchers of the University of Cambridge is the first to capture a detailed image of a mysterious pocket of rock at the boundary layer with Earth’s core, some three thousand kilometers beneath the surface.
The mysterious rock formation that lies almost immediately beneath the Hawaiian Islands is one of numerous ultra-low velocity zones, so named because earthquake waves slow to a crawl as they travel through.
The study, which was published in Nature Communications today, is the first to demonstrate in detail the complex internal variability of one of these pockets, offering information on the topography of Earth’s deep interior and the processes that operate there.
“Of all Earth’s deep interior features, these are the most fascinating and complex,” said Lead author Zhi Li, adding, “We’ve now got the first solid evidence to show their internal structure – it’s a real milestone in deep earth seismology.”
The inside of the Earth is layered like an onion, with the iron-nickel core at the center, surrounded by a thick layer known as the mantle, and a thin outer shell — the crust we live on — on top. Despite being solid rock, the mantle is heated enough to flow very slowly. Internal convection currents transport heat to the surface, causing tectonic plate movement and volcanic eruptions.
Scientists used earthquake seismic waves to view beneath the surface of the Earth, with the waves’ echoes and shadows producing radar-like images of deep inner terrain. However, photographs of the structures at the core-mantle boundary, which are crucial for understanding our planet’s internal heat flow, have been blurry and difficult to decipher until recently.
The scientists employed cutting-edge numerical modeling techniques to uncover kilometer-scale structures at the core-mantle boundary. “We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before,” added co-author Dr Kuangdai Leng. This implies they can enhance the image resolution by an order of magnitude above earlier efforts, according to Leng, who is currently stationed at the Science and Technology Facilities Council.
They discovered a 40 percent reduction in the speed of seismic waves traveling beneath Hawaii near the foot of the ultra-low velocity zone. According to the authors, this backs up previous claims that the zone contains significantly more iron than the surrounding rocks, making it denser and slower. “It’s possible that this iron-rich material is a remnant of ancient rocks from Earth’s early history or even that iron might be leaking from the core by an unknown means,” study leader Dr Sanne Cottaar of Cambridge Earth Sciences added.
The new findings may also assist scientists in comprehending what lies beneath and gives rise to volcanic systems such as the Hawaiian Islands. Volcanic hotspots, such as Hawaii and Iceland, have been linked to low-velocity zones at the bottom of the mantle by scientists, who have noticed a correlation. The formation of hotspot volcanoes is a hotly discussed topic, but the most accepted idea indicates that plume-like structures transport hot mantle material all the way to the surface from the core-mantle barrier.
The researchers can now collect rare physical evidence from what is presumably the root of the plume feeding Hawaii, thanks to photos of the ultra-low velocity zone beneath the island. Surface observations would be supported by their discovery of dense, iron-rich rock beneath Hawaii. “Basalts erupting from Hawaii have anomalous isotope signatures which could either point to either an early-Earth origin or core leaking, it means some of this dense material piled up at the base must be dragged to the surface,” Cottaar explained.
Now, scientists need to take pictures of more of the core-mantle boundary to find out if all surface hotspots have a pocket of dense material at the bottom. Where earthquakes occur, and where seismometers are positioned to record the waves, determines where and how the core-mantle border might be targeted.
The findings add to a growing body of evidence suggesting the deep interior of the planet is just as changeable as its surface. “These low velocity zones are one of the most intricate features we see at extreme depths – if we expand our search we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core-mantle boundary,” Li said.
They now intend to use their methods to improve the resolution of imaging of other pockets at the core-mantle boundary, as well as to map new zones. They hope to eventually map the geological landscape across the core-mantle border and comprehend how it relates to our planet’s dynamics and evolutionary history.
Image Credit: Getty
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