The University of Cambridge led the first detailed image of an unusual pocket of rock at the boundary layer with Earth’s core, some three thousand kilometers beneath the surface. The mysterious rock formation, which lies almost directly beneath the Hawaiian Islands, is one of several ultra-low velocity zones, so named because earthquake waves slow to a crawl as they pass through them.
The study, which was published in Nature Communications, is the first to detail the complex internal variability of one of these pockets, shedding light on the landscape of Earth’s deep interior and the processes that operate within it.
“These are the most fascinating and complex of Earth’s deep interior features. We now have the first solid evidence of their internal structure, which is a true milestone in deep earth seismology” Zhi Li, a Ph.D. student at Cambridge’s Department of Earth Sciences, is the lead author.
The interior 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. Although the mantle is solid rock, it is hot enough to move at a glacial pace. Internal convection currents transport heat to the surface, causing tectonic plate movement and igniting volcanic eruptions.
These are the most fascinating and complex of Earth’s deep interior features. We now have the first solid evidence of their internal structure, which is a true milestone in deep earth seismology.
Zhi Li
Scientists use seismic waves from earthquakes to see beneath Earth’s surface — the echoes and shadows of these waves revealing radar-like images of deep interior topography. But until recently, images of the structures at the core-mantle boundary, an area of key interest for studying our planet’s internal heat flow, have been grainy and difficult to interpret.
The researchers used the latest numerical modelling methods to reveal kilometre-scale structures at the core-mantle boundary. According to co-author Dr. Kuangdai Leng, who developed the methods while at the University of Oxford, “We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before.” Leng, who is currently based at the Science and Technology Facilities Council, said that this means they can improve the resolution of the images by an order of magnitude compared to previous work.
They discovered a 40% reduction in the speed of seismic waves traveling at the base of Hawaii’s ultra-low velocity zone. According to the authors, this supports previous claims that the zone contains significantly more iron than the surrounding rocks, implying that it is denser and more sluggish. “It’s possible that this iron-rich material is a remnant of ancient rocks from Earth’s early history, or that iron is leaking from the core through an unknown mechanism,” said Dr Sanne Cottaar of Cambridge Earth Sciences.
The new findings may also aid scientists in understanding what lies beneath and gives rise to volcanic chains such as the Hawaiian Islands. Scientists have discovered a link between the location of the descriptively named hotspot volcanoes, which include Hawaii and Iceland, and the ultra-low velocity zones at the mantle’s core. The origin of hotspot volcanoes has been widely debated, but the most popular theory proposes that plume-like structures transport hot mantle material all the way to the surface from the core-mantle boundary.
With images of the ultra-low velocity zone beneath Hawaii now in hand, the team can also gather rare physical evidence from what is likely the root of the plume feeding Hawaii. Their observation of dense, iron-rich rock beneath Hawaii would support surface observations, “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,” said Cottaar.
More of the core-mantle boundary must now be imaged in order to determine whether all surface hotspots have a pocket of dense material at their base. Where and how the core-mantle boundary can be targeted is determined by earthquakes and seismometers installed to record the waves.
The findings add to a growing body of evidence that the Earth’s deep interior is as variable as its surface. “These low velocity zones are one of the most intricate features we see at extreme depths,” Li said. “If we broaden our search, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core-mantle boundary.”
They intend to use their techniques to improve the resolution of imaging of other pockets at the core-mantle boundary and to map new zones. They hope to eventually map the geological landscape across the core-mantle boundary and understand its relationship to our planet’s dynamics and evolutionary history.
