New research from the University of Cambridge is the first to take a detailed image of an unusual pocket of rock at the boundary layer with the Earth’s core, some three thousand kilometers below the surface.
The enigmatic rocky area, located almost directly beneath the Hawaiian Islands, is one of several very low-velocity zones, so called because seismic waves slow down as they pass through them.
The research, published today in Nature Communicationis the first to reveal in detail the complex internal variability of one of these pockets, shedding light on the landscape of the Earth’s deep interior and the processes that take place there.
“Of all the Earth’s deep interior features, these are the most fascinating and complex. We now have the first hard evidence to show their internal structure – it’s a real milestone in deep Earth seismology,” said lead author Zhi Li, Ph.D. student in the Department of Earth Sciences at Cambridge.
The interior of the Earth is layered like an onion: in the center is the iron-nickel core, surrounded by a thick layer called the mantle, and above that a thin outer shell – the crust on which we live. Although the mantle is made of solid rock, it is hot enough to flow extremely slowly. These internal convection currents transmit heat to the surface, driving the movement of tectonic plates and fueling volcanic eruptions.
Scientists use seismic waves from earthquakes to see below the Earth’s surface – the echoes and shadows of these waves revealing radar-like images of deep interior topography. But until recently, images of structures at the core-mantle boundary, a key area of interest for studying our planet’s internal heat flux, were grainy and difficult to interpret.
The researchers used the latest numerical modeling methods to reveal kilometric structures at the core-mantle boundary. According to co-author Dr Kuangdai Leng, who developed the methods at Oxford University, “We are really pushing the boundaries of modern high-performance computing for elastodynamic simulations, taking advantage of unnoticed wave symmetries or previously unused.” Leng, who is currently based at the Science and Technology Facilities Council, said this means they can improve image resolution by an order of magnitude over previous work.
They observed a 40% reduction in the speed of seismic waves traveling at the base of the very low speed zone beneath Hawaii. According to the authors, this supports existing proposals that the area contains much more iron than the surrounding rocks, meaning it is 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 is leaking out of the core by some unknown means,” said project leader Dr. Sanne Cottaar of Cambridge Earth Sciences.
The new research could also help scientists understand what lies beneath and gives rise to volcanic chains like the Hawaiian Islands. Scientists have begun to notice a correlation between the location of descriptively named hotspot volcanoes, which include Hawaii and Iceland, and very low-velocity areas at the base of the mantle. The origin of hotspot volcanoes has been widely debated, but the most popular theory suggests that plume-like structures transport hot mantle materials from the core-mantle boundary to the surface.
With images of the ultra-low velocity zone beneath Hawaii now in hand, the team can also collect rare physical evidence of 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 isotopic signatures that could indicate either an early Earth origin or core escape, that means some of that dense material has piled up at the base has to be brought to the surface,” Cottaar said.
More of the core-mantle boundary now needs to be imaged to understand if all surface hotspots have a pocket of dense material at the base. Where and how the core-mantle boundary can be targeted depends on where earthquakes occur and where seismometers are installed to record waves.
The team’s observations add to a growing body of evidence that the Earth’s deep interior is just as variable as its surface. “These low-velocity zones are one of the most complex features we see at extreme depths. If we expand our research, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core limit. -coat,” Li said.
They now plan to apply their techniques to improve the resolution of imaging of other pockets at the core-mantle boundary, as well as to map new areas. Eventually, they hope to map the geologic landscape across the core-mantle boundary and understand its relationship to the dynamics and evolutionary history of our planet.
Possible chemical remnants of early Earth lie near the core
Kilometric structure at the core-mantle boundary near Hawaii, Nature Communication (2022). DOI: 10.1038/s41467-022-30502-5
Provided by the University of Cambridge
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