High-resolution imaging reveals puzzling features deep underground

New research led by the University of Cambridge is the first to obtain a detailed ‘image’ of an unusual pocket of rock in the boundary layer with the Earth’s core, about three thousand kilometers below the surface.

The mysterious region of rock, located almost directly below the Hawaiian Islands, is one of several regions of very low velocity – so called because earthquake waves slow to a crawl as they pass through them.

Research published in the magazine on May 19, 2022 Nature Communicationsis the first to reveal in detail the complex internal anisotropy of one of these pockets, shedding light on the landscapes of the Earth’s depths and the processes operating within them.

“Of all the Earth’s deep internal features, this is the most fascinating and complex. We have now obtained the first solid evidence to show its internal structure – it is a true landmark in deep seismology,” said lead author Zhi Li, a PhD student in the Department of Earth Sciences at Cambridge. ground”.

The interior of the Earth is formed like an onion: in the center is the iron-nickel core, surrounded by a thick layer known as the mantle, and above that a thin outer crust – the crust on which we live. Although the mantle is a solid rock, it is hot enough that it flows very slowly. Internal convection currents feed heat to the surface, causing movement of tectonic plates and fueling volcanic eruptions.

Scientists use seismic waves from earthquakes to “see” what’s beneath the Earth’s surface—the echoes and shadows of these waves reveal radar-like images of the deep interior. But, until recently, “images” of structures at the core-mantle boundary, a region of primary interest for studying our planet’s internal heat flow, were grainy and difficult to interpret.

The events and trajectories of the Sdiff rays used in this study. a) A cross section cutting through the center of the ultra-low velocity region in Hawaii, showing ray trajectories for Sdiff waves at 96°, 100°, 110° and 120° for the 1D PREM Earth model. Dashed lines from top to bottom indicate the discontinuities of 410 km, 660 km, and 2,791 km (100 km above the core-mantle boundary). b) Events and Sdiff-ray trajectories on background tomography model SEMUCB_WM1 at a depth of 2791 km. Beach balls for events painted in different colors including 20100,320 (yellow), 20111214 (green), 20120417 (red), 20180910 (purple), 20180518 (brown), 20181030 (pink), 20161122 (gray), stations (triangles), and rays. Sdiff wave trajectories at a hole depth of 2791 km in the lower mantle used in this study. The event used in the short period analysis is highlighted in yellow. The proposed ULVZ location is shown in a black circle. The dashed line shows the cross section drawn in A. Credit: Nature Communications, DOI: 10.1038/s41467-022-30502-5

The researchers used state-of-the-art numerical modeling methods to detect kilometer-scale structures at the core-mantle boundary. According to co-author Dr. Kuangdai Leng, who developed the methods while at[{” attribute=””>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, says that this means they can improve the resolution of the images by an order of magnitude compared to previous work.

The researchers observed a 40% reduction in the speed of seismic waves traveling at the base of the ultra-low velocity zone beneath Hawaii. This supports existing proposals that the zone contains much more iron than the surrounding rocks – meaning 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 even that iron might be leaking from the core by an unknown means,” said project lead Dr Sanne Cottaar from Cambridge Earth Sciences.

Conceptual cartoons of the Hawaiian ultra-low velocity zone (ULVZ) structure. A) ULVZ on the core–mantle boundary at the base of the Hawaiian plume (height is not to scale). B) a zoom in of the modeled ULVZ structure, showing interpreted trapped postcursor waves (note that the waves analyzed have horizontal displacement). Credit: Nature Communications, DOI: 10.1038/s41467-022-30502-5

The research could also help scientists understand what sits beneath and gives rise to volcanic chains like the Hawaiian Islands. Scientists have started to notice a correlation between the location of the descriptively-named hotspot volcanoes, which include Hawaii and Iceland, and the ultra-low velocity zones at the base of the mantle. The origin of hotspot volcanoes has been debated, but the most popular theory suggests that plume-like structures bring hot mantle material all the way 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 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 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 does depend on where earthquakes occur, and where seismometers are installed to record the waves.

The team’s observations add to a growing body of evidence that Earth’s deep interior is just as variable 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,” said Li.

They now plan to apply their techniques to enhance the resolution of imaging of other pockets at the core-mantle boundary, as well as mapping new zones. Eventually, they hope to map the geological landscape across the core-mantle boundary and understand its relationship with the dynamics and evolutionary history of our planet.

Reference: “Kilometer-scale structure on the core–mantle boundary near Hawaii” by Zhi Li, Kuangdai Leng, Jennifer Jenkins and Sanne Cottaar, 19 May 2022, Nature Communications.
DOI: 10.1038/s41467-022-30502-5