High-resolution imagery reveals puzzling features deep inside the Earth

Animation of the layers of the Earth.

New research from the University of Cambridge is the first to get a detailed ‘picture’ of an unusual pocket of rock at the boundary layer with the Earth’s core, some three thousand kilometers below the surface.

The mysterious rocky area, located almost directly beneath the Hawaiian Islands, is one of many ultra-low-velocity zones – so called because seismic waves slow down as they pass through them.

The research, published May 19, 2022, in the journal 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 deep interior features of the Earth, these are the most fascinating and complex.” — Zhi Li

“Of all the deep interior features of the Earth, these are the most fascinating and complex. We now have the first solid evidence to show their internal structure – this is a real milestone in deep Earth seismology,” said lead author Zhi Li, a PhD student in Cambridge’s Department of Earth Sciences.

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” beneath the Earth’s surface – echoes and shadows from these waves reveal 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.

Sdiff Ray Paths and Events

Sdiff ray events and paths used in this study. A) Cross-section through the center of the Hawaiian ultra-low velocity zone, showing the 96°, 100°, 110°, and 120° Sdiff wave ray trajectories for the 1D PREM Earth model. Dotted lines from top to bottom mark the 410 km, 660 km discontinuity and 2791 km (100 km above core-mantle boundary). B) Events and paths of Sdiff rays on the SEMUCB_WM1 bottom tomography model at 2791 km depth. Event beach balls traced in various colors including 20100320 (yellow), 20111214 (green), 20120417 (red), 20180910 (purple), 20180518 (brown), 20181030 (pink), 20161122 (grey), stations ( triangles) and radius trajectories of the Sdiff waves at a drilling depth of 2791 km in the lowest mantle used in this study. The event used in the short-period analysis is highlighted in yellow. The proposed location of the ULVZ is shown in a black circle. The dotted line shows the cross-section drawn in A. Credit: Nature Communications, DOI: 10.1038/s41467-022-30502-5

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 while he was at University of Oxford, “We are really pushing the boundaries of modern high performance computing for elastodynamic simulations, taking advantage of previously unnoticed or unused wave symmetries.” Leng, who is currently based at the Science and Technology Facilities Council, says this means they can improve image resolution by an order of magnitude over previous work.

The researchers observed a 40% reduction in the speed of seismic waves traveling at the base of the very low speed zone beneath Hawaii. This confirms existing proposals that the area contains much more iron than the surrounding rocks, meaning it is denser and slower. “It is 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 Dr Sanne Cottaar, head of Cambridge Earth Sciences project.

Hawaiian Structure of the Ultra-Low Velocity Zone (ULVZ)

Concept 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 not to scale). B) A zoom in on the modeled ULVZ structure, showing the interpreted trapped post-cursor waves (note that the analyzed waves have horizontal displacement). Credit: Nature Communications, DOI: 10.1038/s41467-022-30502-5

The 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 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 rocks beneath Hawaii would support surface observations. “Erupting Hawaiian basalts have anomalous isotopic signatures that could indicate either an early terrestrial origin or core leakage, meaning some of this dense base-piled material must be dragged 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 set up 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 search, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core boundary. -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. Ultimately, 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.

Reference: “Kilometer-Scale Structure on the Core-Mantle Boundary Near Hawaii” by Zhi Li, Kuangdai Leng, Jennifer Jenkins, and Sanne Cottaar, May 19, 2022, Nature Communication.
DOI: 10.1038/s41467-022-30502-5

Betty K. Park