Animation of the earth’s layers.
A new study led by the University of Cambridge is the first to obtain a detailed “image” of an unusual rock pocket in the boundary layer with the earth’s core, about three thousand kilometers below the surface.
The mysterious rock zone, which is located almost immediately below the Hawaiian Islands, is one of several areas with ultra-low speeds – so-called because the earthquake waves slow down to creep as they pass through them.
The study, published May 19, 2022 in the journal Nature Communications, is 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 taking place in it.
Of all the deep inner elements of the Earth, these are the most fascinating and complex.
“Of all the deep inner elements of the Earth, these are the most fascinating and complex. We already have the first solid evidence that shows their internal structure – this is a real cornerstone in deep earthly seismology, “said lead author Ji Lee, a PhD student at the Department of Earth Sciences in Cambridge.
The interior of the Earth is layered like an onion: in the center is the iron-nickel core, surrounded by a thick layer known as the mantle, and on top – a thin outer shell – the crust on which we live. Although the mantle is hard rock, it is hot enough to flow extremely slowly. These internal convective flows deliver 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 – the echoes and shadows of these waves reveal radar-like images of deep internal topography. But until recently, the “images” of the core-mantle boundary structures, an area of key interest in studying our planet’s internal heat flux, were grainy and difficult to interpret.
Events and Sdiff rays used in this study. A) Cross section intersecting the center of the Hawaiian ultra-low velocity zone, showing the beam paths of Sdiff waves at 96 °, 100 °, 110 ° and 120 ° for a 1D model of the Earth PREM. The broken lines from top to bottom mark a break of 410 km, 660 km and 2791 km (100 km above the core-mantle border). B) Events and Sdiff rays of the background tomographic model SEMUCB_WM1 at a depth of 2791 km. Beach balls from events drawn in a variety of colors, including 20100320 (yellow), 20111214 (green), 20120417 (red), 20180910 (purple), 20180518 (brown), 20181030 (pink), 20161 (2016 rayangy1) Sdiff Wave trails at a drilling depth of 2791 km in the lowest mantle used in this study. The event used in the short-term analysis is highlighted in yellow. ULVZ’s proposed location is shown in a black circle. A dotted line shows the cross section plotted in A. Credit: Nature Communications, DOI: 10.1038 / s41467-022-30502-5
Researchers have used the latest numerical modeling methods to reveal kilometer-scale structures at the core-mantle boundary. According to co-author Dr. Quangdai 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 wave symmetries unnoticed or unused before. Leng, who is currently based on the Council for Scientific and Technological Facilities, says this means they can improve image resolution by an order of magnitude compared to previous work.
Researchers have seen a 40% reduction in the speed of seismic waves that travel at the base of the ultra-low-speed zone below Hawaii. This supports existing suggestions that the area contains much more iron than the surrounding rocks – which means 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 that iron may have leaked from the core in an unknown way,” said project leader Dr San Cotaar of Cambridge Earth Sciences.
Conceptual cartoons of the structure of the Hawaiian Ultra-Low Speed Zone (ULVZ). A) ULVZ on the core-mantle border at the base of the Hawaiian plume (height is not to scale). B) increase in the modeled ULVZ structure showing interpreted captured postcursor waves (note that the analyzed waves have a horizontal shift). Credit: Nature Communications, DOI: 10.1038 / s41467-022-30502-5
The study may also help scientists understand what lies beneath and gives rise to volcanic chains such as the Hawaiian Islands. Scientists have begun to notice a correlation between the location of descriptively called hot volcanoes, which include Hawaii and Iceland, and the ultra-low-speed areas at the base of the mantle. The origin of hot spot volcanoes has been debated, but the most popular theory suggests that jet-like structures bring hot mantle material all the way from the core and mantle boundary to the surface.
With images of the ultra-low-speed zone below Hawaii now in hand, the team can also gather rare physical evidence of what is likely to be the root of the jet feeding Hawaii. Their observation on a dense, iron-rich rock below Hawaii would support superficial observations. “The basalts erupting from Hawaii have anomalous isotopic signatures that could point to either an early Earth origin or a leaking core, which means that some of this dense material accumulated at the base must be extracted to the surface. “Kotaar said.
More than the core-mantle boundary must now be imaged to see if all surface hotspots have a pocket of dense material at the base. Where and how the boundary between the core and the mantle can be directed depends on where the earthquakes occur and where the seismometers are installed to record the waves.
The team’s observations add to the growing body of evidence that the Earth’s deep interior is as volatile as its surface. “These low-speed zones are one of the most complex features we see at extreme depths – if we expand our demand, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core-mantle boundary,” he said. Lee.
Now they plan to apply their techniques to improve the resolution of images of other pockets on the border of the core and mantle, as well as mapping new areas. Ultimately, they hope to map the geological landscape across the core-mantle boundary and understand its connection to the dynamics and evolutionary history of our planet.
Reference: “Kilometer structure of the core-mantle boundary near Hawaii” by Zhi Li, Kuangdai Leng, Jennifer Jenkins and Sanne Cottaar, May 19, 2022, Nature Communications.DOI: 10.1038 / s41467-022-3050
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