Research led by the University of Cambridge has succeeded for the first time in obtaining a detailed picture of a unusual rock structure in the boundary zone with the Earth’s core, about 3,000 kilometers below the surface. This solid body, located where its presence was not expected, could explain the existence of volcanoes like those in Hawaii and others scattered around the Earth.
This enigmatic area of rock, which lies almost directly below the Hawaiian Islandsis one of several ‘ultra-low velocity zones’—so called because seismic waves slow down as they pass through them—that have been discovered.
The research, published this week in Nature Communicationsis the first to reveal in detail the complex internal variability of one of these zones, thus shedding light on the landscape of the deep interior of the Earth and the processes that operate within it.
“Of all the intraterrestrial features, these are the most fascinating and complex. We now have the first hard evidence showing its internal structure: is a real milestone in deep earth seismologysaid lead author Zhi Li, a student in Cambridge’s Department of Earth Sciences.
The interior of the Earth has layers structured like an onion: in the center is the core of iron and nickel, surrounded by a thick layer known as the mantle, and on top of it is a thin outer layer—the crust in which we live. .
Although the mantle is solid rock, it is hot enough to flow extremely slowly. These internal convection currents feed heat to the surface, driving the movement of tectonic plates and also fueling volcanic eruptions.
Scientists use seismic waves to see what is under 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 too grainy, poorly defined, and difficult to interpret.
Now, researchers have used the latest numerical modeling methods to thus being able to reveal structures with much higher resolution at the boundary between the core and the mantle.
“We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of previously unnoticed or unused wave symmetries,” said Kuangdai Leng, who developed the methods while at Oxford University.
A large bag of iron outside the core
They observed a 40% reduction in the speed of seismic waves traveling at the base of the ultra-low-velocity zone below Hawaii. According to the authors, this supports the view that the area contains much more iron than the surrounding rockswhich means that it is denser and slower.
«It is possible that this iron-rich material is a remnant of ancient rocks from early Earth history or even that the iron is escaping from the core by an unknown means.», indicated the person in charge of the project, Sanne Cottaar, of the University of Cambridge.
The new research could also help scientists understand what lies below us and what gives rise to volcanic chains like the Hawaiian Islands. In fact, it has begun to notice a correlation between the location of certain volcanoes, such as those in Hawaii and Iceland, and ultra-low velocity zones at the base of the mantle.
The origin of hot-spot volcanoes has been widely debated, but the most accepted theory suggests that the plume-like structures draw material from the hot mantle, at the core boundary, that reaches the surface.
With the new images obtained from the ultra-low-velocity zone under Hawaii, scientists can also collect physical evidence of what is likely the root of the column that feeds Hawaii.
The detection of dense, iron-rich rock would support observations made on the surface: “Basalts gushing from Hawaii have anomalous isotopic signatures that could point to an early Earth origin or a core leak,” Cottaar explained.
Additional images of the core-mantle boundary are now needed to see if all surface hotspots have a pocket of dense material beneath them., inside the Earth. Where and how the core-mantle boundary can be located depends on where earthquakes occur and where seismometers are set up to record the waves.
The team’s observations add to a growing body of evidence showing that Earth’s deep interior is as variable as its surface. The next step will be to apply these new techniques to improve the resolution of images of other areas of rock at the core-mantle boundary, as well as to map new areas.
Reference study: https://www.nature.com/articles/s41467-022-30502-5
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