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A new study led by the University of Cambridge is the first to take a detailed picture of an unusual pocket of rock near the Earth’s core, about 3,000 km below the surface.
The mysterious area of rock is almost right under the Hawaiian Islands. It is one of several places called “ultra-low velocity zones” because earthquake waves move very slowly through them.
The research, which was published today in Nature Communications, is the first to show in detail how one of these pockets changes on the inside. This sheds light on the deep interior of Earth and the processes going on there.
“These are the most interesting and complicated parts of the Earth’s deep interior. We now have the first solid evidence of how they are built on the inside. This is a big step forward in the study of deep earth seismology “Zhi Li, a Ph.D. student at Cambridge’s Department of Earth Sciences and the study’s main author, said this.
The inside of the Earth is layered like an onion. At the centre is the iron-nickel core, which is surrounded by the thick mantle, and on top of that is the thin crust that we live on. Even though the mantle is solid rock, it moves very slowly because it is so hot. These internal convection currents bring heat to the surface, which moves tectonic plates and makes it possible for volcanoes to erupt.
Scientists use the echoes and shadows of seismic waves from earthquakes to see what’s deep inside the Earth. These waves work like radar to show images of the deep interior topography. But until recently, images of the structures at the core-mantle boundary, which is a key place to study the flow of heat inside our planet, were blurry and hard to understand.
At the boundary between the core and the mantle, there are structures that are a kilometre or more across. The methods were created at the University of Oxford by Dr. Kuangdai Leng, who is also a co-author. He says, “We are really pushing the limits of modern high-performance computing for elastodynamic simulations by taking advantage of wave symmetries that haven’t been noticed or used before.” Leng, who works at the Science and Technology Facilities Council, said that this means they can make the images much clearer than they were before by a factor of a thousand.
At the bottom of the ultra-low velocity zone beneath Hawaii, they saw that the speed of seismic waves slowed by 40%. According to the authors, this backs up the idea that the zone has a lot more iron than the rocks around it, which makes it heavier and slower. The project’s leader, Dr. Sanne Cottaar from Cambridge Earth Sciences, said, “It’s possible that this iron-rich material is a piece of old rock left over from Earth’s early days, or iron could be leaking out of the core through some unknown process.”
The new research could also help scientists figure out what lies below volcanic chains like the Hawaiian Islands and causes them to form. Scientists have started to notice a link between where the hotspot volcanoes, like Hawaii and Iceland, are located and where the ultra-low velocity zones are at the base of the mantle. Many people have different ideas about where hotspot volcanoes come from, but the most popular theory is that plume-like structures bring hot mantle material all the way from the boundary between the core and the mantle to the surface.
Now that the team has pictures of the ultra-low velocity zone under Hawaii, they can also collect rare physical evidence from what is probably the source of the plume that feeds Hawaii. Their finding of dense, iron-rich rock under Hawaii would back up what people have seen on the surface. “Basalts erupting from Hawaii have strange isotope signatures that could point to either an early-Earth origin or a leaking core,” said Cottaar. “This means that some of the dense material piled up at the bottom must be pulled to the surface.”
Now, scientists need to take pictures of more of the core-mantle boundary to find out if all surface hotspots have a pocket of dense material at the bottom. How and where to look for the core-mantle boundary depends on where earthquakes happen and where seismometers are set up to record the waves.
The team’s observations show that the deep interior of Earth is just as changeable as its surface. “These low velocity zones are one of the most complicated features we see at great depths,” said Li. “If we expand our search, we’re likely to find structural and chemical complexity at the core-mantle boundary that keeps getting more complicated.”
Now, they want to use their methods to improve the resolution of images of other pockets at the boundary between the core and the mantle and to map new areas. Eventually, they want to map the geological landscape across the core-mantle boundary and figure out how it fits into the way our planet works and how it has changed over time.
Further information: Kilometer-scale structure on the core–mantle boundary near Hawaii, Nature Communications (2022). DOI: 10.1038/s41467-022-30502-5
Journal information: Nature Communications
Source: University of Cambridge