New research has obtained the first detailed “image” of an unusual pocket of rock in the core boundary layer, about 3,000 kilometers below Hawaii. It suggests that the deep interior of the Earth is as variable as its surface.
An enigmatic area of rock lies almost directly below the Hawaiian Islands: it’s one of the Ultra Low Velocity Zones (ULVZ), so named because seismic waves slow down as they pass through them.
New research, led by the University of Cambridge, has described in detail, in an article published in Nature Communications, the complex internal variability from one of these pockets, shedding light on the landscape of the deep interior of the Earth and the processes that take place inside it.
“Of all the deep interior features of the Earth, these are the most fascinating and complex. We now have the first hard evidence showing its internal structure: it is a real milestone in deep Earth seismology”, explains the main author Li Zhi.
The interior of the Earth is layered like an onion: in the center is the core of iron and nickel, surrounded by a thick layer known as mantle. On top is a thin outer layer: the Cortex in which we live.
Although the mantle is solid rock, it is hot enough to flow slowly. These internal convection currents provide heat to the surface, driving the movement of tectonic plates and fueling volcanic eruptions.
Scientists use seismic waves from earthquakes to “see” what’s going on below the Earth’s surface: echoes and shadows from these waves reveal radar-like images of the deep interior topography.
But, until recently, those images Structures at the core-mantle boundary, a key area of interest for studying our planet’s internal heat flux, have been grainy and difficult to interpret.
In the new study, the researchers used the latest methods of numerical modeling to reveal kilometer-scale structures at the core-mantle boundary. Numerical modeling is a technique based on numerical computation, used in many fields of study to validate or refute conceptual models.
With the precision afforded by this method, 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 area contains much more iron than surrounding rocks, meaning it is denser and slower moving.
“It is possible that this iron-rich material is a remnant of ancient rocks from the Earth’s early history, or even that the iron is escaping from the core by an unknown means,” says the project leader, Sanne Cottaar.
Beneath the volcanic chains
The new research helps scientists better understand what gives rise to volcanic chains, such as those found under the Hawaiian Islands, one of the best-known and most studied geologic areas in the world.
Scientists have begun to notice a correlation between the location of volcanic sites known as hot spotssuch as those in Hawaii and Iceland, and the ultra-low velocity zones identified at the base of the mantle.
Although the origin of hot-spot volcanoes is still a source of debate, the most popular theory suggests that plume-like structures bring hot mantle material from the core-mantle boundary to the surface.
With new images of the ultra-low velocity zone below Hawaii now obtained, the team is able to collect previously unseen physical evidence of what is likely the root of the column of volcanoes that feeds Hawaii.
Their observation of dense, iron-rich rock beneath Hawaii would support surface observations.
“The erupting Hawaiian basalts have anomalous isotope signatures that could point to an early Earth origin or a core leak, meaning that some of this dense material accumulated at the base must be washed to the surface,” Cottaar said.
More images of the core-mantle boundary are now needed to discover whether all surface hotspots have a pocket of dense material at the bottom.
Where and how the core-mantle boundary can be focused 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 Earth’s deep interior is as variable as its surface.
“These low-velocity zones are one of the most intricate features we see at extreme depths: if we broaden our search, we are likely to see increasing levels of complexity, both structural and chemical, at the core-mantle boundary.” , said. Li.
The researchers plan to apply their techniques to improve the resolution of images of other pockets at the core-mantle boundary, as well as to map the geologic landscape across the core-mantle boundary to understand its relationship to the dynamics and evolutionary history of our planet.
Kilometer-scale structure on the core–mantle boundary near Hawaii. ZhiLi, et al. Nature Communications, Volume 13, Article number: 2787 (2022). DOI: 10.1038/s41467-022-30502-5