There are two enormous provinces of unusual rock that sit at the bottom of the mantle, just above the Earth’s core. One of them is located underneath Africa and one is under the Pacific Ocean. They’re called Large Low Shear Velocity Provinces (LLSVPs) and they may shape hotspot formation and volcanic activity across the globe.
The LLSVPs are areas where seismic shear waves propagate much more slowly. They extend laterally for thousands of miles (we think), and they may be up to 1,000km “tall.” Geologists have considered a variety of potential explanations for the origin of the LLSVPs, and now a team has put forth a new argument: The LLSVPs may represent the remains of Theia, the protoplanet thought to have smashed into the world some 4.5 billion years ago, creating the Moon. There are a number of hotspots around the world associated with the margins and boundaries of the LLSVPs:
Image by DR Davies et al, The Australian National University
The hotspots associated with the LLSVP sometimes create a type of lava known as ocean island basalts, which are often compared with mid-ocean ridge basalts. In some cases, ocean island basalts are found with isotopic ratios that are believed to reflect the primordial Earth, especially when they’re located above one of the LLSVPs.
This suggests that at least some of the material down at the mantle/core boundary has been there since the formation of the planet. Mid-ocean ridge basalts are more likely to contain magma drawn from the upper layers of the mantle. This material has typically melted, cooled, and then subducted and melted again more than once across billions of years. This cycling results in magma with different isotope ratios and characteristics compared with the magma welling up from the mantle/core boundary.
Qian Yuan, a Ph.D. student in geodynamics at Arizona State University (ASU), presented his hypothesis on the topic at the Lunar and Planetary Science Conference. According to him, the Theia impactor could have formed the LLSVPs if Theia’s mantle material was 1.5 – 3.5 percent heavier than Earth’s. Under this model, some parts of Theia’s original mantle remained contiguous and never completely mixed with the Earth. This is not a problem; it is very difficult to create a Theia – Earth impact model that achieves uniform mixing, even if you assume a post-impact global magma ocean. Yuan’s work suggests Theia’s mantle material would form 3-15 percent of the mantle volume of the Earth, which lines up with the 3-9 percent of the mantle the LLSVPs are thought to occupy.
Image by Sanne Cottaar, CC BY-SA 4.0. Animated version available.
Alternate Explanations
The Theia impact hypothesis is not the only explanation for the LLSVPs. A number of causes have been proposed. They may be plumes of upwelling magma or represent differentiation that occurred entirely on Earth early in its history. They may be created by thermochemical convection or be comprised of ancient slabs of subducted ocean crust that fell to the bottom of the core/mantle boundary hundreds of millions to billions of years ago.
There’s even a chance that the LLSVPs don’t exist, at least not in their currently theorized size and shape. We track seismic waves as they propagate through the Earth to learn about its composition and structure, but it’s not the same as taking an X-ray. Some researchers have argued that the enormous size and unusual shape of the LLSVPs are due to resolution limits in our seismic data.
Image by Sanne Cottaar, CC BY-SA 3.0
The idea that we might find pieces of Theia inside the mantle is a solid one, even if the LLSVPs turned out to be something other than they’re currently theorized to be. There are other, smaller pockets of low seismic velocities inside the mantle. They’re often near also associated with the LLSVPs, but much smaller. These ultra-low-velocity zones are thought to be enriched with iron. They may represent core fragments of other planetesimals that struck Earth during its formation and became trapped in the mantle. The long-term sequestration of such primitive material would explain why we occasionally find lava that looks as if it came straight from the early solar system.
If the LLSVPs or ultra-low-velocity zones prove to be of extraterrestrial origin, it would mean the planetesimals that helped form the Earth have continued to shape its geology ever since. It’s one thing to know the Moon was created in an impact some 4.5 billion years ago, and another to imagine that some of the core of the planetesimal that shaped our entire Earthly existence might still exist itself, trapped below an ocean of liquid rock.
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