Scientists reveal the evolutionary history of regions of the deep mantle continent size

New research reveals that two continent-sized regions of the Earth’s deep mantle have distinctive history and resulting chemical composition, in contrast to the general assumption that they are the same. The findings can be read in the journal Scientific Reports. The study was led by researchers from Cardiff University, Oxford University, Bristol University and the University of Michigan.

Seismicians have long known that seismic waves produced by earthquakes do not move all parts of the Earth at the same speed. This principle allowed us to visualize the interior of our planet, even at depths that humans cannot access, using techniques similar to those employed in CT scans for medical imaging.

Deep in the mantle (the layer between the Earth’s iron core and its silica-controlled crust) is a vast area beneath the Pacific Ocean and African continent, with seismic waves much slower than average. These “large and low speed capabilities” (LLVPS) are larger than the continent, with a height of up to 900 kilometers and a width of several thousand kilometers.

One common hypothesis is that LLVP is composed of a marine crust that is forced into the mantle of the subduction zone. This crustal material was then stirred with the mantle for millions of years and accumulated to form LLVP.

Researchers have assumed that both LLVPs are similar to nature, as seismic waves usually pass through them in a similar way. However, a new study co-authored by Dr. Paula Koelemeijer (Department of Geoscience, University of Oxford) challenged this view by modeling the formation of LLVPs over time.

By combining a model of mantle convection, this study, which involves reconstructing how tectonic plates have moved over the Earth’s surface over the past billion years, was able to show that African LLVPs are older than Pacific LLVPs and are composed of better mixed materials. The resulting density differences could also explain why LLVPs in Africa are more diffused and taller than their Pacific counterparts.

“The numerical simulations are not perfect, so we run multiple models for different parameters. Each time, we see that the Pacific LLVP is rich in the subducted oceanic crust.

The model for this study also shows that Pacific LLVP has been consistently supplemented with fresh marine crustal material for 300 million years. In contrast, LLVPs in Africa do not receive new material at the same rate, and the material is mixed more with the surrounding mantle, reducing its density.

Until now, these differences have been overlooked. This is because temperature is the dominant control of how fast seismic waves travel through the material. The model presented in this study shows that both LLVPs actually have the same temperature. This underscores the importance of combining various scientific disciplines to closely examine the inner workings of a planet.

“The fact that these two LLVPs have different compositions but different temperatures is key to the story, explaining why they look like the same earthquake. It’s also fascinating to see the link between the movement of plates on the surface of the earth and the structure of our Earth’s 3,000 km deep.”

The high temperature of LLVPS and its location in the deep mantle on either side of the planet mean that it affects the way heat is extracted from the Earth’s nucleus. This affects the convection of the outer core. This is a process that drives magnetic fields and protects us on the surface from harmful cosmic rays.

If the LLVPs in Africa and Pacific Oceans are different, heat may not be extracted symmetrically, leading to magnetic field instability. This makes it important to understand the structure of LLVPs and how they affect heat extraction from the core.

Scientists need to explain this asymmetry of mantle density within models of deep Earth. This poses challenges for observations as the data used often only provides information about the symmetrical structure of the Earth.

Dr. Koelemeijer said, “We need to look for data that can now constrain the proposed asymmetry by density. For example, we need to use observations of the Earth’s gravity field.”