Seismic studies have long revealed that earthquake-generated waves propagate through Earth's interior at varying speeds, offering a window into the otherwise inaccessible deep mantle. Beneath the Pacific Ocean and the African continent, researchers have identified immense LLVPs-each spanning thousands of kilometers across and reaching up to 900 kilometers in height-where seismic waves travel significantly slower than through surrounding regions.
One prevailing theory suggests that LLVPs originate from oceanic crust that has been subducted into the mantle, gradually mixing with surrounding material over millions of years. Historically, scientists believed both LLVPs shared a similar composition and age due to their comparable seismic properties. However, the new study, co-authored by Dr. Paula Koelemeijer of the University of Oxford, challenges this notion by simulating their formation over time.
Using a sophisticated mantle convection model that integrates tectonic plate movements spanning the last billion years, the research team has shown that the African LLVP is composed of older, more thoroughly mixed material, while the Pacific LLVP contains a significantly higher proportion-around 50% more-of younger subducted oceanic crust. This distinction in composition influences their density and structure, explaining why the African LLVP appears taller and more diffuse compared to its Pacific counterpart.
"As numerical simulations are not perfect, we have run multiple models for a range of parameters. Each time, we find the Pacific LLVP to be enriched in subducted oceanic crust, implying that Earth's recent subduction history is driving this difference," explained Dr. James Panton of Cardiff University, lead author of the study.
The study further indicates that the Pacific LLVP has been continually supplied with new oceanic crust for the past 300 million years due to its encirclement by subduction zones, collectively known as the Pacific Ring of Fire. In contrast, the African LLVP receives fresh material at a slower rate, leading to a greater degree of mixing with the surrounding mantle and a lower density overall.
Previously, these differences went unnoticed because seismic wave speeds are primarily influenced by temperature. The study's models demonstrate that, despite their compositional disparities, both LLVPs maintain similar temperatures, which explains their comparable seismic characteristics. This underscores the importance of multidisciplinary approaches in understanding Earth's deep structure.
"The fact that these two LLVPs differ in composition, but not in temperature, is key to the story and explains why they appear to be the same seismically. It is also fascinating to see the links between the movements of plates on the Earth's surface and structures 3,000 km deep in our planet," said Dr. Paula Koelemeijer.
These findings carry significant implications for Earth's thermal dynamics and geomagnetic stability. The high temperatures and strategic positioning of the LLVPs influence heat transfer from Earth's core, thereby affecting convection currents in the outer core. This process plays a crucial role in generating Earth's magnetic field, which shields the planet from harmful cosmic radiation. If heat is extracted asymmetrically due to differences between the African and Pacific LLVPs, it could lead to instability in the geomagnetic field, further highlighting the importance of understanding these deep mantle structures.
Dr. Koelemeijer emphasized the need for further research: "We now need to look for data that can constrain the proposed asymmetry in density, for example using observations of Earth's gravitational field."
Research Report:Unique composition and evolutionary histories of large low velocity provinces
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