Photo credit: www.sciencedaily.com
A recent investigation conducted by scholars from Cardiff University, the University of Oxford, the University of Bristol, and the University of Michigan has unveiled that two extensive regions in the deep mantle of Earth possess unique historical and chemical characteristics, countering the previous notion that they were homogenous. The results of this research have been published in Scientific Reports.
Seismologists have established that seismic waves produced by earthquakes experience varying speeds as they traverse different areas in Earth’s interior. This principle has enabled scientists to create a visual representation of the planet’s internal structure, akin to the imaging techniques utilized in CT scans for medical evaluation.
Located deep within the mantle, which lies between Earth’s iron core and its silica-rich crust, are expansive zones beneath the Pacific Ocean and the African landmass characterized by significantly slower seismic wave propagation. These regions, known as “Large Low-Velocity Provinces” (LLVPs), exceed continental dimensions, reaching heights of up to 900 kilometers and extending thousands of kilometers across.
Historically, researchers have hypothesized that these LLVPs are composed of oceanic crust that has descended into the mantle through subduction zones, leading to a mixing process over millions of years that formed the distinct LLVPs we observe today.
Traditionally, it was believed that the LLVPs shared similarities in their chemical composition and ages, due to the analogous manner in which seismic waves travel through them. However, a groundbreaking study co-authored by Dr. Paula Koelemeijer from the University of Oxford scrutinizes this assumption by modeling the evolution of the LLVPs over geological time.
This research integrates models of mantle convection with a historical reconstruction of tectonic plate movements over the last billion years. The results indicate that the African LLVP is composed of older and more homogenized material compared to its Pacific counterpart, which contains a higher proportion of younger, subducted oceanic crust—about 50% more—which causes a greater disparity with the surrounding mantle. Consequently, the African LLVP is observed to be more diffuse and taller than the Pacific LLVP.
“Due to the imperfections of numerical simulations, we’ve executed multiple models across various parameters. Each iteration consistently shows that the Pacific LLVP is enhanced in subducted oceanic material, suggesting that recent subduction processes shape this distinction,” stated Dr. James Panton, the lead author of the study.
The models also suggest that the Pacific LLVP has been continuously supplied with fresh oceanic material for the past 300 million years, encircled by a series of subduction zones identified as the Pacific Ring of Fire. In contrast, the African LLVP has not experienced comparable rates of replenishment, resulting in a deeper mixing with the surrounding mantle and a corresponding decrease in density.
These discrepancies had been largely neglected until now, as temperature was previously considered the primary factor influencing the speed of seismic waves. The findings from this study indicate that both LLVPs possess similar temperature levels, explaining their seismic resemblance despite the differences in composition. This discovery underscores the significance of interdisciplinary scientific methods in examining the intricacies of Earth’s inner structure.
Dr. Koelemeijer highlighted, “The key aspect of our findings is that these LLVPs differ in material composition, yet maintain similar temperatures, which clarifies their seismic similarities. It’s intriguing to observe how surface tectonic plate movements relate to deep-seated structures that dwell nearly 3,000 kilometers below.”
The elevated temperatures of the LLVPs and their strategic positions within the deep mantle influence the extraction of heat from Earth’s core, thereby affecting convection in the outer core. This process is crucial for sustaining the planet’s magnetic field, which shields the surface from harmful cosmic radiation. If significant differences exist between the African and Pacific LLVPs, it could disrupt the symmetrical extraction of heat, potentially leading to instabilities in the magnetic field. Thus, it becomes crucial to comprehend the architecture of the LLVPs and their roles in moderating core heat dynamics. Future scientific inquiries must factor in this newfound asymmetry in mantle density, presenting challenges for observational data which often focus on symmetrical Earth structures.
Dr. Koelemeijer concluded, “We must seek out data that can verify the proposed density asymmetry, perhaps through analyses of Earth’s gravitational field.”
Source
www.sciencedaily.com