Deep within Earth’s mantle, there are two giant blobs. One sits under Africa, while the other is almost precisely opposite the first, under the Pacific Ocean. But these two blobs are not evenly matched.
The blobs, more formally referred to as Large Low-Shear-Velocity Provinces (LLSVPs), are each the size of a continent and 100 times taller than Mt. Everest. These blobs were discovered back in the 1980s using instruments that measure seismic waves.
Interestingly, while there have been a number of hypotheses advanced regarding their influence on the Earth’s magnetic field, mantle convection, and hotspot volcanism to name a few, we actually know very little about them.
To this day, little is known about why the blobs exist, where they came from, or what led to their odd shapes. However, a new set of geodynamic models may have landed on a possible answer to part of the mystery.
Arizona State University scientists Qian Yuan and Mingming Li of the School of Earth and Space Exploration set out to learn more about these two blobs using geodynamic modeling and analyses of published seismic studies.
Prior studies on the blobs had suggested that the two blobs may not have been created equal. But none of this research had used global data sets that could easily compare the two. So the two scientists examined 17 global seismic-wave data sets to determine the height of each blob.
They found that the African blob extends about 620 miles (1,000 kilometers) higher than the Pacific blob. That’s a difference of roughly 113 Mount Everests.
To try and understand this vast difference in heights, the researchers then used computer modeling to figure out which features of the blobs could explain these differences.
They found the density of the blobs themselves and the viscosity of the surrounding mantle was key factor. Viscosity refers to the ease with which the mantle rocks can be deformed.
The scientists say this may explain the large differences in height between the two blobs. The one under the African continent must be of a lower density, and therefore, less stable, than that of the blob under the Pacific Ocean, indicating that the two may have different compositions and evolution, reports Live Science.
“Our calculations found that the initial volume of the blobs does not affect their height,” lead author Yuan said. “The height of the blobs is mostly controlled by how dense they are and the viscosity of the surrounding mantle.”
“The Africa LLVP may have been rising in recent geological time,” co-author Li added. “This may explain the elevating surface topography and intense volcanism in eastern Africa.”
The unstable nature of the blob under the African continent, for example, may be related to continental changes in topography, gravity, surface volcanism, and plate motion.
“Our combination of the analysis of seismic results and the geodynamic modeling provides new insights on the nature of the Earth’s largest structures in the deep interior and their interaction with the surrounding mantle,” says Yuan, according to Science Alert.
“This work has far-reaching implications for scientists trying to understand the present-day status and the evolution of the deep mantle structure, and the nature of mantle convection.”
