The lower mantle of the Earth, beneath the Pacific and Indian Oceans, is dominated by two vast, continent-sized regions exhibiting low shear velocities and increased density. These massive blob structures, known as Large Low-shear Velocity Provinces (LLSVPs) or Superplumes, are hundreds of kilometers in height, and thousands of kilometers in width. The first one (called the African LLSVP) is under the East African Rift Valley between the Red Sea and Mozambique, and the second one (called the Pacific LLSVP) is primarily under the Nazca plate in the Pacific Ocean near South America. Experiments revealed that shear waves (a type of seismic waves) travel slowly through these two anomalous megastructures vis-à-vis the mantle surrounding them.
How did LLSVPs originate?
The exact origin of LLSVPs is not known. Several hypotheses have been proposed over the years. One of the earliest hypotheses was the Thermal Piles Hypothesis, which suggests that LLSVPs are purely thermal features caused by the accumulation of hot material at the base of the mantle. Another one was the Chemical Piles Hypothesis, which considered LLSVPs to be primordial reservoirs resulting from mantle convection, or recycled oceanic crust (and other subducted material) accumulating at the Core-Mantle Boundary (CMB) for millions of years. The most widely accepted hypothesis is the Thermo-Chemical Piles Hypothesis, which regards LLSVPs are thermo-chemical structures.
One of the most important hypotheses proposed in recent years is the Giant Impact Theory. This theory suggests that LLSVPs were formed as a result of the collision between Earth and Theia (an ancient planet about the size of Mars) about 4.5 billion years ago. Notably, the same collision is also believed to be the reason for the moon’s formation—as Theia collided with the Earth, it broke up, and part of the debris that got flung into orbit eventually coalesced to form the moon. Some of the debris started sinking into the Earth, gradually forming the LLSVPs over a long period of time. (See this research paper for details.)
The ‘Gravity Hole’ in the Indian Ocean
A significant gravitational anomaly exists in the Indian Ocean. The sea levels covering over 3 million square kilometers are about 100 meters less than the global average. This is due to a gravity hole (i.e., a significant dip in the Earth’s gravity) around that area, a phenomenon known as the Indian Ocean Geoid Low (IOGL.) Recent studies suggest that the African LLSVP may be the primary reason behind the occurrence of IOGL. Low-density molten material rises at the edges of the sinking remnants of the African blob, leading to the gravity hole phenomenon. Read this report for details.
Understanding the IOGL is critical to understanding important aspects like mantle plumes, subducted slabs (and their role in shaping geoids), the complex thermo-chemical interactions within the mantle, and how processes from hundreds of millions of years ago continue to influence the modern-day Earth.
Other Effects of LLSVPs
The Pacific LLSVP, among other anomalies, has a prominent gap (about 20° wide between the central and eastern Pacific) that remains unsolved to this date. Some scientists believe that this slab could potentially be explained by the findings from a recent study. An enormous slab of seafloor of 410 to 660 kilometers has been discovered inside the Pacific Ocean. Made of dense rock, it is estimated that this slab has been sinking for over 120 million years, and continues to do so at about 0.5 to 1 centimeter per year.
Recent research also suggests that the African LLSVP is relatively closer to the surface, and much more unstable than the Pacific one. Some scientists believe that this factor could be a major reason for the large earthquakes and supervolcano eruptions that Africa witnessed for hundreds of millions of years. Some studies also suggest that the African LLSVP has been a major factor in the breakup of supercontinents.
Moreover, smaller structures with even slower seismic velocities than LLSVPs have been identified near the LLSVPs. Known as Ultra-low Velocity Zones (ULVZs), these regions, along with LLSVPs, are believed to massively influence the thermal and chemical interactions between the Earth’s core and mantle, which in turn drives the Earth’s magnetic field.
Closing Comments
LLSVPs represent one of the most compelling geophysical & geochemical anomalies on Earth. Many questions about their origin, composition, and long-term evolution remain unanswered. While some progress has been made in recent years, more research is needed to develop a comprehensive understanding of the Earth’s interior (e.g., core-mantle interactions, mantle dynamics, and plate tectonics), the Earth’s thermo-chemical evolution, the forces behind hyper-volcanic activity, and other areas. Additionally, LLSVPs may also provide a deeper insight into planetary processes that shape the Earth, and other rocky planets.