Far below Africa and the Pacific Ocean, the deepest mantle contains two structures so large that geophysicists often compare them with continents. They are called large low-shear-velocity provinces, or LLSVPs. Despite the nickname “blobs,” they are not hollow chambers, buried continents, or neatly bounded masses of molten rock. They are enormous regions in which seismic waves travel differently from the surrounding lower mantle.
Both LLSVPs rest on the core–mantle boundary, nearly 2,900 kilometers below Earth’s surface. One lies mainly beneath Africa and parts of the Atlantic and Indian Ocean regions; the other occupies a broad area beneath the Pacific. They extend thousands of kilometers sideways and rise hundreds to more than a thousand kilometers above the boundary in some seismic models. Their scale makes them among the most prominent structures detected anywhere inside the solid Earth.
Seismic Waves Revealed the Hidden Provinces
No instrument can be lowered into the lower mantle, so the LLSVPs were discovered indirectly. Earthquakes send compressional P waves and shear S waves through the planet. Networks of seismometers record when those waves arrive, and researchers compare the observations with the travel times expected through a more uniform Earth.
When the same pattern appears across many earthquakes and stations, scientists can reconstruct three-dimensional variations in seismic velocity through a method called seismic tomography. It works somewhat like a medical scan, although the data, geometry, and resolution are very different.
The African and Pacific provinces stand out because shear waves generally travel more slowly through them than through neighboring mantle. The velocity reduction is not identical throughout either structure. Their interiors contain smaller-scale variations, and their boundaries can appear unusually sharp in some regions. That complexity is one reason scientists no longer treat them as simple, homogeneous piles.
Slow Seismic Waves Do Not Reveal a Single Cause
Temperature affects seismic speed: hotter mantle is generally less rigid, allowing shear waves to travel more slowly. This initially encouraged interpretations in which the LLSVPs were mainly hot regions generated by mantle convection.
Composition can produce similar signals. Changes in iron content, mineral proportions, recycled oceanic crust, partial melting near the very base of the mantle, or chemically distinct material left from earlier stages of Earth’s history can all alter seismic velocity and density. The central problem is therefore not detecting the provinces but determining what they are made of.
Many modern models treat them as thermochemical structures, meaning that both temperature and composition matter. Their apparent sharp margins are especially important because a purely thermal anomaly tends to blend more gradually into surrounding mantle. Chemically distinct material, if sufficiently dense, can remain concentrated near the bottom of the mantle while convection deforms and stirs its edges.
Their Material May Have More Than One History
Several competing ideas attempt to explain where the provinces came from. One family of hypotheses traces at least part of their material to the earliest Earth. Dense residues could have formed when a deep magma ocean crystallized, leaving chemically unusual material near the core–mantle boundary.
Another possibility emphasizes plate tectonics. Oceanic plates descend at subduction zones and can carry altered basaltic crust, sediments, and other surface-derived material deep into the mantle. Over immense spans of time, some of this material may reach the lowermost mantle and accumulate or mix into preexisting reservoirs.
Recent mineral-physics work has strengthened the case that recycled oceanic crust could contribute to the low seismic velocities. Experiments and calculations under extreme pressure indicate that some silica-rich phases expected in subducted basalt may transmit shear waves more slowly than older models predicted. That makes substantial accumulations of ancient oceanic crust more compatible with the seismic signature of the LLSVPs.
These explanations are not necessarily mutually exclusive. The provinces could contain primordial material, recycled crust, products of chemical exchange near the core, and ordinary mantle in different proportions.
Africa and the Pacific May Not Be Twins
The two provinces look broadly similar in global tomography, but they may have followed different histories. Numerical simulations published in recent years suggest that the Pacific structure may have received relatively young subducted oceanic crust more continuously because it is surrounded by long-lived Pacific subduction systems.
The African province, by contrast, may contain older material that has been mixed for longer periods. Such results come from geodynamic models rather than direct sampling, so they are hypotheses to be tested against seismic observations, mineral physics, and the chemistry of volcanic rocks.
Other models suggest differences in density or internal layering that could affect how easily each province moves or changes shape. The African LLSVP is often portrayed as having taller, more irregular topography, while the Pacific structure appears broader in many reconstructions. Seismic resolution remains uneven, however, and exact outlines depend partly on the tomography model being used.
The Edges May Help Organize Mantle Upwelling
LLSVPs matter because they may influence what rises out of the deep mantle. Many reconstructed mantle plumes and major volcanic centers appear above or near their margins. In this view, the boundaries of the provinces help focus hot buoyant material into upwellings that can travel through the mantle.
A 2026 analysis of oceanic seamounts added another line of evidence. It found that seamount volcanism has been more voluminous on oceanic crust that previously passed above the basal mantle provinces, with the relationship stronger above the Pacific structure. The pattern does not prove that every hotspot is controlled by an LLSVP, but it strengthens the connection between deep mantle structure and long-term intraplate volcanism.
The provinces may therefore be both archives and active participants in mantle circulation. Material delivered downward by subduction can interact with them, while hot material may rise from their surroundings. This links processes at Earth’s surface with structures nearly 2,900 kilometers below.
The “Blobs” Are Maps of an Unfinished Problem
Calling the LLSVPs continent-sized blobs is useful for conveying their scale, but it can create the wrong mental picture. Seismology does not reveal two solid objects with fixed walls. It reveals immense three-dimensional regions whose temperature, chemistry, density, and internal structure differ from average mantle in ways that are still being resolved.
Their origin could record events from Earth’s early differentiation, billions of years of subduction, chemical exchange at the core–mantle boundary, or some combination of all three. As seismic imaging improves and high-pressure experiments reproduce conditions at the bottom of the mantle more accurately, these two provinces are becoming less mysterious in outline while remaining deeply uncertain in composition.