The lower mantle is the largest single layer inside Earth, stretching from about 660 kilometers below the surface down to the core-mantle boundary at roughly 2,900 kilometers depth. That span accounts for more than half the planet’s volume and most of its mass. Despite sitting beneath our feet at all times, it remains one of the hardest parts of Earth to study directly. Almost everything known about it comes from seismic waves, high-pressure lab experiments, and rare mineral inclusions trapped inside diamonds that formed deep underground and were carried to the surface by volcanic eruptions. What researchers have pieced together over the past few decades paints a picture that is far more dynamic and chemically varied than the uniform, sluggish rock layer once imagined.
Where the Lower Mantle Begins
The boundary that marks the top of the lower mantle sits at the 660-kilometer discontinuity, a depth where seismic waves change speed abruptly. That speed change happens because the minerals present at shallower depths transform under immense pressure into denser crystal structures. Ringwoodite, the dominant mineral in the transition zone just above, breaks down into two new phases: bridgmanite and periclase. This transformation is one of the most important phase changes in the planet’s interior, and it defines the start of a chemical and physical environment unlike anything above it.
Below 660 kilometers, pressure and temperature rise steadily. Revised estimates place the temperature just below the discontinuity at roughly 1,960 K and find it climbing to about 2,590 K near the base of the mantle at 2,800-kilometer depth.1Journal of Geophysical Research: Solid Earth. A Revised Adiabatic Temperature Profile for the Mantle Pressure rises from around 24 gigapascals at the top to roughly 136 gigapascals at the bottom. Those conditions are impossible to replicate easily in a laboratory, which is why understanding the lower mantle has advanced in fits and starts.
What the Lower Mantle Is Made Of
Two minerals dominate: bridgmanite and ferropericlase. Bridgmanite, a magnesium-iron silicate with a perovskite crystal structure, makes up the majority of the lower mantle and is widely considered the most abundant mineral in Earth. Ferropericlase, a simpler magnesium-iron oxide, fills in much of the rest. Together, these two minerals make the lower mantle broadly similar in bulk composition to a silicate-rich rock called pyrolite, though the exact proportions are debated. Mineral-physics models that match seismic wave speeds to lab measurements point to a magnesium-to-silicon ratio of about 0.9 to 1.0, which is slightly lower than what you would expect if the lower mantle were chemically identical to the upper mantle.2Annual Review of Earth and Planetary Sciences. The Composition of Earth’s Lower Mantle That discrepancy suggests the two halves of the mantle may not share exactly the same recipe.
High-pressure experiments using synchrotron X-rays have revealed that the microstructure of these minerals matters, not just their identities. When researchers squeezed samples to lower-mantle pressures inside diamond anvil cells, they found that ferropericlase forms an interconnected network around larger bridgmanite grains. That configuration has implications for how easily material flows at these depths and may help explain why some subducting slabs stall and mantle plumes get deflected in the upper part of the lower mantle.3PubMed Central. Exploring microstructures in lower mantle mineral assemblages with synchrotron x-rays
Davemaoite and the Hidden Heat Budget
A third mineral, far less abundant but geochemically critical, rounds out the picture. Calcium silicate perovskite had long been predicted to exist in the lower mantle but was never confirmed as a natural mineral until 2021, when researchers identified it inside a diamond from Botswana and named it davemaoite. The discovery was significant because davemaoite concentrates elements that do not fit easily into bridgmanite or ferropericlase, including thorium, uranium, and potassium. Those elements are radioactive, and their decay produces heat.4PubMed. Discovery of davemaoite, CaSiO(3)-perovskite, as a mineral from the lower mantle
How much davemaoite is present in any given region of the lower mantle therefore affects the local heat budget. Regions enriched in davemaoite would generate more internal heat, potentially influencing convection patterns and even the thermal history of the core. One recent study argues that concentrations of radioactive uranium in davemaoite during Earth’s early history could have helped keep the core hot for longer, possibly delaying the formation of the solid inner core.5PubMed Central. Persistence of davemaoite at lower-mantle conditions Davemaoite-enriched domains near the base of the mantle may also serve as reservoirs for certain isotopic signatures that show up in ocean island basalts, the lavas erupted at places like Hawaii and Iceland.
New measurements of davemaoite’s elastic properties suggest that variations in its abundance could explain some of the large-scale seismic anomalies detected in the deep mantle, linking it to the initiation of mantle plumes and the chemistry of surface volcanism.6PubMed. Elasticity of davemaoite as a primary contributor to lower-mantle heterogeneities
The D” Layer and the Core-Mantle Boundary
The lowest few hundred kilometers of the mantle, just above the liquid iron outer core, behave differently from the rest. This region, called the D” (D-double-prime) layer, has puzzled seismologists for decades with its sharp velocity changes and unusual wave behavior. A major breakthrough came when experiments showed that bridgmanite transforms into a new crystal structure called post-perovskite at the extreme pressures and temperatures near the core-mantle boundary. That phase transition can account for the D” discontinuity, the strong anisotropy in shear waves, and other seismic oddities observed in the region.7Reviews of Geophysics. Postperovskite phase transition and its geophysical implications
Within D”, seismologists have also detected ultra-low velocity zones, or ULVZs, where seismic waves slow dramatically over very thin patches. These zones are only tens of kilometers thick but can stretch laterally over hundreds of kilometers. Waveform analyses indicate that iron enrichment, likely accumulated through billions of years of subducted material settling at the base of the mantle, is the most plausible explanation for these anomalies.8Geophysical Journal International. Ultralow velocity zones at the core–mantle boundary in the subduction regions consistent with the iron-rich origins revealed by ScP waveform complexities
The lowermost mantle also acts as an electromagnetic intermediary between the core and the rest of the planet. Torsional waves generated in Earth’s liquid core appear to vanish when they reach the equator, and modeling shows that an electrically conducting layer at the base of the mantle can absorb those waves entirely, transferring angular momentum from the core to the mantle.9Geophysical Research Letters. Electrical conductivity of the lowermost mantle explains absorption of core torsional waves at the equator This coupling may influence small variations in Earth’s rotation over time.
Large Low-Velocity Provinces and the Theia Hypothesis
Two continent-sized structures sit at the base of the mantle, one beneath Africa and one beneath the Pacific, where seismic waves travel noticeably slower than in the surrounding rock. These large low-velocity provinces, or LLVPs, are among the most dramatic features in Earth’s deep interior. Their origin has been debated for decades. One line of thinking holds that they formed from the crystallization of a basal magma ocean, a global layer of molten rock that lingered above the core early in Earth’s history. As that magma ocean slowly solidified, chemical differentiation could have produced the dense, compositionally distinct piles now observed.10PubMed Central. Present-day Earth mantle structure set up by crustal pollution of the basal magma ocean
A more dramatic hypothesis connects the LLVPs to the Moon-forming giant impact. Computational models show that asteroid-sized fragments of Theia, the Mars-sized body thought to have collided with early Earth, could sink to the core-mantle boundary and persist there for 4.5 billion years as large thermochemical structures that resemble the seismically observed LLVPs.11The Planetary Science Journal. Survival of Asteroid-sized Debris from the Moon-forming Impactor in Earth’s Deep Mantle with Implications for Its Solar System Provenance If true, pieces of the body that created the Moon may still be sitting at the bottom of the mantle. The idea remains speculative but is consistent with multiple independent geodynamic simulations.
Whatever their origin, LLVPs appear to influence where mantle plumes form. Three-dimensional convection models show that strong plumes preferentially originate at or near the edges of these piles, and that such plumes tend to be hotter than those forming on top of the piles. This spatial relationship may explain why many large igneous provinces on Earth’s surface, massive volcanic events responsible for flood basalts, tend to erupt above LLSVP margins rather than their centers.12Earth and Planetary Science Letters. The source location of mantle plumes from 3D spherical models of mantle convection
How Subducted Slabs Navigate the Lower Mantle
When an oceanic plate dives beneath another plate at a subduction zone, it does not simply plunge straight to the bottom. Seismic imaging shows that slabs interact with the 660-kilometer boundary in complicated ways, sometimes stalling above it, sometimes punching through, and sometimes getting temporarily trapped in the uppermost lower mantle before eventually sinking deeper. A global survey of slab images identified four distinct stages: stagnation above 660 kilometers, active penetration through the boundary, trapping at depths between 660 and 1,000 kilometers, and descent into the deep lower mantle. Most slabs were found at stage one or stage three, suggesting those are relatively stable configurations.13Journal of Geophysical Research: Solid Earth. Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity
Why some slabs stall while others sink is partly a question of viscosity. Evidence suggests that viscosity does not simply jump at 660 kilometers but increases gradually over the uppermost 300 kilometers or so of the lower mantle.14Earth and Planetary Science Letters. Linked influences on slab stagnation: Interplay between lower mantle viscosity structure, phase transitions, and plate coupling A slab entering that zone encounters progressively stiffer rock, which can slow or deflect it. Compositional differences also matter. Numerical models show that if the lower mantle contains about eight percent more basaltic rock than the upper mantle, slabs can become neutrally buoyant at around 1,000 kilometers depth, naturally explaining the stagnation pattern observed in seismic data.15PubMed Central. Compositional mantle layering revealed by slab stagnation at ~1000-km depth
Water in the Deep Mantle
The lower mantle is not bone-dry, but water down there exists in an unfamiliar form. Rather than liquid or ice, water is incorporated as hydrogen atoms locked inside the crystal structures of minerals, present in amounts measured in parts per million. The top of the lower mantle, where subducted oceanic crust arrives, is a particularly interesting zone for water transport. Experiments show that aluminous stishovite, a high-pressure form of silica found in subducted basaltic crust, can hold up to about 3,600 parts per million of water by weight. Meanwhile, the bridgmanite and periclase that dominate the surrounding peridotitic mantle are nearly dry at those depths.16Earth and Planetary Science Letters. Water solubility in nominally anhydrous minerals in a silica-rich system at the top lower mantle This means water entering the lower mantle via subduction is concentrated in specific mineral hosts within the slab itself, not spread evenly through the rock.
Another potential water carrier is pyrope garnet, which can contain up to about 2,000 parts per million of water in the transition zone and the topmost lower mantle.17PubMed Central. The role of pyrope garnet in water transport into the topmost lower mantle These findings together suggest that water cycling into the deep Earth is a selective process, carried by particular minerals in particular geological settings rather than being a uniform background feature. Whether significant water penetrates much deeper than the top of the lower mantle remains an open question, and the answer has implications for mantle rheology, melting, and even the origin of deep-focus earthquakes.
Iron Spin Crossover and Thermal Conductivity
One of the stranger things that happens inside the lower mantle involves the iron atoms in ferropericlase. Under increasing pressure, the electrons in iron’s outer shell rearrange from a “high-spin” state to a “low-spin” state, a quantum mechanical transition that changes the mineral’s physical properties in measurable ways. This spin crossover affects sound velocity, density, and heat capacity, which means it shows up in seismic data as subtle anomalies.
The effect on thermal conductivity is especially interesting. Multiple experimental studies have found that the spin crossover reduces the thermal conductivity of ferropericlase, disrupting the normal pattern where conductivity increases steadily with pressure.18American Mineralogist. Thermal conductivity anomaly in spin-crossover ferropericlase under lower mantle conditions and implications for heat flow across the core-mantle boundary Both lattice thermal conductivity (heat carried by vibrations in the crystal) and radiative conduction (heat carried by infrared photons passing through the mineral) are reduced by the transition.19Earth and Planetary Science Letters. Thermal conductivity of ferropericlase in the Earth’s lower mantle Samples with high iron content show a particularly dramatic drop: one study found thermal conductivity falling by a factor of 1.8 across the spin transition at about 53 gigapascals.20PubMed Central. Effects of iron on the lattice thermal conductivity of Earth’s deep mantle and implications for mantle dynamics
A drop in thermal conductivity at mid-mantle depths would slow the escape of heat from the core and could influence convection patterns. Because ferropericlase forms that interconnected network mentioned earlier, its conductivity matters disproportionately; heat tends to follow the path of least resistance, and the ferropericlase network provides that path. A conductivity dip in the middle of the lower mantle could act as a partial thermal blanket, affecting how efficiently the mantle cools over geologic time.
The Temperature Gradient Is Not What You Might Expect
Introductory geology courses often describe the mantle’s temperature as following an adiabat, a smooth, gradually rising curve set by the self-compression of rock under its own weight. In reality, the lower mantle’s temperature profile is more complicated. Convection simulations in three-dimensional spherical geometry consistently produce a temperature gradient in the lower mantle that is slightly below adiabatic through most of its thickness, with a superadiabatic (steeper) portion in the bottom several hundred kilometers where the mantle sits against the hot core.21Earth and Planetary Science Letters. How flat is the lower-mantle temperature gradient? The subadiabatic interior means the bulk of the lower mantle is slightly cooler than simple theory predicts, while the base runs a bit hotter. That thermal structure has consequences for estimating the heat flux out of the core and for understanding how vigorously the deepest mantle convects.
Diamonds as Messengers from the Deep
Nearly everything discussed so far relies on indirect methods: seismic tomography, computer simulations, and lab experiments that recreate lower-mantle pressures for fractions of a second. But a handful of actual physical samples exist. Superdeep diamonds, formed at depths below 660 kilometers, occasionally carry tiny mineral inclusions that preserve the chemistry of the lower mantle. When these diamonds are brought to the surface by kimberlite eruptions, the inclusions provide direct evidence of what exists down there.
Analyses of superdeep diamonds from the Juina-5 kimberlite in Brazil found inclusions comprising the entire phase assemblage expected to crystallize from basaltic rock under lower-mantle conditions. Their compositions require that the host diamond traveled from the lower mantle to the upper mantle, carrying its cargo of trapped minerals intact.22PubMed. Deep mantle cycling of oceanic crust: evidence from diamonds and their mineral inclusions Broader surveys of deep diamond inclusions have identified three main groupings: one resembling the bulk silicate mantle, one that looks like recycled oceanic crust, and one with a carbonatitic signature. The first group, composed of ferropericlase, bridgmanite-derived phases, calcium silicate perovskite, and stishovite, strongly dominates the collection.23Earth-Science Reviews. Mineralogy of the lower mantle: A review of ‘super-deep’ mineral inclusions in diamond These inclusions confirm, in real rock, what lab experiments predict about lower-mantle mineralogy.
Primordial Signatures and Isotopic Reservoirs
Some ocean island basalts carry helium with unusually high ratios of helium-3 to helium-4. Because helium-3 is primordial, left over from Earth’s formation, and helium-4 is continuously produced by radioactive decay, a high ratio implies a source that has been largely isolated from the rest of the mantle for billions of years. The lower mantle has long been the prime suspect for harboring such a reservoir. Analysis of noble gas data from ocean island basalts supports the idea that a well-isolated primordial reservoir exists somewhere in the mantle rather than in the core.24PubMed Central. Primitive noble gases sampled from ocean island basalts cannot be from the Earth’s core
Whether that reservoir requires the lower mantle to be physically sealed off from the upper mantle is another matter. Modeling work has shown that primitive noble gas signatures can be preserved even in a convecting mantle, without needing a hidden layer or strict convective isolation.25Nature. Preserving noble gases in a convecting mantle Dense piles like the LLVPs could act as long-lived storage sites for primordial material without requiring a rigid boundary. The lower mantle, in other words, does not have to be a sealed box to keep ancient chemistry intact; it just needs regions that convection does not efficiently stir. The combination of higher viscosity, compositional density, and geometric stability at the base of the mantle appears to be enough.