Earth’s mantle is the thick layer of mostly solid rock between the planet’s thin outer crust and its metallic core, making up roughly 84 percent of Earth’s total volume and extending from about 5 to 70 kilometers below the surface down to approximately 2,900 kilometers deep. Despite being largely solid, the mantle behaves over geological timescales more like an extremely sluggish fluid, slowly circulating in a process that drives plate tectonics, volcanic eruptions, and earthquakes at the surface. The mantle is far from a uniform slab of rock, though, and its internal layering, chemistry, and hidden reservoirs of water and ancient gases have kept geoscientists busy for more than a century.
Where the Mantle Begins and Ends
The mantle’s upper boundary is marked by a feature called the Mohorovičić discontinuity, or Moho for short, named after the Croatian seismologist who identified it in 1909. The Moho is defined by a sharp jump in the speed at which earthquake waves travel: compressional waves speed up from roughly 6.5 kilometers per second in the deepest crust to about 8 kilometers per second in the uppermost mantle.1Tectonophysics. Contrasts of seismic velocity, density and strength across the Moho That velocity jump reflects a change in rock type. The crust is dominated by lighter minerals like feldspar and quartz, while the mantle is dominated by denser, olivine-rich rock called peridotite.
An important wrinkle is that the seismic Moho and the petrological Moho are not always in the same place. In regions where the crust has been thickened by mountain-building, the deepest crustal rocks can be squeezed into denser mineral forms that look, to a seismic wave, like mantle material. So the boundary where rock composition actually changes from crustal to mantle can sit at a slightly different depth than the boundary where wave speeds jump.2Terra Nova. Evolution of the petrological and seismic Moho‐implications for the continental crust‐mantle boundary For most practical purposes, the two are close enough to treat as one, but the distinction matters when geologists try to reconstruct the deep roots of mountain belts.
At the bottom, the mantle gives way to the outer core at roughly 2,900 kilometers depth. The lowermost 200 to 300 kilometers of the mantle, a region called the D” (pronounced “D double-prime”) layer, is an extraordinarily complicated zone where silicate rock interacts with the liquid iron alloy of the core. Experiments at high pressures and temperatures suggest that chemical reactions between these two materials make D” extremely heterogeneous, which helps explain why seismic waves behave so erratically there.3PubMed. Earth’s Core-Mantle Boundary: Results of Experiments at High Pressures and Temperatures The thermal and chemical gradients in this boundary layer also influence the magnetic field generated in the core beneath it.
What the Mantle Is Made Of
If you could somehow scoop out a handful of upper-mantle rock, you would mostly be holding peridotite, a dense, greenish rock dominated by the mineral olivine, with smaller amounts of pyroxene and garnet.4Geophysical Research Letters. Mineralogy and composition of the upper mantle Olivine is a magnesium-iron silicate, and its behavior under pressure is what gives the mantle many of its defining features. The upper mantle is not perfectly uniform, though. There is significant lateral variation in composition from one region to another, reflecting billions of years of melting, recycling, and mixing.
As you go deeper, the crushing pressure forces olivine to rearrange into denser crystal structures. At roughly 410 kilometers, olivine transforms into a phase called wadsleyite and then, deeper still, into ringwoodite. Below about 660 kilometers, the dominant minerals shift again. The lower mantle is thought to be made largely of a mineral called bridgmanite (a magnesium-iron silicate with a perovskite crystal structure), along with a smaller proportion of ferropericlase and calcium silicate perovskite. Single-crystal measurements of bridgmanite with aluminum and iron have been used to constrain what the lower mantle’s chemical makeup actually is, because matching the mineral’s elastic properties to seismic observations lets researchers work backward to a likely composition.5Nature. Evidence for a Fe3+-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data Diamond-anvil-cell experiments have also explored how calcium and magnesium-iron perovskites share elements at extreme pressures, finding that iron increasingly dissolves into the calcium perovskite phase as pressure climbs.6Physics of the Earth and Planetary Interiors. Approach to the mineralogy of the lower mantle by a combined method of a laser-heated diamond anvil cell experiment and analytical electron microscopy
Internal Layers and Key Boundaries
Geoscientists typically divide the mantle into the upper mantle, the transition zone, and the lower mantle, with each division marked by changes in mineral structure rather than abrupt shifts in raw chemistry.
The transition zone sits between about 410 and 660 kilometers depth. At 410 kilometers, olivine’s transformation into a higher-pressure crystal form produces a sharp discontinuity that seismometers can detect worldwide. At 660 kilometers, a different set of mineral transformations marks the floor of the transition zone, with the dominant phases converting to the bridgmanite-plus-ferropericlase assemblage of the lower mantle. These two boundaries respond to temperature in opposite ways: the 410-kilometer discontinuity moves deeper in hotter regions, while the 660-kilometer discontinuity moves shallower.7Earth and Planetary Science Letters. Reconciling Pacific 410 and 660 km discontinuity topography, transition zone shear velocity patterns, and mantle phase transitions That anti-correlation is a useful diagnostic: when seismologists map the depths of these boundaries globally, the pattern helps them infer where the mantle is hotter or cooler than average.
Just below the crust, the uppermost mantle forms part of the rigid lithosphere, which includes the crust and moves as tectonic plates. Beneath the lithosphere lies the asthenosphere, a zone where the mantle is slightly weaker and can flow more readily. One longstanding question is why the asthenosphere is mechanically soft. Laboratory experiments have shown that even a tiny fraction of melt, around 0.2 percent, can reproduce the low seismic velocities observed in the asthenosphere, supporting the idea that partial melting plays a real role there.8PubMed Central. Experimental evidence supports mantle partial melting in the asthenosphere In some localized areas, the presence of dissolved volatile elements like water and carbon dioxide seems necessary to explain especially extreme velocity drops.
How the Mantle Moves
The mantle convects. Heat from the core and from radioactive decay within the mantle itself creates temperature differences that drive slow circulation, with hotter rock rising and cooler rock sinking. This is the engine behind plate tectonics: cold, dense oceanic plates sink back into the mantle at subduction zones, while hotter material rises elsewhere, sometimes punching through the surface as volcanic hotspot chains.
A major debate in Earth science has been whether the mantle convects as a single layer from top to bottom or whether the upper and lower mantle circulate separately with the 660-kilometer boundary acting as a barrier. Three-dimensional computer models of convection in a spherical shell find that whole-mantle flow produces patterns dominated by narrow, sheet-like downflows (resembling subduction zones) in a background of broader, weaker upwelling, which matches what seismic imaging shows inside the real Earth. Layered-convection models, by contrast, produce complicated roll-like patterns in the upper shell that do not resemble the observed mantle structure.9Journal of Geophysical Research: Solid Earth. Three‐dimensional spherical models of layered and whole mantle convection Parameterized cooling studies also find that whole-mantle convection can satisfy a wide range of constraints on Earth’s heat loss and thermal history, while layered convection satisfies a much narrower parameter range, if any at all.10Geochemistry, Geophysics, Geosystems. Cooling of the Earth: A parameterized convection study of whole versus layered models
The consensus today leans strongly toward whole-mantle convection, but the lower mantle does not behave identically to the upper mantle. Its viscosity is estimated to be about 30 times higher, which slows flow and affects how efficiently the planet cools and releases gases trapped in its interior.11Earth and Planetary Science Letters. Whole-mantle versus layered mantle convection and the role of a high-viscosity lower mantle in terrestrial volatile evolution Seismic tomography, which builds three-dimensional images of the mantle by analyzing how earthquake waves slow down or speed up along different paths, has confirmed that subducted slabs of oceanic lithosphere do sink through the 660-kilometer boundary and penetrate deep into the lower mantle, overcoming the resistance of the phase transition there.12PubMed Central. Mantle dynamics and seismic tomography
Giant Structures Near the Core
Seismic tomography has also revealed two continent-sized blobs of anomalously slow seismic velocity sitting on top of the core, one beneath Africa and one beneath the Pacific. These are called large low-shear-wave-velocity provinces, or LLSVPs, and they are among the largest structures inside the planet, spanning thousands of kilometers in width and reaching roughly 1,800 kilometers in height.13Geochemistry, Geophysics, Geosystems. Compositional layering within the large low shear‐wave velocity provinces in the lower mantle Their origin is one of the most actively debated topics in deep-Earth science.
One hypothesis is that the LLSVPs are purely thermal, just hotter-than-average mantle that has pooled above the core. But their seismic signatures, particularly the strong drop in shear-wave speed and the ratio between shear and compressional velocity changes, are difficult to explain with temperature alone. Mineral physics experiments suggest that enrichment of an iron-rich form of bridgmanite, about 10 to 15 percent by volume, could reproduce the observed seismic anomalies. Geodynamic simulations show that piles with this composition, roughly 1.5 percent denser than the surrounding mantle, could survive as stable thermochemical structures throughout Earth’s history without being stirred away by convection.14Nature Communications. Formation of large low shear velocity provinces through the decomposition of oxidized mantle Some researchers have proposed that these provinces are compositionally layered, with a primordial domain near the core-mantle boundary and a shallower domain composed partly of recycled oceanic crust.15Geochemistry, Geophysics, Geosystems. Compositional layering within the large low shear‐wave velocity provinces in the lower mantle
Water Hidden in the Deep Mantle
One of the more surprising discoveries about the mantle is that it stores a significant amount of water, not as liquid oceans but locked inside the crystal structures of high-pressure minerals. Ringwoodite, the dominant mineral between about 520 and 660 kilometers depth, can hold up to 2 to 3 weight percent of water in its crystal lattice, making the transition zone a potentially enormous water reservoir.16Earth and Planetary Science Letters. Seismically invisible water in Earth’s transition zone? Wadsleyite, the phase just above it, has similarly large water-storage capacity. Measurements of electrical conductivity in these minerals have been used to estimate how much water the transition zone actually contains, since dissolved water changes how well the minerals conduct electricity.17Nature. Water content in the transition zone from electrical conductivity of wadsleyite and ringwoodite
The total amount of water stored in the transition zone could rival the volume of Earth’s surface oceans, though estimates vary widely and the water content is probably patchy rather than uniform. This deep water cycle matters because water lowers the melting point of rock, weakens minerals, and affects how easily the mantle flows. Water dragged into the mantle by subducting plates can be released back toward the surface through volcanic activity, so the mantle acts as both a sink and a source for water over geological time.
Primordial Gases and What They Reveal
Helium isotopes have given scientists an unexpected window into the mantle’s history. The ratio of helium-3 to helium-4 in volcanic rocks varies dramatically depending on where the rock originated. Mid-ocean-ridge basalts, formed by melting the upper mantle, have a helium-3 to helium-4 ratio of about eight times the atmospheric value. But lavas from volcanic hotspots like Hawaii and Iceland can have ratios up to 50 times the atmospheric value.18PubMed. Primordial helium entrained by the hottest mantle plumes Since helium-3 is mostly primordial (left over from Earth’s formation and not produced in significant quantities by radioactive decay), the high ratios at hotspots have been interpreted as evidence that deep mantle plumes tap into a reservoir of ancient, relatively undegassed material that has survived billions of years of convective stirring.
This interpretation is not without challengers. Some researchers have argued that helium isotope ratios alone cannot reliably distinguish a deep mantle plume source from other processes, and that the data do not necessarily require a pristine, isolated lower-mantle reservoir.19Earth and Planetary Science Letters. Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components? The debate continues, but the weight of evidence from multiple isotope systems and seismic imaging has generally favored the idea that some portions of the deep mantle retain a chemical memory of Earth’s earliest days.
Pieces of the Mantle We Can Actually Touch
Nobody has drilled to the mantle. The deepest borehole ever made, the Kola Superdeep Borehole in Russia, reached about 12 kilometers before the rock became too hot and plastic to drill further, and the Moho was still far below. But pieces of the mantle reach the surface through other means.
Kimberlite eruptions, the violent volcanic events that deliver diamonds to the surface, also bring up chunks of mantle rock called xenoliths. Studies of pressure and temperature conditions recorded in these xenoliths and the minerals around them show a strong concentration of highly depleted rock between 140 and 190 kilometers depth, and most diamonds originate from harzburgites within that layer.20Geology. Diamond, subcalcic garnet, and mantle metasomatism: Kimberlite sampling patterns define the link These samples have been analyzed for decades, providing direct chemical and mineralogical information about the uppermost mantle beneath continents.
On the ocean floor, tectonic processes also expose mantle rock. At slow-spreading mid-ocean ridges, large-offset faults called detachment faults can strip away the overlying crust and bring peridotite and gabbro up to the seafloor in structures known as oceanic core complexes.21Oxford Academic. Geophysical signatures of oceanic core complexes Submersibles and dredging operations have collected mantle peridotite from these exposures, giving scientists another direct look at what lies beneath the crust. Ancient slabs of oceanic mantle rock called ophiolites have also been thrust onto land during tectonic collisions. Oman’s Samail ophiolite, for example, preserves a complete cross-section of oceanic crust and upper mantle rock that geologists have studied for decades.
The Mantle’s Role in Earth’s Long-Term Cycles
The mantle is not just a passive layer between crust and core. It actively regulates Earth’s surface conditions over billions of years by cycling carbon, water, and other volatiles between the surface and the deep interior. Petrological studies of komatiites, unusual volcanic rocks erupted in the Archean eon more than 2.5 billion years ago, provide evidence for an early Earth carbon and water cycle involving ingassing during a cooler Hadean period and outgassing during the hotter Archean.22Journal of Petrology. Petrological Evidence from Komatiites for an early Earth Carbon and Water Cycle The mantle has been absorbing and releasing volatiles ever since, with the balance between subduction (which pushes material down) and volcanism (which brings it back up) affecting atmospheric composition and climate over timescales of hundreds of millions of years.
This volatile cycling also connects to habitability. The amount of water and carbon dioxide the mantle stores and releases helps determine whether a planet keeps liquid water on its surface. Earth’s mantle happens to be in a regime that has maintained surface oceans for most of the planet’s history, but small differences in composition, temperature, or convection style could tip the balance. Venus, for instance, has a mantle that may differ from Earth’s in its deepest regions. Modeling based on geodetic data suggests that Venus’s lowermost mantle may lack the perovskite-to-post-perovskite phase transition found in Earth’s D” layer if its iron oxide content is below a certain threshold, which would make the two planets’ deep interiors structurally distinct despite their similar sizes.23Journal of Geophysical Research: Planets. Possible Deep Structure and Composition of Venus With Respect to the Current Knowledge From Geodetic Data Understanding what makes Earth’s mantle work the way it does is, in part, understanding why our planet ended up hospitable while its neighbors did not.