Earth's mantle

• 1. Introduction

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  Earth's mantle has the consistency of caramel. That is the strange truth about the layer of silicate rock that sits between the crust and the outer core. It is almost entirely solid, yet on geologic time scales it flows like a thick, viscous fluid. This single layer holds 86% of the mass of the entire planet. It stretches 2,900 kilometers thick, accounting for about 46% of Earth's radius and roughly 84% of its volume. Beneath your feet lies a world no human has ever reached. Drilling expeditions have failed and been abandoned. Probes have been imagined that would melt their way downward for decades. So how do scientists know anything about a place they cannot visit? What minerals exist under crushing pressure and searing heat? And what ancient collision might lie buried near the bottom of it all? The answers come from seismic waves, rare rock samples, and a few continent-sized mysteries hidden deep below.

• 2. A Layered Interior

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  Andrija Mohorovičić noticed something odd in 1909: a sudden jump in seismic velocity that marked the top of the mantle. That boundary now carries his name, the Mohorovičić discontinuity, or simply the Moho. From there the mantle divides into three major layers, each defined by abrupt changes in how seismic waves move through rock. The upper mantle begins at the Moho and runs down to about 410 kilometers. The transition zone follows, spanning roughly 410 to 660 kilometers. Below that lies the lower mantle, reaching down to about 2,891 kilometers. The upper mantle splits further by how the rock behaves rather than by what it is made of. The rigid lithospheric mantle sits at the top, and the more ductile asthenosphere lies beneath it. They are separated by the lithosphere-asthenosphere boundary. The lithosphere, combining that rigid mantle with the overlying crust, forms the tectonic plates that move across the asthenosphere. Ocean crust lithosphere runs about 100 kilometers thick, while continental crust lithosphere generally measures 150 to 200 kilometers. The deepest 200 kilometers of the lower mantle form a strange zone called D double-prime, written D". Its seismic properties are anomalous, unlike the mantle just above it. This region also contains large low-shear-velocity provinces and ultra low velocity zones, features that point toward the next layer's buried secrets.

• 3. Minerals Under Pressure

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  Peridotite dominates the upper mantle, a rock built mostly from olivine, clinopyroxene, orthopyroxene, and an aluminous phase. That aluminous phase shifts with depth. It begins as plagioclase in the uppermost mantle, becomes spinel, and then turns to garnet below about 100 kilometers. As depth increases through the upper mantle, the pyroxenes grow less stable and transform into majoritic garnet. Olivine transforms at the top of the transition zone, undergoing isochemical phase changes into wadsleyite and then ringwoodite. These high-pressure forms of olivine carry a remarkable trait. Unlike ordinary nominally anhydrous olivine, they can store large amounts of water inside their crystal structure. That capacity has fueled a striking hypothesis: the transition zone may hold a vast quantity of water locked in mineral form. Ringwoodite breaks down at the base of the transition zone, decomposing into bridgmanite and ferropericlase. Bridgmanite, once called magnesium silicate perovskite, makes up most of the lower mantle alongside ferropericlase. Minor amounts of calcium perovskite, calcium-ferrite structured oxide, and stishovite round out the mix. In the lowest 200 kilometers, bridgmanite transforms again, isochemically becoming post-perovskite, a mineral discovered in the depths where ordinary rock gives way to extremes.

• 4. Ghosts Of Theia

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  Two continent-sized anomalies hide in the lowermost mantle, revealed by seismic images of Earth's interior. They show low seismic velocities, and they appear denser and likely different in composition from the mantle around them. These zones are not random noise in the data. They may be buried relics of Theia, the body proposed to have struck the early Earth in the Giant-impact hypothesis. That collision is thought to have formed the Moon. If the interpretation holds, fragments of another world came to rest near Earth's core and have stayed there ever since.

• 5. Reading The Unreachable

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  The chemical composition of the mantle resists certainty because it is largely inaccessible. Yet mantle rocks do surface in rare circumstances. Ophiolites bring them up when sections of oceanic lithosphere are obducted onto a continent. Mantle rock also rides upward as xenoliths, fragments trapped inside basalts or kimberlites as those rocks form. Measured samples of the depleted MORB upper mantle reveal its makeup by mass percent. Silica leads at 44.71%, followed by magnesium oxide at 38.73% and iron oxide at 8.18%. Aluminum oxide accounts for 3.98% and calcium oxide for 3.17%, with smaller traces of chromium, nickel, manganese, sodium, titanium, phosphorus, and potassium oxides. Most such estimates rest on rocks from only the uppermost mantle. Whether the lower mantle shares this bulk composition remains a matter of debate. A 2018 study proposed something stranger still. An exotic form of water called ice VII can form from supercritical water in the mantle. It happens when diamonds carrying pressurized water bubbles move upward, cooling the water to the conditions ice VII requires. That cooling turns out to matter, because temperature governs nearly everything about how the mantle behaves.

• 6. Heat And Crushing Weight

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  Temperatures in the mantle climb from about 500 kelvin at the boundary with the crust to roughly 4,200 kelvin at the core-mantle boundary. That is around 230 degrees Celsius at the top and about 3,900 degrees Celsius at the bottom. The rise is rapid in the thermal boundary layers at the top and bottom, and gradual through the mantle's interior. Those temperatures far exceed the melting points of mantle rocks at the surface, where representative peridotite melts near 1,500 kelvin. Yet the mantle stays almost exclusively solid. The reason is pressure. Enormous lithostatic pressure prevents melting, because the solidus, the temperature where melting begins, rises as pressure rises. Pressure builds from a few hundred megapascals at the Moho to 139 gigapascals at the core-mantle boundary. Every layer must support the weight of all the material above it. This balance of heat and weight sets the stage for the mantle's slow, churning motion.

• 7. The Slow Churn

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  Hot material rises while cooler, heavier material sinks, driven by the temperature difference between Earth's surface and the outer core. The crystalline rocks deform slowly under high pressure and heat, creeping over millions of years in a viscous-like flow. This convective circulation moves the mantle. Rising columns are called mantle plumes, and the surface above them is predicted to sit at high elevation and show hot spot volcanism. An alternative idea, the plate hypothesis, instead explains that volcanism through passive extension of the crust letting magma leak upward. Downward motion concentrates at convergent plate boundaries known as subduction zones. The convection itself is a chaotic process in the fluid-dynamics sense, and it is thought to be integral to plate motion. Plate motion differs from continental drift, which describes only the movement of the crustal parts of continents. Descending lithosphere is an essential component of mantle convection, coupling the plates to the deep interior. Viscosity generally grows with depth, but the relationship is far from linear, with layers of dramatically reduced viscosity in the upper mantle and near the core. Estimates for upper mantle viscosity span a wide range depending on depth, temperature, composition, and stress. The geophysicist Keith Bullen introduced the name D double-prime over 50 years ago for the unusual region just above the core. It may consist of subducted slab material that descended and came to rest there, or of the post-perovskite polymorph found in those depths.

• 8. Chasing The Mantle

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  Earthquakes should not happen deep inside the mantle, yet they do. Below about 50 kilometers, hot and high-pressure conditions ought to inhibit seismicity, since the mantle behaves as viscous and incapable of brittle faulting. Still, in subduction zones, earthquakes are observed down to 670 kilometers. Proposed mechanisms include dehydration, thermal runaway, and phase change. Cool material sinking from the surface can lower the geothermal gradient, strengthening the surrounding mantle and allowing quakes between 400 and 670 kilometers deep. Project Mohole launched the first attempt to drill into the mantle, only to be abandoned in 1966 after repeated failures and cost over-runs. Its deepest penetration reached about 180 meters. Exploration favors the seabed over land because oceanic crust is far thinner than continental crust. The Deep Sea Drilling Project ran from 1968 to 1983, coordinated by Scripps Institution of Oceanography at the University of California, San Diego. Its vessel, the Glomar Challenger, gathered data supporting seafloor spreading and helping prove plate tectonics. Scientific planning fell to the Joint Oceanographic Institutions for Deep Earth Sampling, JOIDES, whose advisory group held 250 scientists from around the world. The Ocean Drilling Program continued from 1985 to 2003, then gave way to the Integrated Ocean Drilling Program. On the 5th of March 2007, scientists aboard the RRS James Cook sailed to a stretch of the Mid-Atlantic Ridge with exposed mantle rock, midway between the Cape Verde Islands and the Caribbean Sea. That site lies about three kilometers beneath the surface and spans thousands of square kilometers. The same year, the Japanese vessel Chikyū attempted to drill up to 7,000 meters below the seabed. A 2005 proposal imagined a different approach entirely: a tungsten probe one meter across, heated by a cobalt-60 radioactive interior, melting its way down while acoustic signals tracked its progress. In 2023, JOIDES Resolution drilled into the Atlantis Massif and recovered cores reaching 1,268 meters deep, including 886 meters of mostly peridotite. Researchers debate whether seawater altered the samples into deep lower crust, but the rock never melted into magma, making it a closer analogue to true mantle than any xenolith yet found.