Earth's mantle

Deep beneath Earth's crust lies the mantle, a thick layer of silicate rock that extends all the way down to the outer core. This layer holds about 86% of the planet’s total mass, weighing in at 4.01×10²⁴ kg, and accounts for roughly 84% of Earth’s volume. Though it is mostly solid, over the immense stretches of geologic time it flows like a very thick, viscous fluid—often compared to caramel in consistency. When the mantle partially melts at mid-ocean ridges, it gives rise to oceanic crust; at subduction zones, similar partial melting creates continental crust.

The mantle’s structure is defined by changes in how seismic waves travel through it. The uppermost part, together with the overlying crust, forms the rigid lithosphere—the tectonic plates that glide over the more ductile asthenosphere below. Oceanic lithosphere is about 100 km thick, while continental lithosphere ranges from 150 to 200 km. Below the asthenosphere, the mantle becomes relatively rigid again. Deeper still, the mantle is divided into three main layers based on seismic velocity jumps: the upper mantle (from the Moho down to 410 km), the transition zone (410–660 km), and the lower mantle (660–2,891 km). Within the transition zone, the minerals wadsleyite and ringwoodite are stable; in the lower mantle, bridgmanite and post-perovskite dominate. The lowermost 200 km of the lower mantle is called the D″ region, noted for its unusual seismic behavior and containing large low-shear-velocity provinces and ultra low velocity zones.

The top of the mantle is marked by a sudden increase in seismic velocity, first observed by Andrija Mohorovičić in 1909 and now called the Mohorovičić discontinuity, or simply the Moho. The upper mantle is mostly peridotite, made up of varying amounts of olivine, clinopyroxene, orthopyroxene, and an aluminous phase that shifts from plagioclase near the top to spinel and then garnet below about 100 km. As depth increases, pyroxenes become less stable and transform into majoritic garnet. At the top of the transition zone, olivine changes into wadsleyite and then ringwoodite—minerals that can hold much more water in their crystal structure than olivine can. This has led to the idea that the transition zone might contain a large amount of water. At the base of the transition zone, ringwoodite breaks down into bridgmanite and ferropericlase, while garnet also becomes unstable. The lower mantle is mainly bridgmanite and ferropericlase, with smaller amounts of calcium perovskite, calcium-ferrite structured oxide, and stishovite. In the deepest 200 km, bridgmanite transforms into post-perovskite.

Seismic images have revealed two continent-sized anomalies in the lowermost mantle, where seismic waves travel more slowly. These zones are denser and likely compositionally different from the surrounding mantle, and they may be remnants of Theia—the hypothetical planet that collided with Earth to form the Moon, according to the Giant-impact hypothesis.

Determining the mantle’s exact chemical composition is tricky because it’s mostly out of reach. Rare exposures occur in ophiolites (where pieces of oceanic lithosphere have been thrust onto continents) or as xenoliths—fragments of mantle rock trapped within basalts or kimberlites. Most estimates of mantle composition come from these uppermost samples, and it’s debated whether the deeper mantle, especially the lower mantle, shares the same bulk composition. Over Earth’s history, the mantle’s composition has changed as magma was extracted to form oceanic and continental crust. A 2018 study also suggested that an exotic form of water called ice VII can form in the mantle when diamonds containing pressurized water bubbles rise and cool the water to the necessary conditions.

Temperatures in the mantle range from about 500 K (230 °C) at the boundary with the crust to roughly 4,200 K (3,900 °C) at the core-mantle boundary. The temperature rises quickly in the thermal boundary layers at the top and bottom, and more gradually through the interior. Even though these temperatures far exceed the melting point of mantle rocks at the surface (around 1,500 K for peridotite), the mantle remains almost entirely solid because the enormous pressure—from a few hundred megapascals at the Moho to 139 GPa at the core-mantle boundary—raises the melting point.