The Earth’s mantle is the thick layer of rock situated between the outer crust and the dense, metallic core. It begins just beneath the Mohorovičić discontinuity (Moho) and extends down to nearly 2,900 kilometers deep. Comprising 84% of the planet’s total volume, the mantle is the largest internal reservoir of material on Earth. Its slow, churning movement drives plate tectonics, which is responsible for earthquakes, volcanoes, and the formation of continents.
The Primary Chemical Components
The mantle is predominantly composed of silicate rocks, rich in Silicon (Si), Oxygen (O), Magnesium (Mg), and Iron (Fe). Oxygen is the most abundant element by weight, followed by Silicon and Magnesium, typically bound together in oxide forms. The primary rock type making up the upper mantle is peridotite, an ultramafic igneous rock rich in magnesium and iron silicates.
Peridotite is defined by its dominant mineral constituents: olivine and pyroxene. Olivine, a magnesium-iron silicate, is the most common mineral in the upper mantle. The presence of Aluminum (Al) and Calcium (Ca) leads to the formation of accessory minerals such as garnet and spinel.
The chemical makeup of the mantle changes subtly with depth, though it is considered a single chemical layer. As pressure increases deeper into the planet, the atoms within these silicate compounds are squeezed into denser, more tightly packed crystal structures.
Physical Layering and State
The mantle is divided into distinct physical layers characterized by changes in rock strength (rheology) and mineral structure. The uppermost part forms the rigid lithosphere, which is coupled with the crust to form the tectonic plates. Directly beneath the lithosphere lies the asthenosphere, where temperatures are close to the rock’s melting point, causing the material to behave plastically.
The asthenosphere is solid rock, but its lower viscosity allows it to slowly deform and flow over geological timescales, enabling the movement of the overlying lithospheric plates. This layer extends to a depth of about 410 kilometers, marking the beginning of the mantle transition zone.
The transition zone, spanning from 410 to 660 kilometers deep, is defined by significant density increases due to pressure-induced phase changes in the mantle minerals. The crystal structure of olivine breaks down and reorganizes into denser forms known as wadsleyite and then ringwoodite. These minerals are chemically identical to olivine but possess different atomic arrangements.
The 660-kilometer boundary marks the base of the transition zone and the top of the lower mantle. The lower mantle extends down to the core-mantle boundary at 2,900 kilometers and is primarily composed of the extremely dense mineral bridgmanite (formerly perovskite), along with magnesiowüstite. This largest mantle layer is subject to immense pressure, which forces its constituent minerals into highly compact structures.
Seismic Methods for Observation
Since direct sampling of the deep mantle is impossible, scientists rely on seismology to infer its internal structure and composition. Earthquakes generate seismic waves (P-waves and S-waves) that travel through the Earth’s interior. The speed and path of these waves change abruptly when they encounter boundaries between layers with different densities or states of matter.
By deploying seismometers globally, researchers analyze the arrival times of these waves to map out the mantle’s internal discontinuities. The Moho is identified by a sharp increase in seismic wave velocity, marking the density change from crustal rock to the denser peridotite of the upper mantle. The 410 km and 660 km boundaries, which define the transition zone, are also identified by sudden velocity changes caused by mineral phase transformations.
Scientists also study fragments of the mantle brought to the surface, known as xenoliths, which are carried upward within volcanic magma. These rock samples, mostly peridotite, provide direct chemical and mineralogical evidence to validate models created from seismic data. The combined analysis of seismic wave behavior and xenolith properties allows for a detailed understanding of the deep Earth’s composition and physical state.