Earth's mantle

The Earth's mantle is a massive layer of silicate rock. It sits between the thin outer crust and the liquid outer core. This layer is vital to how our planet functions. It accounts for 86% of the Earth's total mass. It also makes up about 84% of the Earth's total volume.

Although the mantle is predominantly solid, it behaves like a viscous fluid over geologic time scales. Scientists often compare its consistency to thick caramel. This slow, flowing movement is what drives many of the geological processes we see on the surface.

Heat and pressure drive the movement of the mantle through a process called convection. Heat from the core causes thermal expansion in the material at the bottom. This expansion reduces the density of the rock. As a result, hot material rises in shapes known as mantle plumes. When this material reaches the surface, it cools down. This cooler, heavier material then sinks back toward the bottom.

This continuous circulation is an integral part of how tectonic plates move. The sinking of lithosphere at subduction zones is an essential component of this convective cycle.

The mantle is divided into distinct layers based on its physical properties and seismic velocity. The upper mantle begins at the Mohorovičić discontinuity, or "Moho." This boundary was first noted by Andrija Mohorovičić in 1909. The upper mantle is split into two rheological layers. The first is the rigid lithospheric mantle. The second is the more ductile asthenosphere. The lithosphere and the crust together form the tectonic plates. These plates move over the flowing asthenosphere. Below the asthenosphere, the mantle becomes relatively rigid again.

Deep below the upper mantle lies the transition zone. This zone exists between approximately 410 and 660 kilometers deep. In this region, minerals like wadsleyite and ringwoodite are stable. These minerals can store a large amount of water within their crystal structures. Below this is the lower mantle, which extends to the core-mantle boundary. The lower mantle is composed primarily of bridgmanite and ferropericlase. At the very bottom, in the last 200 kilometers, lies the D" (D-double-prime) region. This area has anomalous seismic properties and may contain unique mineral forms like post-perovskite.

The composition of the mantle is difficult to study directly because it is inaccessible. Most estimates come from sampling the uppermost mantle. One common rock found there is peridotite. This rock is made of minerals like olivine and pyroxene. Scientists also study the mantle through xenoliths. These are fragments of mantle rock embedded in volcanic rocks like basalt.

Other mantle sections can be found in ophiolites, where oceanic lithosphere has been pushed onto a continent.

Extreme conditions exist throughout the mantle. Temperatures range from 230 °C at the top to 4,200 K at the core-mantle boundary. Pressure also increases significantly with depth. It rises from a few hundred megapascals at the Moho to 135 gigapascals at the bottom. Even though temperatures exceed the melting point of surface rocks, the mantle stays mostly solid. This is because the enormous lithostatic pressure increases the temperature required for melting. However, partial melting does occur at mid-ocean ridges and subduction zones. This melting produces both oceanic and continental crust.

Recent scientific discoveries have added even more mystery to the deep mantle. Seismic images show two continent-sized anomalies in the lowermost mantle. These zones have low seismic velocities and are denser than the surrounding material. Some scientists believe these might be remnants of Theia. Theia was an impactor that collided with Earth during the Moon-forming event.

This connection links the deep mantle to the very beginning of our planet's history. Understanding these deep layers helps us connect the movements of the surface to the ancient history of the Earth.