Earth's Mantle

Earth's mantle is the thick layer of hot, solid rock between the thin crust and the metallic core. It extends to a depth of about 2,900 kilometers, makes up around 84 percent of the planet's volume and about two thirds of its mass, and it drives most geological activity at the surface.

The mantle is mostly silicate rock rich in magnesium and iron, with the upper part dominated by olivine and pyroxene, as in the rock peridotite. A boundary called the Mohorovičić discontinuity, found in 1909, marks the base of the crust. Seismic waves reveal a transition zone between about 410 and 660 kilometers, where minerals change into denser forms, and a lower mantle below. Temperatures rise from about 1,000 °C near the top to around 3,700 °C at the boundary with the core.

Although it is solid, the mantle flows very slowly over geological time, at a few centimeters a year, like a thick putty. This convection carries heat from the core, and its upper rigid part, along with the crust, forms the lithosphere whose plates move across the surface, described by plate tectonics. Hot plumes may rise from deep in the mantle to feed volcanic hotspots.

Partial melting of the mantle makes magma, which produces basalt at mid-ocean ridges and, rarely, unusual rocks such as kimberlite, which carries diamonds up from great depth. Carbon and water are stored in the mantle and returned to the surface by volcanism, which makes it an important reservoir in the long-term carbon cycle and biogeochemical cycles.

• Lithosphere — the rigid shell above it
• Plate Tectonics — the surface motion it drives
• Peridotite — its main rock type