Lower mantle

This article is about the third and bottom-most layer of Earth's mantle. For the third layer of the atmosphere, see Mesosphere.

The lower mantle, historically also called the mesosphere, occupies about 56% of the total volume of Earth, and is the region from 660 to 2,890 km (410 to 1,800 mi) below Earth's surface; between the transition zone and the outer core. The preliminary reference Earth model (PREM) separates the lower mantle into three sections, the uppermost (660–770 km (410–480 mi)), mid-lower mantle (770–2,700 km (480–1,680 mi)), and the D layer (2,700–2,890 km (1,680–1,800 mi)).

Pressure and temperature in the lower mantle range from 24–127 GPa (3,500,000–18,400,000 psi) and 1,900–2,600 K (1,630–2,330 °C; 2,960–4,220 °F). It has been proposed that the composition of the lower mantle is pyrolitic, containing three major phases of bridgmanite, ferropericlase, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase, which may affect both mantle plume dynamics and lower mantle chemistry. The mantle moves at about 1 cm (0.39 in) per year.

The upper boundary is defined by the sharp increase in seismic wave velocities and density at a depth of 660 km (410 mi). At a depth of 660 km (410 mi), ringwoodite γ-((Mg,Fe)2SiO4) decomposes into Mg-Si perovskite and magnesiowüstite. This reaction marks the boundary between the upper mantle and lower mantle. This measurement is estimated from seismic data and high-pressure laboratory experiments. The base of the mesosphere includes the D″ zone which lies just above the mantle–core boundary at around 2,700–2,890 km (1,678–1,796 mi).

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