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Reference.org
Lower mantle
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<p>The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2890 km below <a href="/facts/Earth/HLLmDfj0">Earth's surface</a>; between the <a href="/facts/Transition_zone_(Earth)/ID58Q6W7">transition zone</a> and the <a href="/facts/Earth%2527s_outer_core/1U5pw3La">outer core</a>. The <a href="/facts/Preliminary_reference_Earth_model/NcI4jdR6">preliminary reference Earth model</a> (PREM) separates the lower mantle into three sections, the uppermost (660–770 km), mid-lower mantle (770–2700 km), and the D layer (2700–2890 km). Pressure and temperature in the lower mantle range from 24–127 GPa and 1900–2600 <a href="/facts/Kelvin/E3trzC4C">K</a>. It has been proposed that the composition of the lower mantle is <a href="/facts/Pyrolite/FQmNU4M7">pyrolitic</a>, containing three major phases of <a href="/facts/Silicate_perovskite/behIjuQ2">bridgmanite</a>, <a href="/facts/Ferropericlase/4w5s4lC6">ferropericlase</a>, 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 <a href="/facts/Mantle_plume/wLXJ0Q4a">mantle plume</a> dynamics and lower mantle chemistry. </p><p>The upper boundary is defined by the sharp increase in <a href="/facts/Seismic_wave/1JTeLfac">seismic wave</a> velocities and <a href="/facts/Density/92p6hh9I">density</a> at a depth of 660 kilometers (410 mi). At a depth of 660 km, <a href="/facts/Ringwoodite/LG79eVdM">ringwoodite</a> (γ-(Mg,Fe)2SiO4) decomposes into <a href="/facts/Silicate_perovskite/behIjuQ2">Mg-Si perovskite</a> and <a href="/facts/Magnesiow%25C3%25BCstite/4w5s4lC6">magnesiowüstite</a>. This reaction marks the boundary between the <a href="/facts/Upper_mantle/5I9jq18K">upper mantle</a> and lower mantle. This measurement is estimated from seismic data and high-pressure laboratory experiments. The base of the mesosphere includes the <a href="/facts/D%25E2%2580%25B3/EWHuuEdT">D″</a> zone which lies just above the <a href="/facts/Core%25E2%2580%2593mantle_boundary/EWHuuEdT">mantle–core boundary</a> at approximately 2,700 to 2,890 km (1,678 to 1,796 mi). </p>