Earth's mantle is hotter than molten rock yet stays almost entirely solid
Near the core, mantle rock reaches about 4,200 kelvin, far above the temperature at which peridotite melts at the surface. Crushing pressure keeps it solid. Over millions of years, though, this solid creeps like caramel, and two continent-sized blobs near its base may be buried remains of the planet that struck Earth to form the Moon.
The mantle is a 2,900-kilometre shell of silicate rock between crust and outer core, holding 86 percent of Earth's mass and 84 percent of its volume. Seismic jumps divide it into an upper mantle down to 410 km, a transition zone to about 660 km, and a lower mantle reaching roughly 2,891 km. Its top, the Moho, is named after Andrija Mohorovičić, who spotted the sudden speed-up of seismic waves in 1909. Pressure climbs from a few hundred megapascals there to about 139 gigapascals at the core boundary.
Minerals change with depth. The upper mantle is mostly peridotite, rich in olivine. In the transition zone, olivine rearranges into wadsleyite and then ringwoodite, forms that can lock water into their crystals, which has led to the hypothesis that this zone stores a great deal of water. Deeper still, bridgmanite dominates, turning into post-perovskite in the lowest 200 km, the odd D double-prime layer named by geophysicist Keith Bullen. A 2018 study even proposed that ice VII, an exotic form of water, can form inside rising diamonds.
Heat from below drives slow, chaotic convection: hot material rises in plumes while cold slabs sink at subduction zones, and this circulation is bound up with plate motion. Plumes are thought to lift the surface and feed hot-spot volcanoes, though a rival plate hypothesis explains such volcanism by crustal stretching. Rock this hot should not snap, yet earthquakes occur down to 670 km inside sinking slabs, perhaps through dehydration, runaway heating or mineral phase changes.
No one has drilled into it. Project Mohole was abandoned in 1966 after reaching only about 180 m, while the Deep Sea Drilling Project, running from 1968 to 1983 with the ship Glomar Challenger, gathered evidence that helped prove plate tectonics. In 2005 a borehole reached 1,416 m beneath the seabed, and in 2007 Japan's Chikyū aimed for 7,000 m. Geologists otherwise study mantle rocks carried up in volcanic rocks or thrust onto continents in ophiolites.
Source: Earth's mantle
