Mantle
The mantle is the thick layer of hot, dense silicate rock between Earth's crust and core, making up about 84 percent of the planet's volume.
Definition
The mantle is the thick layer of hot rock between Earth's crust and its core, mostly solid but so hot that it slowly flows, like very thick taffy, over millions of years. It extends from the base of the crust to the top of the core, about $2{,}900\ \text{km}$ down, makes up about $84\%$ of Earth's volume, and is composed of dense iron- and magnesium-rich silicate rock (peridotite). The upper mantle includes the rigid part of the lithosphere and the weaker asthenosphere below it, and heat from the core and from radioactive decay drives slow convection currents. Seismic discontinuities divide it further: the upper mantle to $410\ \text{km}$ (olivine-dominated), the transition zone from $410$ to $660\ \text{km}$ where olivine transforms to wadsleyite, ringwoodite, and then bridgmanite plus ferropericlase, and the lower mantle to $2{,}890\ \text{km}$. Mantle rock deforms by solid-state creep with viscosities of order $10^{19}$ to $10^{22}\ \text{Pa}\cdot\text{s}$, allowing convection at a few centimeters per year. The D double-prime layer just above the core-mantle boundary is thermally and chemically heterogeneous and may be the source of mantle plumes.
Example
If Earth were a peach, the skin would be the crust, the juicy flesh would be the mantle, and the pit would be the core. Mantle convection moves the tectonic plates: hot mantle rising under the Mid-Atlantic Ridge and under Yellowstone brings magma to the surface, while cold slabs sink back into the mantle beneath the Andes. Seismic tomography images the cold, subducted Farallon slab sinking through the mantle beneath eastern North America, and warm upwellings beneath Hawaii and Iceland. Ringwoodite inclusions in diamonds show that the transition zone stores water, perhaps as much as the oceans, bound in mineral structures.
Key Insight
The mantle is not a sea of lava; it is solid rock that creeps so slowly you could never see it move. Temperature in the mantle ranges from about $1{,}000^\circ C$ near the top to over $3{,}500^\circ C$ at the core boundary, yet the enormous pressure keeps nearly all of it solid. The mantle can be solid and still flow because, over geologic time, crystal defects migrate through the lattice under stress. Glaciers and even old window glass show the same principle: solid does not mean rigid at every timescale.