Core-Mantle Boundary

The boundary between Earth's silicate mantle and its liquid iron-nickel outer core, 2,891 kilometers below the surface, observed through the discontinuity in seismic wave velocities at that depth. P-waves slow sharply in the outer core and S-waves do not travel through its liquid at all; the uppermost outer core is thought to be about 500 to 1,800 Kelvin hotter than the overlying mantle, and variations in the boundary's thermal properties may affect the flows of the outer core's iron-rich fluids that are ultimately responsible for Earth's magnetic field.