A sharp and flat section of the core-mantle boundary

The structure of a 350-kilometer-long portion of the core-mantle boundary (CMB) beneath the northeast Pacific was determined by measuring seismic waves that change from shear waves to compression waves after reflecting off the CMB. This section of the CMB is characterized by an amplitude of less than 500 meters, an absence of sharp radial gradients in the 400 kilometers above the boundary and a mantle-to-core transition of less than 1 kilometer in thickness. Hence the mantle in this region is probably not chemically heterogeneous.