Global average models have P and S velocity gradients reduced. May be zone of return convective flow, chemical reaction zone between silicate mantle and liquid iron alloyed outer core, and contain slab remnants. Convection of conducting liquid induces magnetic field. P waves travel through the inner core faster in polar directions than equatorial directions. Why the lattice would asume a preferred orientation is not yet known.
Tomography prodcues a three-dimensional image of the Earth.
No detectable variations in the outer core. Wedge of mantle material beneath island arcs and in front of subducting slabs is slow.