Ultra-low velocity zone

The zones appear to correlate with edges of the African and Pacific large low-shear-velocity provinces (LLSVPs)[1] as well as the location of hotspots.

[2] [3] ULVZs are discovered by the delay and scattering of body waves that reflect and diffract on or are refracted by the core-mantle boundary.

Even though ULVZs are discovered in places, it remains difficult to map out their extent and constrain their density and velocity.

[9] Both silicate perovskite[10] and periclase[11] (which are thought to be present in the lowermost mantle) show reduced velocities with increasing iron at these pressures and temperatures.

In a general mantle convection setting, the density contrast as well as the amount of material available would control the morphology/shape of the ULVZ.

Additionally, the ULVZ appears to lie in a gap of the Pacific LLSVP [16] (not represented in the illustration here), leading to the hypothesis that this slow material is pushed to the center by surrounding large piles.