User:Sanne.cottaar/Lowermost Mantle

= Lowermost mantle (or D '' ) =

The lowermost mantle refers to the lowest couple hundred kilometers of the mantle above the core-mantle boundary. This region is the lower thermal boundary layer in global mantle convection. An alternative name is D , which comes from Bullen (1950) terming the lower mantle the D-layer, which later got split into the D' layer for most of the lower mantle, and the D  layer for the lowermost mantle.

A seismic velocity jump is observed tens to hundreds of kilometers above the core-mantle boundary, this is often referred to as the D '' -discontinuity and is often interpreted to be a mineral phase transition from bridgmanite to post-perovskite.

The lowermost mantle shows heterogeneity in seismic velocities. There are two broad, thick regions that show anomalously slow wave speeds, referred to as Large Low Shear Velocity Provinces. There are also thin regions (tens of km) of extremely slow wave speeds, referred to as Ultra Low Velocity Zones.