Effects on the core of structure within d”

The structure of the D” layer is identified as due to the presence of rafts of dense mantle differentiates, termed crypto-continents, at the base of an otherwise homogeneous lower mantle. The remaining areas of the core-mantle boundary, termed crypto-oceans, provide softened material that feeds mantle plumes and moves the crypto-continents about at a rate comparable to continental drift at the surface of the Earth. The crypto-continents act as blankets on their areas of the core and they are almost certainly more iron-rich and consequently more highly conducting than normal lower mantle material. There are two possible mechanisms for the crypto-continents to act as anchors for stationary features of the geomagnetic field, thermal control of core motions and electromagnetic freezing of magnetic flux. The time scale for changes in the distribution of crypto-continents is 108 years. It appears just possible that they provide a thermoelectric bias on field polarity and so influence geomagnetic reversals on this time scale. It is pointed out that the laterally variable properties of D” disallow consideration of a stably stratified layer at the top of the core, because a laterally varying heat flux through it would establish an implausibly rapid thermal wind and secular variation.