Observations of the electrical Moho

Alan G Jones

Step changes in electrical conductivity at the crust-mantle boundary are difficult to identify with confidence due to the commonly-occuring conducting lower crustal layer. However, there are regions in the world where the conductance within this layer is suppressed, thereby permitting resolution of possible change at depths that can be associated with the seismic Moho. Laboratory measurements and mineral physics suggest that conductivity should decrease, i.e. resistivity increase, at the transition between lower crustal to upper mantle rocks. This increase is not commonly observed however, rather and perplexingly an increase in conductivity with increasing depth. Examples will be shown of observations of the electrical Moho, and a possible explanation will be given of the higher conductivity in the uppermost mantle in terms of smaller grain size.