IGPP, La Jolla
poster/oral:
Electrical conductivity is an important physical property of Earth, being intimately related to the thermodynamic and chemical state of the planet's interior and to the generation and propagation of geomagnetic fields. Some point defects responsible for electrical conduction also determine rheological properties of minerals. Our knowledge of Earth conductivity comes from two sources; observations of the attenuation of both externally and internally generated magnetic fields, and both low and high pressure laboratory studies of rocks and mineral phases.
Crustal conductivity is extremely heterogeneous and is primarily dependent on fluid content. Lithospheric mantle also varies at least four orders of magnitude but appears to depend mainly on temperature, and the high conductivity of basaltic melt is exploited in electrical studies of volcanic systems, particularly at mid-ocean ridges. Conductivity of the deeper mantle is to first order radially symmetric and associated with pressure, with a large, probably discontinuous, jump in conductivity at 670 km from around 0.001 S/m to about 2 S/m. Evidence suggests that lower mantle conductivity is very uniform, although D" is too thin to be easily studied using either external or internal magnetic field variations. Perturbations from radial symmetry in the upper mantle are perhaps associated with subducting and spreading plate boundaries, and maybe there is a modest change in conductivity associated with the alpha to beta spinel transition at 450 km. Although most, if not all, of the mantle can be studied using induction of external field variations, direct estimates of core conductivity cannot be made. Indirect estimates of core conductivity agree surprisingly well and are between 200000 and 500000 S/m.