The electric moho

Since Mohorovicic(1) discovered a dramatic increase in compressional seismi c velocity at a depth of 54 km beneath the Kulpa Valley in Croatia, the 'Mo ho' has become arguably the most important seismological horizon in Earth o wing to its role in defining the crust-mantle boundary. It is now known to be a ubiquitous feature of the Earth, being found beneath both the continen ts and the oceans, and is commonly assumed to separate lower-crustal marc r ocks from upper-mantle ultramafic rocks. Electromagnetic experiments conduc ted to date, however, have failed to detect a corresponding change in elect rical conductivity at the base of the crust, although one might be expected on the basis of laboratory measurements(2). Here we report electromagnetic data from the Slave craton, northern Canada, which show a step-change in c onductivity at Moho depths. Such resolution is possible because the Slave c raton is highly anomalous, exhibiting a total crustal conductance of less t han 1 Siemens-more than an order of magnitude smaller than other Archaean c ratons. We also found that the conductivity of the uppermost continental ma ntle directly beneath the Moho is two orders of magnitude more conducting t han laboratory studies on olivine would suggest, inferring that there must be a connected conducting phase.