At the dawn of the 1950’s, Tikhonov (USSR) and Cagniard (France) independently determined that by measuring the geoelectric and geomagnetic fields at ground level, one could reconstruct the variations of the electrical conductivity below ground level as a function of depth, the origin of the magnetotelluric (MT) method. While initially limited to one- and later two-dimensional investigations of geoelectric structure, by the early 2000’s three- and even four-dimensional reconstructions of geoelectric structure became tractable. By combining constraints on the electrical properties of the crust and mantle, with those from derived quantities such as density, seismic/elastic properties and others, substantial advances have been made in understanding complex tectonic, volcanic and mineralogical systems. In today’s seminar we’ll highlight key outcomes of several ambitious projects to study the evolution and structure of the continent, and the nature of volcanic and geothermal systems in Cascadia and the Yellowstone systems.