A map of Earth’s viscous crust

On long scales of length and time, Earth’s crust and upper mantle flow like a stiff liquid. To understand how the rocks deform under geologic stresses, you need to know their viscosity—a property that depends on the rocks’ temperature, strain rate, and composition. Among those features, variations in composition, specifically trace amounts of water and magma, are the most difficult to determine but exert a strong influence on the rocks’ behavior (see the article by Marc Hirschmann and David Kohlstedt, Physics Today, March 2012, page 40). The hotter, wetter, or more molten a rock, the weaker it is. Fortuitously, the same factors that weaken a rock and lower its viscosity also make it more electrically conductive. Since the 1950s, researchers have been able to infer resistivity profiles as a function of depth in crustal and mantle rocks from variations in magnetic and electric fields they measure at Earth’s...