Formation, Composition, and Evolution of the Earth’s Core

The Earth’s core formed by multiple collisions with differentiated protoplanets. The Hf-W (hafnium-tungsten) isotopic system reveals that these collisions took place over a timescale of tens of megayears (Myr), in agreement with accretion simulations. The degree to which the iron and silicates re-equilibrated during each collision is uncertain and affects the apparent core age derived from tungsten isotopic measurements. Seismological data reveal that the core contains light elements in addition to Fe-Ni, and the outer core is more enriched in such elements than the inner core. Because O is excluded efficiently from solid iron, O is almost certainly an important constituent of the outer core. The identity of other elements is less certain, despite intensive measurements of their effects on seismic velocities, densities, and partitioning behavior at appropriate pressures and temperatures. Si and O are very likely present, with perhaps some S; C and H are less likely. Si and Mg may have exsolved over time, potentially helping to drive the geodynamo and producing a low-density layer at the top of the core. Radioactive elements (U, Th, K) are unlikely to be present in important concentrations. The cooling of the core is controlled by the mantle’s ability to extract heat. The geodynamo has existed for at least 3.5 gigayears (Gyr), placing a lower bound on the heat flow out of the core. Because the thermal conductivity of the core is uncertain by a factor of ~3, the lower bound on this heat flow is similarly uncertain. Once the inner core started to crystallize, additional sources of energy were available to power the geodynamo. Inner core crystallization likely started in the time range 0.5 to 2.0 Gyr Before Present (BP); paleomagnetic arguments have been advanced for inner core growth starting at several different epochs within this time range.