The Earth is divided into crust, mantle, and core, with the core further divided into the inner and outer portions. The inner core, discovered by Inge Lehmann in 1936, has a radius of 1220 km (758 mi) and a density about 13 g/cm³. It represents about 1.7% of the mass of the Earth. The inner-core boundary represents a phase transition from liquid iron in the outer core to solid iron in the inner core. Despite its size, the inner core plays an important role in the Earth's dynamical system. For example, scientists believe the solidification of the outer-core materials, that is, the growth of the inner core, drives the convection in the outer core and powers the geodynamo, a physical process that generates the Earth's magnetic field. Most knowledge about the inner core (its discovery, radius, and density) has come from the study of seismic waves, or seismology. Recently, much has been learned by studying variations in seismic waves due to the direction in which they are propagated (anisotropy) and the part of the inner core in which they travel (hemisphericity).