The innermost core of the earth, which consists of highly compressed iron in a solid state, is known to have an extremely low degree of rigidity in regard to shear-the impact of twisting or other forces. The iron at the center of the earth therefore behaves largely like a fluid, which lacks all resistance to shear, making it easy for shifts to take place in the matter in the earth’s core. One consequence is that the seismic waves that move along the surface of the inner core move unexpectedly slowly.
This dual nature of iron has been an enigma to researchers for more than 50 years, since iron in laboratory experiments has not evinced any tendency whatsoever to behave like a fluid under high pressure. The reason for this is the much lower temperatures in laboratory experiments compared with the center of the earth.