At various points throughout Earth's history, the planet's solid exterior has drifted about in relation to the planetary rotation axis. This solid body drift, which is known as “true polar wander,” results in a wholesale shift in the orientation of Earth's landmasses and is different from the motion of individual tectonic plates (“tectonic drift”) or of the magnetic pole (“apparent polar wander”). Sorting out when, in which direction, and at what rate the Earth's solid exterior has rotated in this way depends on having a stable frame of reference to which observations of relative motion can be compared. To develop such a frame, researchers rely on hot spots, regions of recurrent volcanism that are known to produce long, largely linear island chains—such as the Hawaiian islands—as an overlying tectonic plate passes overhead. Hot spots are fed by magma plumes from the deep mantle and hence tend to be long-lived and relatively stable. Hot spots have long been used to understand the motion of tectonic plates.