Plates move away from oceanic ridge and descend beneath a trench at the subduction zone. The rates and directions of plate motions have were originally determined by computing the distance of oceanic floor of known age from the oceanic ridge system. Some volcanic islands in the interiors of plates form above fixed plumes of magma rising from the mantle. The locations of these mantle plumes are known as hot spots . The islands form as the plate moves over the magma source, much like a tectonic conveyer belt.
Islands are progressively older with increasing distance from the hot spot.
The relationship between age and distance yields the rate of plate motion. Kauai has travelled from the location of the plume to its present site over the last 5 million years. A submarine volcano, Loihi, is forming over the current position of the plume. Today satellite technology is used to determine the current rates of plate motion. Satellites anchored in space can record tiny movement of fixed sites on Earth, thus constraining the motions of plates.