Rates of Plate Movement

The whole machinery of plate tectonics only becomes useful if we can say how fast the plates are actually moving. The ocean floor itself provides the clock. As magma rises at mid-oceanic ridges and cools, it records the direction of Earth's magnetic field at that moment. Because the magnetic field has reversed its polarity many times over geological history, the seafloor is laid down in alternating strips of normal and reverse magnetism, running parallel to the ridge. These strips act like a tape recorder, and by measuring their width and knowing the dates of magnetic reversals, scientists can calculate how quickly the seafloor has spread.

These rates are not uniform across the globe. They vary considerably from one ridge to another, and the textbook gives the two extremes to frame the range.

• The Arctic Ridge has the slowest rate of plate movement, at less than 2.5 cm per year.
• The East Pacific Rise, specifically near Easter Island in the South Pacific, has the fastest rate, at more than 15 cm per year. This location is roughly 3,400 km west of Chile. …