Measuring Plate Motion

We can tell from several different lines of evidence that the lithospheric plates move.

Most obviously, we can measure their motions directly, thanks to the GPS satellite network. Where we can put GPS sensors close to plate boundaries, such as in Iceland (which sits on a spreading center) or California (where the San Andreas fault system separates two plates), the numbers are quite precise. (map of current plate motions)

We can extend measurements into the geologic past in several ways. Two methods, one for oceanic rocks and another for continental rocks, are based on the Earth's magnetic field. In every volcanic eruption, the iron-bearing minerals (mostly magnetite) become magnetized by the field as they cool. The direction they're magnetized in points to the nearest magnetic pole.

Oceanic lithosphere forms continuously by volcanic eruptions at spreading ridges. Therefore the whole oceanic plate bears a consistent magnetic signature. When the Earth's magnetic field reverses direction, as it does for reasons not fully understood, the new rock takes on the reversed signature. Thus most of the seafloor has a striped pattern of magnetization, as if it were a piece of paper emerging from a fax machine. The differences in magnetization are slight, but sensitive magnetometers on ships can detect them. Because we know the dates of all the magnetic reversals, magnetic maps of the seafloor translate precisely to age maps. (ocean floor age map)

Unfortunately, the seafloor is relatively young because eventually it is subducted beneath other plates. For times before about 150 million years ago we must rely on the other magnetic method, using continental rocks. As plate movements have rotated the continents, the ancient rocks turned with them, and where their minerals once indicated north they now point somewhere else. If you plot these apparent poles on a map, they appear to wander away from true north as the ages go back in time. In fact, north does not change, and the wandering paleopoles tell a story of wandering continents.

These three methods—GPS, seafloor magnetization and paleopoles—combine into a smooth, integrated timeline for the motions of the lithospheric plates, a tectonic travelogue.

Plate Tectonics in a Nutshell