Plate tectonics is one of the most fascinating aspects of our planet. We know much of the story from the way magnetic stripes flip-flop on ocean-floor magnetic observatories, but we also have other clues like rock samples that reveal how fast and where plates are moving.
The fastest plate movement takes place along narrow zones between plates called divergent and convergent boundaries. The former produce new crust as the plates pull apart. The latter destroy existing crust as one plate dives underneath the other. These zones can create amazing geographic features. For example, the Red Sea was formed as the African and Arabian plates pulled apart creating a zone known as a divergent boundary.
At these zones the plates generate magma that oozes up from the mantle. This molten material then forms new crust as it spreads into the crack between the plates. The plates then move away from each other as they separate. Eventually this process can even connect two continents as it did when the African and Arabian plates separated to create the Great Rift Valley of Africa and the Red Sea. The whole system is part of a global mid-ocean ridge system that encircles the Earth.
But how do scientists know how fast the plates move? They have a rough idea based on how fast they shift magnetic striping on the ocean floor, and the average rates of movement over geologic time.