In geologic terms, a plate is a large, rigid slab of solid rock. The word tectonics comes from the Greek root “to build.” Putting these two words together, we get the term plate tectonics, which refers to how the Earth’s surface is built of plates. The theory of plate tectonics states that the Earth’s outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material. Before the advent of plate tectonics, however, some people already believed that the present-day continents were the fragmented pieces of preexisting larger landmasses (“supercontinents”). The diagrams below show the break-up of the supercontinent Pangaea (meaning “all lands” in Greek), which figured prominently in the theory of continental drift — the forerunner to the theory of plate tectonics.