As you studied volcanoes, igneous, metamorphic and sedimentary rocks, and earthquakes, you learned how these topics are related to plate tectonics. In this chapter we take a closer look at plates and plate motion. We will pay particular attention to plate boundaries and the possible driving mechanisms for plate motion.
The history of the concept of plate tectonics is a good example of how scientists think and work and how a hypothesis can be proposed, discarded, modified, and then reborn. In the first part of this chapter we trace the evolution of an idea - how the earlier hypothesis of moving continents (continental drift) and a moving sea floor (sea-floor spreading) were combined to form a theory of plate tectonics.