PLATE TECTONICS: THE UNIFYING THEORY

• The Discovery of Plate Tectonics 28
• The Plates and Their Boundaries 31
• Rates and History of Plate Movements 39
• The Grand Reconstruction 42
• Mantle Convection: The Engine of Plate Tectonics 45
• The Theory of Plate Tectonics and the Scientific Method 51

THE LITHOSPHERE—EARTH’S STRONG, rigid outer shell of rock—is broken into about a dozen plates, which slide past, converge with, or separate from each other as they move over the weaker, ductile asthenosphere. Plates are formed where they separate and recycled where they converge in a continuous process of creation and destruction. Continents, embedded in the lithosphere, drift along with the moving plates.

The theory of plate tectonics describes the movements of plates and the forces acting on them. It also explains volcanoes, earthquakes, and the distribution of mountain chains, rock assemblages, and structures on the seafloor—all of which result from events at plate boundaries. Plate tectonics provides a conceptual framework for a large part of this textbook and, indeed, for much of geology.

This chapter lays out the theory of plate tectonics and how it was discovered, describes plate movements today and in the geologic past, and examines how the forces that drive these movements arise from the mantle convection system.