Chapter 2: Plate Tectonics

Wegener marshaled compelling evidence from multiple disciplines: the geometric fit between the coastlines of South America and Africa, paleontological data showing identical fossil species on now-separated continents such as Mesosaurus and Glossopteris, correlating rock formations and mountain systems across the Atlantic Ocean basin, and paleoclimatic indicators revealing ancient glacial deposits in equatorial regions. However, his ideas faced rejection because contemporary scientists found his proposed driving mechanisms implausible. The post-World War II era of oceanic exploration revealed previously unknown features including mid-ocean ridge systems, abyssal trenches, and unexpectedly young seafloor material, providing the observational foundation necessary to revive and fully develop Wegener's concept. The chapter explains that Earth's rigid outer layer, the lithosphere, consists of crust and upper mantle material organized into discrete plates that rest upon the plastic asthenosphere beneath. These plates interact along three fundamental boundary types: divergent boundaries such as the Mid-Atlantic Ridge and East African Rift where new lithosphere forms through seafloor spreading; convergent boundaries where lithosphere is destroyed through subduction, generating volcanic arcs in oceanic-continental settings and massive mountain belts during continental collision; and transform boundaries like the San Andreas Fault accommodating lateral plate motion. Supporting evidence includes deep-sea drilling data showing seafloor age progression from ridge axes, paleomagnetism revealing magnetic reversals in symmetrical oceanic patterns confirming spreading, and hot spot volcanism tracing plate trajectories over stationary mantle plumes. The chapter concludes by describing how geologists and geodesists measure plate velocities through both geological dating techniques and modern GPS technology, demonstrating rates of several centimeters annually. Plate motion results primarily from slab pull, the gravitational sinking of dense oceanic lithosphere, and ridge push, the outward force from elevated ridge systems, operating within larger-scale mantle convection circulation.