Chapter 3: Drifting Continents and Spreading Seas

Wegener marshaled compelling evidence across multiple disciplines: the geometric fit of continental margins, the distribution of ancient glacial deposits in presently tropical regions, paleoclimate indicators preserved in rock types, the presence of identical fossils on now-separated landmasses, and matching rock formations and structural features across ocean basins. Despite this persuasive evidence, the scientific community initially rejected continental drift because Wegener could not explain the mechanism driving such massive crustal movement. The chapter then documents how mid-twentieth-century technological advances transformed the field, particularly detailed seafloor mapping, which revealed the existence of mid-ocean ridges as continuous underwater mountain systems and deep oceanic trenches. Harry Hess synthesized these observations into the seafloor spreading hypothesis, proposing that new oceanic crust continuously forms at ridge axes and moves laterally outward, with older crust descending into trenches through subduction. The chapter explains how heat flow measurements, seismic activity patterns, and the relative youth of oceanic crust compared to continental rocks supported this revolutionary concept. The discovery of paleomagnetism—the fossilized record of Earth's ancient magnetic field orientation preserved in cooling igneous rocks—provided the definitive proof needed to convince the scientific community. Magnetic reversals, the periodic switches in Earth's magnetic polarity, create symmetrical striped patterns of alternating magnetization in seafloor basalt on either side of ridge axes. These marine magnetic anomalies, when dated through drilling and radiometric analysis, allowed scientists to calculate precise seafloor spreading rates and demonstrate that continental plates indeed move at measurable velocities, fundamentally reshaping our understanding of Earth's dynamic planetary system.