Drifting Continents

Evidence for Wegener’s Hypothesis

To support his hypothesis of continental drift, Wegener showed that fossils of the same plant and animal species occurred in similar rock layers on continents separated by thousands of kilometers of ocean. For example, the distribution of Glossopteris fern fossils, shown as a green band in Figure 15.1a, spans South America, Africa, Antarctica, India, and Australia in the same stratigraphic position (Figure 15.1b). Glacial features were found in similar positions and orientations on the continents (Figure 15.1c). Mountain ranges with the exact same geology were separated by vast oceans. Wegener hypothesized that the distribution of these phenomena could only have happened if the continents had split from one super-continent in Earth's past.

His hypothesis of continental drift suggested that the continents plowed their way across the oceanic crust like massive rafts. Wegener's hypothesis was dismissed by many because it lacked a mechanism that drove continental drift. In the late 1930s, geologist Arthur Holmes suggested the decay of radioactive elements in the mantle drove convection currents that moved Earth's crust. Although Holmes’s idea eventually became part of the modern explanation of plate motions, at the time, most geologists continued to reject Wegener’s theory.

Development of Modern Tectonics Theory: Seafloor Spreading

At Wegener's time, most scientists thought the ocean floor was part of the original crust that formed after Earth cooled. This long-held belief was challenged by oceanographic research undertaken in the early 1960s and the discovery of the midocean ridge changed scientists’ attitudes toward Wegener’s idea. The ridge rises over 2000 m (6,500 ft.) from the ocean floor, and extends through all the world's oceans, and mirrors continental coastlines. Dredging the ocean floor brought up young rocks from freshly cooled lava near the ridge. Scientists hypothesized that the midocean ridge is a spreading ridge” where molten rock oozed out of fractures and cooled. Older crust was pushed out of the way as new ocean crust was added in a process called seafloor spreading. By crisscrossing the ridge with magnetometers, geoscientists found that changes in Earth's magnetic polarity were frozen into the rocks formed from the lava extruded onto the ocean floor on either side of the midocean ridge. Parallel stripes of magnetic reversals on either side of the midocean ridge validated the notion that the seafloor is spreading (Video 15.3) and provided a mechanism for Wegener's drifting continents.