6 – Continental Drift, Sea-Floor Spreading, and Plate/Plume Tectonics

This chapter describes the theory of continental drift, sea-floor spreading hypothesis and its verification, and plate/plume tectonics. The theory of continental drift was proposed by Alfred Wegener in 1912. In addition to fitting the pieces of the puzzle, Wegener used a variety of arguments to demonstrate a past linkage of continents that are now separated. Wegener also provided insight into orogenesis, or the mountain building process, which is one of the most important problems in solid earth science. Sea-floor spreading hypothesis explains why the thickness of the ocean sediment was one order of magnitude thinner than the estimate obtained by multiplying the age of the sea by the sedimentation rate. It also explains why rock older than Cretaceous had never been discovered on the sea floor. The most decisive support of sea-floor spreading was provided by the geomagnetic anomalies of the sea floor. There are basically three types of plate boundary—namely, divergent, convergent, and transform fault types. The transform faults displacing mid-oceanic ridges are under water, but the San Andreas Fault in North America, Alpine Fault in New Zealand, and North Anatolia Fault in Turkey are the most prominent transform plate boundaries on land often characterized by strike-slip seismicity. The representatives of divergent plate boundaries are spreading mid-oceanic ridges.