A Perspective
Plate tectonics, which has so profoundly influenced geologic thinking since the early 1970s, provides valuable insight into the mechanisms by which the Earth's crust and mantle have evolved. Plate tectonics is a unifying model that attempts to explain the origin of patterns of deformation in the crust, earthquake distribution, continental drift, and mid-ocean ridges, as well as providing a
mechanism for the Earth to cool. Two major premises of plate tectonics are:- the outermost layer of the Earth, known as the lithosphere, behaves as a strong, rigid substance resting on a weaker region in the mantle known as the asthenosphere
- the lithosphere is broken into numerous segments or plates that are in motion with respect to one another and are continually changing in shape and size (Figure 1.1/Plate 1). The parental theory of plate tectonics, seafloor spreading, states that new lithosphere is formed at ocean ridges and moves away from ridge axes with a motion like that of a conveyor belt as new lithosphere fills in the resulting crack or rift. The mosaic of plates, which range from 50 to over 200 km thick, are bounded by ocean ridges, subduction zones (in part coUisional boundaries), and transform faults (boundaries along which plates slide past each other) (Figure 1.1/Plate 1, cross-sections). To accommodate the newly-created lithosphere, oceanic plates return to the mantle at subduction zones such that the surface area of the Earth remains constant. Harry Hess is credited with proposing the theory of seafloor spreading in a now classic paper finally published in 1962, although the name was earlier suggested by Robert Dietz in 1961. The basic idea of plate tectonics was proposed by Jason Morgan in 1968. Many scientists consider the widespread acceptance of the plate tectonic model as a 'revolution' in the Earth Sciences. As pointed out by J. Tuzo Wilson in 1968, scientific disciplines tend to evolve from a stage primarily of data gathering, characterized by transient hypotheses, to a stage where a new unifying theory or theories are proposed that explain a great deal of the accumulated data. Physics and chemistry underwent such revolutions around the beginning of the twentieth century, whereas the Earth Sciences entered such a revolution in the late 1960s. As with scientific revolutions in other fields, new ideas and interpretations do not invalidate earlier observations. On the contrary, the theories of seafloor spreading and plate tectonics offer for the first time unified explanations for what, before, had seemed unrelated observations in the fields of geology, paleontology, geochemistry, and geophysics.