Plate tectonics: principal elements

Abstract

The modern concepts of plate tectonics were formulated rapidly into a working hypothesis in the early to middle 1960s. Elements of the modern paradigm had been proposed since the early part of this century, but the ideas were too revolutionary at that time and did not take hold for want of a unifying principle. Despite the supporting evidence for continental drift, it was not widely accepted as a viable tectonic model because it seemed mechanically impossible. Isostasy required a sialic continent to have a root, in contrast to the simatic ocean crust. Seismic-refraction observations confirmed the evidence from gravity that continental crust is commonly 35 to 50 km thick, whereas oceanic crust is generally only about 6 to 8 km thick. How could continents with such a deep keel plow through the rigid oceanic crust especially because granite is fundamentally weaker than basalt. Many Earth scientists were left in a quandary of apparently conflicting evidence. As recently as the middle 1960s, most universities dealt pedagogically with continental drift in the way that universities today deal with the expanding-Earth model. A tectonic mechanism, a dynamic model, was needed before a significant part of the geologic community would accept the geometric reconstructions and the various kinematic scenarios promoted by the advocates of continental drift.