Plate Tectonics Now and in the Past

Abstract

The concepts known as plate tectonics that began to develop in the 1960s built on a foundation of information that included: • The earth’s mantle is rigid enough to transmit seismic P and S waves, but it is mobile to long-term stresses. • The earth’s temperature gradient is so high that convective overturn must occur in the mantle. • The top of the mobile part of the mantle is a zone of relatively low velocity at depths of about 100 to 200 km. This zone separates an underlying asthenosphere from a rigid lithosphere, which includes rigid upper mantle and crust. • Seismic activity, commonly accompanied by volcanism, occurs along narrow, relatively linear, zones in oceans and along some continental margins. • The zones of instability surround large areas of comparative stability. • Ocean lithosphere is continually generated along mid-ocean ridges and destroyed where it descends under the margins of continents and island arcs. This causes oceans to become larger, but shrinkage of oceans can occur where lithosphere is destroyed around ocean margins faster than it is formed within the basin. • Some of the belts of instability are faults with lateral offsets of hundreds of kilometers. • Some continental margins are unstable (Pacific type), but others are attached to oceanic lithosphere without any apparent tectonic contact (Atlantic type). • Different areas containing continents and attached oceanic lithosphere move around the earth independently of each other. Most of this chapter consists of a summary of plate tectonics in the present earth, including processes along plate margins and the types of rocks formed there (readers who want more detailed information are referred to Rogers, 1993a; Kearey, 1996; and Condie, 1999). We also briefly discuss plumes and then finish with a word of caution about interpreting the history of the ancient and hotter earth with the principles of modern plate tectonics. Starting from the body of continually expanding information summarized above, numerous earth scientists in the 1960s and 1970s began to establish a conceptual framework that would organize scientific thinking about the earth’s tectonic processes. This required a new terminology, and it arrived rapidly (Oreskes, 2002). Geologists decided to call the stable areas “plates” and the unstable zones around them “plate margins.” Thus, the concept became known as “plate tectonics.” Plates are essentially broad regions of lithosphere, although the failure to detect low-velocity zones under many continents leaves unresolved questions.