PLATE TECTONICS

The theory, or paradigm, that the Earth’s lithosphere consists of discrete rigid slabs or plates that move laterally over the weak asthenosphere. At plate margins, plates can move apart, creating new lithosphere through sea-floor spreading[Page 851](divergent or constructive plate margin); move together, consuming old lithosphere through subduction (convergent or destructive plate margin); or slide past each other, conserving the lithosphere (conservative plate margin). Most of the Earth’s tectonic activity, volcanoes and earthquakes are confined to plate margins, and plate interiors are essentially stable. Plate outlines can be defined using the distribution of earthquakes, volcanoes and certain major features of global geomorphology: mid-ocean ridges (MORs) coincide with constructive plate margins, and deep oceanic trenches, volcanic island arcs and active continental margins with destructive plate margins. There are seven major plates— North American, South American, Eurasian, African, Indo-Australian, Pacific and Antarctic—and numerous minor plates (see Figure). Most include areas of both continent and ocean, so the edges of continents (continental margins) do not necessarily correspond to plate margins: passive continental margins occur within plates. Plates move at rates of centimetres to tens of centimetres per year, giving rise to continental drift.

Only lithosphere-bearing, relatively thin and dense oceanic crust is created and destroyed in plate tectonics. Where oceanic crust and continental crust converge, the oceanic crust is subducted and destroyed, creating an active continental margin or continental margin orogen. If two fragments of continental crust are brought together, a continent-continent collision zone develops: sedimentary deposits at the continental margins are subjected to compression, deformation and uplift, forming high fold mountains in an orogenic belt (orogen). This plate-tectonic model for orogenesis—the wilson cycle—has completely replaced geosyncline theory.

The driving force behind plate tectonics is not fully understood. Igneous processes at plate margins contribute to a convective release of geothermal heat, but this is unlikely to drive plate movements. It seems more likely that the sinking of old, dense oceanic lithosphere drags the remainder of the plate behind it. It is now widely accepted that some form of plate tectonics operated in the archaean, but because the Earth’s interior was hotter then, the rates, scales and products are likely to have been significantly different (see greenstone belt, komatiite). During permian and triassic times, the continental masses were assembled as a single supercontinent (pangaea (pangea)), which has fragmented through mesozoic and cenozoic (cainozoic) times. A previous supercontinent configuration, rodinia, existed in the Late proterozoic. It is not known if plate tectonics operates on any other bodies in the solar system.

Although a theory of continental drift was formulated by Alfred Wegener in the early twentieth century, based largely on geological evidence, the development of the plate-tectonics paradigm in the early 1960s came about through the coincidence of several major discoveries. After the Second World War, accurate echo sounders were used to map the ocean floor, leading to the detailed charting of features such as mid-ocean ridges and oceanic trenches. radiometric dating of rock samples from the ocean floor gave surprisingly young ages, nowhere greater than 200 million years, and thus younger than most continental rocks. The theory of sea-floor spreading, developed in the late 1950s, was substantiated by the discovery of oceanfloor magnetic anomalies that were symmetrical either side of the mid-ocean ridges: these were interpreted in terms of ocean-floor forming at mid-ocean ridges and spreading laterally, while the polarity of the Earth’s [Page 852]magnetic field repeatedly switched between normal and reverse states (see geomagnetic polarity reversal). deep-sea drilling showed that the thickness of oceanic sediments increased with distance from the mid-ocean ridge and hence with the age of the oceanic crust. earthquake studies showed an inclined zone of earthquake generation in the Earth’s interior on the same side of oceanic trenches as active volcanoes occur: this was termed a Benioff zone. The locations of earthquake epicentres were found to lie along narrow zones that coincided with the sites of active volcanoes. Studies of seismic waves showed that the sense of displacement on faults that cut the mid-ocean ridges was consistent with crust spreading on each side of the mid-ocean ridge: these were called transform faults. The rapid development and acceptance of plate tectonics led to a genuine revolution in the earth sciences.

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