Earth Materials. John O'Brien. Читать онлайн. Newlib. NEWLIB.NET

Автор: John O'Brien
Издательство: John Wiley & Sons Limited
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Жанр произведения: География
Год издания: 0
isbn: 9781119512219
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areas of 105–108 km2 and thicknesses that average ~103 km; some plates are smaller and microplates are smaller still. The fact that they overlie a weaker, slowly flowing asthenosphere permits them to move very slowly. Each plate is separated from adjacent plates by plate boundary segments that end in triple junctions (McKenzie and Morgan 1969) where three plates are in contact (Figure 1.3).

      Each type of plate boundary produces a characteristic suite of features and Earth materials. This relationship between the kinds of Earth materials formed and the plate tectonic settings in which they are produced provides a major theme of the chapters that follow.

      1.5.2 Divergent plate boundaries

      Divergent plate boundaries occur where two plates are moving apart relative to their boundary (Figure 1.4a). Such areas are characterized by horizontal extension and vertical thinning of the lithosphere. Horizontal extension in continental lithosphere is marked by continental rift systems and in oceanic lithosphere by the oceanic ridge system.

       Continental rifts

Schematic illustration of principal types of plate boundaries: (a) divergent; (b) convergent; (c), transform; thick lines represent plate boundaries and black arrows indicate relative motion between plates; (d) hybrid convergent-transform boundary; red arrows show components of convergent and transform relative motion. Schematic illustration of major features of continental rifts include rift valleys, thinned continental crust (blue) and lithosphere (brown) with volcanic-magmatic activity from melts generated in rising asthenosphere (burnt orange). Schematic illustration of model showing the growth of ocean basins by sea floor spreading from the ridge system following the complete rifting of continental lithosphere along a divergent plate boundary.

       Oceanic ridge system

Schematic illustration of map of the ocean floor showing the distribution of the oceanic ridge system.

      Source: World Ocean Floor Manuscript Map; drawn by Berann, H.C., US Library of Congress, public domain after Heezan, Bruce C. and Tharpe, Marie.