tect - 1 Continental Drift and Plate Tectonics Continental...

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1 Continental Drift and Plate Tectonics Continental Drift In 1912, Alfred Wegener suggested that all continents were once joined together as one supercontinent that he called Pangea (Fig. 1). According to Wegener, Pangea began to break up into smaller continents that drifted apart through the superocean Panthalassa about 200 million years ago. Other scientists (e.g. Suess and Alexander du Toit) who had different versions of the continental drift hypothesis, introduced the names Gondwanaland and Laurasia for supercontinents. Nowadays, Pangea is considered to have first broken up into two supercontinents: Gondwanaland, which later broke up to Africa, India, Australia, Madagascar, Antarctica and South America; and Laurasia, which included North America, Europe and Asia. The ocean between these two supercontinents is called Tethys (Fig. 2). Wegener's evidence for continental drift included: 1- The good geometrical fit of the continents, particularly when the continental shelves are considered. 2- Geological evidence: similar rock types and structures were found on different continents that seem to fit (Fig. 3). These rocks (or structures) now on different continents were found to have formed at the same time. 3- The occurrence of identical fossils on widely separated land masses or continents. 4- Paleoclimatic evidence: Rocks that once formed by the movement of ice sheets (glacial deposits) occur in present - day warm regions as Africa and Australia, whereas rocks of the same age in present - day cold regions as e.g. Pennsylvania, formed under tropical conditions. This observation led Wegener to conclude that Africa and Australia were at one time located near the South Pole, whereas parts of North America were near the equator at the same time, and have since drifted northwards to their present-day positions. Plate Tectonics The concept of plate tectonics may be considered a revival of the continental drift theory with one major modification: the earth's "outer layer" consists of several rigid lithospheric plates consisting of continental as well as oceanic crust and the uppermost part of the mantle (in contrast to Wegener's theory). These plates slide on top of a weaker plastic layer (the asthenosphere; Fig. 4) moving towards, away from or past each other. Accordingly, there are three types of plate boundaries: (i) Divergent (constructive) boundaries where oceanic crust is being created at mid oceanic ridges (Fig. 5a), a process known as " sea floor spreading ". (ii) Convergent (destructive) boundaries, where crustal material is dragged down through trenches along subduction zones into the mantle (i.e. where crustal material is being destroyed; " subduction "; Fig. 5b). Convergent plate boundaries are of three types: (a) ocean - ocean marked by the formation of island arcs (Fig. 6a) (b) ocean - continent, marked by the generation of volcanic arcs on the continents along the plate margin (Fig. 6b) (c) continent - continent collision zones, marked by the development of mountain ranges (Fig. 6c).
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This note was uploaded on 02/27/2012 for the course GEOLOGY 110 taught by Professor Staff during the Fall '08 term at Marshall.

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tect - 1 Continental Drift and Plate Tectonics Continental...

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