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7.4: Plate Tectonics

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  • Accurate world maps were not completed until the middle of the 16th century. Back then, people were mapping coastlines from the decks of ships, and physical features such as mountains and rivers by traversing them on foot.  Soon after the world was well mapped, people began to recognize that certain coastlines seemed to ‘fit’ together (most notably the east coast of South American and the west coast of Africa). As data on the rock types and ages around the world were compiled, the relationships between the continents became more clear. 

    In 1912, a German named Alfred Wegener proposed the theory of continental drift. Wegener provided evidence for how all the earth’s present-day landmasses were united in a single supercontinent he dubbed ‘Pangaea’ about 225 million years ago. Wegener’s ideas were built upon and modified as later scientific discoveries added to his theory.  More was learned about the composition of the continental and ocean crust, and as the bottoms of the oceans were thoroughly mapped, scientists began to recognize features in the ocean floor which pointed to a more complex theory of the movement of the continents.  Ocean bottom imaging revealed deep trenches around the rim of some continents, and linear ridges running down the middle of some ocean floors.  These observations led to the theory of plate tectonics.

    Figure 7.4.1 Plate Tectonics

    The theory of plate tectonics provides us with a comprehensive model of the earth’s inner workings. According to the model, the earth’s rigid outer shell, the lithosphere, is broken into several individual pieces called plates.  From Wegener’s model we understood that these rigid plates are slowly and continually moving. The plate tectonics theory provides a mechanism for this motion: the circulation of molten rock in the earth’s mantle.  The circulation of material in the mantle is not unlike the circulation in the atmosphere.  Hot material from deep in the mantle rises because it is more buoyant, this drives powerful thermal cycles which push the plates laterally. The thermal circulations in the earth's mantle provided the critical mechanism that brought together the theory of Plate Tectonics (see schematic below)

    Ultimately, this movement of the earth’s lithosphere plates generates earthquakes, volcanic activity and the deformation of large masses of rock into mountains. Because each plate moves as a distinct unit, interactions between plates occur at their boundaries.

    You may want to search for images that illustrate the fourteen plates that currently cover the earth’s surface. Image used with permission (Public Domain; USGS).

    There are three distinct types of plate boundaries:

    • Divergent boundaries are zones where plates move apart, leaving a gap between them. One example of divergent boundaries between ocean plates is the mid-ocean ridge in the middle of the Atlantic. Another example of a divergent boundary between two continental plates is the East African Rift zone, part of which has formed the Persian Gulf. 
    • Convergent boundaries are zones where plates move together. When an ocean plate meets a continental plate, the heavier ocean place is pushed beneath the continental plate.  When two continental plates collide, they smash into each other, forming a large mountain belt (like the Himalayas).
    • Transform boundaries form where plates slide past each other, scraping and deforming as they pass.

    Each plate is bounded by a combination of these zones. Look for more graphic examples of plate motion.

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