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the Mid-Atlantic Ridge (Original)

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Author Topic: the Mid-Atlantic Ridge (Original)  (Read 9772 times)
Carolyn Silver
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« Reply #60 on: July 28, 2008, 10:59:39 pm »

Plate tectonics -- a failed revolution

Plates in motion?

According to the classical model of plate tectonics, lithospheric plates move over a relatively plastic layer of partly molten rock known as the asthenosphere (or low-velocity zone). The lithosphere, which comprises the earth's crust and uppermost mantle, is said to average about 70 km thick beneath oceans and to be 100 to 250 km thick beneath continents. A powerful challenge to this model is posed by seismic tomography, which produces three-dimensional images of the earth's interior. It shows that the oldest parts of the continents have deep roots extending to depths of 400 to 600 km, and that the asthenosphere is essentially absent beneath them. Seismic research shows that even under the oceans there is no continuous asthenosphere, only disconnected asthenospheric lenses.
    The crust and uppermost mantle have a highly complex, irregular structure; they are divided by faults into a mosaic of separate, jostling blocks of different shapes and sizes, and of varying internal structure and strength. N.I. Pavlenkova concludes: 'This means that the movement of lithospheric plates over long distances, as single rigid bodies, is hardly possible. Moreover, if we take into account the absence of the asthenosphere as a single continuous zone, then this movement seems utterly impossible' [1]. Although the concept of thin lithospheric plates moving thousands of kilometers over a global asthenosphere is untenable, most geological textbooks continue to propagate this simplistic model, and fail to give the slightest indication that it faces any problems.


Figure 1. Seismotomographic cross-section showing velocity structure across the North American craton and North Atlantic Ocean. High-velocity (colder) lithosphere, shown in dark tones, underlies the Canadian shield to depths of 250 to 500 km. (Reprinted with permission from Grand [2]. Copyright by the American Geophysical Union.)

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