Message #94297

[Carolyn Silver]

The driving force of plate movements was initially claimed to be mantle-deep convection currents welling up beneath midocean ridges, with downwelling occurring beneath ocean trenches. Plate tectonicists expected seismotomography to provide clear evidence of a well-organized convection-cell pattern, but it has actually provided strong evidence against the existence of large, plate-propelling convection cells in the mantle. The favored plate-driving mechanisms at present are 'ridge-push' and 'slab-pull', but their adequacy is very much in doubt.
    Thirteen major plates are currently recognized, ranging in size from about 400 by 2500 km to 10,000 by 10,000 km, together with a proliferating number of microplates (over 100 so far). Plate boundaries are identified and defined mainly on the basis of earthquake and volcanic activity. The close correspondence between plate edges and belts of earthquakes and volcanoes is therefore to be expected and can hardly be regarded as one of the 'successes' of plate tectonics! A major problem is that several 'plate boundaries' are purely theoretical and appear to be nonexistent, including the northwest Pacific boundary of the Pacific, North American, and Eurasian plates, the southern boundary of the Philippine plate, part of the southern boundary of the Pacific plate, and most of the northern and southern boundaries of the South American plate.

Continental drift

Geological field mapping provides evidence for horizontal crustal movements of up to several hundred kilometers. Plate tectonics, however, claims that continents have moved up to 7000 km or more since the alleged breakup of Pangaea. Satellite measurements of crustal movements have been hailed by some geologists as having proved plate tectonics. Such measurements provide a guide to crustal strains, but do not provide evidence for plate motions of the kind predicted by plate tectonics unless the relative motions predicted among all plates are observed. However, many of the results have shown no definite pattern, and have been confusing and contradictory, giving rise to a variety of ad-hoc hypotheses. For instance, distances from the Central South American Andes to Japan or Hawaii are more or less constant, whereas plate tectonics predicts significant separation. The practise of extrapolating present crustal movements tens or hundreds of millions of years into the past or future is clearly a hazardous exercise.
    A 'compelling' piece of evidence that all the continents were once united in one large landmass is said to be the fact that they can be fitted together like pieces of a jigsaw puzzle. However, although many reconstructions have been attempted, none are entirely acceptable. In the Bullard et al. computer-generated fit, for example, there are a number of glaring omissions. The whole of Central America and much of southern Mexico -- a region of some 2,100,000 km² -- has been left out because it overlaps South America. The entire West Indian archipelago has also been omitted. In fact, much of the Caribbean is underlain by ancient continental crust, and the total area involved, 300,000 km², overlaps Africa. The Cape Verde Islands-Senegal basin, too, is underlain by ancient continental crust, creating an additional overlap of 800,000 km². Several major submarine structures that appear to be of continental origin are also ignored, including the Faeroe-Iceland-Greenland Ridge, Jan Mayen Ridge, Walvis Ridge, Rio Grande Rise, and the Falkland Plateau.

Figure 2. The Bullard fit. Overlaps and gaps between continents are shown in black. (Reprinted with permission from Bullard et al. [3]. Copyright by The Royal Society.)