Figure 4. Major seismotectonic belts/'plate boundaries' (broken lines) compared with an icosahedron. (Reprinted with permission from Spilhaus [8]. Copyright by the American Geophysical Union.)
Seafloor spreading and subduction
According to the seafloor-spreading hypothesis, new oceanic crust is generated at midocean ridges by the upwelling of molten material from the earth's mantle, and as the magma cools it spreads away from the flanks of the ridges. The horizontally moving plates are said to plunge back into the mantle at ocean trenches or 'subduction zones'.
The ocean floor is far from having the uniform characteristics that conveyor-type spreading would imply. The mantle is asymmetrical in relation to the midocean ridges and has a complicated mosaic structure independent of the strike of the ridge. N.C. Smoot and A.A. Meyerhoff showed that nearly all published charts of the world's ocean floors have been drawn deliberately to reflect the predictions of the plate-tectonics hypothesis, and the most accurate charts now available are widely ignored because they do not conform to plate-tectonic preconceptions [9]. Side-scanning radar images show that the midocean ridges are cut by thousands of long, linear, ridge-parallel fissures, fractures, and faults. This strongly suggests that the ridges are underlain at shallow depth by interconnected magma channels, in which semi-fluid lava moves horizontally and parallel with the ridges rather than at right-angles to them.
The oldest known rocks from the continents are just under 4 billion years old, whereas -- according to plate tectonics -- none of the ocean crust is older than 200 million years (Jurassic). This is cited as conclusive evidence that oceanic crust is constantly being created at midocean ridges and consumed in subduction zones. There is in fact abundant evidence against the alleged youth of the ocean floor, though geological textbooks tend to pass over it in silence.
Scientists involved in the Deep Sea Drilling Project were apparently motivated by a strong desire to confirm seafloor spreading. They have given the impression that the basalt (layer 2) found beneath the sedimentary sequences (layer 1) at the bottom of many deep-sea drillholes is basement, with no further, older sediments below it. Yet in some cases there is clear evidence that the basalt is a later intrusion into existing sediments. The ocean floor needs to be drilled to much greater depths -- up to 5 km -- to see whether there are Triassic, Paleozoic, or Precambrian sediments below the so-called basement.
Plate tectonics predicts that the age of the oceanic crust should increase systematically with distance from the midocean ridge crests. However, the dates exhibit a very large scatter. On one seamount just west of the crest of the East Pacific Rise, the radiometric dates range from 2.4 to 96 million years. Although a general trend is discernible from younger sediments at ridge crests to older sediments away from them, this is in fact to be expected, since the crest is the highest and most active part of the ridge; older sediments are likely to be buried beneath younger volcanic rocks. The basalt layer in the ocean crust suggests that magma flooding was once ocean-wide, but volcanism was subsequently restricted to an increasingly narrow zone centered on the ridge crests. Such magma floods were accompanied by progressive crustal subsidence in large sectors of the present oceans, beginning in the Jurassic.
