Geoscience Reference
In-Depth Information
the plates outward from a midoceanic ridge: “ridge-push.” But the more geolo-
gists learned about plate movement and the Earth's mantle, the less satisfactory
ridge-push appeared. How could warm, ductile rock push more solid rock out of
the way? And why do the forces at the ridge crest seem to be caused by tension,
not compression?
These doubts led scientists to come up with an alternative theory. In this view,
the dominant force in plate tectonics is drag and downward suction in the zone
where the dense oceanic lithosphere sinks into the mantle: “slab-pull.” (The ocean-
ic lithosphere comprises the outer fifty to hundred kilometers of the crust and
mantle beneath the ocean basins.) One convincing piece of evidence for slab-pull
is that plates attached to subduction zones, like the Pacific Plate, which is nearly
surrounded by the zones, move faster than plates that are not attached to sub-
duction zones, like the North American and giant Eurasian plates. But then what
moves those two plates? Some believe that the slight downward slope attributable
to the contraction and shrinking of the cooling lithosphere as it moves away from
the ridge causes spreading whether or not there is an adjacent subduction zone. In
that case, the ultimate driver is gravity. Still others, hearkening back to the days
of Daly, Taylor, and Wegener, speculate on the part played by tidal drag caused by
the Moon and Sun, on the “flight from the poles,” on the role of the Coriolis effect,
and so on. The “lack of a mechanism” persists, yet still the plates and the contin-
ents move. One day, scientists will know why.
