A Questioning Look at Plate Tectonics Theory

J. Marvin Herndon (2005) Teaching Earth Dynamics: What's Wrong with Plate Tectonics Theory?

Early in the 20^th Century, Alfred Wegener (1880-1930) proposed that the continents at one time had been united, but subsequently had separated and drifted through the ocean floor to their present positions. Wegener’s theory of continental drift was systematically ignored for fifty years. Whoa! Ignored? What’s wrong here? Science is about controversy, about debate. It is about new ideas. Ignoring an important new contradiction to prevailing ideas is really bad science. In fact, it is anti-science. Any important, promising new scientific idea should be discussed and debated. If possible, experiments or theoretical considerations should be made to test the idea. If the new idea is found to be wrong, it should be refuted in the scientific literature, ideally in the journal where it was originally published; otherwise, it should be acknowledged. New ideas often beget other new ideas and science advances. Early on, students should learn about this process and should be exposed to the excitement and debate that is a natural part of good science.

Finally, after half a century of silence, debate erupted in the 1960s. Alfred Wegener’s idea of continental drift re-emerged, cast into a new form called plate tectonics theory, with more detail and with new supporting observational data. Plate tectonics is based upon the idea that the crust of the Earth is composed of a set of movable plates that are in motion. The theory also embraces the idea of seafloor spreading.

In plate tectonics theory, basalt rock, observed erupting from the mid-oceanic ridges, is thought to creep slowly across the ocean basin and to subduct, to plunge into the Earth, typically into submarine trenches. Indeed, there is convincing evidence for such ocean floor movement. When basalt rock cools, its magnetic minerals act like magnetic tape recorders, locking in a record of the prevailing direction of the Earth’s magnetic field. The observed pattern of magnetic stripes across the ocean floor reflects reversals of the geomagnetic field, recorded as new seafloor cooled and moved away from the mid-oceanic ridge. This might appear to be convincing evidence of plate tectonics theory. But does that mean plate tectonics theory is totally correct? Not necessarily. Look deeper and look questioningly.

Imagine new seafloor being produced at the mid-oceanic ridges, moving away slowly, and ultimately plunging into the Earth (subducting). If the Earth is not getting any larger, then what is happening to all that seafloor that is plunging downward? According to plate tectonics theory, the Earth’s mantle is convecting, turning over and over in an endless loop. But is it? Is it really? Is there any direct evidence that the mantle is convecting? The idea of convection comes from what is observed when liquids are heated from beneath. The heated liquid expands a little, becoming slightly less dense than the cooler liquid, and thus tends to rise, to float to the surface. Cool liquid rushes in to fill the space that had been occupied by the heated liquid. In a large volume, a geometric pattern of (typically hexagonal-shaped) convection cells often develops, such as in the laboratory-produced example at the right.