Twenty years ago geologists were certain that the data correlated perfectly with the then-reigning model of stationary continents. The handful of geologists who promoted the notion of continental drift were accused of indulging in pseudoscientific fancy. Today, the opinion is reversed. The theory of moving continents is now the ruling paradigm and those who question it are often referred to as stubborn or ignorant. This "revolution" in our concept of the earth's character is a striking commentary on the human nature of scientists and on the flexibility that scientists allow in use of the geological data.
Plate Tectonics
The popular theory of drifting continents and oceans is called "plate tectonics."1Ā (TectonicsĀ is the field of geology which studies the processes which deform the earthās crust.) The general tenets of the popular theory may be stated as follows. The outer lithospheric shell of the earth consists of a mosaic of rigid plates, each in motion relative to adjacent plates. Deformation occurs at the margins of plates by three basic types of motion: horizontal extension, horizontal slipping, and horizontal compression. Sea-floor spreading occurs where two plates are diverging horizontally (e.g., the Mid-Atlantic Ridge and East Pacific Rise) with new material from the earth's mantle being added between them to form a new oceanic crust. Transform faulting occurs where one plate is slipping horizontally past another (e.g., the San Andreas fault of California and the Anatolian fault of northern Turkey). Subduction occurs where two plates are converging with one plate underthrusting the other producing what is supposed to be compressional deformation (e.g., the Peru-Chile Trench and associated Andes Mountains of South America). In conformity with evolutionary-uniformitarian assumption, popular plate tectonic theory supposes that plates move very slowly ā about 2 to 18 centimeters per year. At this rate it would take 100 million years to form an ocean basin or mountain range.
Fitting of Continents
The idea that the continents can be fitted together like a jigsaw puzzle to form a single super continent is an old one. Especially interesting is how the eastern "bulge" of South America can fit into the southwestern "concavity" of Africa. Recent investigators have used computers to fit the continents. The "Bullard fit"2Ā gives one of the best reconstructions of how Africa, South America, Europe, and North America may have once touched. There are, however, areas of overlap of continents and one large area which must be omitted from consideration (Central America). There are a number of ways to fit Africa, India, Australia, and Antarctica (only one can be correct!). Reconstructions have been shown to be geometrically feasible which are preposterous to continental drift (e.g., rotation of eastern Australia fits nicely into eastern North America).3
Those who appreciate the overall fit of continents call the evidence "compelling," while others who note gaps, overlaps, or emissions remain skeptical. It is difficult to place probability on the accuracy of reconstructions and one's final judgment is largely subjective.
Sea-Floor Spreading
Evidence suggesting sea-floor spreading is claimed by many geologists to be the most compelling argument for plate tectonics. In the ocean basins along mid-ocean ridges or rises (and in some shallow seas) plates are thought to be diverging slowly and continuously at a rate of several centimeters yearly. Molten material from the earth's mantle is injected continuously between the plates and cools to form new crust. The youngest crust is claimed to be at the crest of the ocean rise or ridge with older crust farther from the crest. At the time of cooling, the rock acquires magnetism from the earth's magnetic field. Since the magnetic field of earth is supposed by many geologists to have reversed numerous times, during some epochs cooling oceanic crust should be reversely magnetized. If sea-floor spreading is continuous, the ocean floor should possess a magnetic "tape recording" of reversals. A "zebra stripe" pattern of linear magnetic anomalies parallel to the ocean ridge crest has been noted in some areas and potassium-argon dating has been alleged to show older rocks farther from the ridge crest.
There are some major problems with this classic and "most persuasive" evidence of sea-floor spreading. First the magnetic bands may not form by reversals of the earth's magnetic field. Asymmetry of magnetic stripes, not symmetry, is the normal occurrence.4Ā It has been argued that the linear patterns can be caused by several complex interacting factors (differences in magnetic susceptibility, magnetic reversals, oriented tectonic stresses).5
