Years before the advent of plate tectonics—the widely accepted theory, developed in the mid-1960s, the holds that the major features of Earth's surface are created by the horizontal motions of Earth's outer shell, or lithosphere—a similar theory was rejected by the geological community. In 1912, Alfred Wegener proposed, in a widely debated theory that came to be called continental drift, that Earth's continents were mobile. To most geologists today, Wegener's The Origin of Continents and Oceans appears an impressive and prescient document, containing several of the essential presumptions underlying plate tectonics theory: the horizontal mobility of pieces of Earth's crust; the essential difference between oceanic and continental crust; and a causal connection between horizontal displacements and the formation of mountain chains. Yet despite the considerable overlap between Wegener's concepts and the later widely embraced plate tectonics theory, and despite the fact that continental drift theory presented a possible solution to the problem of the origin of mountains at a time when existing explanations were seriously in doubt, in its day Wegener's theory was rejected by the vast majority of geologists.

Most geologists and many historians today believe that Wegener's theory was rejected because of its lack of an adequate mechanical basis. Stephen Jay Gould, for example, argues that continental drift theory was rejected because it did not explain how continents could move through an apparently oceanic floor. However, as Anthony Hallam has pointed out, many scientific phenomena, such as the ice ages, have been accepted before they could be fully explained. The most likely cause for the rejection of continental drift—a cause that has been largely ignored because we consider Wegener's theory to have been validated by the theory of plate tectonics—is the nature of the evidence that was put forward to support it. Most of Wegener's evidence consisted of homologies—similarities of patterns and forms based on direct observations of rocks in the field, supported by the use of hammers, hand lenses, and field notebooks. In contrast, the data supporting plate tectonics were impressively geophysical—instrumental| determinations of the physical properties of Earth garnered through the use of seismographs, magnetometers, and computers.


The author cites Hallam (see highlighted text) on the ice ages primarily in order to


provide an example of a geologic phenomenon whose precise causes are not understood by geologists today

criticize the geological community for an apparent lack of consistency in its responses to new theories

offer evidence held to undermine a common view of why Wegener's theory was not accepted in its day

give an example of a modern scientist who believes that Wegener's theory was rejected because it failed to adequately explain the mechanical basis of continental drift

support Gould's rationale for why Wegener's theory was rejected by most geologists in the early twentieth century

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