Direct and Indirect Evidence

At the California Institute of Technology in the mid-1940s, a young Henry William Menard—later an expert on submarine physiography and director of the U.S. Geological Survey—learned about continental drift from Beno Gutenberg. For although most American earth scientists considered the question of drift settled, many Europeans did not. Among them was “Dr. G,” famous for his pioneering work on microseisms (the continual seismic disturbances that form the background “noise” of seismographs) and deep-focus earthquakes, who had come to Caltech from Germany in 1930. In 1939, he edited Internal Constitution of the Earth, part of a series entitled Physics of the Earth sponsored by the National Research Council. Gutenberg’s chapter, “Hypotheses on the Development of the Earth’s Crust and their Implications,” focused on the evidence for a plastic crustal substrate and “currents” within it. More than just an idea, he argued, subcrustal currents were necessary—in the past and at present — to account for both isostasy and horizontal crustal dislocations: “Many writers have expressed the belief that the strength of the interior of the earth prevents any currents today. The results of geophysical research, however, leave no doubt that such currents still exist. . . . [either] as a consequence of changes produced by disturbances at the surface [or as ] the primary cause of movement at the surface.” Gutenberg’s course at Caltech reflected these views. The strength of the crust was “enough to support [the] highest mountains,” he explained in class, but isostasy demonstrated that this strength “decreases downwards, and below 40 km or so plastic flow may occur.” This flow was implicated in both geological and seismological processes. Among the forces causing earthquakes, for example, Gutenberg suggested “elastic rebound as a release of shear due to sub-crustal flow and contraction of the crust & possibly differential movements in the crust from continental drift.” He noted that the energy release associated with earthquakes was “of the same order of magnitude as that due to temperature gradient,” which suggested that the most likely cause of plas tic flow was internal temperature differentials. One preexamination review sheet asked students for the meaning of isostasy and of “Wegener’s hypothesis.”