ABSTRACT

In the 1950s and 1960s came the first concerted international effort to study the Earth as a whole. It was found that the ocean floors are traversed by mountain ranges thousands of miles long. These oceanic ridges are zones of frequent earthquakes; periodically magma spews forth from fissures in them to spill down their slopes and widen the seabeds on either side. Scientists perceived that the lines of seismic activity along the oceanic ridges—and along the great circum-pacific earthquake belt as well as along certain earthquake belts extending overland—mark, in effect, the boundaries of immense 'plates' on which the continents 'drift'. Today the once-despised theory of continental drift is accepted everywhere as fact.