Unlike most physics or chemistry experiments, which are run under carefully controlled laboratory conditions, the important experiments in Earth science are performed by Earth itself; scientists simply observe and interpret the results. Fortunately, Earth repeats its experiments, but seldom with the same results because the conditions of each experiment differ. The subduction of ocean plates, for instance, occurs at some dozen zones around the world, each displaying enough similarities with the others that researchers in the 1960s were able to form a conceptual model of the process.
The evidence for subduction is so compelling that it isn’t seriously questioned, although considerable effort remains directed toward understanding the causes of the observed variations. Mid-ocean ridges, where the plates diverge and hot mantle rock underneath flows upward to bridge the gap, are similarly understood in a plate-tectonic context. Together, ocean-plate spreading and subduction are the dominant elements of plate tectonics and Earth’s...