The 1960s revealed a new and revolutionary idea in geological thought: that the continents drift with respect to one another. After having been dismissed for decades as absurd, the concept gradually became part of geology's basic principles. We now know that the Earth's crust and upper mantle consist of a small number of rigid plates that move, and there are significant boundaries between pairs of plates, usually known as earthquake belts.
Plate tectonics now explains much of the structure and phenomena we see today: how oceans form, widen, and disappear; why earthquakes and volcanoes are...