In 1967, Jason Morgan discovered the theory of plate tectonics — the idea that rigid plates pave the Earth’s surface, moving relative to one another with the continents and oceans in tow. That January, sitting in his office at Princeton University, Morgan read a new article in Science by the geologist H. William Menard, who had mapped long cracks called “fracture zones” on the Pacific Ocean floor. “I instantly saw the pattern that all the fracture zones had a common pole that they were concentric about,” Morgan, 81, told Quanta in an email. “They had all been formed with a rotation about the same pole.”

Having learned spherical trigonometry back in high school in Georgia, Morgan was able to calculate that the Pacific seafloor’s hairline fractures all curved around a point just north of Siberia. This particular location has no significance today, he said, “but the idea of ‘poles’ to describe the opening [or rotation] of ocean was now set in my mind.”

Earth’s surface motion — the source of virtually all its features — had long puzzled scientists. The coastlines of the continents seemed to match up as if they were dispersed jigsaw pieces, an observation that instigated the fuzzy theory of “continental drift” in the early 1900s. But scientists had no idea how continents actually drifted, beyond some vague notion that they got dragged through the oceans by the fluid rock of Earth’s mantle below. How did the continents maintain their rigidity as they drifted? And what went on below the oceans? For Morgan, the “pole of opening” associated with the fracture zones served as a key.

Standing at a drafting table in his office, he calculated that simple rotations of rigid plates on a sphere could describe the pattern of features in the Pacific, indicating that “ocean floors also moved as rigid objects — that they were equally strong as continents,” he said. The fracture zones of the Pacific were just relics of long-ago processes, but Morgan set out to test his hypothesis in the Atlantic, where the seafloor is presently spreading from the mid-ocean ridge to the east and west. “Did it have a pattern describable by a rotation? For the rest of January and in February and March, I focused on the Atlantic, and by April had prepared a talk on rigid plates for the annual American Geophysical Union meeting in Washington, D.C.”