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Contents Introduction An Accumulation of Anomalies Radioactivity and Isostasy Continental Drift A Revolution Deferred Root Causes of Rejection

In 1968, Bryan Isacks, Jack Oliver, and Lynn R. Sykes coined the term global tectonics to describe the new theory that was remaking the science of geology.¹ The previous two decades had been a fruitful time for geologists. American scientists and the United States Navy had collaborated to study the ocean floor in detail for the first time, allowing geologists to apply the techniques, instruments, and theories they had developed on land to the unexplored reaches of the seafloor. Based on the data collected on these expeditions, geologists made a surprising finding: The continents, big as they were, were moving. What's more, geologists believed they could explain how.

The layer of the Earth we live on is broken into a dozen or so rigid slabs (called tectonic plates by geologists) that are moving relative to one another.

The slow displacement of continents across the surface of the Earth is one of the core principles of plate tectonics, the elegant theory that shapes how geologists understand the Earth's surface. Plate tectonic theory posits that the Earth's crust is composed of a few dozen rigid, laterally extensive plates floating on a partially molten layer at the top of the mantle. A given plate may consist of continental crust, oceanic crust, or both. Continental crust is thick (20-100 km), granitic, and buoyant; oceanic crust is thin (5-10 km), basaltic, dense, and low-lying. Plates move relative to one another in response to convection currents in the underlying mantle. Where convection currents bring hot material to the surface, plates are driven apart and eruptions of basalt form new ocean floor. Where the mantle cools and sinks, old ocean crust sinks with it, producing ocean trenches, earthquakes, and volcanic arcs. Where two continents come together, the crust compresses and thickens, forming mountain belts. Each plate moves at a rate of only a few centimeters per year.

The main types of plate boundaries. (Cross section by José F. Vigil from This Dynamic Planet, a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.)

The development of plate tectonics during the 1960s is the signature achievement of the science of geology, remarkable for both the small period of time over which the much of the supporting evidence was accumulated and for the explanatory power of the resulting theory. Yet there is a curious prelude: The idea of mobile continents was first proposed as part of a wide-reaching theory of the surface geology of the Earth more than forty years prior to the plate tectonics revolution. In 1912, the German meteorologist Alfred Wegener introduced the theory of continental drift, which imagined the continents splitting, colliding, and plowing through a plastic substrate. While less effective than plate tectonics in explaining the full range of geological phenomena, continental drift was nevertheless far more powerful than contemporary theories of the Earth. Yet the majority of Wegener's peers never accepted that the continents moved, and his theory was never widely adopted. Why was this? How was it that plate tectonics succeeded so quickly where continental drift had failed for so long? How can the disparate fates of the two theories be reconciled?

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