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As scientists intensified their studies of the ocean floor in the 1940s and 1950s, they discovered what causes continents to move: sea-floor spreading. The mechanism was first described in 1960 by Princeton professor Harry Hess. Even with the new ocean-floor studies, his data were so scant that he described his paper as "an essay in geopoetry." Yet Hess's basic conclusions are the foundation of modern plate tectonics.

An Essay in Geopoetry

Hess realized that new sea floor is constantly being formed at spreading ridges and destroyed in distant trenches. Huge tectonic plates carry the continents piggyback style, explaining the illusion of continental drift. This 1960 drawing by Hess shows how the upwelling of material from the mantle creates new sea floor. Notice how the new oceanic crust is carried away by two plates moving in opposite directions.

Avalanche of Understanding

In the early 1960s, scientists around the world refined Hess's notion into the modern concept of plate tectonics. They determined the ages of sea-floor rocks, produced images of ocean-floor features, located underwater earthquakes, and — most importantly — identified magnetic stripes on the sea floor, incontrovertible proof that the sea floor is constantly spreading.

Plate tectonics sent scientists to sea. This ship, the Glomar Challenger, was outfitted exclusively for scientific drilling of the sea floor.