The movement of the crustal plates of the Earth's surface is called plate tectonics. The Earth's surface is fractured into eight major plates and several smaller ones, which move relative to one another. The plates appear to be propelled by flowing magma underneath the surface. The ocean floors are formed of heavier material, while the continents, formed of lighter rocks, sit on the surfaces of the heavier rocks. The lighter continental rock accumulated during Earth's early history, especially during the Archaean Eon about 3 billion years ago. Plate tectonics is now understood as the driving force of continental drift. The theory of continental drift, now confirmed by the mechanism of plate tectonics, has had a revolutionary impact on explaining the evolutionary patterns of life on Earth.

Oceans can form or widen because new ocean floor crust forms along the fracture (a mid-ocean ridge) where crustal plates separate. Underwater volcanic eruptions exude new plate material along the mid-ocean ridge. As a result, the youngest crust material is right next to the mid-ocean ridge, and the oldest material is furthest from the ridge. Geologists can determine the actual ages of volcanic rocks by radioactivity and the relative ages of rocks by determining the orientation of the magnetic fields of the rocks. Both the ages, and the magnetic field patterns, of the rocks of the Atlantic Ocean floor form bands of increasing age away from the Mid-Atlantic Ridge. When geologists Drummond Matthews and Fred Vine from Cambridge University presented this evidence in 1963, their paper was denied publication at first, but when the evidence was finally accepted, it became perhaps the single most important factor in convincing geologists that the theory of plate tectonics was correct.

The Eurasian Plate and the North American Plate have been separating for more than 120 million years, at about the same rate that fingernails grow. Eurasia and North America, parts of which remained connected until about 50 million years ago, are now separated by the North Atlantic Ocean. During this same time, South America and Africa have separated. The Mid-Atlantic Ridge runs along the ocean floor like a gigantic backbone. In Iceland, the Mid-Atlantic Ridge emerges. . .

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