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See Inside September 2009

Tectonic Plates

The long, strange trip of continental drift

Alfred Wegener’s idea of continental drift wandered in the wilderness for the first few decades after he wrote about it in his 1915 book, The Origin of Continents and Oceans. Although some geologists marshaled further evidence for the theory, most remained skeptical because no plausible mechanism seemed capable of sending huge landmasses plowing through the ocean crust on long journeys across the surface of the earth.

The modern concept of moving tectonic plates emerged in 1962, proposed by Harry H. Hess of Princeton University. Hess had captained a U.S. Navy transport ship during World War II and used the vessel’s sonar to map the Pacific Ocean floor along his travels. He hypothesized that all the earth’s crust—oceanic as well as continental—was mobile, driven by convective motions in the underlying layer known as the mantle. New crust forms at mid-ocean ridges, where hot magma from the mantle wells up and crystallizes. The young crust spreads from the ridges, and old crust sinks back down at deep ocean trenches. In this way, the crust and the uppermost, solid portion of the mantle (together known as the lithosphere) are divided into moving plates.

Hess’s ideas became accepted after studies found the magnetism of rock on the ocean floor matched predictions: the earth’s magnetic field, which sporadically reverses polarity, leaves its imprint in solidifying rock, producing bands of alternating magnetism parallel to ocean ridges.

Continental drift thus has its roots in the immense heat coming from the planet’s interior. Radioactive decay still produces the heat today. Yet scientists estimate that three billion years ago twice as much heat was emerging, leading to numerous hotspots with magma welling up, fragmenting the early lithosphere into many small tectonic plates. The first continents may have been not much larger than Iceland and a lot like it in other ways, too: for 16 million years or so Iceland (below) has been forming above a hotspot on the Mid-Atlantic Ridge.

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