고등학교 영어Ⅱ (NE능률 오선영) 연계 글

In 1912, the German meteorologist Alfred Wegener argued that today's continents were joined long ago and had been drifting apart ever since. Others before him had noticed that the coasts of South America and Africa look as though they could fit together. Wegener therefore gathered evidence of other kinds, such as matching mountain ranges and fossils of identical species on opposite shores of the Atlantic Ocean. It was this broad collection of evidence, rather than the matching shape of the coasts alone, that made his proposal a serious scientific theory. In his view, drifting continents also explained puzzling clues about past climates, such as coal deposits from tropical plants in frozen Antarctica.

Most geologists, however, dismissed the theory, partly because they held that the positions of continents and oceans had never changed. It was the lack of a believable force that critics saw as the theory's greatest weakness. Wegener pictured the continents plowing through the rock beneath the oceans, but critics argued that solid rock could not travel in this way without breaking apart. By the time he died on an expedition in Greenland in 1930, he had spent years searching for more evidence, yet most scientists still rejected his idea.

In the 1950s, new instruments let ships map the ocean floor in far greater detail. If it had not been for these instruments, the deep seafloor would have remained largely unknown. The new surveys revealed the mid-ocean ridge, a chain of underwater mountains that nearly circles the planet. They also detected parallel stripes of rock with alternating magnetic properties along each side of the ridge.

In the early 1960s, scientists suggested that new seafloor forms at the ridge as molten rock rises and cools. This rock then moves slowly away on both sides, so the seafloor grows older with distance from the ridge. Because the planet's magnetic field has reversed many times, each strip of rock preserves the field's direction from the time the strip cooled. Had it not been for tiny magnetic grains in this rock, these reversals would have left no trace. This hypothesis, called seafloor spreading, explained why the sequence of stripes on one side mirrors that on the other.

These findings grew into plate tectonics, a theory in which continents and seafloor ride together on moving plates. Scientists accepted the theory even while disagreeing about what force drives the plates. It was mainly seafloor evidence, then, that turned Wegener's rejected idea into accepted science. Without his bold idea, however, that evidence would have lacked a framework.