The Plate Tectonics Revolution

As late as the 1960s, US geology textbooks explained mountains using the principle of isostatic equilibrium according to which mountains are like icebergs floating on land. As icebergs have a smaller section above the surface and a deeper part beneath the surface, so with mountains.

Changes in mountains were a function of vertical, local forces.

The platetectonics revolution began with Alfred Wegner’s theory of continental drift. He noticed the similar shape of the South American and African coasts and gathered data showing that on opposite sides of the Atlantic similar species of animal and similar mineral deposits were found. He postulated that the continents had drifted apart.

But his theories were considered speculative and rejected by most scientists because he lacked any explanation for the force that could account for such drift.

The next line of evidence bolstering Wegner’s theory was the discovery of the mid-Atlantic ridge as the result of sonar searches for German submarines during World War II. Prior to the War it was assumed that the ocean floor was a flat featureless plain, “the abyssal plain.”

Far from it. The mid-Atlantic ridge is the largest single feature on the planet earth.

The next set of data leading toward the platetectonic revolution was the study of the ages of the islands in the Atlantic. It turned out there was a linear relationship between the age of the island and the distance from the ridge in a symmetrical relationship on either side of the ridge.

Finally came the discovery of a pattern of regular reversal of the magnetic field frozen in the rock on either side of the mid-Atlantic ridge.