By Robert Hazen, Ph.D., George Mason University
The plate tectonics revolution resulted primarily from the merging of many separate lines of evidence. Any one of these pieces of evidence, taken alone, wouldn’t have been enough to convince the geological community; but it’s the eventual accumulation and weight of overwhelming evidence that swings the tide, and that’s what happened with plate tectonics.
Shapes of Continents
By far the oldest, and first, observational evidence for moving continents is clearly the shape of Africa and Europe, as they fit into North America and South America.
Early in the century, as more and more ocean soundings were taken along the coasts, just off the coasts of continents, it was realized that the continents actually extend out, sometimes several hundred kilometers from the actual coastline. There is the continental shelf, and then you have a sudden drop-off into the deep ocean.
If you map the continental shelves, the fit — particularly between South America and Africa — is strikingly good. Maybe the fit of the continents is pointing to something like continental drift. .
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Geological Formations and Global Control
Then there are very striking match-ups of geological formations—rock types, mineral deposits—across the coasts, and that became the second kind of evidence, which is very, very important in the acceptance of continental drift.
Most geologists and paleontologists believe in the rocks they collect in the field; that’s their first evidence, and these were specimens you could take and hold in your hand and see: they almost match up, like pieces of a puzzle.
The third piece of evidence was the non-uniform distribution of earthquakes and volcanoes around the globe. People had long recognized that some areas are highly prone to earthquakes, while other regions seem particularly stable, where you never see an earthquake.
The same thing is true with volcanoes. While in some places, you see volcanoes all the time, there are other places where you’ll never see a volcano. But why is that so? There must be some global control, people were starting to think, but they weren’t sure what that global control was.
Learn more about the signs of constant change.
Relationship Between Volcanoes and Earthquakes
Another factor that began to come to light was the curious relationship between volcanoes and earthquakes. In many regions, including Japan, the Philippines, the western coast of South America, you find strong earthquakes at the coast, and they’re shallow.
As you go inland, the earthquakes get deeper and deeper, until the earthquakes can be 100 or 200 kilometers deep. Also, as you go inland 100 or 200 kilometers, you start seeing a series of volcanoes—not right at the coast, but rather inland, like Mount St. Helens and Mount Rainier.
These are mountains that are not right on the Oregon and the Washington coasts; the Cascade volcanoes are inland by 100 kilometers or more, associated also with occasional, deep earthquakes. What’s this relationship? What’s happening? That was a clue, but it didn’t yet fit in to the puzzle of plate tectonics.
Discovery of the Mid-Atlantic Ridge
An absolutely key piece of the puzzle came shortly after World War II; that’s when the Navy declassified sonar technology, which had been used to track enemy submarines. When they declassified it, it became a vital technology for understanding the ocean floor.
You can use sonar to bounce sound waves off the ocean bottom and measure the depth—a very simple idea, but critically important. The conventional wisdom, in the 1950s, was that the ocean bottom was an absolutely flat, featureless plain, called the abyssal plain. There were no features in the bottom of the ocean.
Well, the first sonar traverses of the Atlantic Ocean revealed an astonishing chain of mountains, called the Mid-Atlantic Ridge. This chain— the longest mountain range on Earth, tens of thousands of miles long—runs the full length of the North and the South Atlantic, and continues on, past the tip of South America and Africa.
Learn more about uniformitarianism.
The discovery of the Mid-Atlantic Ridge, and its subsequent widespread recognition, was due in large measure to one person; that’s the American geologist Bruce Heezen, who lived from 1924 to 1977. He was trained as a paleontologist, but he spent most of his career at Columbia documenting this ocean-floor topography and preparing visually dramatic maps.
Preparing visually dramatic material convinced people, much more than any dry scientific article. By preparing a map that showed these mountain ranges in stark topography, he was able to immediately show the importance and the impact of this range of mountains.
Heezen correctly recognized the Mid-Atlantic Ridge as an extensional feature—that is, a place where the crust is actually moving apart—but he incorrectly ascribed this extension to the fact that he thought the Earth was expanding; as the Earth expands, the continents split apart, and get farther and farther apart, because the Earth itself is getting larger.
Common Questions about Evidence of the Plate Tectonic Revolution
By far the oldest, and first, observational evidence for moving continents is the shape of Africa and Europe, as they fit into North America and South America.
The Mid-Atlantic Ridge is the longest mountain range on Earth, tens of thousands of miles long, that runs the full length of the North and the South Atlantic, and continues on, past the tip of South America and Africa. It was discovered when the first sonar traversed the Atlantic Ocean.
Bruce Heezen was an American geologist who lived between 1924 and 1977. The discovery of the Mid-Atlantic Ridge, and its subsequent widespread recognition, was due in large measure to his designing a visually dramatic map of the ridge to show its importance and impact.