 "Photo: Katrina Perry")
The theory of plate tectonics illustrates how rock plates from five to 25 miles thick move at fractions of inches per year over Earth. Plates sink into the earth, and new crust is created by molten rock solidifying at ocean ridges. This motion brings continents together and breaks them apart. Now, scientists using the Global Positioning System are able to directly measure the movements of the plates. The theory, however, took decades to be accepted.
Alfred Wegner first published the theory of continental drift in a 1912 paper. He matched coastlines together like a jigsaw puzzle and saw that they fit remarkably well.
"[Continental drift] started from asking 'How can we explain the distribution of things? How can we explain the origin of mountains and ocean basins?'" said Mark Cloos, a UT geology professor.
Many thought the theory was ridiculous.
"In North America, it died out as even a concept. If you go look at the textbooks from the 1930s, 1940s and 1950s, to even have a paragraph on the subject of continental drift was being generous," Cloos said. Early on, prominent physicist Harold Jeffreys discounted the theory. By studying seismic waves, he established the earth's crust was rigid. He extrapolated the rigidity of the earth to geological timescales, and he believed there was no way for the continents to move through such a hard shell.
"The theory said it was impossible, and the people who made the observations simply said, 'Wegner's seeing things.' The big breakthrough that happened in the 1950s is that we started studying the oceans, using a large array of technologies that had been put together in World War II," Cloos said.
After WWII, much of the instrumentation used to look for submarines was available to study the ocean bottom. New observations forced scientists to re-evaluate their ideas.
"We started to discover that the oceans, which everyone had pretty much assumed were like the continents except under water, are completely different," Cloos said. "The hills are all volcanoes, the oldest rocks are cretaceous and 150 million years old, and the biggest mountain belt on Earth runs right down the middle of the Atlantic - the ocean ridge system."
A new picture of the ocean floor was developing in the 1950s. The next major clue came from the magnetism of rocks. When a rock is formed the magnetic pieces inside it are frozen in place, aligned in the direction of the earth's magnetic field. If a rock formation spanning hundreds of miles is then broken apart as a continent separates, the original orientation of the magnetic field when the rock was formed will be preserved. The magnetic fields frozen in rocks from different time periods show that the continents have not always been sitting still, facing the same direction.
"By the late '50s it became apparent that every continent has a different apparent position of the pole, and if the pole is the only thing that moves around they should all be showing the same migration over time," Cloos said.
The paleomagnetic poles gave the first independent evidence for the changing position of the continents. Another clue came from studying the magnetic field at the bottom of the ocean. By dragging a magnetometer, a device that measures a magnetic field, along the bottom of the ocean floor the magnetic field of the rocks on the ocean floor can be measured. The result was a startling series of magnetic stripes.
"It all started to come together in a famous paper by Harry Hess that came out in 1962 and became known as seafloor spreading. The ocean ridges are the sites where there is upwelling and new ocean crust is being created like a conveyor belt," Cloos said. "In addition, we discovered that the magnetic pole flips. So the conveyor belt is making new crust, and there are pole flips on the 100 thousand- to million-year timescale, changing the orientation of the magnetism in the rock on the seafloor. Those are the magnetic stripes."
According to the theory of plate tectonics, which encompasses the majority of tectonic activity, earthquakes and volcanoes should occur on fault lines where one plate meets another. Geologists knew that some places were more prone to earthquakes than others, but in the 1960s, technological developments greatly improved scientists' ability to locate earthquakes.
"It became apparent by the mid-'60s that earthquakes, for the most part, occur in very narrow belts," Cloos said.
One discovery that led to the acceptance of plate tectonics was geologic samples from the ocean floor, provided by Project Mohole. Arthur Maxwell, former director of the UT Institute for Geophysics and professor emeritus of geological sciences, was the chief scientist on Project Mohole.
"The story that Art Maxwell told me is that almost everyone on the ship didn't believe in seafloor spreading, but they went and they drilled this set of holes across the south Atlantic, and lo and behold, every single one came in right at the time predicted by seafloor spreading," Cloos said.
In other words, the sediment's age agreed with what was predicted by seafloor spreading, providing direct evidence that the floor was indeed spreading.
Between 1960 and 1968, the picture of the surface of the earth had drastically changed. Jeffreys went to his grave, however, holding the belief that plate tectonics was a myth.
"It became apparent that the story of what is going on is frozen in the oceans and that the continents are relatively passive," Cloos said. "That was a revolutionary perspective."
While the major picture of the plate tectonics was understood in 1969, the resulting phenomena are far from being understood. What Cloos calls second-order details, such as the location of the next big earthquake or which volcano will erupt next, are the current areas of research.
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