Continental Drift and Plate Tectonics
The slow discovery of plate tectonics was one of the greatest breakthroughs of modern science, and it changed our understanding of how this planet functions. But the concept of plate tectonics and the theory of continental drift that preceded it were quite radical. It took almost a century to develop and confirm the theory, and it was not widely accepted even among scientists until the 1960s. This is a brief story of how the theory evolved and why there was so much resistance. (additional text under video)
The Earth’s crust is divided into seven large and a number of smaller chunks called “plates.”
At the boundaries where these plates meet, there are different types of activity, together known as plate tectonics. The mechanisms at the plate boundaries are responsible for some of the most important functions of our planet.
- movement of the continents over time and distribution of oceans
- emergence of mountain ranges like the Andes and the Himalayas
- volcanoes and earthquakes
- circulation of minerals from Earth’s core to the surface
- and important features of our atmosphere.
Geologists today know that plate tectonics make Earth an extraordinary place –a planet that can support life. No other planet in our solar system has plate tectonics.
DEVELOPMENT OF THE THEORY
Millions of years ago the present-day continents were one huge land mass. Over time that massive formation broke apart and the individual chunks drifted apart and regrouped in different ways several times. But an understanding of how the continents became separate entities took centuries.
If you look at continents like South America and Africa, for example, you will notice that they seem to fit together. Map-makers and geologists observed that out long ago. But scientists also discovered ancient fossils that were very similar on widely separated continents. And they noticed that rock formations seemed to match up across different continents, suggesting that continents were once joined. But how could huge land masses possibly move? There was no easy answer.
For a long time, there were two major theories. One was that the positions of the continents formed early in Earth’s history and they never moved. The other major theory was that geological cycles caused periodic contractions of the earth, which in turn caused dry land to sink in some places while the seafloor rose in others. The first theory –that the continents had never moved— was largely dismissed over time as geological studies improved. But the second theory – contraction cycles—lasted for decades.
In the early 1900s an Austrian geologist, Edward Suess, suggested that the long cooling period of early Earth caused the planet to contract over time. These contractions wrinkled the planet -rather like a dried up apple-- and reduced the surface. This periodic wrinkling led to a cycle in which huge areas of the land might collapse into the early oceans, while submerged areas of Earth’s crust rose to become land masses. This rising-sinking pattern would create new ocean basins and continents and the pressure of the contractions would form huge mountain ranges. Suess also proposed that the continents in the southern hemisphere had once formed a huge landmass which he called Gondwana. The submerge-and-surface cycle gradually broke up Gondwana, resulting in the modern continents of South America, Southern Africa, India, Antarctica and Australia. To explain the similarity of fossils across these continents, Suess proposed that for some time as during the breakup of Gondwana the land masses were probably connected by land bridges.
There were some variations on the Contraction Theory, but the important point is that this rise-and-fall concept developed long before scientists learned that active tectonic plates on Earth’s crust actually moved continents.
In 1910, an American geologist, Frank Bursley Taylor, proposed that the obvious “fit” between the South American and Africa continents indicated an original landmass that somehow drifted apart over eons. From his study of mountain ranges on these continents, he concluded that such huge masses must have been caused by the collision of continents. The movement of the continents, according to Taylor, must be caused by the gravitational pull of the moon during the Cretaceous period, 145 million years ago. Taylor could not convince the American scientists.
At about the same time, a German meteorologist, Alfred Wegener, offered an alternative to the contraction theory. There were, he noted, two important problems. One was that the heating-cooling cycle fundamental to the contraction theory was not possible because radioactive decay deep in the Earth produced continuous heat. Another problem was that the geological formations underlying the huge mountain ranges showed compression patterns that Suess’s theory could not explain.
Wegener developed several key ideas, together known as the theory of Continental Drift. He suggested that 200-300 million years ago, in the Paleozoic and Mesozoic eras, there was a single supercontinent, which he called Pangea (which means “all Earth”). Paleoclimate changes during this long period led to the breakup of Pangea into Gondwana and Laurasia supercontinents. Wegener explained continental drift theory in his famous 1915 book The Origin of Continents and Oceans. In this work he presented supporting evidence from fossils, geological formations and paleoclimate indicators.
But the Continental Drift theory worked out independently by Taylor and Wegener, was ignored or rejected. Very little was known at the time about the earth’s crust, especially since most of it was under the sea. So it was difficult for scientists in the early 1900s to understand how massive continents could glide along the surface of the planet. Continental drift was controversial –or rejected—for decades.
But over time, further evidence supporting continental drift accumulated. In 1937, Alex du Toit, a South African geologist, made a detailed investigation of geological matchups in the continental shelves of southern Africa and South America. He explained his discoveries and the very clear links between the continents that originated in Gonwanda In his 1937 book Our Wandering Continents. Du Toit’s evidence was conclusive.
In the 1920s, British geologist Arthur Holmes moved the plate tectonics theory forward with two important contributions. He introduced radiometric dating of minerals, ensuring accuracy of the analysis of geological formations and the types of minerals associated with geological deposits. He also elaborated on the continental drift theory by proposing that plate junctions lay under the seabed, and that “convection cells” in the Earth’s mantle generated intense heat energy that could move the crust. This hypothesis was a major advance toward a viable explanation of continental movement.
However, despite the accumulating evidence for continental drift and the force of plate tectonics that moved the continents, many geologists had difficulty accepting how the huge continents could move. Part of the problem was poor communication between European and American scientists in a period of international political tensions. But intellectual resistance to a radical theory of continental shift was the enduring problem -and it was independent of political context.
Steady technological innovations during the World War II period advanced the investigations of the ocean floor. And those detailed investigations revealed rifts in the seabed that were unambiguous indications that the Earth’s crust did move. The problem for several more decades was understanding the source of such massive movements. In the 1960s further investigations provided geological details of the rift patterns. The major studies were done by Harry Hess, Fred Vine and Drummond Matthews. According to their hypothesis, the Earth’s crust is divided into plates that shift over time. The movement causes the seafloor to spread where the plates meet. They found that the rifts were associated with periodic reversals in Earth’s magnetic field, and those reversals were recorded in the seafloor crust. Geologists can calculate when and where plate movements occurred over millions of years.
In the 1960s, these and other scientific advances confirmed at last the theory of “plate tectonics,” –the source of the energy and mechanics of plate movement on the Earth’s crust. Continental drift over millions of years was caused by plate tectonics. And plate tectonics also explained how the movement of the plates create volcanoes and earthquakes, and how the collision between continents gave rise to huge mountain ranges.
TYPES OF PLATE BOUNDARIES
Ultimately, geologists established three basic types of plate boundaries with their respective effects. They are called Convergent, Divergent, and Transform boundaries.
•In Divergent boundaries two plates move away from each other. This causes earthquakes along the boundaries, and magma (molten rock) from deep in the Earth’s mantle rises to the surface, dragging minerals and gases up to be incorporated in new crust.
•In Convergent boundaries the plates move toward each other, and the collision causes subduction zones, where one plate slides under another, dragging material down into Earth’s mantle. This causes earthquakes, volcanoes and the rise mountains. Subduction is important for circulation of minerals and gases.
•In Transform boundaries or “shearing” the plates slide past each other. Typically, this splits apart and grinds up rock formations. Undersea canyons are examples of this process.
Despite the solid evidence of plate tectonics, some very prominent scientists continued to reject the notion that continents could drift. One of the most famous deniers was Harold Jeffreys. Long past the time when plate tectonics became part of standard geological theory, Jeffreys argued that there was “no force strong enough” to move continents across the crust of the planet.
Today, geologists understand also the link between plate tectonics and the movement of land masses, and also zones of earthquakes and volcanoes. And because of plate tectonics, atmospheric gases and minerals that are essential for the development and sustaining life on this planet are recycled.
It took a long time, but today plate tectonics is a central component of geological science.