Lissa graduated from NC State University with a Bachelor's Degree in geology in 2015. Learning and writing about science is her passion.
How Does the Theory of Plate Tectonics Work?
The theory of plate tectonics is a major cornerstone in the field of geology. In this theory, the Earth's crust and upper mantle, together forming a layer called the lithosphere, is divided into several plates. These plates glide over the weaker part of the mantle, called the asthenosphere, over time, and the plates can collide into each other, building large mountain belts like the Himalayas, or one plate is subducted and goes under the other, where it is melted and recycled into new magma.
Plates can also rift apart, creating two or more smaller plates, or they can move past each other. See the diagram below to see the different ways tectonic plates interact with each other. Plate tectonics is a relatively new concept. Our modern idea of it was formulated in the 1960s, but it has its roots in an earlier theory called continental drift.
Alfred Wegener and the Theory of Continental Drift
In the early 20th century, Alfred Wegener, a German geophysicist and professor, came up with the theory of continental drift. Wegener traveled a lot during his career as a scientist and his time in the army weather service during World War I, and recorded many observations about the geological features he saw. In the year 1915, he published The Origins of Continents and Oceans, a book which explained three reasons for his continental drift hypothesis:
- The coastlines of certain continents, like the west coast of Africa and the east coast of South America, match up like pieces of a jigsaw puzzle. When you look at the shapes of the underwater continental shelves, this becomes even more obvious. Wegener found that certain rock units matched on the coastlines of certain continents, and concluded that the continents were once connected in one supercontinent, Pangaea.
- Wegener noticed that there were fossils of land animals that existed on several continents. These animals could not possibly swim across the vast oceans that separate modern continents. Coal beds were also discovered on Antarctica, formed from plants that grew in warm weather swamps. This made Wegener conclude that Antarctica was once farther north than it is now, away from the south pole.
- There is evidence of glacial movement in places that nowadays are too warm to be covered by ice. South Africa is warm and dry, yet glacial deposits dot the landscape, and scour marks gouge the bedrock. Glaciers would not survive the journey through the ocean, so it made more sense for Wegener to include a polar ice cap over the area in his model.
Reception of the Continental Drift Theory
Alfred Wegener's theory of continental drift had mixed reviews. Scientists in the southern hemisphere had seen the similarities in the rocks and fossils on both sides of the Atlantic Ocean, so they believed Wegener was correct. However, northern hemisphere scientists had not seen the evidence themselves, so they were more skeptical about the concept.
A glaring flaw in Wegener's theory was that he could not explain how the continents moved around. In his point of view, the continents plowed through oceanic crust like a fork cuts through a piece of cake. Skeptics pointed out that continental crust was not as dense as oceanic crust, and would not survive that kind of force. And where would that force even come from?
Wegener's hypothesis was rejected by the greater scientific community, and he would have faded into obscurity if not for new data that was discovered in the 1950s...
New Technology Leads to the Theory of Plate Tectonics
After World War 2, technology had advanced considerably, and geologists were now able to explore the topography of the Atlantic ocean floor. In the middle of the Atlantic Ocean, Harry Hess and Robert Dietz discovered a long submarine mountain belt called the Mid-Atlantic Ridge. With data on the magnetism of the ocean floor, the scientists had learned that the oceanic crust around this ridge was actually younger than crust close to the continental margins. The youngest crust at the ridge's center cools and falls when it is created, and is pushed aside as more crust is formed. This concept is called seafloor spreading, and it rekindled interest in Alfred Wegener's work. Eventually, the two concepts merged into the theory of plate tectonics.
What is the Cause of Plate Tectonics?
Plates were discovered to be moved by several forces, one of them being seafloor spreading. Scientists later discovered the effect of slab pull, where the weight of denser plates colliding with lighter plates pulls them underneath the lighter plate, sinking into the mantle and disintegrating.
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The main force that drives all of the spreading and subducting of plates, the ultimate cause of plate tectonics, is convection currents in the mantle. Heat rises through the mantle from the molten outer core, rising up to create mid-ocean ridges and volcanic hotspots, and where the mantle is downwelling, becoming cooler and heavier, you can find subduction zones.
The motion of magma in the mantle causes plates to move, which causes volcanoes to form and earthquakes to occur along plate boundaries. By analyzing the movement of tectonic plates, you get a window into the inner workings of the Earth.
Plate Tectonics Can Explain Volcanic Island Arcs, Large Mountain Belts, and Seamount Chains
In addition to volcanoes and earthquakes, the theory of plate tectonics can also explain the creation of volcanic island arcs, large mountain belts, and seamount chains.
Volcanic island arcs, like the Aleutian Islands of Alaska, form at convergent boundaries where two oceanic plates collide. One plate bends and slides under the other, forming an oceanic trench where sediment and pieces of crust accumulate in an accretionary wedge. As the plate subducts, the temperature and pressure on it increase, and water is released from minerals in the subducting plate. The release of this water causes the asthenosphere to melt, and the magma from this process rises up into the overlying plate, creating an island arc on the surface.
Large mountain belts like the Himalayas are created in collisions of two continental plates. Because both plates have equal densities and thicknesses, neither one can subduct under the other, and the plates buckle and fold, creating immense mountain belts and high-elevation plateaus.
Seamount chains like the Hawaiian islands are created by the movement of a plate over a hot spot. At a hot spot, magma melts and rises into the overlying plate, producing volcanoes. Since the plate is moving over the hot spot, a chain of volcanoes displaying the movement of the plate will be created. Older volcanoes will be further away from the hot spot, and if they are above the surface, erosion and subsiding of the cooled crust can bring them back down below sea level.
Plate Tectonics Can Help to Predict Future Continental Configurations
Like the field of history, in the field of geology scientists can look to the past to notice trends and predict future events. Some interesting predictions have come from the theory of plate tectonics, assuming that current plate motions continue:
- California's landmass west of the San Andreas Fault will continue to slide northwest, eventually bringing Los Angeles to where San Francisco is in 15 million years.
- Africa will eventually collide with Europe in 50 million years, closing the Mediterranean Sea.
- Australia will move north and collide with the islands of Indonesia, forming a larger continent several hundred million years from now.
- Eventually the Pacific Ocean will close together as the Atlantic Ocean widens, forming a new supercontinent known variously as Novopangaea, Amasia, or Pangaea Ultima. This is forecast to happen 250 million years from now.
These predicted events could come to fruition, but who knows? Conditions could change and the world could look totally different from what is predicted. All we can do is hope humans, or whatever evolves from us, are there to see it.
© 2019 Lissa Clason