5. The theory of plate tectonics
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Hess's observations about the extension of the ocean floor, along with the analysis of the global distribution of earthquakes and volcanoes, led to the theory of plate tectonics :
The lithosphere is divided into large pieces called plates. They cover the surface of the Earth and fit together like a jigsaw puzzle.
Most volcanoes and earthquakes, which are caused by the internal geological activity of the Earth, are located at the borders between the plates.
The ocean floor is continually being formed at ocean ridges, by sea-floor spreading, and being destroyed at the trenches, by subduction.
Plate movement moves the continents. Where two plates move apart new oceans are generated; where they come together and collide, mountain ranges rise.
It is possible to locate a point on the Earth's surface with an accuracy of a few centimetres using satellites.
With the new information about the ocean floor, earthquake and volcano distribution, it was clear to Hess that the whole lithosphere (the continental and oceanic) was constantly moving, and not just the continents, as Wegner had thought. Recently, thanks to global positioning system (GPS), the movements of the plates have been verified.
Plate tectonics explained many geological phenomena that appeared not to be related to each other:
The distribution of seismic and volcanic activity.
The present and previous distribution of the continents and oceans.
The formation of mountain ranges.
The formation and destruction of ocean floor.
The distribution of minerals and fossil fuels.
In addition to describing the movement of the continents to their present position, plate tectonics allows us to make predictions about their future movement, as shown in the diagram below:
Predicted location of the continents in 150 million years
5.1. How do the plates move?
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The theory of plate tectonics has been accepted by the scientific community for more than 50 years, even though it is still not completely clear how the movements of the plates are produced. The current theory of plate movement has evolved over time from the original hypothesis.
5.1.1. Original explanation
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It was widely accepted that the lithosphere 'floats' on the asthenosphere. It was thought that the convection currents in the asthenosphere caused tectonic plate movement. It was believed that in the areas where hot currents rise and separate, ridges are formed.In the areas where currents cool and descend, trenches are formed.
Diagram of convection currents according to the original theory
Video 6: Plate tectonics
5.1.2. Current explanation
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Forces that act upon the oceanic lithosphere
New seismic tomography techniques have increased scientists' understanding of the dynamics of the Earth's mantle and how the plates move. The lithospheric plates do not just float on the asthenosphere, but also actively contribute to their own movement in two ways:
On elevated ridges, the force of gravity pulls down on both sides of the plate.
Once a plate is being subducted, the weight of the plate pulls it down lower. The plate sinks down to the edge of the core.
In conclusion, heat rises from the core and makes the mantle ductile. It also generates convection currents in the mantle. These convection currents, combined with the forces of gravity and weight that act on the plate, cause the plates to move.
Earth's internal heat, that keeps the Earth geologically alive, was created from the impact of all the different fragments that formed our planet, and from the disintegration of different radioactive elements found in the core.
5.2. The Wilson cycle
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Wegener had no idea that there was at least one other supercontinent before the formation of Pangaea.
Canadian geologist John Tuzo Wilson (1909-1993) was the first to propose its existence. He suggested that throughout the history of the Earth there have been two cyclic processes of rifting and reuniting of supercontinents, this process is called the Wilson cycle.
The African Rift Valley and the Red Sea
5.2.1. Continental rifting and opening of ocean basins
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A clear example of continental rifting is found in the Rift Valley in East Africa. As you can see on the map on the left, the continental rift is at a divergent plate boundary. When the plates move apart, magma rises up through the fractures as lava, pushing the continental lithosphere on either side further apart. If the process of rifting continues, this continent will end up splitting in two.
This is what has happened in the Red Sea. The Red Sea Rift separates the Arabian Peninsula from Africa. It once was a continental rift, but now it is an oceanic rift, generating oceanic lithosphere.
The Atlantic is an example of an expanding ocean. Its size is increasing steadily due to the production of new oceanic lithosphere (at the Mid-Atlantic Ridge) and the fact that it is no longer surrounded by subduction zones.
5.2.2. Closing of ocean basins
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The oceanic lithosphere becomes progressively older the further it is from the ridge. It also becomes colder and denser. Eventually, this causes the lithosphere to sink down into the mantle, which results in the formation of trenches and subduction zones. From this stage onwards, the ocean, which until this point had been increasing in size, starts to shrink. This is the current situation in the Pacific Ocean, which has many trenches at its boundaries and is getting progressively smaller every year.
5.2.3. Continental collision
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40 million years ago the Indian continent collided with the southern edge of Asia. Marine sediment had accumulated on the edge of both continents when there was an ocean between them. This sediment folded and deformed to create what is now the Himalayas. The marine fossil remains in these mountains are evidence that these rocks once formed part of an ancient ocean floor.
1. A dome forms . The heat under the continent causes the crust to expand and lift up.
2. Continental rift stage . Large fractures appear which make the lithosphere thinner and lead to the formation of a continental rift.
3. Narrow sea stage . The continent has completely split and separated. New oceanic lithosphere and a small ridge begin to form between the two sides.
4. Maturing ocean stage . The divergence continues and the formation of oceanic lithosphere due to sea-floor spreading increases. The Atlantic Ocean is an example of this stage.
5. Shrinking ocean stage . The ocean begins to close because of subduction on its boundaries. The Pacific Ocean is currently at this stage in the cycle.
6. Convergence stage . The ocean has almost closed. The continents converge with marine sediment on their edges.
7. Continental collision stage . The boundaries of both continents and the sediment trapped between them are deformed.
8. Final stage . The continental masses are joined together and a mountain range is formed from the collision of the continents. Eventually the plate boundary will become inactive.
Video 7: Geology: The Wilson cycle
Key concepts
Plate tectonics is a global theory that describes the movement of the lithosphere.
The internal heat of the Earth and forces acting on the plates (gravity and their own weight) cause their movement.
The cyclical opening and closing of oceans can be described by the Wilson cycle.
