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  • The theory of plate tectonics > Tectonic plates
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          5. The theory of plate tectonics

          Talking book

          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.
                
          image
          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:

           

          image
          Predicted location of the continents in 150 million years

          5.1.How do the plates move?

          Talking book

          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.

                 
          image
          Convection currents

          5.1.1. Original explanation

          Talking book

          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.

           

          image
          Diagram of convection currents according to the original theory
          Video 6: Plate tectonics

          5.1.2. Current explanation

          Talking book
                
          image
          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

          Talking book

          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.

               
          image
          The African Rift Valley and the Red Sea

          5.2.1. Continental rifting and opening of ocean basins

          Talking book

          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

          Talking book

          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

          Talking book

          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.

          image
          1. A dome forms. The heat under the continent causes the crust to expand and lift up.
          image
          2. Continental rift stage. Large fractures appear which make the lithosphere thinner and lead to the formation of a continental rift.
          image
          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.
          image
          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.
          image
          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.
          image
          6. Convergence stage. The ocean has almost closed. The continents converge with marine sediment on their edges.
          image
          7. Continental collision stage. The boundaries of both continents and the sediment trapped between them are deformed.
          image
          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.
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          The theory of plate tectonics
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          Understand

          How has GPS helped to confirm the movement of the continents?

          It is possible to locate a point on the Earth's surface with an accuracy of a few centimetres using satellites.

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          Activity 48

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          Analyse

          Look at the map and compare it to a present-day map. Where do you think new oceans, mountain ranges and continental rifts will form?

          Predicted location of the continents in 150 million years

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          How will the relationship between the African and Eurasian plates change?

          Predicted location of the continents in 150 million years
          • Both plates will travel north, however, the African Plate will travel faster and collide with the Eurasian Plate, closing the Mediterranean Sea in the process and generating a new large mountain range.

          • Correct answer
            Wrong answer
          • The African Plate will move north and the Eurasia Plate south, eventually colliding against each other and closing the Mediterranean Sea.

          • Correct answer
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          • Both plates will move south, eventually colliding and creating a new mountain range.

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          What is predicted will happen to the Australian continent? What will form in its place?

          Predicted location of the continents in 150 million years
          • It will move east and collide with South America, creating a new large mountain range.

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          • I will move west without colliding with Africa, but it will close the Pacific Ocean.

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          • It will collide with South East Asia, creating a large mountain range. The archipelagos found between them now will be trapped in the middle, making the mountain range even bigger.

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          Look at the diagram and explain how convection currents are generated, taking into account the differences in temperature and density that take place.

          Convection currents
          When heat is applied to the water in the pot, the water at the bottom of the pot becomes  dense as the particles move and distances (which in turn makes them float to the top). As the water at the bottom of the pot become dense than the water at the top, the water at the top . Then the process starts again and continues until no more heat is applied. The movement of water going up and down are convection currents. 

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          What two forces are acting on the plate to cause it to subduct?

          Forces that act upon the oceanic lithosphere
          • The weight of the plate that subducts.

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          • The force of the two plates colliding.

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          • Gravity

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          What force do you think is more important? Explain your answer.

          Forces that act upon the oceanic lithosphere

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          Why is the Moon, despite being so close to Earth, a geologically dead object, with almost no internal activity?

          • It has very little internal activity (internal heat), although it is not considered to be a geologically dead object.

          • Correct answer
            Wrong answer
          • Because it cooled down much faster due to its smaller size and as a result it no longer has 'geological fuel'.

          • Correct answer
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          • It has no internal acitivity. However, geological processes do occur on its surface and therefore it is not considered to be a geologically dead object.

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          What type of plate movement is there at the Rift Valley and the Read Sea Rift above?

           

          The African Rift Valley and the Red Sea
          • The Rift Valley is separating but the Red Sea Rift is not moving.

          • Correct answer
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          • Both areas are colliding against each other.

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          • Both areas have divergent plate boundaries (the continents are moving away from each other).

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          Where on the Earth do we find steps 2 and 3 of the Wilson cycle?

          • The Rift Valley is at the continent rift stage (step 2), whereas the Red Sea is at the narrow sea stage (step 3).

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          • The Aleutian Islands are at the continental rift stage (step2), whereas the Rocky Mountains are at the narrow sea stage (step 3).

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          • The Kuril Islands are at the continental rift stage (step 2), whereas the Andes are at the narrow sea stage (step 3).

          • Correct answer
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          Explain how the African Rift Vally and Red Sea will look in the future.

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          In your own words, explain how the movement of the tectonic plates causes the opening of new oceans and the closing of others.

           

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          Marine fossils have been found in the Pyreness, the Alps and the Himalayas. How did they get there?

          This is because when two continents , the marine sediment found in their , so the marine sediment forms part of the new mountain range formed during . Therefore, any marine fossils originally found in the marine sediment can then be found on the mountain range. 

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