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  • Studying the ocean floor > Tectonic plates
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          3. Studying the ocean floor

          At the beginning of the 1960s, it was said that we knew more about the landscape of the Moon than the ocean floor. But during the Cold War, nuclear submarines needed accurate maps of the ocean floor. This led to the development of ocean floor maps.

          Talking book
                 
          image
          Ship using sonar
          • Sonar

          Detailed maps of the ocean floor are created using sonar. Sonar sends out sound waves and measures the time it takes for the echo to return from an object. From this measurement, the distance of objects can be calculated in order to map the surface of the ocean floor. This map shows the main ocean floor landforms.

          image

          3.1.Oceanic landfroms

          Talking book

          The use of sonar, deep sea drilling and the marine magnetometer has completely changed our vision of the ocean floor and our understanding of the dynamics of the Earth. Maps of the ocean floor show that there are many different landforms, from flat plains (abyssal plains) to large volcanic mountains. Notable landforms are the Mid-Ocean Ridge and ocean trenches.

          • Mid-Ocean Ridge. All the ocean ridges connect to form an enormous mountain chain of over 60000km long and 2000km wide. Along its length there is a central fault known as a rift, crossed by numerous perpendicular fractures called transform faults.
          • Trenches. These are narrow, deep channels usually found next to continental boundaries or volcanic island arcs, especially in the Pacific. The deepest, the Challenger Deep, part of the Mariana Trench, reaches 11km.
          • Other landforms
          • The abyssal plain is 4 km deep.
          • Island arcs are archipelagos associated with trenches. They are some of the most active volcanic areas on the planet. For example, Japan, Aleutian Islands, Mariana Islands and the Philippines.
          • Seamounts and guyots are ancient underwater volcanic landforms. Seamounts have pointed tops and guyots have a flat top.
          • Volcanic archipelagos, such as the Hawaiian Islands, are chains of volcanic islands. They are not associated with trenches, like island arcs, but with hot spots.

          Maps also show big differences between different oceans, especially between the Atlantic and the Pacific.

          The Atlantic Ocean does not have many trenches, while the Pacific Ocean is almost completely surrounded by a large network of trenches. Some of these trenches are located along the continent; and some are located next to island arcs and inland seas.

          image
          image

           

           

          Video 2: The ocean floor revealed

          3.2.The composition of the ocean floor

          Talking book

          Before our exploration of the oceans, some scientists believed that the oceans were huge bowls that filled up with sediment. However, areas were found with no sediment or with a very thin layer.

          During the 1960s, several oceanographic expeditions, such as the DSDP (Deep Sea Drilling Project), took samples from the ocean floor that revealed new information:

                
          image
          Pillow lava
          • The ocean floor is made up of volcanic rock covered in marine sediment. The volcanic rock has pillow-shaped formations, called pillow lavas, and igneous rocks. They were found just under a thin layer of marine sediment.
          • These rocks are very young; there are no ocean floor rocks older than 180 million years, which is very little compared to the Earth's age (4.5 billion years). When they looked at where the different aged rocks were, they found that the rocks closest to an ocean ridge were younger than those further away from the ridge.

          The diagram below shows the differences between the oceanic and continental crust.

           

          image
          The difference between the oceanic and continental crust

           

          The diagram also shows that the continental crust is not only the areas of the continents above water; it also occupies the underwater shallow areas that surround them, such as the continental shelf.

          Video 3: Scientific deep sea drilling and coring technology
          Key concepts
          • Until the 1960s, the geography of the ocean floor was virtually unknown.
          • Notable ocean landforms are ocean ridges and trenches.
          • The oceanic crust is younger, thinner and denser than the continental crust.
          1

          Activity 25
          Apply

          2

          Activity 26
          Understand

          3

          Activity 27
          Analyse

          4

          Activity 28
          Create

          5

          Activity 29
          Analyse

          6

          Activity 30
          Analyse

          7

          Activity 31
          Create

          8

          Activity 32
          Create

           
          Studying the ocean floor
          Tectonic plates
          Without background sound
          Logo

          Activity 25

          P

          Apply

          Use the diagram to explain how sonar works.

          Ship using sonar

            1 attempt
          Done
          Studying the ocean floor
          Tectonic plates
          Without background sound
          Logo

          Activity 26

          0/15
          Understand

           

          Use an atlas or Internet application such as Google Maps or Google Earth to find the names of the Island arcs (1-8) and the ocean ridges (A-G).

          South Georgia and the South Sandwich Islands

          Kuril Islands

          Southwest Indian Ridge

          East Pacific Rise

          Aleutian Islands

          Philippines

          Lesser Anntilles (Caribbees)

          Southeast Indian Ridge

          Mid-Atlantic Ridge

          Pacific Antarctic Ridge

          Juan de Fuca Ridge

          Tonga

          Central Indian Ridge

          Japan

          Mariana Islands

          Island arcs Ocean ridges
          1. A.
          2. B.
          3. C.
          4. D.
          5. E.
          6. F.
          7. G.
          8.  

           

            South Georgia and the South Sandwich Islands Kuril Islands Southwest Indian Ridge East Pacific Rise Aleutian Islands Philippines Lesser Anntilles (Caribbees) Southeast Indian Ridge Mid-Atlantic Ridge Pacific Antarctic Ridge Juan de Fuca Ridge Tonga Central Indian Ridge Japan Mariana Islands

            3 attempts
          Done
          Studying the ocean floor
          Tectonic plates
          Without background sound
          Logo

          Activity 27

          0/5

          Analyse

          What differences can you see between these two oceans?

          • There is a trench next to the continent and the continent has volcanoes.

          • There are no trenches nor volcanoes near the coast of the continent.

          • There is a volcanic mountain range in the continent.

          • The continental slope connects the continental shelf with the abyssal plain.

          • There is a guyot and a volcanic island.

          Ocean on the left

            Ocean on the left

            Ocean on the right

              Ocean on the right

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                3 attempts
              Done
              Studying the ocean floor
              Tectonic plates
              Without background sound
              Logo

              Activity 28

              P

              Create

              Guyots are extinct and submerged volcanoes, with flat tops. Make a hypothesis to show the formation of guyots. What effect do you think their formation had on isostasy?

                1 attempt
              Done
              Studying the ocean floor
              Tectonic plates
              Without background sound
              Logo

              Activity 29

              0/1

              Analyse

              Compare the thickness, age and density of the continental and oceanic crust.

              • Oceanic crust is thinner, younger and less dense than the continental crust.

              • Correct answer
                Wrong answer
              • Oceanic crust is thinner, older and denser than the continental crust.

              • Correct answer
                Wrong answer
              • Oceanic crust is thinner, younger and denser than continental crust.

              • Correct answer
                Wrong answer

                3 attempts
              Done
              Studying the ocean floor
              Tectonic plates
              Without background sound
              Logo

              Activity 30

              0/1

              Analyse

              What happens to the thickness of sediment further away from the ridge? What could be the cause of this?

              The difference between the oceanic and continental crust
              • The thickness decreases because, the further away we go from the ridge, the older the ocean floor is, so sediment has been depositing on it for longer.

              • Correct answer
                Wrong answer
              • The thickness increases because the further away we go from the ridge, the younger the ocean floor is, so sediment has been depositing on it for longer.

              • Correct answer
                Wrong answer
              • The thickness of sediment near and far way from the ridge is always the same, as the sediment has been depositing everywhere at the same time.

              • Correct answer
                Wrong answer

                3 attempts
              Done
              Studying the ocean floor
              Tectonic plates
              Without background sound
              Logo

              Activity 31

              P

              Create

              Describe the landforms found on the ocean floor that are not found on the continents.

                1 attempt
              Done
              Studying the ocean floor
              Tectonic plates
              Without background sound
              Logo

              Activity 32

              P

              Create

              Find information about the first big oceanographic expedition carried out by the Challenger, in the 19th century. Compare its technology with modern oceanographic ships.

                1 attempt
              Done