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.

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• 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.

3.1.Oceanic landfroms

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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.

 

 

3.2.The composition of the ocean floor

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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:

      

• 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.

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.