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Oceanic Pressure

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Oceanic Pressure
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In 2012, the film director James Cameron, who directed the film Titanic, made headlines when he traveled to Challenger Deep. This is the deepest known point on the ocean floor, almost 11,000 meters below the surface. Cameron was the first person to travel there alone, and he made the trip in a deep-sea diving vehicle named Deepsea Challenger. The vehicle pictured above is another deep-sea diving vehicle, named Alvin. Alvin is no longer used for ocean exploration, but by the time it was retired, it had made thousands of dives. Alvin was even used to explore the Titanic on the North Atlantic Ocean floor.

Q : Why must divers use special diving machines such as Alvin and Deepsea Challenger to go deep below the surface of the ocean? Why can’t they just use scuba diving gear?

A : The pressure of the water is too great that far below the surface. It would crush divers long before they could reach the bottom.

Water Pressure and Depth

Pressure is the amount of force acting on a given area. As you go deeper in the ocean, the pressure exerted by the water increases steadily. That’s because there is more and more water pressing down on you from above. The Figure below shows how pressure changes with depth. For each additional meter below the surface, pressure increases by 10 kPa. At 30 meters below the surface, the pressure is double the pressure at the surface. At a depth greater than 500 meters, the pressure is too great for humans to withstand without special equipment to protect them. At nearly 11,000 meters below the surface, the pressure is tremendous. You can see a video showing changes in water pressure with increasing depth at this URL:

http://www.youtube.com/watch?v=dL08xX4lBQg

Diagram illustrating pressure in relation to depth

Adjusting to Changing Pressure

Scuba divers can dive without special vehicles because they don’t go very deep below the surface of the water. Nonetheless, because of the pressure of the water, scuba divers who go deeper than about 40 meters must return to the surface slowly. They must stop for several minutes at one or more points in their ascent. That’s what the divers in the Figure below are doing. The stops are needed to let the pressure inside their body adjust to the decreasing pressure of the water as they swim closer to the surface. If they were to rise to the surface too quickly, the gases dissolved in their blood would form bubbles and cause serious health problems.

Q : Why would dissolved gases form bubbles as pressure decreases?

A : Less gas can dissolve in a fluid at lower pressure. Therefore, as pressure decreases, gases come out of solution and form bubbles.

Scuba divers need to return to the surface slowly

Summary

  • As you go deeper in the ocean, the pressure exerted by the water increases. For each additional meter below the surface, pressure increases by 10 kPa.
  • Water pressure makes it difficult to explore the deep ocean. The pressure is too great for divers unless they travel in pressure-resistant deep-sea vehicles. Even scuba divers have to take time to adjust to the changes in pressure as they return to the surface.

Vocabulary

  • pressure : Result of force acting on a given area.

Explore More

Read about Challenger Deep in the article at the following URL. Watch the video about James Cameron’s solo dive to the Challenger Deep in the Deepsea Challenger. Also take the virtual video tour of the Marianas Trench. Then answer the questions below.

http://www.extremescience.com/deepest-ocean.htm

  1. Compare the depth of the ocean at the bottom of the Marianas Trench to the height of Mount Everest.
  2. How deep below the surface of the ocean did James Cameron travel in Deepsea Challenger? How great was the water pressure there?
  3. How and why did the size of Deepsea Challenger change as it went very deep below the ocean’s surface?
  4. How can you feel water pressure changes with depth in a swimming pool?

Review

  1. Describe how water pressure in the ocean changes as depth increases.
  2. Why does water pressure change with depth in this way?
  3. How can divers safely travel deep below the surface of the ocean to explore the ocean floor?
  4. Why do scuba divers have to return to the surface slowly if they go deeper than 40 meters below the surface?

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