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Seafloor

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Seafloor

Is it true that we know more about the dark side of the Moon than we do about the oceans?

It's true! Why do you think so? The oceans are deep, dark, frigid, and under extraordinarily high pressure at all but the surface. It's hard to imagine an environment that's less hospitable to human life! Yet, as you will see, we know quite a bit about the oceans and this is due mostly to technology. Rovers, like the one pictured, allow scientists to go to places that are too inhospitable or dangerous for human life.

Studying the Seafloor

Scuba divers can only dive to about 40 meters, and they cannot stay down there for very long. Although this is good for researching the organisms and ecosystems very near a coast, most oceanic research requires accessing greater depths.

Seafloor Bathymetry

How do scientists create bathymetric maps like the one of Loihi volcano in Hawaii shown in "Concept Studying Earth Science V: Maps"? Early explorers mapped a small amount of the seafloor by painstakingly dropping a line over the side of a ship to measure the depth at one tiny spot at a time. Then, during World War II, battleships and submarines carried echo sounders to locate enemy submarines ( Figure below ). Echo sounders produce sound waves that travel outward in all directions, bounce off the nearest object, and then return to the ship. By knowing the speed of sound in seawater, scientists calculate the distance to the object based on the time it takes for the wave to make a round trip.

This echo sounder has many beams and creates a three dimensional map of the seafloor. Early echo sounders had a single beam and created a line of depth measurements.Echo sounders now have many beams to get a more detailed and more rapid picture of the seafloor.

Sampling Remotely

Samples of seawater from different depths in the water column are needed to understand ocean chemistry. To do this, bottles are placed along a cable at regular depths and closed as a weight is dropped down the cable. The water trapped in the bottle can be analyzed later in a laboratory ( Figure below ).

A Niskin bottle being deployed off the side of a research ship.

Scientists are also interested in collecting rock and sediment samples from the seafloor. A dredge is a giant rectangular bucket that is dragged along behind a ship to collect loose rocks. Gravity corers are metal tubes that fall to the seafloor and slice into the sediments to collect a sample. The research vessel, the Joides Resolution , drills deep into the seafloor to collect samples of the sediment and ocean crust. Scientists analyze the samples for chemistry and paleomagnetism.

Submersibles

Samples of seawater and rocks can be collected directly by scientists in a submersible . These subs can take scientists down to make observations. The subs have arms for collecting samples. The human operated vehicle Alvin can dive up to 4,500 m beneath the ocean surface and has made more than 4,400 dives since 1964 ( Figure below ).

Alvin allows two people and a pilot to make a nine hour dive.

Remotely Operated Vehicles

To avoid the expense, dangers, and limitations of human missions under the sea, remotely operated vehicles , or ROVs, allow scientists to study the ocean’s depths by using small vehicles carrying cameras and scientific instruments. ROVs were used to study the Titanic , which would have been far too dangerous for a manned sub to enter. Scientists control ROVs electronically with sophisticated operating systems.

Footage of the NOAA Titanic Expedition of 2004 is visible in this video: http://www.youtube.com/watch?v=6Z7REEnwKOQ .

Summary

  • Most of the ocean is less well known than the dark side of the Moon because it is inhospitable and inaccessible.
  • Echo sounders use sound waves to make bathymetric maps.
  • Submersibles and ROVs allow scientists to view otherwise inhospitable regions either directly or remotely.

Practice

Use this resource to answer the questions that follow.

1. What is the only truly uncharted area of Earth?

2. How were soundings taken in the past?

3. List the advantages of using multi-beam sonar.

4. How is texture captured?

5. What is groundtruthing and why is it necessary?

6. Why is this project important?

Review

1. How does an echo sounder work?

2. Why is an ROV better for some tasks than a submersible? Why is a submersible better for some tasks than an ROV?

3. How do marine geologists collect rock and sediment samples?

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