14.3 The Ocean Floor

Lesson Objectives

• Describe how scientists study the ocean floor.
• Identify major features of the ocean floor.
• List resources found on the ocean floor.

Vocabulary

• abyssal plain
• continental shelf
• continental slope
• mid-ocean ridge
• oceanic trench
• seamount
• sonar

Introduction

Vast, unknown spaces still exist for humans to explore. Of course, outer space is still mostly unknown. But some of the most interesting and hardest to reach places are much closer to home. They are on the ocean floor.

Only a tiny fraction of the ocean floor has ever been studied. Why? Humans can’t travel deep below the water’s surface without special vehicles. The pressure of the water is too great. Total darkness and extreme cold make it even more difficult. That’s why people have worked for decades to invent technology for studying the ocean floor.

Studying the Ocean Floor

Scientists study the ocean floor in various ways. Scientists — or their devices — may actually travel to the ocean floor. Or they may study the ocean floor from the surface. One way is with a tool called sonar.

Using Sonar

Did you ever shout and hear an echo? If you did, that’s because the sound waves bounced off a hard surface and back to you. The same principle explains how sonar works. A ship on the surface sends sound waves down to the ocean floor. The sound waves bounce off the ocean floor and return to the surface, like an echo. Figure below show how this happens.

Sound waves travel through ocean water, but they bounce off the ocean floor. They move through ocean water at a known speed. Can you use these facts to explain how sonar works?

Sonar can be used to measure how deep the ocean is. A device records the time it takes sound waves to travel from the surface to the ocean floor and back again. Sound waves travel through water at a known speed. Once scientists know the travel time of the wave, they can calculate the distance to the ocean floor. They can then combine all of these distances to make a map of the ocean floor. Figure below shows an example of this type of map.

A map of a 10,000 foot-high undersea volcano in Indonesia made by multibeam solar.

Traveling to the Ocean Floor

Only a specially designed vehicle can venture beneath the sea surface. But only very special vehicles can reach the ocean floor. Three are described here and pictured in Figure below:

• In 1960, scientists used the submersible Trieste to travel into the Mariana Trench. They succeeded, but the trip was very risky. Making humans safe at such depths costs a lot of money. People have not traveled to this depth again. In 2012, the film director, James Cameron, dove to the bottom of the Mariana Trench by himself in a submersible that he had built for the purpose.
• The vehicle named Alvin was developed soon after Trieste. The submersible has made over 4,000 dives deep into the ocean. People can stay underwater for up to 9 hours. Alvin has been essential for developing a scientific understanding the world's oceans.
• Today, remote-control vehicles, called remotely operated vehicles (ROVs) go to the deepest ocean floor. They don’t have any people on board. However, they carry devices that record many measurements. They also collect sediments and take photos.

Vehicles for Underwater Exploration. These special vehicles have been used to study the ocean floor.

Features on the Ocean Floor

Scientists have learned a lot about the ocean floor. For example, they know that Earth’s tallest mountains and deepest canyons are on the ocean floor. The major features on the ocean floor are described below. They are also shown in Figure below.

• The continental shelf is the ocean floor nearest the edges of continents. It has a a gentle slope. The water over the continental shelf is shallow.
• The continental slope lies between the continental shelf and the abyssal plain. It has a steep slope with a sharp drop to the deep ocean floor.
• The abyssal plain forms much of the floor under the open ocean. It lies from 3 to 6 kilometers (1.9 to 3.7 miles) below the surface. Much of it is flat.
• An oceanic trench is a deep canyon on the ocean floor. Trenches occur where one tectonic plate subducts under another. The deepest trench is the Mariana Trench in the Pacific Ocean. It plunges more than 11 kilometers (almost 7 miles) below sea level.
• A seamount is a volcanic mountain on the ocean floor. Seamounts that rise above the water surface are known as islands. There are many seamounts dotting the seafloor.
• The mid-ocean ridge is a mountain range that runs through all the world’s oceans. It is almost 64,000 kilometers (40,000 miles) long! It forms where tectonic plates pull apart. Magma erupts through the ocean floor to make new seafloor. The magma hardens to create the ridge.

The features of the ocean floor. This diagram has a lot of vertical exaggeration.

Resources from the Ocean Floor

The ocean floor is rich in resources. The resources include both living and nonliving things.

Living Resources

The ocean floor is home to many species of living things. Some from shallow water are used by people for food. Clams and some fish are among the many foods we get from the ocean floor. Some living things on the ocean floor are sources of human medicines. For example, certain bacteria on the ocean floor produce chemicals that fight cancer.

Nonliving Resources

Oil and natural gas lie below some regions of the seafloor. Large drills on floating oil rigs must be used to reach them. This is risky for workers on the rigs. It is also risky for the ocean and its living things. An oil rig explosion caused a massive oil leak in the Gulf of Mexico in 2010. Oil poured into the water for several months. The oil caused great harm to habitats and living things, both in the water and on the coast. The oil spill also hurt the economy of Gulf Coast states. The effects of the oil spill are still being tallied.

There are many minerals on the ocean floor. Some settle down from the water above. Some are released in hot water through vents, or cracks, in the seafloor. The minerals in hot water settle out and form metallic chimneys, as in Figure below. These metals could be mined, but they are very deep in the sea and very far from land. This means that mining them would be too expensive and not worth the effort.

Metals from the ocean crust are brought by hot water onto the seafloor to create chimneys, as shown in this photo.

Some types of minerals form balls called nodules. Nodules may be tiny or as big as basketballs. They contain manganese, iron, copper, and other useful minerals. As many as 500 billion tons of nodules lie on the ocean floor! However, mining them would be very costly and could be harmful to the ocean environment.

Lesson Summary

• Studying the ocean floor is difficult because the environment is so hostile. The seafloor can be studied indirectly with tools such as sonar. It can be studied directly using special vehicles. Some vehicles carry scientists and their devices to the ocean floor. Other vehicles are operated remotely.
• Features of the ocean floor include the continental shelf and slope, abyssal plain, trenches, seamounts, and the mid-ocean ridge.
• The ocean floor is rich in resources. Living things on the ocean floor are used for food or medicines. Nonliving resources include oil, gas, and minerals.

Lesson Review Questions

Recall

1. Why is it difficult to study the deep ocean floor?

2. What is sonar? How can it be used to study the seafloor?

3. What are the continental slope and continental shelf?

4. Gives examples of living resources from the ocean floor.

5. Identify nonliving resources on or below the ocean floor.

Apply Concepts

6. Create a diagram to show how sonar is used to find the distance from the surface to the ocean floor.

Think Critically

7. Relate features of the ocean floor to plate tectonics.

Points to Consider

Many organisms live on the ocean floor. Others live elsewhere in the ocean.

• Where else do organisms live in the ocean?
• How might organisms that live on the ocean floor differ from those that live in other parts of the ocean?