Sea Secrets
Sea Connections
Ocean Market
Pollution Solution
Stranded Along the Coast
Reflections on the Sea

OBJECTIVES

• Identify some of the features of the ocean, including a continental shelf, a deep ocean plain, a trench, and a mid-ocean ridge.
• Draw a profile of the ocean using data points.
• Infer the conditions of some undersea locations.

MATERIALS

• Student Pages
• pencil, straightedge
• Optional: world map, globe
• Optional: small aquarium, sand, water, salt, metal objects

SUBJECTS

social studies, oceanography, mathematics

PROCEDURE

1. To introduce Sea Secrets, ask students to name the highest and lowest places on Earth's crust. They may name Mt. Everest and the Grand Canyon, respectively. Tell them that the tallest mountains and deepest canyons are found in the ocean. The Hawaiian Islands would dwarf Mt. Everest. These islands are merely the tops of huge mountains that have their base on the deep floor of the Pacific. Also in the Pacific is the deepest trench on the Earth, the Mariana Trench. It measures eleven kilometers below the sea's surface-seven times the depth of the Grand Canyon. The Pacific, named by Magellan because it looked peaceful, is the largest feature on Earth. It can look tranquil sometimes; but at other times huge waves roll, typhoons blow, and tsunamis strike the coast. Spin a globe and have students observe how great an area of Earth the Pacific occupies. Spin the globe again, and have students find and name the Atlantic, Indian, and Arctic Oceans.

2. Ask students to speculate on how people have learned about the ocean. They may know that many a sailor who went out to sea on a great clipper ship never actually went into the water-many of them could not swim. They measured depth using ropes and did their best to stay out of the briny deep. Tell students how the HMS Challenger went out to sea in 1872, and for three years the crew mapped and charted the many mountains and valleys of the ocean floor. Today we have a "window" into the gloom of the deep sea and can map the contours using sophisticated equipment. We use sonar-sound waves that bounce off the bottom of the ocean and back up to a research ship. The longer it takes for the sound to bounce back, the deeper the ocean floor. In some ways, robotic submersibles are like spacecraft charting the unknown regions of space. We have core samplers that drill holes into the ocean bottom and bring up layers of sediment.

3. Have students imagine they are taking the Johnson Sea Link, a submersible research vessel, into the gloomy darkness of the abyss. By five hundred feet below the surface, it is already dark on the sunniest day. Off the coast of Florida, it might take an hour to get down to the seafloor. The incredible water pressure of a million pounds per square inch actually squeezes the thick walls of the small capsule. Creatures of fantastic shapes are viewed for the first time by human eyes as they pass through the craft's lights. Robotic arms carefully funnel gauze-like invertebrate animals into sampling containers. The animals are brought up to the surface and studied.

4. Discuss with students how mapping, photographing, and taking core samples from the ocean floor have helped scientists to conclude how the oceans formed. Earth's crust is both younger and thinner beneath the ocean than it is under the continents. That is because new ocean floor is continually forming at the mid-ocean ridges. These ridges wrap around Earth like the seams on a baseball. Coming off the sides of the ridges are "rift valleys," from which molten rock from within Earth pours out like lava from a volcano. When it cools, it forms new ocean floor. As the rock cools, magnetic particles in the lava are frozen, pointing in the direction of the North Pole. Scientists have matched these particles with periods of pole shifts in the geologic record. They have also matched the particles on both sides of the ridge, showing that they have spread apart. The discovery of these matching "magnetic stripes" in the rocks surrounding the mid-ocean ridges propelled the theory of plate tectonics into the forefront of geology.

5. One interesting connection between biology and plate tectonics involves the life cycle of green sea turtles. The fact that South American green sea turtles swim to tiny Ascension Island in the middle of the Atlantic to lay eggs may help prove the theory of plate tectonics. Some scientists hypothesize that the ancestors of these animals made this journey before the continents were so far apart. The turtles have continued this behavior over many generations as the distance across the Atlantic has gradually increased by several centimeters each year.

6. Tell students that the deep, flat portions of the ocean floor are referred to as the abyss or the abyssal plain. Trenches occur where one of Earth's crustal plates is sliding under another. The continental slope is the area of ascent that leads to the continental shelf, the underwater edge of a continent.

7. Hand out the student pages. Have students examine the map of the Atlantic Ocean. Explain that it shows what the ocean would look like if all the water were removed. Compare the area of the map to a wall map of the world, then have students find the labeled continental shelves, abyssal plains, continental slopes, trenches, and ridges. Parts of the Atlantic trenches are more than eight kilometers deep. To make that distance more meaningful to students, use the distance between two familiar landmarks in your area (eight kilometers equal five miles). Or have them figure out how many school buses (each about ten meters long) would have to line up to cover a distance of eight thousand meters (eight hundred). Tell them that parts of the Atlantic Ocean are that deep from the surface down to the ocean floor.

8. When students seem ready to work on their own, have them plot the data points to draw an ocean profile between Florida and Dakar, Senegal. (These data are approximate and have been simplified somewhat.) Make sure students understand that a profile is a side view of the ocean floor
if you made the journey along a straight line between two locations.

9. Tell students that this treasure hunt for the location of gold is based on real salvage efforts to bring up the cargo of a World War II Japanese submarine torpedoed en route to Europe with supplies of precious metals. It takes the work of people of many nations to accomplish such a task. This activity raises the issue of respect for the dead who were lost with the vessel as well as more temporal and legal disputes about who owns bounty that lies in international waters. Tell students that a nation has legal rights to waters two hundred miles off its coast. At one time countries claimed a mere three miles because that was as far as a cannonball could reach. Later, the limit was extended to twelve miles, then two hundred.

10. When students finish their profiles, have them answer the questions on the student page. They will find that the Nares Deep and Cape Verde Basins are the deepest areas in this section of the ocean and that they are separated by the Mid-Atlantic Ridge. The sunken sub is located at data point 11
at a depth of five and a half kilometers. To work on such a project, they would likely operate from the Cape Verde Islands, the closest landmass. They would probably choose to use a remotely operated vehicle because of the great depth and risk. Perhaps they would choose to erect a memorial to those who died in the sub. Gold survives well under adverse conditions because it does not react easily with other compounds and thus does not corrode like other metals or deteriorate like organic materials.

11. Challenge students to make a model ocean profile using a small aquarium, sand, and water. They can make their own seawater by dissolving thirty-five grams of salt for every liter of water. They might bury a metallic object at a specific location and challenge other students to find it without disturbing other parts of the ocean floor.

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