Stranded Along the Coast
Reflections on the Sea
- 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.
- Student Pages
- pencil, straightedge
- Optional: world map, globe
- Optional: small aquarium, sand, water, salt, metal objects
social studies, oceanography, mathematics
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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