1. Today, ships use echo sounding, or sonar, to find the
distance to the ocean floor. By tracking how long it takes for the
signal to reach the bottom and echo back to the ship, scientists
can measure the waters depth. OCEANOGRAPHY Chapter 23 The Ocean
Floor
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2. The recorder traces a continuous profile of the area over
which the ship is sailing. Such profiles help oceanographers make
accurate and detailed maps of the ocean floor.
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3. The intensity of the reflected sound is also measured to
determine sea floor composition.
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4.Scientists have used such data to pinpoint places along
continental slopes that could experience landslides triggering
tsunamis.
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5. Scientists gather samples of the ocean floor to study its
composition. They have discovered that the ocean floor consist of
layer upon layer of sediment.
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6. By studying these layers of sediment, scientists can learn a
great deal about how Earths atmosphere and climate have changed
over millions of years.
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7. Some sediment samples are retrieved through a process called
core sampling, in which a hollow instrument removes a long cylinder
of material, called a core, from the ocean floor.
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8. Today, satellites provide greater range and speed in the
mapping process. A satellite can gather far more data more quickly
than a sea going vessel.
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9. Although a signal sent from a satellite cannot reach the
ocean floor, it can bounce off the oceans surface.
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10. The ocean surface varies depending upon what lies beneath
it; it is slightly higher over mountains and slightly lower over
undersea trenches.
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11.Using a device called a radar altimeter, satellites detect
variations to within centimeters.
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12. The continental shelf is that part of a continent that
extends from the shoreline to the continental slope. Continental
shelves are very flat and their widths vary.
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13. The continental slope begins at the shelf edge, where water
depth begins to increase rapidly.
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14. The continental rise descends gradually from the
continental slope to the ocean floor. It is considered part of the
ocean basin rather than part of the continent.
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15. Where the oceanic plate is sliding beneath, or subducting
under, the continental plate there is an active continental
margin.
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16. At an active continental margin, the continental slope is
very steep and falls away into a deep oceanic trench.
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17. Active margins encircle the Pacific Ocean.
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18. Passive continental margins are not located on plate
boundaries. At passive continental margins, there are no deep ocean
trenches or coastal mountains.
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19.Submarine Canyons, some bigger than the Grand Canyon, slice
from the continental shelf clear to the ocean floor. Geologists
believe that submarine canyons were formed during the Ice Age when
sea level was perhaps 100 meters lower than it is now. Rivers
running to the sea cut valleys to the edge of the, then exposed,
continental shelf.
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20. Some submarine canyons may be the result of powerful
turbidity currents.
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21. Turbidity Currents, triggered by earthquakes or simply
gravity, are great landslides of mud.
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22. A feature of the sea floor that is the flattest of all
Earths surface areas is the abyssal plain.
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23.Abyssal plains are composed of sediments, most of which came
from the continents. The sediments in an abyssal plain can be over
one kilometer thick.
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24. Abyssal hills are small rolling hills that often occur in
groups next to ocean ridges.
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25. Deep-SeaTrenches are long narrow steep troughs that run
parallel to active continental margins, or to volcanic island chain
called island arcs. Trenches exist at subduction zones where, when
plates converge, one plate sinks beneath another plate. Deep-Sea
Trenches are common sites of earthquakes and volcanic
activity.
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26.A deep-ocean vent is a geyser that erupts underwater. When
the hot vent water mixes with the cold ocean water, minerals
carried from beneath the surface precipitate onto the surrounding
sea floor, creating thick deposits.
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27. The most obvious feature of the ocean basin are mid-ocean
ridges, great undersea mountain ranges. They form a chain over
50,000 kilometers long that wraps around the planet and crosses
every ocean.
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28. Mid-ocean ridges form at divergent plate boundaries where
two lithospheric plates are moving apart. Magma rises between the
plates and cools, forming a ridge of new sea floor.
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29. Seamounts are cone-shaped mountain peaks that rise above
the deep ocean floor.
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30. Volcanic in origin, seamounts seem to be related to plate
boundary activity. Isolated seamount groups probably originated
over hot spots.
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31. The Hawaiian Islands, a famous chain of hot- spot
volcanoes, were once a chain of seamounts, some of which have risen
above sea level.
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32.Flat topped seamounts are called guyots. Wave action removed
their tops when they projected above sea level. Later, the sinking
of the oceanic crust lowered the tops of the guyots below sea
level.
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33.A coral atoll is a ring shaped coral island. An atoll forms
when a coral reef develops around a volcanic island. Corals are
tiny sea animals that live in shallow warm water. As the mountain
sinks completely below sea level, new corals continue to form on
top of the old coral, leaving behind an atoll (circular reef) with
a central lagoon where the mountain top once was.
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34.There are four main classes of ocean floor sediments.
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35. Terrigenous sediments come from continental rocks and
minerals.
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36. Biogenous sediments come from living sources; they are
oozes, composed mostly of shells and skeletons from tiny marine
animals.
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37. Hydrogenous sediments form when chemical reactions cause
minerals to crystallize from seawater. Manganese nodules are best
known of the hydrogenous sediments. Manganese nodules are rich in
manganese and iron oxides and contain small amounts of nickel,
cobalt, copper, and other metals.
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38. In the layers of sediment is a record of Earths history
going back millions of years.
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39.By studying the remains of sea creatures in the sediments,
scientists can determine: a. The extent of former polar ice sheets
b. The history of water temperatures c. Past behaviors of
prevailing winds d. The pattern of changes in Earths climate
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40. The sediments, unique organisms, magnetic records, and
resources available on the ocean floor represent a significant
portion of the Earths hydrosphere, biosphere, and geosphere.