Transcript The Ocean Floor

OCEANOGRAPHY
Chapter 23 – The Ocean Floor
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
water’s depth.
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.
3. The intensity of the reflected sound is also
measured to determine sea floor composition.
4.Scientists have used such data to pinpoint
places along continental slopes that could
experience landslides triggering tsunamis.
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.
6. By studying these layers of sediment, scientists
can learn a great deal about how Earth’s
atmosphere and climate have changed over
millions of years.
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.
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.
9. Although a signal sent from a satellite cannot
reach the ocean floor, it can bounce off the
ocean’s surface.
10. The ocean surface varies depending upon
what lies beneath it; it is slightly higher over
mountains and slightly lower over undersea
trenches.
11. Using a device called a radar altimeter,
satellites detect variations to within
centimeters.
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.
13. The continental slope begins at the shelf
edge, where water depth begins to increase
rapidly.
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.
15. Where the oceanic plate is sliding beneath,
or subducting under, the continental plate there
is an active continental margin.
16. At an active continental margin, the
continental slope is very steep and falls away
into a deep oceanic trench.
17. Active margins encircle the Pacific Ocean.
18. Passive continental margins are not located
on plate boundaries. At passive continental
margins, there are no deep ocean trenches or
coastal mountains.
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.
20. Some submarine canyons may be the result
of powerful turbidity currents.
21. Turbidity Currents, triggered by earthquakes
or simply gravity, are great landslides of mud.
22. A feature of the sea floor that is the flattest
of all Earth’s surface areas is the abyssal plain.
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.
24. Abyssal hills are small rolling hills that often
occur in groups next to ocean ridges.
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.
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.
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.
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.
29. Seamounts are cone-shaped mountain peaks
that rise above the deep ocean floor.
30. Volcanic in origin, seamounts seem to be related
to plate boundary activity. Isolated seamount groups
probably originated over hot spots.
31. The Hawaiian Islands, a famous chain of hotspot volcanoes, were once a chain of seamounts,
some of which have risen above sea level.
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.
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.
34. There are four main classes of ocean floor
sediments.
35. Terrigenous sediments come from
continental rocks and minerals.
36. Biogenous sediments come from living
sources; they are oozes, composed mostly of
shells and skeletons from tiny marine animals.
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.
38. In the layers of sediment is a record of
Earth’s history going back millions of years.
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 Earth’s climate
40. The sediments, unique organisms, magnetic
records, and resources available on the ocean
floor represent a significant portion of the
Earth’s hydrosphere, biosphere, and geosphere.