1. CHAPTER 2
UNDERSEA LANDSCAPES

2. Weight
Until the 20th century, most knowledge
of the ocean floor came from heaving a
lead weight overboard with a hemp rope
and looking at the muds, weeds, oozes,
and sediments retrieved.

3. Depth Soundings
Sigsbee Sounding Machine 1875
This became the basic model for the
next 50 years

4. World War I efforts to combat German
successes with submarines led to:
Echo Sounder
Hydrophone
(sends signal-listens for echo)
(listens passively)

5. Echo Sounder
A device for measuring depth of
water by sending sound waves
down from the surface and recording
the time until the echo returns from
the bottom

6. Searching for submarines in the North
Atlantic led to the realization of the
diverse ocean floor geography.
Intense effort to chart the ocean floor
commenced and continues today.

7. Relief (of the Earth)
The differences in elevation and
slope between the higher and
lower parts of the land surface of
a given area

8. Topography
Graphic representation on a map
of the surface features of a place
or region, indicating their relative
positions and elevations

9. Contour Lines
~ Lines provide elevations
~ Spacing indicates relative slopes

10. Earth’s Two Main Levels of Relief
Continents
Deep Ocean Floor

11. DEEP OCEAN FLOOR
Also known as:
~ Deep sea
~ Deep ocean basin
~ Abyss

12. Abyssal
Plain
Island
Guyot
Seamount
Canyon
Depth Soundings
~ Described in
terms of its
individual
features
Fracture
Ridges
Ridge
Trench

13. Earth
71% covered by
water but only
two-thirds is deep
oceanic basin
Average depth ~ 12,000± feet (2.5 miles)
Some trenches ~ 37,000+ feet (7 miles)

14. Echo sounders measure the time it
takes a sound pulse to travel to the
ocean floor and return. The deeper
the floor, the longer the return.
(Note: two ocean ridges & one rift valley)

15. Sound travel averages 4,800 ft/sec in
water. If an echo return takes 2 seconds,
it traveled 9,600 feet. Since it is a round
trip, the depth is 4,800 feet.

16. You transmitted a sound in water and
the echo returned in 5 seconds.
a. How far did the sound travel?
b. How deep is the ocean floor?

17. You transmitted a sound in water and
the echo returned in 5 seconds.
a. How far did the sound travel?
The sound traveled 24,000 feet
(4,800 x 5)
b. How deep is the ocean floor?
The ocean floor is at 12,000 feet
(to floor and back) (24,000 ÷ 2)

18. Ocean Floor - Three Distinct Areas

19. Continental Shelf
The relatively shallow (up to 200
meters), submerged border of a
continent that slopes gradually
and extends to a point of steeper
descent to the ocean floor

20. Shelf
Land
Most maritime nations have agreed that
the continental shelf is a part of the land
out to a depth of 200 meters (656 feet).

21. North Carolina
about 75 miles
Shelf
Land
Continental shelves drop off at 7 to 10
feet per mile and average about 42
miles in width.

22. California
under 1 mile
Shelf
Land

23. Japan
Peru
Shel
f
Lan
d
Some areas of the world have virtually no
continental shelf as the oceanic plate
subducts below a continental plate.

24. Ganges River
Basin
Indus River
Basin
Oman
Tapi River
Basin
Shelves may
not always
have a
gradual
slope.
Sediment such as
rocks, sand, mud,
silt, clay, and
gravel may
cover shelves.
Coarse sand is the most common.

25. The sunlit continental
shelves provide food,
fuel, and minerals.

26. Most food, oil, and mineral explorations
are on continental shelves, the area
most liable for human conflicts.

27. Continental Slope
Floor
Shelf
Land
Regardless of the
distance from land,
at some point the
shelf suddenly
drops (slopes) to
the ocean floor.

28. Continental Slope
The steep descent of the seabed
from the continental shelf to the
abyssal zone

29. Continental slopes descend 100 to 500 feet
per mile, flattening out near the floor.
This is where the Earth’s continental crust
(granitic material) meets the oceanic crust
(basaltic material + sediment).

30. Granitic Rock:
~hard, speckled whitish or gray color
~rough appearance
~magma cooled slowly under the
Earth’s surface (intrusive)
~most common rock exposed on surface
Basaltic Rock:
~hard, dark color
~smooth to glassy appearance
~lava cooled rapidly above the
Earth’s surface (extrusive)
~most common rock on Earth’s crust,
covering the oceanic crust’s floor

31. If continental slope meets oceanic crust
at subduction zone:
Basalt
+ sediment
e
lop
S
Granite
Granite is lighter relative to Basalt and rises
to mountainous heights, often with volcanic
activity created by the intense pressures.

32. Deep Ocean Floor
• A bleak and uncomfortable world for
humans
• No light
• No plant life
• Extreme pressure
• Cold
• Sediments - mud, clay, sand, gravel

33. ~ Andes Mountains
~ Peru-Chile Trench
8-mile descent in less than 100 miles
PERU
CHILE

34. Continental slope features are similar to
continental surface features but on a
larger scale. Some of the features are:
Submarine
canyons,
cliffs, valleys,
terraces,
plateaus, and
drop-offs of
several
thousand of
feet.

35. Submarine Canyon
A deep underwater valley with
steep sides

36. Terrace
A nearly level strip of land with a more
or less abrupt descent along the
margin of the sea, a lake, or a river

37. Continental Slopes
Submarine canyons are often carved
out by past glaciers, tidal currents,
other underwater currents, and
landslides.

38. Continental Slopes
Rapidly moving underwater currents
called turbidity currents, carry debris
and sediments that scour the canyon
walls much like river or wind erosion
does on continental surfaces.

39. Larger than Arizona’s Grand Canyon,
New York’s Submarine Hudson Canyon
extends from depths of 300 feet to some
7,000 feet, 150 miles off shore.
Hudson
River
Long Island
Hudson Canyon
New Jersey
Continental Slope
Abyss

40. The Hudson River deposits an
enormous plain of mud called
a submarine fan.

41. Submarine Fan
Mud, silt, and other
sediment deposited
when rivers empty
into the ocean
Sometimes extending
hundreds of miles out
to sea

42. Mississippi River
Indus
Ganges
The world’s great rivers extend similar
fans hundreds of miles out to sea.

43. Indus River Delta
Ganges River Delta
Mississippi
River Delta
Submarine fans
extending hundreds
of miles may also
deposit enough
sediment to create
deltas above the
surface.

44. The deep ocean floor begins at the
bottom of the continental slope and
extends seaward as the true bottom.

45. Continental Rise
Where turbidity currents deposit
enough sediment at the base of the
continental slope to change angle of
the slope.

46. Pacific
Ocean
Atlantic
Ocean
Indian
Ocean
The deep ocean floor covers one-third
of the Atlantic and Indian Oceans, and
three-quarters of the Pacific Ocean.

47. Pacific
Ocean
East of Japan
Atlantic
Ocean
East of United States
Most of the Pacific deep ocean basin
consists of rolling hills, while plains
are widespread in the Atlantic.

48. Atlantic
Ocean
East of Canada
Turbidity currents flow through
connecting canyons and channels
carrying land sediment.
Heavier sediments create the rise,
lighter sediments settle on the plains.

49. Every deep ocean has impressive
mountain ranges called ridges. The
Mid-Atlantic Ridge soars more than
6,000 feet.

50. Mid-Ocean Ridge
Many underwater earthquakes occur in a
rift running down the ridge's centerline.

51. Major plate margins abut
the Mid-Ocean Ridge

52. Almost all of the small Pacific Islands
are tops of former volcanic mountains
and true oceanic islands. They did not
break away from continents.

53. The surface remnant of a volcanic peak
that eroded or sank is known as an
atoll. An old crater’s center filled with
water is a lagoon.

54. In some cases, coral
islands subside and
disappear beneath
the sea surface,
leaving a peaked
seamount.
t
amoun
Se
Found in all oceans
but most common
in the Pacific.

55. Seamount
An underwater mountain rising
from the ocean floor and having
a flat topped or peaked summit
below the surface of the sea

56. In other cases,
volcanic activity
that does not
reach the sea
surface can
also leave an
underwater
mountain
called a
seamount.

57. Accurate seamount location and charting
is critical for navigation.
In January 2005, SSN San Francisco (SSN
711) collided with just such a seamount.

58. Seamounts with flattened tops
are guyots.
Guyots are found only in the Pacific.

59. Guyot
A flat topped seamount
Guyots show evidence of having
been above the surface, worn by
ocean waves, and gradually
subsiding under their own weight
as the sea rose.

60. The Hawaiian Islands are part of a
chain created by volcanic eruption as
the Pacific Plate moves northwest.

61. Loihi
Seamount
Southeast of the big island, Hawaii,
the newest seamount is actively
building to be the next island.

62. Kilauea
Mauna Loa
Two of the
world’s most
active volcanoes
are on the big
island of Hawaii.

63. High enough to have snow when dormant,
Mauna Loa’s lava flow is 13,677 feet above
the Pacific Ocean but more than 31,000 feet
from the ocean floor.

64. South Pacific
Caroline
Islands
Gilbert
Islands
Samoan
Islands
Society
Islands
Similar Volcanic Island Chains

65. Sediments
Ocean floor sediments consist of three
general types:
• Oozes
• Clays
• Land-derived muds

66. Oozes
Composed of marine shells, and
skeletons of minute animals,
oozes are found in warm, shallower
waters of the equatorial areas and
the Atlantic.

67. Clays
Volcanic, meteorite, and airborne dusts
make up the dark brown or reddish clay
of the deep, cold, parts of the ocean
basin as in the North Pacific.

68. Muds
Land-derived
materials
delivered by
river flow create
the muds that
empty into the
oceans and
spread over the
abyssal plains.

69. Sediment does not always remain
stationary. Underwater landslides
and water flow may scour sediment
in some areas and deposit great
thickness in others.

70. Fine sediment buildup on
the deep seabed is very
slow:
1 inch every 2,500 years.

71. Volcanic eruptions spread sediments
for miles. The ash and dust may circle
the globe for years before falling to the
Earth's surface.

72. Icebergs, river and shore ice entrap
Detritus, loose material (stone
fragments,
silt, etc.) that is worn away from
Rocks that falls to the ocean floor
when they melt.

73. Mineral crystals often solidify or encrust
around tiny objects on the sea bottom,
forming nodules, or lumps of metal.

74. The most valuable nodule is one
that contains manganese, used
chiefly as an alloying agent in steel to
give it toughness

75. Manganese
A grayish white, usually hard
and brittle, metallic element that
resembles iron but is not magnetic
A valuable oxidizing agent
Used chiefly as an alloying agent
in steel to give it toughness

76. Abundant Manganese Areas

77. Most manganese is found at depths over
12,000 feet making retrieval difficult.

78. Most seafloor
sediment
samples are
obtained by
using a coring
tube.

79. Studying cores with tiny animal shells
in the ooze and sediment provides a
history of that part of the ocean.
Fossils give clues about geological age
and temperatures of the sea.

80. Deep water drilling showed:
~ North Atlantic began to form 200
million years ago, South Atlantic
150 million years ago.
~ Earth’s surface is made up of
moving plates.

81. Q.1. Who invented the hydrophone?
A.1. A team of U.S. Navy scientists

82. Q.2. What is the purpose of a relief
map of the Earth’s surface?
A.2. To show elevations of the
Earth’s surface by using colors
or numbers

83. Q.3. The average width of the
continental shelves is about
how many miles from the shore
line?
A.3. 42 miles

84. Q.4. What is the average depth of
the ocean?
A.4. 2.5 miles deep

85. Q.5. In what part of the ocean would
you find sea vegetation?
A.5. The continental shelf

86. Q.6. What are ocean ridges?
A.6. Ocean ridges are enormous
mountain ranges running along
the center of the ocean floor.

87. Q.7. The enormous cracks that are
found in the ocean are referred
to as what?
A.7. Rifts

88. Q.8. What is the drop rate of the
continental slope?
A.8. 100 to 500 feet per mile

89. Q.9. What type of materials makeup
the ocean floor?
A.9. Oozes, clay, and land-derived
muds

90. Q.10. What are the three (3) areas
the ocean floor is divided into?
A.10. The continental shelf,
continental slope, and deep
ocean basin

91. Q.11. What is the deep ocean floor
sometimes referred to?
A.11. The abyss

92. Q.12. What is a submarine fan?
A.12. The delta of a large river in
which an enormous plain of
mud can be deposited
hundreds of miles out to sea

93. Q.13. How were most of the Pacific
islands formed?
A.13. Through volcanic eruption

94. Q.14. What is a guyot, and how is it
formed?
A.14. A flat-topped undersea
mountain believed to have
formed from repeated volcanic
eruptions