Name:____________________
Features on the Ocean Floor (ch. 18)
(Textbook p. 322-337)
1. The H.M.S. Challenger measured the ocean depths using a____________on a line, while
modern ships continuously measure the oceans depths using ______________ recording
the time taken to reach the bottom and return.
2. An undersea feature that is actually the first part of the continental margin is the
______________________ that extends to a depth of approximately ___________.
3. The continental slopes are cut by many huge gullies called ___________________ that
may have formed during the ____________ when sea level was perhaps 100 meters
lower than today.
4. Some geologists think that the gullies and valleys on the continental slope may have
formed when landslides of mud and sand called _______________________ rushed at
high speed down the slopes.
5. Some seamounts that appear to have their tops sheared off are called _______________.
They seem to be seamounts that may have once risen above sea level and have been worn
down by ______________.
6. Fractures which cut across sections of the mid-ocean ridges where it bends around
regions, like the bend in Africa, are called ______________________.
7. Continental rise: (p. 329)
a. What is it: _________________________________________________________
__________________________________________________________________
__________________________________________________________________
b. They are found on (active/passive) margins only because on (active/passive)
margins the sediments are trapped in the ____________.
8. Circle the correct word that makes each statement true.
a. Abyssal plains in the ocean show the remains of many (volcanoes/ earthquakes)
b. The world’s largest mountain range is the (continental shelf/ the mid-ocean ridge)
c. The mid-ocean ridge was formed where tectonic plates (push together/ move
apart)
d. Undersea earthquakes occur at (seamounts/ the mid-ocean ridge)
e. The continental slope rises sharply from the (deep plains/ continental shelf)
9. What are seamounts? In which ocean are most seamounts formed?
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
10. Match each term in Column A with its description in Column B. You will use some
terms more than once.
Column A
Column B
Trench
Rift
Submarine canyon
______________ 1. Tectonic plates moving apart form a canyon
______________ _2. Dense sediment filled water forms a canyon
_______________ 3.The sinking of part of the ocean floor forms a
canyon at a subduction boundary
_______________ 4. A canyon found on the continental shelf
_______________ 5. Deepest of all canyons
11. How does sediment from land get to deep plains thousands of kilometers from the shore?
________________________________________________________________________
________________________________________________________________________
SECTION B: IDENTIFICATION OF OCEAN FEATURES
For each of the following terms, locate where it would be on the profile of the seafloor, and next
to each word write the letter
_____Continental shelf
_____Guyot
_____Seamount
_____Continental rise
_____Abyssal plain
A
B C D
_____abyssal hills
_____continental slope
_____ trench
_____ rift valley
E
F
G
I
J
SECTION C: MAPPING THE OCEAN FLOOR
Oceanographers study the ocean floor using sonar. Sonar, or sound waves, are sent from a ship
to the ocean bottom. The waves bounce off the bottom and back up to a receiver located on the
ship at the surface. The deeper the ocean, the longer it takes to receive the returning sonar
vibrations. Oceanographers know how fast sound waves travel in water. Thus, they know how
far the waves travel during their round trip to the surface. Half this distance is the depth of the
ocean at that spot. The formula used to find ocean floor depth is: D = ½ t X v
“D” stands for depth, “t” is the time between the sending and receiving, and “v” is the velocity or
speed of the sound waves in water. The speed of sound waves in water is 1524 meters per
second.
D=½t*v
D
D = depth
t = time
v = velocity (1524 meters per
second)
Example: A research ship sends a sonar signal to the bottom. The ship receives the signal back
in 4 seconds. How deep is the ocean at this point?
D=½t*v
D = unknown
t = 4 sec.
v = velocity (1524 meters per second)
D=½t*v
D = ½ (4 sec) * 1524 m/sec
D = 3048 m
The ocean is 3048 meters deep at
this location
The table below gives you some sonar sounding data. The data was gathered on a cruise from
South America to Africa. Calculate the ocean floor depth for each sounding. Then plot these
depths on the graph which follows. Smoothly connect the depth points and you will have a
cross-sectional map of the South Atlantic Ocean floor.
Station
Time for
signal to
return (sec)
6.0
5.2
4.0
2.4
3.2
A
B
C
D
E
Depth of
Ocean (m)
Station
Time for
signal to
return (sec)
2.4
4.0
5.2
6.0
F
G
H
I
Depth of
Ocean (m)
Cross-sectional Map of South Atlantic Ocean
A
West
B
C
D
E
F
G
3000km
H
Depth (m)
0 (sea level)
2000
4000
6000
I
East
1. From what point do oceanographers measure ocean depth? (ocean floor/sea level)
2. At which station (s) is the ocean the deepest? ________________
3. At which station(s) is the ocean the most shallow? _______________
4. What large ocean floor feature is evident in the cross-sectional map? ________________
5. What is the name of the ocean floor feature located directly below station E? _________