Mapping the Ocean Floor
Background Information:
SOund NAvigation and Ranging or SONAR is used to find and identify objects in water
and to find water depth. It is used in water because sound waves taper off less in
water than radar (radio) or light waves. Sonar was first used in World War I to
detect submarines. Later, in the 1920’s, sonar was used to map deep water
areas.
Active sonar emits a pulse of sound into the water. When the
sound wave encounters an object, the sound bounces off the object and
Sonar
returns an “echo”. Since the speed of sound in water is known (about 1524
m/sec), and the time it takes for the sound to travel from the ship to the object
and back is recorded by the ship, the distance (depth) to the object can be
Sea floor
determined. This method of determining depth is known as echosounding.
Usually, a ship makes a series of paths across an area. As the ships moves across the
area, it continually sends and records sound waves. In this way, many depth readings are
collected to use for making a topographic map of the sea floor.
There are two areas used in this investigation. The first is the Mariana Trench located
just east of the Mariana Islands in the western Pacific Ocean basin. The trench is about 2,550
km (1,580 mi) long and on average 70 km (40mi) wide. Its deepest part is about 11,000 meters
(close to 7 miles). The trench lies along a subduction zone, where the Pacific Plate is being
subducted beneath the Phillippine Plate.
The second is the Mid Atlantic Ridge, which is an underwater mountain range found in
the Atlantic Ocean. The Ridge sits on a bulge caused by convection currents in the
asthenosphere. The range runs from the Arctic Ocean to the Southern tip of Africa. The Mid
Atlantic Ridge results from divergent plate boundaries (North American from the Eurasian and
South American from the African) where magma rises from the mantle between the plates. The
heat from the magma causes the crust on either side of the rift to expand, forming the ridges.
The average rate at which the plates move apart is about 2.5 cm per year, a little faster than the
rate at which fingernails grow.
Materials:
Graph paper, pencil, blue, brown map pencils
Data sheets for Mariana Trench and Mid-Atlantic Ridge
Procedure:
1. Using the graphs provided, label the x-axis with distance and the y-axis with depth. Don’t
forget the units!
2. The heavy line across the graph represents sea level (0 meters).
3. Plot the depth of each recorded sounding at each distance on the graph paper.
4. Connect the plotted graph points to create a profile of the sea floor along the Mid-Atlantic
Ridge.
5. Repeat the procedures for the Mariana Trench.
Mid Atlantic Ridge
Distance
from N.
America
(km)
Mid Atlantic Ridge
The data table
shows the depth of
the Atlantic Ocean
measured by a
research vessel
traveling East along
the 39oN latitude.
Analysis:
0
160
200
500
800
1050
1450
1800
2000
2300
2400
2600
3000
3200
3450
3550
3600
3700
3950
4000
4100
4350
4500
5000
5300
5450
5500
5600
5650
Depth to sea
floor
(m)
0
-165
-1800
-3500
-4600
-5450
-5100
-5300
-5600
-4750
-3500
-3100
-4300
-3900
-3400
-2100
-1330
-1275
-1000
0
-1300
-3650
-5100
-5000
-4200
-1800
-920
180
0
Mariana Trench
Distance
from S.
Guam
(km)
0
8
16
24
32
40
48
56
64
72
80
88
96
104
112
120
128
136
144
152
160
168
176
184
192
200
208
216
224
232
240
Depth to sea
floor
(m)
+404
-1280
-1000
-33
-1000
-3000
-3915
-4555
-2000
-2791
-3000
-2791
-2658
-3000
-3200
-2584
-3100
-4200
-7100
-8200
-11022
-10000
-9000
-8000
-7000
-5900
-5100
-4588
-4200
-3840
-3800
Mariana Trench
The data table
shows the depth of
the deepest portion
of the Mariana
Trench. Data was
collected by the
Challenger II
research vessel as it
traveled East along
the 11oN latitude
line.
1. Shade in the sea floor with a brown map pencil on both graphs.
2. Shade in the ocean water with a blue map pencil on both graphs.
3. Using your knowledge of plate tectonics, reference maps and your textbook, label the
following on your graph:
Mid Atlantic Ridge
Ocean Name
2 plates on each side of
ridge
Kind of plate boundary
Mariana Trench
Ocean Name
2 plates on each side of
trench
Kind of plate boundary
Arrows to show direction
of plate movement
Arrows to show direction
of plate movement
Landforms: ridge, new
crust material, rift valley
Landforms: trench,
subduction zone
Conclusions:
Write two paragraphs. The first paragraph should explain the plate tectonics along the
Mid-Atlantic Ridge. The second paragraph should explain plate tectonics along the
Mariana Trench.
Application:
How are the two maps alike?
How are they different?
How does the process taking place on each map determine the topography?
Mid Atlantic Ridge
Latitude 39o N
1000
0
1000
2000
3000
4000
5000
6000
0
Sea level
16
0
200
5
0
0
800
1
0
5
0
1450
1
8
0
0
2000
2
5
0
0
2400
2
6
0
0
3000
3
2
0
0
3450
3
5
5
0
3600
3
7
5
0
3950
4
0
0
0
4100
4 4350
3
5
0
4
5
0
0
5000
5
3
0
0
5450
5600
Mariana Trench
Latitude 11o N
2000
1000
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
0
8
Sea level
16
24
32
40
48
56
64
72
80
88
96
104
112
120
128
136
144
152
160
168
176
184
192
200
208
216
224
232
240