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EXERCISE 2.7
EXERCISE 2.7
A Tale of Two Ridges (continued)
A Tale of Two Ridges
Name:
Name:_
Section:
Section:
Course:
Date:
Course:
Date
The figure below shows bathymetric maps of parts of the East Pacific Rise and Mid-Atlantic Ridge at the same scale.
(a) Mid-ocean ridges typically have a rift valley at their axes-a valley created when two continents split apart. Which
Examine these ocean ridge segments and answer the following questions:
of the two ridges has the deepest and longest rift valley?
Meters
2000
10
-2000
4000
Different spreading rates cause variations in the shape of the ocean floor because of the way the cooling oceanic litho-
-6000
sphere behaves. Lithosphere near the ridge axis is young, thin, and still hot, and therefore has a lower density than older,
colder lithosphere far from the axis. As a result, the ridge-axis area &quot;floats&quot; relatively high on the underlying asthenosphere,
and the water above it is relatively shallow. As seafloor spreading moves the lithosphere away from the ridge axis, the rocks
cool and get thicker and denser. The farther it is from the ridge axis, the lower the lithosphere sits on the asthenosphere,
and the deeper the water will be. This concept is known as the age-versus-depth relationship.
(b) Keeping in mind the age-versus-depth relationship, why is the belt of shallow sea wider over the East Pacific Rise
than over the Mid-Atlantic Ridge?
(c) On the graph provided, plot ocean depth (on the vertical axis) against distance from the ridge axis (on the
horizontal axis) for both the East Pacific Rise and the Mid-Atlantic Ridge. Use five to ten points for each ridge,
marking the points you use on the bathymetric maps on page 38. Connect the points for the East Pacific Rise with
500 km
red pencil and those for the Mid-Atlantic Ridge with green pencil to make a cross section of each ridge.
-2,500
(a) The East Pacific Rise in the South Pacific Ocean.
-3,000
3, 500
Meters
-4,000
2000
-4,500
10
1,000
500
0
500
1,000
-2000
-4000
Distance from ridge axis (km)
6000
(d) Does the rate at which depth increases with distance from the ridge stay the same over time, decrease over time, or
increase over time?
2.5.2 Continental Rifting
A new ocean forms when a divergent boundary develops beneath a continent. The
continental crust first thins, then breaks into two pieces separated by a mid-ocean
500 km
ridge. As seafloor spreading proceeds, an ocean basin grows between the fragments
of the original continent. This process, called continental rifting, is in an early stage
today in eastern Africa, as you will see in Exercise 2.8.
(b) The Mid-Atlantic Ridge off South America.
(continued)
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEATURES
39

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