Laboratory Manual for Introductory Geology 4e

EXERCISE 2.7
A Tale of Two Ridges (continued)
Name:
Course:
Section:
Date:
(a) Mid-ocean ridges typically have a rift valley at their axes—a valley created when two continents split apart. Which
of the two ridges has the deepest and longest rift valley?
Different spreading rates cause variations in the shape of the ocean floor because of the way the cooling oceanic lithosphere
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 “floats” 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
red pencil and those for the Mid-Atlantic Ridge with green pencil to make a cross section of each ridge.
22,500
Depth (m)
23,000
23,500
24,000
24,500
1,000 500 0
500
1,000
Distance from ridge axis (km)
(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
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.
2.5 PROCESSES AT PLATE BOUNDARIES REVEALED BY EARTH FEATURES
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