Download Fault-Block Mountains

Mountains can form as rocks move
along faults.
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In the southwestern United States and northwestern Mexico, hundreds
of mountain ranges line up in rows. The ranges, as well as the valleys
between them, formed along nearly parallel normal faults. Mountains
that form as blocks of rock move up or down along normal faults are
called fault-block mountains.
How can the movement of rocks along faults lead to the
formation of mountains?
Fault-block mountains form as the lithosphere is stretched and
pulled apart by forces within Earth. The rocks of the crust are cool
and rigid. As the lithosphere begins to stretch, the crust breaks into
large blocks. As stretching continues, the blocks of rock move along
the faults that separate them. The illustrations on page 259 show how
this process forms fault-block mountains.
Fault-Block Mountains
How do fault-block mountains form?
Fault-block mountains form along normal faults as blocks of continental
crust are pulled apart. In this activity, you will use wooden blocks to demonstrate
the processes that form fault-block mountains.
PROCEDURE
1
Use the triangular blocks to demonstrate how
movements along normal faults form two
mountains separated by a valley. Start
with the blocks arranged as shown.
Move the outer blocks apart to form
two mountains separated by a valley. Draw a diagram of your results.
2 Use the rectangular blocks to demonstrate how a row of tilted
fault-block mountains forms along normal faults. (Hint: You can
tilt the blocks as they move.) Draw a diagram of your results.
WHAT DO YOU THINK?
• How do your diagrams show that fault-block mountains form as
the crust is being stretched?
• Along which type of plate boundary would fault-block mountains
be most likely to form—divergent, convergent,
or transform? Explain.
CHALLENGE Why do fault-block mountains not form at strike-slip faults?
258 Unit 2: The Changing Earth
SKILL FOCUS
Modeling
MATERIALS
• 3 triangular
blocks
• 3 rectangular
blocks
TIME
15 minutes