1
An area of the lithosphere can arch upward when, for example, it
is heated by material rising in the mantle beneath it. As the crust
stretches, it breaks into many blocks separated by faults.
2
As the lithosphere is pulled apart, some blocks tilt. The edges of
the blocks that tilt upward form mountains, and the edges that tilt
downward form valleys. Other blocks drop down between faults,
forming valleys. The edges of the blocks next to blocks that drop
down are left standing high above the valleys as mountains.
Fault-block mountains form as stress repeatedly builds up in the
crust and then is released during earthquakes. Even the most powerful
earthquakes can move blocks of rock only a few meters up or down at
one time. Fault-block mountains can be kilometers high. Millions of
years and countless earthquakes are needed for them to form.
Describe two ways that blocks of rock can move along faults and
form mountains.
Fault-Block Mountains
Fault-block mountains form as the crust stretches and breaks into
blocks that move along faults.
1
Stretching Begins
The crust breaks into blocks as it is stretched.
2
Blocks Tilt or Drop Down
As the crust is stretched more, the blocks
move along the normal faults between them.
This block has
dropped down.
This block
has tilted.
Chapter 8: Mountains and Volcanoes 259