Earth Dynamics
Mountain Building
The Mountain-Building Cycle
Mountain ranges form slowly, and they change slowly.
Mountains are the result of many different plate collisions
over millions of years. Because of the different plate
collisions, mountains are made of many different types of
rocks. The processes of weathering and erosion can remove
part or all of a mountain.
Converging Plates
When plates collide at a plate boundary, folds, faults, and
uplift combine to form mountains. After millions of years,
the forces that first caused the plates to move together can
become inactive. Two old continents form a single new
continent, and the plate boundary becomes inactive.
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Because there is no compression at the convergent plate
boundary, the mountains no longer increase in size. The
formation of a new continent from two existing continents
is shown in the figure at the top of the next page.
Formation of a New Continent
New
continent
Continent 1
Subduction
zone
Continent 2
Mantle
Mantle
Collisions and Rifting
Continents are always changing because Earth’s tectonic
plates are always moving. When continents split at a
divergent plate boundary, they often break close to the place
where they first collided. First a large split, or rift, forms. The
rift grows, and seawater flows into it. In time, an ocean
forms.
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Eventually plate motion changes again, and the continents
will collide. New mountain ranges form on top of or next to
older mountain ranges. The cycle of repeated collisions and
rifting can create old and complicated mountain ranges,
such as the Appalachian Mountains. Rocks that make up
mountain ranges record the history of plate motion and
collisions that formed the mountains.
Weathering
The Appalachian Mountains are an old mountain range
that stretches along most of the eastern United States. They
are not as high or as rugged as the Rocky Mountains in
the west because they are much older. They are no longer
growing. Weathering has rounded the peaks and lowered
the elevations.
Erosion and Uplift
Over time, natural processes wear away mountains, smooth
their peaks, and reduce their height. Even so, some mountain
ranges are hundreds of millions of years old. Why don’t the
ranges completely disappear?
Recall how isostasy works. As a mountain erodes, the crust
under it must rise to restore the balance between what is left
of the mountain above the surface and the root that floats
within the mantle. Therefore, rocks deep under continents
rise slowly toward Earth’s surface. In old mountain ranges,
metamorphic rocks that formed deep below the surface
became exposed on the top of mountains. When the material
above the rocks eroded, the rocks rose due to isostasy.
Types of Mountains
You learned in the first lesson that stresses caused by
plate movement can pull or compress crust. This is one way
that plate motion forms many types of mountains. But the
effects of plate movement help change the positions of
rocks. Plate movement can also change the rocks within a
mountain range.
Folded Mountains
Rocks that are deeper in the crust are warmer than rocks
closer to Earth’s surface. Deeper rocks are also under much
more pressure. When rocks are hot enough or under enough
pressure, folds form instead of faults. Folded mountains are
made of layers of rocks that are folded. Folded mountains form as
continental plates collide, folding and uplifting layers of
rock. When erosion removes the upper part of the crust,
folds are exposed on the surface.
Fault-Block Mountains
Sometimes tension stresses within a continent form
mountains. As tension pulls crust apart, faults form. At the
faults, some blocks of crust fall and others rise. Fault-block
mountains are parallel ridges that form where blocks of crust move up
or down along faults.
A fault-block mountain might appear as a high, craggy
ridge next to a valley. Somewhere between the two is a fault
where huge movement once occurred. The parallel ridges of
fault-block mountains run perpendicular to the direction of
the stress. If the ridges run north to south, then the tension
that created them pulled east and west.
Uplifted Mountains
The granite on top of the Sierra Nevada’s Mount Whitney
was once 10 km below Earth’s surface. Now it is on top of
a 4,400-m-tall mountain! How did this happen? Mount
Whitney is an uplifted mountain. When large regions rise
vertically with very little deformation, uplifted mountains form.
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The arrangement of the folds is not accidental. You can
show this with a piece of paper. Gently push the ends of a
sheet of paper toward one another to form a fold. The fold is
a long ridge that is perpendicular to the direction in which
you pushed. Folded mountains are similar. The folds are
perpendicular to the direction of the compression that
created them. Like your paper, when folds run up and down
in a mountain, the compression must have come from
the sides.
Formation of Uplifted Mountains
Compression
causes uplift.
Uplifted mountain
Crust
Upper
mantle
Part of cold mantle
starts to detach.
Lower
mantle
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The rocks in the Sierra Nevada are made of granite.
Granite is an igneous rock that forms several kilometers
below Earth’s surface. Uplift and erosion have exposed it.
Scientists do not fully understand how uplifted mountains
form. One hypothesis is shown in the figure above. It
proposes that cold mantle under the crust breaks loose or
detaches from the crust. It then sinks deeper into the mantle.
The sinking mantle pulls the crust and causes compression
closer to the surface. As the crust thickens due to the
compression, the upper part of the crust rises as it maintains
isostasy. Sometimes it rises high enough to form huge
mountain ranges. Geologists are designing experiments to test
this hypothesis.
Volcanic Mountains
You might not think of volcanoes as mountains, but
scientists consider volcanoes to be special types of mountains.
In fact, volcanic eruptions formed some of the largest
mountains on Earth. As molten rock and ash erupt onto
Earth’s surface, they harden. Over time, many eruptions can
build huge volcanic mountains such as the ones that make
up the Hawaiian Islands.
Not all volcanic mountains erupt all the time. Some
volcanic mountains are dormant. This means that they might
erupt again someday. Some volcanic mountains will never
erupt again.