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The Mountain-Building CycleMountain 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 PlatesWhen 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.

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.

What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you’ve read this lesson, reread the statements to see if you have changed your mind.

Before Statement After

5. Metamorphic rocks formed deep below Earth’s surface sometimes can be located near the tops of mountains.

6. Mountain ranges can form over long periods of time through repeated collisions between plates.

Key Concepts • How do mountains change

over time?• How do different types of

mountains form?

Make Flash Cards For each head in this lesson, write a question on one side of a flash card and the answer on the other side. Quiz yourself until you know all the answers.

Earth Dynamics

Mountain Building

LESSON 3

CHAPTER 8

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Reading Check 2. Identify Where do plates tend to break apart?

Collisions and RiftingContinents 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.

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.

WeatheringThe 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 UpliftOver 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.

Key Concept Check3. Describe How can mountains change over time?

Visual Check1. Locate Highlight the inactive plate boundary in the figure on the right.

Formation of a New Continent

Continent 1

Continent 2

Newcontinent

MantleMantle

Subductionzone

Reading Essentials Earth Dynamics 121

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Key Concept Check4. Specify How do folded and fault-block mountains form?

Types of MountainsYou 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 MountainsRocks 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.

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.

Fault-Block MountainsSometimes 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 MountainsThe 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.

ACADEMIC VOCABULARYperpendicular(adj) being at right angles to a line or plane

Make a three-tab book to describe how different types of mountains form. Provide an example of each type.

FoldedMountain

UpliftedMountain

Fault-blockMountain

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Visual Check5. Name What force is creating the uplift?

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 MountainsYou 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.

Key Concept Check7. Describe How do uplifted and volcanic mountains form?

An equation showing two equal ratios is a proportion. Some mountains in the Himalayas are rising 0.001 m/y. How long would it take the mountains to reach a height of 7,000 m?a. Set up a proportion.

0.001 m _______ 1y = 7,000 m _______ xy

b. Cross multiply.0.001x = 7,000

c. Divide both sides by 0.001.

0.001x ______ 0.001 = 7,000

d. Solve for x.x = 7,000,000 y

6. Use Proportions If the uplift rate of Mount Everest is 0.0006 m/y, how long did it take Mount Everest to reach a height of 8,848 m?

Math Skills

Formation of Uplifted Mountains

Crust

Lower

mantle

Upper

mantle

Uplifted mountain

Part of cold mantle

starts to detach.

Compression

causes uplift.

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