BAChapter 3: Mountains and Volcanoes 77

VOCABULARYfolded mountain p. 80fault-block mountain p. 82

KEY CONCEPT

Movement of rock builds mountains.

EXPLORE Folding

How does rock fold?

PROCEDURE

Make three flat layers of clay on top of a sheet of newspaper. Put a block ateither end of the clay.

Hold one block still. Push on the other blockto slowly bring the blocks closer together.

WHAT DO YOU THINK?• What happened to the clay when you

pushed on the block?• What shape did the middle layer

of clay form?• If a large block of rock reacted to

pressure in a similar way, what kind of landform would result?

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MATERIALS• 2 or 3 colors of

modeling clay• ruler• 2 blocks• newspaper

Most mountains form along plate boundaries.

A shallow sea once covered the area that is now Mount Everest, Earth’stallest mountain. If you were to climb Mount Everest, you would bestanding on rocks containing the remains of ocean animals. MountEverest also contains rocks that formed far away at a spreading centeron the sea floor. How can rocks from the sea floor be on top of amountain on a continent? Plate tectonics provides the answer.

Recall that an oceanic plate sinks when it collides with a continentalplate. Some sea-floor material scrapes off the sinking plate and ontothe continent. As continental mountains form, material once at thebottom of an ocean can be pushed many kilometers high.

BEFORE, you learned

• Major geologic events occur attectonic plate boundaries

• Most faults are located alongplate boundaries

NOW, you will learn

• How the folding of rock canform mountains

• How movement along faultscan form mountains

Mountain Ranges and BeltsA mountain is an area of land that rises steeply from the land aroundit. A single mountain is rare. Most mountains belong to ranges—longlines of mountains that were formed at about the same time and bythe same processes. Ranges that are close together make up mountainbelts. For example, the Rocky Mountain belt in western North Americacontains about 100 ranges.

Most of the world’s major mountain belts are located alongtectonic plate boundaries. But mountain belts like the Appalachians(AP-uh-LAY-chee-uhnz) in eastern North America are in the interiorof plates. Mountains such as these were formed by ancient plate collisions that assembled the present-day continents.

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Major Mountain Belts

ATLANTIC OCEAN

ATLANTIC OCEAN

PACIFICOCEAN

PACIFICOCEAN

INDIANOCEAN

ROCKY M

OUNTAIN

BELT

Major mountain belts mark the locations of present or past plate boundaries.

Mountains rise highabove the land aroundthem.

Most mountains are ingroups called mountainranges.

Closely spaced mountainranges make up moun-tain belts.

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RockyMountainBelt

Urals

Appalachians

AndesTasmanBelt

NorthAmericanCordillera

CaledonianBelt

AlpsHimalayas

Never SummerMountain Range

MountCumulus

MountCumulus

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Mountains Wear Down

Mountains, Rocks, and SedimentAt the same time that some processes push mountains up, other processes wear them down. At Earth’s surface, water and wind breakrocks apart and move the pieces away. As long as mountains arepushed up faster than they wear down, they grow taller. For this reason, young mountains tend to be tall and steep. But eventuallymountain-building processes slow, then end. Water and wind takeover. Given enough time, all mountains become rounded hills, andthen they are gone. Countless mountains have formed and worn awaythroughout Earth’s long history.

Rocks break down into loose pieces that can be carried by water or wind. These pieces are called sediments. For example, sand on abeach is sediment. Thick layers of sediments can build up in low-lyingareas, such as valleys, lakes, or the ocean. Pieces of sediments form sedimentary rock as they are pressed together or joined by naturalcement.

The land becomes flatter as mountains wear down and valleys fillwith sediments. If tectonic plates were to stop moving, eventually thesurfaces of all the continents would be completely flat.

Mountains wear down as water and wind break theirrocks into sediments and carry them away.

How do mountains wear away?

Young Mountains

Most young mountains are rugged. But even as they form, their rocks are being broken apart.

Old Mountains

Most old mountains are rounded.Lower areas around them con-tain thick layers of sediments.

Chapter 3: Mountains and Volcanoes 79

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Mountains can form as rocks fold.Though people usually do not think of rocks as being able tobend and fold, they can. Think of a wax candle. If you benda candle quickly, it will break. If you leave a candle proppedup at an angle, over many days it will bend. If the candle isin a warm area, it will bend more quickly. Rocks also bendwhen stress is applied slowly. Rocks deep in the crust are athigh temperatures and pressures. They are particularly likelyto bend rather than break.

check your reading Under what conditions are rocks likely to bend and fold?

Remember that tectonic plates move only a few centimeters each year. The edge of a continent along a convergent boundary is subjected to stress for a very long time as another plate pushesagainst it. Some of the continent’s rocks break, and others fold.As folding continues, mountains are pushed up. A is a mountain that forms as continental crust crumples and bends into folds.

Folded mountains form as an oceanic plate sinks under the edgeof a continent or as continents collide. One example is the Himalaya(HIHM-uh-LAY-uh) belt, which formed by a collision between Indiaand Eurasia. Its formation is illustrated on page 81.

Convergent Boundary Develops At one time an ocean separatedIndia and Eurasia. As India moved northward, oceanic lithospheresank in a newly formed subduction zone along the Eurasian Plate.Along the edge of Eurasia, folded mountains formed. Volcanoesalso formed as magma rose from the subduction zone to the surface.

Continental Collision Begins Eventually the sea floor was completely destroyed, and India and Eurasia collided. Subductionended. The volcanoes stopped erupting because they were no longersupplied with magma. Sea-floor material that had been added tothe edge of Eurasia became part of the mountains pushed up bythe collision.

Collision Continues India and Eurasia continue to push together.Their collision has formed the Himalayas, the world’s tallestmountains. They grow even higher as rock is folded and pushed upfor hundreds of kilometers on either side of the collision boundary.

Earthquakes can also be important to the upward growth of foldedmountains. A great deal of rock in the Himalaya belt has been pushedup along reverse faults, which are common at convergent boundaries.

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folded mountain

80 Unit: The Changing Earth

reading tip

Eurasia is the landmassconsisting of Europe and Asia.

VOCABULARYMake a word triangle for folded mountainin your notebook.

Formation of Himalayas

The Himalayas are being pushed higher by an ongoingcontinental collision.

In each illustration, where is the boundary between India and Eurasia?

Convergent Boundary Develops1

Continental Collision Begins2

The Collision Continues3

Chapter 3: Mountains and Volcanoes 81

As India began moving toward Eurasia 200 million years ago, aconvergent boundary developed along the edge of Eurasia. Theoceanic lithosphere between the two continents sank into a subduction zone.

Currently, the Himalayas are growing more than one centimeterhigher each year.

The sea floor was completely destroyed about 50 million yearsago, and India and Eurasia collided.

Himalayas

Volcanoes formed as magmarose from the subductionzone to the surface.

Subduction stopped afterthe continents collided. Nomore magma formed.

A remnant of sea floor crustremains deep under themountains.

Folded moun-tains formed asoceanic andcontinentalplates pushedtogether.

Crust along theedges of bothcontinents wascrumpled andfolded intomountains.

As the collisioncontinues, thecrust keepsfolding. Also,earthquakesare common.

India

Eurasia

Himalayas

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Mountains can form as rocks move along faults.

In the southwestern United States and northwestern Mexico, hundreds of mountain ranges line up in rows. The ranges, as well as the valleysbetween them, formed along nearly parallel normal faults. Mountainsthat form as blocks of rock move up or down along normal faults arecalled

check your reading How can the movement of rocks along faults lead to the formation of mountains?

Fault-block mountains form as the lithosphere is stretched andpulled apart by forces within Earth. The rocks of the crust are cooland rigid. As the lithosphere begins to stretch, the crust breaks intolarge blocks. As stretching continues, the blocks of rock move alongthe faults that separate them. The illustrations on page 83 show howthis process forms fault-block mountains.

fault-block mountains.

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CONTENT FRAMEAdd information aboutfault-block mountains toyour content frame.

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 demonstratethe processes that form fault-block mountains.

PROCEDURE

Use the triangular blocks to demonstrate howmovements along normal faults form twomountains separated by a valley. Startwith the blocks arranged as shown.Move the outer blocks apart to formtwo mountains separated by a valley. Draw a diagram of your results.

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?

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Fault-Block MountainsFault-Block MountainsSKILL FOCUSModeling

MATERIALS• 3 triangular

blocks• 3 rectangular

blocks

TIME15 minutes

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Fault-Block Mountains

An area of the lithosphere can arch upward when, for example, it is heated by material rising in the mantle beneath it. As the cruststretches, it breaks into many blocks separated by faults.

As the lithosphere is pulled apart, some blocks tilt. The edges ofthe blocks that tilt upward form mountains, and the edges that tiltdownward form valleys. Other blocks drop down between faults,forming valleys. The edges of the blocks next to blocks that dropdown are left standing high above the valleys as mountains.

Fault-block mountains form as stress repeatedly builds up in thecrust and then is released during earthquakes. Even the most powerfulearthquakes can move blocks of rock only a few meters up or down atone time. Fault-block mountains can be kilometers high. Millions ofyears and countless earthquakes are needed for them to form.

check your reading Describe two ways that blocks of rock can move along faults andform mountains.

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Chapter 3: Mountains and Volcanoes 83

Fault-block mountains form as the crust stretches and breaks intoblocks that move along faults.

Stretching Begins1

The crust breaks into blocks as it is stretched.

Blocks Tilt or Drop Down2

As the crust is stretched more, the blocks move along the normal faults between them.

This block hasdropped down.

This block has tilted.

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The Sierra Nevada in California is a fault-block mountain range.The range moved up along a normal fault along its eastern edge. Theblock on the other side of the fault dropped down. This combinationof upward and downward movement formed the steep eastern side ofthe Sierra Nevada. The western side of the range tilts down gentlytoward California’s Central Valley.

In summary, both folded mountains and fault-block mountainsform over millions of years. Folded mountains are pushed up by slow,continual stress that causes rock to gradually bend. Fault-block moun-tains form, earthquake by earthquake, as stress built up in the crust isreleased by the movement of rock. Folded mountains form wherecontinental crust is being compressed, and fault-block mountainsform where it is being stretched.

KEY CONCEPTS1. How is the formation of

mountain belts related to tectonic plate boundaries?

2. How do folded mountainsform?

3. How do fault-block mountainsform?

CRITICAL THINKING4. Analyze The Ural Mountain

belt is no longer along the edgeof a tectonic plate. Would youexpect the Urals to be tall andsteep or low and rounded?Why?

5. Synthesize How could it bepossible for a mountain rangeto be continually pushed upbut not get any higher?

CHALLENGE6. Analyze This graph shows

how the heights of two moun-tains changed as they formed.Which line shows the forma-tion of a folded mountain? afault-block mountain? Explain.

84 Unit: The Changing Earth

Time

Hei

ght

The Sierra Nevadamoved up along oneside of the fault.

Approximate location of fault

The land on theother side of thefault dropped down.