308 Chapter 11

11.1 Rock Deformation

Reading StrategyComparing and Contrasting After youread the section, compare types of faults bycompleting the table below.

Key ConceptsWhat determines thestrength of a rock?

What are the types ofstresses that affect rocks?

What are the three maintypes of folds?

What are the main typesof faults?

Vocabulary◆ deformation◆ stress◆ strain◆ anticline◆ syncline◆ monocline◆ normal fault◆ reverse fault◆ thrust fault◆ strike-slip fault

Mountains, like those shown in Figure 1, provide some of the mostspectacular scenery on our planet. It is theorized that all continentswere once mountainous masses and grow by the addition of mountainsto their edges. As geologists unravel the secrets of mountain formation,they also gain a deeper understanding of the evolution of Earth’s con-tinents. However, if continents do grow by adding mountains to theiredges, then how do mountains exist in the interior of continents?

Factors Affecting DeformationEvery body of rock, no matter how strong, has a point at which it willbend or break. Deformation is a general term that refers to all changes inthe original shape and/or size of a rock body. Most crustal deformationoccurs along plate margins. Plate motions and interactions at plateboundaries create forces that cause rock to deform.

Stress is the force per unit area acting on a solid. When rocks areunder stresses greater than their own strength, they begin to deform,usually by folding, flowing, or fracturing. The change in shape orvolume of a body of rock as a result of stress is called strain. How canrock masses be bent into folds without being broken? When stress isgradually applied, rocks first respond by deforming elastically. Changesthat result from elastic deformation are recoverable. Like a rubberband, the rock will return to almost its original size and shape oncethe force is removed. Once the elastic limit or strength of a rock is sur-passed, it either flows or fractures. The factors that influence thestrength of a rock and how it will deform include temperature, con-fining pressure, rock type, and time.

Figure 1 Mountain Ranges Thispeak is part of the KarakoramRange in Pakistan.

Types of Fault Description

Normal fault a.

b. c.

d. e.

f. g. ??

??

??

?

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FOCUS

Section Objectives11.1 Identify the factors that

determine the strength of arock and how it will deform.

11.2 Explain how rockspermanently deform.

11.3 Distinguish among the typesof stresses that affect rocks.

11.4 List the three main types offolds and identify the maintypes of faults.

Build VocabularyWord Parts The terms anticline andsyncline have the same Greek root word,klinein, meaning “to lean.” Anti- means“opposite” or “against.” In an anticline,the layers bend downward in oppositedirections from the crest. Syn- is a Greekprefix meaning “together with,” so asyncline has layers that dip toward eachother.

Reading Strategya. hanging wall block moves downrelative to footwall block; high anglefaultb. Reverse faultc. hanging wall block moves up relativeto footwall block; high angle faultd. Thrust faulte. hanging wall block moves up andover the footwall block; low angle faultf. Strike-slip faultg. movement is horizontal and parallelto the trend of the fault surface; usuallyconsists of a zone of roughly parallelfractures

INSTRUCT

Factors AffectingDeformationIntegrate PhysicsForce Remind students that force isdefined as a push or pull exerted onan object. A force has magnitude; forexample, you can push hard or gentlyon an object. Force also has direction;you can push on an object to the left,to the right, up, or down.Logical

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Temperature and Pressure Rocks deform permanentlyin two ways: brittle deformation and ductile deformation. Rocksnear the surface, where temperatures and confining pressures are low,usually behave like brittle solids and fracture once their strength isexceeded. This type of deformation is called brittle failure or brittledeformation. You know that glass objects, wooden pencils, china plates,and even our bones show brittle failure once their strength is exceeded.

At depth, where temperatures and confining pressures are high,rocks show ductile behavior. Ductile deformation is a type of solid-state flow that produces a change in the size and shape of an objectwithout fracturing the object. Objects that display ductile behaviorinclude modeling clay, bee’s wax, caramel candy, and most metals. Forexample, a copper penny placed on a railroad track will be flattenedand deformed without breaking by the force applied by a passing train.Ductile deformation of a rock that is strongly aided by high tempera-ture and high confining pressure is somewhat similar to thedeformation of a penny flattened by a train.

Rock Type The mineral composition and texture of a rock alsogreatly affect how it will deform. Rocks like granite and basalt that arecomposed of minerals with strong internal molecular bonds usuallyfail by brittle fracture. Sedimentary rocks that are weakly cemented ormetamorphic rocks that contain zones of weakness—such as folia-tion—are more likely to deform by ductile flow. Rocks that are weakand most likely behave in a ductile manner when under force includerock salt, gypsum, and shale. Limestone, schist, and marble are of inter-mediate strength and may also behave in a ductile manner.

Time In nature small stresses applied over long time spans play animportant role in the deformation of rock.You can see the effects of timeon deformation in everyday settings. For example, marble benches havebeen known to sag under their own weight over a span of a hundredyears or so. Forces that are unable to deform rock when firstapplied may cause rock to flow if the force is maintained over a longperiod of time.

Types of StressRocks are exposed to many different forces due to plate motions.

The three types of stresses that rocks commonly undergo aretensional stress, compressional stress, and shear stress. Look atFigure 2. When rocks are squeezed or shortened the stress is compres-sional. Tensional stress is caused by rocks being pulled in oppositedirections. Shear stress causes a body of rock to be distorted.

What is brittle deformation?

Unstressed

Compressionalstress

Tensional stress

Shear stress

Figure 2 Undeformed material ischanged as it undergoes differenttypes of stress. The arrows showthe direction of maximum stress.A Compressional stress causes amaterial to shorten. B Tensionalstress causes a material to bestretched or to undergoextension. C Shear stress causes amaterial to be distorted with nochange in volume.

A

B

C

D

Types of Stress

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Build Reading LiteracyRefer to p. 306D, which provides theguidelines for KWL (Know-Want toKnow-Learned).

KWL Have students make a KWL chartcontaining three columns entitled“What I Know,” “What I Want to Know,”and “What I Learned.” Before readingthis section, have students fill in the firstcolumn with what they know about thefactors affecting deformation of rocks.They should have prior knowledgeabout rocks from Chapters 2 and 3. Thesecond column should be filled out asstudents read pp. 308 and 309. Finally,students should fill in the third columnafter they have finished reading thissection. The material in this columncan take the form of an outline of thematerial under the head FactorsAffecting Deformation.Verbal, Logical

Types of StressUse VisualsFigure 2 Have students examine thediagrams. Ask: What happens to theshape of a rock that undergoescompressional stress? (It shortens.)What happens to the shape of a rockthat undergoes tensional stress? (Itstretches or extends.) What happens tothe shape of a rock that undergoesshear stress? (The rock becomesdistorted.)Visual

Build Science SkillsUsing Models Havestudents model thethree kinds of stress bypulling or pushing on amarshmallow or piece of foam rubber.The material can be cut in half to modelshear stress. Students will be able toobserve how shape changes withtensional and compressional stress andhow the shape becomes distorted withshear stress.Visual, Kinesthetic

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Answer to . . .

Brittle deformation isdeformation where the

strength of a material is exceeded, andthe material breaks or fractures.

Customize for Inclusion Students

Visually Impaired Many of the conceptsin this section can be modeled for visuallyimpaired students. Faults can be representedby using blocks of wood. Folds can be modeledusing construction paper or sheets of flexiblefoam rubber. Types of stress can be modeled

using marshmallows or foam rubber, as in theBuild Science Skills activity on this page. Rockdeformation can be illustrated by using a stickof chewing gum. When a stick of gum is cold,an applied stress causes it to crack and break.When the gum is warm, it will bend easily.

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FoldsDuring mountain building, flat-lying sedimentary and volcanic rocksare often bent into a series of wavelike ripples called folds. Folds in sed-imentary strata are much like those that would form if you were to holdthe ends of a sheet of paper and then push them together. In nature,folds come in a wide variety of sizes and shapes. The three maintypes of folds are anticlines, synclines, and monoclines.

Anticlines The two most common types of folds are anticlines andsynclines. An anticline is most commonly formed by the upfolding,or arching, of rock layers, as shown in Figure 3.

Synclines Often found in association with anticlines are down-folds, or troughs, called synclines. Notice in Figure 3 that the limb ofan anticline is also a limb of the adjacent syncline. Folds do not con-tinue forever. Instead their ends die out much like the wrinkles in cloth.

Monoclines Although we will discuss folds and faults separately,in the real world folds are generally closely associated with faults.Examples of this close association are broad, regional features calledmonoclines. Monoclines are large, step-like folds in otherwise hori-zontal sedimentary strata. Monoclines seem to occur as sedimentarylayers have been folded over a large faulted block of underlying rock.Monoclines are prominent features of the Colorado Plateau area inColorado, New Mexico, Utah, and Arizona, as shown in Figure 4 onthe next page.

What is a syncline?

Normal limb

Overturnedlimb

Symmetrical fold Asymmetrical fold Overturned fold

Anticline Anticline

Syncline Syncline

Figure 3 Anticlines andSynclines The upfolded orarched structures are anticlines.The downfolds or troughs aresynclines. Notice that the limb ofan anticline is also the limb of theadjacent syncline.

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FoldsUse VisualsFigure 3 Have students examine thediagram of the types of folds. Ask:Where would you expect to find theoldest rock layer in an anticline? (inthe center of the fold) Where would youexpect to find the oldest rock layer ina syncline? (on the outside of the fold)How would you describe a symmetricalfold? (a fold in which the two sides aremirror images) How would you describean overturned fold? (a fold in which onelimb is tilted beyond the vertical)Visual, Logical

Making an AnticlinePurpose Students observe how ananticline is produced.

Materials stack of construction papersheets in several colors

Procedure Make a stack containingseveral sheets of colored constructionpaper. Each colored sheet will representa rock layer. Lay the stack on a table.Place your two hands on the stack, oneat each end of the stack. Press down onthe paper with your hands and slowlypush them together.

Expected Outcomes The entire stackof paper will form an anticline. Rocklayers will be visible when viewed fromthe side.Kinesthetic, Visual

Students may have trouble rememberingthe direction in which an anticline and asyncline fold. The terms were namedaccording to the direction of the limbsin relation to the axis of the fold.Remind students that anti- means“opposite” or “against.” In an anticline,the layers bend downward in oppositedirections from the crest. The prefix syn-means “together with,” so a synclinehas layers that dip toward each other.Students can remember the directionof these folds if they think of an antclimbing up a hill. The word anticlinecontains the word ant.

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Section 11.1 (continued)

The Colorado Plateau is a high, sparselyvegetated region of deep canyons, mesas, andplateaus. It encompasses almost 363,000 sq kmat the four corners—the area where Utah,Colorado, Arizona, and New Mexico cometogether. The Colorado Plateau includes theColorado River and its tributaries. The highSierra Nevada mountain range located to thewest of the Plateau prevents moisture-laden

air masses from reaching the region. This rainshadow effect causes the region to be verydry; the average precipitation is about 25 cmper year. Because plant cover is so sparse,the area has been eroded by fast-movingstreams, which has exposed the bare rocksthat contribute to the area’s beauty.

Facts and Figures

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Recall that faults are fractures in the crust alongwhich movement has taken place. Small faults can be recog-nized in road cuts where sedimentary beds have been offset a fewmeters, as shown in Figure 5. Faults of this size usually occur as singlebreaks. By contrast, large faults, like the San Andreas fault in California,have displacements of hundreds of kilometers and consist of manyinterconnecting fault surfaces. These fault zones can be many kilome-ters wide and are often easier to identify from high-altitudephotographs than at ground level.

The rock surface that is immediately above the fault is commonlycalled the hanging wall, and the rock surface below the fault is calledthe footwall. The major types of faultsare normal faults, reverse faults, thrustfaults, and strike-slip faults.

Normal Faults A normal fault occurswhen the hanging wall block moves downrelative to the footwall block. Most normalfaults have steep dips of about 60°, as shownin Figure 6A on the next page. These dipsoften flatten out with depth. The movementin normal faults is mainly in a vertical direc-tion, with some horizontal movement.Because of the downward motion of thehanging wall block, normal faults result inthe lengthening, or extension, of the crust.

Figure 4 Monocline A Monocline located nearMexican Hat, Utah. B Thismonocline consists of bentsedimentary beds that weredeformed by faulting in thebedrock below.

B

Figure 5 Normal Fault Faultingcaused the vertical displacementof these beds located near Kanab,Utah. Arrows show the relativemotion of rock units. Observing Which side of thefault is the hanging wall?

A

Build Science SkillsUsing Models Havestudents model theformation of amonocline by holdingtwo wooden blocks together so the“fault” between them runs at 60° angle.Have another student carefully drapethe towel or fabric over the woodenblocks. The student holding the blocksshould slide them along the “fault” untilthe towel forms a fold. Ask: What doesthe towel represent? (sedimentary rocklayers) What do the wooden blocksrepresent? (rock blocks)Kinesthetic, Logical

Use VisualsFigure 5 Have students examine thephotograph of a normal fault. Ask:Which side of the fault is the footwall?(the right side) How can you tell whichside is the footwall? (A normal faultoccurs when the hanging wall block movesdown relative to the footwall. In thepicture, the right side is higher than theleft side, so it must be the footwall.) Canyou tell from the photo which side ofthe fault has moved? (No, there is noway to tell. The two sides of the fault havemoved relative to each other but it isimpossible to know whether one side hasmoved up or the other side has moveddown.)Visual, Logical

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Answer to . . .

Figure 5 The hanging wall is on theleft side.

A syncline is a down foldor trough.

The terms hanging wall and footwall werecoined by prospectors and miners whoexcavated shafts and tunnels along fault zonesbecause these are frequent sites of ore

deposits. In these tunnels, the miners wouldwalk on the rocks below the mineralized faultzone (the footwall) and hang their lanterns onthe rocks above (the hanging wall).

Facts and Figures

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Reverse Faults and Thrust Faults A reverse fault is a faultin which the hanging wall block moves up relative to the footwall block.Reverse faults are high-angle faults with dips greater than 45°. Thrustfaults are reverse faults with dips of less than 45°. Because the hangingwall block moves up and over the footwall block, reverse and thrustfaults result in a shortening of the crust, as shown in Figure 6B and 6C.

Most high-angle reverse faults are small. They cause only local dis-placements in regions dominated by other types of faulting. Thrustfaults, on the other hand, exist at all scales. In mountainous regionssuch as the Alps, northern Rockies, Himalayas, and Appalachians,thrust faults have displaced rock layers as far as 50 kilometers over adja-cent rocks. The result of this large-scale movement is that older rocksend up on top of younger rocks.

Normal faults occur due to tensional stresses, and reverse andthrust faults result from compressional stresses. Compressional forcesgenerally produce folds as well as faults. These compressional forcesresult in a thickening and shortening of the rocks.

What are the major types of faults?

Q How do you determine whichside of a fault has moved?

A For the fault shown inFigure 5, did the left side movedown, or did the right sidemove up? Since the surface atthe top of the photo has beeneroded flat, either side couldhave moved, or both sidescould have moved, with oneside moving more than theother. That’s why geologiststalk about relative motion acrossfaults. In this case, the left sidemoved down relative to the rightside, and the right side movedup relative to the left side.

FootwallHanging

wall

Footwall

Hanging wall

Footwall

Hangingwall

Normal fault (tensional)Reverse fault (compressional)

Thrust fault (compressional) Strike-slip fault (shear)

AB

C D

Figure 6 A Normal fault B Reverse fault C Thrust fault D Strike-slip fault Interpreting Diagrams Whichtype of fault would causeextension in an area?

Four Types of Faults

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Use VisualsFigure 6 Have students examine thediagrams. Tell them to imagine twohouses located side by side. Ask: Willthe houses be closer together orfarther apart if a normal fault formsbetween them? Why? (They will befarther apart because normal faults aretensional.) Will the houses be closertogether or farther apart if a reversefault forms between them? Why?(They will be closer together becausereverse faults are compressional.) Will thehouses be closer together or fartherapart if a thrust fault forms betweenthem? Why? (They’ll be closer togetherbecause thrust faults are compressional.)What will be the relative location ofthe houses if a strike-slip fault formsbetween them? (The houses will movefarther apart, but they will no longer beside by side. One house will be somewhatbehind the other.)Visual, Logical

Build Reading LiteracyRefer to p. 246D in Chapter 9, whichprovides the guidelines for relatingcause and effect.

Relate Cause and Effect Ask: Whattype of faulting would you expect tofind in an area where continentalplates are diverging? (a normal fault)Why would you find this type of fault?(When continental plates diverge, thecrust is pulled apart. This creates tensionalforces that stretch or expand the crust.)What type of faulting would youexpect to find in an area where platesare subducting or colliding? Why?(a reverse fault or a thrust fault becauseplate subduction and collision causecompressional forces, which cause thecrust to shorten)Logical

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Section 11.1 Assessment

Reviewing Concepts1. What factors determine the strength of a

rock?

2. In what ways do rocks deform? Explain thedifferences in these deformations.

3. Describe the different types of stress.

4. List the three types of folds.

5. Explain the direction of movement in thefour types of faults.

Critical Thinking6. Inferring What type of deformation would a

rock in the lower part of the mantle be morelikely to undergo? Explain.

7. Comparing and Contrasting How is ananticline different from a syncline?

8. Applying Concepts What type of faultsshould be most common at a spreading oceanridge? Explain.

Strike-Slip Faults Faults in which the move-ment is horizontal and parallel to the trend, or strike,of the fault surface are called strike-slip faults, asshown in Figure 6D. Because of their large size andlinear nature, many strike-slip faults produce a tracethat is visible over a great distance. Rather than asingle fracture, large strike-slip faults usually consist ofa zone of roughly parallel fractures. The zone may beup to several kilometers wide. The most recent move-ment, however, is often along a section only a fewmeters wide, which may offset features such as streamchannels. Crushed and broken rocks produced duringfaulting are more easily eroded, often producing linearvalleys or troughs that mark the locations of strike-slip faults. Scientific records of strike-slip faulting weremade following surface ruptures that produced largeearthquakes. Strike-slip faults are commonly causedby shear stress. The San Andreas fault in Californiaand the Great Glen fault in Scotland are well-knownexamples of strike-slip faults.

Joints Among the most common rock structuresare fractures called joints. Unlike faults, joints arefractures along which no appreciable movement has occurred.Although some joints have a random orientation, most occur inroughly parallel groups, as shown in Figure 7. Joints usually form as theresult of large-scale regional stresses.

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Compressional Stress Review the typesof plate boundaries in Chapter 9. At whichtype of boundary would compressionalstresses be the dominant force?

Figure 7 Joints These joints arefound in Arches National Park,near Moab, Utah. The joints inthe sandstone stand out becausechemical weathering is enhancedalong them.

It is commonly believed that Californiais in danger of sliding into the PacificOcean due to the movement of the SanAndreas Fault. Have students examinethe map on p. 325 and note the generaldirection of the fault. (northwest) Remindthem that the San Andreas Fault is astrike-slip fault. Ask: Is it possible thatthe San Andreas Fault will causeCalifornia to slide into the ocean?Explain. (No, the movement in strike-slipfaults is horizontal and parallel to thetrend, so the southern California coastis moving north, not west.)Visual, Logical

ASSESSEvaluateUnderstandingTo assess students’ knowledge of thesection content, ask: How does a jointdiffer from a fault? (A fault is a fracturealong which movement has occurred. Ajoint is a fracture along which no movementhas occurred.) How does a reverse faultdiffer from a thrust fault? (A thrustfault is a reverse fault with a dip of lessthan 45°. A reverse fault usually has a dipgreater than 45°.)

ReteachHave students make a table listing thefour factors that influence the strengthof a rock. For each factor, studentsshould write a brief description of howthat factor influences rock deformation.

Compressional forces are dominant atconvergent plate boundaries.

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Answer to . . .

Figure 6 A normal fault causesextension or stretching in an area.

normal, reverse, thrust,and strike-slip faultsthe rock. Shear stress is a force that causes a

rock body to be distorted.4. anticlines, synclines, and monoclines5. In normal faults, the hanging wall movesdown relative to the footwall and the move-ment is mainly vertical. In reverse and thrustfaults, the hanging wall moves up relative tothe footwall. In reverse faults the movementis mainly vertical; in thrust faults, the move-ment is mainly horizontal. Strike-slip faultsmove parallel to the trend of the fault, andthe movement is mainly horizontal.

Section 11.1 Assessment

1. temperature, confining pressure, rocktype, and time2. brittle deformation which causes an objectto fracture, and ductile deformation, whichchanges the shape and size of the objectwithout fracturing it3. Compressional stress is a force that com-presses, shortens, or squeezes a rock.Tensional stress is a force that pulls a rockapart, causing extension or lengthening of

6. ductile deformation because of high tem-peratures and pressures in the lower mantle7. Anticline: Rock layers are folded upwardsto form an arch. Syncline: Rock layers arefolded downwards to form a trough.8. Normal faults; the plates at an ocean ridgeare being pulled apart or are undergoingextension, which results in normal faults.

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