HSES 1eTE C03.qxd 5/14/04 3:48 PM Page 75 3.3 ...hal.pcs.k12.mi.us/JaneAttachments/Documents/113201024715...It is very durable and resistant to chemical weathering. Clastic sedimentary

3.3 Sedimentary Rocks

Reading StrategyOutlining Copy this outline beneath theoutline you made for Section 3.2. Completethis outline as you read. Include points abouthow each of these rocks form, some of thecharacteristics of each rock type, and someexamples of each.

Key ConceptsDescribe the majorprocesses involved in theformation of sedimentaryrocks.

What are clasticsedimentary rocks?

What are chemicalsedimentary rocks?

What features are uniqueto some sedimentaryrocks?

Vocabulary◆ erosion◆ deposition◆ compaction◆ cementation◆ clastic sedimentary

rock◆ chemical

sedimentary rock

All sedimentary rocks begin to form when existing rocks are brokendown into sediments. Sediments, which consist mainly of weatheredrock debris, are often transported to other places. When sediments aredropped, they eventually become compacted and cemented to formsedimentary rocks. The structures shown in Figure 9 are made ofthe sedimentary rock called sandstone. It is only one of many types ofsedimentary rocks.

Figure 9 Sedimentary Rocks inCanyonlands National Park,Utah The rocks shown hereformed when sand and othersediments were deposited andcemented. Weathering processescreated this arch.

II. Sedimentary RocksA. Clastic Rocks

1.2.

B. Chemical Rocks1.2. ?

?

??

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FOCUS

Section Objectives3.8 Describe the major processes

involved in the formation ofsedimentary rocks.

3.9 Distinguish between clasticsedimentary rocks andchemical sedimentary rocks.

3.10 Identify the features that areunique to some sedimentaryrocks.

Build VocabularyLINCS Have students List the parts ofthe vocabulary words that they know.For example, cement is part ofcementation. Next, they should Imaginea mental picture of the term’s meaningand describe the image in their ownwords. Sediments held together bycement might be an image forcementation. Students should thenmake a Note of a familiar “sound-alike”word. They can Connect the terms bymaking up a short story about themeaning of the term that incorporatesthe sound-alike word. Lastly, studentsshould conduct a Self-test by quizzingthemselves on the vocabulary terms.

Reading StrategyA.1. rock made up of weathered bits ofrocks and mineralsA.2. Common example of clasticsedimentary rock is shale.B.1. rock that forms when dissolvedminerals precipitate from waterB.2. Common example of chemicalsedimentary rock is limestone.

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Reading Focus

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Section 3.3

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76 Chapter 3

Formation of Sedimentary RocksThe word sedimentary comes from the Latin word sedimentum, whichmeans “settling.” Sedimentary rocks form when solids settle out of afluid such as water or air. The rocks shown in Figure 10 formed whensediments were dropped by moving water. The sediments eventuallybecame cemented to form rocks. Several major processes contributeto the formation of sedimentary rocks.

Weathering, Erosion, and Deposition Recall that weath-ering is any process that breaks rocks into sediments. Weathering isoften the first step in the formation of sedimentary rocks. Chemicalweathering takes place when the minerals in rocks change into newsubstances. Weathering also takes place when physical forces breakrocks into smaller pieces. Living things, too, can cause chemical andphysical weathering.

Weathered sediments don’t usually remain in place. Instead,water, wind, ice, or gravity carries them away. Erosion

involves weathering and the removal of rock. When an agentof erosion—water, wind, ice, or gravity—loses energy, itdrops the sediments. This process is called deposition.Sediments are deposited according to size. The largest sedi-

ments, such as the rounded pebbles in the conglomerate inFigure 10A, are deposited first. Smaller sediments, like the piecesof sand that make up the sandstone in Figure 10B, are dropped

later. Some sediments are so small that they are carried greatdistances before being deposited.

Compaction and Cementation After sediments aredeposited, they often become lithified, or turned to rock.Compaction and cementation change sediments into sedi-mentary rock. Compaction is a process that squeezes, orcompacts, sediments. Compaction is caused by the weight ofsediments. During compaction, much of the water in the sed-iments is driven out.

Cementation takes place when dissolved minerals aredeposited in the tiny spaces among the sediments. Much of thecement in the conglomerate shown in Figure 10A can be seen with theunaided eye. The cement holding the sand grains together in the sand-stone in Figure 10B, however, is microscopic.

Briefly describe the five major processes involved inthe formation of sedimentary rocks.

A

B

Figure 10 Although these tworocks appear quite different, bothformed when sediments weredropped by moving water. A Conglomerate is made ofrounded pebbles cementedtogether. B Sandstone is made ofsand grains cemented together.

76 Chapter 3

INSTRUCTBuild Reading LiteracyRefer to p. 64D in Chapter 3, whichprovides the guidelines for directedreading/thinking activity (DRTA).

DRTA Before students read this section,have them preview the key concepts,vocabulary terms, and headings. Ask:What do you think you will learn inthis section? (Sample answer: aboutsedimentary rock formation, clasticsedimentary rocks, and chemicalsedimentary rocks) What type ofquestions might a teacher ask aboutthis topic? (Sample answer: How dosedimentary rocks form? How aresedimentary rocks classified?) List thesequestions on the board. As studentsread the section, pause to discuss theanswers to the questions.Verbal

Formation ofSedimentary Rocks

Some students may think that rocks arestronger than the agents of mechanicaland chemical weathering. To help dispelthis misconception, place a few drops ofvinegar on a sample of limestone. Havestudents observe the resulting chemicalreaction. Ask: What do you thinkwould happen if the acid continued todrip on the rock over a long period?(The rock would eventually break down orbe chemically weathered.)Visual, Logical

Build Science SkillsObserving Providesmall groups of studentswith 250-mL beakers,stirrers, sand, water,gravel, and soil. Tell students to half-fillthe beakers with water. They shouldthen pour about a handful of eachmaterial into the water. Have them stirthe mixture, then observe what happensto the materials. Ask: Which materialssettled on the bottom? Which settledon the top? (The heavier materialssettled on the bottom; the smaller, lightermaterials settled on the top.) What doesthis activity model? (the settling out ofsediments from a fluid, such as water or air)Kinesthetic, Visual

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Section 3.3 (continued)

Customize for English Language Learners

Compile a classroom library using up-to-datemagazines and newspaper articles. Selectreading materials that correspond to chaptercontent. For example, try to find articles thatdiscuss different types of rocks. Avoid academicjournals and reference materials with high

reading levels. Provide opportunities for ELLstudents to read the articles in class. This willgive them a broader context in which to placethis chapter’s vocabulary terms and keyconcepts.


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Classification of Sedimentary RocksJust like igneous rocks, sedimentary rocks can be classified into

two main groups according to the way they form. The first groupincludes rocks that are made of weathered bits of rocks and minerals.These rocks are called clastic sedimentary rocks. The second groupforms when dissolved minerals precipitate from water solutions. Theserocks are called chemical sedimentary rocks.

Clastic Sedimentary Rocks Many different minerals arefound in clastic rocks. The most common are the clay minerals andquartz. This is because clay minerals, like those that make up much ofthe shale in Figure 11A, are the most abundant products of chemicalweathering. Quartz, which is a major mineral in the breccia shown inFigure 11B, is a common sedimentary mineral for a different reason.It is very durable and resistant to chemical weathering.

Clastic sedimentary rocks can be grouped according to the size ofthe sediments in the rocks. When rounded, gravel-size or larger parti-cles make up most of the rock, the rock is called conglomerate. If theparticles are angular, the rock is called breccia. Sandstone is the namegiven to rocks when most of the sediments are sand-size grains. Shale,the most common sedimentary rock, is made of very fine-grained sed-iment. Siltstone is another fine-grained rock.

Chemical and Biochemical Sedimentary RocksChemical sedimentary rocks form when dissolved substances precip-itate, or separate, from water solution. This precipitation generallyoccurs when the water evaporates or boils off leaving a solid product.Examples of this type of chemical rock are some limestones, rock salt,chert, flint, and rock gypsum.

Describe the major types of clastic sedimentaryrocks.

A B

Figure 11 A Shale and B brecciaare common clastic sedimentaryrocks. This sample of shalecontains plant fossils.Formulating Hypotheses Howdo you think this breccia mighthave formed?

For: Links on sedimentary rocks

Visit: www.SciLinks.org

Web Code: cjn-1034

Classification ofSedimentary Rocks

Chemical WeatheringPurpose Students will observe howchemical weathering can change theminerals in rocks.

Materials calcium tablet, 250-mLbeaker, vinegar

Procedure Half-fill the beaker withvinegar. Place the calcium tablet into thevinegar. Allow students to observe thereaction.

Expected Outcome Students willobserve that the calcium fizzes, foams,and eventually dissolves in the vinegar.Explain that chemical weathering breaksdown rocks in a similar, though slower,fashion.Visual

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Answer to . . .

Figure 11 Rocks were weathered.The larger fragments were deposited.Fine-grained sediments were depositedlater. Little compaction occurredbecause of the size of the angularsediments. Dissolved minerals enteredthe spaces among the sediments andheld them together to form the breccia.

Weathering breaksexisting rocks into

smaller pieces. Erosion is the processwhereby sediments are moved fromplace to place. Deposition occurs whensediments are dropped by erosionalagents. Compaction is the process ofsqueezing sediments. Cementation isa process that “glues” sedimentstogether to form sedimentary rocks.

Conglomerates andbreccias are made

mostly of gravel-sized sediments.Sandstone is made mostly of sand-sizegrains. Shale and siltstone are fine-grained rocks in which clay-size orsmaller particles are the majorcomponents.

Download a worksheet onsedimentary rocks for studentsto complete, and find additionalteacher support from NSTA SciLinks.


78 Chapter 3

About 90 percent of limestones are formed from biochemical sed-iments. Such sediments are the shells and skeletal remains of organismsthat settle to the ocean floor. The coquina in Figure 12 is one obviousexample. You can actually see the shells cemented together. Anotherbiochemical rock is chalk, the material used to write on a chalkboard.

Features of Some Sedimentary RocksSedimentary rocks, like other types of rocks, are used to unravel whatmay have happened in Earth’s long history. The many unique fea-tures of sedimentary rocks are clues to how, when, and where therocks formed. Each layer of a sedimentary rock, for example, recordsa period of sediment deposition. In undisturbed rocks, the oldest layersare found at the bottom. The youngest layers are found at the top of therocks. Ripple marks like the ones shown in Figure 13A may indicatethat the rock formed along a beach or stream bed. The mud cracks inFigure 13B formed when wet mud or clay dried and shrank, leaving arock record of a dry environment.

Fossils, which are the traces or remains of ancient life, are uniqueto some sedimentary rocks. Fossils can be used to help answer manyquestions about the rocks that contain them. For example, did the rockform on land or in the ocean? Was the climate hot or cold, rainy ordry? Did the rock form hundreds, thousands, millions, or billions ofyears ago? Fossils also play a key role in matching up rocks from dif-ferent places that are the same age.

To summarize, sedimentary rocks are rocks that form as the resultof four major processes. Weathering produces particles called sedi-ments. Wind, water, ice, and gravity erode and deposit these sediments.Over time, the sediments are compacted and cemented to form rocks.Sedimentary rocks can be classified according to how they form. A gen-eral classification scheme based on a rock’s formation, texture, andcomposition is shown in Table 2.

Figure 12 This biochemical rock,called coquina, is a type oflimestone that is made ofhundreds of shell fragments.

Figure 13 A Ripple marks and B mud cracks are features ofsedimentary rocks that can be used to learn about theenvironments in which the rocks formed.

A B

78 Chapter 3

Build Science SkillsDesigningExperimentsHave students work insmall groups to designan experiment to show how sedimentaryrocks form when dissolved mineralsprecipitate from water. Students shoulddevelop a hypothesis and procedure,listing controls, safety measures, andmaterials to be used. A sampleexperiment might involve placing tablesalt in water, then heating the wateruntil it evaporates. If time permits, allowstudents to carry out their experiments.Logical, Interpersonal

Features of SomeSedimentary RocksUse VisualsFigure 13 Ask: Based on itsappearance, what can you inferabout the rock labeled A? (It mayhave formed along a beach or streambed.) What can you infer about therock labeled B? (It may have formedwhen wet mud or clay dried and shrank.)Visual

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Section 3.3 (continued)

Unlike other chemical sedimentary rocks, whichare rich in calcite or silica, coal is made mostlyof organic matter. When coal is viewed undera magnifying glass, chemically altered leaves,bark, and wood are visible. The materialsprovide evidence that coal is the end productof the burial of large amounts of plant materials

over long periods of time. The initial stage ofcoal formation is the accumulation of largequantities of plant remains in a swampyenvironment. Coal then undergoes successivestages of formation. With each stage, highertemperatures and pressures drive off impuritiesand volatiles.

Facts and Figures


Section 3.3 Assessment

Reviewing Concepts1. Contrast weathering, erosion, and

deposition.

2. Name four clastic sedimentary rocks andexplain how these rocks form.

3. Name four chemical sedimentary rocksand explain how these rocks form.

4. Explain how three different features ofsedimentary rocks can be used to determinehow, where, or when the rocks formed.

5. What is compaction?

6. Where do the cements that hold sedimentstogether come from?

Critical Thinking7. Applying Concepts Briefly describe how the

rock shown in Figure 12 may have formed.

8. Predicting Which type of sediments do youthink would undergo more compaction—grains of sand or grains of clay? Explain yourchoice.

9. Formulating Conclusions Suppose youfound a sedimentary rock in which ripplemarks were pointing toward the ground.What could you conclude about the rock?

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Sedimentary Rocks Choose one of thesedimentary rocks pictured in this section.Find out how the rock is useful to people.

Texture(grain size) Sediment Name Rock Name

Coarse(over 2 mm)

Gravel(rounded fragments)

Gravel(angular fragments)

Medium(1/16 to 2 mm) Sand

Conglomerate

Breccia

Sandstone

Fine(1/16 to

1/256 mm)

Very fine(less than1/256 mm)

Mud

Mud

Siltstone

Shale

Coquina

Chalk

Chert (light colored)Flint (dark colored)

Rock Gypsum

Rock Salt

Bituminous Coal

Calcite, CaCO3

Quartz, SiO2

GypsumCaSO4•2H2O

Halite, NaCl

Altered plantfragments

Fine to coarsecrystalline

CrystallineLimestone

Visibleshells and shell

fragments looselycemented

Various size shellsand shell fragments

cemented withcalcite cement

Microscopic shellsand clay

Very finecrystalline



Fine-grainedorganic matter

FossiliferousLimestone

Biochemical

Li

mestone

Texture(grain size)Composition Rock Name

Clastic Sedimentary Rocks Chemical Sedimentary Rocks

Travertine

Table 2 Classification of Major Sedimentary RocksUse VisualsTable 2 Ask: How does the texture ofgravel compare with that of sand?(Gravel has a coarse texture; grain size ismore than 2 mm. Sand has a mediumtexture; grain size is 1/16 to 2 mm.)What type of detrital sedimentaryrock has a very fine texture? (shale)What is the chemical composition ofchalk? (calcite) Which chemicalsedimentary rock is made up ofhalite? (rock salt)Visual

ASSESSEvaluateUnderstandingGive students samples of sandstone,siltstone, shale, breccia, andconglomerate. Have them usemagnifying glasses to classify therocks according to grain size.

ReteachReview Table 2. As you discuss thedifferent types of textures and chemicalcompositions, explain how eachsedimentary rock likely formed.

Sample answer: Shale is used inconstruction. Coal is used as anenergy resource.

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3. Most limestones, rock salt, rock gypsum,flint, and chert are chemical sedimentaryrocks that form when dissolved minerals pre-cipitate from water.4. Each layer of a sedimentary rock records aperiod of deposition. Ripple marks indicatethat a rock bed formed in water. Mud cracksare indicative of unusually dry periods. Fossilscan be used to determine if a rock formed onland or in the ocean, if the climate was hotor cold, or rainy or dry, and when the rockcontaining them formed.5. Compaction is the process that squeezes,or compacts, sediments.

Section 3.3 Assessment

1. Weathering is any process in which rocksare broken down into smaller pieces. Erosioninvolves the weathering and removal ofsediments. Deposition is the dropping ofsediments by agents of erosion.2. Conglomerate, breccia, sandstone, shale,and siltstone are clastic rocks. Clastic rocksform when bits of weathered materials arecompacted and cemented together.

6. Cements are dissolved minerals thatare deposited in the tiny places amongthe sediments.7. Animals with shells died. The shells accu-mulated and became cemented to form asedimentary rock.8. Because they are smaller, clay particlesundergo more compaction than sand-sizeparticles.9. Ripple marks indicate that a rock formedin water. And, because the ripple marks werepointing down, one can infer that the rockhas been overturned from its original position.
