Chap05 Igneous Rocks

8/10/2019 Chap05 Igneous Rocks

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Cooling Rates When lava flows on Earths surface, it is exposedto air and moisture. Under these conditions, the lava cools quickly,and there is not enough time for large crystals to form. Thus, extru-sive igneous rocks such as obsidian have no visible mineral grains.In contrast, when magma cools slowly beneath Earths surface,there is sufficient time for large crystals to form. Thus, intrusiveigneous rocks such as gabbro may have crystals larger than 1 cm.Youll investigate the effects of cooling rate on crystal size in theGeoLab at the end of this chapter.

TEXTUREOften, its easier to observe the sizes of mineral grains than it is to observe theirshapes. Geologists solve this problem by making thin sections, which are slices of rock so thin that light can pass throughthem. As shown in the thin section inFigure 5-12, many mineral grains haveinterlocking edges. As the grains crystallizefrom magma, they grow together and formthese irregular edges. Although irregular

crystal shapes are characteristic of many igneous rocks, well-shaped crystals canform under certain conditions. Duringfractional crystallization, the minerals thatform early in the process float in a liquidand have space in which to grow distinctcrystal shapes.

5.2 Classifying Igneous Rocks 10

AFigure 5-11 Obsidian coolsquickly and has no visiblemineral grains (A). Thewhite patches that charac-terize this snowflake obsid-ian are mineral crystals thatformed from impurities asthe obsidian cooled. Gabbro

cools slowly and thus largemineral grains form (B).

B

Figure 5-12 The interlock-ing edges of mineral grainsare evident in this thin sec-tion of diorite.
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5.2 Classifying Igneous Rocks 11

A

EnvironmentalConnection

C

suddenly intruded higher in the crust, or if it erupted onto Earthssurface, the remaining magma would cool quickly and form smallercrystals. Youll explore other characteristics of igneous rocks in theProblem-Solving Labon the previous page.

IGNEOUS ROCKS AS RESOURCES

Igneous rocks have several characteristics that make them espe-cially useful as building materials. The interlocking grain textures of igneous rocks help to give them strength. In addition, many of theminerals found in igneous rocks are resistant to weathering. Graniteis among the most durable of igneous rocks. Some common con-struction uses of granite are shown in Figure 5-14. Youll learn moreabout uses of other igneous rocks in the Science & Technology featureat the end of this chapter.

ORE D EPOSITSAs you learned in Chapter 4, ores are minerals that contain a usefulsubstance that can be mined at a profit. Valuable ore deposits areoften associated with igneous intrusions. Sometimes, these oredeposits are found within igneous rock, such as the layeredintrusions mentioned earlier. Other times, ore minerals arefound in the rocks surrounding intrusions. These type of deposits sometimes occur as veins.

Veins Recall from Bowens reaction series that the fluidleft during magma crystallization contains high levels of silicaand water. This fluid also contains any leftover elements that

Figure 5-14 The columnsin the Rhodes Memorial inCape Town, South Africa(A); the kitchen tiles (B);and the Vietnam Memorialin Washington, D.C. (C) arall made of granite.

B


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were not incorporated into the common igneous minerals. Some

important metallic elements that are not included in common min-erals are gold, silver, lead, and copper. These elements, along with thedissolved silica, are released at the end of magma crystallization in ahot, mineral-rich fluid that fills cracks and voids in the surroundingrock. This fluid solidifies to form metal-rich quartz veins, such as thegold-bearing veins found in the Sierra Nevada mountains of Cali-fornia. An example of gold-bearing quartz is shown in Figure 5-15.

Pegmatites Vein deposits may contain other valuable resourcesin addition to metals. Veins of extremely large-grained minerals,such as the one shown in Figure 5-16A, are called pegmatites . Oresof rare elements such as lithium and beryllium are found in peg-matites. In addition to ores, pegmatites can produce beautiful crys-tals. Because these veins fill cavities and fractures in rock, mineralsgrow into voids and retain their shapes. Some of the worlds mostbeautiful minerals have been found in pegmatites. An example isshown in Figure 5-16B.

112 CHAPTER 5 Igneous Rocks

Figure 5-15 This sample ofgold-bearing quartz camefrom El Dorado County,California.

Figure 5-16 Pegmatitesare veins of extremelylarge-grained minerals (A).Stunning crystals such asthis garnet are oftenfound in pegmatites (B).

B

Using NumbersA granite slab has adensity of 2.7 g/cm 3.What is the massof a 2-cm thickcountertop that is0.6 m x 2.5 m? Howmany pounds is this?

A


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ModelingCrystal Formation

The rate at which magma cools has an effect on the grainsize of the resulting igneous rock. Observing the crystal-lization of magma is difficult because molten rock is very hot and the crystallization process is sometimes very slow. Other materials, however, crystallize at lower temperatures. These materials can be used to model crystal formation.

ProblemModel the crystallization of mineralsfrom magma.

Materialsclean, plastic petri dishessaturated alum solution200-mL glass beaker

magnifying glasspiece of dark-coloredconstruction paper

thermometerpaper towelswaterhot plate

ObjectivesIn this GeoLab, you will: Determine the relationship between

cooling rate and crystal size. Compare and contrast different

crystal shapes.

Safety Precautions

The alum mixture can cause skin irrita-tion and will be hot when it is firstpoured into the petri dishes. If splatter-ing occurs, wash skin with cold water.Always wear safety goggles and anapron in the lab.

Preparation

1. As a group, plan how you couldchange the cooling rate of a hotsolution poured into a petri dish.For instance, you may want to put onesample in a freezer or refrigerator fora designated period of time. Assign

each group member a petri dishto observe during the experiment.

2. Place a piece of dark-colored con-struction paper on a level surfacewhere it wont be disturbed. Place thepetri dishes on top of the paper.

Procedure


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1. How did you vary the cooling rate of the solutions in the petri dishes?Compare your methods with thoseof other groups. Did one methodappear to work better than others?Explain.

2. Use a magnifying glass or binocularmicroscope to observe your alumcrystals. What do the crystals look like? Are all the crystals the same size?

3. Compare your drawings and petridish with those of other students in your group. Which petri dish hadthe smallest average crystal size?Describe the conditions under whichthat petri dish cooled.

4. Do all the crystals have the sameshape? Draw the most commonshape. Share your drawings withother groups. Describe any patternsthat you see.

1. What factors affected the size of thecrystals in the different petri dishes?How do you know?

2. Infer why the crystals changed shapeas they grew.

3. How is this experiment differentfrom magma crystallization? Howis it the same?

4. Describe the relationship betweencooling rate and crystal formation.

Analyze

Conclude & Apply

3. Carefully pour a saturated alum solu-tion that is about 95C to 98C, or just below boiling temperature, intoeach petri dish so that it is half-full.Use caution when pouring the hotliquid to avoid splatters and burns.

4. Observe the petri dishes. Record yourobservations in your science journal.

Draw what you observe happening inthe petri dish assigned to you.

5. Every 5 minutes for 30 minutes,record your observations of yourpetri dish. Make accurate, full-sizeddrawings of any crystals that beginto form.

GeoLab 1


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In addition to scalpels, obsidian is used tomake knives. Other igneous rocks, such asgranite, are used in the construction indus-try. Go to the Earth Science Web Site atearthgeu.com to research and write areport about some common uses forigneous rocks.

Reading Analysis

scalpel meet at a single point, which gives the

scalpel its fine, sharp edge. This sharpness allowsthe scalpel to divide rather than tear flesh.Unlike the stainless steel scalpel, the obsidianscalpel creates such a small incision it barely leaves a scar. For this reason, obsidian scalpelsare particularly well-suited to plastic surgery.

DisadvantagesAt present, only a few doctors use obsidian

scalpels, largely because these handmade-toolsare relatively expensive. The price of the scalpelsis high because only a few knappers are producingthe scalpels, and each scalpel takes days or evenweeks to complete. The average obsidian scalpelmay cost $20. In contrast, stainless steelscalpels, which can be mass-produced, costapproximately $2 each.


KnappingKnapping is the process of shaping a rock by

using a mallet-like instrument to break off piecesof the rock. For thousands of years, this techniquehas been used to shape obsidian into tools anddecorative pieces. This fast-cooling, extrusiverock has a conchoidal fracture, which allows therock to break in predictable ways. Knappers usethree techniques, sometimes in combination, toshape obsidian. Percussion flaking involves usinga hammer or mallet to shape the rock. Pressureflaking involves using specially designed tools to

pry off flakes of the rock. Lastly, in indirect per-cussion, a tool called a punch is placed on theedge of the rock. Flaking results when the punchis struck by a hammer or mallet.

Obsidian ScalpelsKnapping is carried on today by skilled arti-

sans. Some of these modern knappers have takentheir craft into the medical field, and are creatingscalpels made from obsidian. Because obsidianscalpels are handmade, their surfaces look some-what rough compared with traditional stainlesssteel scalpels. However, obsidian scalpels actually have a much sharper, smoother edge than stain-less steel scalpels. When viewed under anelectron microscope, the edges of an obsidian

Cutting-Edge SurgeryW hen we hear the word technology, we often envision complicated gadgets. But one of the earliest forms of technology centered around common rocks. To better hunt their prey, for instance, our early ances- tors created razor-sharp arrowheads and spears from the igneous rock obsidian. Today, theres a new use for this old rock: plastic surgery.
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SummaryVocabularyBowens reaction

series (p. 103)extrusive (p. 99)fractional crystall

tion (p. 103)igneous rock (p. 9intrusive (p. 99)lava (p. 99)partial melting

(p. 102)

Vocabularyfelsic (p. 107)kimberlite (p. 113mafic (p. 107)pegmatite (p. 112porphyritic (p. 11ultramafic (p. 108

Main Ideas Igneous rocks are formed by the cooling and crystallization of

magma. Intrusive rocks form inside Earths crust, and extrusiverocks form on Earths surface. Extrusive rocks, which cool morerapidly than intrusive rocks, are generally more fine grained.

Magma is a slushy mix of molten rock, gases, and mineral crys-tals. The elements found in magma are the same major elementsfound in Earths crust: oxygen (O), silicon (Si), aluminum (Al),iron (Fe), magnesium (Mg), calcium (Ca), potassium (K), andsodium (Na).

Silica (SiO2) is the most abundant compound in magma.Magmas are classified as basaltic, andesitic, and rhyolitic, basedon the amount of SiO2 they contain.

Different minerals melt and crystallize at different temperaturesin the processes of partial melting and fractional crystallization.Minerals crystallize from magma in a sequential pattern knownas Bowens reaction series.

Main Ideas Igneous rocks are classified as felsic, mafic, intermediate, and

ultramafic, depending upon their mineral compositions. Felsicrocks such as granite are light-colored, have high silica contents,and contain quartz and feldspars. Mafic rocks such as gabbroare dark-colored, have lower silica contents, and are rich in ironand magnesium. Intermediate rocks have moderate silica levels.Ultramafic rocks have low silica contents and very high levels of iron and magnesium. Igneous groups can be further identified bycrystal size and texture.

Early forming minerals may have well-shaped crystals, whilelater-forming minerals have irregular shapes. Porphyritic texturescontain both large and small crystals.

Igneous rocks such as granite are often used as building mate-rials because of their strength, durability, and beauty.

Valuable ore deposits and gems are often associated withigneous intrusions. Ores of rare elements such as lithium andberyllium are found in veins of extremely large-grained mineralscalled pegmatites. Diamonds are found in rare types of igneousintrusions known as kimberlites.

SECTION 5.1

What are

igneous rocks?

SECTION 5.2

ClassifyingIgneous Rocks

Study Guide 1earthgeu.com/vocabulary_puzzlemaker
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1. What term describes igneous rocks that crystallizeinside Earth?a. magma c. lavab. intrusive d. extrusive

2. What magma type contains the greatest amountof SiO2?a. basaltic c. rhyoliticb. andesitic d. peridotitic

3. What igneous rock has no visible crystals as aresult of rapid cooling?a. gabbro c. obsidianb. andesite d. pegmatite

4. What type of ultramafic rock sometimes containsdiamond?a. pegmatite c. graniteb. kimberlite d. rhyolite

5. What minerals are associated with the rightbranch of Bowens reaction series?a. olivine and pyroxene c. mica and feldsparsb. feldspars d. quartz and biotite

6. What is the last mineral to crystallize frommagma?a. biotite c. olivineb. plagioclase d. quartz

7. What are veins of extremely coarse-grainedigneous rocks called?a. gabbros c. pegmatitesb. layered intrusions d. crystals

8. What effect does a fast cooling rate have ongrain size in igneous rocks?a. It forms fine-grained crystals.b. It forms large-grained crystals.c. It forms light crystals.d. It forms dark crystals.

Understanding Main Ideas 9. What term describes magma that flows out ontoEarths surface?

a. layered intrusion c. crystallizationb. lava d. ultramafic

10. Which of the following affects the melting tem-perature of magma?a. ore deposits c. oxygen contentb. silica content d. potassium content

11. Which of the following does not affect the forma-tion of magma?a. temperature c. volumeb. pressure d. mineral composition

12. What are some uses of igneous rocks in the con-struction industry?

13. Why do scientists theorize that kimberlites origi-nate deep within Earths crust or mantle?

14. What are the three main types of magma? Whatfactor determines these classifications?

15. Describe Bowens reaction series. Be sure to dis-cuss the two branches of the series.

16. Why are olivine and calcium-rich plagioclaseoften found together in igneous rocks?

GET TO THE ROOT OF THINGS . If youdont know the definition of a word, you caninfer its meaning by examining its roots, pre-fixes, and suffixes. For instance, words that startwith non-, un-, a-, dis-, and in- generally reversewhat the rest of the word means.Words thatend in -ly are usually adverbs, and thus, aredescriptive terms.

Test-Taking Tip

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1. Which of the following is most abundant inmagma and has the greatest effect on itscharacteristics?a. Ob. Cac. Ald. SiO2

2. Which process describes how different minals form at different rates?a. partial melting

b. Bowens reaction seriesc. fractional crystallizationd. geothermal gradient

USING TABLES Use the table to answerquestions 3 and 4.

3. Rock A is most likely what kind of rock?a. felsicb. maficc. ultramaficd. intermediate

4. Which type of rock is rock B?a. granite

b. dioritec. gabbrod. pegmatite

17. Why is magma usually a slushy mixture of crys-tals and molten rock?

18. What is unusual about peridotite and dunite?

Use the table below to answer questions 19-21.

19. Which rock is most likely granite?20. Which rock is an ultramafic rock?21. Rock 4 is fine grained. What type of rock is it?

22. What characteristics of igneous rocks make themgood building materials?

23. How is it possible for magma to have a highersilica content than the rock from which itformed?

24. Would you expect to find plagioclase feldspar orbiotite in a greater variety of igneous rocks?

Explain.25. Which would make a lighter-colored kitchen

counter, granite or gabbro? Why?26. Why are mineral deposits often found around the

perimeter of igneous intrusions?

Thinking Critically

Applying Main Ideas Standardized Test Practice

Mineral Mineral Percentage

Rock 1 Rock 2 Rock 3 Rock 4

Quartz 5 35 0 0Potassium 0 15 0 0

feldsparPlagioclase 55 25 0 55

feldsparBiotite 15 15 0 10

Amphibole 25 10 0 30Pyroxene 0 0 40 5Olivine 0 0 60 0

Rock Composition

SilicaColor Content Composition

Rock A light high quartz andfeldspars

Rock B dark low iron andmagnesium

Characteristics of Rocks
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Chap05 Igneous Rocks

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