98

What You’ll Learn• How magma melts and

crystallizes to formigneous rocks.

• How igneous rocks areclassified.

• How igneous rocks areused.

Why It’s ImportantThis photograph shows a microscopic view ofgabbro, a type of igneousrock. Igneous rocks arethe most abundant rocksin Earth’s crust. Manyimportant mineral andmetal deposits are associ-ated with igneous rocks.

IgneousRocksIgneousRocks

55

To find out more aboutigneous rocks, visit theEarth Science Web Site at earthgeu.com

5.1 What are igneous rocks? 99

Rocks are mixtures of minerals,organic matter, and other materials.Sometimes, it’s possible to identifythe different minerals in a sample ofrock.

1. Examine a sample of granite froma distance of about 1 m. Recordyour observations.

2. Use a magnifying glass or stereo-microscope to observe the granitesample. Record your observations.

Observe In your science journal, draw a picture ofwhat you saw through themagnifying glass or stereo-microscope. Include a scalefor your drawing. How manydifferent minerals did youobserve in the rock? Whatminerals can you identify?Describe the sizes and shapesof the minerals. Do you seeany evidence that these min-erals crystallized from moltenrock? Explain.

Identifying MineralsDiscovery LabDiscovery Lab

OBJECTIVES

• Compare and contrastintrusive and extrusiveigneous rocks.

• Describe the composi-tion of magma.

• Discuss the factors thataffect how rocks melt andcrystallize.

VOCABULARY

igneous rocklavaextrusiveintrusivepartial meltingfractional crystallizationBowen’s reaction series

In the Discovery Lab, you examined the minerals in a piece of gran-ite. Granite is an igneous rock formed from magma, which, as youlearned in Chapter 4, is molten rock below Earth’s surface. Igneousrocks are formed from the crystallization of magma. The termigneous comes from the Latin word ignis, which means “fire” becauseearly geologists often associated igneous rocks with fiery lava flows.Lava is magma that flows out onto Earth’s surface.

TYPES OF IGNEOUS ROCKSIf you live near an active volcano, you can literally watch igneous rocksform. A hot, molten mass of rock may solidify into solid rockovernight. Fine-grained igneous rocks that cool quickly on Earth’s sur-face are called extrusive igneous rocks. Coarse-grained igneous rocksthat cool slowly beneath Earth’s surface are called intrusive igneousrocks. Granite is the most common intrusive igneous rock. Initially,scientists did not believe that granite was igneous in origin because itwas coarse grained and thus unlike the fine-grained surface rocks thatformed from lava. In the late 1700s, however, careful study of graniterock formations revealed that they cut across other rock formations.

What are igneous rocks? 5.15.1

These cross-cutting relationships are evidence that the granite wasintruded, or forced into, existing rocks. Cross-cutting relationshipsand other geologic clues provided direct evidence that granites wereigneous rocks that formed underground. Figure 5-1 shows the condi-tions under which granite and other igneous rocks form.

COMPOSITION OF MAGMAMagma is often a slushy mix of molten rock, gases, and mineral crys-tals. The elements found in magma are the same major elementsfound in Earth’s crust: oxygen (O), silicon (Si), aluminum (Al), iron(Fe), magnesium (Mg), calcium (Ca), potassium (K), and sodium(Na). Of all the compounds found in magma, silica (SiO2) is themost abundant and has the greatest effect on magma characteristics.As summarized in Table 5-1, magmas are classified as basaltic,andesitic, and rhyolitic, based on the amount of SiO2 they contain.Silica content affects melting temperature and also impacts howquickly magma flows.

100 CHAPTER 5 Igneous Rocks

Figure 5-1 Lava coolsquickly on Earth’s surfaceand forms fine-grainedigneous rocks such as rhyo-lite (A). Magma coolsslowly beneath Earth’s sur-face and forms coarse-grained igneous rocks suchas granite. (B).

A

B

ORIGINS OF MAGMAIn the laboratory, most rocks must be heated to temperatures of800°C to 1200°C before they melt. In nature, these temperatures arefound in the upper mantle and lower crust. Where does this heatcome from? Scientists theorize that the remaining energy fromEarth’s molten formation and the heat generated from the decay ofradioactive elements are the sources of Earth’s thermal energy.

Factors That Affect Magma Formation The main factorsinvolved in the formation of magma are temperature, pressure, watercontent, and mineral composition. Temperature generally increaseswith depth in Earth’s crust. This temperature increase, known as thegeothermal gradient, is plotted in Figure 5-2A. Oil-well drillers andminers, such as those shown in Figure 5-2B, have firsthand experiencewith the geothermal gradient. Temperatures encountered whendrilling deep oil wells can exceed 200°C.

Pressure also increases with depth. This is a result of the weight ofoverlying rock. Laboratory experiments show that as pressure on arock increases, its melting point also increases. Thus, a rock may meltat 1100°C at Earth’s surface, but the same rock will melt at 1400°Cunder the intense pressure found at a depth of 100 km.

The third factor that affects the formation of magma is water con-tent. Rocks and minerals often contain small percentages of water,which changes the melting point of the rocks. As water contentincreases, the melting point decreases.

5.1 What are igneous rocks? 101

Table 5-1Types of Magma

SiO2

Group content

Rhyolitic 70 percent

Andesitic 60 percent

Basaltic 50 percent

100

200

300

Geothermal Gradient

Dep

th (

km

)

0

500° 1000° 1500° 2000°0°

Temperature (°C)

Continentalcrust

Oceaniccrust

A B

Figure 5-2 Differences in mineral compositioncause the geothermal gradient to be higher inoceanic crust than in continental crust (A), asyou’ll learn on the next page. Also, the geother-mal gradient causes temperatures in deep minesto be quite high (B).

Figure 5-3 Granite’shigher water content andmineral composition causeit to melt at lower temper-atures than basalt.

102 CHAPTER 5 Igneous Rocks

10

20

15

5

25

30

35

40

Melting Temperatures

Incr

easi

ng

pre

ssu

re/d

epth

(km

)

0

200° 800° 1000° 1200° 1400° 1600°600°400°

Melting temperature (°C)

Granite

Basalt

Mineral content also impacts how magma is formed. Differentminerals have different melting points. For example, rocks formed ofolivine, calcium feldspar, and pyroxene melt at higher temperaturesthan rocks containing quartz and potassium feldspar. In general,oceanic crust is rich in iron and magnesium and therefore melts athigher temperatures than continental crust, which contains higherlevels of silicon and aluminum. Rocks melt only under certain con-ditions—the right combination of temperature, pressure, and com-position must be present. Figure 5-3 shows the melting curves ofboth granite and basalt. As you can see, granite has a lower meltingpoint. This is because it contains more water than basalt and is madeup of minerals that melt at lower temperatures.

HOW ROCKS MELTSuppose you froze bits of candle wax and water in an ice-cube tray.If you took the tray out of the freezer and left it at room temperature,the ice would melt but the candle wax would not. Why? The two sub-stances have different melting points. Rocks melt in a similar waybecause the minerals they contain have different melting points.

Partial Melting Because different minerals have different melt-ing points, not all parts of a rock melt at the same time. This explainswhy magma is often a slushy mix of crystals and molten rock. Theprocess whereby some minerals melt at low temperatures while otherminerals remain solid is called partial melting. Partial melting is

illustrated in Figure 5-4. As each group of minerals melts, differentelements are added to the magma “stew,” thereby changing its com-position. If temperatures are not great enough to melt the entirerock, the resulting magma will have a different chemistry from thatof the original rock. This is one way in which different types ofigneous rocks form.

Fractional Crystallization When magma cools, it crystallizesin the reverse order of partial melting—the first minerals to crystal-lize from magma are the last minerals to melt during partial melting.The process wherein different minerals form at different tempera-tures is called fractional crystallization. This process, which is illus-trated in Figure 5-5, is similar to partial melting in that thecomposition of magma may change. However, during fractionalcrystallization, the changes occur because as each group of mineralscrystallizes, it removes elements from the remaining magma insteadof adding new elements.

BOWEN’S REACTION SERIESIn the early 1900s, Canadian geologist N. L. Bowen demonstratedthat as magma cools, minerals form in predictable patterns.Bowen’s reaction series illustrates this relationship between cool-ing magma and mineral formation. Bowen discovered two mainpatterns, or branches, of crystallization. The first pattern is char-acterized by a continuous, gradual change of mineral composi-tions in the feldspar group. The second pattern is characterized byan abrupt change of mineral type in the iron-magnesium groups.Figure 5-6 on page 104 illustrates Bowen’s reaction series.

Feldspars In Bowen’s reaction series, the right branch represents thefeldspar minerals, which undergo a continuous change of composition.

5.1 What are igneous rocks? 103

Solid crystals

Moltenrock

A.Solid rock B.Partially melted rock

K-feldspar

Quartz

Hornblende

Biotite

Plagioclase

Si

Si

Si

MgAl

O

OO

Na

Magma

KFe

Fe

Ca

Si

Si

Mg

Mg ,Fe

Mg ,Fe

Al

O

O

Na

Na

OlivinecrystalsK

Fe

Ca

Ca

Figure 5-4 A rock is madeup of different mineralsthat melt at different tem-peratures (A). Thus, duringthe melting process, someminerals are molten whileothers remain solid (B).

Figure 5-5 Magma is madeup of different mineralsthat crystallize at differenttemperatures (A). Thus,during the crystallizationprocess, some mineralsbecome solid while othersremain molten (B).

A

B

Figure 5-6 In the leftbranch of Bowen’s reactionseries, Fe-Mg mineralschange to different miner-als during the crystallizationprocess. In the right branch,calcium-rich feldsparschange gradually tosodium-rich feldspars.

As magma cools, the first feldspars to form are rich in calcium.As cooling continues, these feldspars react with magma, and theircalcium-rich compositions change to sodium-rich compositions. Insome instances, as when magma cools rapidly, the calcium-rich coresare unable to react completely with the magma. The result is a zonedcrystal that has sodium-rich outer layers and calcium-rich cores, asshown in Figure 5-7.

Iron-Rich Minerals The left branch of Bowen’s reaction seriesrepresents the iron-rich minerals. These minerals undergo abruptchanges during fractional crystallization. For example, when a

104 CHAPTER 5 Igneous Rocks

Discontinuous

reactionseries

ofFe-M

gm

inerals

Conti

nuou

sre

actio

nse

ries

Incr

easi

ngso

dium

cont

ent

of

pla

gioc

lase

feld

spar

of

pla

gioc

lase

feld

spar

Simultaneous crystallization

Olivine

Pyroxene

Amphibole

Biotite mica

Sodium-rich

Calcium-rich

Potassium feldsparMuscovite mica

Quartz

Magma

typesEarly, high-

temperature

(~1000°C)crystallization

Late, low-

temperature(~600°C)

crystallization

Basaltic(low silica)

Andesitic

Rhyolitic

(high silica)

Calcium-richcore

Sodium-richouter layers

A

Figure 5-7 When magmacools quickly, a feldsparcrystal may not have time to react completely with the magma and it retains a calcium-rich core (A).The result is a crystal withdistinct zones (B).

B

magma rich in iron and magnesium cools to around 1800°C, olivinebegins to crystallize. Olivine continues to form until the temperaturedrops to 1557°C. At that temperature, a completely new mineral,pyroxene, begins to form. All the olivine that previously formedreacts with the magma and is converted to pyroxene. Similar mineralchanges have been observed in amphiboles and biotite.

As minerals form in the order shown in Bowen’s reaction series,elements are removed from the remaining magma. Silica and oxygen,the most abundant elements in magma, are left over at the end of thereaction series. When the remaining melt, enriched with silica andoxygen, finally crystallizes, quartz is formed. Quartz often occurs inveins, as shown in Figure 5-8, because it crystallizes as the last liquidportion of magma is squeezed into rock fractures.

Crystal Separation As is often the case with scientific inquiry,Bowen’s reaction series led to more questions. For example, if olivineconverts to pyroxene during cooling, why is olivine found in rock?Geologists hypothesize that under certain conditions, newly formedcrystals are separated from magma, and the chemical reactionsbetween the magma and the minerals stop. Crystal separation canoccur when crystals settle to the bottom of the magma body, andwhen liquid magma is squeezed from the crystal mush to form twodistinct igneous bodies with different compositions.

Layered Intrusions In some magma bodies, the minerals form intodistinct bands in the order shown in Bowen’s reaction series. The result

Figure 5-8 Quartz, the lastmineral to crystallize, oftenforms in rock veins whenthe remaining magma issqueezed into rock fracturesand cools.

5.1 What are igneous rocks? 105

Topic: Bowen’s ReactionTo find out more aboutBowen’s reaction series,visit the Earth Science Web Site at earthgeu.com

Activity: Explain whichtypes of minerals weathermore quickly than others.What unique rocks demon-strate this weatheringprocess?

106 CHAPTER 5 Igneous Rocks

1. Compare and contrast magma and lava.What two types of igneous rock areformed as each cools?

2. Make a data table that lists the eight majorelements found in most magma. Includethe chemical symbol of each element.

3. What are the factors that affect the for-mation of magma?

4. Compare the ways in which iron-magnesium minerals and feldspars crystallize from magma.

5. Thinking Critically Geologists have foundzoned pyroxene crystals that have magnesium-rich cores and iron-rich outerlayers. Which has a higher melting tempera-ture, magnesium-rich pyroxene or iron-richpyroxene? Explain your reasoning.

SKILL REVIEW

6. Comparing and Contrasting Compare andcontrast how partial melting and frac-tional crystallization can change the com-position of magma. For more help, referto the Skill Handbook.

Layered intrusion

Metal-rich layer

Metal-rich layer

Figure 5-9 The settling ofcrystals, flowing currents inmagma, and temperaturedifferences may cause theformation of a layeredintrusion, which sometimeshas metal-rich layers.

is a layered intrusion, as shown in Figure 5-9. Geologists are uncertainhow these layers form. The settling of crystals, flowing currents in themagma, and temperature gradients within the magma chamber mayall play a role. Layered igneous intrusions can be valuable sources ofrare metals. Some have very high concentrations of elements such asplatinum, chromium, nickel, or gold. For instance, a layered intru-sion in Montana called the Stillwater Complex is the only source ofplatinum in the United States. Platinum is a critical component incatalytic converters, which are used to reduce the amount of pollu-tants that vehicles emit.

earthgeu.com/self_check_quiz

5.2 Classifying Igneous Rocks 107

OBJECTIVES

• Classify different typesand textures of igneousrocks.

• Recognize the effects ofcooling rates on the grainsizes of igneous rocks.

• Describe some uses ofigneous rocks.

VOCABULARY

felsic porphyriticmafic pegmatiteultramafic kimberlite

5.25.2 Classifying Igneous Rocks

Igneous rocks are broadly classified as intrusive or extrusive.However, geologists further classify these rocks by their mineralcompositions. In addition, physical properties such as grain size andtexture serve as clues for the identification of various igneous rocks.

MINERAL COMPOSITIONAs shown in Table 5-2, the three main groups of igneous rocks—felsic, mafic, and intermediate—are classified according to theirmineral compositions. Felsic rocks such as granite are light-colored,have high silica contents, and contain quartz and the feldspars ortho-clase and plagioclase. Mafic rocks such as gabbro are dark-colored,have lower silica contents, and are rich in iron and magnesium. Maficrocks contain plagioclase, biotite, amphibole, pyroxene, and olivine.Diorite is a good example of an intermediate rock with moderateamounts of biotite, amphibole, and pyroxene.

25%

75%

50%

100%

Min

eral

co

mp

osi

tio

n(p

erce

nta

ge

by

volu

me)

0%

FelsicExtrusiveIntrusive

Intermediate Mafic Ultramafic Texture

Obsidian

Rhyolite

Granite

Pegmatite

Diorite Gabbro Peri-dotite

Dun-ite Coarse-grained

Verycoarse-grained

Andesite Basalt Fine-grained

Basaltic glass Glassy(non-crystalline)

Potassium feldspar(pink to white)

Quartz(clear to white)

Plagioclase feldspar(white to gray)

Biotite (black)Pyroxene(green)

Olivine(green)

Amphibole(black)

Table 5-2 Classification of Igneous Rocks

Ultramafic Rocks Two unusual igneousrocks, peridotite and dunite, have low silicacontents and very high levels of iron andmagnesium, and thus, they are classified asultramafic rocks. Some scientists theorizethese ultramafic rocks, shown in Figure 5-10,are formed by the fractional crystallizationof olivine and pyroxene. The minerals mayhave been separated from magma and didnot convert to another mineral upon reach-ing a particular temperature. Anotherhypothesis is that ultramafic rocks representpieces of the upper mantle that have beenbrought close to Earth’s surface. In theMiniLab on this page, you’ll analyze the min-eral compositions of various igneous rocks.

GRAIN SIZEIn addition to differences in their mineralcompositions, igneous rocks differ in thesizes of their grains. Figure 5-11 comparesobsidian, a glassy extrusive rock, and gab-bro, a coarse-grained intrusive rock. Whatmight account for the lack of visible crystalsin obsidian and the large crystals of gabbro?

108 CHAPTER 5 Igneous Rocks

How do igneous rocks differ?Compare and contrast the different char-acteristics of igneous rocks.

Procedure1. Using the igneous rock samples provided

by your teacher, carefully observe the fol-lowing characteristics of each rock: color,grain size, texture, and, if possible, min-eral composition.

2. Design a data table in your science journalto record your observations.

Analyze and Conclude1. Classify your rock samples as extrusive or

intrusive rocks.2. What characteristics do the extrusive rocks

share? How do they differ? What charac-teristics do the intrusive rocks share? Howdo they differ?

3. Classify your rock samples as felsic, inter-mediate, mafic, or ultramafic.

Figure 5-10 Dunite (A) andperidotite (B) are ultramaficrocks. They have low silica contents and high levels of iron and magnesium.

A

B

Cooling Rates When lava flows on Earth’s 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 Earth’s surface,there is sufficient time for large crystals to form. Thus, intrusiveigneous rocks such as gabbro may have crystals larger than 1 cm.You’ll investigate the effects of cooling rate on crystal size in theGeoLab at the end of this chapter.

TEXTUREOften, it’s easier to observe the sizes ofmineral grains than it is to observe theirshapes. Geologists solve this problem bymaking thin sections, which are slices ofrock 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 irregularcrystal shapes are characteristic of manyigneous 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 109

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. Gabbrocools 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.

110 CHAPTER 5 Igneous Rocks

AFigure 5-13 The sample ofgranite has crystals of thesame size (A). In contrast,the other granite sample has a porphyritic texture withcrystals of different sizes (B).

Estimate mineral compositionIgneous rocks are classified by their mineralcompositions. In this activity, you’ll esti-mate the different percentages of mineralsin an igneous sample, then use your resultsto classify the rock.

Analysis1. Using the diagram of the thin section

shown here, design a method to esti-mate the percentages of the mineralsin the rock sample.

2. Make a data table that lists the miner-als and their estimated percentages.

Thinking Critically3. Use Table 5-2 to determine which

type of igneous rock the thin sectionrepresents.

4. Compare your estimates of the per-centages of minerals in the rock withthose of your classmates. Hypothesizewhy the estimates vary. What are somepossible sources of error?

5. What could you do to improve theaccuracy of your estimate?

Interpreting Scientific Illustrations

Porphyritic Texture Compare the crystal textures of the rocksshown in Figure 5-13. One of the rocks has grains of two differentsizes. This rock has a porphyritic texture, which is characterized bylarge, well-formed crystals surrounded by finer-grained crystals ofthe same mineral or different minerals.

What causes minerals to form both large and small crystals in thesame rock? Porphyritic textures indicate a complex cooling historywherein a slowly cooling magma suddenly began cooling rapidly.Imagine a magma body cooling slowly deep in Earth’s crust. As itcools, the resulting crystals grow large. If this magma were to be

Plagioclasefeldspar

Quartz

Biotite

Amphibole

B

5.2 Classifying Igneous Rocks 111

A

EnvironmentalConnection

C

suddenly intruded higher in the crust, or if it erupted onto Earth’ssurface, the remaining magma would cool quickly and form smallercrystals. You’ll explore other characteristics of igneous rocks in theProblem-Solving Lab on the previous page.

IGNEOUS ROCKS AS RESOURCESIgneous rocks have several characteristics that make them espe-

cially useful as building materials. The interlocking grain textures ofigneous 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. You’ll learn moreabout uses of other igneous rocks in the Science & Technology featureat the end of this chapter.

ORE DEPOSITSAs 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 ofdeposits sometimes occur as veins.

Veins Recall from Bowen’s 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 columns in the Rhodes Memorial inCape Town, South Africa(A); the kitchen tiles (B);and the Vietnam Memorialin Washington, D.C. (C) areall made of granite.

B

were not incorporated into the common igneous minerals. Someimportant 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 world’s 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 often found in pegmatites (B).

B

Using Numbers A granite slab has adensity of 2.7 g/cm3.What is the mass of a 2-cm thick countertop that is 0.6 m x 2.5 m? Howmany pounds is this?

A

Kimberlites Diamond is a valuable mineral foundin rare, ultramafic rocks known as kimberlites,named after Kimberly, South Africa, where the intru-sions were first identified. These unusual rocks are a variety of peridotite. They likely form deep in the crust at depths of 150 to 300 km or in the mantle because diamond and other min-erals found in kimberlites can form only under very high pressures.

Geologists hypothesize that kimberlite magma is intruded rapidlyupwards towards Earth’s surface, where it forms long, narrow,pipelike structures. These structures extend several kilometers intothe crust, but they are only 100 to 300 m in diameter. Most of theworld’s diamonds come from South African mines, such as the oneshown in Figure 5-17.

5.2 Classifying Igneous Rocks 113

B

A

1. Describe the three major groups ofigneous rocks. What are ultramafic rocks?

2. Why does rhyolite have smaller crystalsthan granite?

3. What chemical property is most com-monly used to classify igneous rocks? Listtwo physical properties that you could useto identify igneous rocks.

4. Why is gold often found in veins ofquartz that are in and around igneousintrusions?

5. Thinking Critically Would quartz or pla-gioclase be more likely to form a well-shaped crystal in an igneous rock? Explain.

SKILL REVIEW

6. Concept Mapping Use the followingterms to construct a concept map aboutigneous rock classification. For more help,refer to the Skill Handbook.

granite ultramafic diorite

types of igneous rocks

mafic felsic

peridotite intermediate

gabbro

Figure 5-17 Diamonds are found in kimberlites (A), such as those in the Kimberly Diamond Mine in South Africa (B).

earthgeu.com/self_check_quiz

114 CHAPTER 5 Igneous Rocks

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 hotand the crystallization process is sometimes very slow. Othermaterials, however, crystallize at lower temperatures. Thesematerials can be used to model crystal formation.

ProblemModel the crystallization of mineralsfrom magma.

Materialsclean, plastic petri dishessaturated alum solution200-mL glass beakermagnifying glasspiece of dark-colored

construction paperthermometerpaper 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 dish to observe during the experiment.

2. Place a piece of dark-colored con-struction paper on a level surfacewhere it won’t be disturbed. Place thepetri dishes on top of the paper.

Procedure

1. How did you vary the cooling rate ofthe solutions in the petri dishes?Compare your methods with those of 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 looklike? Are all the crystals the same size?

3. Compare your drawings and petridish with those of other students inyour group. Which petri dish had the 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? How is 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 95°C to 98°C, orjust 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 115

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 at earthgeu.com to research and write a report about some common uses forigneous rocks.

Reading Analysis

scalpel meet at a single point, which gives thescalpel 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 barelyleaves 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.

116 CHAPTER 5 Igneous Rocks

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 topry 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 Scalpels Knapping 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 actuallyhave a much sharper, smoother edge than stain-less steel scalpels. When viewed under an electron microscope, the edges of an obsidian

Cutting-Edge SurgeryWhen we hear the word technology, we often envision complicatedgadgets. But one of the earliest forms of technology centered aroundcommon rocks. To better hunt their prey, for instance, our early ances-tors created razor-sharp arrowheads and spears from the igneous rockobsidian. Today, there’s a new use for this old rock: plastic surgery.

Summary

VocabularyBowen’s reaction

series (p. 103)extrusive (p. 99)fractional crystalliza-

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

(p. 102)

Vocabularyfelsic (p. 107)kimberlite (p. 113)mafic (p. 107)pegmatite (p. 112)porphyritic (p. 110)ultramafic (p. 108)

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

magma. Intrusive rocks form inside Earth’s crust, and extrusiverocks form on Earth’s 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 Earth’s 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 Bowen’s 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 ofiron 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 117earthgeu.com/vocabulary_puzzlemaker

118 CHAPTER 5 Igneous Rocks

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 Bowen’s 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 ontoEarth’s 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 Earth’s crust or mantle?

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

15. Describe Bowen’s 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 youdon’t 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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Assessment 119

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 miner-als form at different rates?a. partial meltingb. Bowen’s reaction seriesc. fractional crystallizationd. geothermal gradient

USING TABLES Use the table to answer questions 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. graniteb. 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 higher silica 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 kitchencounter, granite or gabbro? Why?

26. Why are mineral deposits often found around theperimeter 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 0feldspar

Plagioclase 55 25 0 55feldsparBiotite 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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