Lesson 22: Exploring Igneous Rock Formation

240 STC/MS™ CATASTROPHIC EVENTS

INTRODUCTIONThe super-hot water of a geyser cools as itmakes its way to the earth’s surface. As thewater cools, some of it evaporates, leaving min-eral crystals behind. Over time, these mineralsaccumulate on the walls of the channels andcracks through which the geyser erupts. Themore the geyser erupts, the more mineral crys-tals build up around the hot spring. And thelonger it takes for the water to evaporate, thelarger the mineral crystals become.This same process occurs in the cracks of a

volcano. Molten rock cools and crystallizes—either on the ground as lava or below ground asmagma. This is how igneous rocks form.In this lesson, you will take another look at

three of the igneous rocks from Lesson 21. Youwill examine the size of the crystals in eachrock and discuss how each rock is different. Youwill examine how the rate at which a solution

22Exploring Igneous RockFormation

LESSON

OBJECTIVES FOR THIS LESSON

Examine the texture, or crystal size, of

three igneous rock samples under

magnification.

Conduct tests to analyze the size of

crystals that form from a solution under

different cooling conditions.

Using information gained in the lab,

hypothesize about how each of three

rock samples formed.

On the basis of their properties and

origin, identify igneous rocks by name.

Read to learn more about the rock cycle.

Sulfur vapor escaping from this vent in the Kilauea

volcano cooled to form yellow crystals.

R.L.CHRISTIANSEN/USGS

STC/MS™ CATASTROPHIC EVENTS 241

cools affects the size of the crystals it produces.You will then relate your findings to the forma-tion of igneous rocks, which, you will find, aremade of beautiful and often colorful mineralcrystals.

Getting Started

1. Brainstorm what you know and want tolearn about crystals. Your teacher willrecord your ideas.

2. Collect a tote tray for your group. Takefrom it one small cup of magnesium sul-fate crystals, three hand lenses, one loupe,and four sheets of black constructionpaper. Place a few crystals on the blackpaper and observe them closely.

3. Discuss your observations. Answer the fol-lowing questions:

A. What is the shape of each crystal?

B. In what ways are the crystals the sameor different from one another? Why doyou think this is so?

C. How are these crystals different fromothers you have seen?

D. How do you think the crystals formed?

4. Fold your black paper like a chute andpour the crystals back into the small cup.Save the paper.

MATERIALS FORLESSON 22

For you1 copy of StudentSheet 22.1b:InvestigatingCrystallization—Observation Sheet

1 pair of safetygoggles

For your group1 copy of StudentSheet 22.1a:InvestigatingCrystallization—Planning Sheet

2 copies of InquiryMaster 22.1:Igneous RockDescriptions

1 small cup ofmagnesium sulfate

1 double-eye loupe3 hand lenses4 sheets of blackpaper

1 set of rocks,labeled #1, #2,and #5

1 metric ruler2 petri dishes2 index cards1 long pipette1 pair of scissors

4 ComparingCatastrophicEvents ActivityBoards

1 resealable plasticbag containingComparingCatastrophicEvents ActivityCardsor

4 sheets ofComparingCatastrophicEvents ActivityCards

1 resealable plasticbag


LESSON 22 EXPLORING IGNEOUS ROCK FORMATION

Inquiry 22.1Investigating Crystallization

PROCEDURE

1. Look at the materials set up in front ofthe room. Predict what would happen ifyou mixed the magnesium sulfate crystalswith water and heated the solution.

2. Before you begin, review the Safety Tipswith the class.

3. Watch as student volunteers create a mag-nesium sulfate solution.

4. Discuss with the class what you alreadyknow about solutions, solvents, andsolutes. Answer these questions:

A. What does it mean when we say thatsomething “goes into solution”?

B. In most cases, how does the tempera-ture of the water affect the solute’s abilityto go into solution?

C. How does stirring the water affect thesolute’s ability to go into solution?

D. What is a “saturated” solution?

5. Now answer these questions:

A. What would happen to the solution ifit were allowed to cool?

B. Is there any way to get back the crys-tals that dissolved in the water?

C. Do you think the rate at which thesolution cools will affect the size of crys-tals that form? Explain your prediction.

5. Remove the three igneous rock sampleslabeled #1, #2, and #5 from your totetray. Spend a few minutes examining thesize of the crystals, or texture, of eachrock. Discuss your observations withyour group using the following questionsas your guide:

A. What are the properties of the rocks?

B. What do you notice about the texture,or crystal size, of each rock?

C. How are the rocks alike and different?

D. On the basis of what you alreadyknow about crystals, how might the sizeof the crystals reveal how each rock wasformed?

6. Report to the class what you observedabout the texture, or crystal size, of eachrock. Although each of these rocks isigneous, they look different because theircrystal size is different. This is an indica-tion that they formed under different con-ditions. You will investigate this conceptduring Inquiry 22.1.

SAFETY TIPS

Magnesium sul-

fate, the chemi-

cal you will use

in this inquiry, is

safe when han-

dled properly. Be

sure to follow all

basic lab proce-

dures when

using it.

Wear your safety

goggles during

this inquiry.

Be careful not to

get the magne-

sium sulfate

solution on your

clothes or skin.

Be careful when

you collect the

solution from the

hot pot.



8. Have one member collect the solutionfrom the hot pot area. Bring a long pipetteand two petri dishes. Very carefully, putabout 10 to 20 mL of solution in eachpetri dish, just enough to cover the bot-tom of the dish, as shown in Figure 22.1.

9. Add a “pinch” of crystals (approximately10 granules) to each petri dish, as shownin Figure 22.2. These “seed” crystals willhelp other crystals begin to grow.

6. With your teacher, review ProcedureSteps 7 through 11. Listen as yourteacher discusses how to cool one petridish quickly and another slowly. Onegroup will be selected to cool their dishesin two stages.

7. Collect one copy of Student Sheet 22.1afor your group. Your group is responsiblefor recording your experimental design onthe sheet before you complete the inquiry.Write down the question you will try toanswer and predict what you think willhappen. Then describe the materials andprocedures you will use, what you willkeep the same in each setup, what youwill measure, and how you will measure it.

Figure 22.1 Use the pipette to collect the hot solution,

and release it carefully into the petri dish.

Figure 22.2 Seed the solution using extra crystals.



REFLECTING ON WHAT YOU’VE DONE

1. Collect one copy of Student Sheet 22.1b.Discuss the sheet with your teacher.

2. Notice that the petri dishes are labeled#1, #2, or #3. Examine your own andother groups’ petri dishes and recordyour observations on Student Sheet22.1b. Do not complete the last columnyet. Use a metric ruler to measure thecrystal size, if possible. Explain why youthink the crystals are this size.

3. Discuss your observations of the variouscrystal sizes. Answer these questions:

A. What observations did you makeabout the contents of each petri dish?

B. What observations did you makeabout the crystal size in each petri dish?

C. Did the rate of cooling (that is, howfast each petri dish cooled) affect crystalsize? If so, how?

4. Collect your tote tray and remove thethree igneous rock samples (#1, #2, and#5), hand lenses, and loupe. Try to drawsome conclusions about how each ofthese rocks formed, given your observa-tions of the petri dishes. Again, use yourruler and loupe to measure the crystals.

5. Your teacher will give your group scissorsand two copies of Inquiry Master 22.1,which provides information on each rockstudied in this lesson. Cut the mastersinto four tables so that each group mem-ber has one. With your group, identifyeach of the rocks by name. Record youridentifications in the last column ofStudent Sheet 22.1b.

10. Set out one of your petri dishes in an areawhere it will cool quickly and the otherdish in an area where it will cool slowly.Use index cards to label each dish either“Fast Cooling #1” or “Slow Cooling #2.”Put your group number on the cards aswell. Circle the number. (If your group istesting the two-stage cooling, label yourtwo petri dishes “Two-Stage Cooling #3.”)

11. Clean up. Replace all rock samples andobservation tools in the tote tray. Youwill use them again during “Reflecting onWhat You’ve Done.” If your teacherteaches another class, do the following:

A. Refill the small cups with magnesiumsulfate.

B. Put out new petri dishes and index cards.

C. Do not throw away the black paper. Itwill be used in Lesson 23.



B. Shuffle all four sets of cards. Put the cardsinto one pile in the center of your group.Each group member should have anActivity Board.

C. To play the game, each student selects 10activity cards from the pile to place on hisor her Activity Board in the appropriatecells. The remaining cards should remainin a pile in the center of the group.

D. Working clockwise, one student at a timeselects a card from the pile and places iton his or her board before play rotates tothe next student. (Refer to Table 18.1:Comparing Catastrophic Events, if youneed to.) Return any cards you cannotuse to the discard pile.

E. The game ends when the first student fillsone row on his or her ComparingCatastrophic Events Activity Board. Thecards must be placed correctly to win.Use Table 18.1 as a key.

F. When you are finished, replace yourgroup’s cards in the resealable plastic bag.If it has not already been done for you,mark on the outside of the bag the num-ber of sets of cards in that bag.

6. Apply the results of this investigation tothe earth by answering these questions:

A. Using what you learned from thisinvestigation, how quickly or slowly doyou think igneous rocks form under theearth’s surface? What type of rock mightthis form?

B. Using what you learned from thisinvestigation, how quickly or slowly doyou think igneous rocks form on theearth’s surface? What type of rock mightthis form?

C. If fast-cooling lava forms fine-grainedvolcanic rocks such as basalt, whyaren’t all volcanic rocks basalt?

D. On the basis of the evidence gatheredfrom your investigation, how do youthink each rock sample formed?

7. Read “The Rock Cycle,” on pages250–251.

8. Look ahead to the next lesson, in whichyou will examine pieces of volcanic rockcalled ash.

9. Play the “Comparing CatastrophicEvents” game with your group by doingthe following:

A. Collect your group’s bag of activity cards.If your cards are not cut, you will need tocut the four sheets of ComparingCatastrophic Events Activity Cards intoindividual cards.



or nearly three-fourths of all the world’s gey-sers. In all, Yellowstone has 10,000 geothermalfeatures (“geo” means earth and “thermal”means heat). Besides geysers, Yellowstone’sgeothermal features include hot springs andbubbling mud pots.Why does Yellowstone have so many hot

springs? Most of Yellowstone sits inside anancient caldera. The volcano’s last major erup-tion, which created the caldera, happened600,000 years ago. Smaller lava flows from thevolcano gradually filled up most of the caldera.Rock under the earth’s surface can stay hot forthousands of years. Heat from molten rock afew kilometers below the surface heats thegroundwater. The groundwater is held in aporous type of rock, and the heated water

In the 1800s, fur trappers who ventured intothe Rocky Mountains came back and told of“the place where hell bubbles up.” No onebelieved them. One of the trappers, a Virginiannamed Jim Bridger, told people about findinga column of water as thick as his body thatspouted 60 feet in the air. People called hima liar. But explorers later confirmed the trap-pers’ stories. They had found the roaring gey-sers (springs that spout hot water and steam)and craters of boiling mud in the area that isnow Yellowstone National Park. Today, severalmillion people every year come to view thesewonders at Yellowstone.Yellowstone Park, located in Wyoming and

Montana, is the hottest, most active geyser areain the world. It contains more than 500 geysers,

Earth’s Waterworks

Hot springs are one of the geothermal features found at Yellowstone National Park.

DAVIDE.WIEPRECHT/U.S.DEPARTMENTOFTHEINTERIOR/USGS/

DAVIDA.JOHNSTONCASCADESVOLCANOOBSERVATORY,VANCOUVER,WA



travels upward untilit bursts through theearth’s surface like afountain.Old Faithful,

Yellowstone’s mostfamous geyser, got itsname because it nor-mally erupts on aregular basis—onaverage, every 79minutes. The erup-tions are so regularbecause the watersupply and the struc-ture of the rockremain fairly con-stant over time. YetOld Faithful isn’tcompletely pre-dictable. The timebetween eruptionsactually variesbetween 45 and 105minutes, dependingon the amount ofsuper-hot water leftin the spongy rockwhen the geyser runsout of steam.Recently, scientists

lowered a video cam-era and other instru-ments into the ventof Old Faithful. Theyfound that for thefirst 20 or 30 secondsof each eruption(which lasts for sev-eral minutes), steamand boiling waterrocket through thenarrowest under-ground cracks at thespeed of sound!Old Faithful is the most famous geyser in Yellowstone National Park.

DAVIDE.WIEPRECHT/U.S.DEPARTMENTOFTHEINTERIOR/USGS/DAVIDA.JOHNSTONCASCADESVOLCANOOBSERVATORY,VANCOUVER,WA



water out of the earth, erupting sometimes forup to several hours.If the super-hot water mixes with cool

groundwater that is not under pressure, it risesto the surface as a hot spring. When hot springsbecome choked with pieces of weathered rock(sediment) that break off from the surroundingrock, bubbling mud pots are the result.If the super-hot water rises to the surface with

no resistance, it begins to boil and erupts at thesurface as steam. This thermal feature, called afumarole, is like a geyser, except that it is mostlysteam.

When Geysers Lose Their SteamSome old geysers lose their steam. The super-hot water carries minerals that, through time,accumulate on the walls of the undergroundchannels and cracks. Like arteries, the cracks

What Makes a Geyser Go?Rainwater trickles through cracks into porousrock, where it collects like water in a sponge.Heat from magma a few kilometers beneath theearth’s surface rises and heats the water in theporous rock. The porous rock layer is like apressure cooker: it has lots of heat from themagma and lots of pressure from the weight ofwater and rock above it. The water in theporous rock can reach temperatures of 310 °Cwithout boiling because of the tremendouspressure.This super-hot water rises into pockets of

groundwater that are also under pressure.Steam forms, more pressure builds, and bubblesrise. Steam keeps building until a spout of hotwater and vapor explodes to the surface andshoots high into the air. More super-hot waterthen bursts into steam and blasts more ground-

A geyser forms when magma heats up groundwater that is under pressure.

Magma

Water table

Water

Heat

Steam

Groundwaterrefills emptychambers



Earthquake Predictor?In 1990, a scientist from the CarnegieInstitution in Washington, D.C., workedwith a person who owned property neara geyser in California to study patternsin the geyser’s eruption cycle. Thegeyser didn’t always erupt on schedule.For 15 years, the property owner hadcollected data on the geyser’s eruptions.The scientist compared the data withrecords of thousands of earthquakes inCalifornia and found that major changesin the geyser’s activity coincided withthree large earthquakes that occurredwithin 248 kilometers of the geyser. Inall cases, changes in the geyser hap-pened 1 to 3 days before the earthquake.

The scientist hypothesized that under-ground movements that caused theearthquakes may also have affected thegeyser’s water supply.Earthquakes often hit the Yellowstone

National Park area, shaking and movingthe geysers’ “plumbing system” andchoking off the water supply. An earth-quake in 1995 moved heat and wateraway from Steamboat Geyser and redi-rected it to Monarch Geyser, which hadbeen dormant for 81 years. Suddenly,Monarch began blowing off steam.Could a geyser be an earthquake detec-

tor? It’s possible, but much more studyneeds to be done on this subject.

become clogged, and the steam and watercan no longer escape.For one Yellowstone geyser, named

“Porkchop,” the pressure was too much.It spouted water and steam for years.Then one day it blew rocks the size of TVsets into the air and stopped gushing forgood. �

When the water from Castle Geyser in

Yellowstone National Park evaporates, a mineral

called silica is left behind. This mound of silica

is called a geyserite.

S.R.BRANTLEY/USGS



There are three basic types of rocks: igneous,sedimentary, and metamorphic. Each typeforms in a different way, and one type of rockcan be changed into another in a continuousprocess called the rock cycle. (See the illustra-tion.) Not all rocks follow this full cycle, andthere are always shortcuts along the way.Igneous rocks form when magma or lava

cools and crystallizes on or below the earth’ssurface. After a rock has been exposed on thesurface of the earth, for example, through avolcanic eruption, the earth’s natural processesbegin to break it down. This may happen whenrain, wind, or snow erode the rock or break itinto smaller pieces. Rocks can also be chemi-cally broken down by acid in groundwater.They may be physically broken down by plant

Have you ever thought about the life of a rock?Where did it come from? What did it look likewhen it was new? What will it look like in ahundred years? What will it look like in a thou-sand years?Throughout history, rocks have moved

around from place to place. For example, onemight find a rock containing fossils of a deep-sea creature on the summit of a mountain. Howcould that be? Rocks move around as a result ofthe earth’s forces. Wind, water, volcanic erup-tions, and plate movements see to it that rocksnever stay in one place. As the changing earthmoves them about, the rocks change. Overthousands of years, one rock can change intoanother kind of rock. The new rock may have atotally different appearance.

THE ROCK CYCLE

The rock cycle

At the earth’s surface,rocks are broken downto form sediments.

Sediments are depositedin layers and compacted.

SEDIMENTARYROCKS

Heat and pressuredeep in the earth

METAMORPHICROCKS

Melting

MAGMA

Cooling

IGNEOUSROCKS



rock can be transformed into one of the others.(See the table, “Examples of Rock FormationDue to the Rock Cycle.”) For example, anigneous rock, such as granite, can changeunder high heat and pressure into a metamor-phic rock called gneiss (say “nice”). This iswhy these two rocks are often mistaken for oneanother. At other times, a metamorphic rockcan become weathered before melting into anigneous rock. This might happen if the meta-morphic rock lifts up during mountain building.Rocks continuously travel through the rock

cycle and along its shortcuts. So the next timeyou pick up a rock, think about where it hasbeen and what it was in the past. And remem-ber: “Just because it is ‘gneiss,’ don’t take it for‘granite!’” �

roots or humans. The tiny pieces ofbroken igneous rocks are called sedi-ments. Over time, sediments canbecome compacted and cementedtogether into sedimentary rocks.Sandstone is an example of a sedi-mentary rock. These rocks some-times move deep into the earthwhen one ocean plate sinks belowanother plate.Plate movement puts great heat

and pressure on rock. If either sedi-mentary or igneous rocks changeinto something else because of theearth’s great heat and pressure, wesay they have metamorphosed, orchanged, into a new kind of rock,called metamorphic rock. Sometimesthere is so much heat and pressure that therock melts and creates magma, the raw materi-al from which igneous rocks form. Or the rockmight become ductile and bend during moun-tain building. Folds, like the one shown in thephoto above, develop in the rock when oneplate collides with another. If the pressurecomes from opposite directions, the mineralsin the rock might line up with one another.Lines in rocks are a sign that pressure and heatwere applied to the rock over time. Heat andpressure can metamorphose any type of rock—sedimentary, igneous, and even other meta-morphic rocks!As the earth’s ever-changing surface goes

through episodes of uplift, erosion, deposition,and burial, each of the three basic kinds of

Folded gneiss is a metamorphic rock. The folds develop in the rock as

one plate collides with another. This rock may at one time have been

igneous (such as granite) or sedimentary (such as sandstone). Heat

and pressure from the plate collision changed the rock into gneiss.

©CHIPCLARK

Examples of Rock Formation Due to the Rock Cycle

Starting Rock First Process Second Process New Rock

Igneous or metamorphic Uplift/mountain building Erosion/sedimentation Sedimentary

Sedimentary or metamorphic Deep burial Melting Igneous

Igneous or sedimentary Deep burial Recrystallization Metamorphic

Lesson 22: Exploring Igneous Rock Formation

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Transcript of Lesson 22: Exploring Igneous Rock Formation