Field Guide to Plutonic and Metamorphic Rocks. Earth Science Curriculum Project Pamphlet Series PS-5

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Romey, Williar D.Field Guide to Plutonic and Metamorphic Rccks. EarthScience Curriculum Prcject Pamphlet Series PS-5.

American Geological Inst., Washington, D.C.National Science Foundation, Washingtcn, D.C.

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DESCRIPTORS *Earth Science; Field Studies; Field Trips; *Geology;*Geophysics; Instructional Materials; *ScienceActivities; *S4ence Course Improvement Project;

Science Educaticn; Science Instructicn; SecondaryEducation; *Secondary School Science

IDENTIFIERS *Earth Science Curriculum Project; National ScienceFgundation

ABSTRACTSuggested are methcds for the collection cf field

evidence about processes that form plutonic and metamorphic rock.

Desc-iption and discussion of these types of rocks arc prtvided. The

.ing and execution of a successful field trip is discussed.Advanced field projects are also discus"sed. Included arc five

appendices, references, and a glessary. (PE)

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Plutonic and metamorphic rocks appear inMany areas at the earth's surface. Samples fromdeep drill holes show that these rocks underliethe layered rocks that cover most land. Oftenthey are beautiful rocks, containing interestingstructures, unusu l minerals, and, occasionally,.large. well-formed cryctals. Yet nowhere has

man been able to observe these rocks in theprocess of formation. Field evidence and experi-ments lea.1 geologists to believe that plutonicand metarkahic rocks form deep below thesurface, wher-rtemperatures and pressures are

. high. Looking for field evidence ahniit processesthat form these rocks is the subject of this palm,phlet.

Dr. William D. Romey is Director of the EarthScience Curriculum Project and Adjunct Asso-ciate Professor of Geology and of ScienceEducation at Syracuse University. He has con-

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ducted field and laboratory studies on plutonicand metamorphic rocks in northern California,in the Adirondack Highlands of New York, andin Norway. He held a National Science Founda-tion Science Faculty Fellowship at the Geolog-ical Museum, University of Oslo. Norway, andhas lectured in the United States,1Norway. the

A; Soviet Union. and Australia on both geology andon science education.

Copyright 1971 American Geological Institute

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FIELD GUIDE TO

Plutonic and

Metamorphic

Rocks

William D. Romey

Series Editor: Robert E. Boyer

EARTH SCIENCE CURRICULUM PROJECT

Sponsored by the American Geological InstituteSupported by the National Science FoundationEndorsed by the Council on Educationin the Geological Sciences

HOUGHTON MIFFLIN COMPANY BOSTON.1*

New York Atlanta Geneva. III Dallas Palo Alto

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Contentt

Introductionplutonic RocksMetamorphic Rocks

Conditions in the Earth's Interior Promote Metamorphism

"Reading" a Metamorphic Rock

Migmatites

Taking a Field TripSetting Your GoalsField EquipmentFinding OutcropsVisit to an OutcropActivities fo'r Areas Without Suitable Outcrops

Advanced Field ProjectsDescribing a SectionThe Grade of Metamorphism

Special Features of Plutons

Mapping Metamorphic and Plutonic Rocks

Cross-Country Geology

AppendicesIKey to Rock-Forming Minerals

IIIdentification of Metamorphic 'and Plutonic Rocks

HIPercentage Compositkon of Rocks

IVTypical Mineral Assemblages and Probable Parent Rocks

VMaking Measurements on Tilted Rocks

ReferencesGlossary

= Plutonic* andMetamorphicRocks

INTRODUCTIONAll of the many kinds of rocks at the earth's

surface contain clues that can help you determinesomething about how they formed. It is easy toimagine that a piece of sandstone, for example,formed when grains of sand were cemented orsqueezed together, perhaps like a snowball. Youmay readily conclude that rocks on the sides of avolcano formed when lava spewed out from acrater, cooled, and became solid, preserving howmarkings like those in the sauce on a hot-fudge

supdae.Other kinds of rocks, however, called plutonic

and metankirphic rocks, cannot be seen forming atthe surface of the earth, even though we can findthem there now. These rocks are generally exposedin mountain ranges, or in what are thought to be the

roots Of ancient mountain ranges. For these !ea-sohs, sand from laboratory evidence, both plutonicand metamorphic rocks are believed to haveformed deep below the earth's surface. How didthese rocks become exposed to view? Duringmountain building, the crustal layers must havebeen gradually uplifted, and weathering and ero-sion must have slowly stripped away the uppercover of rocks.

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2 / PLUTONIC AND METAMORPHIC RC,CKS

When the materials that make up sandstone,

limestone, and other sedimentary rocks that

accumulate at the earth's surface are burieddeeply under other rock layers, they are sub-jected to high temperatures and pressures. Thesechanges in conditions cause minerals in the rockmass to react chemically. If the rock is not actuallymelted, the resulting rocks, such as schist, gneiss,and marble. arc inetamorpltic rocks. They common-ly show a pattern or orientation in the arrangementof their grains, like the sedimental/ rocks frornwhich they formed. Pare spaces that existedbetween grains of the original sedimentary Tocksdisappear as the grans are squeezed togetherand chemical reactions tape place.

If sedimentary or metamorphic rocks are buriedstill deeper. they may become so hot that' theymelt, forming magma. Later ,:ooling of the magma,perhaps several kilometers below the surface,produces ign('ous (fire-formed) rocky.

When you look at a rock, however, it may not bepossible to tell how it formed. Rock that has beenheated almost to the melting point is often ;ndistin-guishable from rock containing the same mineralsthat has actually been melted. Geologists lumpthese kinds of rock into a category called plutonic

Figure 1. Granite. (A)Outcrop. (B) Cluse-upof cutcrop. (C) ;Thinslice seen under amidroscope (cross- ,polarized light).

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PLUTONIC AND METAMORPHIC ROCKS / 3

rocks (from Pluto, the Roman god of the under-world). A sample of a plutonic rock, such as granite.is probably igneous in origin, but it may be meta-morphic. However, a metamorphic *tonic rocklooks more like other plutonic rocks than like ametamorphic rock. Plutonic rocks tend to have

.1 large grains with little or no pattern or orientation.Table I gives a summary of the characteristic§of the different classes of rocks.

Table I. ('haracteiistics of Rock Types

Rock T})pe Pore Spaces Orientation of Grains

Sedimentary yes yesMetamorphic no yesPlutonic no little or none

Laboratory work and field evidence about thestructure and appeara'nce of large plutonic rockbodies are the basis for arguing whether theyformed from magma or by metamorphism thatstopped short of. melting. This field guide is in-tended to help you learn to observe plutonic andmetamorphic rocks and to find clues as to howthey formed. In turn, you will also learn some-thing about what must be happening below theearth's surface even today.

PLUTONIC ROCKS

Plutonic rocks commonly occur in large. blob-shaped masses called plutons that may range insurface area from a few square meters to manytens or even hundreds of square kilometers. andmay extend several kilometers below the surface.The rocks in plutons are usually massive ar,dcontain mineral grains large enough to he seenreadily with the naked eye (Figure H. Mineralgrains characteristically grow together in an

interlocking fashion called crystalline texture

(Figures (1.3 and 1 C). Some plutonic rocks have

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crude layering; others have mineral grains ar-ranged vaguely parallel to each other suggesting

that these grains floated in magmamuch as logs

float in a sluggish stream.Plutonic rocks commonly contain the same kinds

and amounts of minerals found in many lava flows,

Quartz, feldspar, and a few dark-colored minerals,

in various, .proportions, are the main minerals in

plutonic rocks. (See Appendix L) Gradations found

in some places between lava flows and plutonic

rocks lead field workers to cOnclude that plutons

forthed from melted rock that crystallized below

the surface. The central parts of some thick lava

flows have the same mineral content and texture

as some plutonic rocks. Rocks in the root zones

of deeply eroded volcanoes look much like plutonic

rocks from within the crust.In many places, bodies of plutonic rock that are

large in surface area, but relatively thin like a

table top, cut across the layering and structure

of other rocks (Figure 2). The layers must have

been there first, and the plutonic rock moved in,

or intruded, later. Such cross-cutting plutonic

rocks are therefore younger than the rocks they

intrude.Granite is the most abtmdant plutonic rock ex-

posed at the earth's surface. Although granite

bodies cover extensive areas on the continents,

estimates or rock densities from measurements of

the earth's gravitational attraction suggest that

most granite bodies arc relatively shallow, probably

extendim; downward only a few kilometers or, at

most, a few tens of kilometers. The magmas from

which many granites crystallized probably came

from the melting of sedimentary, metamorphic,

and volcanic rocks pushed downward into the root

tones of newly forming mountain chains.

Some geologists believe that a few granites

formed by a process called Kranitization. in which

chemical compommts of older rocks separate,migrate, and recrystallize without actual melting

to produce granite that is metamorphic in origin.

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Where there is no clear field and laboratoryevidence that the plutonic rock formed by crystal-lization from a liquid, a granitization origin musthe considered. Nonetheless, 7granitized granitesmay still be classified with the plutonic rocks, forthey have indeed formed at a depth within the crust.As is true throughout nature, we must recognizemany gradations among rock types and rocf.k-forming processes. Plutonic and metamorphicrocks gra& into one another. for they form in muchthe same envieonment.

METAM6RPHC ROCKSMost metamorphic rocks look very diffe-ent from

plutonic rocks. One reason is that the mineralgrains grow together in different ways in the two

Figure 2. Intrusion ofdark-colored plutonicrock cutting metamor-phic rock.

5


groups of rocks. Compare Figure I , which shows atypical plutonic rock, with Figure 3, which illus-trates major kinds of metamorphic rocks. In metarmorphic rocks, minerals are commonly alignedparallel to each other, enabling the rock to spliteasily in certain directions. Layering of variduskinds can be seen in most metamorphic rocksalthough in many these layers are crumpled and

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Figure 3 Metamor-phic rocks. (A) Slatefence, outcrop of slate,and close-up of slateoutcrop

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crop, close-up of lay-ers, and thin slice ofone of the dark layers,seen under a micro-scope.

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'folded. Such folding may involve large racit masses,antl be clearly .visible from a distance (Figure ..i

or ii 'my, be 'too small to be' seen except through j. .._,,

a microscope. . I r 7

Metatorphic rocks contain a greater variety Pf':mineralithan do plutonic rocks, because great hetlt

'1breaks -down many minerals. Whereas quarti,7.z

feldspar, and a few dark-colored minerals are the. -.)

only Minerals present in large quantities in vest', Aziplutonic 'reeks, metamorphic rocks maycofitain-,-)

many different minerals in almost any proportions. f . i

However, ctrtain grOups of minerals are commonlyassociated in certain kinds of metamorphic rocks,

...; eand the Cistinctive mineral combinations nrovkle, ,{

t important clues about how these rocksforined. .

The word metamorphic is taken from the Greek -words meta, meaning change, and morphe, meaningform. A metarnomhic rock, then, has been cop-verted 'from one kind of rock into another. But,hovii-are rocks metamorphosed (changed)? LaboratorW,

_ experiments reveal that when rocks are subjecteflto high temperature and pressure, chemical.:.reactions may form characteristic metamorph)cminerals from the minerals originally in the reek.

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Figure 4. Foldedrocks. (A) Folded lay-ers weathered out. (B)Folds and pinched-outblobs seen in a roadcut.

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PLUTONIC AND METAMORPHIC ROCK'S / 9

Conditions in the Earth's Interior PromoteMetamorphism

Measurements made in deep, mines and drillholes reveal that temperatures within the earth'supper crust increase downward at an average rateof about 30°C per kilometer. At this rate, whattemperature would you expect at'a depth of tenkilometers? At 20 kilometers? Below about 20kilometersfrom the surface,.however, the tempera-ture evidently increases' more slowly, since rocksbegin to melt at temperatures greater than about700°C, and there is no evidence tv siiggest that theearth is mainly tnolten at depths as shallow as

' 30 or 40 kilometers. Indeed, evidence based onthe study of vibrations caused by earthquakcwaves indicates that at these depths the earthbehaves as a solid.

Pressure also increases rapidly with depth,caused by the weight of overlying rock and thecompivssion 'of water vapor and other gasestrapped in its pore spaces. Imagine the .veightof a cc:umn of rock one meter square and tenkilometers high! Great pressures may also beproduced by the processes of mountain building, asevidenced by tight folding.

But, how do stdimentary and volcanic rocks atthe earth's surface get 'down to depths wherehigh temperature and pressure can convert theminto metamorphic rocks, or,even melt them? These

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10 / PLUTONIC AND MSTAMORPHIC ROCKS

rocks commonly accumulate in basins located atthe edges of continents. Present geologic theoriessuggest that sea-floor spreading may cause thesebasins to be ',silted slowly under the continentaledge as layer upon layer of sediments is deposited.In somi, basins piles of sediments, may becomemany kilometers thick. As the sediments are moveddownward, the increased temperature and highpressure cause the minerals to react chemically. to he metamorphosed.

Metamorphism can also occur when magmacomes into contact with other rocks, as when alava flow pours from volcano The kind of bakingthat occut:s in this .way is caHed coma( t meta-morphism. It can be seen at thY earth's surfacewhere a lava flow has spreadover soil or rockgiving it a."baked" appearance.

0

"Reading" a Metamorphic Rock

All metamorphic rocks were once other rockt:sedimentary volcanic, plutonic. or even an earliergeneration of metamorphic rocks. If you look at ametamorphic rock with the right kinds of questionsin mind, you may he able to tell what kind of rockit was beforeits "parent" rock. The Parent rockof marble is probably limestone (both are composedalmost entirely a the mineral calcite). and quartiiteis likely to be the "offspring" of quartz sandstone.Other examples a parent rocks and their meta-morphic offspring are given in Appendix II.

For many metamorphic rocks the ancestry kmore difficult to determine. Three or four possibleparent rocks may have to be considered. Forexample. a sandstone made of grains derived fromthe erosion of granite may' have the same chemicalcomposition as reid granite. and the metamorphicoffspring of these two rock types' could he identical.Furthermore. during nwtamorphism some atomsmay move out of one mineral (w rock and accumu-late elsewhere to form new minends,there. If this

PLUTONIC-AND METAMORPHI8 ROCKS /11

occurred, the metamorphic roceomposition maygive a wrong idea about the fiattire of the parentrock. In spite of these difficutties, the compositionof a metamorphic rock commonly yields useful

11information about the rock's.origin.

In the early 1900's several geologists recognizedthat metamorphic rocks vhich apparently hadidentical parent rocks lookyd very different. Thesegeologists ground up s'amMes of slates, phyllites,schists, and gneisses (Five 3)all metambrphierocks of quite differenQappearanceand cheth-ically analyzed the resulting powders. Theyrealized that the differunt combination's of dif-

Tferent minerals all had th.e same chemiFal composi-tion as mud or clay. Ally did these offspring ofthe same parent materialtlook so different from eachother and contain different minerals (Appendix I I)?Perhaps these diffoant metamorphic rocks hadbeen buried to different depths. If clay or mudwere buried deeper and deeper it would first beconverted into shale, a sedimentary rock, thenprogressively into the metamorphic rocks 'slate,schist, and finally gneiss. With still deeper burial,it would ultimately melt and form a new plutonicigneous rock. Weathering of any of these rockswould form clay again.

Geological experimenters have since subjected,clays to high temperatures and pressures undercarefully controlled conditions in the laboratory.In this way they have been able to confirm andrefine conclusions originally bEted on field work.jvlodern field and experimental studies have madeit possible to recognize certain key groupings ofminerals that allow the probable depth at whichthe rocks have been metamorphosed to be inferred.However. it is important to realize that actual

.pressures and temperatures within the earth'scrust are not accurately known. Thus it is notpossible to infer accurately the depThs at whichthe rocks formed, although good estimates of thesedepths can he made.

1 6


Figure 5. Diagramshowing temperaturesand depths of burialrequired -4o producevarious grades ofmetamorphism. Typi-cal rock types charac-teristic of the variousgrades are listed inAppendix IV.

Various groups of key minerals and the probabletemperatures and pressures at which they formed,with approximate depths within the crust, areindicated in Figure 5. In the zone of low-pressuremetamorphism by baking, the commonest rocksare fine-grained, flinty hornfelses. The low-graderocks include slates, phyllites, and some schistscommonly containing chlorite, epidote, and mica.The medium-vade rocks include schists, amphib-olites. and somc.., gneisses. Garnet, ,mic.as, horn-blende. pyroxene, staurolite, kyanite. and silli-manite may be present in certain combinations.The high-grade metamorphic rocks include mainlycoarse-grained gnekses with little or no mica orhornblende. They are characterized especiallyby sillimanite, garnet, and pyroxene. Tne- highest-

1 7

PLUTOMC AND MVAMORPHIC ROCKS /13

grade metamorphic rocks are gneisses composedentirely of light pink garnet and deep green py-roxene. Rocks in any of the grades below thehighest may also contain quartz and feldspars,deknding on the overall composition of the rock.

Migmatites

Some areas contain mixed rocks. mixtures ofplutonic and metamorphic rocks 6.:itted

tiles. like the ones shown in Figure 6. Somemigmatites are metamorphic rocks that contain afew isolated patches of granite or other plutonicrock. Others are mainly plutonic, but containblocks of metamorphic rock. Mixed rocks mayhave formed by partial melting of metamorphic

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Figure 6. Migmatites.(A) Partially recrystal-lized masses of meta-morphic rock in gran-ite. (B) Rounded,bouldew-like pieces ofdark plutonic arvimetamorphic rocks ingranite.

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Figure 7. Structuresin mixed rocks. (A)

Gradations betweenmetamorphic rocks, atthe left of each dia-gram, and plutonicrocks at the right.(B) Intrusive relation-ships The gray-col-ored material has

probably been squeez-ed into the other. (C)Granitic rock (gray-colored) containingrotated blocks oflayered metamorphicrock In places thecrystals in the graniteare aligned parallel tothe layers in the meta-morphic rock (D)Block diagram show-ing a contact betweenlayered metamorphicrock and granite

rock, by injection of granitic material formed else-where, or by' granitization of parts of the rock.Figure 7 diagrammatically illustrates some rela-tionships of mixed rocks you may find.

Migmatites, as well as plutonic and metamorphicrocks. are more abundant than they appear to be.Sedimentary rocks cover about three quarters ofthe earth's land surface. However, studies ofearthquake waves and data from drill holes snowthat probably more than 95 percent of the earth'scrust below the surface consists of metamorphicand plutonic rocksmainly metamorphic rocks inthe upper part and plutonic rocks at greaterdepths. Great masses of granite and other plutonicrocks may he hundreds of kilometers long and wide,like those in the Sierra Nevada and Coast Rangesof the western United States. At the edges anddraped over the tops of these masses are meta-morphic rocks. Commonly a /one of migmatitesoccurs along the contact between the plutons andthe adjacent metamorphic rocks.

} 15 CM I

PLUTONIC AND METAMORPHIC.ROCKSJ 15

TAKING A FIELD TRIPSetthirg Your Goals

Before taking a field trip, decide what you wishto study. Certain questions should be in your mindbefore you begin. Here are some specific' goals:

1) Determine where in your area metamorphicarid plutonic rocks are found.

2) See how they differ from sedimentary rocks.3) Make a collection of common metamorphic

and plutonic rocks.4) Collect and study metamorphic minerals.5) Determine whether the rocks in one outcrop

can be 'related to rocks in other outcrops.6) Study the groups of minerals present in

metamorphic rocks and make conclusionsabout the temperature and pressure conditionsunder which ttr rocks formed.

7) Make a simple geologic map of your area toshow the surface distribution of differentkinds of metamorphic and plutonic rocks.

8) Work out the sequence of events that hasaffected an area of' metamorphic rocks.

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9) Discover whether metatriorphic and plutonicrocks are used as building stone in your town.

The detailed suggestions in the following sectionSare organized in approximate order of difficulty.Easier suggestions are listed first, and the morecomplicated ones are included under AdvancedField Projects. Before undertaking an advancedproject, make sum that you first master the tech-nicues suggested here.

Field Equipment

A list of suggested basic equipment for field tripsfollows. How much of it you will need depends onwhat you plan to do. The main things needed arecuriosity, willingness to observe carefully, and animagination.

Hand lens (prefei ably 10-power)Small sledge hammer (I; or 3-pound head) or

geology pickPlastic glasses or goggles to protect your eyes

while hammering on rocksSmall packsack for carrying samples, paper or

cloth bags for the samples, and a felt markingpen

Notebook"lopographic map of the area you will visit. Such

maps can usually be purchased at local book-stores, or ordered from the U.S. GeologicalSurvey. (See References.)

Compass (necessary only if mapping is planned,or if you will he in areas away from roadswhere .you might otherwise become lost)

Small 'dropper bottle containing dilute hydro-chloric acid ( in, which may he obtained atdnig stores (needed only in areas where marbleis present)

Sturdy shoes or hootsPencils, pen, and rulerCamera. if availableif you go onto private property, be sure you get

permission from the owner. Most people will letyou onto their property if you ask them .first.

1.

METAMORPHIC.

PLUTONIC

SOUTHERN LIMIT OF GLACILRS

Finding Outcrops

In parts of the United States, metamorphic andplutonic rocks are buried beneath rocks such assandstone. limestone, shale, and volcanic rocks.Areas where you can expect to find exposures ofmetamorphic and plutonic rocks at the surfaceare shown in Figure 8. Why do you think meta-morphic and plutonic rocks are so abundant inmountainous areas and so rare in the interior ofthe continent'? More detailed information should 11,.available on geologic maps of your local area (,rstate, or on other available geologic maps. Some ofthese maps are listed in Selected Maps and EarthSciem.e Publications. (See References.) Or. writedirectly to the state geological survey in your stalecapital for information on the availability of geo-logic maps. Geology faculty members at local col-leges will suggest suitable mups if you phone orwrite to them..

Road cuts are one of the best places to look forfresh rock exposures. But he careful of traffic anddon't knock chunks of rock onto the road! Quarriesare particularly good places to observe fresh rock

9 2

Figure 8. Schematicgeologic map of theUnited States showingapproximate locationof main areas of plu-tonic and metamor-phic rocks.(Note: Small darkareas representingplutonic rocks shouldalso appear in the.reg ions )esignatedas metamorphic inWisconsin. Minne-sota. Vermont, NewHampshire, RhodeIsland Connecticut,New York. and Massa-chusetts Cape Codshould not be shownas containing meta-morphic rocks.)

17

PLUTONIC AND METAMORPHIC ROCKS

surfaces, but be careful of falling rocks and crum-bling quarry edges. Stream valleys and rocky ares.along a seacoast or lake shore are also good placesto expect exposures. Hilltops may provide out-crops.

Even though there may be no outcrops of plu-tonic or metamorphic rocks nearby, you mayfind boulders of these rocks in area3 in thoseparts of the northern United States that werecovered a few thousand years ago by the Pleisto-cene ice sheets (Figure 8). Where do you thinkthese boulders came from?

Visit to an Outcrop

First, locate the of!tcrop on your topographicmap or road map. Give it a number on the map andin your notebook so that you can readily relateyour observations to a definite locality.

Next, look over the entire outcrop from a dis-tance in order to see major features. At a roadcut, stand across the road and observe the outcrop.Describe important observed features like the fol-lowing carefully in your notebook and sketch theoutcrop, marking specially interesting things younotice. .11 you have a camera, photograph theoutcrop.

LayeringDoes the outcrop contain distinctlayers or does it appear massive and unlayered? Iflayered. what distinguishes the d'fferent layersa difference in color. size of grains, kind of vegeta-tion that grows on various parts of the rock? (SeeField Guide to Layered Rocks, in References.-)

FoldingAre the layers horizontal or tilted? Iftilted, in which direction and how many degree'sFrom horizontal'? Are there obvious folds in thelayers? IF so, describe them. (Appendix V describesways of measuring the tilt of the layers.)

IntrusionsDo intrusions cut across the layer-ing or across the parallel arrangement of minerakin the rocks? Are mixed rocks present'?

Breakage patternsDo the rockq break into.Hocks or split readily into platy fragments?

2 3


WeatheringAre the rocks weathered or fresh?(See Field °C.1 aid(, to Rock Weathering, in Refer-ences.) Before moving up to the outcrop, viewit from several places. Any outcrop presents aproblem in solid geometry, and you may see rela-tionships on one surface that do not appear onanother. In Figure 9, for example, the surfacelabeled "A" suggests a uniform and unlayeredrock. The surface labeled "B", however, revealsthat the rock is actually layered.

Now move close to the outcrop. Do you observelayering that was not obvious from your overview'?Describe th e. texture of the rock, how the individualmineral grains fit together. Are individual mineralgrains within the rock flat and parallel like sheetsin a pad of paper? If so, the rock texture is jbliated.Are elongated mineral grains aligned parallel toeach other like- pencils in a bundle? If so, thetexture is lineated. Do the minerals define more orless parallel planes? Are these planes horizontal.vertical, yr tilted?

Break off pieces of rock from various layers orparts of the outcrop. On your sketch of the out-crop, number ihe locations of rocks examined.

Figure 9. Sketch ofan outcrop. A, theweathered, massive-looking surface. B, aside view that revealslayering.

/;+'""'W

"7.3r 4 .011/41.-. 414

' 20 / PLUTONIC AND ME TAMORPHIC ROCKS

Use your hand lens and the ,mineral identifica-

tion key (Appendix I) to identify the variousminerals. Estimate the relative amounts of thedifferent minerals, using the percentage estimationkey in Appendix III. Then name the rock usingthe rock identification key in Appendix 11.

Label samples of various rock types, keying themto your sketch of the outcrop. You can study thesesamples more carefully at home. Use Appendix IIor IV t$sidentify the likely parent rocks of variousmetamorphic l'ock types you sampled.

Visit other nearby outcrops. How do they re-semble or differ from the first outcrop?

\ctivities for Areas Without SuitableOutcrops

In areas north of the limit of the Pleistoceneice sheet (shown in Figure 8), you may find boul-ders of Metamorphic and plutonic rocks eVellthough no nearby outcrops ocsur. Go to a fieldcontaining boulders or to boulders piled along afence. How much information can you gain fromobserving single boulders? How many differentkinds of rock can you find?

If possible, find gravel exposed in a road cut,gravel pit, or stream bank. Collect 100 pebblesand boulders; do Otot choose special ones, buttake 011 pebbles and boulders within a small areauntil you have 100. Use the rock identificationchart (Appendix II) to identify .the rock types.

What percentage of the pebbles is plutonic? Whatpercentage is metamorphic? What percentage is

neither plutonic nor metamorphic? What is thenearest source of the plutonic and metamorphicmaterial? How many different types of plutonicand metamorphic rocks do you find?

Pebble counts will force you to look carefullyat the rock fragments of a gravel bed. Geologistswho study glacial deposits commonly make pebblecounts and from them recognize deposits fromvarious sources and of different ages.


Another goo& place to observe plutonic andmetamorphic rocks is in cemeteries. Just remem-ber not to use hammers. How many different rocktypes do you observe in a cemetery? Is there adifference in rock type between older and newertombstones? Which have been least durable? Why?(For further suggested activities, see Field Guideto .Rock Weatlwring, in References.)

How many different plutonic and metamorphicrocks can you find that have been used as buildingstone or in sidewalks and curbstones? Do you finda higher percentage of plutonic. and metamorphicrocks or ofsedirnentary and volcanic. rocks'? Doyou, find thpecific rock types are used forspecial Purposes? Try to discover a reason :sorwhat you observe.

Many museums contain displays of rocks andminerals. Visit displays in nearby college or univer-sity geology departments, state geological surveyoffices, or mining company offices. Compare yoursamples with the display specimens to verifyyour identifications. If you do not agree with themuseum's identification, do not hesitate to chal-lenge it.

ADVANCED FIELDPROJECTS

A single outcrop is like one piece of a jigsawpuzzle. You win get more satisfaction from puttingthe puzzle together than from looking at a singlepiece. Observing single outcrops, identifying rocks.and making collections are interesting activitiesin themselves, but by themser es they rarely leadto solutions to bigger problems. To develop aregional geologic history or an understanding ofmajor processes operating below the surface.examine several outcrops and relate your observa-tions from one outcrop to the next.

2 6

22 / PLUTONIC AND, ME1'AN1ORPHIC ROCKS

Figure 10. Views of,two adjacent out-crops. Note how it ap-pears that the layerscan be matched upeyeri though they arenot in contact.

Degcribing a Section

In layered rocks, measure the thiykness of dis-tinct layers.i`n several nearby outcrops (Figure 10)..Using notebook sketches, arrange the rayei-s fromeach outcrop in their proper sequence If severtiloutcrops contain layers of marble, for example..are they of the sarrie thickness at each outcrop? Canthe'rock types above and below a marble layer inone outcrop be found in the same relative positionsin other outcrops? Does a distinctive rock type,such as amphibolite or quartzite (see Appendix IIfor description), characterize a certain level in thesequence of layers? Use the measui-ements androck descriptions from several outcrops to relateoutcrops to each other. The process of comparingrelated iayered sections from different places iscalled matching or correlating. (See -Field Guideto Layered Rocks, in References.) Try to build acomposite sequence of layers for a whole area.

Once you have determined the relative positionsof various distinctive rock types in a sequence oflayers, study the Minei ak in each layer.and attemptto determine the parent rocks. For example, sup-pose that in several outcrops you find an easilyrecognizable sequence or layers including a layerof marble, a layer of quartl-mica schist, and a

0 7

24 / PLUTCNIC AND METAMORPHIC ROCKS

Figure 11! Constructing a map to showchanges in metamor-phic grade. Light linesrepresent ropds; darklines represent bound-aries between areas ofequat metamdrphicgrade (Appendix IV);numbered dots repre-sent sample-collect-ing points.

The dashed lines in Figure .11. separatai areascontaining mineral assemblages of ditfefentgrades. These dashed lines sepasating rocks ofdifferent grades are called isograds, lines of equalgrades or amounts of metamorphism.

With this information you can make inferencesabout the rocks you are studying. Thew yOu canbegin to answer the following questions:

I ) What rocks probably existed ,in the areabefore metamorphism? Possible answer:shaly and limy sedimentary. rocks.

1) How deeply were these rocks buried? Towhat maximum temperatures were theysubjected? Possible answer: From Figure 5you can infer that the low-grade rocks haveprobably been burieci at least 15 kilometersdeep and have been subjected to te'mperaturesabove 350°C.

3) Which part of the area was once subjectedto the highest temperatures and prestures,and was thus probably most deeply buried?Possible answer: The southern part, because


layer of quartzite. What parent rocks do thesethree rock types iMply? From Appendix 11 you,might infer that this was probably a sequence oflimestone, shale, and sandstone.

The Grade of Metamorphism

Collect several samples from outcrops of meta-morphic rocks. Using Appendices 1 and II, iden-tify the minerals present. Pay special attentionto the groups of minerals that are found together ina single rock type. Consult Appendix IV to attemptto determine the grade or degree of metamorphismand the pafent rocks of each group of minerals.(Note: The tables presented in this pamphlet aresimplified. More detailed tables are found in someof the sources in the References.)

How many different parent rocks are repre-sented? Plot yotir data on a map like that in FigureI . Note that stations I and 4 of this figure contain

.mineral assemblages, series of minerals closelyassociated within a single rock type. common inlow-grade metamorphic rocks. Assemblage I is

muscovitz-chlorite-quartz-plagioclase: assemblage2 is calcite-dolomite-quartz. Appendix 1 V indicatesthat these are low-grade metamorphic rocks.

At stations 2, 1, 5. 6, 7. and 8. assemblage 1 is

kyanite garnet -,muscovite -biotite -quartz: assem-bhAge 2 is caleite-pyioxene-deep red garnet. Ac-cording to Appendix ly these are medium-gradernetarorphic rock S.

At stations 9, 10, and I I, assemblage I is silli-nianite - garnet potassium feldspar - plagioclase -quartz. assemblage 2 is plagioclase-pyroxene.These assemblages indicate high-grade meta-morphic rocks (Appendix

,4\semblage I for each zone was seen in rocksprobably derived from shaly mecent rocks. As-semblage 2 for each zone was seen M rocksprobably derived front limy parent rocks. .f he

conclusiom on parent roc`ks are also based onAppendix IV.


it contains iiigher-grade metamorphic rocks.Note that if a granite pluton were found justsouth of the map area, heat and hot liquidfrom the cooling pluton might have influencedthe metamorphism.

Special Features of Plutons

In plutonic rocks, look for aH contacts of dif-ferent rock types. The surface or zone wherethe plutonic rock meets surrounding rocks is animportant contact. Commonly, metamorphic rockssurround plutons, but. some plutons are in contactwith sedimentary rocks, volcanic rocks, or otherplutonic rocks. Careful observation of the outercontacts of plutons may suggest whether the plutonis older or younger than adjacent rocks.

If fragments of the surrounding rock appear tohave fallen into the pluton. if intrusions can betraced outward from the pluton intoatdjacent rocks,and if the rocks surrounding the pluton were bakedby contact metamorphism. the pluton is youngerthan the rocks surrounding it. If, on the other hand,fragments of the plutonic rock occur in the over-lying rocks, the pluton is probably older, and thesurrounding rocks were deposited later on top ofthe pluton.

Some plutonic rock masses are nearly uniformin composition. You may be able to examine thisrock for kilometers without seeing much changein rock type or structure. Other plutons are madeup of several rock types. Contacts between thevarious types may be sharp or gradational. WatchcarefuHy for changes in color or structure of therock.

Although plutonic rocks, and some metamorphicrocks, may appear uniform over wide areas, subtlezoning and slight changes in rock type may occur.To determine the nature or such variations, collectsamples at regular intervals. Set up a grid pattern

3 o

Figure 12. Sample ofa geologic map froman area of igneous andmetamorphic rocks,Contour lines are notshown. In the fieldyou would've them to!coati) stations and todraw contacts be-tween rock types Thpgranite is hom415-neous and a samplinggrid has been ruled 6nit.

26

on your map (Figure 12), dividing the map intosquares, perhaps half a kilometer on each side.Collect a sample at each corner of the grid'squares.At 'home, place all the samples in their properrelative positions. Do you detect differences whichcasual observation did not disclose? Are rocks inone part of the pluton slightly darker in color, dothey contain grains of a different size, or do theycontain greater amounts of a particular mineralsuch as quartz or poursium feldspar? Once youdiscover subtle vwiations, you can develop aneye for recognizing them in the field.

31

.PLUTONIC AND METAMORPHIC ROCKS / 21

Mapping. Metamorphic and Plutonic, Rocks

To make a more complete field study, preparea geologic map. First develop skill in identifyingrocks, locating your position on the map, andmaking basic observations at individual outcrops.A geologic inap shows in detail where various rocktypes are located and indicates their structure.Think of the map as-what you would see if youphotographed a three-dimensional model of thearea.

Firg you need a base map of the study area. Ifa topographic map is not available, a road mapmay do. The more detailed you want the geologicmap to be, the better and more detailed the basemap must be. Information is then plotted on thismap.

A geologic map of an area of metamorphic.andplutonic rocks in northern California is shownin Figure 12. The process was begun by firstexamining individual outcrops. At stations I and 2the mapper found diorite, a plutonic rock describedin Appendix II, and at stations 3 and 4 he foundhornblende schist or amphibolite. Having estab-lished the rock types of these four stations, hethen drew a contact line on the map. This linerepresents the outcrop of the zone that, separatesdiorite from hornblende schist. North of this con-tact, until granite appears, the rock is diorite: tothe south. until quartz-mica schist appears, it ishornblende schist. The mapper continued to makenew observations and record them on the map untilhe had outlined the boundaries of rock units hecould recognize. If contacts are traced carefully,they may outline major structural features as wellas rock types. Note the folds in the lower half ofHgure 12 and try to visualize them in three di-mensions.

Observe the direction and inclination from thehorizontal of layered rocks shown by appropriatesymbols on the map (stations 3 and 4. Figure 12).The longer line. called the strike line, represents

Mgr

28 / PLUTONIC AND METAMORPHIC ROCKS1111

7 `;, ,:.,,7,11N.;

the direction you would read from a compass if

you sighted the compass along a horizontal linedrawn on the surface of the layers. This line actu-ally has two possible directions: the words east and

west merely describe opposite ends of the samelines. The triangle or short line points in the direc-tion a drop of water would roll down the sloping.surface of a single layer; the number shows the dip,

the angle of inclination in degrees from the hori-

zontal, Appendix V describes how to measurestrike and dip.

A possible detailed set of field notes for ;he mapin Figure 12 is shown in Figure 13,

While working in the field, think of several pos-sible ways to explain the features you observe. Inthe example of Figure 12, the rock et station 3

° was described as metamorphic and the rock at.

station 1 as plutonic. Somewhere between the twostations there must be a contact between the tworock types. Four hypotheses should be considered,

and others could be proposed:1) The diorite may be older than adjacent rocks

and may represent a platform on which theletter were deposited.

2) The diorite may be younger, and thereforeintrusive into the older metamorphic rocks.

3) The diorite may be the same age as the sur-rounding rocks, but "granitized."

4) A fault may exist between the diorite and'theadjacent rocks which makes it difficult to tell

the age relationships.Every bit of evidence bearing on the problem

should be examined to determine which explana-tions it supports or contradicts. For example, the

presence of diorite dikes ,etnting across the meta-

morphic rocks favors the second explanation.Examine each bit of new data in this way. Siftingseveral pieces of evidence may make it clear that

the second hvothesis is most reasonable. How-

ever, a better hypothesis than the ones previouslyconsidered may be developed later on.

MM.

11.3.1.0 11501\ aiviS : ana Vadransilc, 15rtitnirtt;scms LLS .Geol,StunittA

Atak,i.az. .&4cT tittvOn 4nci ke.rnarks

Lano -CoLiation Outcrop 7.5 m. longsup. c LifC -cac.t..., Rock.,4e.sh. tkt

eAttpt, -czt u.pptx: .Ya -mdtrLiak tc.h vi_s u.rnbLii Latiessf i01( atLOA Vitt\ ViornoLntou.s

ci 4scsigtton: .t-anular .abr - -

Aram -m Tn. 161uart7.. abstrtt,-1-katodase.-Qttaispar 6% e)iotitt-10.7e-Horn bicn 8E. as%Rock. tfamt... Thor

. ToLiation:-(doum on .

.bip: 700 kJ.)

Ma) 3 0

1.0 U.) outcrop 30 cm,Grams'a . an t.t.mathEri...8.

black .whe.rf...Qresk .

Rock ntAtd away,4..cam\ 5 t2S.1. t- 3 mm.Tiorn bLancit. 60% Pi.aiilociase.. 25Garnd 5 %

Gar /lel rnphiPlatiocLas,E. ac.nE.t Schisb)__

t(07e.: C ontact bakoa...r bio at.,. phN)oLitR.,= is bet,k4uxk si.auons

i. and . tfot txposth so I orkn o wh at rEtakionship isInt.Lotex the- too .

rilt.'IMIC .1 L13 Sdmple

r1c)te-

t), WI (1r cling 1Yr-

',1,1 it' ! 1( In

1.)

\\ 1)),1 he pc,111.1\ imi

\ \ t:\ itLIk \\ Ill\ lI di' a

111,tp \\ 1111 \ till

:129


Figure 14. How to usea hand lens. Hold thespecimen a few centi-meters away from theeye and in good light.Study at least threesurfaces of the speci-men at nearly rightangles to each other.

Cross-Country Geology

When you take weekend trips or vacations, carrya road map and note the location and types .ofmetamorphic and plutonic rocks you observe.Order a copy of the state geologic map by writingto the state geological survey. For cross-countrytrips, carry a set of the geological highway maps ofthe states you will cross. (See References.) Followyour route on these geologic maps and keep a note-book of your ro4dside observations.

APPENDICES

IKey to Rock-Forming Minerals

To use the table, apply all tests described toinifividual mineral grains and not to the whole rock.Use a hand lens (10- to 20-power). The techniquefor using a lens is shown in Figure 14.

If the mineral is colored, first lOok in the sectionsof the table referring to colored minerals. If youcannot make a satisfactory identification, try thesection dealing with colorless, white. or light-colored minerals.

Describe the cleavage of the mineral. Clvarageis the tendency of a mineral to split along closely-spaced. flat surfaces. Some minerals have severaldifferent directions of cleavage. For example.when you break a crystal of halite, common tablesalt, it will split into small fragments that haveright angles between all flat surfaces. Use yourhand lens to examine some common table salt.Halite has three directions of cleavage. Someminerals have no cleavage. Instead, they breakalong irregular surfaces. Quart/. for example.breaks the way glass does. It has a con(lwidalor Nhell-like fracture.

' )u

PLUTONIC AND METAMORPHIC ROCKS /31

Determine the hardness of the mineral; Canyou scratch it with your fingernaih a penny, or apocket knife? If it is harder than a knife blade,consider it hard.

In using the table,I) First look up the proper color in column I.

2) Within the proper color group in column 2,

locate the compartment containing mineralswith the right cleavage.

3) In column 3, within the same cleavagecompartment, find minerals of the samehardness.

4) Then examine any special' properties untilyou locate a mineral name in the last column.

For an identification to be correct, the mineralname you choose must fit all properties directlyto its left in columns 4, 3, 2, and I. For example,suppose you find a green mineral with good cleav-age. The mineral cannot be scratched with a knife.Upon close examination you find that it has twocleavage directions at about right angles to eachother. The name of the mineral, according to thistable, is potassium feldspar.

What do you do if you can't decide on the propername?

I ) Take a sample of the rock containing the min-eral and determine what it might be by com-paring it with display specimens in a museum.

2) Try using tables from a more detailed min-eralogy book. (See References.) You mayhave found a mineral that is not included in

Appendix I.3) If you still cannot identify the mineral, ask

a geologist or earth science instructor forhelp. Sometimes geologists have to examineminerals under a microscope, applying variousspecial tests. They may even need X-raystudies to identify the mineral.

(Note: Some mit Is appear in more than onesection of the tab!, because they may vary incolor or other properties.)

36

*

Color Cleavage Hardness Other Distinguishing- Characteristics

Name

red or brown

good cleavagecan be scratched §plits into thin sheets (1 perfect cleav- BIOTITEby penny or age) which are nearly transparent, bend MICAeasily by knife easily, and snap back; usually dark

brown or black

no good cleavage cannot be scratchedby knife

=1110commonly well-formed crystak growtogether to form a cross; usuallyrather dull

STAUROLITE

almost spherical, 12-sided crystals when GARNETwell formed; glassy looking; also maybe green. white.black

stubby. square crystals; commonly has ANDALUSITEblack inclusioft in shape of a cross;when altered it may be softer than aknife; red, brown, or olive green .conchoidal fracture; glassy: small stubby OLIVINEgrains; common in marble; in otherrocks generally green or yellow

long, flat crystals can be scratched bycan he scratched by knife parallel to the long direction, butknife but not by not paraHel to the width; pale blue but

blue good cleavage penny may also be colorless

(for palecolors see also"colorless, etc.".)

moderate cleavage

1K Y AN IIE

cannot be scratched ckavage may be poor; varjous CORDIERITEby knife tades of blue; glassy looking

green orblack

(for palecolors see ako"colorless. etc.-)

good cleavage

can be scratchedby fingernail

can be scratched bypenny but not byfingernail

',Pperfect cleavage; splits into pear ysheets that bend but do not snap back CHLORITE

IN01100...

black; 1 perfect cleame; splits into BIOTITEsheets that bend and snap back MICA

about same hardnessas knife: mineraland krife willprobvoly scratcheach other

2 good cleavages; cleavage planes formIN' angle; _crystals usually longer thanthey are wide: usually dark but somerarer varieties are pale green, white,blue, or brown

HORNBLENDE(AMPHIBOLE)

2 good cleavages; cleavage planes form900 angle: crystals usually stubby

cannot he scratchedby knife

2 good cleavages at almost 90° angle:usually white or pink,occasionally bright green

PYROXENE

POTASSI UMFELDSPAR

1 perfect cleavage, I poor cleavage:glassy looking: usually pistachio-green

no good cleavage cannot be scratchedb knife

3 8

almost spherical I2-sided crystalswhen well formed: more commonly redor brown

stubby. square crystals: commonly hasblack inclusions in shape of a cross:when altered it is softer than a knife.

conchoidal fracture: glassy: small.stubby grains: green and ellowishvarieties common in gabbros.

EPIDOTE

GARNET

ANDALUSITE

OLIVINE

Color Cleavage Hardness Other DistinguishingCharacteristics

61

Name

colorless,white, orlight colored

goodcleavage

INIMI.I.M.-Mi....

can be scratchedby penny

white, pearly, or transparent; splitsinto thin transparent sheets whichbend easily and snap back; I perfectcleavage: flat, hexagonal crystalsoccasionally found

MUSCOVITEMICA

3 good cleavages; mineral breaks intoperfect rhomb-shaped fragments; fizzeswhen hydrochloric acid is droppedon it: usually white or colorless:may also be tinted gray, green. blue.yellow, pink

CALCITE

cannot be scratchedby penny but canbe scratched byknife

easily scratched with knife; 3 goodcleavages; breaks into perfectrhomb-shaped fragments; large pieces

,will not fizz in col ydrochloric acid,but a powder of e mineral willfizz in acid; hot acid will also causemineral to fizz; commonly pale pink.but may be colorless, white, gray,green. brown, or black .

DOLOMITE"

long, flat crystals can be scratchedby a knife parallel to the longdirection of crystal, but notparallel to the width: pale blue orcolorless

KYANITE

colorless. white.or lightcolored (cont.)

good cleavage(cont.)

cannot he scratchedby knife

common rock-forming tnitteral;2 good cleavages at nearly right angles;roughly rectangular, blocky crystals;narrower edges contain paraRCNtriationsthat look like deep scratches(usually seen only under hand lens):usually white or clear; may begray, dull green, yellow, or pink

PLAGIOCLASEFELDSPAR(SODIUM-CALCIUMFELDSPAR)

common rock-forming mineral;2 good cleavages at aboutright angles: roughly rectangular.blocky crystals; distinguished fromplagioclase by lack of striations:usually pink or white; occasionatlycolorless or bright green

POTASSIUMFELDSPAR

good cleavage: long slender crystals:may be white, pale brown, pale green SILLIMANITE

no goodcleavage

I poor cleavage: various shades orblue CORDIERITE

almost spherical. 12-sided crystals whenwell formed; may be white. or pale pink:usually darker red, brown, or greencannot he scratched

hy knife

4 0

GARNET

one of the most common rock-formingminerals; conchoidtt fracture; well-formed crystals usually pointed,hexagonal: ulmally white, colorless, orpale gray; some varieties pink, darkergray, violet, brown, or pale blue

QU ARTZ

PLUTONIC AND METAMORTIC ROCKS / 37

II--1dentification of Metamorphic and4 Plutonic Rocks

1--

ow.

,The following terms are useful in describingt;te texture of metamorphic and plutonic rocks.Texture is the:way the grains lit together.

Schistose or foliatedRocks that split easilyinto platy fragments. The terms sehistosity andfithation refer to sheettike layering of mineralgrains. Rocks that show this feature generallycontain flakes of mica or other flat minerals ar-ranged in parallel sheets t-Figure 3). Depending onthe size of the grains, rocks with schistose texturesare given spedfic names.

1 Slate is so fine-grained that individual min-eral grains cannot be seen, even with a handlens. it splits easily into hard, flat platesthat are commonly used as roofing tiles andchalk boards. Slate has a dull luster.

2) Phyllite has slightly larger mineral grainsthan _slate. Tiny mica flakts may be seen onthe flat rock surfaces. Because of the coarsercrystals. phyllite has a shinier appearancealong the surface of splitting than slate.

3) Seli;st contains grains large enough to he

easily seen with the naked eye.LineatedRocks consisting of elongated, pen-

cil-shaped minerals with their long dimensionsoriented parallel to each other.

GranularMineral grains that look square.rectangular. or round under the hand lens. Micaflakes ot elongated minerals may he present, hut

they are not arranged in parallel fashion. Granularrocks do not split into thin platy fragments thewa!, schist.s do. Most plutonic and some meta-morphic rocks have granular textures &italics 1

and 3C).When examined carefully under a hand lens or

microscope almost all metamorphic and plutonicrocks reeal complelely interlocking grains withno open spaces hetween them.

4 1

36 / PLUTONIC anD METAMORPHIC ROCKS

To use the table, 'first identify the texture ofthe rock from the foregoing descri.ptions. Thenidentify the/minerals present and estimate theirrelative pecentages and grain size. With thesedata, move across the table from the,. left-handcolumn to the proper name at .the right. Add thenames of the most abundant minerals to give acomplete rock name. A schist with 45 percentquartz, 35 percent muscovite mica, and 20 percentgarnet is called a quartz-muscovite-garnet schist.

The rocks from slate through gneiss are meta-'morphic. Those from granite down the columnare plutonic. However, some gneisses grade intoplutonic rocks, and some granites may be meta-morphic. In doubtful cases such names as "granitegneiss" or "gneissic granite" may.be used.

In order to give plutonic rocks more completenames, add the names of abundant minor minerals(excluding the essential ones). For example, agranite containing 30 percent quartz, 40 percentpotassium feldspar, 15 percent plagioclase feld-spar, 10 percent muscovite mica. and 5 percentbiotite mica should be called a muscovite-biotitegranite, since the >vord granite already includes thequartz, potassium feldspar, and plagiociase.

(Note: This table presents a simplified fieldc ssitication. In order to assign more precise

les, see References.)

cZ..

Texture Grain Size Essential Minerals Name ProbableParentRock

fine-grained=less than1 mmmedium-grained=1-5 mmcoarse-grainedore than5 mm

grains too fine to beseen with hand lens.shiny surface

nonethe name"slate" relatesonly to texture

SLATE shale

grai ns large enough nonenameto be seen with hand relates onlylens to texture

'PH YLLITE

tohated virtually all minerals nonename(schistose) large enough to be relates only

seen easily; rock to textureflaky

SC H 1ST

usually line-grained serpentineand roughly foliated mineralshut may be granular: (chlorite andgreen to black in iibrous mineralscolor: greasy looking like asbestos)

SERPEN"LYN I TE

shale

shalevolcanic rockshalysandstone

peridotiteor pyroxenite(see below)

lineated(elongatedminerals

, parallel)

medium- to coarse-grained

usually amphiboleand plagioclase

4-3

AMPHIBOLITE volcanic rockor impurelimestone

Masaa1%.

Texture

-Grain Size Essential Minerals Name Probable

ParentRock

..\

granular.usually layeredor streakylooking

very fine-grained; mayhave some large, well-formed crystals sur-rounded by fine-grainedmaterial; commonlyflinty looking

none-namerelates onlyto texture

HORNFELS can beanything

tine- to medium-grained

mostly quartz(rock very hard;gray or white,sometimes red)

fine to coarse

QUARTZITE sandstone

calcite or dolomite MARBLE limestone(rock sott; usuallyreacts with NC I)

medium tocoarse

commonly containsqvartz, feldspar,did dark minerals

GNEISS

granular. medium tounfoliated or coarseonly slightlyfoliated

quart/. potassiumfeldspar.plagioclase

GRANITE

feldspar-richsandstone,granite

crustal rockhas probablybeen meltedor granitized

granular,unfoliated oronly slight I yfoliated(cont.)

medium to coarse(cont.)

quartz absent,plagioclase,dark minerals lessthan 50%

DIORITE from amelt?

quartz absent,plagioclase,dark minerals morethan 50%

GABBRO melt fromsub-crustal orlower crustalrocks?

quartz absent. .

potassium feldspar,'SYENITE from

crustalrocks?

quartz absent,feldspar absent,dark minerals only

PERIDOTITE(predominantlyolivine)

sub-crustalrocks?

PY ROX EN ITE(predominantlypyroxene)

sub-crustalrocks?

HORN-B LEN DITE(predominantlyhornblende orother amphibole)

sub-crustalor crustalrocks?

very coarse: somevery large, well-formed crystals:occurs usually asintrusions or pods

PEGMATITE(depending onminerals presentcall these granite-gabbro pegmatite,etc.)

melt fromcrustal orsub-crusudrocks


<

I117Percentage Compositkm of Rocks

To use the iharts (Figure 15), 'assume, for ex-ample, that you wish to estimate the total percent-age of -dark minerals present in a rock. First look,carefully with either your naked eye or a hand lensat the hand specimen of the rock. Note the distri-bution ratio of dark-colored minerals to light-colored minerals. Then compare this sample withthe charts. Try to make accurate estimates for allmajor minerals you have distinguished. With prdc-tice, you can develop an accuracy'within about fiveto ten percent.

4 6

O'


47.

Figure 15. Charts toassist you in estimat-ing percentages. Notethe difference in ap-pearance between afew large dark areasand many small darkareas.

r-

---',7'Metamorphic Shaly or Clayey Parent Rocks Limy Parent Iron- and Magne-

Grade Parent Rocks Rich in Ouartz Rocks (Limestone, sium-rich Parentand Feldspar Dolomite) Rocks (Gabbro,(Sandstone, Basalt)

Granite, Vol-canic Rocks).

IVTypical Mineral Assemblages andProbable Parent Rocks

(Note: More complete .lists can be found inPetrography or in other college petrology text-books. See References.)

Low-pressure muscovite-biotite-.

quartz-potassium calcite-garnet- plagioclase-Metamorphism by quartt-andalusite feldspar-plagioclase- pyroxene. and other hornblende-Baking

andaiusite-cordierite-potassium feldspar-quartz

biotite calcium-bearingminerals (garnetusually red-brownor pale green)

pyroxene

muscovite-chlorite- quartz-sodic plag- calcite-dolomite- sodic plagio-'Low-grade quartz -plagioclase ioclase-epidote- quartz clase7chlorite-

(sodium-rich) (muscovite) calcite

biotite-muscovite- quartz-sodic plagio- calcite-amphibole- sodic plagio-quartz-sodicplagioclase

clase-epidote epidote-quartz clase-chlorite-amphibole-epidote

Medium-grade

garnet-biotite-muscovite-quartz-sodium-calciumplagioclase

kyanite-garnet-muscovite-biotite-quartz

staurolite-garnet-biotite-muscovite-quartz

sillimanite-biotite-muscovite-garnet-quartz

quartz-potassiumfeldspar-sodium-calcium plagioclase-biotite-muscovite

salcite-pyroxene-quartz

calcite-pyroxene-garnet (garnetusually deep red)

+11....

hornblende-sodium-calciumplagioclase-epidote-quartz-biotite

hornblende-sodium-calciumplagioclase-garnet

H igh-grade

sillimanite-(kyanite)-garnet-potassiumfeldspar-plagioclase-quartz

quartz-potassiumfeldspar-plagioclase-:sillimanit&-garnet

quartz-potassiumfeldspar-plagioclase-pyroxene-garnet

calcite-pyroxene-quartz-plagioclase

plagioclase-pyroxene

plagioclase-pyroxene-garnet

olivine-pyroxene-amphibole

Very high-grade

green pyroxene-garnet


Figure 16. Strike anddlp of tilted layers.

VMaking Measurements on Tilted Rocks

Figure 16 shows what strike and dip are. Tomeasure the strike, place a clinometer (Figure 17)on the top of a layer. Be sure to measure the actualtop of a single layer, or the foliation plane, and notmerely the rounded top of an outcrop. Move theclinometer until it reads zero degrees. Draw a linealong the flat surface of the rock layer parallelto the edge of the clinometer. This is a horizontalline on the surface of the layer and is thereforethe strike line of the layer. Could you measure thestrike of a horizontal layer? Why? Measure thecompass bearing of the strike line you have drawnon top of the layer, and record this figure as thedirection of strike.

5 0

PLUTONIC AND METAMORPHIC ROCKS 4?

In order to measure the dip of the layer, put theclinometer at a right angle to the strike line (Figures16 and 17). Read the dip angle directly from theclinometer and record it in your notebook. Thedip is actually the maximum angle of tilt of thelayer measured from the horizontal, so you canalso measure it by simply nioving the clinometeraround on the surface of a layer until you reach Figure 17. Construc-the maximum possible angle for the given surface. tion of a simple clino-

meter: Tape a protrac-Why measure. str_ike..and dip? Strike and diptartgmo pium-ofsymbols on a map help you remember how the cardboard or plywood

layers shmt into the ground. Furthermore, from as shown. Drill a holesuch measurements you can project the position through point A andof the layers underground and make inferences attach a piece of

about their shapes. string. Weight thestring with a piece ofmetal. Place the clino-meter on any inclinedsurface as shown andread the angle. Notethat the clinometershould register zerodegrees when edgeBC of the chnometeris horizontal.

j.

48/PLUTONIC AND METAMORPHIC ROCKS

RefeLrences

Boyer, Robert E. Field Guide to Rock Weathering.Boston, Houghton Mifflin Company, 1971.

Compton, Robert R. Manual of Field Geology.New York. John Wiley & Sons, 1962.

Earth Science Curriculum Project. Investigatingthe Earth. Boston, Houghton Mifflin Company,1967.

Freeman. Tom. Field Guide to Layered Rocks.Boston. Houghton Mifflin Company, 1971.

Leet. L. Don, and Sheldon Judson. Physical Geol-ogy, 3rd ed. Englewood Cliffs. N.J., Prentic-e-Hall, Inc., 1965.

Longwell. Chester R.. Richard R. Flint, and JohnE. Sanders. Physical Geology, text ed. NewYork, John Wiley & Sons, 1969.

Pearl, Richard M. Hmi. to Know the Minerals andRocks. New York. McGraw-Hill Book Com-pany. 1965. (Paperback)

Pough. Frederick H. A Field Guide to Rocks andMinerals. 3rd ed. Boston, Houghton Mifflin

Company. 1960.

Ramsey. W.. and Raymond A. Burckley. ModernEarth Science. New York, Holt. Rinehart &Winston, Inc., 1969.

52

PLUTONIC AND METAMORPHIC ROpK$ /40

Tuttle, 0. "The Origin of Granite." ScientificAtnerkan. April. 1955. (Also Scientific Amer-ican Offprint #8193. W.H. Freeman & Com-pany. San Francisco.)

Williams. Howel, F. Turner, and C. Gilbert.Petrography. San Francisco, W.H. Freeman &

Company, 1954.

Zim, Herbert S., and Paul R. Sahffer. Rocks undMinerals. New York, Golden Press, Inc.,

1957. (Paperback)

MapsAmerican Association of Petroleum Geologists,

P.O. Box 979, Tulsa. Oklahoma 74101. Geo-logical Highway Maps.

Distribution Section. U.S. Geological Survey,1200 South Eads Street. Arlington. Va. 22202

(maps of areas east of the Mississippi River)and Distribution Section, U.S. GeologicalSurvey. Federal Center. Denver, Colo. 80225(maps of areas west of the Mississippi River).Topographic maps. (State topographic indexesare free on request.)

Matthews. William H., Ill. Selected Maps andEarth Sience Publications. Earth ScienceCurriculum Project Reference Series: RS-4.Englewood Cli(N. N.J.. Prentice-Hall. Inc..1965.

r

41"

RP PLUTONIC-AND, METAMORPHIC ROCKS

1

urmr .

twItOr;P":1I

MO.

Glossary

clinometera device for measurtog the tilt of rocklayers.

contact metamorphismchemical changes withoutmelting-in minerals, and changes in their ar-rangement in a rock. It is caused by exposureof a rock to great heat, as when molten rockintrudes another rock.

crystalline texturean arrangement of the mineralgrains forming a rock in which the edges ofgrains are tightly irfterlocking. with no freespaces between them. This texture is found inmost plutonic and metamorphic rocks: it is

also found in some sedimentary rocks, as insome limestones.

dip--the angle at which a rock layer is tilted intothe ground. measured from the horiiontal andat a right angle to the strike.

foliated texture (sehlstostI)hitving Mat mineralgrains that lie parallel tveach other like sheetsin a pad of paper.

granitizationformation of granite from otherrock types several kilometers iv-low theearth's surface in environments where temper-ature and pressure are high enough to allowchemical elements to move about withoutactual melting of the rock.

igneous roCkrock that crystallired from molten. rock liquid or magma.

intrusiona body of rock that hip; been injected orhas owed into other rocks. Many intrusionsare thought to have occurred when the in-truding rock was molten. "Solid- rock can

5 4


also become relatively plastic or doughyand t intruded.

isograd line drawn on a map. separating rockswhich have undergone different amounts ofmetamorphism and are of different meta-morphic grade.

lineated texturehaving elongated mineral grainsarranged parallel to each other like a bundleof pencils.

magmamelted rock.metamorphic gradean index describing how much

a metamorphic rock differs from' the rock itcame from. For example, high-grade meta-morphic rock is rock that has been subjectedto high temperature and pressure.

metamorphic rockrock that has formed underconditions of high temperature and pressuregenerally several kilometers below the earth'ssurface, having an interlocking arrangementof mineral grains known as crystalline texture.Chemical rearrangements and changes inshapes of mineral grains have occurred pith-out actual melting of the rock from whichthey formed. .Metamorphic rocks commonlyhave mineral grains arranged parallel toeach other. Examples are slate, schist, andgneiss.

migmatite (mixed rock)rock in which plutonicrock types and metamorphic rock types aremixed together in various ways.

mineral assembiagethe particular minerals thatoccur together in a given rock.

5,5


mineralsMaturally occurring homogeneous solidswhich make up rocks.

parent rockthe rock from which a metamorphicrock is thought to have formed. For example,marble fortnsiotn the parent rock limestone,and quartzite 'forms from quartz sandstone.

plutona large body of plutonic rock, such as,granite or gabbro, ranging in surface areafrom a few ,hundred square meters to manyhundreds of square kilometers.

plutonic rocknormally coarse-grained rock thathas crystallized deep below the earth's surface.having an interlocking arrangement of mineralgrains known as crystalline texture. Plutonicrocks are commonly of igneous'origin, hut mayhave formed without melting and thereforemay actually be metaMorphic rocks. Exam-ples are granite and gabbro.

regional metamorphisnichemical changes with-out melting in minerals and changes in theirarrangement in a rock; it occurs over a wideregion and is generally related to foldingand regional increases in temperature andpressure rather than the immediate proximityof a molten mass.

sedimentary rockrock composed of particles ofsand. clay, or other sediment,, pressed orcemented together. Generally, sedimentaryrocks are layered and their grains are arranged

, in a pattern with pore spaces between them.strike linea horizontal line drawn on a tilted

layer.

544

PLUTONIC AND METAMORPHIC ROCKS /

PICTURE CREDITS

All photos are from the collection of the author',Dr. William D. Romey.

Pages 14-15, adapted from Robert RManual of Field Geology. New York,& Sons, Inc., 1962.

Page 47. adapted from R. a TerryChan er, Journal of SedimentaryVolu 25, 1955.

. ComptOn,John Wiley

and G. V.Petrology,

5 7

ESCP

or

Pamphlet Spries

PS-1 . Field Guide to Rock Weathering

PS-2 Field .Guide to Soils

PS-3 Field Guide to Layered Rocks

PS-4 Field Guido to Fossils

PS-5 Field Guide to Plutonic and Metamorphic"Rocks*

PS-6 Color of Minerals

PS-7 Field Guide to Beaches

PS-8 Field Guide to Lakes

P5-9 Field Guide to Astronomy Without a Tele-scope

PS-10 Meteorites

.5

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Field Guide to Plutonic and Metamorphic Rocks. Earth Science Curriculum Project Pamphlet Series PS-5

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