& rholR ~ c o n d i u o n ~ . -d%pusdydh& m,mytokecp J(Nt1y~mindhowrhed,emiatl f composition 6Ta so& an che temperamre, pressure, d water pmenc eaOh

wntiibute to the mctmorphic p m c a and the resultant metamorphic rock We also discuss hydrothermally deposited rocks and minerals, which are

found in associauon &th bath igneo$ and mcramo'phic rocks. Nydrorher- mal ore deposits, while not votumctridy significant, are of p t importance to the worldb supply of metals.

Because nearly dl rnrcamorphic rocks form deep within the earth's cwt , they prwide geologists with many clues about u)nditions at depth. Therefbre, understanding meramorphism will help you when we consider geologic procesw involvingthe arch's i n t d forcas. Metamorphic rocks are a feature of the oldest exposed rocks of the continents and of major mountain belts. They are especially important in providing evi&ncc of what happens during subduction and plate convergence.

From your study so fa^ of eveh materials and the rock cyde, you know that rocks dungr, given enough time, when their physical environment changes radially. In chapter 11, you ~w how deeply buried mcks melt (or ~arrially melt) to form magma when tempcnnua ye high eno What happens m roekc that are deeply buried but arc not %t enough m melt? They become mmmorphosed. McmmorphLm refers to h g c s m mclrs that rake place in the cardis interior. T k changes may be new M(-

cum, new mineral assemblages, or both. Tivlsformations occur in the solid state (meaning the rock does not melt).

The new rock, the metamorphic rock, in nearly all cases has a texture clearly different from that of the on'giml mck, or parent rodr. When limestone is metamorphosed to mar- ble, for example, the fine grains of calcite coalesce and recrys- tallize into larger calcite crystals. The calcite crystals are interlocked in a mosaic pattern that gives marble a texture distinctly different from that of the parent limestone. If the limestone is composed entirely of calcite, then metamor- ~ h i s m into marble involves no new minerals, only a change in texture.

More commonly, the various elements of a parent rock react chemically and crystallize into new minerals, thus mak- ing the metamorphic rock distinct both minerdogically and texturally from the parent rock. This is because the p m t rock is unstable in its new environment. The old minerals recrystallize into new ones that are at equilibrium in the new environment. For example, day minerals form at the earth's surface (see chapter 12). Therefore, they are stable at the low temperature and pressure conditions both at and just below the earth's surface. When subjected to the temperatures and pressures deep within the earth's crust, the day minerals of a shale can recrystallize into coarse-pined mica. Another example is that under appropriate temperature and pressure conditions, a quartz sandstone with a calcite cement meta- morphoses as follows:

CaCO, + SiO, + CaSi03. + CO, calcite quartz wollaston~te carbon

(a mineral) dioxide gas

No one has observed metamorphism taking place, just as no one has ever seen a gnnite pluton form. What, then, leads us to believe that metamorphic rocks form in a solid state (i.e., without melting) at high pressure and temperature? Many metamorphic rocks found on the earth's surface exhibit con- torted banding (figure 15.1). Commonly, banding in mcwnor- ~ h i c rocks can be demonstrated to have orieinallv been hat-lying sedimentary layering (even though the rick dc since recrystallized). These rocks, now had and brittle, would ahancr if smashed with a hammer. But they must have been ductile (or phrric), capable of being bent and molded under stress, to have , been folded into such contotted patterns. Because high temper- ature and pressure are necessary to make rocks ductile, a reason- able conclusion is that these r& formed at considerable depth, where such conditions exist. Moreover, crystalliition of a magma would not produce contorted layering.

Metamorphic rock from Qremlmd. Metamorphism took pl 3.700 mllllon years a g ~ t is one of the oldest rocks on Photo by C. C Plurnmmr

Factors Controlling the - Characteristics of Metamorphic Rock - A metamorphic rock owes its characteristic texture and 1 ular mineral content to several factors, the most imp being (1) the composition of the parent rock before met phism, (2) temperature and pressure during metamorF (3) the effects of tectonic forces, and (4) the effects of such as water.

Composition of the Parent Rock Usually no new elements or chemical compounds are ad( the rock during metamorphism, except perhaps water. ( somatism, discussed later in this chapter, does involve the tion of other elements.) Therefore, the mineral content metamorphic rock is controlled by the chemical compc

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Figure 15.2 Confining pressure. The diver's suit is pressurized to counteract hydrostatic pressure. Object (cube) has a greater volume at low pressure than at high pressure.

the earth's crust is compressed by strong confining pressure, called lithostutic pressure, which forces grains closer together and eliminates pore space.

Any new mineral that has crystallized under high-pressure conditions tends to occupy less space than did the mineral or minerals from which it formed. The new mined is denser than *

its low-pressure counterparts because the pressure forces atoms closer together into a more closely packd crystal structure.

B

Figure i s.a (A ) Compressive stress. More force is exerted in the directi the arrows than elsewhere on the ball. (6) Shearing. Parall arrows indicate the direction of forces.

If the forces on a body are stronger or wcakcr in di tions, a body is subjected to di5mmtial mess. stms ten& to deform objects into oblong or flatten you squeeze a rubber ball between your thumb and fi, the ball is under differential stress (figure 15.3A). If you a ball of dough, it will remain Aattened after you s because dough is plastic. To illustrate the differ confining pressure and differential stress, visualize with water. If you place a ball of putty underwater of the drum, the ball will not change its shape (its volum dccrease slightly due to the weight of the overlying w-atcr) rate the putty ball out of the water and place it under the The putty will be flattened into the shape of a p the differential stress. In this case, the putty is sub pwsiuc d~j%mFialstmr or, more simply, camp-vc s is the do& ball shown in figure 15.54).

~i f fennda l stress is alsocaused b i rhearim, which c a d parts of a body to move or slide relatiGe to one Gother ac plane. An example of shearing is when you spread out a of c a d s on a table with your hand moving parallel to the Figure 15.3B shows that initially dough is flattened at a angle to the shear force (the moving hands). As shearing tinues, the flattened dough is rotated tavardpumIlelism t shear force. In contrast, compressive stress flattens objectsp pcndimhr to the applied force.

Dz5watial Stress filiation ..

Most metamorphic rocks show the cffcctr of tectonic forces. Differential stress has a very important influence on the When forces are applied to an object, the object is under suess. of a metamorphic rock because it forces the constituents of

phosed conglomerate in which the pebbles have been

to become parallel to one another. For instance, the peb- osed conglomerate shown in figure 15.4 hericd but have been flattened by dif- ck has a planar texture, it is said to be

Foliation is manifested in various ways. If a platy (such a9 mica) is crystallizing within a rock that is

ss, the mineral grows in such a way s parallel to the direction of shearing or perpen- direction of compressive stress (figure 15.5). Any

grow against shearing is either up or forced into alignment. Minerals that crystallize lclike shapes (for example, hornblende) behave simi- rowing with their long axes parallel to the plane of

or perpendicular to compressive stress. The three very

mencary rock or given off by a cooling pluton.

Metamo*phirm

Elongate mlnoraln

C Flgum 15.5 Orientation of platy and elongate mlnerals in metamorphic rock. ( A ) Platy minerals~randokly orlented (e.g., clay mlnerals before metamorphism). No differential stress involved. (6) Platy mlnerals (e.g., mlca) and elongate minerals (e.g., amph bole) nave ~rvstallized under the Influence of corn~ressive stress. (C ) Platy ark elongate minerals developed under the Influence of shearing.

Platv minerals such as mica

b Needlelike mlnerak such as amphibolo

Flgum 16.6 Schistose texture.

Water is thought to help triggcr metamorphic chemical reac- tions. Water is a sort of intn-rock rapid transit for ions. Under high pmure, it moves between grains, dissolves ions fmrn one mineral, and then carries these ions elsewhere in the rock where

, Mcramorphic Rocks, and Hydmtherrnal Rocks 367

A

Flgum I S.7 Photomicrographs of metamorphic rocks in thin sections of ( A ) nonfoliated rock and (8) foliated rock. Photo by C. C. Plummer

they can react with the ions of a second mined. The n w min- eral that forms is stable under the existing conditions.

Time The effect of time on metamorphism is hard to comprehend. Most metamorphic rocks are composed predominantly of sili- cate minerals, and silicate compounds arc notorious for their sluggish chemical reaction rates. Recently, garnet crystals taken from a metamorphic rock collected in Vermont were analyzed and scientists calculated a growth rate of 1.4 millimeters per million years. The garnets' growth was sustained over a 10.5 million year pcriod. Many laboratory attempts to duplicate metamorphic reactions believed to occur in nature have been frustrated by the time element. The several million years dur- ing which a particular combination of temperature and pres- sure may have prevailed in nature are impossible to duplicate.

Classification of Metamorphic Rocks As we noted before, the kind of metamorphic rock that forms is determined by the metamorphic environment (primarily the particular combination of pressure, stress, and temperature) and by the chemical constituents of the parent rock. Many kinds of rnctamorphic rocks exist because of the many possible combinations of these factors. These rocks are classified based on broad similarities. (Appendix B contains a systematic pro- cedure for identifying common metamorphic rocks.)

First consider the texture of a metamorphic rock. Is it utedor nonfiliuted (figure 15.7)? If thc rock is nonfoliated named on the basis of its composition. For example, a no1 ated quam-rich metamorphic rock is a quartzite; one I

posed almost entirely of calcite is a mdrble. If the rock is foliated, you must determine the type 01

ation. For example, a schistose rock is called a schist. Bui name tells us nothing about what minerals ate in this roc we add adjectives to describe the composition. Thus, aga mica schist (whose parent was shale) is easily distinguisl from an urnphibole schirt (a metamorphic product of ba The relationship of texture to rock name is summariu table 15.1.

j!

Types of Metamorphism - -

The two most common types of metamorphism are co mcta~orphism and rcgional metamorphism. These arc cussed next. Hydrothermal metamorphism, in which water plays a major role, is discussed later in this chapter.

Contact Metamorphism Contact metamorphirm (also known as thermal meta phism) is metamorphism in which high temperature i, dominant factor. Confining pressure may influence which minerals crystallize. However, the confining pressure is us relatively low. This is because contact metamorphism m takes place not too far beneath the earth's surface (less tha kilometers). Contact metamorphism occurs adjacent to a

of magma intrudes relatively cool country ally, only microscopically visible micas develop. Sometimes a ess can be thought of as the "baking of country few minerals grow large enough to be seen with the naked eye; to an intrusive contact; hence the term mn*rct these are minerals that are especially capable of crystallizing . The zone of contact metamorphism (also called under the particular temperature attained during metamor- sually quite narrow--generally from 1 to 100 phism. Hornfels can also form from basalt, in which casc ifferential stress is rarely significant; therefore, amphibole, rather than mica, is the predominant fine-grained

typically are nonfoliatcd. mineral produced. rlng contact metamorphism, shale is changcd into the Limestone recrystallizes during metamorphism into mu- e-grained metamorphic rock h o d . Chuacteristi- blc, a coarse-grained rock composed of interlocking calcite

Mehamorphirm, Metamo*pkrc Rocks, and HydmthmMI Rorkr

crystals (figure 15.8). Dolomite, less commonly found, recrys- tallizes into a dolomitic marbk. Marble has long been vaJued as a building material and as a material for sculpture, pardy because it is easily cut and polished and partly because it reflects light in a shimmering pattern, a result of the cycllent cleavage of the individual calcite crystals. Marble is, however, highly susceptible to chemical weathering (see chapter 12).

Q d t e is produced when grains of quartz in sandstone are welded together while the rock is subjected to high temper- ature. This makes it as difficult to break along grain boundaries as through the grains. Therefore quartzite, being as hard as a single quartz crystal, is difficult to crush or break. It is the most durable of common rocks used for construction, both becausc of its hardness and becausc quartz is not susceptible to chemi- , cal weathering.

Marble and quartzite also form under conditions of regional metamorphism. When grains of calcite or quartz recrystailizc, they tend to be equidimensional, rather than elongate or platy. For this reason, marble and quartzite do not usually exhibit foliation, even though subjected to differential stress during metamorphism.

Chapm 15

Regional Metamorphism The great majority of the metamorphi earth's surface arc products of regiond known as dynumothcwnal met phiim that takes place at cons (generally greater than 5 kil rocks are almost always foliated during recrystallization. Metam the most intensely deformed por They are visible when once deeply b ranges are exposed by erosion. Furth the continents are underlain by met to be the roots of ancient mount to plains or rolling hills.

Temperatures during regi widely. Usually, the temperatures a 800°C. Temperature at a particula extent on depth of burial and the region (see box 15.3). Locally, temperature may also inc because of heat from friction (due to shearing) or from

oto by c c Plummar

- psn've Metamorphism how ro& are changed by regional metamorphiam,

at what happens to shale duringpmgmdiuc rmtnmop

I Regional M.trmorpMc Rocks that Form under Approximately Slmllar I

C Tectonic

loroes (w6ull in

compressive stma)

Schematic cross section representing an approximately 30- kilometer portion of the earth's crust during mstamorphism. Rock names given are those produced from shde.

tallizc into equally fine-grained micas. Under differential stress, the old and new platy minerals are aligned, creating slaty cleavage in the rock. A slate indicates that a relatively cool and brittle rock has been subjected to intense tectonic activity.

, Mltamolphic Rocks, and Hydmthrrnml Roch 371

eigun 15.1 0 Slate outcrop in Antarctica. Photo by P. 0. Rowley, U.S. Geological Survey

Slate. Photo by C. C. Plummer

Because of the ease with which it can be split into thin, flat sheets, slate is used for making chalkboards, pool tables, and roofs.

Ph~llite is a rock in which the newly formed micas are larger than in slate, but still cannot be seen with the naked eye.

Figure 11.1 2 Phyllite, exhibiting a crinkled, s~lky-looking surface. Photo by C. C. Plummet

f3 This requires a further increase in temperature wet needed for slate to form. The very fine-grained mica i m ~ silky sheen to the rock, which may otherwise closely re81 slate (figure 15.12). However, the slaty cleavage may be kled in the process of conversion of slate to phyllite.

A schist is characterized by megascopically visible, ap imately parallel-oriented minerals. Platy or elongate ml that crystallize from the parent rock are clearly visible I nated eye. Shale may recrystallize into several mincralq distinct varieties of schist. Which minerals form depen the particular combination of temperature and pressun wiling during recrystallization. For instance, if the roc micu rchist, metamorphism probably took place at only sl higher temperatures and pressures than those at which a lite forms. A garnet-mica $chist (figure 15.13) indicates th temperature and pressure were somewhat greater than I

sary for a mica schist to form (see box 15.2). In a gncias, a rock consisting of light and dark miner

ers or lenses, the highest temperatures and pressures changed the rock so that minerals have separated into I Platy or elongate minerals (such w mica or amphibole) il layers alternate with layers of Light-colored minerals of n ticular shape. Within the light-colored layers warse fell have crystallized. In composition, a gneiss may resemble ite or diorite, but it is distinguishable from those plr rocks by its foliation (figure 15.14).

Temperature conditions under which a gneiss de approach those at which granite solidifies. It is not surp~ then, that the same minerals are found in gneiss and in gi In fact, a previously solidified granite can be convertec gneiss under appropriate pressure and temperature cond and if the rock is under differential stress.

If the temperature is high enough, partial melting o may take place, and a)aagma is sweated out into layers \ the foliation planes of the solid rock. After the magma ! fies, the rock bcwmes a migmatitc, a mixed igneou

neiss. . , - -

metamorphic rock (figure 15.15). A migmatite thought of as a "twilight zone" rock that is neith igneous nor entirely metamorphic.

-I Plate Tectonics and Metamorphism The model of the earth's crust and upper mantle that fmm plate tectonic theory allows us to explain man) 0 k ~ e d chcteristics of metamorphic rocks. To dem the relationship between regional metamorphism and p tonics, we will look at what is bclicved to take place at a gent boundary in which oceanic lithosphere is sul

I beneath continend lithosphere, as shown h figure 15.1 Differential stress, which is responsible for fa

I occurs wherever rocks are being squ;ezed benveen I

plates or whcrcvcr rocks are sliding past one another, ing is expected in the subduction zone where the I crust slides beneath the continental lithosphere

I 15.16). In the chapter on mountains (chapter describe the new concept ofgra~itutionul ~olkapse und ing, in which the central part of a mountain belt b too high after plate convergence and is gravitationall] ble. This forces rock downward and outward as sh~

Flmum 1 J.15 the arrows in figure 15.16 (see also figure 5.20). At

Migmatlte In the Danlels Range, Antarctica. levels the plastic rock flows during mctarnorphism an

Photo by C C Plummer Gravitational

g

e 1,. 'z iE t B e

8

Figun lB.16 Metarnorphlsrn across a convergent plate boundary. From W. d. Ernst. Metamorphiamand Plate Tectonic Regimes. Stroudsberg, Pa.: Dowden, Hutchinson, & Ross, 1875; p. 425. Reprinted by permiss the publisher.

tion should develop parallel to the direction of flowage. Also, vertical foliation may develop throughout much of the region due to the compressive stress in which the crust is caught, as if in avise (see figure 15.9).

Confining pressure is directly related to depth. For this reason, we would expect the same pressure at a depth of 20 kilometers beneath a volcanic area as beneath the relatively cool rodts of a plate's interior.

Metamorphosed rocks indicate wide ranges of tempera- tures for a given pressure. This is understandable if we realize that the geothermal gradient is not the same everywhere in the world. Each of the three places (A, B, and C) in figure 15.16 would have a different geothermd gradient. If you were somehow able to push a thermometer through the litho- sphere, you would find the rock is hotter at shallower depths in areas with higher geothermal gradients than at places where the geothermal gradient is low. As indicated in figure 15.16, the geothermal gradient is higher progressing down-, ward through an active volcanic-plutonic complex (for instance, the Cascade Mountains of Washingon and Ore- gon) than it is in the interior of a plate (beneath the Great Plains of North America, for example).

Heat, then, is the most variable of the controlling fac- i tors of metamorphism. Figure 15.16 shows the temperatures :calculated for a convergent boundary. A line connecting tpoints that have the same temperature is called an isotherm. Note how the 300°C, 60O0C, and 1100°C isotherms change

adically across the subduction zone. This is because that were cool because they were relatively near the

s surface have been transported rapidly to depth. pidly" in the geologic sense-movement being around

crnlyeat.) The oceanic lithosphere along a subduction zone as not had time to heat up and come into thermal equilib- lum with the relatively hot rocks elsewhere at these depths. n the top of the subducted oceanic slab, the extra heat pro- ded by friction along with the heat that is normal for the crlying continental lithosphere and asthenosphere cause

isotherms to rise sharply toward the surface. The hcrms are bowed upward in the region of the volcanic-

utonic complex because magma created along the lower els of the subduction zone works its way upward and ngs heat from the asthenosphere into the mantle and t of the continental lithosphcrc.

understand why metamorphic rocks can form under wide variety of temperatures and pressures, study fig- .16. You may observe that the bottom of line A is at a of about 50 kilometers, and if a hypothetical ther- ter were here, it would read just over 300' because it be just below the 300° isotherm. Compare this to ver-

line C in the volcanic-plutonic complex. The confining ure at the base of this line would be the same as at the of line A, yet the temperature at the base of line Cwould

well over GOO0. The minerals that could form at the base tine A would not be the same as those that could form at e ' Therefore, we would expect quite different metamw-

).

Mctumorphinr

phic rocks in the two places, even if the parent rock had been the same (box 15.3).

Hydrothermal Processes Rocks that have precipitated from hot water or have been altered by hot water passing through are hard to classify. As described earlier, hot water is involved to some extent in most metamorphic processes. Beyond metamorphism, hot water also plays an important role creating new rocks and minerals. These form entirely by precipitation of ions derived from hydrothermal solutions. Hydrothermal minerals can form in void spaces or between the grains of a host rock. An aggregate of hydrothermal minerals, a hydrothermd rock, may crystallize within a preexisting fracture in a rock to form a hydrothermal win.

Hydrothermal processes are summarized in table 15.3. The "dry" metamorphism referred to is relatively rare. Most of the examples given so far in this chapter probably took place with water present and so would be classed as ''wet" metamorphism.

Hydrothermal Activity at Divergent plate Boundaries

-

Hydrothermal processes are particularly important at mid- oceanic ridges (which are also divergent plate bounbriu). As shown in figure 15.17, cold seawater mover downward through cracks in the basaltic crust and is eyeled upward by heat from magma beneath the ridge crest. Very hot water returns to the ocean at submarine hot springs. Hot water trav- eling through the basalt, gabbro, and ultnmafic rodts of the oceanic lithosphere helps metamorphose these mcks whiie they are dose to the diverging boundary

e Metamorphic Rocks, and Hydnothermal Rocks

. " filmed in the Pacific, where some springs sp& clouds of fine- grained ore minerals that look l i bladc smoke (f igu~ 15.18).

The metals in rift-valley hot springs are predominantly - - iron, copper, and zinc, wi& smaller amounts bf manganese, gold, and silver. Although the mounds arc nearly solid metal

Figun 15.1 7 Cross section of a mid-ooeanlc ridge (diverging plate boundary). Water descends through fractures in the oceanic lithosphere, is heated by magma and hot igneous rocks, and rises.

376 C h p w I5 hnp://auruur mhhc.com/~arthsci/g~ohgy/plummw

carried by the water md.participatc in metamorphic rcmionr. Large feldspar crystals may gow,in .schist due to the addidan

the ions m imroducdd h m a d i n g magna. Some hptb mitt ~ommndally mined depuaioofmntls nlch a imn, rut& sw, eoppu, Id, dab and dher arc d u d #r rnetuohtiCm. 7Y 15.19 &OW how mrgnecite (imn midc) on Mls haw rmed &tot& moawrmatisnt Ions rhr md&tx&pomdbywatetmdrmctHithm'IhenLin(hc har tack Ekmcnb airhin the hort & arc rlmtdfuhcwrEy diswhndmtofthchderrodr*nd~lPccdbythehciioia bm& in by the ftuid. b u afthe mlubi l i~&itc , mat* ble commonly rcrva ~1 a host for mecmmatic on deposits.

Zone of contan

Flguro 15.19 "lack smoker" or submarine hot spring on the crest of the mid- Development of a contact metasomatic deposit of iron oceanic ridge in the Pacific Ocean near 21 'North latitude. The (magnetite). (A) Magma intrudes country rock (limestone), and 'amok# is a hot plume of metalllc sulfMe minerals being marble forms along contact. (6) As magma solidifies, gases discharged Into cold seawater from a chimney 0.5 meters high. bearing ions of imn leave the magma, dissolve some of the The large mounds around the chimney are metal depos~ts. The marble, and deposit lron as magnetite. instruments in the foreground are attached to the small submarine from which the picture was taken. - Photo by U.S. Geological Survey

Hydrothermal Rocks and Minerals Quartz veins (figure 15.20) arc especially common where igneous activity has occurred. These can form from hot water given off by a cooling magma; but probably they are mostly produced by ground water heated by a pluton and circulated by convection, as shown in figure 15.21. Where the water is hottest, the most material (notably silica) is dissolved. As water vapor continues upward through increasingly cooler ro& dur- ing its journey toward the earth's surface, pressure decreases and heat is lost. Fewer ions can be carried in solution, and so

Flguro 16.20 Ore-bearing velns In a mlne.

Photo by C. C Plurnmer

, .

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veins form. Cold water descends, is heated, dissolves material, ascends, and deposits material as water cools and pressure drops

HM water released Hat wmr raleased from solidifying magma

at a converging boundary. Seawater trapped In the oceanic crust is carried downward and released upon heating at various depths the subduction zone.

orr dcposit~ and world (see box 15.4)

are not as common and are composed of calcite or a Sources of Water owing is a logical

ard from the earth's rocks; however, the

penetrate is quite

tectonics can account for water at deeper levels in the lithosphere as seawater trapped in the oceanic wust can be car-

percolate upward between the ried to considerable depths through subduction (figure 15.22). ry fine grains of ore mineral Water trapped in sediment and in sedimentpry rocks lying on

Metamo*phism, Metdmorphic Rocks, and Hydmthmnal Rock3 -@

bvalt may be carried down with the descending crust. How- fissures. In h e process of ascending, water assists in rhe mera- ever, recent studies indicate that most of the water is carried by morphism of rocks, dissolves minerals, and carries the ions to hydrous minerals (amphibole, for example) in the basaltic interact during memomarism, or it deposits quartz and other w t . When the rocks get hot enough h e hydrous minerals minerals in fissures as veins. The water can also lower the melt- recrystallize, releasing water. The water vapor works its way ins points of rocks at depth, allowing magma to form '9-

upward through the overlying continental lithosphere through described in the chapter on igneous roeks).

tamorphic rocks form from other rocks are suhjected to high temperature gen- y accompanied by high confining pres-

e. Recrystallization takes place in the state although water, usually present. metamorphic reactions. Foliation in

etamorphic rocks is due to d@rcntial (either compnssiur stress or shraring).

tc, phyllitc, schist, and gneiss are foliated d distinguished from one another by the

Conkut metamorphic rocks are produced g metamorphism usually without signif- differential strcss but with high tempen- Contact metamorphism occurs in mdcr ediately adjacent to intruded magmas.

Rrgionul memmo~hism, which involves heat. confining pressure, and differential stress, has created most of the metamorphic rock of the earth's crust. Different parent rocks as well as widely varying combinations of pressure and temperature result in a large variety OF metanlorphic rocks. Combinations of minerals in a rock can indicate what thc pressure and temperature conditions were during metanlorphism. Extreme metamor- phism, where the rock partially melts, can result in m i p t i t e s .

Hydrothermal veins form when hot water precipitates nlaterial that crystallizes into minerals. During memromarism, hot water introduces ions into a rock being

metamorphosed, changing the chemical composition of the mctasomatizcd mdt from that of the parent rock.

Plate tectonic theory accounts for the featurcs observed in metamorphic rocks and relates their development to other activities in the earth. In particular, plate tectonics explains (1) the deep burial of rocks origi- nally formed at or near the earth's surface; (2) the intense squeezing necessary for the differential stress, implied by foliated rocks; (3) the presence of water deep within the lithosphere; and (4) the wide variety of pres- sures and temperatures believed to be present during metamorphism.

isotherm 375 marble 369 metamorphic facics 376 metamorphic rock 301 metamorphism 364 metasomatism 377 migmatitc 372 parent rock 384 phyllite 372 quartzite 370

regional metamorphism 370 schist 372 schistose 367

' ' , ;' !. shearing 366 , , . , ' , .>; .

, .: slate 371 . ,

slaty 367 slaty cleavage 367 stress 366 vein 375

Mctnmorphimr, Mctnmotphic Rocks, dnd H y d m t b m l Rockr

Use the qucrtions below to prepare for exams based on this chapter. 1. What are the effms on metamorphic minerals and textUte6 of

temperature, canfining pressure, and difirential smss?

2. What are the various sources of h a t for metamorphism?

3. How do regional metamorphic mcka differ in texture from canmct metamorphic rocks?

I : Testing Your Knowledge

4. Why is such a variety of combinations of prcssure and temperanue environments possible during metamorphism?

5. How would you distinguish

a. schist and gneiss? b. slate and phyllite?

c, quartzite and marble? d, granite and gneiss?

6. Why is an edifice built with blocks of quvt~ire more durable than one built of marble blocks?

I 7. Mnvnorphiim of limestone may contribute to global warming by the dcasc of (3 oxygen (b) sulfuric acid (c) nitrogen (d) CO,

I 8. Shearing is a type of (a) compressive strew (b) canfining

pressure (c) lithoscatic pressure (d) differential stress

9. Metamorphic rocks with a p h texture (the constituents of the rock are parallel to one another) arc said to be (a) concordant (b) foliated (c) discordant (d) nonfoliatcd

11. Which is not a foliated metamorphic rock? (a) gn& (c) quartzite (d) slate

12. Limestone recrystallizes during metamorphism into (a) (b) marble (c) quartzite (d) schist

13. Quam sandatone is changed during mmorphism (3 hornfels (b) marblo (c) quamire (d) schut

14. The correct sequence of rocks that uc formed when s undergoes progressive mctvnorphism is (a) slate phyUite (b) phyllite, date, schist, gneiss (c) slate, gneiss (d) schist, phyllite, slate, gneiss

15. The major difference h n metamorphism and metasomatism is (a) temperature at which each akcs (b) the m i n e d involved (c) the area or region i (d) mewmatism is metamorphism coupled wi introduction of ions from an external so-

16. Ore bodies at divergent plate boundaries can be crratad (a) contaa metamorphism (b) regional metamorphism : (c) hydrothermd processes

17. Metamorphic rocks with the same mineral asremblage to the same (a) metamorphic facies (b) prognssive metamorphism (c) schistnsity

I . . . . . .

10. Metamorphic rocks arc classified primarily on (a) texture-the 18. A metamorphic rock that has undergone partid me1

presence or absence of foliation (b) mineralogy-the presence produce a mixed igneour-metamorphic rock is a (a)

or absence of qumz (c) environment of deposition (d) chemical (b) hornfels (c) schist (d) migmtite

camposition

mantle be regarded as metamorphic deposits? rocks rather than igneous to&? 3. Whnt happens to originally horizontal 4. Where in the euth's crust would yo

2. Where were the metals before they were layers of scdimcntary rode when b y are expect most migmadtes to form?

Blatt, H., and R. C. Tracy 1996. * Pem/ou: Igneous, Srdimmtaq and Mctamorphic. 2d ed. New York: W. H. Freeman. Hibbard, M. J. 1995. Pcmgnzpby m pemgcncsis. Englmood Cliffs, New Jersey: Prenticc-Hall. Kerrick, D. M., cd. 1991. Conuct meramorphum. Reviews in Mineralogy,

vol. 26. Wdington, D.C.: Mineralogical Society of America. Mason, R. 1989. Pemlop of thc metamorphic rocks 2d ed. London: Unwin Hyman. Yardlcy, B. W D. 1989. An intducrion to metumorphicprtrplog)l h e x , England: Longman.

Chapter 15

h r r p : l / m v w . ~ e n c & u b c c d - ~ medmetunorphic.hm1

University of British Columbiab Metamotphic Rock Homr Page. 7% site meant for a geology course on the study rodts. Although it is at a more advanced Icvcl, it can be used to reinforce some of concepts m e n d in this chapter.

~ p : l ~ ~ ~ . b . u n c . e d d ~ u n i a l "Metamorphic microtextuns." Click on see nrcellent photomicrog+., . 1gMetAtlulmainmcnu.html terms covercd in this chapter (e.g., foliation, through a polarizing microscope

University of North Cmlinak Atks ofRocks, gneiss, phyllire, marble, quartzite, slate) to Minerah, and Twm. Click on

rdetamotphum, Metamotphic Rocks, and H~mtkerinal Rockt