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m , m y t o k e c p J(Nt1y~mindhowrhed,emiatl
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. Nydrorhermal 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 c w t ,
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.
-d%pusdydh&
f
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 ~arriallymelt) 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 marble, for example, the fine grains of calcite coalesce and recrystallize 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 minto 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 making 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 metamorphoses as follows:
CaCO, + SiO,
calcite
quartz
+
CaSi03. +
CO,
wollaston~te
carbon
(a mineral)
dioxide gas
Metamorphic rock from Qremlmd. Metamorphism took pl
3.700 mllllon years a g ~is tone of the oldest rocks on
Photo by C. C Plurnmmr
Factors Controlling
- the
Characteristics of
Rock
Metamorphic
-
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.,
A metamorphic rock owes its characteristic texture and 1
without melting) at high pressure and temperature? Many
ular mineral content to several factors, the most imp
metamorphic rocks found on the earth's surface exhibit conbeing (1) the composition of the parent rock before met
torted banding (figure 15.1). Commonly, banding in mcwnorphism, (2) temperature and pressure during metamorF
~ h i crocks can be demonstrated to have orieinallv been
(3) the effects of tectonic forces, and (4) the effects of
hat-lying sedimentary layering (even though the rick dc since
such as water.
recrystallized). These rocks, now h a d 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 , Composition of the Parent
Usually no new elements or chemical compounds are ad(
been folded into such contotted patterns. Because high temperthe rock during metamorphism, except perhaps water. (
ature and pressure are necessary to make rocks ductile, a reasonsomatism, discussed later in this chapter, does involve the
able conclusion is that these r& formed at considerable
tion of other elements.) Therefore, the mineral content
depth, where such conditions exist. Moreover, crystalliition of
metamorphic rock is controlled by the chemical compc
a magma would not produce contorted layering.
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B
Figure is.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.
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.
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 f e n n d astress
l
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.
When forces are applied to an object, the object is under suess.
Differential stress has a very important influence on the
of a metamorphic rock because it forces the constituents of
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.
Elongate
mlnoraln
phosed conglomerate in which the pebbles have been
C
to become parallel to one another. For instance, the pebosed conglomerate shown in figure 15.4
hericd but have been flattened by difck 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 perpendirection 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 simirowing with their long axes parallel to the plane of
or perpendicular to compressive stress. The three very
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
~rvstallizedunder the Influence of corn~ressivestress. ( 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.
mencary rock or given off by a cooling pluton.
Water is thought to help triggcr metamorphic chemical reactions. 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
Metamo*phirm,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 mineral 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 silicate 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 during which a particular combination of temperature and pressure 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 procedure 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~orphismand 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
ess can be thought of as the "baking of country
to an intrusive contact; hence the term mn*rct
.The zone of contact metamorphism (also called
sually quite narrow--generally from 1 to 100
ifferential stress is rarely significant; therefore,
typically are nonfoliatcd.
rlng contact metamorphism, shale is changcd into the
e-grained metamorphic rock h o d . Chuacteristi-
ally, only microscopically visible micas develop. Sometimes a
few minerals grow large enough to be seen with the naked eye;
these are minerals that are especially capable of crystallizing
under the particular temperature attained during metamorphism. Hornfels can also form from basalt, in which casc
amphibole, rather than mica, is the predominant fine-grained
mineral produced.
Limestone recrystallizes during metamorphism into mublc, a coarse-grained rock composed of interlocking calcite
Mehamorphirm, Metamo*pkrc Rocks, and HydmthmMI Rorkr
crystals (figure 15.8). Dolomite, less commonly found, recrystallizes 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 temperature. 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 chemical 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
I
Regional M.trmorpMc Rocks that
Form under Approximately Slmllar
oto by c
c
I
Plummar
C
Tectonic
loroes
(w6ull in
compressive
stma)
- psn've Metamorphism
how ro& are changed by regional metamorphiam,
at what happens to shale duringpmgmdiuc rmtnmop
Schematic cross section representing an approximately 30kilometer 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
Figure 11.12
Phyllite, exhibiting a crinkled, s~lky-lookingsurface.
Photo by C.C. Plummet
eigun 15.10
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~lliteis a rock in which the newly formed micas are
larger than in slate, but still cannot be seen with the naked eye.
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 d e
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
I
I
I
Flmum 1J.15
Migmatlte In the Danlels Range, Antarctica.
I
The model of the earth's crust and upper mantle that
fmm plate tectonic theory allows us to explain man)
0 k ~ e chcteristics
d
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
beneath continend lithosphere, as shown h figure 15.1
Differential stress, which is responsible for fa
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
crust slides beneath the continental lithosphere
15.16). In the chapter on mountains (chapter
describe the new concept ofgra~itutionul~olkapseund
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 s h ~
the arrows in figure 15.16 (see also figure 5.20). At
levels the plastic rock flows during mctarnorphism an
Photo by C C Plummer
Gravitational
1,.
g
e
'zEit
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 temperatures 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 lithosphere, 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 Oregon) 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 facitors 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 equiliblum with the relatively hot rocks elsewhere at these depths.
n the top of the subducted oceanic slab, the extra heat proded 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 volcanicutonic 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 therter were here, it would read just over 300' because it
be just below the 300° isotherm. Compare this to verline 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-
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 midoceanic 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 traveling through the basalt, gabbro, and ultnmafic rodts of the
oceanic lithosphere helps metamorphose these mcks whiie
they are dose to the diverging boundary
'
).
Mctumorphinre Metamorphic Rocks, and Hydnothermal Rocks
.
"
filmed in the Pacific, where some springs sp& clouds of finegrained ore minerals that look l i bladc smoke (figu~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://auruurmhhc.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 h
p
t
b
mitt ~ommndallymined depuaioofmntls nlch a imn, rut&
sw, eoppu, Id, dab and dher arc d u d #r
rnetuohtiCm.
15.19 &OW how mrgnecite (imn midc)
7Y
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 mlubili~&itc, mat*
ble commonly rcrva ~1a host for mecmmatic on deposits.
on Mls haw rmed &tot&
Zone of contan
Flguro 15.19
"lack smoker" or submarine hot spring on the crest of the midoceanic ridge in the Pacific Ocean near 21 'North latitude. The
'amok# is a hot plume of metalllc sulfMe minerals being
discharged Into cold seawater from a chimney 0.5 meters high.
The large mounds around the chimney are metal depos~ts.The
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& during 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
Development of a contact metasomatic deposit of iron
(magnetite). (A) Magma intrudes country rock (limestone), and
marble forms along contact. (6)As magma solidifies, gases
bearing ions of imn leave the magma, dissolve some of the
marble, and deposit lron as magnetite.
-
,
.
a
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
percolate upward between the
ry fine grains of ore mineral
tectonics can account for water at deeper levels in the
lithosphere as seawater trapped in the oceanic wust can be carried to considerable depths through subduction (figure 15.22).
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. However, recent studies indicate that most of the water is carried by
hydrous minerals (amphibole, for example) in the basaltic
w t . When the rocks get hot enough h e hydrous minerals
recrystallize, releasing water. The water vapor works its way
upward through the overlying continental lithosphere through
tamorphic rocks form from other rocks
are suhjected to high temperature geny accompanied by high confining prese. 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 signifdifferential strcss but with high tempenContact metamorphism occurs in mdcr
ediately adjacent to intruded magmas.
fissures. In h e process of ascending, water assists in rhe meramorphism of rocks, dissolves minerals, and carries the ions to
interact during memomarism, or it deposits quartz and other
minerals in fissures as veins. The water can also lower the meltins points of rocks at depth, allowing magma to form
described in the chapter on igneous roeks).
'9-
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 OFmetanlorphic rocks. Combinations
of minerals in a rock can indicate what thc
pressure and temperature conditions were
during metanlorphism. Extreme metamorphism, 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 originally 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 pressures 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, dndH y d m t b m l Rockr
'
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,,
.
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. , ,' .,:.>;
.
,
I:
Testing Your Knowledge
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?
4. Why is such a variety of combinations of prcssure and
temperanue environments possible during metamorphism?
5. How would you distinguish
b. slate and phyllite?
a. schist and gneiss?
d, granite and gneiss?
c, quartzite and marble?
I
6. Why is an edifice built with blocks of quvt~iremore durable
than one built of marble blocks?
7. Mnvnorphiim of limestone may contribute to global warming
by the dcasc of (3oxygen (b) sulfuric acid (c) nitrogen (d) CO,
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
10. Metamorphic rocks arc classified primarily on (a) texture-the
presence or absence of foliation (b) mineralogy-the presence
or absence of qumz (c) environment of deposition (d) chemical
camposition
I
I
mantle be regarded as metamorphic
rocks rather than igneous to&?
2. Where were the metals before they were
*
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,
Chapter 15
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 m m o r p h i s m
(3hornfels (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 so16. 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
18. A metamorphic rock that has undergone partid me1
produce a mixed igneour-metamorphic rock is a (a)
(b) hornfels (c) schist (d) migmtite
deposits?
3. Whnt happens to originally horizontal
layers of scdimcntaryrode when b y are
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.
4. Where in the euth's crust would yo
expect most migmadtes to form?
hrrp:l/mvw.~enc&ubccd-~
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
1gMetAtlulmainmcnu.html
terms covercd in this chapter (e.g.,foliation,
University of North Cmlinak Atks ofRocks,
gneiss, phyllire, marble, quartzite, slate) to
Minerah, and T w m . Click on
.
see nrcellent photomicrog+.,
through a polarizing microscope
rdetamotphum, Metamotphic Rocks, and H~mtkerinalRockt