METAMORPHICMETAMORPHICROCKSROCKS

►►You have learned that increasing pressureYou have learned that increasing pressureand temperature during burial causeand temperature during burial causerecrystallizationrecrystallization and cementation ofand cementation ofsediments.sediments.

►►Whathappens whenWhathappens whenrocksare buriedatrocksare buriedat

evengreater depths?evengreater depths?

CAUSESOF METAMORPHISMCAUSESOF METAMORPHISM►► Pressure and temperature increasePressure and temperature increase

with depth.with depth.►► When temperature or pressureWhen temperature or pressure

becomes high enough, rocks meltbecomes high enough, rocks meltand form magma.and form magma.

►► But what happens if the rocks doBut what happens if the rocks donot quite reach the melting point?not quite reach the melting point?

►► When high temperature andWhen high temperature andpressure combine to alter thepressure combine to alter thetexture, mineralogy, or chemicaltexture, mineralogy, or chemicalcomposition of a rock withoutcomposition of a rock withoutmelting it, a metamorphic rockmelting it, a metamorphic rockforms.forms.

►► The word metamorphism is derivedThe word metamorphism is derivedfrom the Greek words meta,from the Greek words meta,meaning "change," andmeaning "change," and morphemorphemeaning "form.meaning "form.““

►► During metamorphism, a rockDuring metamorphism, a rockchanges form while remainchanges form while remain­­ingingsolid.solid.

►► The high temperatures required for metamorphismThe high temperatures required for metamorphismultimately are derived from Earth's internal heat,ultimately are derived from Earth's internal heat,either through deep burial or from nearby igneouseither through deep burial or from nearby igneousintrusions.intrusions.

►► The high pressures required for metamorphismThe high pressures required for metamorphismcan be generated in two ways:can be generated in two ways:from vertical pressure caused by the weight of overlyingfrom vertical pressure caused by the weight of overlying

rock, orrock, orfrom the compressive forces generated as rocks arefrom the compressive forces generated as rocks are

deformed during mountain building.deformed during mountain building.

TYPESOF METAMORPHISMTYPESOF METAMORPHISM►► Different combinationsDifferent combinations

of temperature andof temperature andpressure result inpressure result indifferent types ofdifferent types ofmetamorphism, shownmetamorphism, shownin the figure on thein the figure on theright.right.

►► Each combinationEach combinationproduces a differentproduces a differentgroup of metamorphicgroup of metamorphicminerals and textures.minerals and textures.

►► When high temperatureWhen high temperatureand pressure affect largeand pressure affect largeregions of the Earth'sregions of the Earth'scrust, they produce largecrust, they produce largebelts of regionalbelts of regionalmetamorphismmetamorphism

►► Regional metamorphism can beRegional metamorphism can below grade, intermediate grade,low grade, intermediate grade,and high grade.and high grade.

►► The grade of regionalThe grade of regionalmetamorphism reflects themetamorphism reflects therelative intensity of temperaturerelative intensity of temperatureand pressure, with lowand pressure, with low--gradegrademetamorphism reflecting themetamorphism reflecting thelowest temperature and pressure.lowest temperature and pressure.

►► The figure below shows theThe figure below shows theregional metamorphic belt thatregional metamorphic belt thathas been mapped in thehas been mapped in thenortheastern United States.northeastern United States.

►► Geologists have divided the beltGeologists have divided the beltinto zones based upon theinto zones based upon themineral groups found in themineral groups found in therocks.rocks.

►► Some of the key minerals used toSome of the key minerals used tomap metamorphic zones aremap metamorphic zones arelisted in Knowing thelisted in Knowing thetemperatures that certain areastemperatures that certain areasexperienced when, rocks wereexperienced when, rocks wereforming can help geologistsforming can help geologistslocate economically valuablelocate economically valuablemetamorphic minerals such asmetamorphic minerals such asgarnet.garnet.

►► When molten rocks, suchWhen molten rocks, suchas those in an igneousas those in an igneousintrusion, come in contactintrusion, come in contactwith solid rock, a localwith solid rock, a localeffect calledeffect called contactcontactmetamorphismmetamorphism occurs.occurs.

►► High temperature andHigh temperature andmoderatemoderate--toto--low pressurelow pressurecause the mineralcause the mineralassemblages that areassemblages that arecharacteristic of contactcharacteristic of contactmetamorphism.metamorphism.

►► Ht picture on the rightHt picture on the rightshows zones of differentshows zones of differentminerals surrounding anminerals surrounding anintrusion.intrusion.

►► Why do you think these zones occur?Why do you think these zones occur?►► Because temperature decreases with distanceBecause temperature decreases with distance

from an intrusion, metamorphic effects alsofrom an intrusion, metamorphic effects alsodecrease with distance.decrease with distance.

►► Recall that minerals crystallize at specificRecall that minerals crystallize at specifictemperatures.temperatures.

►► Minerals that crystallize at high temperatures areMinerals that crystallize at high temperatures arefound closest to the intrusion, where it is hottest.found closest to the intrusion, where it is hottest.

►► Contact metamorphism from extrusive igneousContact metamorphism from extrusive igneousrocks is limited to thin zones.rocks is limited to thin zones.

►► Normally, lava cools too quickly for the heat toNormally, lava cools too quickly for the heat topenetrate very far into surface rocks.penetrate very far into surface rocks.

►► When very hot water reacts with rock and altersWhen very hot water reacts with rock and altersits chemistry and mineralogyits chemistry and mineralogy hydrothermalmetamorphism occurs.occurs.

►► The word hydrothermal is derived from the GreekThe word hydrothermal is derived from the Greekwords hydro, meaning "water," and thermal,words hydro, meaning "water," and thermal,meaning "heat.meaning "heat.““

►► Hydrothermal fluids can disHydrothermal fluids can dis­­solve some minerals,solve some minerals,break down others, and deposit new minerals.break down others, and deposit new minerals.

►► Hydrothermal metamorphism is common aroundHydrothermal metamorphism is common aroundigneous intrusions and near active volcanoes.igneous intrusions and near active volcanoes.

METAMORPHICTEXTURESMETAMORPHICTEXTURES

►►Metamorphicrocks areMetamorphicrocks areclassifiedinto twotexturalclassifiedinto twotexturalgroups:groups:

FoliatedandFoliatedandNonfoliatedNonfoliated

FOLIATEDFOLIATED►► Wavy layers and bands of minerals characterize foliated metamorpWavy layers and bands of minerals characterize foliated metamorphic rocks.hic rocks.►► High pressure during metamorphism cause,, minerals with flat orHigh pressure during metamorphism cause,, minerals with flat or needlelikeneedlelike

crystals to form with their long axes perpendicular to the presscrystals to form with their long axes perpendicular to the pressure, as shownure, as shownin the photo below.in the photo below.

►► This parallel alignment of minerals creates the layer observed iThis parallel alignment of minerals creates the layer observed in foliatedn foliatedmetamorphic rocks.metamorphic rocks.

►► The two most common types of foliated metamorphic rock are:The two most common types of foliated metamorphic rock are: schist, which is derived frown shale, andschist, which is derived frown shale, and gneiss, which is derived from granites.gneiss, which is derived from granites.

NONFOLIATEDNONFOLIATED►► Unlike foliated rocks,Unlike foliated rocks, nonfoliatednonfoliated

metamorphic rocks lackmetamorphic rocks lack minarelminarelgrains with long axes in onegrains with long axes in onedirection.direction.

►► NonfoliatedNonfoliated rocks are composedrocks are composedmainly of minerals that form withmainly of minerals that form withblocky crystal shape: Two commonblocky crystal shape: Two commonexamples ofexamples of nonfoliatednonfoliated rocksarerocksare :: quartzite andquartzite and marble.marble.

►► Quartzite is a hard, lightQuartzite is a hard, light--coloredcoloredrock formed by the metamorphismrock formed by the metamorphismof quartzof quartz--rich sandstone.rich sandstone.

►► Marble is formed by theMarble is formed by themetamorphism of limestone.metamorphism of limestone. Some marbles have very smoothSome marbles have very smooth

textures that are formed bytextures that are formed byinterlocking grains of calciteinterlocking grains of calcite

Such marbles are sought bySuch marbles are sought byartists for sculptures.artists for sculptures.

►►Rocks with foliated textures have parallelRocks with foliated textures have parallelbands of minerals.bands of minerals.

►►Rocks without mineral bands areRocks without mineral bands arenonfoliatednonfoliated..

MINERALCHANGESMINERALCHANGES►►How do minerals change without melting?How do minerals change without melting?

►► Think back to the concept of fractional crystallization, weThink back to the concept of fractional crystallization, wediscussed.discussed.

►► Minerals are stable at certain temperatures and crystallizeMinerals are stable at certain temperatures and crystallizefrom magma at different temperatures.from magma at different temperatures.

►► Scientists have discovered that these stability ranges alsoScientists have discovered that these stability ranges alsoapply to minerals in solid rock.apply to minerals in solid rock.

►► During metamorphism, the minerals in a rock change intoDuring metamorphism, the minerals in a rock change intonew minerals that are stable under the new temperaturenew minerals that are stable under the new temperatureand pressure conditions.and pressure conditions.

►► Minerals that change in this way are said to undergo solidMinerals that change in this way are said to undergo solid--state alterations.state alterations.

►► Scientists have conScientists have con­­ducted experiments to identify theducted experiments to identify themetamorphic conditions that create specific minerals.metamorphic conditions that create specific minerals.

►► When these same minerals are found in rocks, scientistsWhen these same minerals are found in rocks, scientistsare able to interpret the conditions inside the crustare able to interpret the conditions inside the crustduring the rocks metamorphism.during the rocks metamorphism.

►► These conditions are temperature and pressure related,These conditions are temperature and pressure related,with low temperatures and pressures resulting in lowwith low temperatures and pressures resulting in low--grade metamorphism and so on. (as displayed below)grade metamorphism and so on. (as displayed below)

COMPOSITIONALCHANGESCOMPOSITIONALCHANGES

►► Most metamorphic rocks reflect the original chemicalMost metamorphic rocks reflect the original chemicalcomposition of the parent rock.composition of the parent rock.

►► Gneiss, for example, has the same general chemicalGneiss, for example, has the same general chemicalcomposition as granite.composition as granite.

►► In some instances, however, the chemistry of a rock canIn some instances, however, the chemistry of a rock canbe altered along with its minerals and texture.be altered along with its minerals and texture.

►► This occurs because hot fluids migrate in and out of theThis occurs because hot fluids migrate in and out of therock during metamorphism, which can change the originalrock during metamorphism, which can change the originalcomposition of the rock.composition of the rock.

►► Chemical changes are especially common during contactChemical changes are especially common during contactmetamorphism near igneous intrusions.metamorphism near igneous intrusions.

►► Hydrothermal fluids invade the surrounding rocks andHydrothermal fluids invade the surrounding rocks andchange their mineralogy, textures, and chemistry.change their mineralogy, textures, and chemistry.

►► Valuable ore deposits of gold, copper, zinc, tungsten, andValuable ore deposits of gold, copper, zinc, tungsten, andlead are formed in this manner.lead are formed in this manner.

Found in your earth science reference tablepage #7

►Rocksfound intheschemefor

metamorphicrockidentificationchart