PetrologyPetrology

All about rocks!All about rocks!

Essential QuestionsEssential Questions

What is the rock cycle?What is the rock cycle? What are the three types of rocks?What are the three types of rocks?

The Rock CycleThe Rock Cycle

MagmaMagma Magma is molten rock material below the Magma is molten rock material below the

surfacesurface– Lower density causes magma to move upward

to the surface– Magma at the surface is lava

There are 4 types of magma based upon the amount of silica present– Felsic– Intermediate– Mafic– Ultramafic

Silica controls the viscosity of the magma

Felsic MagmaFelsic Magma

SiOSiO22 >70% >70% Fe-Mg: ~ 2%Fe-Mg: ~ 2% TemperatureTemperature: < 900°C: < 900°C ViscosityViscosity: High: High EruptiveEruptive behaviorbehavior: explosive or : explosive or

effusiveeffusive DistributionDistribution: hot spots in : hot spots in

continental crust, continental riftscontinental crust, continental rifts

Intermediate MagmaIntermediate Magma

SiOSiO22 ~ 60% ~ 60% Fe-Mg: ~ 3%Fe-Mg: ~ 3% TemperatureTemperature: ~1000°C: ~1000°C ViscosityViscosity: Intermediate: Intermediate EruptiveEruptive behaviorbehavior: explosive or : explosive or

effusiveeffusive DistributionDistribution: convergent plate : convergent plate

boundaries, island arcsboundaries, island arcs

Mafic Magma Mafic Magma

SiO22 < 50% FeO and MgO typically < 10 wt%Temperature: up to ~1300°CViscosity: LowEruptive behavior: gentleDistribution: divergent plate

boundaries, hot spots, convergent plate boundaries

Ultramafic MagmaUltramafic Magma

SiOSiO22 < 45% < 45% Fe-Mg >8% up to 32%MgOFe-Mg >8% up to 32%MgO TemperatureTemperature: up to 1500°C: up to 1500°C ViscosityViscosity: Very Low: Very Low EruptiveEruptive behaviorbehavior: gentle or very : gentle or very

explosive (kimberilites)explosive (kimberilites) DistributionDistribution: divergent plate : divergent plate

boundaries, hot spots, convergent boundaries, hot spots, convergent plate boundaries plate boundaries

What are Sedimentary Rocks?What are Sedimentary Rocks?

A kind of rock formed when a layer of A kind of rock formed when a layer of sediment becomes solid.sediment becomes solid.

Sedimentary RocksSedimentary RocksEarth Revealed SeriesEarth Revealed Series

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FormationFormation

CompactionCompaction– Is caused by the weight of the overlying Is caused by the weight of the overlying

rocks squeezing the sediments togetherrocks squeezing the sediments together CementationCementation

– Is a chemical process through which Is a chemical process through which water carries and deposits dissolved water carries and deposits dissolved minerals in the small spaces between minerals in the small spaces between the sediment particles.the sediment particles.

Types of Sedimentary RocksTypes of Sedimentary Rocks Chemical sedimentary rocksChemical sedimentary rocks

– Such as rock limestone, halite, and gypsum Such as rock limestone, halite, and gypsum – are formed from minerals left behind after water are formed from minerals left behind after water

evaporates.evaporates. Clastic sedimentary rocksClastic sedimentary rocks

– Such as conglomerate, breccias, and sandstoneSuch as conglomerate, breccias, and sandstone– are formed when little pieces of broken-up rock pile up are formed when little pieces of broken-up rock pile up

over time and are pressed and cemented together.over time and are pressed and cemented together. Organic sedimentary rocksOrganic sedimentary rocks

– Such as coal and organic limestone, contain the remains Such as coal and organic limestone, contain the remains of materials that were once alive. of materials that were once alive.

– Deposits of shells, dinosaur bones, and compressed Deposits of shells, dinosaur bones, and compressed plant remains become beds of coal.plant remains become beds of coal.

Sedimentary Rock CharacteristicsSedimentary Rock Characteristics

Often contains fossils Often contains fossils May react with acid May react with acid Often has layers, flat or curvedOften has layers, flat or curved Has great color variety Has great color variety Particle size may be the same or varyParticle size may be the same or vary Usually has pores between pieces Usually has pores between pieces May have cross-bedding, mud cracks, May have cross-bedding, mud cracks,

worm burrows, raindrop impressions worm burrows, raindrop impressions

What is a Metamorphic Rock?What is a Metamorphic Rock?

A kind of rock formed when A kind of rock formed when sedimentary rock or igneous rock sedimentary rock or igneous rock undergoes a change due to pressure undergoes a change due to pressure or heat in the earth.or heat in the earth.

Metamorphic RocksMetamorphic RocksEarth Revealed SeriesEarth Revealed Series

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FormationFormation

Can occur beneath the surface of the Can occur beneath the surface of the earth, where the magma from the earth, where the magma from the asthenosphere comes in contact with asthenosphere comes in contact with the bottom of he crust of the earth.the bottom of he crust of the earth.

Metamorphism can occur to varying Metamorphism can occur to varying degrees depending upon the degrees depending upon the levels/amounts of heat, pressure, levels/amounts of heat, pressure, and chemically active fluids.and chemically active fluids.

Types of Metamorphic RockTypes of Metamorphic Rock

Foliated metamorphic rocksFoliated metamorphic rocks– Form with layered bands.Form with layered bands.– Examples include slate, schist, and Examples include slate, schist, and

gneissgneiss Nonfoliated metamorphic rocksNonfoliated metamorphic rocks

– Form without layered bandsForm without layered bands– Examples include marble and quartzite.Examples include marble and quartzite.

Metamorphic Rock CharacteristicsMetamorphic Rock Characteristics

Rarely has fossils Rarely has fossils May react with acid May react with acid May have alternate bands of light and dark May have alternate bands of light and dark

minerals minerals May be composed of only one mineral, ex. May be composed of only one mineral, ex.

marble & quartzitemarble & quartzite Usually made of mineral crystals of Usually made of mineral crystals of

different sizesdifferent sizes Rarely has pores or openings Rarely has pores or openings May have bent or curved foliation May have bent or curved foliation

What are Igneous Rocks?What are Igneous Rocks?

Rock formed by the cooling of melted Rock formed by the cooling of melted materials, such as magma inside the materials, such as magma inside the earth and lava about the ground.earth and lava about the ground.

Intrusive Igneous RocksIntrusive Igneous RocksEarth Revealed SeriesEarth Revealed Series

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FormationFormation formed through the cooling and formed through the cooling and

solidification of magma or lavasolidification of magma or lava may be formed below the surface as may be formed below the surface as

intrusive rocks or on the surface as intrusive rocks or on the surface as extrusive rocksextrusive rocks

formed as a result of activity at plate boundaries.

Igneous Rock Textures– Refers to the size, shape, and arrangement of

mineral grains– Size relates to cooling rate, and indicates an

intrusive or extrusive origin

Igneous Rock CharacteristicsIgneous Rock Characteristics Classified by texture and compositionClassified by texture and composition Normally contains no fossils Normally contains no fossils Rarely reacts with acid Rarely reacts with acid Usually has no layering Usually has no layering Usually made of two or more minerals Usually made of two or more minerals May be light or dark colored May be light or dark colored Usually made of mineral crystals of Usually made of mineral crystals of

different sizes different sizes Sometimes has openings or glass fibersSometimes has openings or glass fibers May be fine-grained or glassy (extrusive) May be fine-grained or glassy (extrusive)

Igneous Rock typesIgneous Rock types (based on texture) (based on texture)

Aphanitic: – fine grained, rapid

cooling Phaneritic:

– coarse grained– slow cooling

Porphyritic: – phenocrysts and

groundmass– two stage cooling

history

Glassy: – no crystal structure– Obsidian

Vesicular: – gas cavities– Scoria, Pumice

Pyroclastic: – fragments

generated by explosive volcanism

– Tuff, Breccia

Igneous rocks based Igneous rocks based on magma typeon magma type

Rocks produced from felsic magma are:– composed of relatively composed of relatively

high quantities of high quantities of sodium, aluminum, and sodium, aluminum, and potassium and are potassium and are composed of more than composed of more than 65% silica65% silica

– light in color because of light in color because of the dominance of the dominance of quartz, potassium and quartz, potassium and sodium feldspars, and sodium feldspars, and plagioclase feldspar plagioclase feldspar minerals minerals

– granite, granodiorite, granite, granodiorite, dacite, dacite, andand rhyolite rhyolite

Rocks produced from intermediate magma are:– composed of silica composed of silica

amounts between 53 to amounts between 53 to 65% 65%

– composed composed predominantly of the predominantly of the minerals plagioclase minerals plagioclase feldspar, amphibole, feldspar, amphibole, and pyroxene and pyroxene

– AndesiteAndesite and and dioritediorite

Igneous rocks based on magma Igneous rocks based on magma type continued…type continued…

Rocks produced from Rocks produced from mafic magma are:mafic magma are:– composed of relative composed of relative

low amounts of silica (< low amounts of silica (< 45%) and are 45%) and are dominated by the dominated by the minerals olivine, minerals olivine, calcium-rich plagioclase calcium-rich plagioclase feldspars, and pyroxenefeldspars, and pyroxene

– Basalts Basalts andand gabbros gabbros

Rocks produced from Rocks produced from ultramafic magma are:ultramafic magma are:– < 45% silica, generally < 45% silica, generally

>18% MgO, high FeO, >18% MgO, high FeO, low potassium low potassium

– composed of usually composed of usually greater than 90% mafic greater than 90% mafic minerals (dark colored, minerals (dark colored, high magnesium and high magnesium and iron content)iron content)

– PeridotitePeridotite