86
Rocks are made of MineralsRocks are made of Minerals(Minerals in Granite)(Minerals in Granite)
Three Types of RocksThree Types of Rocks1.1. IgneousIgneous – Crystallized from hot, molten – Crystallized from hot, molten
rock. rock. • ExamplesExamples: granite, basalt : granite, basalt
2.2. SedimentarySedimentary – Fragments of sediment laid – Fragments of sediment laid down by water or wind become compressed down by water or wind become compressed or cemented into layers over time. or cemented into layers over time.
• ExamplesExamples: sandstone, shale, limestone : sandstone, shale, limestone
3.3. MetamorphicMetamorphic – Rocks changed by heat – Rocks changed by heat and/or pressure or chemical activity.and/or pressure or chemical activity.
• ExamplesExamples: gneiss, schist, slate, marble : gneiss, schist, slate, marble
Proportions ofProportions ofRock Types on the EarthRock Types on the Earth
Igneous & Metamorphic Rocks = Crystalline RocksIgneous & Metamorphic Rocks = Crystalline Rocks
The Rock Cycle: The Rock Cycle: Part of the Earth SystemPart of the Earth System
• The loop that involves the The loop that involves the processes by which rocks processes by which rocks originate and change to originate and change to other rocks.other rocks.
• Illustrates the various Illustrates the various processes and paths as processes and paths as rocks change both on the rocks change both on the surface and inside the surface and inside the Earth.Earth.
• Illustrates Illustrates interrelationship among interrelationship among many parts of the Earth many parts of the Earth System.System.
Igneous Igneous RocksRocksChapter 3Chapter 3
What Can Igneous What Can Igneous Minerals/Rocks Tell Us?Minerals/Rocks Tell Us?
1.1. Magma Composition, Viscosity, Temperatures, Magma Composition, Viscosity, Temperatures, PressuresPressures
2.2. Volcano Types and Eruptive BehaviorVolcano Types and Eruptive Behavior3.3. Tectonic SettingTectonic Setting4.4. MagnetismMagnetism5.5. PaleomagnetismPaleomagnetism6.6. Latitude – Magnetic DeclinationLatitude – Magnetic Declination7.7. Polar Reversals – Orientation of Earth’s Magnetic Field Polar Reversals – Orientation of Earth’s Magnetic Field
and Timing of Reversalsand Timing of Reversals8.8. Seafloor SpreadingSeafloor Spreading9.9. Plate MovementsPlate Movements10.10. Changes in Atmospheric Chemistry – OxygenChanges in Atmospheric Chemistry – Oxygen11.11. Radiometric Age DatingRadiometric Age Dating
Origin of Origin of Igneous RocksIgneous Rocks
How Do Igneous Rocks Form?How Do Igneous Rocks Form?
Igneous rocks Igneous rocks form from the form from the cooling and cooling and crystallization crystallization of magma or of magma or lava (molten lava (molten rock)rock)..
MagmaMagma is is molten rock molten rock
that is that is generated in generated in deep in the deep in the
Earth.Earth. Magma that Magma that reaches the reaches the surface is surface is
called called lavalava..
How Does Magma Originate?How Does Magma Originate?
• MagmaMagma originates from originates from partial partial meltingmelting of rocks at various levels in of rocks at various levels in the Earth’s the Earth’s crustcrust and and upper mantle.upper mantle.
• Plate tectonicsPlate tectonics plays a major plays a major role in the role in the generation of generation of most magma.most magma.
Generating Magma from Generating Magma from Solid RockSolid Rock
1.1. Role of HeatRole of Heat
2.2. Role of PressureRole of Pressure
3.3. Role of VolatilesRole of Volatiles
Generating Magma from Solid RockGenerating Magma from Solid Rock• Role of Heat:Role of Heat:
– Temperature increases Temperature increases within Earth’s upper crust within Earth’s upper crust (called the (called the geothermal geothermal gradientgradient) average between ) average between 2020ooC to 30C to 30ooC per kilometer. C per kilometer.
– Rocks in the lower crust Rocks in the lower crust and upper mantle are near and upper mantle are near their melting points.their melting points.
– Any additional heat may Any additional heat may induce melting:induce melting: 1.1. from rocks descending into from rocks descending into
the mantlethe mantle2.2. heating or frictionheating or friction3.3. or rising heat from the or rising heat from the
mantlemantle
• Role of Pressure:Role of Pressure:– A reduction in confining pressure causes the A reduction in confining pressure causes the
lowering of a rock’s melting temperature.lowering of a rock’s melting temperature.
Generating Magma from Solid RockGenerating Magma from Solid Rock
– When confining When confining pressures drop, pressures drop, decompression decompression meltingmelting occurs. occurs.
– May occur when a May occur when a rock ascends as a rock ascends as a result of result of convective convective upwellingupwelling..
Decompression MeltingDecompression Melting
Insert Mantle Melting and Plate Tectonics: Decompression Melting Animation #52
Affect of Pressure and VolatilesAffect of Pressure and Volatiles
Insert Mantle Melting Pressure-Temperature Graphs Animation #53
• Role of Volatiles:Role of Volatiles:– Volatiles (primarily water) cause rocks to melt at Volatiles (primarily water) cause rocks to melt at
lower temperatureslower temperatures..
Generating Magma from Solid RockGenerating Magma from Solid Rock
– Effect of Effect of volatiles is volatiles is magnified by magnified by increased increased pressure.pressure.
Affect of Pressure and VolatilesAffect of Pressure and Volatiles
Insert Mantle Melting Pressure-Temperature Graphs Animation #53
Role of Volatiles – Wet MeltingRole of Volatiles – Wet Melting• Effect of volatiles is magnified by increased Effect of volatiles is magnified by increased
pressure.pressure.
• This is particularly important where oceanic This is particularly important where oceanic lithosphere descends into the mantle.lithosphere descends into the mantle.
• Increased heat and Increased heat and pressure drive water pressure drive water from the subducting from the subducting slab.slab.
• These volatiles are very These volatiles are very mobile and migrate into mobile and migrate into the wedge of hot mantle the wedge of hot mantle – lowering the melting – lowering the melting temperature and temperature and causing melting.causing melting.
• Mantle-derived basaltic magma buoyantly Mantle-derived basaltic magma buoyantly rises due to its rises due to its lesser densitylesser density (hotter). (hotter).
• In a continental setting, basaltic magma may In a continental setting, basaltic magma may ““pondpond” beneath crustal rocks, which have a ” beneath crustal rocks, which have a lower density.lower density.
Role of Volatiles – Wet MeltingRole of Volatiles – Wet Melting
• Crustal rocks are Crustal rocks are near their melting near their melting point.point.
• Increased heatIncreased heat from from basaltic magma basaltic magma causes melting of causes melting of crustal rockscrustal rocks..
• Forms secondary Forms secondary silica-rich magmas.silica-rich magmas.
Wet MeltingWet Melting
Insert Mantle Melting and Plate Tectonics: Decompression Melting Animation #52
Components of Components of MagmaMagma
Three Components of MagmaThree Components of Magma
1.1. A A liquid liquid portion, called portion, called meltmelt, that is composed , that is composed of mobile ions.of mobile ions.
2.2. SolidsSolids, if any, are silicate , if any, are silicate mineralsminerals that have that have already crystallized from the melt.already crystallized from the melt.
3.3. VolatilesVolatiles, which are , which are gases dissolvedgases dissolved in the in the melt that are confined melt that are confined under immense under immense pressurepressure exerted by overlying rocks. exerted by overlying rocks.
• water vapor (Hwater vapor (H22O)O)
• carbon dioxide (COcarbon dioxide (CO22))
• sulfur dioxide (SOsulfur dioxide (SO22))
Crystallization of Crystallization of Magma:Magma:
Formation of Igneous Formation of Igneous RocksRocks
Crystallization of Magma:Crystallization of Magma:Formation of Igneous RocksFormation of Igneous Rocks
1.1. Cooling of magma results in the Cooling of magma results in the systematic arrangement of ions into systematic arrangement of ions into orderly patterns.orderly patterns.
2.2. As heat is lost, ions lose their mobility As heat is lost, ions lose their mobility (vibrate less vigorously) and begin to (vibrate less vigorously) and begin to pack closer and closer together until the pack closer and closer together until the forces of chemical bonds will confine forces of chemical bonds will confine them to and orderly crystalline them to and orderly crystalline arrangement.arrangement.
3.3. When magma cools, silicon and oxygen When magma cools, silicon and oxygen atoms link together first to form atoms link together first to form Si-O Si-O tetrahedratetrahedra..
4.4. Further cooling – Tetrahedra bond with Further cooling – Tetrahedra bond with other ions to form other ions to form embryonic crystal embryonic crystal nucleinuclei..
5.5. Further cooling – Further cooling – Nuclei growNuclei grow as ions lose as ions lose their mobility and join the crystalline their mobility and join the crystalline network.network.
6.6. The silicate minerals resulting from The silicate minerals resulting from crystallization form in a predictable order.crystallization form in a predictable order.
Crystallization of Magma:Crystallization of Magma:Formation of Igneous RocksFormation of Igneous Rocks
• Bowen’s Reaction SeriesBowen’s Reaction Series explains this predictable order of explains this predictable order of crystallization of silicate minerals and how this relates to crystallization of silicate minerals and how this relates to the evolution of magma and igneous compositions.the evolution of magma and igneous compositions.
• Demonstrates that as a magma cools, minerals crystallize Demonstrates that as a magma cools, minerals crystallize in a systematic fashion based on their in a systematic fashion based on their melting pointsmelting points..
Bowen’s Reaction Series explains the evolution Bowen’s Reaction Series explains the evolution of magma and igneous rock compositionsof magma and igneous rock compositions
Evolution of MagmasEvolution of Magmas• Bowen’s Reaction Series: Bowen’s Reaction Series: Divided into two Divided into two
branches: branches: discontinuous seriesdiscontinuous series and and continuous seriescontinuous series
• Discontinuous Reaction SeriesDiscontinuous Reaction Series– The upper left branch indicates that as magma cools, The upper left branch indicates that as magma cools,
olivineolivine is the first mineral to crystallize. is the first mineral to crystallize.– Olivine chemically reacts with the remaining melt to formOlivine chemically reacts with the remaining melt to form
pyroxenepyroxene..– Single tetrahedra of olivine link together with additional Single tetrahedra of olivine link together with additional
tetrahedra to form single-chains structures of the tetrahedra to form single-chains structures of the pyroxene mineral.pyroxene mineral.
– Pyroxene reacts with the remaining melt to form the Pyroxene reacts with the remaining melt to form the double-chain structure of double-chain structure of amphiboleamphibole..
– Amphibole reacts to form the sheet structure of Amphibole reacts to form the sheet structure of biotitebiotite..– Discontinuous – each step forms a different silicate Discontinuous – each step forms a different silicate
structure.structure.
Evolution of MagmasEvolution of Magmas
• Bowens Reaction Series shows that Bowens Reaction Series shows that during crystallization, the composition during crystallization, the composition of the liquid portion of the magma of the liquid portion of the magma continually changes.continually changes.– Composition changes due to removal of Composition changes due to removal of
elements by earlier-forming minerals.elements by earlier-forming minerals.– Minerals remain in contact with the Minerals remain in contact with the
remaining melt and will chemically react remaining melt and will chemically react and evolve into the next mineral in the and evolve into the next mineral in the sequence.sequence.
• The The silica silica component component of the melt becomes of the melt becomes enrichedenriched as as crystallization crystallization proceeds.proceeds.
• Forms increasingly Forms increasingly complex silicate complex silicate structures.structures.
Evolution of MagmasEvolution of Magmas• Bowen’s Reaction Series Bowen’s Reaction Series
Continuous Reaction Series:Continuous Reaction Series:– The right branch indicates Ca-The right branch indicates Ca-
rich plagioclase feldspar reacts rich plagioclase feldspar reacts with Na ions in the melt to with Na ions in the melt to become progressively more Na-become progressively more Na-rich.rich.
– Na ions diffuse into feldspar Na ions diffuse into feldspar crystals and replace Ca ions in crystals and replace Ca ions in the crystal lattice.the crystal lattice.
– Continuous – no steps, same Continuous – no steps, same silicate structure, same family of silicate structure, same family of minerals – minerals – solid solution seriessolid solution series
– Rapid cooling prohibits complete Rapid cooling prohibits complete replacement producing zoned replacement producing zoned crystals with Ca-rich cores and crystals with Ca-rich cores and Na-rich rims.Na-rich rims.
– ““Mini”-continuous reaction Mini”-continuous reaction series occurs within each step of series occurs within each step of the discontinuous series – olivine, the discontinuous series – olivine, pyroxene, amphibole. pyroxene, amphibole.
• Magmatic Differentiation by Magmatic Differentiation by Crystal Settling (Fractional Crystal Settling (Fractional Crystallization)Crystallization)– More dense, early-formed crystals More dense, early-formed crystals
sink toward the bottom of the magma sink toward the bottom of the magma chamber. chamber.
– Separation of a melt from earlier Separation of a melt from earlier formed crystals formed crystals halts the chemical halts the chemical reaction process along BRSreaction process along BRS..
– Produces one or more stages of Produces one or more stages of crystallization.crystallization.
– Forms of one or more Forms of one or more secondary secondary magmasmagmas from a single from a single parental parental magmamagma. .
Processes Responsible Processes Responsible for Deviations from BRSfor Deviations from BRSBRS is highly idealized, assumes magma BRS is highly idealized, assumes magma cools slowly in an unchanging environmentcools slowly in an unchanging environment
Fractional CrystallizationFractional Crystallization
Insert Fractional CrystallizationAnimation #33
• AssimilationAssimilation– Changing a magma’s composition by the incorporation of Changing a magma’s composition by the incorporation of
foreign matterforeign matter into a magma ( into a magma (xenolithsxenoliths).).
Processes Responsible for Processes Responsible for Deviations from BRSDeviations from BRS
– In near surface In near surface environments, the force of environments, the force of injecting magma may injecting magma may fracture surrounding brittle fracture surrounding brittle rocksrocks ( (host rockhost rock). Dislodged ). Dislodged blocks become incorporated blocks become incorporated into the magma.into the magma.
– In deeper environments, In deeper environments, magma may be hot enough magma may be hot enough to to melt and assimilatemelt and assimilate host host rock near its melting rock near its melting temperature.temperature.
Processes Responsible for Processes Responsible for Deviations from BRSDeviations from BRS
– Two chemically Two chemically distinct magmas may distinct magmas may produce an produce an intermediate intermediate compositioncomposition quite quite different from either different from either original magma.original magma.
– Mixing Mixing aided by aided by convective flowconvective flow in the in the magma chamber.magma chamber.
• Magma MixingMagma Mixing– Involves two bodies of magma intruding Involves two bodies of magma intruding
one another.one another.
Magma compositions will Magma compositions will vary depending on the vary depending on the source materialsource material that is that is melted and the melted and the events events (history)(history) that affect that affect crystallization along crystallization along
BRS.BRS.
Origin of Magma CompositionsOrigin of Magma Compositions1.1. Silica-Poor Magmas (Basaltic)Silica-Poor Magmas (Basaltic)
– Primary/Primitive MagmasPrimary/Primitive Magmas• Partial Melting of Mantle (Asthenosphere)Partial Melting of Mantle (Asthenosphere)
2.2. Silica-Rich and Intermediate Magmas Silica-Rich and Intermediate Magmas (Andesitic and Rhyolitic)(Andesitic and Rhyolitic)
– Secondary/Evolved to Highly Evolved Secondary/Evolved to Highly Evolved MagmasMagmas
• Partial Melting of CrustPartial Melting of Crust• Fractional Crystallization of Basaltic MagmaFractional Crystallization of Basaltic Magma• Assimilation or Magma MixingAssimilation or Magma Mixing
• Most originate from direct Most originate from direct partial meltingpartial melting (incomplete melting) of (incomplete melting) of ultramafic rock ultramafic rock (peridotite) in the (peridotite) in the mantlemantle..
• Not evolved. Not evolved. • Primary/Primitive Primary/Primitive
Magmas –Magmas – earliest stages earliest stages along BRS – olivine, along BRS – olivine, pyroxene, Ca-plagioclase.pyroxene, Ca-plagioclase.
• Basaltic magmas form at Basaltic magmas form at mid-ocean ridges and rift mid-ocean ridges and rift zoneszones by by decompression decompression meltingmelting or at or at subduction subduction zones zones by by wet meltingwet melting..
Origin of Basaltic MagmasOrigin of Basaltic Magmas
• Mantle-derived basaltic Mantle-derived basaltic magmas migrates upward, magmas migrates upward, melts, and melts, and assimilates more assimilates more silica-rich rockssilica-rich rocks in the crust in the crust generating magma of generating magma of andesitic composition.andesitic composition.
• Andesitic magma may also Andesitic magma may also evolve by evolve by magmatic magmatic differentiationdifferentiation (crystal (crystal settling).settling).
• Evolved Magmas. Evolved Magmas. • Secondary Magmas –Secondary Magmas –
intermediate stages along intermediate stages along BRS – pyroxene, BRS – pyroxene, amphibole, biotite, amphibole, biotite, plagioclase, and minor plagioclase, and minor quartz.quartz.
Origin of Andesitic MagmasOrigin of Andesitic Magmas
• Most likely form as the Most likely form as the end end product of crystallization of product of crystallization of andesitic magmaandesitic magma..
• Or product of Or product of partial melting partial melting of silica-rich continental of silica-rich continental rocksrocks..
• Highly Evolved Magmas. Highly Evolved Magmas. • Secondary/Teritary Magmas –Secondary/Teritary Magmas –
late stages along BRS – late stages along BRS – orthoclase, quartz, muscovite, orthoclase, quartz, muscovite, plagioclase, biotite, and lesser plagioclase, biotite, and lesser amphibole.amphibole.
• Higher in silica and therefore Higher in silica and therefore more viscous than other more viscous than other magmas.magmas.
• Because of their viscosity, they Because of their viscosity, they lose their mobilitylose their mobility before before reaching the surface.reaching the surface.
• Tend to produce Tend to produce large large plutonic structuresplutonic structures..
Origin of Granitic MagmasOrigin of Granitic Magmas
Classification of Classification of Igneous RocksIgneous Rocks
Classification of Igneous RocksClassification of Igneous Rocks
• Igneous rocks are typically classified byIgneous rocks are typically classified by– TextureTexture– Mineral CompositionMineral Composition
• The The environmentenvironment during crystallization during crystallization can be roughly inferred fromcan be roughly inferred from texturetexture..
• TheThe source and history source and history can be roughly can be roughly inferred from theinferred from the mineral composition mineral composition..
Igneous TexturesIgneous Textures
Igneous TexturesIgneous Textures
• TextureTexture in igneous rocks is in igneous rocks is determined by the determined by the – sizesize, ,
– shapeshape, and , and – arrangementarrangement of mineral grains. of mineral grains.
Igneous TexturesIgneous Textures
• Factors Affecting Crystal Size Factors Affecting Crystal Size 1.1. Rate of cooling – Time Rate of cooling – Time
2.2. Amount of Amount of silicasilica (SiO (SiO22) present) present
3.3. Amount of Amount of dissolved gasesdissolved gases
4.4. FluidsFluids (water and other volatiles) (water and other volatiles) presentpresent
Rate of CoolingRate of Cooling
• The rate of cooling is determined by the The rate of cooling is determined by the environment:environment:– ExtrusiveExtrusive:: Rocks formed from Rocks formed from lavalava at the at the
surface are classified as surface are classified as extrusiveextrusive or or volcanic volcanic rocks.rocks.
– IntrusiveIntrusive:: Rocks formed from Rocks formed from magmamagma that that crystallizes at depth are termed crystallizes at depth are termed intrusiveintrusive, or , or plutonic rocks.plutonic rocks.
The rate of cooling is determined by the environment:The rate of cooling is determined by the environment:– ExtrusiveExtrusive: : Cools quickly at surface – Cools quickly at surface – fine-grainedfine-grained– IntrusiveIntrusive: : Cools slowly subsurface – Cools slowly subsurface – course-grainedcourse-grained
Rapid Rate of CoolingRapid Rate of CoolingAphanitic (Fine-Grained) TextureAphanitic (Fine-Grained) Texture
• Rapid Rapid rate of cooling of rate of cooling of lava or magma. lava or magma.
• Causes ions to quickly Causes ions to quickly lose mobility and readily lose mobility and readily combine with existing combine with existing crystals.crystals.
• Promotes development of Promotes development of numerous embryonic numerous embryonic nucleinuclei that all compete that all compete for available ions.for available ions.
• Forms Forms many microscopic many microscopic crystalscrystals..
• Commonly characterized Commonly characterized by color – by color – lightlight,, intermediateintermediate,, or dark or dark..
Vesicular Texture – Vesicular Texture – Rapid Cooling W/ VolatilesRapid Cooling W/ Volatiles
Vesicular Basalt
• Aphaniutic Rocks Aphaniutic Rocks may contain may contain vesiclesvesicles (voids from (voids from gas bubbles).gas bubbles).
• Form in the upper Form in the upper zone of the lava zone of the lava flow where… flow where…
• rapid cooling rapid cooling “freezes” the lava “freezes” the lava preserving opening preserving opening produced by produced by expanding gas expanding gas bubbles.bubbles.
• Very Fast RateVery Fast Rate – – molten material is molten material is quenched such quenched such that ions are that ions are unable to arrange unable to arrange into a crystalline into a crystalline network.network.
• Forms Forms glassglass rock – rock – obsidian, scoria, or obsidian, scoria, or pumicepumice..
Very Fast Rate of CoolingVery Fast Rate of CoolingGlassy TextureGlassy Texture
Obsidian
Pyroclastic TexturesPyroclastic Textures
• Various fragments ejected during a violent Various fragments ejected during a violent volcanic eruption: volcanic eruption: – Very fine-grained Very fine-grained ashash– CrystalsCrystals,, glass fragments glass fragments,, pumice pumice – Bombs Bombs – – streamlined molten blobsstreamlined molten blobs that that
solidified in air.solidified in air.– Blocks Blocks –– large angular fragments large angular fragments torn from the torn from the
walls of the vent.walls of the vent.• Textures often appear to more similar to Textures often appear to more similar to
sedimentary rocks (sedimentary rocks (clasticclastic).).
Pyroclastic RocksPyroclastic Rocks• TuffTuff – Composed of ash-sized fragments that – Composed of ash-sized fragments that
solidified before impact and cemented later.solidified before impact and cemented later.• Welded TuffWelded Tuff – Composed hot, fine glass shards – Composed hot, fine glass shards
that fused together upon impact.that fused together upon impact.• Volcanic BrecciaVolcanic Breccia – Particles larger than ash. – Particles larger than ash.
• Slow RateSlow Rate of cooling of cooling magma.magma.
• Permits movement of ions Permits movement of ions until they join an existing until they join an existing crystalline structure.crystalline structure.
• Promotes the growth of Promotes the growth of fewer but larger crystalsfewer but larger crystals..
• Mass of intergrown Mass of intergrown crystals.crystals.
• Large, visible crystals can Large, visible crystals can be identified without a be identified without a microscope.microscope.
Slow Rate of CoolingSlow Rate of Cooling Phaneritic (Coarse- Phaneritic (Coarse-
Grained) TextureGrained) Texture
• Exceptionally coarse-Exceptionally coarse-grainedgrained igneous igneous rocks.rocks.
• Large crystal size Large crystal size generated by slow generated by slow cooling rates and cooling rates and dissolved fluids.dissolved fluids.
• Crystals are all Crystals are all larger larger than 1 cm in than 1 cm in diameterdiameter..
• Crystals can be as Crystals can be as large as 1 meter or large as 1 meter or more. more.
Very Slow Rate of Cooling w/ VolatilesVery Slow Rate of Cooling w/ VolatilesPegmatitic TexturePegmatitic Texture
• Pegmatites form in Pegmatites form in late stages of crystallization of late stages of crystallization of granitic magmasgranitic magmas (highly evolved). (highly evolved).
• Water and volatilesWater and volatiles are present in unusually large are present in unusually large percentage:percentage:– Chlorine, fluorine, and sulfurChlorine, fluorine, and sulfur
• Also contain significant amounts rare elements:Also contain significant amounts rare elements:– lithium, cesium, boron, berylium, uranium, lithium, cesium, boron, berylium, uranium,
• Ion migration is enhanced in this fluid-rich Ion migration is enhanced in this fluid-rich environment forming large crystals.environment forming large crystals.
• Magma “stewing in its own juices.”Magma “stewing in its own juices.”• May produce semi-precious gems such as beryl, May produce semi-precious gems such as beryl,
topaz, and tourmaline.topaz, and tourmaline.
Very Slow Rate of Cooling w/ Volatiles Very Slow Rate of Cooling w/ Volatiles Pegmatitic TexturePegmatitic Texture
• Minerals form byMinerals form by different different cooling ratescooling rates..
• Large crystals, called Large crystals, called phenocrystsphenocrysts, are embedded in a , are embedded in a matrix of smaller crystals, called matrix of smaller crystals, called the the groundmassgroundmass..
• A rock with porhyritic texture is A rock with porhyritic texture is called acalled a porphyry porphyry..
• This type of rock typically has a This type of rock typically has a 2-phase cooling history that 2-phase cooling history that affected the rate of affected the rate of crystallization.crystallization.
Two-Phased Cooling Two-Phased Cooling Porphyrytic TexturePorphyrytic Texture
The texture of a particular The texture of a particular igneous rock is ultimately igneous rock is ultimately
determined by the determined by the environmentenvironment from which it crystallized.from which it crystallized.
Mineral Compositions Mineral Compositions of Igneous Rocksof Igneous Rocks
Bowen’s Reaction Series explains the order of silicate Bowen’s Reaction Series explains the order of silicate mineral crystallization and the how this governs the mineral crystallization and the how this governs the evolution of magma and igneous rock compositionsevolution of magma and igneous rock compositions
Igneous CompositionsIgneous Compositions
• UltramaficUltramafic Composition: Composition:– Rare composition that is high in magnesium and Rare composition that is high in magnesium and
iron.iron.
• Low silica content – Low silica content – approximately approximately 45%45%..
• Main constituent of Main constituent of the the upper mantleupper mantle..
Ultramafic RocksUltramafic Rocks: : PeridotitePeridotite
• Ultramafic igneous rocks Ultramafic igneous rocks are composed entirely of are composed entirely of dark (or dark (or ferromagnesianferromagnesian) ) silicate minerals:silicate minerals:– OlivineOlivine
– Pyroxene (Augite) Pyroxene (Augite)
– Minor Ca-PlagioclaseMinor Ca-Plagioclase
Igneous CompositionsIgneous Compositions
• Basaltic Basaltic (or(or Mafic Mafic) Composition:) Composition:– MaficMafic ( (mamagnesium and gnesium and feferrum, for iron) rrum, for iron)
– Silica deficient – Silica deficient – approximately approximately 50 percent50 percent..
– More More densedense than granitic than granitic rocks.rocks.
– Comprise the Comprise the ocean floorocean floor as well as many as well as many volcanic volcanic islandsislands..
– Also found on continents.Also found on continents.
Mafic RocksMafic Rocks: : GabbroGabbro, , BasaltBasalt
• Mafic igneous rocks are Mafic igneous rocks are composed primarily of composed primarily of dark (or dark (or ferromagnesianferromagnesian) ) silicate minerals:silicate minerals:– OlivineOlivine– Pyroxene (Augite) Pyroxene (Augite) – Amphibole (Hornblende)Amphibole (Hornblende)– Ca-PlagioclaseCa-Plagioclase
Igneous CompositionsIgneous Compositions• Andesitic Andesitic (or(or Intermediate Intermediate) Composition:) Composition:
– Intermediate silica content (Intermediate silica content (~60%~60%) between granite ) between granite (65%) and basalt (50%). (65%) and basalt (50%).
• Contain at least 25 Contain at least 25 percent dark silicate percent dark silicate minerals.minerals.
• Associated with Associated with explosive volcanic explosive volcanic activityactivity near near continental margins.continental margins.
Intermediate RocksIntermediate Rocks: : DioriteDiorite, , AndesiteAndesite
• Intermediate igneous rocks are composed of dark Intermediate igneous rocks are composed of dark and light silicate minerals:and light silicate minerals:– Pyroxene (Augite) Pyroxene (Augite) – Amphibole (Hornblende)Amphibole (Hornblende)– Biotite MicaBiotite Mica– PlagioclasePlagioclase– Minor QuartzMinor Quartz
Igneous CompositionsIgneous Compositions
• Granitic Granitic or (or (FelsicFelsic) Composition:) Composition:
– FelsicFelsic ( (felfeldspar and dspar and sisilica).lica).
– Contains high Contains high amounts of silica amounts of silica (SiO(SiO22) – ) – 65 percent65 percent or more.or more.
– Major constituents Major constituents of of continental crustcontinental crust..
Felsic RocksFelsic Rocks: : GraniteGranite, , RhyoliteRhyolite
• Felsic igneous rocks are composed of primarily Felsic igneous rocks are composed of primarily light with fewer dark silicate minerals:light with fewer dark silicate minerals:– Quartz Quartz – Muscovite and Biotite Micas Muscovite and Biotite Micas
– Plagioclase and Orthoclase FeldsparsPlagioclase and Orthoclase Feldspars– Amphibole (Hornblende)Amphibole (Hornblende)
Igneous Rock Classification Igneous Rock Classification
FelsicFelsic IntermediateIntermediate MaficMafic UltramaficUltramafic
FFiinnee
RhyoliteRhyolite AndesiteAndesite BasaltBasalt
CCooaarrssee
GraniteGranite DioriteDiorite GabbroGabbro PeridotitePeridotite
The mineral composition of a The mineral composition of a particular igneous rock is particular igneous rock is
ultimately determined by the ultimately determined by the composition of the magma composition of the magma
(source material and history)(source material and history) from which it crystallized.from which it crystallized.
Glossary of Glossary of Igneous RocksIgneous Rocks
Naming Igneous RocksNaming Igneous Rocks
Naming Igneous RocksNaming Igneous Rocks
Igneous Rock Classification Igneous Rock Classification
FelsicFelsic IntermediateIntermediate MaficMafic UltramaficUltramafic
FFiinnee
RhyoliteRhyolite AndesiteAndesite BasaltBasalt
CCooaarrssee
GraniteGranite DioriteDiorite GabbroGabbro PeridotitePeridotite
Igneous Rock ClassificationIgneous Rock Classification
GraniteGranite• IntrusiveIntrusive and and phaneriticphaneritic. . • Over 25 percent quartz, about 65 Over 25 percent quartz, about 65
percent or more feldspar.percent or more feldspar.• Other minor silicates muscovite, Other minor silicates muscovite,
biotite, amphibole comprise less biotite, amphibole comprise less than 10 percent.than 10 percent.
• May exhibit a porphyritic texture.May exhibit a porphyritic texture.• Very abundant as it is often Very abundant as it is often
associated with mountain associated with mountain building.building.
• The term granite covers a wide The term granite covers a wide range of mineral compositions.range of mineral compositions.
RhyoliteRhyolite• ExtrusiveExtrusive equivalent of equivalent of
granite.granite.• AphaniticAphanitic texture. texture.• May contain glass May contain glass
fragments and vesicles.fragments and vesicles.• May contain May contain phenocrystsphenocrysts
of orthoclase, mica, and of orthoclase, mica, and quartz.quartz.
• Typically Typically red to reddish-red to reddish-purplepurple to gray in color. to gray in color.
• Less common and less Less common and less voluminous than granite.voluminous than granite.
Other Granitic Other Granitic (Felsic) Rocks(Felsic) Rocks
• ObsidianObsidian– Extrusive with glassy textureExtrusive with glassy texture– High silica contentHigh silica content– Dark colored due to presence of Dark colored due to presence of
metallic ionsmetallic ions• PumicePumice
– Extrusive with glassy textureExtrusive with glassy texture– Vesicular texture (more voids than Vesicular texture (more voids than
rock)rock)– Frothy appearance with numerous Frothy appearance with numerous
voids voids – Void shape is typically elongatedVoid shape is typically elongated
• ScoriaScoria– Extrusive with glassy textureExtrusive with glassy texture– Vesicular texture (more rock than Vesicular texture (more rock than
voids) voids) – Void shape is typically more Void shape is typically more
roundedrounded
AndesiteAndesite• Volcanic origin – Volcanic origin –
extrusiveextrusive..• Aphanitic Aphanitic texture.texture.• Often resembles Often resembles
rhyolite.rhyolite.• Typically medium-Typically medium-
gray.gray.• May contain May contain
phenocrystsphenocrysts of of plagioclase and plagioclase and hornblende.hornblende.
DioriteDiorite
• PlutonicPlutonic equivalent of equivalent of andesite.andesite.
• Coarse-grained or Coarse-grained or phaneriticphaneritic..
• Intrusive.Intrusive.• Composed mainly of Na-Composed mainly of Na-
rich plagioclase rich plagioclase (intermediate) feldspar (intermediate) feldspar and amphibole with and amphibole with lesser amounts of biotite.lesser amounts of biotite.
• Salt and pepperSalt and pepper appearance.appearance.
BasaltBasalt• Volcanic origin – Volcanic origin – extrusiveextrusive..
• AphaniticAphanitic texture. texture.
• Composed mainly of pyroxene Composed mainly of pyroxene and calcium-rich plagioclase and calcium-rich plagioclase feldspar with lesser amounts of feldspar with lesser amounts of olivine and amphibole.olivine and amphibole.
• Very dark green to black in Very dark green to black in color.color.
• May contain May contain phenocrystsphenocrysts of of plagioclase and olivine.plagioclase and olivine.
• May exhibit vesicular texture.May exhibit vesicular texture.
• Most common extrusive igneous Most common extrusive igneous rock.rock.
GabbroGabbro
• IntrusiveIntrusive equivalent equivalent of basalt.of basalt.
• PhaneriticPhaneritic texture texture consisting of consisting of pyroxene and pyroxene and calcium-rich calcium-rich plagioclase.plagioclase.
• Makes up a Makes up a significant percentage significant percentage of the oceanic crust.of the oceanic crust.
PyroclasticPyroclastic
• Composed of fragments (Composed of fragments (tephratephra) ejected ) ejected during a volcanic eruption.during a volcanic eruption.– TuffTuff – ash-sized fragments that solidified before – ash-sized fragments that solidified before
impact and cemented later.impact and cemented later.– Welded TuffWelded Tuff – Composed hot fine glass shards – Composed hot fine glass shards
that fused together upon impact.that fused together upon impact.– Volcanic BrecciaVolcanic Breccia – Particles larger than ash. – Particles larger than ash.
• BombsBombs – streamlined fragments that solidified in air. – streamlined fragments that solidified in air.• Large angular Large angular blocksblocks torn from the walls of the vent. torn from the walls of the vent.• Crystals, glass fragments, pumice, ash.Crystals, glass fragments, pumice, ash.
Interlayered Tuff and ObsidianInterlayered Tuff and Obsidian
1996 Eruption of Mount Ruapehu, New Zealand – 1996 Eruption of Mount Ruapehu, New Zealand – Intermediate to Felsic composition magmasIntermediate to Felsic composition magmas
