Igneous Rocks Chapter 4. General characteristics of magma General characteristics of magma Igneous rocks form as molten rock cools and solidifiesIgneous.

Igneous RocksIgneous Rocks

Chapter 4Chapter 4

General characteristics General characteristics of magmaof magma

• Igneous rocks form as molten rock Igneous rocks form as molten rock cools and solidifiescools and solidifies

• General characteristics of General characteristics of magma:magma:• Parent material of igneous rocksParent material of igneous rocks• Forms from partial melting of rocksForms from partial melting of rocks• Magma at surface is called Magma at surface is called lavalava

General characteristics General characteristics of magmaof magma

• General characteristic of magmaGeneral characteristic of magma• Rocks formed from lava = Rocks formed from lava = extrusiveextrusive, or , or

volcanic rocksvolcanic rocks• Rocks formed from magma at depth = Rocks formed from magma at depth =

intrusiveintrusive, or , or plutonic rocksplutonic rocks

General characteristicsGeneral characteristics of magma of magma

• The nature of magmaThe nature of magma• Consists of three components:Consists of three components:

– Liquid portionLiquid portion = = meltmelt– SolidsSolids, if any, are silicate minerals, if any, are silicate minerals– VolatilesVolatiles = dissolved gases in the melt, = dissolved gases in the melt,

including water vapor (Hincluding water vapor (H22O), carbon dioxide O), carbon dioxide

(CO(CO22), and sulfur dioxide (SO), and sulfur dioxide (SO22))

General characteristicsGeneral characteristics of magma of magma

• Crystallization of magmaCrystallization of magma• Cooling of magma results in the Cooling of magma results in the

systematic arrangement of ions into systematic arrangement of ions into orderly patternsorderly patterns

• The silicate minerals resulting from The silicate minerals resulting from crystallization form in a predictable crystallization form in a predictable orderorder

• Texture - size and arrangement of Texture - size and arrangement of mineral grainsmineral grains

Igneous texturesIgneous textures

• TextureTexture is used to describe the is used to describe the overall appearance of a rock based overall appearance of a rock based on the size, shape, and arrangement on the size, shape, and arrangement of interlocking mineralsof interlocking minerals

• Factors affecting crystal sizeFactors affecting crystal size• Rate of coolingRate of cooling

– Slow rate = fewer but larger crystalsSlow rate = fewer but larger crystals– Fast rate = many small crystalsFast rate = many small crystals– Very fast rate forms glassVery fast rate forms glass


• Factors affecting crystal size Factors affecting crystal size • % of % of silicasilica (SiO (SiO22) present) present

• Dissolved gasesDissolved gases


• Types of igneous texturesTypes of igneous textures• AphaniticAphanitic (fine-grained) texture (fine-grained) texture

– Rapid rate of coolingRapid rate of cooling– Microscopic crystalsMicroscopic crystals– May contain May contain vesiclesvesicles (holes from gas (holes from gas

bubbles)bubbles)

• PhaneriticPhaneritic (coarse-grained) texture (coarse-grained) texture– Slow coolingSlow cooling– Large, visible crystalsLarge, visible crystals

Aphanitic textureAphanitic texture

Phaneritic texturePhaneritic texture


• Types of igneous texturesTypes of igneous textures• Porphyritic Porphyritic texturetexture

– Minerals form at different temperaturesMinerals form at different temperatures– Large crystals (Large crystals (phenocrystsphenocrysts) are embedded ) are embedded

in a matrix of smaller crystals (in a matrix of smaller crystals (groundmassgroundmass))

• GlassyGlassy texturetexture– Very rapid cooling of lavaVery rapid cooling of lava– Resulting rock is called Resulting rock is called obsidianobsidian


• Types of igneous texturesTypes of igneous textures• PyroclasticPyroclastic texture texture

– Fragmental appearance produced by violent Fragmental appearance produced by violent volcanic eruptionsvolcanic eruptions

– Often appear more similar to sedimentary Often appear more similar to sedimentary rocksrocks

• PegmatiticPegmatitic texture texture– Exceptionally coarse grainedExceptionally coarse grained– Form in late stages of crystallization of Form in late stages of crystallization of

granitic magmasgranitic magmas

Porphyritic texturePorphyritic texture

Glassy textureGlassy texture

Igneous compositionsIgneous compositions

• Igneous rocks are composed primarily Igneous rocks are composed primarily of silicate mineralsof silicate minerals

• Dark (or Dark (or ferromagnesianferromagnesian) silicates) silicates– Olivine, pyroxene, amphibole, and biotite Olivine, pyroxene, amphibole, and biotite

micamica

• Light (or nonferromagnesian) silicatesLight (or nonferromagnesian) silicates– Quartz, muscovite mica, and feldsparsQuartz, muscovite mica, and feldspars


• GraniticGranitic versus versus basalticbasaltic compositions compositions• GraniticGranitic composition composition

– Light-colored silicatesLight-colored silicates– Termed Termed felsicfelsic ( (fefeldspar and ldspar and sisilica) in lica) in

compositioncomposition

– High amounts of silica (SiOHigh amounts of silica (SiO22))

– Major constituent of continental crustMajor constituent of continental crust


• GraniticGranitic versus basaltic compositionsversus basaltic compositions• Basaltic compositionBasaltic composition

– Dark silicates and calcium-rich feldsparDark silicates and calcium-rich feldspar– Termed Termed maficmafic ( (mamagnesium and gnesium and feferrum, for rrum, for

iron) in compositioniron) in composition– Higher dense than granitic rocksHigher dense than granitic rocks– Comprise the ocean floor and many Comprise the ocean floor and many

volcanic islandsvolcanic islands

Basaltic lava dropping into the ocean along Kilauea Volcano along Basaltic lava dropping into the ocean along Kilauea Volcano along the southeastern coast of the big island of Hawaiithe southeastern coast of the big island of Hawaii


• Other compositional groupsOther compositional groups• IntermediateIntermediate (or (or andesiticandesitic) composition) composition

– Contain 25% or more dark silicate mineralsContain 25% or more dark silicate minerals– Associated with explosive volcanic activityAssociated with explosive volcanic activity

• UltramaficUltramafic composition composition– Rare composition that is high in magnesium Rare composition that is high in magnesium

and ironand iron– Composed entirely of ferromagnesian Composed entirely of ferromagnesian

silicatessilicates


• Silica contentSilica content as an indicator of as an indicator of compositioncomposition

• Exhibits a considerable range in the Exhibits a considerable range in the crustcrust

– 45% to 70%45% to 70%

• Silica contentSilica content influences magma influences magma behaviorbehavior

• Granitic magmas = high silica content Granitic magmas = high silica content and viscousand viscous

• Basaltic magmas = much lower silica Basaltic magmas = much lower silica content and more fluid-like behaviorcontent and more fluid-like behavior


• Naming igneous rocks – granitic rocksNaming igneous rocks – granitic rocks• GraniteGranite

– PhaneriticPhaneritic– Over 25% quartz, about 65% or more Over 25% quartz, about 65% or more

feldsparfeldspar– Very abundant - often associated with Very abundant - often associated with

mountain buildingmountain building– The term granite includes a wide range of The term granite includes a wide range of

mineral compositionsmineral compositions

GraniteGranite


• Naming igneous rocks – granitic rocksNaming igneous rocks – granitic rocks

• RhyoliteRhyolite– Extrusive equivalent of graniteExtrusive equivalent of granite– May contain glass fragments and vesiclesMay contain glass fragments and vesicles– Aphanitic textureAphanitic texture– Less common and less voluminous than Less common and less voluminous than

granitegranite

RhyoliteRhyolite


• Naming igneous rocks – granitic rocksNaming igneous rocks – granitic rocks• ObsidianObsidian

– Dark coloredDark colored– Glassy textureGlassy texture

• PumicePumice– VolcanicVolcanic– Glassy textureGlassy texture– Frothy appearance with numerous voids Frothy appearance with numerous voids

Pumice is very glassy and sharp, with countless vesicles.


• Naming igneous rocks – intermediate Naming igneous rocks – intermediate rocksrocks

• AndesiteAndesite– Volcanic originVolcanic origin– Aphanitic textureAphanitic texture

• DioriteDiorite– Plutonic equivalent of andesitePlutonic equivalent of andesite– Coarse grainedCoarse grained

Andesite

DioriteDiorite


• Naming igneous rocks – basaltic rocksNaming igneous rocks – basaltic rocks

• BasaltBasalt– Volcanic originVolcanic origin– Aphanitic textureAphanitic texture– Composed mainly of pyroxene and calcium-Composed mainly of pyroxene and calcium-

rich plagioclase feldsparrich plagioclase feldspar– Most common extrusive igneous rockMost common extrusive igneous rock

BasaltBasalt


• Naming igneous rocks – mafic rocksNaming igneous rocks – mafic rocks

• GabbroGabbro– Intrusive equivalent of basaltIntrusive equivalent of basalt– Phaneritic texture consisting of pyroxene Phaneritic texture consisting of pyroxene

and calcium-rich plagioclaseand calcium-rich plagioclase– Significant % of the oceanic crustSignificant % of the oceanic crust

GabbroGabbro


• Naming igneous rocks – Naming igneous rocks – pyroclastic pyroclastic rocksrocks

• Composed of fragments ejected during Composed of fragments ejected during a volcanic eruptiona volcanic eruption

• VarietiesVarieties– TuffTuff = ash-sized fragments = ash-sized fragments– Volcanic brecciaVolcanic breccia = particles larger than ash = particles larger than ash

Origin of magmaOrigin of magma

• Highly debated topicHighly debated topic

• Generating magma from solid rockGenerating magma from solid rock• Role of heatRole of heat

– Temperature increases in the upper crust Temperature increases in the upper crust ((geothermal gradientgeothermal gradient) average between ) average between 2020ooC C to 30to 30ooC per kilometerC per kilometer of depth of depth

– Rocks in the lower crust and upper mantle Rocks in the lower crust and upper mantle are near their melting pointsare near their melting points

– Any additional heat may induce meltingAny additional heat may induce melting

Origin of magmaOrigin of magma

• Role of pressureRole of pressure– Increases in confining pressure cause an Increases in confining pressure cause an

increase in a rock’s melting temperature increase in a rock’s melting temperature – When confining pressures drop, When confining pressures drop,

decompression melting occursdecompression melting occurs

• Role of volatilesRole of volatiles– Volatiles (primarily Volatiles (primarily waterwater) cause rocks to ) cause rocks to

melt at lower temperaturesmelt at lower temperatures– Important factor where oceanic lithosphere Important factor where oceanic lithosphere

descends into the mantledescends into the mantle

Decompression meltingDecompression melting

Evolution of magmasEvolution of magmas

• A single volcano may extrude lavas exhibiting A single volcano may extrude lavas exhibiting very different compositionsvery different compositions

• Each volcanic eruption tends to exhibit a Each volcanic eruption tends to exhibit a unique geochemical fingerprint, defined by unique geochemical fingerprint, defined by trace element percentagestrace element percentages

• Bowen’s reaction seriesBowen’s reaction series • Minerals crystallize in a systematic fashion Minerals crystallize in a systematic fashion

based on their melting pointsbased on their melting points• During crystallization, the composition of the During crystallization, the composition of the

liquid portion of the magma continually changesliquid portion of the magma continually changes

Bowen’s reaction seriesBowen’s reaction series


• Processes responsible for changing a Processes responsible for changing a magma’s compositionmagma’s composition

• Magmatic differentiationMagmatic differentiation– Separation of a melt from earlier formed Separation of a melt from earlier formed

crystalscrystals

• AssimilationAssimilation– Changing a magma’s composition by the Changing a magma’s composition by the

incorporation of surrounding rock bodies incorporation of surrounding rock bodies into a magmainto a magma

Magma evolves as the Magma evolves as the hotter minerals crystallize hotter minerals crystallize and settle to the bottom of and settle to the bottom of the magma chamberthe magma chamber


• Processes responsible for changing a Processes responsible for changing a magma’s compositionmagma’s composition

• Magma mixingMagma mixing– Two chemically distinct magmas may Two chemically distinct magmas may

produce a composition quite different from produce a composition quite different from either original magmaeither original magma

Assimilation, magma mixing, Assimilation, magma mixing, and magmatic differentiationand magmatic differentiation


• Partial melting and magma formationPartial melting and magma formation• Incomplete melting of rocks is known as Incomplete melting of rocks is known as

partial meltingpartial melting• Formation of basaltic magmasFormation of basaltic magmas

– Most originate from partial melting of Most originate from partial melting of ultramafic rock in the mantle at oceanic ultramafic rock in the mantle at oceanic ridgesridges

– Large outpourings of basaltic magma are Large outpourings of basaltic magma are common at Earth’s surfacecommon at Earth’s surface


• Partial melting and magma formationPartial melting and magma formation• Formation of andesitic magmasFormation of andesitic magmas

– Produced by interaction of basaltic magmas Produced by interaction of basaltic magmas and more silica-rich rocks in the crust and more silica-rich rocks in the crust

– May also evolve by magmatic differentiationMay also evolve by magmatic differentiation


• Partial melting and magma formationPartial melting and magma formation• Formation of granitic magmasFormation of granitic magmas

– Most likely form as the end product of Most likely form as the end product of crystallization of andesitic magmacrystallization of andesitic magma

– Granitic magmas are more viscous than Granitic magmas are more viscous than other magmas so they tend to lose their other magmas so they tend to lose their mobility before reaching the surfacemobility before reaching the surface

– Tend to produce large plutonic structuresTend to produce large plutonic structures

Igneous Rocks Chapter 4. General characteristics of magma General characteristics of magma Igneous rocks form as molten rock cools and solidifiesIgneous.

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