Ch 4-Igneous Rocks.pptBBBBBBBBBBBBBBBBBB

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1. Igneous RocksIgneous Rocks
Chapter 4Chapter 4

2. 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

3. 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

4. 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 (H
22O), carbon dioxide O), carbon dioxide
(CO(CO
22), and sulfur dioxide (SO), and sulfur dioxide (SO
22))

5. 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

6. 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

7. Igneous texturesIgneous textures
•Factors affecting crystal size Factors affecting crystal size
•% of % of silicasilica (SiO (SiO
22) present) present
•Dissolved gasesDissolved gases

8. Igneous texturesIgneous textures
•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

9. Aphanitic textureAphanitic texture

10. Phaneritic texturePhaneritic texture

11. Igneous texturesIgneous textures
•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

12. Igneous texturesIgneous textures
•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

13. Porphyritic texturePorphyritic texture

14. Glassy textureGlassy texture

15. 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

17. Igneous compositionsIgneous compositions
•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 (SiO
22))
–Major constituent of continental crustMajor constituent of continental crust

18. Igneous compositionsIgneous compositions
•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

19. 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

20. Igneous compositionsIgneous compositions
•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

21. Igneous compositionsIgneous compositions
•Silica contentSilica content as an indicator of as an indicator of
compositioncomposition
•Exhibits a considerable range in the crustExhibits a considerable range in the crust
–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

23. Igneous compositionsIgneous compositions
•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

24. GraniteGranite

25. Igneous compositionsIgneous compositions
•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

26. RhyoliteRhyolite

27. Igneous compositionsIgneous compositions
•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

28. Pumice is very glassy and sharp,
with countless vesicles.

29. Igneous compositionsIgneous compositions
•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

30. Andesite

31. DioriteDiorite

32. Igneous compositionsIgneous compositions
•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

33. BasaltBasalt

34. Igneous compositionsIgneous compositions
•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

35. GabbroGabbro

36. Igneous compositionsIgneous compositions
•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

38. 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 2020
oo
C C
to 30to 30
oo
C 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

39. 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

40. Decompression meltingDecompression melting

41. 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

42. Bowen’s reaction seriesBowen’s reaction series

43. Evolution of magmasEvolution of magmas
•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

44. 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

45. Evolution of magmasEvolution of magmas
•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

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

47. Evolution of magmasEvolution of magmas
•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

48. Evolution of magmasEvolution of magmas
•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

49. Evolution of magmasEvolution of magmas
•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