Geology

2. Rocks are made of MineralsRocks are made of Minerals
(Minerals in Granite)(Minerals in Granite)

3. Three Types of RocksThree Types of Rocks
1.1. 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 compresseddown 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

4. Proportions ofProportions of
Rock Types on the EarthRock Types on the Earth
Igneous & Metamorphic Rocks = Crystalline RocksIgneous & Metamorphic Rocks = Crystalline Rocks

5. The Rock Cycle:The Rock Cycle:
Part of the Earth SystemPart of the Earth System
• The loop that involves theThe loop that involves the
processes by which rocksprocesses by which rocks
originate and change tooriginate and change to
other rocks.other rocks.
• Illustrates the variousIllustrates the various
processes and paths asprocesses and paths as
rocks change both on therocks change both on the
surface and inside thesurface and inside the
Earth.Earth.
• IllustratesIllustrates
interrelationship amonginterrelationship among
many parts of the Earthmany parts of the Earth
System.System.

6. IgneousIgneous
RocksRocks
Chapter 3Chapter 3

7. What Can IgneousWhat 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 Behavior
3.3. Tectonic SettingTectonic Setting
4.4. MagnetismMagnetism
5.5. PaleomagnetismPaleomagnetism
6.6. Latitude – Magnetic DeclinationLatitude – Magnetic Declination
7.7. Polar Reversals – Orientation of Earth’s Magnetic FieldPolar Reversals – Orientation of Earth’s Magnetic Field
and Timing of Reversalsand Timing of Reversals
8.8. Seafloor SpreadingSeafloor Spreading
9.9. Plate MovementsPlate Movements
10.10. Changes in Atmospheric Chemistry – OxygenChanges in Atmospheric Chemistry – Oxygen
11.11. Radiometric Age DatingRadiometric Age Dating

8. Origin ofOrigin of
Igneous RocksIgneous Rocks

9. How Do Igneous Rocks Form?How Do Igneous Rocks Form?
Igneous rocksIgneous rocks
form from theform from the
cooling andcooling and
crystallizationcrystallization
of magma orof magma or
lava (moltenlava (molten
rock)rock)..

10. MagmaMagma isis
molten rockmolten rock
that isthat is
generated ingenerated in
deep in thedeep in the
Earth.Earth.
Magma thatMagma that
reaches thereaches the
surface issurface is
calledcalled lavalava..

11. How Does Magma Originate?How Does Magma Originate?
• MagmaMagma originates fromoriginates from partialpartial
meltingmelting of rocks at various levels inof rocks at various levels in
the Earth’sthe Earth’s crustcrust andand upper mantle.upper mantle.
• Plate tectonicsPlate tectonics
plays a majorplays a major
role in therole in the
generation ofgeneration of
most magma.most magma.

12. Generating Magma fromGenerating 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

13. Generating Magma from Solid RockGenerating Magma from Solid Rock
• Role of Heat:Role of Heat:
– Temperature increasesTemperature increases
within Earth’s upper crustwithin Earth’s upper crust
(called the(called the geothermalgeothermal
gradientgradient) average between) average between
2020oo
C to 30C to 30oo
C per kilometer.C per kilometer.
– Rocks in the lower crustRocks in the lower crust
and upper mantle are nearand upper mantle are near
their melting points.their melting points.
– Any additional heat mayAny additional heat may
induce melting:induce melting:
1.1. from rocks descending intofrom rocks descending into
the mantlethe mantle
2.2. heating or frictionheating or friction
3.3. or rising heat from theor rising heat from the
mantlemantle

14. • Role of Pressure:Role of Pressure:
– A reduction in confining pressure causes theA 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 confiningWhen confining
pressures drop,pressures drop,
decompressiondecompression
meltingmelting occurs.occurs.
– May occur when aMay occur when a
rock ascends as arock ascends as a
result ofresult of convectiveconvective
upwellingupwelling..

15. Decompression MeltingDecompression Melting
Insert Mantle Melting
and Plate Tectonics:
Decompression Melting
Animation #52

16. Affect of Pressure and VolatilesAffect of Pressure and Volatiles
Insert Mantle Melting
Pressure-Temperature
Graphs
Animation #53

17. • Role of Volatiles:Role of Volatiles:
– Volatiles (primarily water) cause rocks to melt atVolatiles (primarily water) cause rocks to melt at
lower temperatureslower temperatures..
Generating Magma from Solid RockGenerating Magma from Solid Rock
– Effect ofEffect of
volatiles isvolatiles is
magnified bymagnified by
increasedincreased
pressure.pressure.

18. Affect of Pressure and VolatilesAffect of Pressure and Volatiles
Insert Mantle Melting
Pressure-Temperature
Graphs
Animation #53

19. Role of Volatiles – Wet MeltingRole of Volatiles – Wet Melting
• Effect of volatiles is magnified by increasedEffect of volatiles is magnified by increased
pressure.pressure.
• This is particularly important where oceanicThis is particularly important where oceanic
lithosphere descends into the mantle.lithosphere descends into the mantle.
• Increased heat andIncreased heat and
pressure drive waterpressure drive water
from the subductingfrom the subducting
slab.slab.
• These volatiles are veryThese volatiles are very
mobile and migrate intomobile and migrate into
the wedge of hot mantlethe wedge of hot mantle
– lowering the melting– lowering the melting
temperature andtemperature and
causing melting.causing melting.

20. • Mantle-derived basaltic magma buoyantlyMantle-derived basaltic magma buoyantly
rises due to itsrises due to its lesser densitylesser density (hotter).(hotter).
• In a continental setting, basaltic magma mayIn 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 areCrustal rocks are
near their meltingnear their melting
point.point.
• Increased heatIncreased heat fromfrom
basaltic magmabasaltic magma
causes melting ofcauses melting of
crustal rockscrustal rocks..
• Forms secondaryForms secondary
silica-rich magmas.silica-rich magmas.

21. Wet MeltingWet Melting
Insert Mantle Melting
and Plate Tectonics:
Decompression Melting
Animation #52

22. Components ofComponents of
MagmaMagma

23. Three Components of MagmaThree Components of Magma
1.1. AA liquidliquid portion, calledportion, 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 havethat have
already crystallized from the melt.already crystallized from the melt.
3.3. VolatilesVolatiles, which are, which are gases dissolvedgases dissolved in thein the
melt that are confinedmelt that are confined under immenseunder 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))

24. Crystallization ofCrystallization of
Magma:Magma:
Formation of IgneousFormation of Igneous
RocksRocks

25. Crystallization of Magma:Crystallization of Magma:
Formation of Igneous RocksFormation of Igneous Rocks
1.1. Cooling of magma results in theCooling of magma results in the
systematic arrangement of ions intosystematic arrangement of ions into
orderly patterns.orderly patterns.
2.2. As heat is lost, ions lose their mobilityAs 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 thepack closer and closer together until the
forces of chemical bonds will confineforces of chemical bonds will confine
them to and orderly crystallinethem to and orderly crystalline
arrangement.arrangement.

26. 3.3. When magma cools, silicon and oxygenWhen magma cools, silicon and oxygen
atoms link together first to formatoms link together first to form Si-OSi-O
tetrahedratetrahedra..
4.4. Further cooling – Tetrahedra bond withFurther cooling – Tetrahedra bond with
other ions to formother ions to form embryonic crystalembryonic crystal
nucleinuclei..
5.5. Further cooling –Further cooling – Nuclei growNuclei grow as ions loseas ions lose
their mobility and join the crystallinetheir mobility and join the crystalline
network.network.
6.6. The silicate minerals resulting fromThe 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

27. • Bowen’s Reaction SeriesBowen’s Reaction Series explains this predictable order ofexplains this predictable order of
crystallization of silicate minerals and how this relates tocrystallization 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 crystallizeDemonstrates that as a magma cools, minerals crystallize
in a systematic fashion based on theirin a systematic fashion based on their melting pointsmelting points..

28. Bowen’s Reaction Series explains the evolutionBowen’s Reaction Series explains the evolution
of magma and igneous rock compositionsof magma and igneous rock compositions

29. Evolution of MagmasEvolution of Magmas
• Bowen’s Reaction Series:Bowen’s Reaction Series: Divided into twoDivided into two
branches:branches: discontinuous seriesdiscontinuous series andand 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 additionalSingle tetrahedra of olivine link together with additional
tetrahedra to form single-chains structures of thetetrahedra to form single-chains structures of the
pyroxene mineral.pyroxene mineral.
– Pyroxene reacts with the remaining melt to form thePyroxene reacts with the remaining melt to form the
double-chain structure ofdouble-chain structure of amphiboleamphibole..
– Amphibole reacts to form the sheet structure ofAmphibole reacts to form the sheet structure of biotitebiotite..
– Discontinuous – each step forms a different silicateDiscontinuous – each step forms a different silicate
structure.structure.

30. Evolution of MagmasEvolution of Magmas
• Bowens Reaction Series shows thatBowens Reaction Series shows that
during crystallization, the compositionduring crystallization, the composition
of the liquid portion of the magmaof the liquid portion of the magma
continually changes.continually changes.
– Composition changes due to removal ofComposition changes due to removal of
elements by earlier-forming minerals.elements by earlier-forming minerals.
– Minerals remain in contact with theMinerals remain in contact with the
remaining melt and will chemically reactremaining melt and will chemically react
and evolve into the next mineral in theand evolve into the next mineral in the
sequence.sequence.

31. • TheThe silicasilica componentcomponent
of the melt becomesof the melt becomes
enrichedenriched asas
crystallizationcrystallization
proceeds.proceeds.
• Forms increasinglyForms increasingly
complex silicatecomplex silicate
structures.structures.

32. Evolution of MagmasEvolution of Magmas
• Bowen’s Reaction SeriesBowen’s Reaction Series
Continuous Reaction Series:Continuous Reaction Series:
– The right branch indicates Ca-The right branch indicates Ca-
rich plagioclase feldspar reactsrich plagioclase feldspar reacts
with Na ions in the melt towith Na ions in the melt to
become progressively more Na-become progressively more Na-
rich.rich.
– Na ions diffuse into feldsparNa ions diffuse into feldspar
crystals and replace Ca ions incrystals and replace Ca ions in
the crystal lattice.the crystal lattice.
– Continuous – no steps, sameContinuous – no steps, same
silicate structure, same family ofsilicate structure, same family of
minerals –minerals – solid solution seriessolid solution series
– Rapid cooling prohibits completeRapid cooling prohibits complete
replacement producing zonedreplacement producing zoned
crystals with Ca-rich cores andcrystals with Ca-rich cores and
Na-rich rims.Na-rich rims.
– ““Mini”-continuous reactionMini”-continuous reaction
series occurs within each step ofseries occurs within each step of
the discontinuous series – olivine,the discontinuous series – olivine,
pyroxene, amphibole.pyroxene, amphibole.

33. • Magmatic Differentiation byMagmatic Differentiation by
Crystal Settling (FractionalCrystal Settling (Fractional
Crystallization)Crystallization)
– More dense, early-formed crystalsMore dense, early-formed crystals
sink toward the bottom of the magmasink toward the bottom of the magma
chamber.chamber.
– Separation of a melt from earlierSeparation of a melt from earlier
formed crystalsformed crystals halts the chemicalhalts the chemical
reaction process along BRSreaction process along BRS..
– Produces one or more stages ofProduces one or more stages of
crystallization.crystallization.
– Forms of one or moreForms of one or more secondarysecondary
magmasmagmas from a singlefrom a single parentalparental
magmamagma..
Processes ResponsibleProcesses Responsible
for Deviations from BRSfor Deviations from BRS
BRS is highly idealized, assumes magmaBRS is highly idealized, assumes magma
cools slowly in an unchanging environmentcools slowly in an unchanging environment

34. Fractional CrystallizationFractional Crystallization
Insert Fractional
Crystallization
Animation #33

35. • AssimilationAssimilation
– Changing a magma’s composition by the incorporation ofChanging a magma’s composition by the incorporation of
foreign matterforeign matter into a magma (into a magma (xenolithsxenoliths).).
Processes Responsible forProcesses Responsible for
Deviations from BRSDeviations from BRS
– In near surfaceIn near surface
environments, the force ofenvironments, the force of
injecting magma mayinjecting magma may
fracture surrounding brittlefracture surrounding brittle
rocksrocks ((host rockhost rock). Dislodged). Dislodged
blocks become incorporatedblocks become incorporated
into the magma.into the magma.
– In deeper environments,In deeper environments,
magma may be hot enoughmagma may be hot enough
toto melt and assimilatemelt and assimilate hosthost
rock near its meltingrock near its melting
temperature.temperature.

36. Processes Responsible forProcesses Responsible for
Deviations from BRSDeviations from BRS
– Two chemicallyTwo chemically
distinct magmas maydistinct magmas may
produce anproduce an
intermediateintermediate
compositioncomposition quitequite
different from eitherdifferent from either
original magma.original magma.
– MixingMixing aided byaided by
convective flowconvective flow in thein the
magma chamber.magma chamber.
• Magma MixingMagma Mixing
– Involves two bodies of magma intrudingInvolves two bodies of magma intruding
one another.one another.

37. Magma compositions willMagma compositions will
vary depending on thevary depending on the
source materialsource material that isthat is
melted and themelted and the eventsevents
(history)(history) that affectthat affect
crystallization alongcrystallization along
BRS.BRS.

38. Origin of Magma CompositionsOrigin of Magma Compositions
1.1. 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 MagmasSilica-Rich and Intermediate Magmas
(Andesitic and Rhyolitic)(Andesitic and Rhyolitic)
– Secondary/Evolved to Highly EvolvedSecondary/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

39. • Most originate from directMost originate from direct
partial meltingpartial melting
(incomplete melting) of(incomplete melting) of
ultramafic rockultramafic rock
(peridotite) in the(peridotite) in the mantlemantle..
• Not evolved.Not evolved.
• Primary/PrimitivePrimary/Primitive
Magmas –Magmas – earliest stagesearliest stages
along BRS – olivine,along BRS – olivine,
pyroxene, Ca-plagioclase.pyroxene, Ca-plagioclase.
• Basaltic magmas form atBasaltic magmas form at
mid-ocean ridges and riftmid-ocean ridges and rift
zoneszones byby decompressiondecompression
meltingmelting or ator at subductionsubduction
zoneszones byby wet meltingwet melting..
Origin of Basaltic MagmasOrigin of Basaltic Magmas

40. • Mantle-derived basalticMantle-derived basaltic
magmas migrates upward,magmas migrates upward,
melts, andmelts, and assimilates moreassimilates more
silica-rich rockssilica-rich rocks in the crustin the crust
generating magma ofgenerating magma of
andesitic composition.andesitic composition.
• Andesitic magma may alsoAndesitic magma may also
evolve byevolve by magmaticmagmatic
differentiationdifferentiation (crystal(crystal
settling).settling).
• Evolved Magmas.Evolved Magmas.
• Secondary Magmas –Secondary Magmas –
intermediate stages alongintermediate stages along
BRS – pyroxene,BRS – pyroxene,
amphibole, biotite,amphibole, biotite,
plagioclase, and minorplagioclase, and minor
quartz.quartz.
Origin of Andesitic MagmasOrigin of Andesitic Magmas

41. • Most likely form as theMost likely form as the endend
product of crystallization ofproduct of crystallization of
andesitic magmaandesitic magma..
• Or product ofOr product of partial meltingpartial melting
of silica-rich continentalof 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 lesserplagioclase, biotite, and lesser
amphibole.amphibole.
• Higher in silica and thereforeHigher in silica and therefore
more viscous than othermore viscous than other
magmas.magmas.
• Because of their viscosity, theyBecause of their viscosity, they
lose their mobilitylose their mobility beforebefore
reaching the surface.reaching the surface.
• Tend to produceTend to produce largelarge
plutonic structuresplutonic structures..
Origin of Granitic MagmasOrigin of Granitic Magmas

42. Classification ofClassification of
Igneous RocksIgneous Rocks

43. Classification of Igneous RocksClassification of Igneous Rocks
• Igneous rocks are typically classified byIgneous rocks are typically classified by
– TextureTexture
– Mineral CompositionMineral Composition
• TheThe environmentenvironment during crystallizationduring crystallization
can be roughly inferred fromcan be roughly inferred from texturetexture..
• TheThe source and historysource and history can be roughlycan be roughly
inferred from theinferred from the mineral compositionmineral composition..

44. Igneous TexturesIgneous Textures

45. Igneous TexturesIgneous Textures
• TextureTexture in igneous rocks isin igneous rocks is
determined by thedetermined by the
– sizesize,,
– shapeshape, and, and
– arrangementarrangement of mineral grains.of mineral grains.

46. Igneous TexturesIgneous Textures
• Factors Affecting Crystal SizeFactors Affecting Crystal Size
1.1. Rate of cooling – TimeRate of cooling – Time
2.2. Amount ofAmount of silicasilica (SiO(SiO22) present) present
3.3. Amount ofAmount of dissolved gasesdissolved gases
4.4. FluidsFluids (water and other volatiles)(water and other volatiles)
presentpresent

47. Rate of CoolingRate of Cooling
• The rate of cooling is determined by theThe rate of cooling is determined by the
environment:environment:
– ExtrusiveExtrusive:: Rocks formed fromRocks formed from lavalava at theat the
surface are classified assurface are classified as extrusiveextrusive oror volcanicvolcanic
rocks.rocks.
– IntrusiveIntrusive:: Rocks formed fromRocks formed from magmamagma thatthat
crystallizes at depth are termedcrystallizes at depth are termed intrusiveintrusive, or, or
plutonic rocks.plutonic rocks.

48. 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

49. Rapid Rate of CoolingRapid Rate of Cooling
Aphanitic (Fine-Grained) TextureAphanitic (Fine-Grained) Texture
• RapidRapid rate of cooling ofrate of cooling of
lava or magma.lava or magma.
• Causes ions to quicklyCauses ions to quickly
lose mobility and readilylose mobility and readily
combine with existingcombine with existing
crystals.crystals.
• Promotes development ofPromotes development of
numerous embryonicnumerous embryonic
nucleinuclei that all competethat all compete
for available ions.for available ions.
• FormsForms many microscopicmany microscopic
crystalscrystals..
• Commonly characterizedCommonly characterized
by color –by color – lightlight,,
intermediateintermediate,, or darkor dark..

50. Vesicular Texture –Vesicular Texture –
Rapid Cooling W/ VolatilesRapid Cooling W/ Volatiles
Vesicular
Basalt
• Aphaniutic RocksAphaniutic Rocks
may containmay contain
vesiclesvesicles (voids from(voids from
gas bubbles).gas bubbles).
• Form in the upperForm in the upper
zone of the lavazone of the lava
flow where…flow where…
• rapid coolingrapid cooling
“freezes” the lava“freezes” the lava
preserving openingpreserving opening
produced byproduced by
expanding gasexpanding gas
bubbles.bubbles.

51. • Very Fast RateVery Fast Rate ––
molten material ismolten material is
quenched suchquenched such
that ions arethat ions are
unable to arrangeunable to arrange
into a crystallineinto a crystalline
network.network.
• FormsForms glassglass rock –rock –
obsidian, scoria, orobsidian, scoria, or
pumicepumice..
Very Fast Rate of CoolingVery Fast Rate of Cooling
Glassy TextureGlassy Texture
Obsidian

52. Pyroclastic TexturesPyroclastic Textures
• Various fragments ejected during a violentVarious fragments ejected during a violent
volcanic eruption:volcanic eruption:
– Very fine-grainedVery fine-grained ashash
– CrystalsCrystals,, glass fragmentsglass fragments,, pumicepumice
– BombsBombs –– streamlined molten blobsstreamlined molten blobs thatthat
solidified in air.solidified in air.
– BlocksBlocks –– large angular fragmentslarge angular fragments torn from thetorn from the
walls of the vent.walls of the vent.
• Textures often appear to more similar toTextures often appear to more similar to
sedimentary rocks (sedimentary rocks (clasticclastic).).

53. 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.

54. • Slow RateSlow Rate of coolingof cooling
magma.magma.
• Permits movement of ionsPermits movement of ions
until they join an existinguntil they join an existing
crystalline structure.crystalline structure.
• Promotes the growth ofPromotes the growth of
fewer but larger crystalsfewer but larger crystals..
• Mass of intergrownMass of intergrown
crystals.crystals.
• Large, visible crystals canLarge, visible crystals can
be identified without abe identified without a
microscope.microscope.
Slow Rate of CoolingSlow Rate of Cooling
Phaneritic (Coarse-Phaneritic (Coarse-
Grained) TextureGrained) Texture

55. • Exceptionally coarse-Exceptionally coarse-
grainedgrained igneousigneous
rocks.rocks.
• Large crystal sizeLarge crystal size
generated by slowgenerated by slow
cooling rates andcooling rates and
dissolved fluids.dissolved fluids.
• Crystals are allCrystals are all largerlarger
than 1 cm inthan 1 cm in
diameterdiameter..
• Crystals can be asCrystals can be as
large as 1 meter orlarge as 1 meter or
more.more.
Very Slow Rate of Cooling w/ VolatilesVery Slow Rate of Cooling w/ Volatiles
Pegmatitic TexturePegmatitic Texture

56. • Pegmatites form inPegmatites form in late stages of crystallization oflate stages of crystallization of
granitic magmasgranitic magmas (highly evolved).(highly evolved).
• Water and volatilesWater and volatiles are present in unusually largeare 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-richIon 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/ VolatilesVery Slow Rate of Cooling w/ Volatiles
Pegmatitic TexturePegmatitic Texture

57. • Minerals form byMinerals form by differentdifferent
cooling ratescooling rates..
• Large crystals, calledLarge crystals, called
phenocrystsphenocrysts, are embedded in a, are embedded in a
matrix of smaller crystals, calledmatrix of smaller crystals, called
thethe groundmassgroundmass..
• A rock with porhyritic texture isA rock with porhyritic texture is
called acalled a porphyryporphyry..
• This type of rock typically has aThis type of rock typically has a
2-phase cooling history that2-phase cooling history that
affected the rate ofaffected the rate of
crystallization.crystallization.
Two-Phased CoolingTwo-Phased Cooling
Porphyrytic TexturePorphyrytic Texture

58. The texture of a particularThe texture of a particular
igneous rock is ultimatelyigneous rock is ultimately
determined by thedetermined by the environmentenvironment
from which it crystallized.from which it crystallized.

59. Mineral CompositionsMineral Compositions
of Igneous Rocksof Igneous Rocks

60. Bowen’s Reaction Series explains the order of silicateBowen’s Reaction Series explains the order of silicate
mineral crystallization and the how this governs themineral crystallization and the how this governs the
evolution of magma and igneous rock compositionsevolution of magma and igneous rock compositions

62. Igneous CompositionsIgneous Compositions
• UltramaficUltramafic Composition:Composition:
– Rare composition that is high in magnesium andRare composition that is high in magnesium and
iron.iron.
• Low silica content –Low silica content –
approximatelyapproximately 45%45%..
• Main constituent ofMain constituent of
thethe upper mantleupper mantle..

63. Ultramafic RocksUltramafic Rocks:: PeridotitePeridotite
• Ultramafic igneous rocksUltramafic igneous rocks
are composed entirely ofare composed entirely of
dark (ordark (or ferromagnesianferromagnesian))
silicate minerals:silicate minerals:
– OlivineOlivine
– Pyroxene (Augite)Pyroxene (Augite)
– Minor Ca-PlagioclaseMinor Ca-Plagioclase

64. Igneous CompositionsIgneous Compositions
• BasalticBasaltic (or(or MaficMafic) Composition:) Composition:
– MaficMafic ((mamagnesium andgnesium and feferrum, for iron)rrum, for iron)
– Silica deficient –Silica deficient –
approximatelyapproximately 50 percent50 percent..
– MoreMore densedense than graniticthan granitic
rocks.rocks.
– Comprise theComprise the ocean floorocean floor
as well as manyas well as many volcanicvolcanic
islandsislands..
– Also found on continents.Also found on continents.

65. Mafic RocksMafic Rocks:: GabbroGabbro,, BasaltBasalt
• Mafic igneous rocks areMafic igneous rocks are
composed primarily ofcomposed primarily of
dark (ordark (or ferromagnesianferromagnesian))
silicate minerals:silicate minerals:
– OlivineOlivine
– Pyroxene (Augite)Pyroxene (Augite)
– Amphibole (Hornblende)Amphibole (Hornblende)
– Ca-PlagioclaseCa-Plagioclase

66. Igneous CompositionsIgneous Compositions
• AndesiticAndesitic (or(or IntermediateIntermediate) 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 25Contain at least 25
percent dark silicatepercent dark silicate
minerals.minerals.
• Associated withAssociated with
explosive volcanicexplosive volcanic
activityactivity nearnear
continental margins.continental margins.

67. Intermediate RocksIntermediate Rocks:: DioriteDiorite,, AndesiteAndesite
• Intermediate igneous rocks are composed of darkIntermediate 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

68. Igneous CompositionsIgneous Compositions
• GraniticGranitic or (or (FelsicFelsic) Composition:) Composition:
– FelsicFelsic ((felfeldspar anddspar and
sisilica).lica).
– Contains highContains high
amounts of silicaamounts of silica
(SiO(SiO22) –) – 65 percent65 percent
or more.or more.
– Major constituentsMajor constituents
ofof continental crustcontinental crust..

69. Felsic RocksFelsic Rocks:: GraniteGranite,, RhyoliteRhyolite
• Felsic igneous rocks are composed of primarilyFelsic igneous rocks are composed of primarily
light with fewer dark silicate minerals:light with fewer dark silicate minerals:
– QuartzQuartz
– Muscovite and Biotite MicasMuscovite and Biotite Micas
– Plagioclase and Orthoclase FeldsparsPlagioclase and Orthoclase Feldspars
– Amphibole (Hornblende)Amphibole (Hornblende)

70. Igneous Rock ClassificationIgneous Rock Classification
FelsicFelsic IntermediateIntermediate MaficMafic UltramaficUltramafic
FF
ii
nn
ee
RhyoliteRhyolite AndesiteAndesite BasaltBasalt
CC
oo
aa
rr
ss
ee
GraniteGranite DioriteDiorite GabbroGabbro PeridotitePeridotite

71. The mineral composition of aThe mineral composition of a
particular igneous rock isparticular igneous rock is
ultimately determined by theultimately determined by the
composition of the magmacomposition of the magma
(source material and history)(source material and history)
from which it crystallized.from which it crystallized.

72. Glossary ofGlossary of
Igneous RocksIgneous Rocks

73. Naming Igneous RocksNaming Igneous Rocks

74. Naming Igneous RocksNaming Igneous Rocks

75. Igneous Rock ClassificationIgneous Rock Classification
FelsicFelsic IntermediateIntermediate MaficMafic UltramaficUltramafic
FF
ii
nn
ee
RhyoliteRhyolite AndesiteAndesite BasaltBasalt
CC
oo
aa
rr
ss
ee
GraniteGranite DioriteDiorite GabbroGabbro PeridotitePeridotite

76. Igneous Rock ClassificationIgneous Rock Classification

77. GraniteGranite
• IntrusiveIntrusive andand phaneriticphaneritic..
• Over 25 percent quartz, about 65Over 25 percent quartz, about 65
percent or more feldspar.percent or more feldspar.
• Other minor silicates muscovite,Other minor silicates muscovite,
biotite, amphibole comprise lessbiotite, amphibole comprise less
than 10 percent.than 10 percent.
• May exhibit a porphyritic texture.May exhibit a porphyritic texture.
• Very abundant as it is oftenVery abundant as it is often
associated with mountainassociated with mountain
building.building.
• The term granite covers a wideThe term granite covers a wide
range of mineral compositions.range of mineral compositions.

78. RhyoliteRhyolite
• ExtrusiveExtrusive equivalent ofequivalent of
granite.granite.
• AphaniticAphanitic texture.texture.
• May contain glassMay contain glass
fragments and vesicles.fragments and vesicles.
• May containMay contain phenocrystsphenocrysts
of orthoclase, mica, andof orthoclase, mica, and
quartz.quartz.
• TypicallyTypically red to reddish-red to reddish-
purplepurple to gray in color.to gray in color.
• Less common and lessLess common and less
voluminous than granite.voluminous than granite.

79. Other GraniticOther Granitic
(Felsic) Rocks(Felsic) Rocks
• ObsidianObsidian
– Extrusive with glassy textureExtrusive with glassy texture
– High silica contentHigh silica content
– Dark colored due to presence ofDark colored due to presence of
metallic ionsmetallic ions
• PumicePumice
– Extrusive with glassy textureExtrusive with glassy texture
– Vesicular texture (more voids thanVesicular texture (more voids than
rock)rock)
– Frothy appearance with numerousFrothy appearance with numerous
voidsvoids
– Void shape is typically elongatedVoid shape is typically elongated
• ScoriaScoria
– Extrusive with glassy textureExtrusive with glassy texture
– Vesicular texture (more rock thanVesicular texture (more rock than
voids)voids)
– Void shape is typically moreVoid shape is typically more
roundedrounded

80. AndesiteAndesite
• Volcanic origin –Volcanic origin –
extrusiveextrusive..
• AphaniticAphanitic texture.texture.
• Often resemblesOften resembles
rhyolite.rhyolite.
• Typically medium-Typically medium-
gray.gray.
• May containMay contain
phenocrystsphenocrysts ofof
plagioclase andplagioclase and
hornblende.hornblende.

81. DioriteDiorite
• PlutonicPlutonic equivalent ofequivalent of
andesite.andesite.
• Coarse-grained orCoarse-grained or
phaneriticphaneritic..
• Intrusive.Intrusive.
• Composed mainly of Na-Composed mainly of Na-
rich plagioclaserich plagioclase
(intermediate) feldspar(intermediate) feldspar
and amphibole withand amphibole with
lesser amounts of biotite.lesser amounts of biotite.
• Salt and pepperSalt and pepper
appearance.appearance.

82. BasaltBasalt
• Volcanic origin –Volcanic origin – extrusiveextrusive..
• AphaniticAphanitic texture.texture.
• Composed mainly of pyroxeneComposed mainly of pyroxene
and calcium-rich plagioclaseand calcium-rich plagioclase
feldspar with lesser amounts offeldspar with lesser amounts of
olivine and amphibole.olivine and amphibole.
• Very dark green to black inVery dark green to black in
color.color.
• May containMay contain phenocrystsphenocrysts ofof
plagioclase and olivine.plagioclase and olivine.
• May exhibit vesicular texture.May exhibit vesicular texture.
• Most common extrusive igneousMost common extrusive igneous
rock.rock.

83. GabbroGabbro
• IntrusiveIntrusive equivalentequivalent
of basalt.of basalt.
• PhaneriticPhaneritic texturetexture
consisting ofconsisting of
pyroxene andpyroxene and
calcium-richcalcium-rich
plagioclase.plagioclase.
• Makes up aMakes up a
significant percentagesignificant percentage
of the oceanic crust.of the oceanic crust.

84. 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 angularLarge 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.

85. Interlayered Tuff and ObsidianInterlayered Tuff and Obsidian

86. 1996 Eruption of Mount Ruapehu, New Zealand –1996 Eruption of Mount Ruapehu, New Zealand –
Intermediate to Felsic composition magmasIntermediate to Felsic composition magmas