lynn s. fichter department of geology/environmental science james madison university fall, 2005
TRANSCRIPT
Lynn S. FichterDepartment of Geology/Environmental Science
James Madison UniversityFall, 2005
Tem
pera
ture
Sp
ecifi
c G
ravit
y
Silic
a C
on
ten
t
Poly
meri
zati
on
Vis
cosit
y
Dis
solv
ed
gasses
Colo
r
Igneous RocksThe Two Most Abundant Rocks
on Earth
Light colored and coarse
Dark colored and fine
Granite
Basalt
Two Ideal MagmasMafic Magma
Felsic to IntermediateMagma
Basalt GraniteExample
ColorPosition inReactionSeries
MineralComposit.
Grain SizeCoolingHistory
Location
Spec. Grav.
LavaChar.
Dark (gray/black) Light (white/pink)
Top of ReactionSeries
Bottom of ReactionSeries
Pyroxene,Ca Plagioclase;Olivine (trace)
Na Plagioclase/ Orthoclase
Quartz; trace mafics
Fine grained, rapid cooling at Earth’s
surface
Large grained, slow cooling deep under
ground.
Ocean Basins Continents
High Low
Very hot, low viscosity
Cool, sticky
Igneous Rock Textures
PhaneriticCoarse grained
AphaniticFine grained
Glassy
Cellular
Aphanitic
Phaneritic
Porphoritic
Phenocryst
1000’s of years
Hoursto days
PHANER-
ITIC
GLASSYto
CELLULAR
APHAN-ITIC or
Porphyritic
Days to months/years
BASALT
GRANITE
GABBRO
RHYO-LITE
DIORITE
ANDE-SITE
SCORIA
OBSIDIANPUMICE
Igneous Rock ClassificationBy Texture and Color . . .
Is convenient . . .
But, it is not a very good classification
Primarily because the color of a rock by itself has no meaning.
There is no theory of rock color, nothing in chemistry or physics that explains why color should change systematically from the top to bottom of Bowen’s Reaction Series
. . . And a place to start . . .
Igneous Rock ClassificationModal Classification
By Texture and percent mineral contentBasalt
Gabbro
Diorite
Diorite
Plagiogranite
Alkaligranite
Olivine
Pyroxene
Amphibole
Biotite
Calcic-Sodic
Orthoclase
Muscovite
Quartz
CalcicPlagioclase
SodicPlagioclase
Mafi
cFe
lsic
In
term
edia
teLig
ht Co
lorInt
ermedi
ate C
olor
Dark
Color
Bowen's Reaction Series And Igneous Rock Classification
Dull, Black
Shiny, Black
Olive Green
Dark Gray
White
Pink
Clear, glassy, grayish
Color does not work for obsidian, which is felsic in composition but light in color
BASALT
PLAGIO-
GRANITE
DIORITE
(porphyry)(porphyr
y)
GABBRO
ANDE-SITE
SCORIAPUMICE
OBSIDIAN
PERIDOTITE
DUNITE
ALKALI-GRANIT
E
SYENITE
RHYOLITE(porphyry)
18%
18+7=25%
25+2=27%
27+52=79%
79+18=97%97+3=100%
91%
9%
Ultramafic means the rock has no feldspar
Feldspar Dominated Rocks
FeldsparAbsentRocks
Igneous Rock ClassificationIn Bowen’s Reaction Series
Identifying Igneous Rocks –
Practically
Fels
ic
Fels
ic
Inte
rmed
.M
afic
Inte
rmed
.M
afic
Ultra
mafi
c
Ultra
mafi
c
Mafi
c
QUARTZ> 20%
Light-colored rocks
MAFICS<10%
MAFICS10-25%
Whitish,light gray,
Salt & Pepperwhite>black
GRANODIORITE
PLAGIOGRANITE
FELD-SPARS
80-100%
Pinkish,Reddish ALKALIGRANITE
PLAGIOCLASE
35-100%
ORTHOCLASE65-100%
Plag. 0-35%Mafics <10%
Quartz 0-5%pinkish SYENITE
Quartz5-20% QTZ. SYENITE
Quartz 0-5%white-ish MONZONITE
Quartz5-20%
QTZ.MONZONITE
Plagio. >90%mid to
dark grayPyroxene
<10% ANORTHOSITE
MAFICS
PLAGIO-CLASE
GABBRO
DIORITEAMPHIBOLE &
BIOTITELight-coloredPLAGIOCLASE
PYROXENE;rare Olivine
Dark gray orbrown-gray
MAFICS90-100%Dark-colored rocks
DUNITEBlack Chromite
crystals typ.Pale-green
glassy grains
PERIDOTITE
PYROXENITEPYROXENE60-100%
OLIVINE40 - 90 %
Dark green, orbrown; black
Light to darkgreen
OLIVINE> 90 %
OLIVINE0-40 %
PYROXENE10-60%
(TONOLITE)PLAGIOCLASE>ORTHOCLASE
PLAGIOCLASE ORTHOCLASE
Dark-GrayPLAGIOCLASE
Salt & Pepperblack white
Ortho 10-35%Mafics <10%
MAFICS/PLAGIOCLASE 50/50%
MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS
PLAGIOCLASE>ORTHOCLASE
ORTHOCLASE>PLAGIOCLASE
pure Ca plag is whiteand is sometimes seen
ModalClassification
Of IgneousRocks
Based on percent mineral
abundance
Modal Classification Of Igneous RocksBased on percent mineral
abundance
Alkaligranite:>20% quartz,Orthoclasse dominates; plagioclase possible;Mafics (biotite/amphibole) <5%
Alkaligranite:>20% quartz,Na plagioclase dominates;Mafics (biotite/amphibole) <5%
Quartz and Na plagioclase often hard to tell apart
Plagiogranite:>20% quartz,Na plagioclase dominates; orthoclase possible but may be absent;Mafics (biotite/amphibole) <5%
Diorite:0 - 5% quartz,Na plagioclase and amphibole in about equal amounts
Diorite:0% quartz,Na plagioclase and amphibole in about equal amounts; this is a darker version but light and dark colors are still about equal.
Gabbro:No quartz,Ca/Na to Ca plagioclase and pyroxene in about equal amounts
Gabbro:No quartz,Ca/Na to Ca plagioclase and pyroxene in about equal amounts
Rhyolite:Minerals cannot be identified in hand specimen
Andesite porphyryMinerals cannot be identified in fine grained groundmass, but phenocrysts of amphibole are visible
Rhyolite porphyryMinerals cannot be identified in fine grained groundmass, but phenocrysts of orthoclase are visible
BasaltMinerals cannot be identified in hand specimen, but come from the top of Bowen’s Reaction Series
Basalt porphyryPhenocrysts are a Ca rich plagioclase
Granodiorite:Half way between a granite (>20% quartz) and a diorite (no quartz). Quartz here is over 20% but mafics and plagioclase are about equal, a diorite feature.
Fels
ic
Fels
ic
Inte
rmed
.M
afic
Inte
rmed
.M
afic
Ultra
mafi
c
Ultra
mafi
c
Mafi
c
QUARTZ> 20%
Light-colored rocks
MAFICS<10%
MAFICS10-25%
Whitish,light gray,
Salt & Pepperwhite>black
GRANODIORITE
PLAGIOGRANITE
FELD-SPARS
80-100%
Pinkish,Reddish ALKALIGRANITE
PLAGIOCLASE
35-100%
ORTHOCLASE65-100%
Plag. 0-35%Mafics <10%
Quartz 0-5%pinkish SYENITE
Quartz5-20% QTZ. SYENITE
Quartz 0-5%white-ish MONZONITE
Quartz5-20%
QTZ.MONZONITE
Plagio. >90%mid to
dark grayPyroxene
<10% ANORTHOSITE
MAFICS
PLAGIO-CLASE
GABBRO
DIORITEAMPHIBOLE &
BIOTITELight-coloredPLAGIOCLASE
PYROXENE;rare Olivine
Dark gray orbrown-gray
MAFICS90-100%Dark-colored rocks
DUNITEBlack Chromite
crystals typ.Pale-green
glassy grains
PERIDOTITE
PYROXENITEPYROXENE60-100%
OLIVINE40 - 90 %
Dark green, orbrown; black
Light to darkgreen
OLIVINE> 90 %
OLIVINE0-40 %
PYROXENE10-60%
(TONOLITE)PLAGIOCLASE>ORTHOCLASE
PLAGIOCLASE ORTHOCLASE
Dark-GrayPLAGIOCLASE
Salt & Pepperblack white
Ortho 10-35%Mafics <10%
MAFICS/PLAGIOCLASE 50/50%
MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS
PLAGIOCLASE>ORTHOCLASE
ORTHOCLASE>PLAGIOCLASE
pure Ca plag is whiteand is sometimes seen
ModalClassification
Of IgneousRocks
Based on percent mineral
abundance
Obsidian: (volcanic glass)No minerals are present since
this cooled too fast for them to form.
Syenite:Orthoclase rich, mafic poor, quartz absent or only a trace.
Dunite:Olivine >90%. As a mineral this would be identified as an olivine.