9 igneous rocks

Lynn S. Fichter
Department of Geology/Environmental Science
J ames Madison University
Fall, 2005



Co lo r

Dis s o lv e d g as s e s

Vis c o s ity

Po ly m e riz atio n

S ilic a Co nte nt

S p e c ific Grav ity

Te m p e rature


Ig ne o us Ro c ks
The Two Mo s t Abundant Ro c ks o n
Earth

Bas alt
Dark colored and fine

Light colored and coarse

Granite


Two Ideal Magmas
Mafic Mag ma

Example
Color
Position in
Reaction
Series

Basalt
Granite
Dark (gray/black) Light (white/pink)
Top of Reaction
Series

Pyroxene,
Ca Plagioclase;
Olivine (trace)
Grain Size Fine grained, rapid
Cooling
cooling at Earth’s
History
surface
Location
Ocean Basins
Spec. Grav.
High

Mineral
Composit.

Lava
Char.

Fe ls ic to Inte rme diate
Mag ma

Very hot, low
viscosity

Bottom of Reaction
Series
Na Plagioclase/
Orthoclase
Quartz; trace mafics
Large grained, slow
cooling deep under
ground.
Continents
Low
Cool, sticky



Igneous Rock Textures

Phaneritic
Coarse grained

Aphanitic
Fine grained

Glassy
Cellular


Porphoritic

Aphanitic

Phe nocrys t

Phaneritic


1000’s of
years

Days to
months/years

PHANERITIC

APHANGLAS S Y
ITIC o r
to
Po rp hy ritic CELLULAR

GABBR BAS ALT
O

Hours
to days

S CORIA

DIORITE ANDES ITE
OBS IDIA
GRANITE RHYON
LITE PUMICE


Ig ne o us Ro c k Clas s ific atio n
By Te xture and Co lo r . . .
Is convenient . . .
. . . And a place to start . . .
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


Ig ne o us Ro c k Clas s ific atio n
Mo dal Clas s ific atio n

By Texture and percent mineral content
Bowen'sReactionSeriesAndIgneousRockClassification

Basalt

Color does not work for obsidian, which
is felsic in composition but light in color

OliveGreen

Calcic
Plagioclase

Olivine
Pyroxene
Amphibole
Sodic
Biotite Plagioclase
Pink

Diorite

White

IntermediateColor

Gabbro

Calcic-Sodic

Intermediate

Shiny, Black

Plagiogranite

Orthoclase
Clear, glassy, grayish

Muscovite

Felsic
LightColor

Dull, Black

Mafic
DarkColor

DarkGray

Quartz

Diorite

Alkaligranite


97+3=100%
79+18=97%

9%

27+52=79%
91%
25+2=27%
18+7=25%
18%

PERIDOTITE
S YENITE ALKALI- PLAGIO- DIORITE GABBRO
GRANITE GRANITE
DUNITE
Ultram afic
ANDE- BAS ALT
RHYOLITE
m e ans the rock
S ITE
(p o rp hy ry )

PUMICE

OBS IDIAN

(p o rp hy ry )

(p o rp hy ry )

S CORIA

has no fe lds par



Ig ne o us Ro c k Clas s ific atio n
In Bo we n’s Re ac tio n S e rie s

Feldspar
Absent
Rocks

Feldspar Dominated Rocks


Identifying
Igneous Rocks
– Practically


MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS

Whitish,
light gray,

PLAGIOGRANITE

Salt & Pepper
white>black

GRANODIORITE

Quartz 0-5%
pinkish

SYENITE

Quartz
5-20%

QTZ. SYENITE

Quartz 0-5%
white-ish

MONZONITE

Quartz
5-20%

QTZ.
MONZONITE

Plagio. >90%
mid to
dark gray

Pyroxene
<10%

ANORTHOSITE

Light-colored

Salt & Pepper
black white

DIORITE

Dark-Gray

Dark gray or
brown-gray

GABBRO

P LAGIOCLAS E
ORTHOCLASE

65-100%

Plag. 0-35%
Mafics <10%

FELD-

SPARS
80-100%

Ortho 10-35%
Mafics <10%

Mafic

Inte
rmed.

P LAGIOCLAS E
35-100%

Ultramafic

Bas e d o n
p e rc e nt
m ine ral
ab und anc e

O RTHOCLAS E

AFICS/
S
MAFIC
M

GIOCLASE
PLA
LA
GIOP
 50/50%
CLASE

MAFICS

90-100%
Dark-colored rocks

A MP HIB OLE &
BIOTITE
P YROXENE;
rare Olivine

PLAGIOCLASE
PLAGIOCLASE

pure Ca plag is white
and is sometimes seen

PYROXENE

O LIVINE

0-40 %

Dark green, or
brown; black

PYROXENITE

O LIVINE

PYROXENE

10-60%

Light to dark
green

P ERIDOTITE

O LIVINE

Pale-green
glassy grains

Black Chromite
crystals typ.

DUNITE

60-100%

40 - 90 %
> 90 %

Felsic

MAFICS

10-25%

(T ONOLITE)

Mafic

>ORTHOCLASE

Inte
rmed.

> 20%

Light-coloredrocks

P LAGIOCLAS E

Mafic

QUARTZ

<10%

ALKALIGRANITE

Ultramafic

MAFICS

Pinkish,
Reddish

>PLAGIOCLASE

Felsic

Mo dal
Clas s ific atio n
Of Ig ne o us
Ro c ks

O RTHOCLAS E


Mo dal Clas s ific atio n Of Ig ne o us
Bas e d o n p e rcRo
e nt c
mks
ine ral ab und anc e



Alkaligranite:
>20% quartz,
Orthoclasse dominates; plagioclase possible;
Mafics (biotite/amphibole) <5%



Alkaligranite:
>20% quartz,
Na plagioclase dominates;
Mafics (biotite/amphibole) <5%

Plagiogranite:
>20% quartz,
Na plagioclase dominates; orthoclase
possible but may be absent;
Mafics (biotite/amphibole) <5%

Quartz and Na plagioclase often hard to tell apart