Igneous Rock Classification
Igneous Rock Classification
I. Igneous Textures—refers
to the size and shape of the
minerals in the rock
A. Phaneritic: contain crystals
large enough to see with
unaided eye.
When magma cools slowly over
hundreds to thousands of years
the minerals crystallize slowly and
have ample time to grow large.
Intrusive Rocks (Plutonic rocks)
crystallized underground; only
place to cool slowly enough to
form large crystals
B. Aphanitic: crystals are
so small they cannot be
seen by the naked eye—or
barely seen.
Magma cools more quickly
Extrusive Rocks (Volcanic
Rocks): crystallized from lava
that flowed out, or extruded, onto
Earth’s surface
C. Porphyritic: rocks with
large crystals surrounded
by regions with much
smaller crystals or areas
with undistinguishable
grains.
Results from initial slow cooling
(large crystals form) followed
abruptly by rapid cooling
(smaller surrounding crystals
form)
Classified as extrusive
Phenocrysts
Groundmass
D. Glassy: very rapid cooling
(explosive volcano or flow into
water), crystals don’t have
time to form
Ions frozen in place randomly:
no internal structure.
1.Pumice: bubbling, highly
gaseous, silica-rich lava cools
very quickly
2. Obsidian: very silica-rich
lavas, containing little gas,
cool very quickly—they lack
internal structure, so they
fracture rather than have
cleavage
Felsic Rocks
felsic from feldspar and silica
II. Igneous Compositions
Classified based on their silica
content (silicon and oxygen)
A. Felsic Rocks
Felsic from feldspar and silica
1. Composition
silica: >65%
Al, K, Na
some amphibole, mica, Naplag, K-spar, quartz
2. Temperature when crystals
start to form = 600-800C
(1100-1475F).
Felsic Rocks
felsic from feldspar and silica
3. Cools on surface = extrusive
(volcanic)
aphanitic = rhyolite, pumice &
obsidian
Cools underground = intrusive
(plutonic) = phaneritic =
granite.
4. Physical properties
 Light color - pink and white with
very few black specks
 Least dense
 Magma viscosity = high
Note: Viscosity = resistance to flow
(opposite of fluidity)
Increases with decreasing temp—
maple syrup example
Increases with increasing silica
B. Intermediate Rocks
B. Intermediate Rocks
1. Composition
silica: 55-65%
+Al, Ca, Na, Fe, Mg
Pyroxene, amphibole, biotite, Ca& Na-plag, sometimes quartz
2. Temperature when crystals
start to form = 800-1000C (14751830F)
3. Extrusive (volcanic) aphanitic
= andesite (2nd most abundant
volc rock)
Intrusive (plutonic) =
phaneritic = diorite
B. Intermediate Rocks
B. Intermediate Rocks
3. Extrusive (volcanic)
aphanitic = andesite (2nd most
abundant volc rock)
Intrusive (plutonic) phaneritic
= diorite
Viscosity = resistance to flow
(opposite of fluidity)
Increases with decreasing temp.,
maple syrup example
Viscosity increases with higher silica
content
4. Physical properties
lighter in color than mafic
rocks—“salt and pepper”
medium density
magma viscosity = medium
C. Mafic Rocks
C. Mafic Rocks: most
abundant rocks of the
Earth’s crust
mafic = derived from
magnesium and ferrum (Latin
for iron)
1. Composition
Silica: 45-55%
Al, Ca, Fe, Mg
Olivine, pyroxene, Caplagioclase
2. Temperature
Crystals start to form = 10001200C (1830-2200F)
C. Mafic Rocks
C. Mafic Rocks: most abundant
rocks of the Earth’s crust
mafic = derived from
magnesium and
ferrum (Latin for iron)
3. Crystallize: surface = extrusive
(volcanic) = aphanitic = basalt
Intrusive (plutonic) =
phaneritic = gabbro
4. Physical properties
Dark in color
Relatively dense
Magma Viscosity = low
Back to Viscosity:
Magmas with the most
silica are most viscous
- hardest time flowing
And magmas with the
least silica are the least
viscous - flow easiest
Resistance to flow (honey vs. water)
Factors that affect viscosity: Composition --->
hi silica is more viscous
Viscosity = resistance
to flow
D. Ultramafic Rocks
(The Mantle)
1. Composition
Dominated by iron &
magnesium silicate minerals
Plus olivine and pyroxene
very little silica <40% Mg, Fe,
Al, Ca
2. Temperature: solid at
temperatures = >1200C
(>2200F)
D. Ultramafic Rocks (The
Mantle)
3. Peridotite: contains 60-100%
olivine - green rock
4. Properties
Usually green in color b/c of
olivine
Hi density
Dominated by iron &
magnesium silicate minerals
Rock Name Color
Composition
Minerals
Temp
Rhyolite
~70%wt SiO2
quartz
~800
low in Mg, Fe
feldspar
Light
(granite)
Occurrence
C
continents
amphibole
mica
Andesite
Salt-pepper
~60%wt SiO2
(diorite)
quartz
~1000
C
continent-
feldspar
ocean
amphibole
boundary
pyroxene
Basalt
(gabbro)
dark
~52%wt SiO2
feldspar
~1200
C
ocean-
hi Mg, Fe
amphibole
ocean
pyroxene
boundary
Same
composition, but
different texture.
Rates of cooling!
Crystallization of
melt forming
minerals.