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GLY 155 Introduction to Physical Geology, W. Altermann
©
Grotzinger • Jordan
Understanding Earth
Sixth Edition
Chapter 4:
IGNEOUS ROCKS
Solids from Melts
© 2011 by W. H. Freeman and Company
GLY 155 Introduction to Physical Geology, W. Altermann
©
Chapter 4:
Igneous
Rocks:
Solids from
Melts
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GLY 155 Introduction to Physical Geology, W. Altermann
©
About Igneous Rocks
• Igneous rocks form from liquid rock
(magma) in several different ways.
• Igneous processes within the Earth
produce intrusive igneous rocks.
• Igneous processes on or near Earth’s
surface produce extrusive igneous rocks.
GLY 155 Introduction to Physical Geology, W. Altermann
©
Lecture Outline
1. How do igneous rocks differ from one
another?
2. How do magmas form?
3. Magmatic differentiation
4. Forms of igneous intrusions
5. Igneous processes and plate tectonics
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GLY 155 Introduction to Physical Geology, W. Altermann
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1. How Do Igneous Rocks Differ
from One Another?
Texture – size of crystals
Coarse-grained rocks
Fine-grained rocks
Mixed texture rocks
GLY 155 Introduction to Physical Geology, W. Altermann
©
1. How Do Igneous Rocks Differ
from One Another?
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GLY 155 Introduction to Physical Geology, W. Altermann
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1. How Do Igneous Rocks Differ
from One Another?
Clues about significance of texture
Early studies of volcanic rocks
Laboratory crystallization studies
Studies of slow cooling in granite
GLY 155 Introduction to Physical Geology, W. Altermann
©
1. How Do Igneous Rocks Differ
from One Another?
Texture is related to rate of cooling.
Intrusive igneous rocks
Extrusive igneous rocks
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GLY 155 Introduction to Physical Geology, W. Altermann
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1. How Do Igneous Rocks Differ from One Another?
Pyroclasts
Volcanic ash
Extrusive rocks
Bomb
Pumice
Mafic
Basalt
Felsic
Rhyolite
Gabbro
Granite
Porphyry
Intrusive rocks
Extrusive pyroclasts
form in violent
eruptions from lava
in the air.
Extrusive igneous
rocks cool rapidly
and are finegrained.
Intrusive igneous
rocks cool slowly,
allowing large,
coarse crystals to
form.
Phenocrysts
Porphyry
Some phenocrysts
grow large, but the
remaining melt
cools faster,
forming smaller
crystals during an
eruption.
GLY 155 Introduction to Physical Geology, W. Altermann
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Chemical and Mineral
Composition of Igneous Rocks
Two basic compositional groups:
Felsic igneous rocks
Mafic igneous rocks
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GLY 155 Introduction to Physical Geology, W. Altermann
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GLY 155 Introduction to Physical Geology, W. Altermann
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Chemical and Mineral
Composition of Igneous Rocks
Four compositional groups:
Felsic igneous rocks
Intermediate igneous rocks
Mafic igneous rocks
Ultramafic igneous rocks
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GLY 155 Introduction to Physical Geology, W. Altermann
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GLY 155 Introduction to Physical Geology, W. Altermann
©
2. How Do Magmas Form?
Why do rocks melt?
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GLY 155 Introduction to Physical Geology, W. Altermann
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2. How Do Magmas Form?
What is a magma chamber?
A rising mass of magma that
pushes aside crustal rocks as
it rises through the crust.
GLY 155 Introduction to Physical Geology, W. Altermann
©
2. How Do Magmas Form?
A temperature of about 1000°C
is required for partial melting of
crustal rocks.
A depth of at least 40 km is
required for temperatures of
1000°C to occur.
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GLY 155 Introduction to Physical Geology, W. Altermann
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3. Magmatic Differentiation
A process by which rocks of
varying composition can arise
from a uniform parent magma.
The first minerals to crystallize
from a cooling magma are the
ones that are the last to melt.
GLY 155 Introduction to Physical Geology, W. Altermann
©
3. Magmatic Differentiation
Fractional crystallization is the
process by which the crystals
are formed in a cooling
magma and are segregated
from the remaining liquid.
Example: basaltic intrusion
like Palisades, New Jersey
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GLY 155 Introduction to Physical Geology, W. Altermann
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3. Magmatic Differentiation
THE PALISADES INTRUSION
An excellent example from South Africa is the Bushveld Intrusion
GLY 155 Introduction to Physical Geology, W. Altermann
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3. Magmatic Differentiation
BOWEN’S REACTION SERIES
Magma
composition
Temperature
~600°C
Orthoclase feldspar
Quartz
Biotite
mica
Pyroxene
Olivine
Sodiumrich
e
l as
ioc ar
a g sp
Pl eld
f
Amphibole
~1200°C
Felsic,
Rhyolitic
(high silica)
Muscovite mica
Simultaneous
crystallization
Calciumrich
Intermediate,
andesitic
Mafic,
basaltic
Ultramafic
(low silica)
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GLY 155 Introduction to Physical Geology, W. Altermann
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BOWEN’S REACTION SERIES
As magma temperature
decreases…
Magma
composition
Temperature
~600°C
Orthoclase feldspar
Quartz
Sodiumrich
Biotite
mica
e
l as
ioc ar
a g sp
Pl eld
f
Amphibole
Pyroxene
~1200°C
Felsic,
Rhyolitic
(high silica)
Muscovite mica
Olivine
Simultaneous
crystallization
Calciumrich
Intermediate,
andesitic
Mafic,
basaltic
Ultramafic
(low silica)
GLY 155 Introduction to Physical Geology, W. Altermann
©
BOWEN’S REACTION SERIES
As magma temperature
decreases…
Temperature
~600°C
…materials
crystallize in
an ordered
series…
Magma
composition
Orthoclase feldspar
Quartz
Biotite
mica
Pyroxene
Olivine
Sodiumrich
e
l as
ioc ar
a g sp
Pl eld
f
Amphibole
~1200°C
Felsic,
Rhyolitic
(high silica)
Muscovite mica
Simultaneous
crystallization
Calciumrich
Intermediate,
andesitic
Mafic,
basaltic
Ultramafic
(low silica)
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GLY 155 Introduction to Physical Geology, W. Altermann
©
BOWEN’S REACTION SERIES
…while plagioclase
feldspar crystallizes,
from calcium-rich to
sodium-rich form…
As magma temperature
decreases…
Temperature
~600°C
…materials
crystallize in
an ordered
series…
Magma
composition
Orthoclase feldspar
Quartz
Sodiumrich
Biotite
mica
e
l as
ioc ar
a g sp
Pl eld
f
Amphibole
Pyroxene
~1200°C
Felsic,
Rhyolitic
(high silica)
Muscovite mica
Olivine
Simultaneous
crystallization
Intermediate,
andesitic
Mafic,
basaltic
Ultramafic
(low silica)
Calciumrich
GLY 155 Introduction to Physical Geology, W. Altermann
©
BOWEN’S REACTION SERIES
…while plagioclase
feldspar crystallizes,
from calcium-rich
sodium-rich form…
As magma temperature
decreases…
Temperature
~600°C
…materials
crystallize in
an ordered
series…
Magma
composition
Orthoclase feldspar
Quartz
Biotite
mica
Pyroxene
Olivine
Sodiumrich
e
l as
ioc ar
a g sp
Pl eld
f
Amphibole
~1200°C
Felsic,
Rhyolitic
(high silica)
Muscovite mica
Simultaneous
crystallization
Calciumrich
Intermediate,
andesitic
Mafic,
basaltic
Ultramafic
(low silica)
…and the composition
of magma changes
from ultramafic to
andesitic.
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GLY 155 Introduction to Physical Geology, W. Altermann
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3. Magmatic Differentiation
GRANITE AND BASALT
Crystallizing
minerals
Magma
chamber A
Magma
chamber B
Partial melting
of country rock
Basaltic
magma
GLY 155 Introduction to Physical Geology, W. Altermann
©
Partial melting creates
a magma of a particular
composition.
Cooling causes minerals
to crystallize and settle.
Crystallizing
minerals
Magma
chamber A
Magma
chamber A
Magma
chamber B
Partial melting
of country rock
A basaltic magma
chamber breaks
through.
Mixing results
in andesitic
magma.
Magma
chamber B
Crystals may
accumulate on
the sides and
roof of the
chamber due
to turbulence.
Basaltic
magma
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GLY 155 Introduction to Physical Geology, W. Altermann
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4. Forms of Igneous Intrusions
Plutons
Discordant intrusions
Batholiths
Stocks
Dikes
Concordant intrusions
Sills
Veins
GLY 155 Introduction to Physical Geology, W. Altermann
©
4. Forms of Igneous Intrusions
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GLY 155 Introduction to Physical Geology, W. Altermann
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Lava flow
(extrusive) Ash falls and pyroclasts (effusive)
Country
rock
Volcano
Volcanic neck with
radiating dikes
Dik
e
Stock
Dikes cut
across layers
of country
rock…
Sill
Sill
Dik
e
Dike
Sill
ton
Plu
lith
tho
Ba
…but sills run
parallel to them.
Batholiths are the largest
forms of plutons, covering
at least 100 km2.
GLY 155 Introduction to Physical Geology, W. Altermann
©
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GLY 155 Introduction to Physical Geology, W. Altermann
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GLY 155 Introduction to Physical Geology, W. Altermann
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5. Igneous Processes
and Plate Tectonics
Magma factories:
Spreading centers
Subduction zones
Mantle plumes
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GLY 155 Introduction to Physical Geology, W. Altermann
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5. Igneous Processes and Plate Tectonics
GLY 155 Introduction to Physical Geology, W. Altermann
©
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GLY 155 Introduction to Physical Geology, W. Altermann
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GLY 155 Introduction to Physical Geology, W. Altermann
©
5. Igneous Processes
and Plate Tectonics
Origin of magma in magma
factories:
Decompression melting in
spreading centers
Fluid-induced melting in
subduction zones
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GLY 155 Introduction to Physical Geology, W. Altermann
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Decompression Melting: Spreading Centers
Pillow lava
Newer, thinner
sediments
Older, thicker
sediments
Sheeted dikes
in basalt
Oceanic
crust
Gabbro
Moho
Peridotite layer
Mantle
Spreading
center
GLY 155 Introduction to Physical Geology, W. Altermann
©
Hot mantle rises,
decompresses,
and melts.
Pillow lava
Newer, thinner
sediments
Older, thicker
sediments
Sheeted dikes
in basalt
Oceanic
crust
Gabbro
Moho
Peridotite layer
Mantle
Spreading
center
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GLY 155 Introduction to Physical Geology, W. Altermann
©
Dikes
A thin dike erupts,
spilling lava in
“pillows.”
Dikes intruding dikes
Hot mantle rises,
decompresses,
and melts.
Pillow lava
Newer, thinner
sediments
Older, thicker
sediments
Sheeted dikes
in basalt
Oceanic
crust
Gabbro
Moho
Peridotite layer
Mantle
Spreading
center
Dikes
A thin dike erupts,
spilling lava in
“pillows.”
Dikes intruding dikes
Hot mantle rises,
decompresses,
and melts.
Pillow lava
Newer, thinner
sediments
Older, thicker
sediments
Sheeted dikes
in basalt
Oceanic
crust
Gabbro
Cold
seawater
Sheeted
dikes
Sediments are
deposited on
the spreading
seafloor.
The gabbro layer
metamorphoses
by contact with
the magma.
Moho
Peridotite layer
Mantle
Dikes intrude
dikes to form
sheeted dikes.
Spreading
center
Heated
seawater
carrying
dissolved
minerals
Magma chamber
Peridotite layer
Mantle
Seawater filters
through the
basalt layer,
where it is heated.
The heated seawater
then rises. Dissolved
minerals precipitate in
the ocean.
Crystals settle out
of the magma,
forming the
peridotite layer.
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GLY 155 Introduction to Physical Geology, W. Altermann
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Fluid-Induced Melting – Subduction Zones
Trench
Oceanic sediments
Magma chamber
Oceanic crust basalt
Oceanic mantle
lithosphere
Asthenosphere
GLY 155 Introduction to Physical Geology, W. Altermann
©
Trench
Oceanic sediments
Magma chamber
Oceanic crust basalt
Oceanic mantle
lithosphere
Asthenosphere
Subducting oceanic crust
carries sediments with it.
Sediment
grains
Water
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GLY 155 Introduction to Physical Geology, W. Altermann
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Trench
Oceanic sediments
Magma chamber
Oceanic crust basalt
Oceanic mantle
lithosphere
Asthenosphere
Subducting oceanic crust
carries sediments with it.
Water remains trapped
as the pressure and
temperature increase.
Sediment
grains
Water
Magma of intermediate
composition is erupted
to form arc volcanoes.
Molten sediments
combine with
lithospheric magma.
Trench
Oceanic sediments
Magma chamber
Oceanic crust basalt
Oceanic mantle
lithosphere
The water
and molten
sediments melt
parts of the
overlying plate.
Asthenosphere
Subducting oceanic crust
carries sediments with it.
H2O
H2O
H2O
Water remains trapped
as the pressure and
temperature increase.
Sediment
grains
Water
…causing the
sedimentary
rocks to melt
at lower
temperatures.
The trapped water is
released as the
temperature increases,…
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GLY 155 Introduction to Physical Geology, W. Altermann
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Thought questions for this chapter
How would you classify a coarsecoarse-grained igneous rock
that contains about 50 percent pyroxene and 50 percent
olivine?
What kind of rock would contain some plagioclase
feldspar crystals about 5 mm long “floating”
floating” in a dark gray
matrix of crystals less than 1 mm?
What differences in crystal size might you expect to find
between two sills, one intruded at a depth of about 12 km,
where country rock was very hot, and the other at a depth
of 0.5 km, where the country rock was moderately warm?
GLY 155 Introduction to Physical Geology, W. Altermann
©
Thought questions for this chapter
Assume that a magma with a certain ratio of calcium to
sodium starts to crystallize. If fractional crystallization
occurs during the solidification process, will the
plagioclase feldspars formed after complete crystallization
have the same ratio of sodium that characterized the
magma?
Why are plutons more likely than dikes to show the
effects of fractional crystallization?
What might be the origin of a rock composed almost
entirely of olivine?
What processes create the unequal sizes of crystals in
porphyries?
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GLY 155 Introduction to Physical Geology, W. Altermann
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Thought questions for this chapter
What observations would show that a pluton solidified
during fractional crystallization?
If you were to drill a hole through the crust of a midmid-ocean
ridge, what intrusive or extrusive igneous rocks might you
expect to encounter at or near the surface? What
intrusive or extrusive igneous rocks might you expect at
the base of the crust?
Water is abundant in the sedimentary rocks and oceanic
crust of subduction zones. How would the water affect
melting in these zones?
Why are granitic and andesitic rocks so plentiful?
GLY 155 Introduction to Physical Geology, W. Altermann
©
Key terms and concepts
Andesite
Basalt
Batholith
Bomb
Concordant intrusion
Country rock
Dacite
Decompression melting
Dike
Discordant intrusion
Extrusive igneous rock
Felsic rock
FluidFluid-induced melting
Fractional crystallization
Gabbro
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GLY 155 Introduction to Physical Geology, W. Altermann
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Key terms and concepts
Granodiorite
Intermediate igneous rock
Intrusive igneous rock
Lava
Mafic rock
Magma chamber
Magmatic differentiation
Obsidian
Ophiolite suite
Partial melting
Pegmatite
Peridotite
Pluton
Porphyry
Pumice
GLY 155 Introduction to Physical Geology, W. Altermann
©
Key terms and concepts
Pyroclast
Rhyolite
Sill
Stock
Tuff
Ultramafic rock
Vein
Viscosity
Volcanic ash
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