1/30/23
Igneous Rocks, Magma and
Intrusive Igneous Structures
GEOL 101-002 / Physical Geology
Prof. Jules Goldspiel
George Mason University / Spring 2023
Jan 30, 2023
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Igneous Rocks, Magma and Intrusive Igneous Structures
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Rocks Overview
Magma, Lava and Igneous Rocks
Igneous Rock Compositions
Igneous Rock Textures
Igneous Rock Naming
Origin of Magma
Evolution of Magmas
Intrusive Igneous Structures
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Focus Questions
• What is the distinction between magma and lava?
– Do magmas contain solids and gases or are they pure liquids?
• How are igneous rocks formed?
• What is the difference between intrusive and extrusive
igneous rocks?
– Are the properties of intrusive igneous rocks different from
extrusive igneous rocks? If so, why?
• What properties are used to classify (name) igneous rocks?
– How does intrusive vs. extrusive relate to the classifications?
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Focus Questions
• What are the differences between felsic and mafic magmas?
– What are the chemical compositional differences?
– What are the gas content differences?
– Do both have the same viscosity?
• Do felsic and mafic magmas erupt in the same manner?
• Do felsic and mafic magmas yield the same type of rocks?
– What is an example of a rock type produced from felsic magmas?
– What is an example of a rock type produced from mafic magmas?
• Do all minerals in igneous rocks crystallize at the same time?
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1. Rocks Overview
• Rock definition
– A rock is a solid material that...
§ Occurs naturally
§ Is composed of one or more minerals or
other natural planetary materials
Granite example (USGS)
Anthracite coal (D. Pizzarelli, USGS)
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1. Rocks Overview
• Rock classification and identification
– Rocks are first classified by how they formed
§ Igneous
§ Sedimentary
§ Metamorphic
– Individual rocks are then identified (classified, named)
by their specific characteristics
§ Composition
§ Texture
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1. Rocks Overview
• Rocks can (and do) transform
from one type to another
– Rock Cycle
Illustration of the Rock Cycle
(USDA, adapted from Plummer
and McGeary, Physical Geology,
1988). See also Lutgens et al. 2018,
Fig. 1.23 and associated video
(https://goo.gl/XMyy2O).
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2. Magma, Lava and Igneous Rocks
• Igneous rocks
– Rocks formed from molten rock
– Magma or lava is the parent material
of igneous rocks
Granite example (USGS, see also
Lutgens et al., 2018, Fig.4.2)
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2. Magma, Lava and Igneous Rocks
• Components of magma and lava
– Liquid
– Solids
§ Mineral crystals
– Volatiles
§ Dissolved gases
o Water vapor (H2O)
o Carbon dioxide (CO2)
o Sulfur dioxide (SO2)
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Lava flow on Hawaii (Hawaii Volcano
Observatory, USGS)
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2. Magma, Lava and Igneous Rocks
• Crystallization
– The formation and growth of a
crystalline solid from a liquid or gas
– For liquid water, this process
occurs as the water freezes
Ice cubes
in an ice
cube tray
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2. Magma, Lava an Igneous Rocks
• Intrusive vs. Extrusive igneous rocks
– Intrusive igneous rocks
§ Formed when molten rock crystallizes
below the surface
§ Also referred to as Plutons, or
plutonic rocks
– Extrusive igneous rocks
§ Formed when molten rock crystallizes
on the surface
§ Also referred to as volcanic rocks
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Illustration showing an active volcanic extrusion
and multiple types of igneous intrusions (USGS).
See also Lutgens et al., 2018, Fig.4.3, and
accompanying tutorial (https://goo.gl/ac0bu5).
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2. Magma, Lava an Igneous Rocks
• Emplacement location affects rock properties
– Intrusive igneous rocks
§ Cool slowly
§ Large crystals
– Extrusive igneous rocks
§ Cool quickly
§ Small (fine) crystals
Illustration showing an active volcanic extrusion
and multiple types of igneous intrusions (USGS).
See also Lutgens et al., 2018, Fig.4.3, and
accompanying tutorial (https://goo.gl/ac0bu5).
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3. Igneous Rock Compositions
• Igneous rocks are composed primarily of silicate minerals
– Silicate minerals can be sorted into two broad
composition categories
§ Light silicates
o Little if any Iron (Fe) and Magnesium (Mg)
o Examples: Quartz, Muscovite, Feldspars
§ Dark silicates
o Rich in Iron (Fe) and/or Magnesium (Mg)
o Examples: Olivine group, Pyroxene group,
Amphibole group, Biotite
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Olivine crystals on basalt,
sample about 4 cm across
(J. Goldspiel, 2020)
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3. Igneous Rock Compositions
• Igneous rock composition classifications
– Rock compositional groupings are based on
the proportion of light and dark silicates
§ Felsic
§ Intermediate
§ Mafic
§ Ultramafic
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3. Igneous Rock Compositions
• Felsic igneous rocks
– Also called Granitic rocks
– Composed almost entirely of light silicates
§ High percentage feldspar
§ High percentage quartz
o Relatively high silica (SiO2) content
– Felsic = feldspar and silica
Granite example (USGS, see also
Lutgens et al., 2018, Fig.4.2)
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3. Igneous Rock Compositions
• Mafic igneous rocks
– Also called Basaltic rocks
– Composed of significant amount of dark silicates
§ At least 45% dark silicates
o Rich in Iron (Fe) and/or Magnesium (Mg)
§ Low percentage quartz
o Relatively low silica content
– Mafic = magnesium and ferrum
Basalt example (USGS)
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3. Igneous Rock Compositions
• Intermediate igneous rocks
– Also called Andesitic rocks
– Composition between
felsic and mafic
Andesitic lava flows dating
to 1510-1570 CE
Southeast flank of Mount St. Helens
on Sep 28, 1979 (R. Hoblitt, USGS)
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3. Igneous Rock Compositions
• Ultramafic igneous rocks
– Composed almost entirely of dark silicates
Peridotite example, an ultramafic rock,
from northern Italy (Mattiabianchi198,
https://commons.wikimedia.org/wiki/
File:Dunite0001.jpg)
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3. Igneous Rock Compositions
• Composition affects flow of magmas and lavas
– Felsic (granitic) magmas/lavas
§ Viscous
§ Tend to erupt explosively
– Mafic (basaltic) magmas/lavas
§ Less viscous
§ Tend to erupt effusively
Silica content
important
Explosive eruption, Mt.
St. Helens, WA, 1980
(M. Doukas, USGS)
Effusive eruption
on Hawaii (R. W.
Decker, USGS)
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3. Igneous Rock Compositions
• Composition affects rock density
– Felsic (granitic) rocks
§ Lower density
– Mafic (basaltic) rocks
§ Higher density
Periodic Table, with Na, Al, K, and Ca circled
in pink, Mg and Fe circled in dashed blue,
and Si circled in dotted red (NIST)
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3. Igneous Rock Compositions
• Composition summary
Grain
Felsic
Intermediate
Mafic
Ultramafic
Coarse
Rocks
Fine
Mineral %
(by Volume)
(Potassium feldspar)
Igneous rock
composition diagram
(UCSD, Scripps Inst.)
See also Lutgens et al.,
2018, Fig.4.5, and
accompanying tutorial
(https://goo.gl/goy5yO)
Minerals
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3. Igneous Rock Compositions
• Composition summary
Grain
Felsic
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Intermediate
Mafic
Ultramafic
Coarse
Fine
75 %
Silica (SiO2) Content
40 %
Low
Magnesium and Iron Content
High
650℃
Temperature at Which Melting Begins
1250℃
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3. Igneous Rock Compositions
• Temperature scale interlude
– Common units and scales
§ Fahrenheit (ºF)
§ Celsius (ºC)
§ Kelvin (K)
– Rock melting temperatures
650ºC = 923K = 1200ºF
1250ºC = 1523K = 2280ºF
Freezing and boiling points of pure water at 1 atm
(https://chem.libretexts.org)
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4. Igneous Rock Textures
• Texture
– Texture is the overall appearance
of a rock based on...
§ Size of mineral grains
§ Shape of mineral grains
§ Arrangement of mineral grains
Granite, an example of a coarsegrained rock (USGS, see also
Lutgens et al., 2018, Fig.4.6)
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4. Igneous Rock Textures
• Texture classifications
– Coarse-grained (a.k.a. Phaneritic)
§ Large crystals (naked eye)
§ Result of slow cooling
Granite, a coarsegrained rock (USGS,
see also Lutgens et
al., 2018, Fig.4.6)
– Fine-grained (a.k.a. Aphanitic)
§ Small crystals (microscopic)
§ Result of fast cooling
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Basalt, a finegrained rock (USGS,
see also Lutgens et
al., 2018, Fig.4.6)
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4. Igneous Rock Textures
• Texture classifications
– Glassy
§ No crystals (no grains)
§ Can result from two processes
o Very rapid cooling (quenching)
o Early polymerization (chain forming)
in high silica content magma/lava
Obsidian, a glassy rock (B. Domangue,
https://commons.wikimedia.org/wiki/
File:Obsidian_-_Igneous_Rock.jpg)
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4. Igneous Rock Textures
• Texture classifications
– Porphyritic
§ Two distinct grain sizes
o Large crystals (phenocrysts)
o Matrix of smaller crystals (groundmass)
§ Result of changing magma/lava environment
Porphyritic basalt, a fine-grained
rock with phenocrysts (USGS)
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4. Igneous Rock Textures
• Texture classifications
– Vesicular
§ Voids in rock
§ Result from gas bubbles
escaping as lava solidifies
Vesicular basalt from Quincy, WA
(L. Kestay, USGS)
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4. Igneous Rock Textures
• Texture classifications
– Pyroclastic (a.k.a. Fragmental)
§ Rock fragments in rock
§ Result of consolidation of rock
fragments ejected during
explosive eruptions
Pyroclastic rock, volcanic breccia type
(Natural Resources, Newfoundland
and Labrador, Canada)
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4. Igneous Rock Textures
• Texture classifications
– Pyroclastic (a.k.a. Fragmental)
§ Three subtypes of pyroclastic textures
o Tuff
» Most fragments < 4 mm (some bigger)
o Welded tuff
» Most fragments < 4 mm (some bigger)
» Fragments fused (welded) together
o Volcanic breccia
» Most fragments > 4 mm
Tuff deposit near Yellowstone National
Park (M. Myers, Montana State Univ.)
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5. Igneous Rock Naming
• Igneous rocks are named according to
their composition and texture
Composition
Texture
Felsic
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Intermediate
Mafic
Ultramafic
Coarse-grained
Granite
Diorite
Gabbro
Peridotite
Fine-grained
Rhyolite
Andesite
Basalt
Komatite
Porphyritic
Granite
Porphyry
Andesite
Porphyry
Basalt
Porphyry
[ -- ]
Glassy
Obsidian
[ -- ]
[ -- ]
[ -- ]
Vesicular
Pumice
Pumice/Scoria Scoria
[ -- ]
Pyroclastic
[*]
[*]
[*]
[*]
Notes:
[ -- ] = Uncommon
[ * ] = Names do not
equate to
composition
(See also Lutgens et al.,
2018, Fig.4.12, and the
accompanying tutorial
(https://goo.gl/VOzSR0))
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5. Igneous Rock Naming
• Igneous rocks are named according to their
composition and texture
– Exception in the case of pyroclastic igneous rocks
§ Type names refer to texture only
§ Type names are used as
modifiers to other igneous
rock types
o e.g., Rhyolitic tuff
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Notes:
[ -- ] = Uncommon
[ * ] = Names do not
equate to
composition
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5. Igneous Rock Naming
Gabbro, ruler in
inches (NASA)
Granite (USGS)
• Igneous rocks are named according to…composition…
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5. Igneous Rock Naming
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Basalt (USGS)
Rhyolite (USGS)
• Igneous rocks are named according to…composition…
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5. Igneous Rock Naming
Granite (USGS)
Rhyolite (USGS)
• Igneous rocks are named according to…texture
Obsidian (Ji-Elie,
http://en.wikipedia.org/
wiki/File:LipariObsidienne_(5).jpg)
Pumice, ruler in cm
(MPF, https://commons.
wikimedia.org/wiki/File:
Teidepumice.jpg)
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5. Igneous Rock Naming
Basalt (USGS)
Gabbro, ruler in
inches (NASA)
• Igneous rocks are named according to…texture
Basalt Porphyry (USGS)
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6. Origin of Magma
• Earth is divided (differentiated) into three major
composition layers (or shells)
– Core
§ Inner core: Solid metal
§ Outer core: Liquid metal
– Mantle
§ Primarily solid rock
– Crust
§ Primarily solid rock
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Earth interior diagram (USGS). See also Lutgens
et al. 2018, Fig. 1.20 and accompanying video
(https://goo.gl/8lwyPV).
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6. Origin of Magma
• Generating magma from solid rock
– Geothermal gradient plays an
important role
§ But…cannot do it alone
Geothermal gradient (red line) and
rock melting curve (blue line) (D.
Boden, 2016, Geologic Fundamentals
of Geothermal Energy). (See also
Lutgens et al. 2018, Fig. 4.17)
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6. Origin of Magma
• Generating magma from solid rock
– Rock temperatures must be pushed
above their melting points
– This can happen in a few ways
§ Increase in regional temperature
§ Decrease in pressure
§ Addition of water
Geothermal gradient (red line) and
rock melting curve (blue line) (D.
Boden, 2016, Geologic Fundamentals
of Geothermal Energy)
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6. Origin of Magma
• Generating magma from solid rock
– Regional temperature increase
§ Heat from nearby magma can melt
surrounding crustal rocks
o Still leaves problem of where the
nearby magma came from
§ Heat from continental collisions can
melt crustal rocks
Geothermal gradient (red line) and
rock melting curve (blue line) (D.
Boden, 2016, Geologic Fundamentals
of Geothermal Energy)
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6. Origin of Magma
• Generating magma from solid rock
– Decrease in confining pressure
§ Decompression Melting
§ Pressure increases with depth
§ Rock melting point increases as pressure
increases
§ Therefore, the melting point of the rocks…
o Increases as rock depth increases
o Or equivalently, decreases as rocks get
closer to surface
Geothermal gradient (red line)
and rock melting curve (blue line)
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6. Origin of Magma
• Generating magma from solid rock
– Decrease in confining pressure
§ When hot, but solid, rocks ascend through
the upper mantle…
o Hot solid rock begins to melt
(becomes molten)
Geothermal gradient (red line)
and rock melting curve (blue line)
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6. Origin of Magma
• Generating magma from solid rock
– Addition of water
§ Adding water to mantle rocks lowers the
rock melting point
o Results in partial melting of mantle rock
o Occurs mainly at convergent boundaries
(subduction zones)
Geothermal gradient (red line)
and rock melting curve (blue line)
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7. Evolution of Magmas
• The chemical composition of the liquid portion of a
magma can change (evolve)
• This can occur as a result of…
– Magmatic differentiation
– Magma mixing
– Assimilation
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7. Evolution of Magmas
• Magmatic differentiation
– Crystallization removes
specific chemical elements
from the liquid
– Composition of remaining
liquid portion then differs
from parent magma
Schematic of magmatic differentiation with crystal settling
(Earth Science Australia). See also Lutgens et al., 2018, Fig. 4.21.
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7. Evolution of Magmas
• Magmatic differentiation
– Minerals crystallize from
a cooling magma in a
systematic sequence
§ Minerals with higher
melting points crystallize
before minerals with
lower melting points
Schematic of magmatic differentiation with crystal settling
(Earth Science Australia).
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7. Evolution of Magmas
• Magmatic differentiation
– The crystallization sequence is known as Bowen’s Reaction Series
⚬
Temperature
of cooling
magma
(Pl
a
Fe gioc
ld s las
pa e
r)
~1200 C
Rock types
produced
Bowen’s Reaction Series
diagram for a mafic magma
(Colivine, https://commons.
wikimedia.org/wiki/File:Bo
wen%27s_Reaction_Series.
png). See also Lutgens et
al., 2018, Fig. 4.20.
(Potassium
Feldspar)
~650 ⚬C
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7. Evolution of Magmas
• Magma mixing
– As two magma masses ascend,
they may mix if they come in
contact
Magma 1
Magma 2
Magma 1
Magma 2
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Magma mixing
schematic, with
Magma 2 rising
faster than Magma 1
(see also Lutgens et
al., 2018, Fig. 4.23)
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7. Evolution of Magmas
• Assimilation
– As magma migrates through the crust,
it may incorporate (assimilate) some of
the surrounding rock
Host
rock
Magma
Magma assimilation
schematic (see also
Lutgens et al., 2018,
Fig. 4.22)
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8. Intrusive Igneous Structures
• Most magma crystallizes below the surface
– This creates intrusive igneous bodies called Plutons
§ Massive plutons (irregular shapes)
§ Tabular plutons (table-like shapes)
Illustration showing volcanic
extrusions and multiple types of
igneous intrusions (USGS). See
also Lutgens et al., 2018, Fig.4.26,
and accompanying animation
(https://goo.gl/2CGehV).
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8. Intrusive Igneous Structures
• Massive plutons
– Batholiths
§ Largest intrusive body
§ Surface area exposure > 100 km2 (40 mi2)
– Stocks
§ Surface area exposure ≤ 100 km2
Half Dome in
Yosemite
National Park,
part of the
Sierra Nevada
Batholith (A.
Demas, USGS)
– Laccoliths
§ Intrusion injected between sedimentary
strata that causes overlying strata to arch
upward
(USGS)
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8. Intrusive Igneous Structures
• Tabular plutons
– Sills
§ Concordant plutons
§ Closely resemble buried lava flows
§ May exhibit columnar jointing
Columnar jointing seen in Giant’s
Causeway, Northern Ireland
(Man vyi, https://en.wikipedia.org/
wiki/Columnar_jointing#/media/File:
Giant's_Causeway_2006_08.jpg)
(USGS)
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8. Intrusive Igneous Structures
• Tabular plutons
– Dikes
§ Discordant plutons
§ Serve as conduits to transport
magma upward
Small dike, Alaska (Jonathan.s.kt,
https://commons.wikimedia.org/
wiki/File:Geological_Dike_CrossIsland_Trail_Alaska.jpg)
(USGS)
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References for Further Reading
• Lutgens, F. K., et al., 2018, Essentials of Geology, 13th Edition.
[Chapter 4]
• Johnson, C., et al., 2017, An Introduction to Geology (online).
[Chapter 4]
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