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ERTH2404
Lecture 4: Igneous Rocks
Dr. Jason Mah
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Reading assignment
Please read Kehews book to complement the
material presented in this lecture:
Chap. 4
2
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Lecture objectives & contents
The relationship between igneous rocks and
other rock types
Properties and classification of igneous rocks
Mineral composition
Mantle melting processes
Intrusive and Extrusive processes
Engineering considerations
3
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Review of Minerals
What is a mineral?
Inorganic
Homogeneous solid
Crystal structure
Characteristic physical properties
Rocks are composed of one or more mineral
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Introduction to Rocks
Three rock types:
Igneous
Sedimentary
Metamorphic
Igneous + metamorphic = 95% of rocks
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The Rock cycle
Theoretical
concept
predating plate
tectonics
Rock types:
Igneous,
sedimentary,metamorphic
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Rock types
Igneous rocks form by cooling and solidificationfrom a liquid called magma
Sedimentary rocks form by the erosion anddeposition of rock fragments
or
by precipitation
Metamorphic rocks form by the alteration ofexisting rocks by heat, pressure or fluids
7
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Igneous Rocks
Crystallization: the process of mineral
formation in a cooling magma
Types:
Extrusive: volcanic (cooling at surface)
small grains
Intrusive: plutonic (cooling at depth)
large grains
8
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Rock cycle: Igneous Rocks
Igneous
rocks are
formed
from anytype of
pre-
existing
rocks
9Source: British Broadcasting Corporation (BBC)
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Igneous Rocks
Volcanic materials:
Magma (ointment in Greek): partially molten rockbelow the Earths surface
Lava(to wash in Latin): magma that reaches thesurface
Three main components
Melt: liquid portion
Pyroclastic (Solids): ash, cinders, bombs, mineralscrystallized from the melt
Gases (Volatiles): H2O, CO2, SO2
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Igneous Rocks
How does the mantle melt?
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Melting and Magmas
What can melt?
Upper mantle
Continental crust
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Mantle Temperatures
Liquidus: Tabovewhich all
liquid
Solidus: T
belowwhich allsolid
13
Magmas form within
200km of the surface
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Mantle Melting 1
Change in depth and pressure: mantle risesvertically, crosses solidus, becomes partially
molten
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(oceanic)
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Mantle Melting 2
H2O added to mantle: shifts melting
temperature, partially melts
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Mantle Melting 3
Mantle plume:
anomalously hot
(200-300C)
Volatile rich
magma travels
from core
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Mantle Melting
1. Lower the pressure: decompression melting
2. Add volatile compounds (H2O, CO2):
dehydration melting
3. Hot mantle plume (decompression,
dehydration)
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Igneous Rock Classification
All properties are closely related to the
cooling environment and magma
behaviour1. Intrusive or Extrusive?
Texture: size, shape, and arrangementof mineral grains
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Extrusive vs Intrusive
Cooling rate controls grain size
Intrusive
Cooling slowly at depth leads to uniformly largegrain size
Extrusive
Cooling quickly at surface, leads to small grain size
Can have two phases of cooling, one at depth andremainder at surface: big and small crystals
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Intrusive (plutonic)
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Plutonic
www.turnstone.ca/shap.htm
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Extrusive (volcanic)
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Volcanic
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Igneous rock identification
1. Intrusive or Extrusive?
2. Mineral composition
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Minerals and Chemistry
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Minerals SiO2Igneous Rock
Type
Olivine, pyroxene < 45% Ultramafic
Olivine, pyroxene,Ca-feldspar
45-57% Mafic
Na-feldspar,
amphibole57-65% Intermediate
Biotite, K-feldspar, quartz
> 65% Felsic
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Minerals and Chemistry
Key characteristic: % of silica (SiO2)
Felsicrocks: rich in silica(SiO2 66%)
(Feldspar + silica) Intermediate rocks: 52% SiO2 65%
Maficrocks: rich in ferromagnesian
minerals (45% SiO2 51%)(Magnesium + ferric)
Ultramafic rocks: 45% < SiO2
24
Pale
Dark
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Minerals and Chemistry
% SiO2 controls magna viscosity and thereforemagma behavior and eruptive style
High % SiO2: Viscous magma
Low-temperature (600-900oC)
Tend to produce large plutonic bodies or explosiveeruptions
Low % SiO2: Fluid magma
High-temperature (1000-1250oC)
Large, peaceful outpouring at the Earths surface
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Bowens Reaction Series
26
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Crystallization temperature
High Temperature: olivine, pyroxene, Ca-feldspar
Intermediate Temperature: amphibole, Na-feldspar, + biotite
Low Temperature: Biotite, Na-K feldspar, quartz
Previous mineral may dissolve as new ones form
Branch depends on presence of element
Felsic or Mafic minerals
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Classification
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Classification
29
Cooling
Slow
Fast
Pale Dark
Low% SiO2
High% SiO2
GabbroIntrusive Phaneritic Granite Diorite
Mafic
Extrusive
Glassy Obsidian
Aphanitic Rhyolite Andesite Basalt
Igneous
rock type
Comp
TextureFelsic Intermediate
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Cooling based textures
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Grain diameter
[mm]
Non-crystalline No grains formed
Fine-grained < 1
Medium-grained 1 5
Coarse-grained > 5
Texture
Crystalline
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Cooling based textures
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Cooling
rate
Grain diameter
[mm]
Very fast Non-crystalline No grains formed
Fine-grained < 1
Medium-grained 1 5Slow Coarse-grained > 5
Fast
Texture
Crystalline
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Cooling based textures
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Cooling
environment
Cooling
rate
Grain diameter
[mm]
Very fast Non-crystalline No grains formed
Fine-grained < 1
Medium-grained 1 5
Intrusive Slow Coarse-grained > 5
FastExtrusive
Texture
Crystalline
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Cooling based textures
Phaneritictexture (intrusive):
Coarse, uniform grains
Slow cooling in the subsurface
Easily seen with eye
Aphanitictexture (extrusive):
fine grained
Fast cooling at the surface
you may need a hand lens to see the crystals
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Extrusive vs Intrusive Rocks
34
Intrusive
Cools slowly: Phaneritic
Granite
Extrusive
Cools quickly: Aphanitic
Rhyolite
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Extrusive vs Intrusive Rocks
35
Extrusive
Cools very quickly: glassy, no crystalline structure
Obsidian
What is this
texture?
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Extrusive vs Intrusive Rocks
36
GraniteRhyolite Obsidian
Do these rocks have anything in common?
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Volatile based textures
37
Cooling
environment
Cooling
rateTexture
Volatiles
escaping?
YES
No
YES
Non-crystalline
Fast
Crystalline
No
Intrusive Slow No
Extrusive
Very fast
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Volatile based textures
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Cooling
environment
Cooling
rateTexture
Volatiles
escaping?
YES
No
YES
Non-crystallinePumiceous
Glassy
Fast
Crystalline
Vesicular
No
Igneous texture
Intrusive Slow NoPhaneritic
two-stage cooling Porphyritic
AphaniticExtrusive
Very fast
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Volatile based textures
Pumice
Highly vesicular
Frothy appearance
Simultaneouscooling anddepressurization
freezes thebubbles
Considered a glass!
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Add it to the mix
40
GraniteRhyolite Obsidian
Pumice
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Volatile based textures
Magma poor in volatiles
Porphyritic texture: grains of two sizes indicate a
two-stage cooling process
Phenocrysts: large grains/crystals formed first and had
time to grow in magma chamber before magma
reached the surface
Matrix: finer grain than phenocryst second in contactwith the atmosphere
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Volatile based textures
42
Porphyritic texture
Phenocryst:
crystals
Matrix: fine
grained,
surrounding
phenocrystsMatthew Nyman, TERC
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Volatile based textures
Magma rich in volatiles
43
Vesicles: small holes on
top of lava flows through
which gases escape
Amygdule: infilling of
vesicle with secondary
mineral
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Question
An ultramafic rock is composed dominantly of
what mineral:
1. Quartz
2. Feldspar
3. Amphibole
4. Olivine
5. Pyroxene
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Question
An ultramafic rock is composed dominantly of
what mineral:
1. Quartz
2. Feldspar
3. Amphibole
4. Olivine
5. Pyroxene
45
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Intrusive & Extrusive processes
We now examine Intrusive processes
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Intrusive processes
Intrusion: movement of magma from a
magma chamber to a different subsurface
location
Plutons: bodies of rocks formed by the
intrusion of magma into older rocks, named
country rocks
47
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Plutonic Igneous Activity
Vast majority ofmagmas solidify atdepth: 87%
Represent magmas thatwere not erupted; nowexposed due to erosion
Half Dome, Yosemite CA
Granite
4,737 ft
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Types of plutons
Plutons are classified according to:
Size and shape
Relation with country rocks
Concordant: parallel to country rock layering
Discordant: cutting across country rock layering
Pluton: large, massive intrusion
Sill, Dyke: thin, tabular intrusions
Batholith: assemblage of plutons
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Pluton classification
50
Concordant
< 100 km
2
Stock> 100 km
2Batholith
TabularThin in one
dimension SillDyke
Thick andbroad
Shape
Non-tabular
Relation with country rock
Discordant
Laccolith
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Intrusions
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Mackenzie dyke swarm
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Volcanic neck
53www.geology.wisc.edu/~maher/air/air03.htm
Ship Rock, New Mexico
dykes
Intrusion landform created when magmahardens within vent and surrounding softerrock eroded
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Batholiths
Non-tabular discordant pluton
Majority have a composition of granodioriteto granite
Represent uplifted, eroded roots ofsubduction-related volcanic complexes
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USGS
Batholiths
55
Yosemite, Half Dome
Granite
4,737 ft
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Intrusive & Extrusive processes
We now examine Extrusive processes
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Extrusive Processes
Volcanism: process by which magma rise intothe crust and is extruded onto the Earthssurface and into the atmosphere
Extruded volcanic material: Lava
Pyroclastic material (tephra): material formed byvolcanic explosion or aerial expulsion from avolcanic vent
Volatiles: mainly H2O, CO2, SO2
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Lava
Lava viscosity
Increases with volatile content
Increases with % SiO2
58
High SiO2Basaltic
Viscious
Low SiO2Felsic lava
Fluid
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Lava
Felsic lava (Si, Al, K, Na)
Flows slowly
Lava flows with jagged upper surface: aa
Basaltic lava (Mg, Fe)
Flows quickly over large distances
Lava flows with smooth upper surface
Pahoehoe: flow wrinkles
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Lava
60
pahoehoeUSGS
aa flow
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Columnar jointing
Columnar jointing: vertical polygonal structure formedduring cooling
Cracks formed during cooling of igneous rocks
Rocks cool from the outside in, causing shrinkage Present in intrusive and extrusive rocks
Common in basaltic lava flows
Side view: columns Fissures grow at 90 to the cooling surface
Top view: hexagons Fissures develop preferentially in 3 directions
at 120 to each other
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Columnar jointing
62
Cooling surface
Sideview
Mapview
120
120
120
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Columnar jointing
63
Photographs: P. Fernberg
Giant Causeway, Northern Ireland
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Tephra
64
Tephra classified according to particle size:
Blocks: ejected as solid fragments with angular shapes
Bombs: ejected as incandescent lava fragments which
were semi-molten when airborne
Unconsolidated
material
Consolidated
material
Tephra Pyroclastic rock
< 2 Ash Ash tuff2 64 Lapili Lapili tuff
Particle
diameter [mm]
> 64 Volcanic brecciaBlocks / Bombs
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Pyroclastic rocks
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Pyroclastic rocks are transitional between igneousand sedimentary rocks
"Igneous on the way up. sedimentary on the way down!"
Tuff: pyroclastic rock formed from volcanic ash andlapili
Unconsolidatedmaterial
Consolidatedmaterial
Tephra Pyroclastic rock
< 2 Ash Ash tuff
2 64 Lapili Lapili tuff
Particle
diameter [mm]
> 64 Volcanic brecciaBlocks / Bombs
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Pyroclastic rocks
Processes converting tephra into
pyroclastic rocks:
When tephra is very hot, particles fuse together
and form a glassy rock Further cementing can occur from agents
transported by groundwater
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Engineering implications
For intrusive igneous rocks
Uniformity and great strength
Dense interlocking network of crystals
Applications: Provide adequate foundation support for
large structures
Water reservoirs
Low permeability
Kitchen countertops
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Engineering implications
For extrusive igneous rocks Variability
May be vesicular, highly permeable
May be formed by interlayering of lava flows andpyroclastic material
Used extensively as engineering material
Concrete, rock fill, railroad ballast,
highway base
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Engineering implications
Fracturing
Columnar jointing allow significant movement of
ground water
Weathering Ferromagnesian minerals present in mafic igneous
rocks may decay if exposed to air and water
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Next: Sedimentary Rocks