erth2404 l4 igneous upload

7/28/2019 ERTH2404 L4 Igneous Upload

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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

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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

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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

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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

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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

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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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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

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Pale

Dark


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% 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

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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

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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

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

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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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

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Intrusive

Cools slowly: Phaneritic

Granite

Extrusive

Cools quickly: Aphanitic

Rhyolite


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Extrusive

Cools very quickly: glassy, no crystalline structure

Obsidian

What is this

texture?


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GraniteRhyolite Obsidian

Do these rocks have anything in common?


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Volatile based textures

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Cooling

environment

Cooling

rateTexture

Volatiles

escaping?

YES

No

YES

Non-crystalline

Fast

Crystalline

No

Intrusive Slow No

Extrusive

Very fast


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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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Pumice

Highly vesicular

Frothy appearance

Simultaneouscooling anddepressurization

freezes thebubbles

Considered a glass!

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Add it to the mix

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GraniteRhyolite Obsidian

Pumice


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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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Porphyritic texture

Phenocryst:

crystals

Matrix: fine

grained,

surrounding

phenocrystsMatthew Nyman, TERC


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Magma rich in volatiles

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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

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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

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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

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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

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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

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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

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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

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Cooling surface

Sideview

Mapview

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120

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Columnar jointing

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Photographs: P. Fernberg

Giant Causeway, Northern Ireland


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Tephra

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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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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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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

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