chapter 4web.nmsu.edu/~dshearer/geology/04_ignrks/mod04-ignrocks-ho.pdf · chapter 4 igneous rocks...

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Chapter 4

Igneous Rocks

and Intrusive Igneous Activity

IntroductionIgneous rocks make up large parts of the continents and all the oceanic crust - indicate plate boundaries and hot spot activity

GraniteMostly quartz and feldsparUsed for kitchen counters, facings on buildings, tombstones, etc

How do igneous rocks differ How do igneous rocks differ

from one another?from one another?

Texture Texture –– size of crystalssize of crystals

CoarseCoarse--grained rocksgrained rocks

FineFine--grained rocksgrained rocks

Mixed texture rocksMixed texture rocks





Texture is related to rate of cooling.Texture is related to rate of cooling.

Intrusive igneous rocks Intrusive igneous rocks ––slow cooling slow cooling –– coarse texturecoarse texture

Extrusive igneous rocks Extrusive igneous rocks ––rapid cooling rapid cooling –– fine texturefine texture

The Properties and Behavior of Magma and Lava

Molten rock material below the Earth's surface is called magma.

If the magma reaches the surface it is called lava.

Magma is erupted as either Lava flows orPyroclastic materials.

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Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks

Igneous texturesIgneous textures

Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks

Volcanic ash Bomb Pumice

IGNEOUS ROCKS 1. Texture

IGNEOUS ROCKS 1a. Texture

Pyroclasts

Bomb

Pumice

Volcanic ash

1. Extrusive pyroclastsform in violent eruptions from lavain the air.

Extrusive rocks

Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks

Volcanic ash Bomb Pumice

Mafic FelsicBasalt Rhyolite

1. Extrusive pyroclastsform in violent eruptions from lavain the air.


Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks

Volcanic ash Bomb Pumice1. Extrusive pyroclastsform in violent eruptions from lavain the air.

Mafic FelsicBasalt Rhyolite 2. Extrusive

igneous rocks cool rapidly and are fine-grained.


Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks


Mafic Felsic

Basalt Rhyolite 2. Extrusive igneous rocks cool rapidly and are fine-grained.

Gabbro Granite


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Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks




Gabbro Granite 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form.

IGNEOUS ROCKS 1a. Texture IGNEOUS ROCKS 1a. TextureMafic Felsic

Basalt Rhyolite

2. Extrusive igneous rocks cool rapidly and are fine-grained.

Gabbro Granite

3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form.

Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks





Phenocrysts

Porphyry


Intrusive rocks

PorphyryPorphyry

Pyroclasts

Extrusive rocks


Mafic Felsic

Basalt Rhyolite 2. Extrusive igneous rocks cool rapidly and are fine-grained.


Phenocrysts

Porphyry

4. Some intrusive porphyritic crystals grow large, but theremaining melt cools faster, forming smaller crystals.



Porphyry

4. Some intrusive porphyritic crystals grow large, but theremaining melt cools faster, forming smaller crystals.

Phenocrysts


Composition of Magma

Silica is the primary constituent of magmasMagmas are differentiated based on the relative proportions of silica, iron, and magnesium.

Felsic – silica rich, iron and magnesium poorIntermediate – intermediate between felsic and mafic Mafic – silica poor, iron and magnesium rich

Parent magma composition largely determines the composition of igneous rocks.

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Chemical and mineral Chemical and mineral

composition of igneous rockscomposition of igneous rocks

Four compositional groups:Four compositional groups:Felsic igneous rocksFelsic igneous rocksIntermediate igneous rocksIntermediate igneous rocksMafic igneous rocksMafic igneous rocksUltramafic igneous rocksUltramafic igneous rocks

IGNEOUS ROCKS 1c. Chemical & Minerals Igneous Rocks 1c. Chemical & Minerals

Igneous Rocks 1c. Chemical & Minerals The Properties and Behavior of Magma and Lava

How Hot Are Magma and Lava?

Measured temperatures of lavas suggest that typical mafic magma is somewhat hotter than 1200ºCfelsic magma is somewhat hotter than 900ºC.

Fig. 4.2, p. 87


Viscosity – Resistance to Flow

Viscosity measures a substance’s resistance to flow.

High viscosity –thick, stiff flow

Examples: Tar, glacial ice, felsic magma

Fig. 4.3 b, p. 88



Viscosity measures a substance’s resistance to flow.

Examples: Water, syrup, mafic magmasMafic magmas are hotter and have less silica. This makes them flow better.

Fig. 4.3 a, p. 88

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The viscosity of magma is controlled primarily by temperature and composition (silica and iron content). Other factors include loss of volatiles, crystallinity, bubble content, and shear stress during movement.

How do magmas form?How do magmas form?Why do rocks melt?Why do rocks melt?

How do magmas form?How do magmas form?

What is a magma chamber?What is a magma chamber?

A rising blob of magma that A rising blob of magma that pushes aside crustal rocks aspushes aside crustal rocks asit rises through the crust.it rises through the crust.

How Does Magma Originate and Change?

Magma accumulates in reservoirs in the lower crust or upper mantle in magma chambers

Molten rock is less dense than solid rock, and so it is buoyant and begins to rise thru the lithosphere.


Later the magma may stall out and crystallize at a level of neutral buoyancy.

Stoping - A process where a large body of magma can push aside the overlying crust and assimilate blocks of crust on its way toward the surface.

Fig. 4.7, p. 92

Magmatic differentiation

Magmatic Differentiation Magmatic Differentiation ––•• A process by which rocks of A process by which rocks of

varying composition can arise varying composition can arise from a uniform parent magma.from a uniform parent magma.

•• The first minerals to crystallize The first minerals to crystallize from a cooling magma are the from a cooling magma are the ones that are the last to melt.ones that are the last to melt.

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Magmatic differentiation

Fractional crystallization Fractional crystallization ––•• the process by which the crystals the process by which the crystals

are formed in a cooling magma are formed in a cooling magma and are segregated from the and are segregated from the remaining liquid.remaining liquid.

•• Example: basaltic intrusion like Example: basaltic intrusion like Palisades, New JerseyPalisades, New Jersey

Magmatic differentiationTHE PALISADES INTRUSION

Basa

ltic

in

tru

sio

n2

45

–2

75

m (

80

0–

90

0 f

t)

SandstoneBasalt

Mostly sodium-richplagioclase feldspar;no olivine

Calcium-rich plagioclase feldspar and pyroxene; no olivine

Olivine

Sandstone

Basalt

Magmatic differentiationTHE PALISADES INTRUSION


Bowen’s reaction series

Bowen’s reaction series describes the sequence of mineral crystallization in a cooling magma.

There are two branches in the reaction series.

Discontinuous seriesContinuous series

Fig. 4.4, p. 89



The discontinuous seriesproduces ferromagnesian minerals.

Given ideal cooling conditions, a mafic magma will yield a sequence of different minerals, each of which is stable within specific temperature ranges.

As each mineral forms, it removes elements from the melt, leaving the remaining melt to form the next mineral in the sequence. Fig. 4.4, p. 89



The continuous seriesproduces a variety of plagioclase feldspars

Substitution occurs in the crystal structure of plagioclase feldspar. Calcium is replaced with sodium during cooling.

These minerals react continuously with the magma to adjust their compositions during cooling.

.Fig. 4.4, p. 89

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Bowen’s reaction series predicts that

olivine will form first at high temperatures and quartz (SiO²) is the last mineral to crystallize from a cooling magma.

.

Fig. 4.4, p. 89



The reverse of Bowen’s reaction series describes the melting of rock

The first mineral to melt would be quartz.

• followed by muscovite• followed by potassium

feldspar• followed by biotite and

Na-rich plagioclase feldspar

Fig. 4.4, p. 89

Fig. 4-4, p. 89Stepped Art

Reaction

Pyroxene(augite)

Reaction

Biotitemica

Reaction

Amphibole(hornblende)

Potassiumfeldspar

Muscovitemica

Quartz

Types ofmagma

Dec

reas

ing

tem

pera

ture

Discontinuous branch

Mafic(45–52% silica)

Con

tinuo

us re

actio

n se

ries

Sodium-richplagioclase

Cont

inuo

us b

ranc

h

Plag

iocl

ase

feld

spar

s

Calcium-richplagioclaseOlivine

Intermediate(53–65% silica)

Felsic(>65% silica)

Igneous activity and plate tectonics

Magmatic Magmatic geosystemsgeosystems::

Island arc plate subductionIsland arc plate subduction

Plate divergencePlate divergence

HotHot--spot volcanismspot volcanism

Continental plate subductionContinental plate subduction

GeomagmaticGeomagmatic systems of Earthsystems of EarthIgneous Rocks: Igneous activity vs plate tectonics How Does Magma

Originate and Change?

The Origin of Magma at Spreading Ridges.

Magmas that forms beneath spreading ridgesas a result of mantle plumes are mafic.

Fig 4.5, p. 92

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Mid-Atlantic Ridge, Iceland

PLATEDIVERGENCE

Basaltic extrusivesBasaltic intrusivesMid-ocean ridge

Partial meltingOf upper mantlePartial melting

Of upper mantle

Rising magmaRising magma

Plate divergence boundaryPlate divergence boundary

Igneous Rocks: Igneous activity vs plate tectonics, Plate Divergence Boundary

Basalticextrusives& intrusives


Subduction Zones and the Origin of Magma

Along subduction zones at convergent boundaries, partial melting of the subducting plate produces intermediate to felsic magmas.

Fig 4.5, p. 92

ISLAND ARCPLATE SUBDUCTION

Mafic to intermediateintrusives (plutonism)Mafic to intermediateextrusives (volcanism)Island arc

volcano

Subduction zone

OceaniclithosphereOceaniclithosphere

Island arc plate subductionIsland arc plate subduction

Igneous Rocks: Igneous activity vs plate tectonics,

Mafic to intermediate

intrusives& extrusives


The Origin of Magma at Spreading Ridges and Subduction Zones

Magmas accumulate in magma chambers A few kilometers beneath divergent boundaries Tens of kilometers beneath convergent boundaries.

Fig 4.5, p. 92

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Continental margin

volcano,Mt. Rainier, Washington CONTINENTAL

PLATE SUBDUCTIONMafic to felsic intrusivesMafic to felsic extrusives

Subductionzone

Continentalmargin volcano

Oceanic

lithosphere

Oceanic

lithosphere

Continental mantle

lithosphere

Continental mantle

lithosphere

Continental crust

Continental crustOceaniccrustOceaniccrust

Igneous Rocks: Igneous activity vs plate tectonics

Mafic to felsicextrusives& intrusives


CONTINENTALPLATE SUBDUCTIONMafic to felsic intrusivesMafic to felsic extrusives

Subductionzone

Continentalmargin volcano

Oceanic

lithosphere

Oceanic

lithosphere

Continental mantle

lithosphere

Continental mantle

lithosphere

Continental crust

Continental crust

OceaniccrustOceaniccrust



Mafic to felsicextrusives& intrusives


Hot Spots and the Origin of Magma.

Magmas can originate from hot spot activityRising plumes in the ocean create volcanic islands far from plate boundaries.Flood basalts, huge thick flat-lying layers of basalt, originate this way.

Hot Spot volcano, Hawaii

HOT-SPOTVOLCANISM

Basaltic extrusivesBasaltic intrusivesHot-spot volcano

Mantle plume(hot spot)Mantle plume(hot spot)

MantleMantle

HotHot--spot volcanismspot volcanism


Basalticextrusives& intrusives

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GeomagmaticGeomagmatic systems of Earthsystems of EarthIgneous Rocks: Igneous activity vs plate tectonics


Processes That Bring About Compositional Changes in Magma

1. Crystal settling - the first formed ferromagnesian minerals can settle to the base of a magma chamber or body, withdrawing some of the iron and magnesium and enriching the remaining magma in other elements. During crystallization, the remaining melt becomes progressively more silica-enriched.

Fig. 4.7a, p. 92

Olivinecrystals

~1200°C

Magma withcomposition A

Olivine crystallizesfirst.

Olivine

Plagioclasefeldspar

Pyroxene

Magma withcomposition B

Pyroxene and plagioclasefeldsparcrystallize.

Olivine

Plagioclasefeldspar

Pyroxene

Magma withcomposition C

A gradient ofpyroxene andfeldspar is established.

~600°C

Plagioclasefeldspar

Magma withcomposition C

Plagioclase feldspar continues to crystallize.

THE PALISADES INTRUSION

Igneous Rocks 4 Magmatic differentiation, exampleHow Does Magma Originate

and Change?

2. Assimilation of country rock, sometimes by stoping, can change a magma's composition, but usually only to a limited extent.

3. Magma mixing can also result in compositional change.

Two different magmas can blend together (mix) if their viscosities are very similar, or they can mingle without blending if their viscosities are very different.

Processes That Bring About Compositional Changes in Magma

Fig. 4.8, p. 93

Crystallizingminerals

Partial meltingof country rock

Basalticmagma

Magmachamber B

Magmachamber A

Partial melting creates a magma of a particular composition.

Cooling causes mineralsto crystallize and settle.

A basaltic magma chamber breaks through.

Magmachamber A

Mixing results in andesitic magma.

Crystals may accumulate on the sides and roof of the chamber due to turbulence.

Igneous Rocks 4 Magmatic differentiation Igneous RocksTheir Characteristics and Classification

Igneous Rock Textures

Minerals begin to crystallize from magma and lava after small crystal nuclei form and grow.

Two broad groups based on texture are: volcanic (extrusive) rocks which have an aphanitic textureplutonic (intrusive) rocks which have a phaneritic texture. Rocks with more complex cooling histories are characterized by porphyritic textures.

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Country rock

Igneous Rocks: Magmatic intrusions

Batholith intrusions

Country rock

Rising magma wedges open overlying country rock.


Country rock


The magma melts the surrounding rock...


Country rock



…which changes the magma’s composition.


Country rock



…which changes the magma’s composition.

The magma also breaks off xenoliths that sink into the magma.


Igneous Rock Textures

Rapid cooling typifies volcanic rock and produces aphanitic textures.

Slow cooling of plutonic magmas produces phaneritic textures with mineral grains that are easily visible without magnification.

Porphyritic textures are characteristic of rocks with complex cooling histories and contain mineral grains of different sizes.

Other igneous rock textures include vesicular, glassy, and pyroclastic.

Igneous RocksTheir Characteristics and Classification

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Fig. 4-9, p. 93Stepped Art

Phenocrysts

Igneous RocksTheir Characteristics and Classification

Composition of Igneous Rocks

Mg-Fe-rich magma is called “mafic,”Gabbros and basalts are

products of mafic magmas

Na, K, Al and H²O rich magma is “felsic.”Granites and rhyolites are products of felsic magmas

Most magma, like most minerals, consists of silicon and oxygen with lesser amounts of other elements, such as magnesium (Mg), iron (Fe), sodium (Na), potassium (K), and aluminum (Al).

Fig. 4.10, p. 93

Igneous Rocks Their Characteristics and Classification

Classifying Igneous Rocks

Depending on whether magma erupts, it can produceplutonic (deep-seated) or volcanic (eruptive) rocks.

It is best to learn the different kinds of igneous rocks as pairs of equivalent plutonic and volcanic compositions; that is, each plutonic rock has its volcanic compositional equivalent, and visa versa.

For example, gabbro is chemically equivalent to basalt.Gabbro is a plutonic rock. Basalt is a volcanic rock.


Composition of Igneous Rocks

After geologists identify a rock as plutonic or volcanic, the name it is given depends on the relative percentages of certain key minerals, such as quartz, orthoclase, plagioclase, olivine, and pyroxene. . Fig. 4.10, p. 93



Ultramafic RocksPeridotite is an ultramafic rock, meaning that it contains more iron and magnesium than basalt and gabbro which are mafic. Its composition is close to that of the upper mantle. Peridotite

Figure 4.11, p. 95



Gabbro/basalt, and granite/rhyolite are pairs of common rock types They represent two ends of the compositional range of most igneous rocks at Earth’s surface.

Gabbro/basalt are maficGranite/rhyolite are felsic

Fig. 4.12b, p. 95Fig. 4.14b, p. 96

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Basalt-Gabbro – Mafic compositionGabbro is chemically equivalent to basalt. Gabbro is a plutonic rock. Basalt is a volcanic rock.

Fig. 4.12, p. 95



Rhyolite-Granite – Felsic compositionGranite is chemically equivalent to rhyolite.Granite is a plutonic rock. Rhyolite is a volcanic rock.

Fig. 4.14, p. 96



Andesite-Diorite – Intermediate compositionDiorite is chemically equivalent to andesite.Diorite is a plutonic rock. Andesite is a volcanic rock.

Fig. 4.13, p. 96


Classifying Igneous RocksIt is best to learn the different kinds of igneous rocks as pairsof equivalent plutonic and volcanic compositions

Diorite

BasaltAndesite porphyry

Granite

Rhyolite porphyry

Gabbro Fig. 4.12, p. 95Fig. 4.13, p. 96Fig. 4.14, p. 96



PegmatitePegmatite is a rock type closely related to granite that contains many minerals not ordinarily found in other igneous rocks. By definition, pegmatites contain crystals that measure at least 1 cm across.Typically they form in water-rich magmas

Pegmatite

4.15b, p. 97


Classifying Igneous Rocks Other Igneous Rocks

TuffComposed of volcanic ash.

ObsidianComposed of volcanic glass.

Pumice & ScoriaComposed of volcanic glassVesicular

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Intrusive Igneous Bodies - Plutons

Plutons are bodies of igneous rock which have been intruded in country rock or have formed in place far beneath the surface.

Concordant plutons include sills and laccoliths. Discordant plutons include dikes, volcanic necks, batholiths and stocks. Each of these plutons is defined by volume and geometry.

Geo-inSight 1., p. 100

Forms of magmatic intrusion

Countryrock

Volcano

Lava flow Ash falls and pyroclasts

Volcanic neck withradiating dikes

StockStock

SillSillDik

eD

ike

BatholithBatholith

PlutonPluton

SillSill

SillSill Dik

eD

ike

Dik

eD

ike


Countryrock

Volcano



StockStock

SillSillDik

eD

ike

BatholithBatholith

PlutonPluton

SillSill

SillSill Dik

eD

ike

Dik

eD

ike

Dikes cut across layersof country rock…


Countryrock

Volcano



StockStock

SillSillDik

eD

ike

BatholithBatholith

PlutonPluton

SillSill

SillSill Dik

eD

ike

Dik

eD

ike


…but sills runparallel to them.


Countryrock

Volcano



StockStock

SillSillDik

eD

ike

BatholithBatholith

PlutonPluton

SillSill

SillSill Dik

eD

ike

Dik

eD

ike


…but sills runparallel to them.

Batholiths are the largest forms of plutons, covering at least 100 km2.


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Dikes and SillsDikes and sills are the most common sheet-like igneous intrusions.

Dikes are discordant features (meaning they cut across layering in the country rock)Sills are concordant (parallel to the rock layers).

Geo-inSight 5., p. 101 Igneous Rocks: Magmatic intrusions

Sill


Sill

A sill runs parallel tocountry rock layers.


Sill



Sill


DikeDike


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Sill


DikeDike

A dike cuts acrosslayers.


Sill

DikeDike


A dike cuts acrosslayers. DikeDike


Laccoliths, Volcanic pipes and necksLaccoliths are sill-like bodies with inflated cores.

Geo-inSight 6. and 7., p. 101


Laccoliths, Volcanic pipes and necksVolcanic pipes are magma-filled, cylindrical feeder channels beneath volcanoes.

Pipes can become volcanic necks with deep erosion.

Geo-inSight 3., p. 100


Laccoliths, Volcanic pipes and necks

Dikes, sills, and laccoliths radiate from many volcanic pipes and necks.

Geo-inSight 1 ., p. 100, Geo-Focus Fig. 2, p. 102


Batholiths and Stocks

Batholiths are plutons that have more than 100 km²in area of exposure. Stocks are somewhat smaller plutonic bodies.

Geo-inSight 2. & 4. , p. 100

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How Are Batholiths Intruded into Earth’s Crust?

Most are composed of granite. How did we get so much granite in the Earth’s crust?

Why are there so many batholiths?Why are they felsic (silica rich) if the source is mafic or ultramafic?

Ideas1. Assimilation – they melt their way into

the crustProblem: insufficient heat to create this much country rock

2. Forceful injection– magma moves the country rock aside and slowly risesPreferred idea

3. Stoping – at shallower depths, the crust is fractured as the magma moves up. Magma replaces the country rock as it moves up. Fig. 4.18, p. 99

EndEnd

chapter 4web.nmsu.edu/~dshearer/geology/04_ignrks/mod04-ignrocks-ho.pdf · chapter 4 igneous rocks...

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