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1
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
How do igneous rocks differ How do igneous rocks differ
from one another?from one another?
How do igneous rocks differ How do igneous rocks differ
from one another?from one another?
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.
2
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.
IGNEOUS ROCKS 1a. Texture
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.
IGNEOUS ROCKS 1a. Texture
Intrusive rocks
PorphyryPorphyry
Pyroclasts
Extrusive rocks
Volcanic ash Bomb Pumice1. Extrusive pyroclastsform in violent eruptions from lavain the air.
Mafic Felsic
Basalt Rhyolite 2. Extrusive igneous rocks cool rapidly and are fine-grained.
Gabbro Granite
IGNEOUS ROCKS 1a. Texture
3
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.
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
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.
Gabbro Granite 3. Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form.
Phenocrysts
Porphyry
IGNEOUS ROCKS 1a. Texture
Intrusive rocks
PorphyryPorphyry
Pyroclasts
Extrusive rocks
Volcanic ash Bomb Pumice1. Extrusive pyroclastsform in violent eruptions from lavain the air.
Mafic 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.
Phenocrysts
Porphyry
4. Some intrusive porphyritic crystals grow large, but theremaining melt cools faster, forming smaller crystals.
IGNEOUS ROCKS 1a. Texture
IGNEOUS ROCKS 1a. Texture
Porphyry
4. Some intrusive porphyritic crystals grow large, but theremaining melt cools faster, forming smaller crystals.
Phenocrysts
The Properties and Behavior of Magma and Lava
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.
4
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
The Properties and Behavior of Magma and Lava
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
The Properties and Behavior of Magma and Lava
Viscosity – Resistance to Flow
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
5
The Properties and Behavior of Magma and Lava
Viscosity – Resistance to Flow
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.
How Does Magma Originate and Change?
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.
6
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
How Does Magma Originate and Change?
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
How Does Magma Originate and Change?
Bowen’s reaction series
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
How Does Magma Originate and Change?
Bowen’s reaction series
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
7
How Does Magma Originate and Change?
Bowen’s reaction series
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
How Does Magma Originate and Change?
Bowen’s reaction series
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
8
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
How Does Magma Originate and Change?
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
How Does Magma Originate and Change?
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
9
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
Continental plate subductionContinental plate subduction
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
Continental plate subductionContinental plate subduction
Igneous Rocks: Igneous activity vs plate tectonics
Mafic to felsicextrusives& intrusives
How Does Magma Originate and Change?
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
Igneous Rocks: Igneous activity vs plate tectonics
Basalticextrusives& intrusives
10
GeomagmaticGeomagmatic systems of Earthsystems of EarthIgneous Rocks: Igneous activity vs plate tectonics
How Does Magma Originate and Change?
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.
11
Country rock
Igneous Rocks: Magmatic intrusions
Batholith intrusions
Country rock
Rising magma wedges open overlying country rock.
Igneous Rocks: Magmatic intrusions
Country rock
Rising magma wedges open overlying country rock.
The magma melts the surrounding rock...
Igneous Rocks: Magmatic intrusions
Country rock
Rising magma wedges open overlying country rock.
The magma melts the surrounding rock...
…which changes the magma’s composition.
Igneous Rocks: Magmatic intrusions
Country rock
Rising magma wedges open overlying country rock.
The magma melts the surrounding rock...
…which changes the magma’s composition.
The magma also breaks off xenoliths that sink into the magma.
Igneous Rocks: Magmatic intrusions
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
12
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.
Igneous Rocks Their Characteristics and Classification
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
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
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
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
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
13
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
Basalt-Gabbro – Mafic compositionGabbro is chemically equivalent to basalt. Gabbro is a plutonic rock. Basalt is a volcanic rock.
Fig. 4.12, p. 95
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
Rhyolite-Granite – Felsic compositionGranite is chemically equivalent to rhyolite.Granite is a plutonic rock. Rhyolite is a volcanic rock.
Fig. 4.14, p. 96
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
Andesite-Diorite – Intermediate compositionDiorite is chemically equivalent to andesite.Diorite is a plutonic rock. Andesite is a volcanic rock.
Fig. 4.13, p. 96
Igneous Rocks Their Characteristics and Classification
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
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks
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
Igneous Rocks Their Characteristics and Classification
Classifying Igneous Rocks Other Igneous Rocks
TuffComposed of volcanic ash.
ObsidianComposed of volcanic glass.
Pumice & ScoriaComposed of volcanic glassVesicular
14
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
Igneous Rocks: Magmatic intrusions
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
Dikes cut across layersof country rock…
Igneous Rocks: Magmatic intrusions
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
Dikes cut across layersof country rock…
…but sills runparallel to them.
Igneous Rocks: Magmatic intrusions
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
Dikes cut across layersof country rock…
…but sills runparallel to them.
Batholiths are the largest forms of plutons, covering at least 100 km2.
Igneous Rocks: Magmatic intrusions
15
Intrusive Igneous Bodies - Plutons
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
Igneous Rocks: Magmatic intrusions
Sill
A sill runs parallel tocountry rock layers.
Igneous Rocks: Magmatic intrusions
Sill
A sill runs parallel tocountry rock layers.
Igneous Rocks: Magmatic intrusions
Sill
A sill runs parallel tocountry rock layers.
DikeDike
Igneous Rocks: Magmatic intrusions
16
Sill
A sill runs parallel tocountry rock layers.
DikeDike
A dike cuts acrosslayers.
Igneous Rocks: Magmatic intrusions
Sill
DikeDike
Igneous Rocks: Magmatic intrusions
A dike cuts acrosslayers. DikeDike
Intrusive Igneous Bodies - Plutons
Laccoliths, Volcanic pipes and necksLaccoliths are sill-like bodies with inflated cores.
Geo-inSight 6. and 7., p. 101
Intrusive Igneous Bodies - Plutons
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
Intrusive Igneous Bodies - Plutons
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
Intrusive Igneous Bodies - Plutons
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
17
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