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Introduction
™ Igneous rocks make up large parts of the continents
and all the oceanic crust - indicate plate boundaries
and hot spot activity
Chapter 4
Igneous Rocks
Granite
™Mostly quartz and
feldspar
™Used for kitchen
counters, facings on
buildings,
tombstones, etc
and Intrusive
Igneous Activity
How do igneous rocks differ
from one another?
How do igneous rocks differ
from one another?
Texture – size of crystals
CoarseCoarse-grained rocks
FineFine-grained rocks
Mixed texture rocks
How do igneous rocks differ
from one another?
Texture is related to rate of cooling.
Intrusive igneous rocks –
slow cooling – coarse 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 or
™Pyroclastic materials.
Extrusive igneous rocks –
rapid cooling – fine texture
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IGNEOUS ROCKS 1. Texture
Igneous textures
Pyroclasts
Volcanic ash
Bomb
Pumice
Pyroclasts
Extrusive rocks
Extrusive rocks
Porphyry
Porphyry
Intrusive rocks
Intrusive rocks
IGNEOUS ROCKS 1a. Texture
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Bomb
IGNEOUS ROCKS 1a. Texture
Pyroclasts
Volcanic ash
Extrusive rocks
Bomb
Mafic
Basalt
Pumice
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Felsic
Rhyolite
Porphyry
Pumice
Intrusive rocks
Volcanic ash
Pyroclasts
Extrusive rocks
IGNEOUS ROCKS 1a. Texture
Pyroclasts
Volcanic ash
Extrusive rocks
Porphyry
Intrusive rocks
Bomb
Mafic
Basalt
Pumice
Felsic
Rhyolite
IGNEOUS ROCKS 1a. Texture
Pyroclasts
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Volcanic ash
Extrusive rocks
2. Extrusive
igneous rocks cool
rapidly and are
fine-grained.
Bomb
Mafic
Pumice
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Felsic
Basalt
Rhyolite
Gabbro
Granite
Porphyry
2. Extrusive
igneous rocks cool
rapidly and are
fine-grained.
Intrusive rocks
2
IGNEOUS ROCKS 1a. Texture
Pyroclasts
IGNEOUS ROCKS 1a. Texture
Mafic
Volcanic ash
Extrusive rocks
Bomb
Pumice
Mafic
Basalt
Felsic
Rhyolite
Gabbro
Granite
Porphyry
Felsic
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
2. Extrusive
igneous rocks cool
rapidly and are
fine-grained.
2. Extrusive
igneous rocks cool
rapidly and are
fine-grained.
Basalt
3. Intrusive igneous
rocks cool slowly,
allowing large,
coarse crystals to
form.
Rhyolite
3. Intrusive igneous
rocks cool slowly,
allowing large,
coarse crystals to
form.
Intrusive rocks
Gabbro
IGNEOUS ROCKS 1a. Texture
Pyroclasts
Volcanic ash
Extrusive rocks
Bomb
Pumice
Mafic
Basalt
Felsic
Rhyolite
Gabbro
Granite
Porphyry
Intrusive rocks
IGNEOUS ROCKS 1a. Texture
Pyroclasts
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Volcanic ash
Extrusive rocks
2. Extrusive
igneous rocks cool
rapidly and are
fine-grained.
Mafic
Pumice
Intrusive rocks
1. Extrusive
pyroclasts
form in violent
eruptions from lava
in the air.
Felsic
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.
Porphyry
3. Intrusive igneous
rocks cool slowly,
allowing large,
coarse crystals to
form.
Phenocrysts
Bomb
Basalt
Phenocrysts
Porphyry
Porphyry
4. Some intrusive
porphyritic crystals
grow large, but the
remaining melt
cools faster,
forming smaller
crystals.
The Properties and Behavior of
Magma and Lava
IGNEOUS ROCKS 1a. Texture
Composition of Magma
Phenocrysts
Porphyry
Granite
4. Some intrusive
porphyritic crystals
grow large, but the
remaining melt
cools faster,
forming smaller
crystals.
™ Silica is the primary constituent of magmas
™ Magmas are differentiated based on the relative
proportions of silica, iron, and magnesium.
™Felsic – silica rich, iron and magnesium poor
™Intermediate – intermediate between felsic and
mafic
™Mafic – silica poor, iron and magnesium rich
™Parent magma composition largely determines the
composition of igneous rocks.
3
Igneous Rocks 1c. Chemical & Minerals
IGNEOUS ROCKS 1c. Chemical & Minerals
Chemical and mineral
composition of igneous rocks
Four compositional groups:
Felsic igneous rocks
Intermediate igneous rocks
Mafic igneous rocks
Ultramafic igneous rocks
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 1200ºC
™felsic magma is
somewhat hotter
than 900ºC.
Fig. 4.2, p. 87
The Properties and Behavior
of Magma and Lava
The Properties and Behavior
of Magma and Lava
Viscosity – Resistance to Flow
Viscosity – Resistance to Flow
™Viscosity measures a
substance’s resistance to
flow.
™Examples: Water,
syrup, mafic magmas
™Mafic magmas are
hotter and have less
silica. This makes them
flow better.
™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
Fig. 4.3 a, p. 88
4
The Properties and Behavior
of Magma and Lava
How do magmas form?
Why do rocks melt?
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 Does Magma
Originate and Change?
How do magmas form?
™Magma accumulates in reservoirs in the lower crust
or upper mantle in magma chambers
What is a magma chamber?
A rising blob of magma that
pushes aside crustal rocks as
it rises through the crust.
How Does Magma
Originate and Change?
™Molten rock is less dense than solid rock, and so it is
buoyant and begins to rise thru the lithosphere.
Magmatic differentiation
™Later the magma may stall out and
crystallize at a level of neutral buoyancy.
Magmatic Differentiation –
• A process by which rocks of
varying composition can arise
from a uniform parent magma.
™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.
• The first minerals to crystallize
from a cooling magma are the
ones that are the last to melt.
Fig. 4.7, p. 92
5
Magmatic differentiation
Magmatic differentiation
Fractional crystallization –
• the process by which the crystals
are formed in a cooling magma
and are segregated from the
remaining liquid.
THE PALISADES INTRUSION
• Example: basaltic intrusion like
Palisades, New Jersey
How Does Magma
Originate and Change?
Magmatic differentiation
Bowen’s reaction series
THE PALISADES INTRUSION
Basaltic intrusion
245–275 m (800–900 ft)
Sandstone
Basalt
Mostly sodium-rich
plagioclase feldspar;
no olivine
Bowen’s reaction series
describes the sequence of
mineral crystallization in a
cooling magma.
Calcium-rich
plagioclase feldspar
and pyroxene;
no olivine
Olivine
Basalt
There are two branches in
the reaction series.
Sandstone
™Discontinuous series
™Continuous series
Fig. 4.4, p. 89
How Does Magma
Originate and Change?
How Does Magma
Originate and Change?
Bowen’s reaction series
Bowen’s reaction series
The discontinuous series
produces 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 series
produces 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
6
How Does Magma
Originate and Change?
How Does Magma
Originate and Change?
Bowen’s reaction series
Bowen’s reaction series
The reverse of Bowen’s
reaction series describes
the melting of rock
™ 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.
™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
Decreasing
Reaction
Amphibole
(hornblende)
Co
Plag ntinuo
iocla us b
se fe ranc
ldsp h
Con
ars
tinu
ous
reac
tion
seri
es
ch
bran
ous
tinu
Pyroxene
(augite)
on
Disc
Reaction
temperature
Types of
magma
Calcium-rich
plagioclase
Olivine
Fig. 4.4, p. 89
Igneous activity and plate tectonics
Magmatic geosystems:
geosystems:
Mafic
(45–52% silica)
Island arc plate subduction
Intermediate
(53–65% silica)
Plate divergence
Reaction
Biotite
mica
Sodium-rich
plagioclase
Potassium
feldspar
HotHot-spot volcanism
Felsic
(>65% silica)
Muscovite
mica
Continental plate subduction
Quartz
Stepped Art
Fig. 4-4, p. 89
Igneous Rocks: Igneous activity vs plate tectonics
Geomagmatic systems of Earth
How Does Magma
Originate and Change?
The Origin of Magma at Spreading Ridges.
™Magmas that forms beneath spreading ridges
as a result of mantle plumes are mafic.
Fig 4.5, p. 92
7
Plate divergence boundary
Mid-Atlantic
Ridge, Iceland
PLATE
DIVERGENCE
Mid-ocean ridge
Basaltic extrusives
Basaltic intrusives
Basaltic
extrusives
& intrusives
Partial melting
Of upper mantle
Rising magma
Igneous Rocks: Igneous activity vs plate tectonics, Plate Divergence Boundary
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
How Does Magma
Originate and Change?
Island arc plate subduction
The Origin of Magma at Spreading Ridges
and Subduction Zones
ISLAND ARC
PLATE SUBDUCTION
Island arc
volcano
Mafic to intermediate
intrusives (plutonism)
Mafic to intermediate
extrusives (volcanism)
Mafic to
intermediate
intrusives
& extrusives
™Magmas accumulate in magma chambers
Subduction zone
Oceanic
lithosphere
Igneous Rocks: Igneous activity vs plate tectonics,
™A few kilometers beneath divergent boundaries
™Tens of kilometers beneath convergent
boundaries.
Fig 4.5, p. 92
8
Continental plate subduction
Continental
margin
volcano,
Mt. Rainier,
Washington
CONTINENTAL
PLATE SUBDUCTION
Mafic to felsic intrusives
Mafic to felsic extrusives
Mafic to felsic
extrusives
& intrusives
Continental
margin volcano
Subduction
zone
Co
nt
Co inen
nt
in tal
lit ent crus
ho al
sp ma t
he
re ntle
Oceanic
crust
ic ere
an h
ce sp
O tho
li
Igneous Rocks: Igneous activity vs plate tectonics
How Does Magma
Originate and Change?
Continental plate subduction
Hot Spots and the Origin of Magma.
CONTINENTAL
PLATE SUBDUCTION
Mafic to felsic intrusives
Mafic to felsic extrusives
Mafic to felsic
extrusives
& intrusives
Continental
margin volcano
Subduction
zone
Oceanic
crust
ic ere
an h
ce sp
O tho
li
Co
nt
Co inen
ta
nt
lc
in
ru
lit ent
st
ho al
sp ma
he
re ntle
Igneous Rocks: Igneous activity vs plate tectonics
Hot Spot volcano, Hawaii
™Magmas can originate from hot spot activity
™Rising plumes in the ocean create volcanic
islands far from plate boundaries.
™Flood basalts, huge thick flat-lying layers of
basalt, originate this way.
HotHot-spot volcanism
HOT-SPOT
VOLCANISM
Hot-spot volcano
Basaltic extrusives
Basaltic intrusives
Basaltic
extrusives
& intrusives
Mantle plume
(hot spot)
Mantle
Igneous Rocks: Igneous activity vs plate tectonics
9
How Does Magma
Originate and Change?
Igneous Rocks: Igneous activity vs plate tectonics
Geomagmatic systems of Earth
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 silicaenriched.
Fig. 4.7a, p. 92
Igneous Rocks 4 Magmatic differentiation, example
THE PALISADES INTRUSION
How Does Magma Originate
and Change?
Processes That Bring About Compositional
Changes in Magma
2. Assimilation of country rock,
~1200°C
~600°C
Plagioclase
feldspar
Plagioclase
feldspar
Olivine
crystals
Pyroxene
Pyroxene
Olivine
Magma with
composition A
Magma with
composition B
Olivine
crystallizes
first.
Pyroxene and
plagioclase
feldspar
crystallize.
Olivine
Plagioclase
feldspar
Magma with
composition C
Magma with
composition C
A gradient of
pyroxene and
feldspar is
established.
Plagioclase
feldspar
continues to
crystallize.
Igneous Rocks 4 Magmatic differentiation
Partial melting creates
a magma of a particular
composition.
Cooling causes minerals
to crystallize and settle.
Crystallizing
minerals
Magma
chamber A
Magma
chamber A
Magma
chamber B
Partial melting
of country rock
A basaltic magma
chamber breaks
through.
Mixing results
in andesitic
magma.
Basaltic
magma
Crystals may
accumulate on
the sides and
roof of the
chamber due
to turbulence.
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. Fig. 4.8, p. 93
Igneous Rocks
Their 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 texture
™plutonic (intrusive) rocks which have a
phaneritic texture.
™Rocks with more complex cooling histories are
characterized by porphyritic textures.
10
Igneous Rocks: Magmatic intrusions
Igneous Rocks: Magmatic intrusions
Rising magma
wedges open
overlying country
rock.
Country
rock
Country
rock
Batholith intrusions
Igneous Rocks: Magmatic intrusions
Rising magma
wedges open
overlying country
rock.
The magma melts
the surrounding
rock...
Country
rock
The magma melts
the surrounding
rock...
…which changes
the magma’s
composition.
Country
rock
Igneous Rocks: Magmatic intrusions
Rising magma
wedges open
overlying country
rock.
Igneous Rocks: Magmatic intrusions
Rising magma
wedges open
overlying country
rock.
The magma melts
the surrounding
rock...
…which changes
the magma’s
composition.
Igneous Rocks
Their Characteristics and Classification
Igneous Rock Textures
™ Rapid cooling typifies volcanic rock
and produces aphanitic textures.
Country
rock
™ Slow cooling of plutonic magmas
produces phaneritic textures with
mineral grains that are easily visible
without magnification.
The magma also
breaks off xenoliths
that sink into the
magma.
™ 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.
11
Igneous Rocks
Their Characteristics and Classification
Composition of Igneous Rocks
Phenocrysts
™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).
Stepped Art
Fig. 4.10, p. 93
Fig. 4-9, p. 93
Igneous Rocks
Their Characteristics and Classification
Igneous Rocks
Their Characteristics and Classification
Composition of Igneous Rocks
Classifying Igneous Rocks
Depending on whether magma erupts, it can produce
™plutonic (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.
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.
.
Igneous Rocks
Their Characteristics and Classification
Igneous Rocks
Their Characteristics and Classification
Classifying Igneous Rocks
Classifying Igneous Rocks
Ultramafic Rocks
™Peridotite 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.
Fig. 4.10, p. 93
Peridotite
™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 mafic
™Granite/rhyolite are felsic
Fig. 4.12b, p. 95
Fig. 4.14b, p. 96
Figure 4.11, p. 95
12
Igneous Rocks
Their Characteristics and Classification
Igneous Rocks
Their Characteristics and Classification
Classifying Igneous Rocks
Classifying Igneous Rocks
Basalt-Gabbro – Mafic composition
Rhyolite-Granite – Felsic composition
™Gabbro is chemically equivalent to basalt.
™Gabbro is a plutonic rock. Basalt is a volcanic rock.
™Granite is chemically equivalent to rhyolite.
™Granite is a plutonic rock. Rhyolite is a volcanic rock.
Fig. 4.12, p. 95
Fig. 4.14, p. 96
Igneous Rocks
Their Characteristics and Classification
Classifying Igneous Rocks
Igneous Rocks
Their Characteristics and Classification
It is best to learn the different kinds of igneous rocks as pairs
of equivalent plutonic and volcanic compositions
Classifying Igneous Rocks
Andesite-Diorite – Intermediate composition
™Diorite is chemically equivalent to andesite.
™Diorite is a plutonic rock. Andesite is a volcanic rock.
Rhyolite porphyry
Andesite porphyry
Basalt
Diorite
Gabbro
Granite
Fig. 4.13, p. 96
Igneous Rocks
Their Characteristics and Classification
Igneous Rocks
Their Characteristics and Classification
Classifying Igneous Rocks
Other Igneous Rocks
Classifying Igneous Rocks
Pegmatite
™Pegmatite 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 waterrich magmas
Fig. 4.12, p. 95
Fig. 4.13, p. 96
Fig. 4.14, p. 96
Tuff
™Composed of volcanic ash.
Obsidian
™Composed of volcanic glass.
Pumice & Scoria
Pegmatite
™Composed of volcanic glass
™Vesicular
4.15b, p. 97
13
Intrusive Igneous Bodies - Plutons
Forms of magmatic intrusion
™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
Igneous Rocks: Magmatic intrusions
Volcanic neck with
radiating dikes
Stock
Ash falls and pyroclasts
Volcano
Volcanic neck with
radiating dikes
Stock
Dike
Sill
Sill
Sill
Sill
to n
Plu
to n
Plu
lith
tho
Ba
lith
tho
Ba
Igneous Rocks: Magmatic intrusions
Lava flow
Sill
Volcanic neck with
radiating dikes
Sill
Dikes cut
across layers
of country
rock…
Dik
e
Sill
Sill
Ash falls and pyroclasts
Volcano
Stock
Dik e
Stock
Country
rock
Dik
e
Volcanic neck with
radiating dikes
Sill
to n
Plu
lith
tho
Ba
Dikes cut
across layers
of country
rock…
Sill
Dik
e
Volcano
Dike
Country
rock
Igneous Rocks: Magmatic intrusions
Ash falls and pyroclasts
Dike
Lava flow
Dikes cut
across layers
of country
rock…
Sill
Dik
e
Dike
Sill
Country
rock
Dik
e
Volcano
Lava flow
Dik e
Country
rock
Igneous Rocks: Magmatic intrusions
Ash falls and pyroclasts
Dik
e
Lava flow
to n
Plu
…but sills run
parallel to them.
lith
tho
Ba
…but sills run
parallel to them.
Batholiths are the largest
forms of plutons, covering
at least 100 km2.
14
Intrusive Igneous Bodies - Plutons
™Dikes and Sills
Dikes 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
A sill runs parallel to
country rock layers.
Sill
Igneous Rocks: Magmatic intrusions
A sill runs parallel to
country rock layers.
Sill
Sill
Igneous Rocks: Magmatic intrusions
A sill runs parallel to
country rock layers.
Sill
Dike
Igneous Rocks: Magmatic intrusions
Igneous Rocks: Magmatic intrusions
15
A sill runs parallel to
country rock layers.
Sill
Sill
A dike cuts across
layers.
A dike cuts across
layers.
Dike
Dike
Dike
Igneous Rocks: Magmatic intrusions
Igneous Rocks: Magmatic intrusions
Intrusive Igneous Bodies - Plutons
Intrusive Igneous Bodies - Plutons
Laccoliths, Volcanic pipes and necks
Laccoliths, Volcanic pipes and necks
™Laccoliths are sill-like bodies with inflated cores.
™Volcanic pipes are magma-filled, cylindrical feeder
channels beneath volcanoes.
™ Pipes can become volcanic necks with deep erosion.
Geo-inSight 6. and 7., p. 101
Intrusive Igneous Bodies - Plutons
Laccoliths,
Volcanic pipes and necks
Geo-inSight 3., p. 100
Intrusive Igneous Bodies - Plutons
Batholiths and Stocks
Dikes, sills, and laccoliths radiate from many
volcanic pipes and necks.
Geo-inSight 1 ., p. 100, Geo-Focus Fig. 2, p. 102
™Batholiths are plutons that have more than 100 km²
in area of exposure.
™Stocks are somewhat smaller plutonic bodies.
Geo-inSight 2. & 4. , p. 100
16
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?
End
™ Why are there so many batholiths?
™ Why are they felsic (silica rich) if the
source is mafic or ultramafic?
Ideas
1. Assimilation – they melt their way into
the crust
Problem: insufficient heat to create
this much country rock
2.
Forceful injection– magma moves the
country rock aside and slowly rises
Preferred 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
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