Chapter 4,Igneous Rocks,
Product of Earth’s Internal Fire
Introduction,What Is an Igneous Rock?
? Igneous rocks vary greatly.
? Some contain large mineral grains.
? Others contain grains so small they can
barely be seen under a high power
microscope.
? Igneous rocks also vary greatly in color.
? All igneous rocks are formed through the
cooling and solidification of magma.
Intrusive Versus Extrusive Igneous Rocks
? Intrusive igneous rocks form when
magma cools within existing rocks in
Earth’s crust.
? Extrusive igneous rocks form when
magma cools on Earth’s surface,where
they have been,extruded.”
Texture In Igneous Rocks
The two most obvious textural features of
an igneous rock are the size of its mineral
grains and how the mineral grains are
packed together.
Sizes of mineral grains:
? Intrusive rocks are coarse-grained.
? Magma that solidifies in the crust cools
slowly and has sufficient time to form large
mineral grains.
? Extrusive rocks are fine-grained
? Magma that solidifies on the surface
usually cools rapidly,allowing
insufficient time for large crystals to
grow.
? Coarse-grained igneous rock is called a
phanerite 显晶岩 (from the Greek word
meaning visible).
Texture In Igneous Rocks
? Igneous rock that contains unusually
large mineral grains (2cm or larger)
is called a pegmatite 伟晶岩,
? Fine-grained igneous rock is called
an aphanite隐晶岩 (from the Greek
word meaning invisible).
Texture In Igneous Rocks
Texture In Igneous Rocks
? The isolated large grains are phenocrysts 斑
晶,
? A porphyry is an igneous rock in which
50% or more of the rock is coarse mineral
grains scattered through a mixture of fine
mineral grains.
Texture In Igneous Rocks
? Glassy rocks.
? Atoms lack time to organize themselves into
minerals.
? A mineraloid forms instead (mineral-like
solid that lacks either a crystal structure or a
definite composition or both).
? Extrusive igneous rocks that are largely or
wholly glassy are called obsidian 黑矅石,
? They display a distinctive conchoidal 贝壳状
fracture (smooth,curved surface).
Glassy Rock,Obsidian from the Jemez Mountains
Mineral Assemblage In Igneous Rocks
? Once the texture of an igneous rock is
determined,its name will depend on its mineral
assemblage,
? All common igneous rocks consist largely of:
? Quartz,
? Feldspar (both potassium feldspar and
plagioclase).
? Mica (both muscovite and biotite).
? Amphibole.
? Pyroxene.
? Olivine.
Color
? The overall lightness or darkness of a rock is a
valuable indicator of its makeup.
? Light-colored rocks are:
? Quartz,
? Feldspar,
? Muscovite.
? Dark-colored rocks are:
? Biotite,
? Amphibole,
? Pyroxene.
Intrusive (Coarse-grained)
Igneous Rocks (1)
? Granite is quartz-bearing rock in which
potassium feldspar is at least 65 percent by
volume of the total feldspar present.
? Granodiorite is quartz-bearing rock in
which plagioclase is 65 percent or more of
the total feldspar present.
Naming Igneous Rock
Intrusive (Coarse-grained)
Igneous Rocks (2)
Granitic rocks include both granite and
granodiorite.
Granitic rocks are only found in the continental
crust.
Granitic magma forms when continental crust is
heated to its melting temperature,
The most common place where such high
temperatures are reached is in the deeper
portions of mountain belts formed by the
collision of two masses of continental crust.
Intrusive (Coarse-grained)
Igneous Rocks (3)
? Diorite:
? The chief mineral in diorite is plagioclase.
? Either or both amphibole and pyroxene
are invariably present.
? Forms in the same way as granite and
granodiorite.
? It is found only in continental crust.
Intrusive (Coarse-grained)
Igneous Rocks (4)
? Dark-colored diorite grades into gabbro.
— In gabbro,dark-colored minerals pyroxene and
olivine exceed 50 percent of the volume of the
rock.
? A coarse-grained igneous rock in which
olivine is the most abundant mineral is
called a peridotite.
? Gabbros and peridodites can be found in
both the oceanic and the continental crust.
Extrusive (Fine-Grained)
Igneous Rocks (1)
? Rhyolites and dacites are quartz-bearing.
? Rhyolites contain a predominance of
potassium feldspar.
? Dacites contain a predominance of
plagioclase.
? Dacites can only be distinguished from
rhyolites through microscopic examination.
Granite Rhyolite
Extrusive (Fine-Grained)
Igneous Rocks (2)
? Andesite:
? An igneous rock similar in appearance to a dacite,
but lacking quartz.
? Named for the Andes,
? Basalt:
? Compositionally equivalent to coarse-grained
gabbro,fine-grained basalt is the most common kind
of extrusive igneous rock,
? The dominant rock of the oceanic crust.
Diorite Andesite
Figure 4.7 B
Pyroclasts,Tephra,And Tuffs (1)
? A fragment of rock ejected during a
volcanic eruption is called a pyroclast.
? Rocks formed from pyroclasts are
pyroclastic rocks.
? Geologists commonly refer to a deposit of
pyroclasts as tephra,a Greek name for ash.
? Tephra is a collective term for all airborne
pyroclasts.
Pyroclasts,Tephra,And Tuffs (2)
? Tephra is igneous when it goes up but
sedimentary when it comes down.
? Tephra particles are categorized by size:
? Bombs,greater than 64 mm in diameter
? Lapilli 火山砾, between 2 and 64 mm
? Ash,smaller than 2 mm.
Gabbro Basalt
Pyroclasts,Tephra,And Tuffs (3)
? Pyroclastic rocks are transitional between
igneous and sedimentary rocks.
? When bomb-sized tephra are transformed
into a rock they are called agglomerates.
? They are called tuffs when particles are
either lapilli or ash.
Lapilli,Intermediate sized tephra
Pyroclasts,Tephra,And Tuffs (4)
Tephra can be converted into pyroclastic
rock in two ways:
Through the addition of a cementing
agent,such as quartz or calcite,
introduced by groundwater.
Through the welding of hot,glassy,ash
particles.
? Welded tuff.
Plutons
? All bodies of intrusive igneous rock,
regardless of shape or size,are called
plutons,after Pluto,the Greek god of the
underworld.
? Plutons are given special names depending
on their shapes and sizes.
Volcanic neck
Plutons
Minor Plutons,Dikes,Sills,and Laccoliths
? A dike is a tabular,sheet-like (thin but
laterally extensive) body of igneous rock that
cuts across the layering or fabric of the rock
into which it intrudes,
? A Sill is tabular and sheet-like,like a dike,
but runs parallel to the layering or fabric of
the rocks into which it intrudes.
Minor Plutons,Dikes,Sills,and Laccoliths
? A laccolith is parallel to the layering of the
rocks into which it intrudes,but forces the
layers of rock above it to bend,forming a
dome,
? A volcanic pipe is the roughly cylindrical
conduit that once fed magma upward to a
volcanic vent,
Major Plutons
? A batholith is the largest kind of pluton,It is an
intrusive igneous body of irregular shape that
cuts across the layering or other fabric of the
rock into which it intrudes.
? The largest batholith in North America,
approximately 1500 km long,is the Coast
Range batholith of British Columbia and
southern Alaska.
? The magma from which a batholith forms
intrudes upward from its source deep in the
continental crust.
Giant intrusions
Xenoliths and Stocks
? Rising magma can dislodge fragments of the
overlying rock,and the dislodged blocks,being
cooler and more dense than the magma,sink,
This process,called stoping,can produce
xenoliths,
? Any rock fragment still enclosed in a
magmatic body when it solidifies is a
xenolith.
? Stocks are irregularly shaped intrusives no
larger than 10 km in maximum dimension.
Locations of Volcanoes
Distribution of Volcanoes (1)
? Rhyolitic magma:
? Volcanoes that erupt rhyolitic magma are
abundant on the continental crust.
? The process that forms rhyolitic magma
does not occur in oceanic crust.
? The process that form rhyolitic magma must
be restricted to continental-type crust
Distribution of Volcanoes (2)
? Andesitic magma:
? Volcanoes that erupt andesitic magma occur
on both oceanic and continental crust.
? A line around the Pacific separates andesitic
volcanoes from those that erupt only
basaltic lava.
? This Andesite Line is generally parallel to
the plate subduction margins.
Where Andesites are Found
? Basaltic magma:
? Volcanoes that erupt basaltic magma occur on
oceanic crust
? The source of basaltic magma,therefore,must
be the mantle.
? Everywhere along the midocean ridges,
volcanoes erupt basaltic magma.
? Some large basaltic volcanoes are not located
along midocean ridges,
? The Hawaiian volcanic chain,formed over the
past 70 million years as the Pacific Plate moved
slowly northwestward across a midplate hot spot.
Distribution of Volcanoes (3)
Origin of Basaltic Magma (1)
? When discussing the origin of basaltic magma,
geologists ask:
? Was the rock that melted to form the magma
wet or dry?
The presence of water lowers the temperature at
which melting begins.
? What kind of rock melted?
The kind of rock that melts controls the
composition of the magma that forms.
? Did the rock melt completely or only partially?
Origin of Basaltic Magma (2)
? The process of forming magma through the
incomplete melting of rock is known as chemical
differentiation by partial melting.
? Basaltic magma is probably either a dry or a water-
poor magma,
? Olivine,pyroxene,and plagioclase do not contain
water in their formula.
? Water content of basaltic magma rarely exceeds 0.2
percent.
? The process must occur in the mantle.
Origin of Basaltic Magma (3)
? Laboratory experiments on the dry
partial-melting properties of garnet
peridotite show that,at asthenospheric
pressures and temperatures (100 km
deep),a 5 to 10 percent partial melts has
a basaltic composition,
? The upper portion of the mantle contains
garnet peridotites.
The effect of pressure on melting
Fractional melting
Origin of Andesitic Magma (1)
? Andesitic magma is close to the average
composition of continental crust.
? Igneous rocks formed from andesitic
magma commonly occur in the
continental crust.
? It is likely that andesitic magma forms by
the complete melting of a portion of the
continental crust.
Origin of Andesitic Magma (2)
? In the laboratory,wet partial melting of mantle
rock under suitably high pressure yields a
magma of andesitic composition.
? Andesitic magma can be extruded from
volcanoes that are far from the continental crust.
? When a moving plate of lithosphere plunges
back into the asthenosphere,it carries with it a
capping of basaltic oceanic crust saturated with
seawater.
Origin of Andesitic Magma (3)
? Wet partial melting that starts at a
pressure that is equivalent to a depth of
about 80 km produces a melt having the
composition of andesitic magma.
? The andesitic line corresponds closely
with plate subduction margins.
Volcanoes
in Japan
Origin of Rhyolitic Magma (1)
? Volcanoes that extrude rhyolitic magma are
confined to the continental crust or to regions
of andesitic volcanism.
? Volcanoes that extrude rhyolitic magma give
off a great deal of water vapor.
? Intrusive igneous rocks formed from rhyolitic
magma (granite) contain significant quantities
of OH-bearing (hydrous) minerals,such as mica
and amphibole.
Origin of Rhyolitic Magma (2)
? The generation of rhyolitic magma probably
involves some sort of wet partial melting of
rock having the composition of andesite,
? Once a rhyolitic magma has formed,it starts to
rise,However,the magma rises slowly because
it is very viscous,with a high SiO2 content (70
percent),
? Most rhyolitic magma solidifies underground
and forms granitic batholiths.
Solidification of Magma (1)
? A magma of a given composition can
crystallize into many different kinds of
igneous rock.
? Solidifying magma forms several
different minerals which start to
crystallize from the cooling magma at
different temperatures.
Solidification of Magma (2)
? Crystal-melt separation can occur in a number of
ways:
? Compression can squeeze melt out of a crystal-melt
mixture,
? Dense,early crystallized minerals may sink to the
bottom of a magma chamber,thereby forming a solid
mineral layer covered by melt.
? However a separation occurs,the compositional
changes it causes are called magmatic
differentiation by fractional crystallization.
Bowen’s Reaction Series (1)
? Canadian-born scientist N,L,Bowen
(1887-1956) first recognized the
importance of magmatic differentiation by
fractional crystallization.
? Bowen argued that a single magma could
crystallize into both basalt and rhyolite
because of fractional crystallization.
Bowen’s Reaction Series (2)
? Bowen knew that plagioclases that crystallize
from basaltic magma are usually calcium-rich
(anorthitic 钙长石 ).
? Plagioclases formed from rhyolitic magma are
commonly sodium-rich (albitic).
? Bowen called such a continuous reaction
between crystals and melts a continuous
reaction series.
Bowen’s Reaction Series (3)
? Bowen identified several sequences of reactions
besides the continuous reaction series of the
feldspars.
? When basalt cools down,one of the earliest
minerals to form is olivine.
? Olivine contains about 40 percent SiO2 by weight.
? Basaltic magma contains 50 percent SiO2.
? Crystallization of olivine will leave the residual
liquid a little richer in silica.
Continuous reaction,center are anorthite,progressing
to albite at the rim
A grain of olivine in a gabbro surrounded by reaction rims
of pyroxene and amphibole,discontinuous reaction
O
P
A
Bowen’s Reaction Series (4)
? The solid olivine reacts with silica in the
melt to form a more silica-rich mineral,
pyroxene.
? The pyroxene in turn can react to form
amphibole.
? Amphibole can react to form biotite.
? Such a series of reactions is called a
discontinuous reaction series.
Product of Earth’s Internal Fire
Introduction,What Is an Igneous Rock?
? Igneous rocks vary greatly.
? Some contain large mineral grains.
? Others contain grains so small they can
barely be seen under a high power
microscope.
? Igneous rocks also vary greatly in color.
? All igneous rocks are formed through the
cooling and solidification of magma.
Intrusive Versus Extrusive Igneous Rocks
? Intrusive igneous rocks form when
magma cools within existing rocks in
Earth’s crust.
? Extrusive igneous rocks form when
magma cools on Earth’s surface,where
they have been,extruded.”
Texture In Igneous Rocks
The two most obvious textural features of
an igneous rock are the size of its mineral
grains and how the mineral grains are
packed together.
Sizes of mineral grains:
? Intrusive rocks are coarse-grained.
? Magma that solidifies in the crust cools
slowly and has sufficient time to form large
mineral grains.
? Extrusive rocks are fine-grained
? Magma that solidifies on the surface
usually cools rapidly,allowing
insufficient time for large crystals to
grow.
? Coarse-grained igneous rock is called a
phanerite 显晶岩 (from the Greek word
meaning visible).
Texture In Igneous Rocks
? Igneous rock that contains unusually
large mineral grains (2cm or larger)
is called a pegmatite 伟晶岩,
? Fine-grained igneous rock is called
an aphanite隐晶岩 (from the Greek
word meaning invisible).
Texture In Igneous Rocks
Texture In Igneous Rocks
? The isolated large grains are phenocrysts 斑
晶,
? A porphyry is an igneous rock in which
50% or more of the rock is coarse mineral
grains scattered through a mixture of fine
mineral grains.
Texture In Igneous Rocks
? Glassy rocks.
? Atoms lack time to organize themselves into
minerals.
? A mineraloid forms instead (mineral-like
solid that lacks either a crystal structure or a
definite composition or both).
? Extrusive igneous rocks that are largely or
wholly glassy are called obsidian 黑矅石,
? They display a distinctive conchoidal 贝壳状
fracture (smooth,curved surface).
Glassy Rock,Obsidian from the Jemez Mountains
Mineral Assemblage In Igneous Rocks
? Once the texture of an igneous rock is
determined,its name will depend on its mineral
assemblage,
? All common igneous rocks consist largely of:
? Quartz,
? Feldspar (both potassium feldspar and
plagioclase).
? Mica (both muscovite and biotite).
? Amphibole.
? Pyroxene.
? Olivine.
Color
? The overall lightness or darkness of a rock is a
valuable indicator of its makeup.
? Light-colored rocks are:
? Quartz,
? Feldspar,
? Muscovite.
? Dark-colored rocks are:
? Biotite,
? Amphibole,
? Pyroxene.
Intrusive (Coarse-grained)
Igneous Rocks (1)
? Granite is quartz-bearing rock in which
potassium feldspar is at least 65 percent by
volume of the total feldspar present.
? Granodiorite is quartz-bearing rock in
which plagioclase is 65 percent or more of
the total feldspar present.
Naming Igneous Rock
Intrusive (Coarse-grained)
Igneous Rocks (2)
Granitic rocks include both granite and
granodiorite.
Granitic rocks are only found in the continental
crust.
Granitic magma forms when continental crust is
heated to its melting temperature,
The most common place where such high
temperatures are reached is in the deeper
portions of mountain belts formed by the
collision of two masses of continental crust.
Intrusive (Coarse-grained)
Igneous Rocks (3)
? Diorite:
? The chief mineral in diorite is plagioclase.
? Either or both amphibole and pyroxene
are invariably present.
? Forms in the same way as granite and
granodiorite.
? It is found only in continental crust.
Intrusive (Coarse-grained)
Igneous Rocks (4)
? Dark-colored diorite grades into gabbro.
— In gabbro,dark-colored minerals pyroxene and
olivine exceed 50 percent of the volume of the
rock.
? A coarse-grained igneous rock in which
olivine is the most abundant mineral is
called a peridotite.
? Gabbros and peridodites can be found in
both the oceanic and the continental crust.
Extrusive (Fine-Grained)
Igneous Rocks (1)
? Rhyolites and dacites are quartz-bearing.
? Rhyolites contain a predominance of
potassium feldspar.
? Dacites contain a predominance of
plagioclase.
? Dacites can only be distinguished from
rhyolites through microscopic examination.
Granite Rhyolite
Extrusive (Fine-Grained)
Igneous Rocks (2)
? Andesite:
? An igneous rock similar in appearance to a dacite,
but lacking quartz.
? Named for the Andes,
? Basalt:
? Compositionally equivalent to coarse-grained
gabbro,fine-grained basalt is the most common kind
of extrusive igneous rock,
? The dominant rock of the oceanic crust.
Diorite Andesite
Figure 4.7 B
Pyroclasts,Tephra,And Tuffs (1)
? A fragment of rock ejected during a
volcanic eruption is called a pyroclast.
? Rocks formed from pyroclasts are
pyroclastic rocks.
? Geologists commonly refer to a deposit of
pyroclasts as tephra,a Greek name for ash.
? Tephra is a collective term for all airborne
pyroclasts.
Pyroclasts,Tephra,And Tuffs (2)
? Tephra is igneous when it goes up but
sedimentary when it comes down.
? Tephra particles are categorized by size:
? Bombs,greater than 64 mm in diameter
? Lapilli 火山砾, between 2 and 64 mm
? Ash,smaller than 2 mm.
Gabbro Basalt
Pyroclasts,Tephra,And Tuffs (3)
? Pyroclastic rocks are transitional between
igneous and sedimentary rocks.
? When bomb-sized tephra are transformed
into a rock they are called agglomerates.
? They are called tuffs when particles are
either lapilli or ash.
Lapilli,Intermediate sized tephra
Pyroclasts,Tephra,And Tuffs (4)
Tephra can be converted into pyroclastic
rock in two ways:
Through the addition of a cementing
agent,such as quartz or calcite,
introduced by groundwater.
Through the welding of hot,glassy,ash
particles.
? Welded tuff.
Plutons
? All bodies of intrusive igneous rock,
regardless of shape or size,are called
plutons,after Pluto,the Greek god of the
underworld.
? Plutons are given special names depending
on their shapes and sizes.
Volcanic neck
Plutons
Minor Plutons,Dikes,Sills,and Laccoliths
? A dike is a tabular,sheet-like (thin but
laterally extensive) body of igneous rock that
cuts across the layering or fabric of the rock
into which it intrudes,
? A Sill is tabular and sheet-like,like a dike,
but runs parallel to the layering or fabric of
the rocks into which it intrudes.
Minor Plutons,Dikes,Sills,and Laccoliths
? A laccolith is parallel to the layering of the
rocks into which it intrudes,but forces the
layers of rock above it to bend,forming a
dome,
? A volcanic pipe is the roughly cylindrical
conduit that once fed magma upward to a
volcanic vent,
Major Plutons
? A batholith is the largest kind of pluton,It is an
intrusive igneous body of irregular shape that
cuts across the layering or other fabric of the
rock into which it intrudes.
? The largest batholith in North America,
approximately 1500 km long,is the Coast
Range batholith of British Columbia and
southern Alaska.
? The magma from which a batholith forms
intrudes upward from its source deep in the
continental crust.
Giant intrusions
Xenoliths and Stocks
? Rising magma can dislodge fragments of the
overlying rock,and the dislodged blocks,being
cooler and more dense than the magma,sink,
This process,called stoping,can produce
xenoliths,
? Any rock fragment still enclosed in a
magmatic body when it solidifies is a
xenolith.
? Stocks are irregularly shaped intrusives no
larger than 10 km in maximum dimension.
Locations of Volcanoes
Distribution of Volcanoes (1)
? Rhyolitic magma:
? Volcanoes that erupt rhyolitic magma are
abundant on the continental crust.
? The process that forms rhyolitic magma
does not occur in oceanic crust.
? The process that form rhyolitic magma must
be restricted to continental-type crust
Distribution of Volcanoes (2)
? Andesitic magma:
? Volcanoes that erupt andesitic magma occur
on both oceanic and continental crust.
? A line around the Pacific separates andesitic
volcanoes from those that erupt only
basaltic lava.
? This Andesite Line is generally parallel to
the plate subduction margins.
Where Andesites are Found
? Basaltic magma:
? Volcanoes that erupt basaltic magma occur on
oceanic crust
? The source of basaltic magma,therefore,must
be the mantle.
? Everywhere along the midocean ridges,
volcanoes erupt basaltic magma.
? Some large basaltic volcanoes are not located
along midocean ridges,
? The Hawaiian volcanic chain,formed over the
past 70 million years as the Pacific Plate moved
slowly northwestward across a midplate hot spot.
Distribution of Volcanoes (3)
Origin of Basaltic Magma (1)
? When discussing the origin of basaltic magma,
geologists ask:
? Was the rock that melted to form the magma
wet or dry?
The presence of water lowers the temperature at
which melting begins.
? What kind of rock melted?
The kind of rock that melts controls the
composition of the magma that forms.
? Did the rock melt completely or only partially?
Origin of Basaltic Magma (2)
? The process of forming magma through the
incomplete melting of rock is known as chemical
differentiation by partial melting.
? Basaltic magma is probably either a dry or a water-
poor magma,
? Olivine,pyroxene,and plagioclase do not contain
water in their formula.
? Water content of basaltic magma rarely exceeds 0.2
percent.
? The process must occur in the mantle.
Origin of Basaltic Magma (3)
? Laboratory experiments on the dry
partial-melting properties of garnet
peridotite show that,at asthenospheric
pressures and temperatures (100 km
deep),a 5 to 10 percent partial melts has
a basaltic composition,
? The upper portion of the mantle contains
garnet peridotites.
The effect of pressure on melting
Fractional melting
Origin of Andesitic Magma (1)
? Andesitic magma is close to the average
composition of continental crust.
? Igneous rocks formed from andesitic
magma commonly occur in the
continental crust.
? It is likely that andesitic magma forms by
the complete melting of a portion of the
continental crust.
Origin of Andesitic Magma (2)
? In the laboratory,wet partial melting of mantle
rock under suitably high pressure yields a
magma of andesitic composition.
? Andesitic magma can be extruded from
volcanoes that are far from the continental crust.
? When a moving plate of lithosphere plunges
back into the asthenosphere,it carries with it a
capping of basaltic oceanic crust saturated with
seawater.
Origin of Andesitic Magma (3)
? Wet partial melting that starts at a
pressure that is equivalent to a depth of
about 80 km produces a melt having the
composition of andesitic magma.
? The andesitic line corresponds closely
with plate subduction margins.
Volcanoes
in Japan
Origin of Rhyolitic Magma (1)
? Volcanoes that extrude rhyolitic magma are
confined to the continental crust or to regions
of andesitic volcanism.
? Volcanoes that extrude rhyolitic magma give
off a great deal of water vapor.
? Intrusive igneous rocks formed from rhyolitic
magma (granite) contain significant quantities
of OH-bearing (hydrous) minerals,such as mica
and amphibole.
Origin of Rhyolitic Magma (2)
? The generation of rhyolitic magma probably
involves some sort of wet partial melting of
rock having the composition of andesite,
? Once a rhyolitic magma has formed,it starts to
rise,However,the magma rises slowly because
it is very viscous,with a high SiO2 content (70
percent),
? Most rhyolitic magma solidifies underground
and forms granitic batholiths.
Solidification of Magma (1)
? A magma of a given composition can
crystallize into many different kinds of
igneous rock.
? Solidifying magma forms several
different minerals which start to
crystallize from the cooling magma at
different temperatures.
Solidification of Magma (2)
? Crystal-melt separation can occur in a number of
ways:
? Compression can squeeze melt out of a crystal-melt
mixture,
? Dense,early crystallized minerals may sink to the
bottom of a magma chamber,thereby forming a solid
mineral layer covered by melt.
? However a separation occurs,the compositional
changes it causes are called magmatic
differentiation by fractional crystallization.
Bowen’s Reaction Series (1)
? Canadian-born scientist N,L,Bowen
(1887-1956) first recognized the
importance of magmatic differentiation by
fractional crystallization.
? Bowen argued that a single magma could
crystallize into both basalt and rhyolite
because of fractional crystallization.
Bowen’s Reaction Series (2)
? Bowen knew that plagioclases that crystallize
from basaltic magma are usually calcium-rich
(anorthitic 钙长石 ).
? Plagioclases formed from rhyolitic magma are
commonly sodium-rich (albitic).
? Bowen called such a continuous reaction
between crystals and melts a continuous
reaction series.
Bowen’s Reaction Series (3)
? Bowen identified several sequences of reactions
besides the continuous reaction series of the
feldspars.
? When basalt cools down,one of the earliest
minerals to form is olivine.
? Olivine contains about 40 percent SiO2 by weight.
? Basaltic magma contains 50 percent SiO2.
? Crystallization of olivine will leave the residual
liquid a little richer in silica.
Continuous reaction,center are anorthite,progressing
to albite at the rim
A grain of olivine in a gabbro surrounded by reaction rims
of pyroxene and amphibole,discontinuous reaction
O
P
A
Bowen’s Reaction Series (4)
? The solid olivine reacts with silica in the
melt to form a more silica-rich mineral,
pyroxene.
? The pyroxene in turn can react to form
amphibole.
? Amphibole can react to form biotite.
? Such a series of reactions is called a
discontinuous reaction series.
