Transcript Chapter 4: Igneous Rocks: Product of Earth`s Internal Fire

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 (1)
 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.
Texture In Igneous Rocks (2)
 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 (from
the Greek word meaning visible).
 Igneous rock that contains unusually large mineral grains
(2cm or larger) is called a pegmatite.
 Fine-grained igneous rock is called an aphanite (from the
Greek word meaning invisible).
Texture In Igneous Rocks (3)
 The isolated large grains are phenocrysts.
 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 (4)
 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.
—They display a distinctive conchoidal fracture (smooth,
curved surface).
Figure 4.5
Texture In Igneous Rocks (5)
 Another common variety of glassy igneous rock is
pumice, a mass of glassy bubbles of volcanic
origin.
 Volcanic ash is also mostly glassy because the
fragments of magma cooled too quickly to
crystallize.
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:
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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.
Figure 4.6
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.
Figure 4.7 A
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
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 particles are categorized by size:
 Bombs: greater than 64 mm in diameter
 Lapilli: between 2 and 64 mm
 Ash: smaller than 2 mm.
 Tephra is igneous when it goes up but
sedimentary when it comes down.
Gabbro
Figure 4.7 C
Basalt
Pyroclasts, Tephra, And Tuffs (3)
 Pyroclastic rocks are transitional between igneous
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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.
Figure 4.8 B
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
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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.
Figure 4.10
Figure 4.11
Minor Plutons: Dikes, Sills, and
Laccoliths
 A dike is a tabular, sheet-like (thin but laterally
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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 (2)
 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.
Figure 4.14
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.
Figure 4.16
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 (including
those places in the ocean where new crust of
continental character is forming.
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.
Figure 4.17
Distribution of Volcanoes (3)
 Basaltic magma:
—Volcanoes
that erupt basaltic magma also occur on both
oceanic and continental 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 is
believed to have formed over the past 70 million years
as the Pacific Plate moved slowly northwestward across
a midplate hot spot.
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 waterpoor 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 partialmelting 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.
Figure B4.1
Figure B4.2
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
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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.
Figure 4.18
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
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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
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
from basaltic magma are usually calcium-rich
(anorthitic).
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.
Figure 4.19 A
Figure 4.19 B
Bowen’s Reaction Series (4)
 The solid olivine reacts with silica in the melt to
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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.
Figure 4.20
Valuable Magmatic Mineral Deposits
(1)
 The processes of partial melting and fractional

crystallization in magmas sometimes lead to
formation of large and potentially valuable
mineral deposits.
An important example of this kind of
concentration process is provided by pegmatites,
especially those formed through crystallization of
rhyolitic magma.
Valuable Magmatic Mineral Deposits
(2)
 Pegmatites may contain significant enrichments

of rare elements such as beryllium, tantalum,
niobium, uranium, and lithium.
Most of the world’s chromium ores were formed
in this manner by accumulation of the mineral
chromite (FeCr2O4).
Valuable Magmatic Mineral Deposits
(3)
 The largest known chromite deposits are in

South Africa, Zimbabwe,and the former Soviet
Union.
Vast deposits of ilmenite (FeTiO3), a source of
titanium, were formed by magmatic
differentiation.
Figure 4.21 B
Valuable Magmatic Mineral Deposits
(4)
 Certain magmas separate into two immiscible


liquids.
One, a sulfide liquid rich in iron, copper, and
nickel, sinks to the floor of the magma chamber
because it is denser.
The resulting igneous rock is rich in copper or
nickel ore.
 Many of the world’s great nickel deposits, in
Canada, Australia, Russia,and Zimbabwe, formed
in this manner.
Figure 4.21
Revisiting Plate Tectonics And The
Earth System (1)
 The melting of a rock increases with pressure. If a

hot mass of rock is under pressure and the pressure
suddenly decreases, decompression melting can
occur.
The oceanic crust varies very little in composition
around the world.
 It is simply referred to as MORB, an acronym for
“midocean ridge basalt.”
 The ridge and seafloor are everywhere covered by
water except in a few places such as Iceland, where
the midocean ridge stands above the sea level.
Revisiting Plate Tectonics And The
Earth System (2)
 In places where a plate collision has caught up and

crushed a fragment of oceanic crust between two
colliding continental masses, the minerals that are
characteristic of basalt are transformed into an
assemblage dominated by a green, fibrous mineral
called serpentine.
Serpentine-dominated fragments of oceanic crust
found on continents are called ophiolites, from the
Greek word for serpent, ophis.
Figure 4.22
Igneous Rock And Life on Earth
 Life requires nutrients such as potassium, sulfur,
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
calcium, and phosphorus.
Magma, which is less dense than the rock from
which it forms by melting, rises buoyantly
upward, bringing with it the nutrients on which
life depends.
A continent unaffected by any process of surface
renewal, such as uplift or volcanic eruptions, but
subjected to erosion for a hundred million years,
would finish with low relief and almost barren
soils.
Figure 4.23