Chapter 5
Igneous Rocks
Section 5.1
What are Igneous Rocks?
• Objectives:
– Summarize igneous rock formation
– Describe the composition of magma
– Identify the factors that affect how rocks melt and
crystallize
– Define:
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Silicate
Lava
Igneous rock
Partial melting
Bowen’s reaction series
Fractional crystallization
I. Igneous Rock Formation
• Magma – molten rock below Earth’s
surface
• Lava – magma that flows out onto Earth’s
surface
• Igneous rocks – form when magma cools
and minerals crystallize
• Rocks must be heated to temperatures of
800oC – 1200oC before they melt
• Temperatures are present in nature in
upper mantle and lower crust
• Theory – heat came from remaining
energy from Earth’s molten formation and
head generated from the decay of
radioactive elements
A. Composition of Magma
• Type of igneous rock that forms
depends on the composition of the
magma
• Magma – often slushy mix of
molten rock, dissolved gases,
mineral crystals
• Common elements present in
magma are same major elements
that are in Earth’s crust:
• Oxygen, Silicon, Aluminum, Iron,
Magnesium, Calcium, Potassium, Sodium
• Silica is most abundant
• Silica has greatest
effect on magma
characteristics
• Magma classification:
based on amount of
silica it contains
– Basaltic –
– Andesitic –
– Rhyolitic -
• Silica content affects
melting temperature
and impacts how
quickly magma flows
• Once magma is free of overlying pressure
of rock layers around it  dissolved gases
are able to escape into atmosphere
– Chemical composition of lava is slightly
different from chemical composition of magma
from which it develops
Mt. Etna, Sicily, Italy
B. Magma Formation
• Magma can be formed by melting
Earth’s crust or by melting within the
mantle
• 4 main factors involved in formation
of magma: temperature, pressure,
water content, mineral content
1. Temperature – generally increases
with depth (geothermal gradient)
2. Pressure – increases with depth
• Result of weight of overlying rock
• As pressure increases  melting point
increases
3. Water Content – changes melting
point
• As water content increases  melting point
decreases
C. Mineral Content
• Different minerals have different
melting points
– Basalt (olivine + calcium feldspar +
pyroxene) melts at higher
temperatures than granite (quartz +
potassium feldspar)
– Granite has lower melting point than
basalt because granite contains more
water and minerals that melt at lower
temperatures
• In general, rocks that are rich in
iron and magnesium melt at higher
temperatures than rocks that
contain higher levels of silicon
D. Partial Melting
• Not all parts of a rock melt at the same
temperature because they contain
different minerals
• This explains why magma is often a
slushy mix of crystals and molten rock
• Partial melting – process whereby
some minerals melt at relatively low
temperatures while other minerals
remain solid
– as each group of minerals melts, different
elements are added to the magma mixture
 changing magma composition
– If temperatures are not high enough to melt
the entire rock  the resulting magma will
have a different composition than that of the
original rock
• This is one way different igneous rocks form
II. Bowen’s Reaction Series
• Bowen’s Reaction Serious – process that
demonstrate as magma cools and crystallizes 
minerals form in predictable patterns
• 2 main patterns / branches of crystallization:
– Right – continuous, gradual change of mineral
compositions in feldspar group
– Left – abrupt change of mineral type in ironmagnesium groups
A. Iron-Rich Minerals
• Left branch
• Undergo abrupt changes as
magma cools and crystallizes
– Olivine = 1st mineral to crystallize
when magma that is rich in iron
and magnesium begins to cool
– When temp decreases enough for
completely new mineral (pyroxene)
to form  olivine that previously
formed reacts with magma and is
converted to pyroxene
– As temp decreases further similar
reactions produce minerals
amphibole and biotite mica
B. Felspars
• Right branch
• Plagioclase feldspars – undergo
continuous change of composition
• As magma cools – 1st feldspars to form
are calcium-rich
• As cooling continues, feldspars react
with magma, and their calcium-rich
compositions change to sodium-rich
compositions
• In some cases, such as when magma
cools rapidly, the calcium-rich cores are
unable to react completely with magma
– result in zoned crystal
III. Fractional Crystallization
• When magma cools  it crystallizes in reverse order of
partial melting
• 1st minerals that crystallize from magma are last minerals
that melted during partial melting
• Fractional crystallization – similar to partial melting in
that the composition of magma can change
• Early formed crystals are removed from the magma and
cannot react with it
• As minerals form and their elements are removed from
the remaining magma  it becomes concentrated silica
• Questions arose from discovery of
Bowen’s reaction series:
– If olivine converts to pyroxene during cooling,
why is olivine found in rock?
• Hypothesis: under certain conditions, newly
formed crystals are separated from magma, and
chemical reactions b/w magma and minerals stop.
Can occur when crystals settle at bottom of
magma body and when liquid magma is squeezed
from crystal mush to form 2 distinct igneous bodies
with different compositions.
Olivine
Peridotite =
olivine +
pyroxene
• As fractional crystallization
continues and more magma is
separated from crystals, the
magma becomes more
concentrated in silica,
aluminum, and potassium
• Last 2 minerals to form are
potassium feldspar and quartz
– Potassium feldspar – one of
most common feldspars in
Earth’s crust
– Quartz – often occurs in veins
because it crystallizes while the
last liquid portion of magma is
squeezed into rock fractures
Section 5.2
Classification of Igneous Rocks
• Objectives:
– Classify different types and textures of igneous rocks.
– Recognize the effects of cooling rates on the grain sizes in
igneous rocks.
– Describe some uses of igneous rocks.
– Define:
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Intrusive rock
Extrusive rock
Basaltic rock
Granitic rock
Texture
Porphyritic texture
Vesicular texture
Pegmatite
kimberlite
I. Mineral Composition of Igneous
Rocks
• Broad classification: intrusive or
extrusive
• Intrusive – when magma cools and
crystallizes below Earth’s surface
– If magma is injected into surrounding rock =
igneous intrusion
– Crystals = generally large enough to see
without magnification
• Extrusive – when magma cools and
crystallizes on Earth’s surface
– A.K.A. – lava flows or flood basalts
– Crystals = small and difficult to see without
magnification
• Classified by mineral compositions,
physical properties (grain size, texture)
A. Classification of Igneous Rocks
•
According to mineral composition
1. Basaltic Rocks – dark-colored, lower
silica content, contain mostly
plagioclase and pyroxene (gabbro)
2. Granitic Rocks – light-colored, high
silica contents, contain mostly quartz,
potassium feldspar, plagioclase
feldspar (granite)
3. Intermediate Rocks – mineral
composition somewhere in between
basaltic and granitic, consist mostly
of plagioclase feldspar and
hornblende (diorite)
4. Ultrabasic – only iron-rich minerals,
such as olivine and pyroxene –
always dark (peridotite)
II. Texture
• Rocks differ in sizes of grains or
crystals
• Texture – refers to size, shape,
and distribution of crystals or
grains that make up a rock
– Rhyolite = fine-grained
– Granite = coarse-grained
• Difference in crystal size
explained by fact that one rock is
extrusive and other is intrusive
A. Crystal Size and Cooling Rates
• Lava flows on Earth’s surface –
cools quickly and not enough time
for large crystals to form = extrusive
igneous rocks (rhyolite)
– Sometimes cooling occurs so quickly
that crystals do not form at all =
volcanic glass (obsidian)
• Magma cools slowly beneath
Earth’s surface = sufficient time for
large crystals to form
• Intrusive igneous rocks can have
crystals larger than 1 cm (granite,
diorite, gabbro)
B. Porphyritic Rocks
• Porphyritic texture – characterized by large, wellformed crystals surrounded by finer-grained
crystals of same mineral or different minerals
• Indicate complex cooling history during which a
slowly cooling magma (forming large crystals)
suddenly began cooling rapidly (remaining
magma forms small crystals)
Rhyolite
w/ white
feldspar &
dark gray
quartz
Basalt w/ olivine
Andesite w/
amphibole
(horneblend)
C. Vesicular Rocks
• Magma contains dissolved gases that escape
when the pressure on magma lessens
• If lava is thick enough to prevent gases bubbles
from escaping  holes (vesicles) are left behind
– Rock that forms looks spongy
– Vesicular texture – spongy appearance
• Examples: pumice, vasicular basalt
III. Thin Sections
• Usually easier to observe sizes of mineral
grains than it is to identify the mineral
– to identify minerals  geologists examine
samples called thin sections
– Thin section – slice of rock (2cm x 4cm x
.033mm thick)
• So thin light can pass through it
• Petrographic microscope – observe
mineral grains b/c they exhibit distinct
properties
– Properties allow geologists to identify minerals
present in rock
• Example: feldspar grains – show a distinct
banding called twinning
• Example: quartz grains – appear wavy as the
microscope stage is rotated
• Example: calcite crystals – become dark
(extinguish) as stage is rotated
IV. Igneous Rocks as Resources
• Cooling and crystallization history of
igneous rocks sometimes results in
unusual but useful minerals
• Can be used in many fields: construction,
energy production, jewelry making
A. Veins
• Ores – minerals that contain useful
mineral that can be mined for a
profit
• Often occur w/in igneous intrusions
– Sometimes occur as veins
– Fluid left during magma crystallization
contains high levels of silica and water
and any leftover elements that were not
incorporated into the common igneous
minerals
• Some important metallic elements not
included in common minerals = gold,
silver, lead, copper
• These + dissolved silica = released at
end of magma crystallization in hot,
mineral-rich fluid that fills cracks and
voids in surrounding rock
• Fluid solidifies to form metal-rich quartz
veins
B. Pegmatites
• Pegmatites – veins of extremely
large-grained minerals
• Ores of rare elements (lithium)
(beryllium) form in pegmatites
• Can produce beautiful crystals
– Minerals grow to voids and retain
their shapes because these veins
fill cavities and fractures in rock
• Mount Rushmore
C. Kimberlites
• Variety of peridotite
• Most likely form deep in crust
or in mantle at depths of 150
– 300km
– Because diamond and other
minerals present in kimberlites
can form only under very high
pressure
• Hypothesis: magma is
intruded rapidly upward
toward surface  forms long,
narrow, pipe-like structures
– Extend many km into crust &
only 100 – 300m diameter
D. Igneous Rocks in Construction
• Especially useful as building materials
– Interlocking grain texture = strong
– Many minerals resistant to weathering
(granite)