Ch04 igneous rocks-fall2007

Rocks are made of Minerals
(Minerals in Granite)

Three Types of Rocks
1. Igneous – Crystallized from hot, molten
rock.

•

Examples: granite, basalt

2. Sedimentary – Fragments of sediment laid

down by water or wind become compressed
or cemented into layers over time.

•

Examples: sandstone, shale, limestone

3. Metamorphic – Rocks changed by heat
and/or pressure or chemical activity.

•

Examples: gneiss, schist, slate, marble

Proportions of
Rock Types on the Earth

Igneous & Metamorphic Rocks = Crystalline Rocks

The Rock Cycle:
Part of the Earth System
• The loop that involves the
processes by which rocks
originate and change to
other rocks.
• Illustrates the various
processes and paths as
rocks change both on the
surface and inside the
Earth.
• Illustrates
interrelationship among
many parts of the Earth
System.

What Can Igneous
Minerals/Rocks Tell Us?
1. Magma Composition, Viscosity, Temperatures,
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.

Pressures
Volcano Types and Eruptive Behavior
Tectonic Setting
Magnetism
Paleomagnetism
Latitude – Magnetic Declination
Polar Reversals – Orientation of Earth’s Magnetic Field
and Timing of Reversals
Seafloor Spreading
Plate Movements
Changes in Atmospheric Chemistry – Oxygen
Radiometric Age Dating

How Do Igneous Rocks Form?
Igneous rocks
form from the
cooling and
crystallization
of magma or
lava (molten
rock).

Magma is
molten rock
that is
generated in
deep in the
Earth.
Magma that
reaches the
surface is
called lava.

How Does Magma Originate?
• Magma originates from partial
melting of rocks at various levels in
the Earth’s crust and upper mantle.

• Plate tectonics
plays a major
role in the
generation of
most magma.

Generating Magma from
Solid Rock
1. Role of Heat
2. Role of Pressure
3. Role of Volatiles

Generating Magma from Solid Rock
• Role of Heat:

– Temperature increases
within Earth’s upper crust
(called the geothermal
gradient) average between
20oC to 30oC per kilometer.
– Rocks in the lower crust
and upper mantle are near
their melting points.
– Any additional heat may
induce melting:
1. from rocks descending into
the mantle
2. heating or friction
3. or rising heat from the
mantle


• Role of Pressure:
– A reduction in confining pressure causes the
lowering of a rock’s melting temperature.
– When confining
pressures drop,
decompression
melting occurs.
– May occur when a
rock ascends as a
result of convective
upwelling.

Decompression Melting
Insert Mantle Melting
and Plate Tectonics:
Animation #52

Affect of Pressure and Volatiles
Pressure-Temperature
Graphs
Animation #53


• Role of Volatiles:
– Volatiles (primarily water) cause rocks to melt at
lower temperatures.
– Effect of
volatiles is
magnified by
increased
pressure.

Role of Volatiles – Wet Melting
• Effect of volatiles is magnified by increased
pressure.
• This is particularly important where oceanic
lithosphere descends into the mantle.
• Increased heat and
pressure drive water
from the subducting
slab.
• These volatiles are very
mobile and migrate into
the wedge of hot mantle
– lowering the melting
temperature and
causing melting.

Role of Volatiles – Wet Melting
• Mantle-derived basaltic magma buoyantly

rises due to its lesser density (hotter).
• In a continental setting, basaltic magma may
“pond” beneath crustal rocks, which have a
lower density.
• Crustal rocks are
near their melting
point.
• Increased heat from
basaltic magma
causes melting of
crustal rocks.
• Forms secondary
silica-rich magmas.

Wet Melting
and Plate Tectonics:
Animation #52

Three Components of Magma
1. A liquid portion, called melt, that is composed
of mobile ions.
2. Solids, if any, are silicate minerals that have
already crystallized from the melt.
3. Volatiles, which are gases dissolved in the
melt that are confined under immense
pressure exerted by overlying rocks.
•

water vapor (H2O)

•

carbon dioxide (CO2)

•

sulfur dioxide (SO2)

Crystallization of
Magma:
Formation of Igneous
Rocks

Crystallization of Magma:
Formation of Igneous Rocks
1. Cooling of magma results in the
systematic arrangement of ions into
orderly patterns.
2. As heat is lost, ions lose their mobility
(vibrate less vigorously) and begin to
pack closer and closer together until the
forces of chemical bonds will confine
them to and orderly crystalline
arrangement.

Crystallization of Magma:
Formation of Igneous Rocks
3. When magma cools, silicon and oxygen
atoms link together first to form Si-O
tetrahedra.
4. Further cooling – Tetrahedra bond with
other ions to form embryonic crystal
nuclei.
5. Further cooling – Nuclei grow as ions lose
their mobility and join the crystalline
network.
6. The silicate minerals resulting from
crystallization form in a predictable order.

•
•

Bowen’s Reaction Series explains this predictable order of
crystallization of silicate minerals and how this relates to
the evolution of magma and igneous compositions.
Demonstrates that as a magma cools, minerals crystallize
in a systematic fashion based on their melting points.

Bowen’s Reaction Series explains the evolution
of magma and igneous rock compositions

Evolution of Magmas

• Bowen’s Reaction Series: Divided into two

branches: discontinuous series and continuous series

• Discontinuous Reaction Series
– The upper left branch indicates that as magma cools,
olivine is the first mineral to crystallize.
– Olivine chemically reacts with the remaining melt to form
pyroxene.
– Single tetrahedra of olivine link together with additional
tetrahedra to form single-chains structures of the
pyroxene mineral.
– Pyroxene reacts with the remaining melt to form the
double-chain structure of amphibole.
– Amphibole reacts to form the sheet structure of biotite.
– Discontinuous – each step forms a different silicate
structure.

Evolution of Magmas
• Bowens Reaction Series shows that

during crystallization, the composition
of the liquid portion of the magma
continually changes.
– Composition changes due to removal of
elements by earlier-forming minerals.
– Minerals remain in contact with the
remaining melt and will chemically react
and evolve into the next mineral in the
sequence.

• The silica component
of the melt becomes
enriched as
crystallization
proceeds.
• Forms increasingly
complex silicate
structures.

Evolution of Magmas

• Bowen’s Reaction Series

Continuous Reaction Series:

– The right branch indicates Carich plagioclase feldspar reacts
with Na ions in the melt to
become progressively more Narich.
– Na ions diffuse into feldspar
crystals and replace Ca ions in
the crystal lattice.
– Continuous – no steps, same
silicate structure, same family of
minerals – solid solution series
– Rapid cooling prohibits complete
replacement producing zoned
crystals with Ca-rich cores and
Na-rich rims.
– “Mini”-continuous reaction
series occurs within each step of
the discontinuous series – olivine,
pyroxene, amphibole.

Processes Responsible
for Deviations from BRS
BRS is highly idealized, assumes magma
cools slowly in an unchanging environment

• Magmatic Differentiation by
Crystal Settling (Fractional
Crystallization)

– More dense, early-formed crystals
sink toward the bottom of the magma
chamber.
– Separation of a melt from earlier
formed crystals halts the chemical
reaction process along BRS.
– Produces one or more stages of
crystallization.
– Forms of one or more secondary
magmas from a single parental
magma.

Fractional Crystallization
Insert Fractional
Crystallization
Animation #33

Processes Responsible for
Deviations from BRS

• Assimilation

– Changing a magma’s composition by the incorporation of
foreign matter into a magma (xenoliths).
– In near surface
environments, the force of
injecting magma may
fracture surrounding brittle
rocks (host rock). Dislodged
blocks become incorporated
into the magma.
– In deeper environments,
magma may be hot enough
to melt and assimilate host
rock near its melting
temperature.

Processes Responsible for
Deviations from BRS

• Magma Mixing

– Involves two bodies of magma intruding
one another.
– Two chemically
distinct magmas may
produce an
intermediate
composition quite
different from either
original magma.
– Mixing aided by
convective flow in the
magma chamber.

Magma compositions will
vary depending on the
source material that is
melted and the events
(history) that affect
crystallization along
BRS.

Origin of Magma Compositions
1. Silica-Poor Magmas (Basaltic)
–

Primary/Primitive Magmas
• Partial Melting of Mantle (Asthenosphere)

2. Silica-Rich and Intermediate Magmas
(Andesitic and Rhyolitic)

–

Secondary/Evolved to Highly Evolved
Magmas
• Partial Melting of Crust
• Fractional Crystallization of Basaltic Magma
• Assimilation or Magma Mixing

Origin of Basaltic Magmas
• Most originate from direct
partial melting
(incomplete melting) of
ultramafic rock
(peridotite) in the mantle.
• Not evolved.
• Primary/Primitive
Magmas – earliest stages
along BRS – olivine,
pyroxene, Ca-plagioclase.
• Basaltic magmas form at
mid-ocean ridges and rift
zones by decompression
melting or at subduction
zones by wet melting.

Origin of Andesitic Magmas
• Mantle-derived basaltic

magmas migrates upward,
melts, and assimilates more
silica-rich rocks in the crust
generating magma of
andesitic composition.
• Andesitic magma may also
evolve by magmatic
differentiation (crystal
settling).
• Evolved Magmas.
• Secondary Magmas –
intermediate stages along
BRS – pyroxene,
amphibole, biotite,
plagioclase, and minor
quartz.

Origin of Granitic Magmas
• Most likely form as the end
•
•
•

•
•
•

product of crystallization of
andesitic magma.
Or product of partial melting
of silica-rich continental
rocks.
Highly Evolved Magmas.
Secondary/Teritary Magmas –
late stages along BRS –
orthoclase, quartz, muscovite,
plagioclase, biotite, and lesser
amphibole.
Higher in silica and therefore
more viscous than other
magmas.
Because of their viscosity, they
lose their mobility before
reaching the surface.
Tend to produce large
plutonic structures.

Classification of
Igneous Rocks

Classification of Igneous Rocks
• Igneous rocks are typically classified by
– Texture
– Mineral Composition

• The environment during crystallization
can be roughly inferred from texture.
• The source and history can be roughly
inferred from the mineral composition.

Igneous Textures
• Texture in igneous rocks is
determined by the
– size,
– shape, and
– arrangement of mineral grains.

Igneous Textures
• Factors Affecting Crystal Size
1.
2.
3.
4.

Rate of cooling – Time
Amount of silica (SiO2) present
Amount of dissolved gases
Fluids (water and other volatiles)
present

Rate of Cooling
• The rate of cooling is determined by the
environment:

– Extrusive: Rocks formed from lava at the
surface are classified as extrusive or volcanic
rocks.
– Intrusive: Rocks formed from magma that
crystallizes at depth are termed intrusive, or
plutonic rocks.

The rate of cooling is determined by the environment:
– Extrusive: Cools quickly at surface – fine-grained
– Intrusive: Cools slowly subsurface – course-grained

Rapid Rate of Cooling
Aphanitic (Fine-Grained) Texture
• Rapid rate of cooling of
•
•
•
•

lava or magma.
Causes ions to quickly
lose mobility and readily
combine with existing
crystals.
Promotes development of
numerous embryonic
nuclei that all compete
for available ions.
Forms many microscopic
crystals.
Commonly characterized
by color – light,
intermediate, or dark.

Vesicular Texture –
Rapid Cooling W/ Volatiles
• Aphaniutic Rocks

may contain
vesicles (voids from
gas bubbles).
• Form in the upper
zone of the lava
flow where…
• rapid cooling
“freezes” the lava
preserving opening
produced by
expanding gas
bubbles.

Vesicular
Basalt

Very Fast Rate of Cooling
Glassy Texture
• Very Fast Rate –

molten material is
quenched such
that ions are
unable to arrange
into a crystalline
network.
• Forms glass rock –
obsidian, scoria, or
pumice.

Obsidian

Pyroclastic Textures
• Various fragments ejected during a violent
volcanic eruption:
–
–
–

Very fine-grained ash
Crystals, glass fragments, pumice
Bombs – streamlined molten blobs that
solidified in air.
– Blocks – large angular fragments torn from the
walls of the vent.

• Textures often appear to more similar to
sedimentary rocks (clastic).

Pyroclastic Rocks

• Tuff – Composed of ash-sized fragments that

solidified before impact and cemented later.
• Welded Tuff – Composed hot, fine glass shards
that fused together upon impact.
• Volcanic Breccia – Particles larger than ash.

Slow Rate of Cooling
Phaneritic (CoarseGrained) Texture
• Slow Rate of cooling
•
•
•
•

magma.
Permits movement of ions
until they join an existing
crystalline structure.
Promotes the growth of
fewer but larger crystals.
Mass of intergrown
crystals.
Large, visible crystals can
be identified without a
microscope.

Very Slow Rate of Cooling w/ Volatiles
Pegmatitic Texture
• Exceptionally coarse-

grained igneous
rocks.
• Large crystal size
generated by slow
cooling rates and
dissolved fluids.
• Crystals are all larger
than 1 cm in
diameter.
• Crystals can be as
large as 1 meter or
more.

Very Slow Rate of Cooling w/ Volatiles
Pegmatitic Texture
• Pegmatites form in late stages of crystallization of

granitic magmas (highly evolved).
• Water and volatiles are present in unusually large
percentage:
– Chlorine, fluorine, and sulfur

• Also contain significant amounts rare elements:
– lithium, cesium, boron, berylium, uranium,

• Ion migration is enhanced in this fluid-rich

environment forming large crystals.
• Magma “stewing in its own juices.”
• May produce semi-precious gems such as beryl,
topaz, and tourmaline.

Two-Phased Cooling
Porphyrytic Texture
• Minerals form by different

cooling rates.
• Large crystals, called
phenocrysts, are embedded in a
matrix of smaller crystals, called
the groundmass.
• A rock with porhyritic texture is
called a porphyry.
• This type of rock typically has a
2-phase cooling history that
affected the rate of
crystallization.

The texture of a particular
igneous rock is ultimately
determined by the environment
from which it crystallized.

Mineral Compositions
of Igneous Rocks

Bowen’s Reaction Series explains the order of silicate
mineral crystallization and the how this governs the
evolution of magma and igneous rock compositions

Igneous Compositions
• Ultramafic Composition:
– Rare composition that is high in magnesium and
iron.

• Low silica content –

approximately 45%.
• Main constituent of
the upper mantle.

Ultramafic Rocks: Peridotite

• Ultramafic igneous rocks
are composed entirely of
dark (or ferromagnesian)
silicate minerals:
– Olivine
– Pyroxene (Augite)
– Minor Ca-Plagioclase

• Basaltic (or Mafic) Composition:
– Mafic (magnesium and ferrum, for iron)
– Silica deficient –
approximately 50 percent.
– More dense than granitic
rocks.
– Comprise the ocean floor
as well as many volcanic
islands.
– Also found on continents.

Mafic Rocks: Gabbro, Basalt

• Mafic igneous rocks are

composed primarily of
dark (or ferromagnesian)
silicate minerals:
–
–
–
–

Olivine
Pyroxene (Augite)
Amphibole (Hornblende)
Ca-Plagioclase

• Andesitic (or Intermediate) Composition:
– Intermediate silica content (~60%) between granite
(65%) and basalt (50%).

• Contain at least 25

percent dark silicate
minerals.
• Associated with
explosive volcanic
activity near
continental margins.

Intermediate Rocks: Diorite, Andesite

• Intermediate igneous rocks are composed of dark
and light silicate minerals:
–
–
–
–
–

Pyroxene (Augite)
Biotite Mica
Plagioclase
Minor Quartz

• Granitic or (Felsic) Composition:
– Felsic (feldspar and
silica).
– Contains high
amounts of silica
(SiO2) – 65 percent
or more.
– Major constituents
of continental crust.

Felsic Rocks: Granite, Rhyolite

• Felsic igneous rocks are composed of primarily
light with fewer dark silicate minerals:
–
–
–
–

Quartz
Muscovite and Biotite Micas
Plagioclase and Orthoclase Feldspars

Igneous Rock Classification
Felsic
F
i
n
e

C
o
a
r
s
e

Intermediate

Mafic

Rhyolite

Andesite

Basalt

Granite

Diorite

Gabbro

Ultramafic

Peridotite

The mineral composition of a
particular igneous rock is
ultimately determined by the
composition of the magma
(source material and history)
from which it crystallized.

Granite
• Intrusive and phaneritic.
• Over 25 percent quartz, about 65
•
•
•
•

percent or more feldspar.
Other minor silicates muscovite,
biotite, amphibole comprise less
than 10 percent.
May exhibit a porphyritic texture.
Very abundant as it is often
associated with mountain
building.
The term granite covers a wide
range of mineral compositions.

Rhyolite
• Extrusive equivalent of
•
•
•
•
•

granite.
Aphanitic texture.
May contain glass
fragments and vesicles.
May contain phenocrysts
of orthoclase, mica, and
quartz.
Typically red to reddishpurple to gray in color.
Less common and less
voluminous than granite.

Other Granitic
(Felsic) Rocks
• Obsidian

– Extrusive with glassy texture
– High silica content
– Dark colored due to presence of
metallic ions

• Pumice

– Vesicular texture (more voids than
rock)
– Frothy appearance with numerous
voids
– Void shape is typically elongated

• Scoria

– Vesicular texture (more rock than
voids)
– Void shape is typically more
rounded

Andesite
• Volcanic origin –
•
•
•
•

extrusive.
Aphanitic texture.
Often resembles
rhyolite.
Typically mediumgray.
May contain
phenocrysts of
plagioclase and
hornblende.

Diorite
• Plutonic equivalent of
•
•
•

•

andesite.
Coarse-grained or
phaneritic.
Intrusive.
Composed mainly of Narich plagioclase
(intermediate) feldspar
and amphibole with
lesser amounts of biotite.
Salt and pepper
appearance.

Basalt
•
•
•

•
•
•
•

Volcanic origin – extrusive.
Aphanitic texture.
Composed mainly of pyroxene
and calcium-rich plagioclase
feldspar with lesser amounts of
olivine and amphibole.
Very dark green to black in
color.
May contain phenocrysts of
plagioclase and olivine.
May exhibit vesicular texture.
Most common extrusive igneous
rock.

Gabbro
• Intrusive equivalent

of basalt.
• Phaneritic texture
consisting of
pyroxene and
calcium-rich
plagioclase.
• Makes up a
significant percentage
of the oceanic crust.

Pyroclastic
• Composed of fragments (tephra) ejected
during a volcanic eruption.
– Tuff – ash-sized fragments that solidified before
impact and cemented later.
– Welded Tuff – Composed hot fine glass shards
that fused together upon impact.
– Volcanic Breccia – Particles larger than ash.
• Bombs – streamlined fragments that solidified in air.
• Large angular blocks torn from the walls of the vent.
• Crystals, glass fragments, pumice, ash.

Interlayered Tuff and Obsidian

1996 Eruption of Mount Ruapehu, New Zealand –
Intermediate to Felsic composition magmas

Ch04 igneous rocks-fall2007

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