Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 4
Igneous Rocks: Solids from Melts
Igneous Rocks
Igneous rocks
Formed from the cooling and
consolidation of magma
•
plutonic (intrusive) —
cooled below the surface
•
volcanic (extrusive) —
cooled on the surface
Coarsely Crystalline Granite
Fig. 4.1
Photomicrograph of Granite
Fig. 4.1
Finely Crystalline Basalt
Fig. 4.1
Photomicrograph of Basalt
Fig. 4.1
Igneous textures
Glassy
no minerals present
Crystalline rocks made of mineral
grains
Porphyritic mixture of coarse and fine
Vesicular
with bubble holes
Composition of melts affects
behavior while still fluid
More SiO2 will increase
viscosity,
making strong temporary
bonds
in magma.
Factors controlling the
viscosity of magmas
• Composition:
higher SiO2; higher viscosity
lower volatiles; higher viscosity
• Temperature:
lower temperature; higher viscosity
Granite (g) Intruding
Metamorphic (m) Rocks
m
g
Fig. 4.2
Magma
Usually a silicate melt (liquid) at high
temperatures (650 to 1200°C).
Mixture of all the elements that make
up minerals plus volatile components:
H2O, CO2, Cl, F, S
These components form gases and will
boil off when pressure is released.
Texture of Igneous Rocks
• Controlled by cooling rate
• Grain size
• Degree of crystallinity
• Vesicularity
Classification of Igneous Rocks
Defined by texture:
• Fine-grained: extrusive or volcanic
• Coarse-grained: intrusive or
plutonic
Pyroclastic Igneous Rocks
Obsidian
Pumice
Ash
Fig. 4.3
Quartz-rich Felsic Porphyry
Fig. 4.4
Classification by
composition and texture
Extrusive
Intrusive
basalt
gabbro
andesite
diorite
rhyolite
granite
Extrusive
Basalt
Rhyolite
Intrusive
Gabbro
Granite
Fig. 4.5
Classification of Igneous Rocks
Determined by composition (both
chemical and mineralogical):
• magnesium (Mg) + iron (Fe) =
mafic
• feldspar + quartz (Si) = felsic
Classification of Igneous Rocks
When we talk about the chemical
composition of a rock we usually speak in
terms of the oxides, e.g.,
Typical basalt Typical
granite
SiO2
50%
70%
Al2O3
15%
12%
FeO+MgO
15%
3%
CaO
8%
2%
K2O+Na2O
5%
8%
Classification of Igneous Rocks
Fig. 4.6
The process of complete melting of a
rock or complete crystallization of a
magma will not change the bulk
composition of the system, but if
either
of these processes goes only part
way,
the composition of the solid and the
liquid can be very different
(especially
for small ƒ).
Partial melting
• Opposite of fractional crystallization
• Last minerals to form will melt at
lowest temperature
• Biggest changes will be for small
degrees of melting
Factors Affecting
Melting
Temperatures
Fig. 4.7
Where do magmas come from?
• Basalts: Broadly speaking, we know that mantle rocks
(45% SiO2) partially melt (10 to 15%) to produce basalts
(50% SiO2).
• With the addition of some water, basalts will partially
melt to produce Andesite (60% SiO2).
Where do magmas come from?
Granites may also be produced by fractional
crystallization of a basaltic magma. But this
works only for a dry granite. However, most
granites contain many hydrous phases
(micas), and some granites are associated
with extensive hydrothermal ore deposits.
Tectonic Settings of
Igneous Activity
Fig. 4.8
Volcanic Island
Arc, Indonesia
Fig. 4.8
Oceanic
Hot Spot
Hawaii
Fig. 4.8
Continental
Volcanic Arc
N. Cascades
Fig. 4.8
Volcanic Island Arc
Java, Indonesia
Lava flow at
Volcanoes National Park,
Hawaii
Continental Volcanic Arc
North Cascades, Washington
Fractional crystallization
• The modification of magma by
crystallization and removal of
mineral
phases.
• Because only certain elements will
go into a given mineral, this will
tend
to change the composition of the
remaining liquid.
Early Crystallization
Fig. 4.9a
Liquids Squeezed from Crystals
Fig. 4.9b
The Palisades Sill
Fig. 4.10
Fig. 4.10
Crystallization
Ideally, crystallization is the
opposite of melting.
In fact, the process is more
complicated than that because
rocks are complex aggregates of
many minerals with different
melting
(crystallization) points.
Simple crystallization
Example: Quartz
When melt reaches the
crystallization temperature of a
mineral, the mineral forms and
undergoes no further changes
with
subsequent cooling.
Continuous crystallization
Example: Plagioclase feldspar
When a mineral begins to crystallize it
takes on a given composition but the
composition of the crystallizing exterior
(and therefore the entire crystal)
changes due to changes in the
composition of the magma.
Continuous Crystallization
Plagioclase Feldspar
Discontinuous crystallization
Examples: Olivine and Pyroxenes
Crystals previously formed react
with
melt to produce new minerals.
Discontinuous crystallization
Olivine  Pyroxene
Bowen’s reaction series
• Series of chemical reactions that take
place in silicate magmas as they cool
• First investigated in the 1920s and
1930s by N. L. Bowen
• Important experiments that help us
understand the evolution of magmas
Bowen’s Reaction Series
Fig. 4.11
Magma Differentiation
Fig. 4.12
Forms of intrusive igneous masses
Plutons can be divided into two
groups:
1) Concordant
2) Discordant
Forms of intrusive igneous masses
Concordant:
Sills
Laccoliths
Discordant:
Dikes
Necks
Forms of intrusive igneous masses
Batholith: Any deep-seated pluton of
coarse-grained rocks that has a surface
exposure of over 100 sq. km that is
mostly granitic. Examples include Sierra
Nevada, Coast Range, Idaho batholiths.
Stock: Same as a batholith, only smaller.
Types of Igneous Structures
Fig. 4.13
Methods of Intruding Magma
Fig. 4.14
Sill
Sill
Fig. 4.15
Dike
Fig. 4.16
Pegmatite Dike
Fig. 4.17
Magma Chamber Beneath
Mid-ocean Spreading Ridge
Fig. 4.18
Volcanism Due to Partial Melting
in a Subduction Zone
Fig. 4.19
Mt. Rainier
Fig. 4.20