Igneous Rocks
Igneous Rocks
 “Ignis” means fire
 Rocks that are formed from
cystallization of magma
 Magma is molten rock

Lava is magma that is on the Earth’s
Surface
Igneous Rocks

Two Types of Igneous Rocks
Extrusive (Exterior)
 Intrusive (Interior)

 Igneous
rocks that solidify into rock
beneath Earth’s surface
 Coarse Grained
 Cool Slowly
 Granite
Extrusive Igneous Rocks
Extrusive (Exterior)
 Igneous rocks
that solidify into
rock on Earth’s
surface
 Fine Grained
 Cool Quickly
Intrusive Igneous Rocks
Intrusive (Interior)
 Igneous rocks
that solidify into
rock beneath
Earth’s surface
 Coarse Grained
 Cool Slowly
 Granite
Igneous Rock Formation
Origin of Magma

Where does the
heat come from
that melts rocks?


Formation of
Earth
Heat from the
decay of
radioactive
elements
Factors that Affect Magma
Formation

Temperature


Pressure

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Increases with depth
Water Content


Increases with depth
Decreases melting
point
Mineral
Composition

Different minerals,
Characteristics of Magma


Slushy Mix of molten
 Compounds in Magma
rock, gases, and
 Silica ( Si02)
mineral crystals
 Most abundant
Common Elements
 Greatest effect on
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Oxygen (O)
Silicon (Si)
Aluminum (Al)
Iron (Fe)
Magnesium (Mg)
Calcium (Ca)
Potassium (K)
Sodium (Na)

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Magma Characteristics
Effects melting temp
Viscosity of Magma
Types of Magma

Based on amount of Silica

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Basaltic
Andesitic
Rhyolitic
Type of Magma
SiO2 Content
Rhyolitic
70%
Andesitic
60%
Basaltic
50%
Melting Rocks

Question

Does a rocks melt like an ice cube, all
at the same time?
Melting Rocks

Rocks melt according to their melting
points.

Example: Ice cube with wax


Which would melt first ice or wax?
This example is known as partial melting.


Partial Melting: some minerals melt at lower
temperatures and other minerals remain
solid
Think of “stew”
Fractional Crystallization
Opposite of Partial Melting
 Last to melt are first to solidify
(crystallize)

Bowen’s Reaction Series

N.L. Bowen
Canadian
 1900’s
 Stated that “as magma cools,
minerals form in predictable patterns”
 Known as Bowen’s Reaction Series

Bowen’s Reaction Series

Two Branches

Feldspars
 Continuous,
gradual change of mineral
compositions

Iron-Rich Minerals
 An
abrupt change of mineral type
Bowen’s Reaction Series
Feldspars


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Continuous
Change
First Feldspars
are rich in
Calcium (Ca)
Sodium (Na)
increases as
cooling continues
Last Feldspars to
form are Sodium
rich (Na)
Iron Rich Minerals

Discontinuous
Change

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Magnesium (Mg)
cools around 18000C,
when olivine
crystallizes, this
continous up to
15570C.
Now Pyroxene begins
to form. All olivine
that was formed is
now turned to
pyroxene
Quartz is the last to
form, because silica
Why do we find Olivine?
Four main groups of igneous rocks based
on magma type/mineral composition

Felsic: high silica content, light colored, from thick & slow moving
magma, contains low amounts of Ca, Fe, and Mg dominant minerals
quartz, potassium rich feldspar
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Intermediate: moderate amount of silica, mixture of colors dominant
minerals: sodium and calcium rich feldspar

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Ex: andesite, diorite
Magma Type:
Mafic: low silica content, dark colored, high levels of Fe & Mg formed from
thinner, more fluid, & hotter magma than Felsic rocks dominant minerals
hornblende, calcium rich feldspar
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Ex: granite, pumice, rhyolite
Magma Type:
Ex: basalt, gabbro
Magma Type:
Ultramafic: very low silica content, dark colors, high levels of Fe & Mg dominant
minerals: olivine, pyroxene
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Ex: peridotite, dunite
Magma Type:
Classifying Igneous Rocks

Igneous rock textures:

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Crystal size is dictated by the rate of cooling
of the magma body. A slower cooling rate
results in larger mineral crystals being
formed in the rocks as they cool.
Porphyritic texture - Large crystals
surrounded by fine grained rock. The rock
initially cools slowly to form some large
crystals and then cools quickly to form the
fine grained rock surrounding them.
The Scheme for
Igneous Rock
Identification
A Brief Tour
Crystal
size
Grain Size
Description
Igneous rocks have
“Intergrown Crystals”
Intergrown
Intergrown
Intergrown
Not Intergrown
Outside
theEarth:
volcano:
Extrusive
Inside the
Intrusive
Where it was formed
Bubbles
?
Yes= Vesicular
Bubbles
?
No=Non-vesicular
Neither Light nor Dark
Very Light
Color
NotVery Light
Light
Not Very Dark Very Dark
Dark
Darkest
Lightest
Very Light
Density
Medium Density
Light
Dense Very Dense
Finding The Minerals
Identify the rock.
Unless you have other information,
work in the middle of the rock’s box.
This is the amount of Potassium Feldspar in the rock.
This is the amount of Quartz in the rock.
This is the amount of Plagioclase Feldspar in the rock.
Use tick marks on a scrap paper to measure the percentage.
Potassium Feldspar 25%
Quartz 40%
Practice
Name a light-colored, fine-grained
rock with no bubbles.
Name a coarse-grained, dense rock.
Igneous Rock Resources
Ore Deposits
 Building Materials
 Other Uses

Ore Deposits
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Veins: streaks of valuable metal within a mineral.
Created when a metal-rich fluid, such as goldquartz, goes through fractional crystallization, the
mineral (quartz) has a lower crystallization temp
and thus solidifies before the gold. The gold
remains liquid and settles between the quartz
crystals forming “gold veins.”
Pegmatites: veins with extremely large grain
crystals. Creates some of the world’s most
precious gems.
Kimberlites: intrusions of magma cooled deep
within earth’s crust. Usually find diamonds with
kimberlites. Named after location of first
discovery, Kimberly, South Africa.
Building Materials

Many IR’s are used in building
materials because of their
interlocking crystals strength
IR’s are fairly weather resistant
 Ex: Granite – building
 Ex: Basalt – crushed up to make
gravel

Other Uses
Pumice – cleaning and polishes
 Obsidian

heated to make perlite, a soil additive
that keeps soil loose.
 scalpels – more precise and
smoother than steel, but 10x the cost
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