IGNEOUS ROCKS
CHAPTER 5
What are Igneous Rocks?
•I G N E O U S R O C K S F O R M F R O M T H E C R Y S T A L L I Z A T I O N O F
MAGMA
•I G N E O U S I S L A T I N F O R “ F I R E ”
•M O S T I G N E O U S R O C K S A R E A S S O C I A T E D W I T H F I E R Y L A V A
FLOWS
•( L A V A I S S I M P L Y M A G M A T H A T F L O W S O N T O T H E S U R F A C E O F
THE EARTH).
Types of Igneous Rocks
Extrusive
 Fine-grained igneous
rocks that cool quickly on
the Earth’s surface
 Rhyolite forms quickly
on the Earth’s surface,
which makes it a
common extrusive
igneous rock
Intrusive
 Course-grained igneous
rocks that cool slowly
beneath the Earth’s
surface
 Granite is the most
common intrusive
igneous rock
Composition of Magma
 Magma is a mixture of molten rock, gases, and mineral crystals
 The major elements that make up the different minerals in the
Earth’s crust are the same major elements that make up magma
 Three types of magma, categorized based on the amount of silica
(SiO2)
Basaltic
• Dark
and fluid
• Flows slowly
• Not explosive
Andesitic
• Thick
Rhyolitic
and chunky
• Highly explosive
• Intermediate flow
• High levels of SiO2
• Intermediate explosions
Magma is formed from the
heat and pressure from
inside the Earth.
Factors that affect magma
formation are:
1.
Temperature
2.
Pressure
3.
Water Content
4.
Mineral Composition
Origins of Magma
How Rocks Melt
Rocks melt in different ways, based on their composition
This explains why magma is a mixture of crystals and molten rock
Partial Melting
 The process where some
minerals melt at low
temperatures while others
remain solid
 As each group of minerals melts,
different elements are added to
the magma, changing the
composition
 If temperatures are not great
enough, only part of the rock will
melt
 The resulting magma will
therefore have a different
chemistry from the original rock
Fractional Crystallization
 The process where different
minerals crystallize at different
rates
 The process is similar to partial
melting; however, during
fractional crystallization the
changes have occurred due to
crystallization
 When fractional crystallization
occurs, elements are removed
from the remaining magma
(rather than being added like
partial melting)
Classifying Igneous Rocks
IGNEOUS ROCKS ARE CLASSIFIED ACCORDING TO:
1.
Mineral compositions
2.
Grain size
3.
Texture
THREE MAIN GROUPS
1.
Felsic (light colored, high silica)
2.
Mafic (Dark Colored, low silica)
3.
Intermediate (Moderate amounts of each)
Grain Size
Varies depending on the
amount of time
lava/magma takes to cool
Outside of Earth, lava
cools quickly and
produces little to no
visible crystals (Obsidian,
Andesite, Basalt)
Inside of Earth, magma
cools slowly and produces
large , coarse, crystals
(Gabbro, Granite, Diorite)
Texture
As grains crystallize from
magma, they grow
together and form
irregular edges.
Although this is
characteristic of many
igneous rocks, wellshaped crystals can also
form under certain
conditions.
Many rocks have
porphyritic texture,
which is characterized by
large, well-formed
crystals surrounded by
finer-grained crystals of
the same or different
material.
To show porphyritic texture, we know that a complex
cooling history took place wherein a slowly cooling
magma suddenly shifted to a rapid cooling magma.
Chances are, the crystals grew so large that they
intruded into the Earth’s crust!
Igneous Rocks as Resources
Usefulness in building materials:
Strength due to interlocking grain textures
 Many minerals in igneous rocks are resistant to weathering
 Beauty of crystals are desired in modern decorating
(granite)
 Ore deposits (found through igneous intrusions)

Veins – found in layered intrusions of surrounding rocks
 Pegmatities – Veins of extremely large-grained minerals
 Kimberlites – Diamonds are found in intrusions of these rocks
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