Warm Up 10/2
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On Front White Board
Igneous Rocks
Chapter 3, Section 2
Formation of Igneous Rocks
Rocks that form when magma hardens
beneath Earth’s surface are called
intrusive igneous rocks
 They have intruded into the existing rocks
 As magma rises it cools, allowing elements
to combine and form minerals, which
combine to form igneous rocks
 When lava hardens, the rocks that form
are called extrusive igneous rocks
 The have extruded onto the surface
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Evolution
of a
Magma
Formation of Igneous Rocks
Intrusive - Granite
Extrusive - Rhyolite
Classification of Igneous Rocks
Texture and composition are two
characteristics used to classify igneous
rocks
 Texture describes the appearance of an
igneous rock based on its size, shape, and
arrangement of interlocking crystals
 Composition describes the proportions of
light and dark minerals in the rock
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Course/Fine-Grained Texture
If the magma cools slowly, it will have
only have a few centers of crystal growth
 Slow cooling results in the formation of
large crystals (course-grained)
 If magma or lava cools rapidly, a large
number of tiny crystals compete for
available ions
 Rapid cooling of magma or lava results in
rocks with small, interconnected mineral
grains (fine-grained)
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Course/Fine-Grained Texture
Course-Grained Gabbro
Fine-Grained Basalt
Glassy/Porphyritic Texture
Glassy Texture – solids produced so
quickly that the ions are not able to align
 Porphyritic Texture – rocks that have
different size minerals, a result of different
rates of cooling
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Glassy/Porphyritic Texture
Glassy Texture - Obsidian
Porphyritic Texture - Andesite
Concept Check
How does the rate of cooling of magma or
lava affect the texture of igneous rocks?
 A slowly cooling magma or lava will
produce rocks in which the mineral grains
are relatively large. Quickly cooling
molten material will result in rocks with
small mineral grains. Lava that cools
rapidly will produce a glassy rock. Rocks
that form as a result of different cooling
rates will have both large and small
mineral grains.
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Granitic/Basaltic Composition
Granitic Composition – Igneous rocks in
which quartz and feldspar are the
dominant minerals
 Most granitic rocks also contain about
10% dark silicate minerals and 70% light
silicate minerals
 Basaltic Composition – rocks that contain
many dark silicate minerals
 Basaltic rocks are rich in iron and
magnesium
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Granitic/Basaltic Composition
Granitic Composition - Granite
Basaltic Composition - Gabbro
Other Compositional Groups
Andesitic Composition – rocks with a
composition between granitic and basaltic
 Andesitic rocks contain at least 25% dark
silicate minerals
 Ultramafic – rocks that are composed
mostly of dark silicate minerals
 Peridotite is an example of an ultramafic
rock, although rare on the crust, it is
thought that the mantle is made mostly of
peridotite
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Other Compositional Groups
Andesitic Composition - Andesite
Ultramafic - Peridotite
Bowen’s Reaction Series
Classification of Igneous Rocks
Concept Check
Describe the main differences between
granitic and basaltic rocks?
 Granitic rocks contain mostly quartz and
feldspar and thus are light-colored.
Basaltic rocks are rich in iron and thus are
dark-colored and more dense.
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Homework
Read Chapter 3, Section 2 (pg. 70-74)
 Do 3.2 Assessment #1-7 (pg. 74)
 Print out “Investigation 3 – Classifying
Rocks Using a Key” for next class
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Assignment
d.
Create a Rock Cycle Poster!
Your poster should have:
Captions (5 points)
Color (5 points)
Drawings of each step (5 points)
Be neat (5 points)
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You may use page 67 of the text to guide you.
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a.
b.
c.