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TRANSCRIPT
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Earth Science Standard 3c: Students know how to
explain the properties of rocks based on the physical
and chemical conditions in which they formed, including
plate tectonic processes.
Classifying Igneous Rocks
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• As magma cools and crystallizes minerals
form in predictable patterns
• Bowen discovered 2 main patterns:
a) Right-branch – continuous, gradual change
of mineral compositions in the feldspar
group
b) Left-branch – minerals rich in iron and
magnesium change abruptly as the
temperature of the magma decreases
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http://ees.as.uky.edu/sites/default/files/elearning/module03swf.swf
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1. Texture (Rate of Cooling)
2. Color (Mineral Composition)
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• Determined by rate of
cooling where and how
fast the lava or magma
cooled
2 Locations of cooling:
a) Intrusive (cooled inside
Earth)
b) Extrusive (cooled on
Earth’s surface)
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• Not exposed to weather since inside Earth
• Very hot within Earth so cooling takes a long time!
a) Intrusive Igneous Rocks
Slow Cooling
• Large mineral grains (can see with human
eye)
• Hundreds to millions of years to cool
• The larger the mineral grains the slower it
cooled!
• Coarse-grained Texture (phaneritic)
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Intrusive:
Course-grained,
interlocking
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• Exposed to weather since form on top of Earth
• Cooler on Earth’s surface so cools more quickly!
b) Extrusive Igneous Rocks
• Very Fast Cooling = NO mineral grains &
Glassy Texture
• Hours to Days
• Fast Cooling = Fine-grained Texture (mineral
grains not seen with human eye (aphanitic)
• Days to Weeks
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• Porphyritic = started cooling slowly and the
cooling sped up
• Some large crystals (phenocrysts)surrounded by a
fine-grained texture
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Intrusive:
Course-grained,
interlocking
Extrusive:
Fine-grained, discrete
crystals, often glassy
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• Determined by mineral composition
4 classifications:
a) Felsic
b) Mafic
c) Intermediate
d) Ultramafic
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a) Felsic = Light colored; white to pink; high silica
b) Mafic = Dark colored; blackish-gray; low silica
c) Intermediate = Gray; between felsic and mafic
d) Ultramafic = black-green; very low silica
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• Felsic = continental crust
• Mafic = oceanic crust
• Intermediate = coastlines
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Ex
trusiv
e
Felsic Intermediate Mafic Ultramafic Texture
Obsidian
Basaltic
Glass
Glassy
(non-
crystalline)
Rhyolite
Andsesite
Basalt
Fine-
grained
Intru
sive
Granite
Diorite
Gabbro
Peridotite
Dunite
Coarse-
grained
Pegmatite
Very
Coarse-
grained
Classification of Igneous Rocks
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Formation of Igneous Rocks
Pyroclasts
Extrusive
Intrusive
Porphyry:
partially crystalline
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Compositional
Classification
Granite
Granite
Quartz
Orthoclase
Biotite
Plagioclase
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Compositional
Classification
Granite
Granite
Granodiorite
Quartz
Amphibole
Plagioclase
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Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Plagioclase
Amphibole
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Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Gabbro
Plagioclase
Pyroxene
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Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Gabbro
Peridotite
Pyroxene
Olivine
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Granite Gabbro
Intrusive Igneous Rocks
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Rhyolite Basalt
Extrusive Igneous Rocks
Where did these rocks form?
How can you tell?
Color?
- http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/5Andesite/AndesiteNCHblPorphyritic.JPG