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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
• 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
http://ees.as.uky.edu/sites/default/files/elearning/module03swf.swf
1. Texture (Rate of Cooling)
2. Color (Mineral Composition)
• 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)
• 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)
Intrusive:
Course-grained,
interlocking
• 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
• Porphyritic = started cooling slowly and the
cooling sped up
• Some large crystals (phenocrysts)surrounded by a
fine-grained texture
Intrusive:
Course-grained,
interlocking
Extrusive:
Fine-grained, discrete
crystals, often glassy
• Determined by mineral composition
4 classifications:
a) Felsic
b) Mafic
c) Intermediate
d) Ultramafic
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
• Felsic = continental crust
• Mafic = oceanic crust
• Intermediate = coastlines
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
Formation of Igneous Rocks
Pyroclasts
Extrusive
Intrusive
Porphyry:
partially crystalline
Compositional
Classification
Granite
Granite
Quartz
Orthoclase
Biotite
Plagioclase
Compositional
Classification
Granite
Granite
Granodiorite
Quartz
Amphibole
Plagioclase
Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Plagioclase
Amphibole
Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Gabbro
Plagioclase
Pyroxene
Compositional
Classification
Granite
Granite
Granodiorite
Diorite
Gabbro
Peridotite
Pyroxene
Olivine
Granite Gabbro
Intrusive Igneous Rocks
Rhyolite Basalt
Extrusive Igneous Rocks
Where did these rocks form?
How can you tell?
Color?