Igneous Rocks
Goals
To understand what your “granite” counter top is
really made out of and to use that knowledge to
make plate tectonic interpretations.
Igneous rocks
• Rocks cooled and crystallized from molten rock
• Made up of an interlocking network of mineral
crystals called grains
Grains
Igneous rocks
• Molten rock is called magma—can be either all
melt or a “slush” of melt and crystals
• Magma erupted onto the Earth’s surface called
lava
Igneous rock compositions
Minerals in igneous rocks tell us a lot about the
tectonic environment in which rocks formed
Calcium plagioclase
feldspar
Potassium feldspar
Pyroxene
Quartz
Ultramafic igneous rocks
•
•
•
•
Lots of iron and magnesium
Very low silica (<45% SiO2)
Contain olivine and pyroxene—Often green
Make up the Earth’s mantle
Peridotite
Green is olivine
Black is pyroxene
This is what the
mantle looks like
Mafic igneous rocks
• Lots of iron, magnesium, and calcium
• Low silica (45–55% SiO2)
• Contain pyroxene, amphibole, and calcium (black
lab) plagioclase—Dark gray to black in color
• Make up oceanic crust
Intermediate igneous rocks
• Contain iron, calcium, and sodium
• Medium silica (55–65% SiO2)
• Contain amphibole, and calcium-sodium
(chocolate lab) plagioclase—Salt-and-pepper or
medium-gray color
• Found in the continents
Felsic igneous rocks a.k.a. silicic
• Contain sodium, aluminum, and potassium
• High silica (>65% SiO2)
• Contain potassium feldspar, quartz, and sodium
(yellow lab) plagioclase—Makes them light
colored
• Make up most of the continents
Mafic to Felsic comparison
A
C
B
Mafic to Felsic comparison
B
Felsic
(silicic)
Mafic
A
Intermediate
C
Igneous composition and temperature
• Bowen’s reaction series: Progression of minerals
that can crystallize as temperature decreases
• Discontinuous: discrete steps between mineral
phases. Each one is entirely different
• Continuous: Plagioclase feldspars can have any
amounts of calcium and sodium
Fractionation
Mafic minerals crystallize first—use up iron and
magnesium and increase concentration of
aluminum, potassium, and sodium.
• Both mafic and felsic rocks crystallized from a
single melt containing all the major elements.
Rock with
concentrated
pyroxene produced
by fractionation
Fractional Crystallization
Igneous rock textures
Texture: Size, shape, and alignment of mineral
grains in a rock
– Provides information about local environment
in which rocks form
– Slower cooling = bigger crystal grains
Intrusive igneous rocks
Cool and crystallize inside the earth
– Earth is a great insulator. Intrusive rocks cool
slowly
– Grow large crystals.
Extrusive igneous rocks a.k.a. volcanic
Cool and crystallize outside the earth
– Water and air remove heat quickly. Extrusive
rocks cool rapidly
– Grow small crystals or form glass.
Intrusive igneous textures
Phaneritic texture: coarse-grained
– All mineral grains >0.5mm in diameter
– Minerals visible to naked eye
– Indicates slow cooling
Intrusive igneous textures
Pegmatitic texture: very coarse-grained
– Largest mineral grains >3cm in diameter
– Indicates very slow cooling
Extrusive igneous textures
Aphanitic texture: fine-grained
– All mineral grains <0.5mm in diameter
– Minerals invisible to naked eye
– Indicates rapid cooling
Extrusive igneous textures
Porphyritic texture: fine and coarse grains
– Isolated grains >0.5mm surrounded by grains
invisible to the naked eye
– Indicates a mixed cooling history
Extrusive igneous textures
Glassy texture: no mineral grains
– Extremely rapid (almost instantaneous) cooling
– Looks like glass
Arrow head made from
volcanic glass
Extrusive igneous textures
Vesicular texture: bubbly
– Gas bubbles trapped in volcanic rock
– Like frozen foam from a coke or beer
– Indicates explosive eruption and very fast
cooling
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Granite: Felsic, phaneritic (intrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Diorite: Intermediate phaneritic
(intrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Basalt: Mafic, aphanitic (extrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic or
porphyritic
Andesite: Intermediate, aphanitic
(extrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic or
porphyritic
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Rhyolite: Felsic, aphanitic (extrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Names of igneous rocks based on
composition and texture
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Gabbro: Mafic, phaneritic (intrusive)
Are these rocks?
Are these rocks?
A. Mafic
B. Intermediate
C. Felsic
A. Phaneritic
B. Aphanitic
Peridotite: Ultramafic, phaneritic rock
• Makes up the Earth’s mantle
• Crystallized from a melt when the Earth formed
• Technically metamorphic rocks
Aphanitic
Phaneritic
Felsic
Rhyolite
Granite
Andesite
Diorite
Intermediate
Mafic
Basalt
Ultramafic
http://www.monanneeaucollege.com/minerauxphoto/rhyolite-rouge.1.net2.jpg
http://academic.brooklyn.cuny.edu/geology/grocha/monument/images/granite.gif
http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/5Andesite/AndesiteHblPorphDrty.jp
http://www.pitt.edu/~cejones/geoimages/2IgneousRocks/IgneousCompositions/4Diorite/DioriteGoreMtnCUp.jpg
http://z.about.com/d/geology/1/0/R/K/basalt.jpg
http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/2Gabbro/GabbroPyroxPlag.jpg
http://www.krc.karelia.ru/results/projects/AK.project/s11.jpg
http://www.gc.maricopa.edu/earthsci/imagearchive/peridotite.htm
Gabbro
Peridotite
Intrusive rock bodies
Dike: Sheet-like intrusive rock body that cuts
across layering in surrounding rocks
Mafic dike in
the Grand
Canyon
Intrusive rock bodies
Sill: Sheet-like intrusive rock body that follows
layering in surrounding rocks
Intrusive rock bodies
Lacolith: Large mushroom- or dome-shaped
intrusive body consisting of a dome-shaped sill
and a smaller feeder dike
Intrusive rock bodies
Batholith: Very large intrusive body with an area of
>100 km2
Sierra Nevada Batholith
Sierra Nevada Batholith
Igneous rocks and plate tectonics
Continental rifts have bimodal volcanism, or,
dominated by felsic and mafic igneous rocks
Igneous rocks and plate tectonics
Oceanic divergent boundaries dominated by mafic
igneous rocks
Igneous rocks and plate tectonics
Oceanic-oceanic convergent boundaries
dominated by mafic and intermediate igneous
rocks
Volcanoes
Trench
Igneous rocks and plate tectonics
Continental-oceanic convergent boundaries
dominated by intermediate igneous rocks
Mt. Rainier, WA
Mt. Shasta, CA