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
Pyroxene
Potassium feldspar
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
PeridotiteGreen 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
B
C
Mafic to Felsic comparison
Mafic
Felsic(silicic)
Intermediate
B
A
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 texturesTexture: 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 texturesAphanitic 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 texturesVesicular 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?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Granite: Felsic, phaneritic (intrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Names of igneous rocks based on composition and texture
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Diorite: Intermediate phaneritic (intrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Names of igneous rocks based on composition and texture
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Basalt: Mafic, aphanitic (extrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Names of igneous rocks based on composition and texture
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic or porphyritic
Andesite: Intermediate, aphanitic (extrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic or porphyritic
Names of igneous rocks based on composition and texture
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Rhyolite: Felsic, aphanitic (extrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Names of igneous rocks based on composition and texture
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?A. Phaneritic
B. Aphanitic
Gabbro: Mafic, phaneritic (intrusive)
Are these rocks?A. Mafic
B. Intermediate
C. Felsic
Are these rocks?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
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Felsic
Intermediate
Mafic
Ultramafic
Granite
Diorite
Gabbro
Peridotite
Rhyolite
Andesite
Basalt
Aphanitic Phaneritic
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 tectonicsOceanic-oceanic convergent boundaries
dominated by mafic and intermediate igneous rocks
Volcanoes Trench
Igneous rocks and plate tectonicsContinental-oceanic convergent boundaries
dominated by intermediate igneous rocks
Mt. Shasta, CA
Mt. Rainier, WA