igneous rocks. goals to understand what your “granite” counter top is really made out of and to...

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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

http://www.monanneeaucollege.com/minerauxphoto/rhyolite-rouge.1.net2.jpghttp://academic.brooklyn.cuny.edu/geology/grocha/monument/images/granite.gifhttp://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/5Andesite/AndesiteHblPorphDrty.jphttp://www.pitt.edu/~cejones/geoimages/2IgneousRocks/IgneousCompositions/4Diorite/DioriteGoreMtnCUp.jpghttp://z.about.com/d/geology/1/0/R/K/basalt.jpghttp://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/2Gabbro/GabbroPyroxPlag.jpghttp://www.krc.karelia.ru/results/projects/AK.project/s11.jpghttp://www.gc.maricopa.edu/earthsci/imagearchive/peridotite.htm

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

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