Igneous Rocks 1 / 51
Jan 20, 2016
Igneous Rocks
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What are Igneous Rocks?
from the Latin word for “fire” - ignis
Thus, rocks that are “fire-formed”
Molten rock (magma) cools to form a solid rock
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What is magma?
Magma is molten rock, and contains:
Melt
Liquid, composed of mobile ions
Solids
Silicate minerals that have already crystallized from
the melt
Volatiles
gases dissolved in the melt,
including water vapor (H2O),
carbon dioxide (CO2), and sulfur
dioxide (SO2)
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Identifying Igneous Rocks
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How to classify igneous rocks
1. Composition
2. Texture
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Igneous Rocks
Rocks formed on the surface
Volcanic (extrusive) rocks
Rocks formed inside Earth
Plutonic (intrusive) rocks
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Igneous Rocks (two types)
Extrusive
Lava and Pyroclastic Debris
Extruded at surface or at very shallow levels.
Granite is Intrusive
Basalt is Extrusive
Intrusive
Magma crystallized slowly within the crust.No exposure to the cool atmosphere.
Plutonic– intrusive igneous rock at great depth within crust or mantle.
Magma Compositions
Dark iron and/or magnesium
Olivine
Pyroxene
Amphibole
Biotite mica
Light potassium, sodium, & calcium
Quartz
Muscovite mica
Feldspars
Magma is composed mainly of silicate minerals
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Bowen’s Reaction Series
Magma crystallizes over a temperature range of several hundred degrees
Minerals crystallize in a predictable order
Last minerals to crystallize are very different in composition from the earlier formed minerals
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Bowen’s Reaction Series
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Bowen’s Reaction Series Animation
Bowen’s Reaction Series
Mg
Fe
Mg
Fe
Mg
Fe
Mg
Fe
Mg
Fe MgFe
Mg
Fe
Ca
K
Na
Ca
K
Na
Ca
K
Na
Ca
K
Na
Ca
K
Na
Na
SiSi
Si
Si
Si
Si
Si
Si
Si
AlAl
Al
AlAl
Al
Al
Al
Olivine
Fe Mg
Pyroxene
Amphibole
Plagioclase
SiBiotite
K
AlCa
Si
Muscovite
Orthoclase
Quartz12 / 51
Partial MeltingSilica-rich compounds melt before other compounds.
Magma DifferentiationFour types:•Magma Mixing•Crystal Settling•Magma Assimilation•Magma Migration
Changing a Magma’s Composition
Crystal settling
Earlier-formed minerals are denser than the liquid portion and sink to the bottom of the
magma chamber
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Changing a Magma’s Composition
Assimilation
Changing a magma’s composition by the
incorporation of foreign matter
(surrounding rock bodies) into a magma
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Changing a Magma’s Composition
Magma mixing
Involves two bodies of magma intruding one another
Two chemically distinct magmas may produce a composition quite different from either original
magma
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Texture is estimated using visual grain size(depends on crystallization history)
Composition is estimated using visual colorCan be misleading
Mafic minerals crystallize early and felsic minerals crystallize late in magma
Minerals at the top of Bowen’s Reaction Series:
• Dark in color• Mafic to ultramafic • Iron and magnesium
Minerals at the bottom of Bowen’s Reaction Series:
• Light in color• Felsic• Sodium plagioclase, potassium
plagioclase, and quartz are light.
Igneous rocks are named on the basis of their texture and composition
Igneous Textures
The size, shape, and arrangement of the minerals. Glassy – Made of solid glass or glass shards. Interlocking crystals – Minerals that fit like jigsaw
pieces. Fragmental – Pieces of pre-existing rocks.
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Igneous Textures
Aphanitic
Phaneritic
Porphyritic
Glassy
Pyroclastic
Pegmatitic
• Rapid rate of cooling of lava or magma
• Microscopic crystals• Typically occurs in extrusive /
volcanic rocks
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Basalt: Aphanitic Texture
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Igneous Textures
AphaniticPhaneritic
Porphyritic
Glassy
Pyroclastic
Pegmatitic
• Slow cooling• Crystals can be identified
without a microscope• Typically occurs in intrusive /
plutonic rocks
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Granite: Phaneritic Texture
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Thin section showing APHANITIC TEXTURE
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Thin section showing PHANERITIC TEXTURE
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Igneous Textures
Aphanitic
PhaneriticPorphyritic
Glassy
Pyroclastic
Pegmatitic
• Minerals form at different temperatures & rates
• Large crystals (phenocrysts) are embedded in a matrix of smaller crystals
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Granodiorite: Porphyritic Texture
Potassium Feldspar Phenocryst
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Igneous Textures
Aphanitic
Phaneritic
PorphyriticGlassy
Pyroclastic
Pegmatitic
• Very rapid cooling of molten rock
• Resulting rock is called obsidian
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Obsidian: Glassy Texture
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Igneous Textures
Aphanitic
Phaneritic
Porphyritic
GlassyPyroclastic
Pegmatitic
• Various fragments ejected during a violent volcanic eruption
• Textures often appear similar to sedimentary rocks
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Igneous Textures
Aphanitic
Phaneritic
Porphyritic
Glassy
PyroclasticPegmatitic
• Exceptionally coarse grained• Form in late stages of
crystallization of granitic magmas
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Quartz Vein: Pegmatitic Texture
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Crystal Shapes
• Euhedral - grains completely bounded by crystal faces
• Subhedral - grains only partly bounded by crystal faces
• Anhedral - grains completely devoid of crystal boundaries
Source: 1, 2
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Igneous Textures
Arrangement of the grains
Equigranular inequigranularSlightly inequigranular
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Igneous Textures
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Magma Compositions
Igneous rocks are often classified by mineral (chemical) composition
Ultramafic
Mafic (or basaltic)
Intermediate (or andesitic)
Felsic (or granitic)
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Ultramafic Compositions
Magma Composition:Rare
high in magnesium and iron
composed entirely of ferromagnesian silicates
Fe
Mg
Fe
MgFe
Mg
Mg
FeOlivine
Pyroxene
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Magma Composition: Mafic: magnesium and ferrum (iron)
Contains substantial dark silicate minerals and calcium-rich plagioclase feldspar
More dense than granitic rocks
Mafic (or basaltic) Compositions
Fe
Mg
Fe
MgFe
Mg
Mg
Fe Olivine
PyroxeneCa
Plagioclase
Ca
Ca
Na
Na
Na
Al
Al
Si
Si
Si
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Mafic (or basaltic) Compositions
Extrusive / Volcanic Rock:
Basalt
Aphanitic texture Composed mainly of pyroxene
and calcium-rich plagioclase Most common extrusive
igneous rock
Intrusive / Plutonic Rock:
Gabbro
Phaneritic texture Composed of pyroxene and
calcium-rich plagioclase Makes up a significant portion
of the oceanic crust
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Intermediate (or andesitic) Compositions
Magma Composition
Contains at least 25% dark silicate minerals
Associated with explosive volcanic activity
Fe
Mg
Fe
Mg
Mg
Fe
Ca
Ca
Ca
Na
Na
Na
Al
Al
Si
Si
Si
Amphibole
Plagioclase
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Intermediate (or andesitic) Compositions
Extrusive / Volcanic Rock:
Andesite
Aphanitic texture Dark grey color
May have plagioclase phenocrysts
Intrusive / Plutonic Rock:
Diorite
Phaneritic texture Composed of intermediate
feldspar and hornblende “Black and white makes
DIORITE”
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Felsic (or granitic) Compositions
Magma Composition
Felsic: feldspar and silica
Composed almost entirely of light-colored silicates
Fe
Mg
Fe
Mg
Mg
Fe
Ca
Ca
Ca
Na
Na
Na Al
SiSi
Amphibole
Plagioclase
Biotite
Muscovite
Orthoclase
Quartz
K
K
K
Al
Si
Al
K AlSi
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Felsic (or granitic) Compositions
Extrusive / Volcanic Rock:
Rhyolite
Aphanitic texture Buff or pink color
May have glass phenocrysts
Intrusive / Plutonic Rock:
Granite
Phaneritic texture Over 25 % quartz, about 65 %
or more feldspar The term “granite” covers a
wide range of mineral compositions
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Varied Compositions
Obsidian
Volcanic Dark colored Glassy texture
Pumice
Volcanic Glassy texture
Frothy appearance with numerous voids
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Varied Compositions
Pyroclastic rocks
Composed of fragments ejected during a volcanic
eruption
Tuff – ash-sized fragments
Volcanic breccia – particles larger than ash
lvo.wr.usgs.gov/gallery/BishopTuff_1.html
www.gc.maricopa.edu 48 / 51
Igneous Rock Classification
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All rocks on Earth have evolved from the first igneous rocks
All Igneous Rocks Result from Magma Differentiation.
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