chapter 4: igneous rocks
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Chapter 4: Igneous Rocks. Introduction. Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock - PowerPoint PPT PresentationTRANSCRIPT
Chapter 4: Igneous RocksChapter 4: Igneous Rocks
IntroductionIntroduction Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock Crystallization = when magma cools, ions
arrange themselves into orderly crystalline structures
Extrusive = erupts on surface “Volcanic” from god Vulcan
Intrusive = cools under ground “Plutonic” from god Pluto
Volatiles = gases & liquids dissolved into magma
Igneous rocks = formed from “fire” Magma = completely or partially molten rock Lava = magma which reaches surface Melt = liquid portion of partially melted rock Crystallization = when magma cools, ions
arrange themselves into orderly crystalline structures
Extrusive = erupts on surface “Volcanic” from god Vulcan
Intrusive = cools under ground “Plutonic” from god Pluto
Volatiles = gases & liquids dissolved into magma
Crystal Size Effected By …Crystal Size
Effected By …
Cooling rate Amount of silica Amount of
dissolved gases
Igneous rocks classified by texture & composition
Cooling rate Amount of silica Amount of
dissolved gases
Igneous rocks classified by texture & composition
Igneous TexturesIgneous Textures
Aphanitic Vesicular
Phaneritic Porphyritic
Phenocrysts Groundmass
Aphanitic Vesicular
Phaneritic Porphyritic
Phenocrysts Groundmass
Glassy Viscosity
Pyroclastic AKA
Fragmental Pegmatitic
Glassy Viscosity
Pyroclastic AKA
Fragmental Pegmatitic
Aphanitic TextureAphanitic Texture Fine-grained
“A” = not “Phaner” = visible
Rapid cooling Characterized by
color Light Intermediate dark
Usually extrusive / volcanic
Fine-grained “A” = not “Phaner” = visible
Rapid cooling Characterized by
color Light Intermediate dark
Usually extrusive / volcanic
Vesicular TextureVesicular Texture
Type of aphanitic rock
Vesicule = void left by gas bubble
Spherical or elongate holes
Usually form in upper layers of extrusive rocks
Type of aphanitic rock
Vesicule = void left by gas bubble
Spherical or elongate holes
Usually form in upper layers of extrusive rocks
Phaneritic TexturePhaneritic Texture
Coarse-grained “Phaner” = visible
Slow cooling Minerals can be
identified Usually intrusive Exposed at surface
after much erosion
Coarse-grained “Phaner” = visible
Slow cooling Minerals can be
identified Usually intrusive Exposed at surface
after much erosion
Porphyritic TexturePorphyritic Texture
Large crystals in matrix of smaller crystals
Phenocryst = large crystals
Groundmass = surrounding smaller matrix
Due to minerals forming at different temps & rates
Large crystals in matrix of smaller crystals
Phenocryst = large crystals
Groundmass = surrounding smaller matrix
Due to minerals forming at different temps & rates
Glassy TextureGlassy Texture
High SiO2 content forms chains which impede movement of ions High viscosity
e.g., Obsidian Quick cooling also
prevents crystallization
e.g., Pele’s hair
High SiO2 content forms chains which impede movement of ions High viscosity
e.g., Obsidian Quick cooling also
prevents crystallization
e.g., Pele’s hair
Pyroclastic Texture
Pyroclastic Texture
AKA Fragmental Texture
Forms from consolidation of rock fragments from violent eruption
Large range in particle sizes
Angular pieces
AKA Fragmental Texture
Forms from consolidation of rock fragments from violent eruption
Large range in particle sizes
Angular pieces Montserrat, 1996
Pegmatitic TexturePegmatitic Texture
Abnormally large crystals > 1 cm
Late stages of crystallization
Rich in volatiles Composition ~
granite
Abnormally large crystals > 1 cm
Late stages of crystallization
Rich in volatiles Composition ~
granite
Igneous CompositionsIgneous Compositions
Composed primarily of silicate minerals Dark
(Ferromagnesian) minerals
Examples: olivine, pyroxene, amphibole, biotite mica
Light (nonferromagnesian) minerals
Examples: quartz, feldspars, muscovite mica
Composed primarily of silicate minerals Dark
(Ferromagnesian) minerals
Examples: olivine, pyroxene, amphibole, biotite mica
Light (nonferromagnesian) minerals
Examples: quartz, feldspars, muscovite mica
Granitic vs. Basaltic RocksGranitic vs. Basaltic Rocks
Granitic Composition Light-colored
silicates Rich in SiO2
Up to 70% AKA Felsic
From feldspar & silica AKA Silicic Major constituent of
continental crust
Granitic Composition Light-colored
silicates Rich in SiO2
Up to 70% AKA Felsic
From feldspar & silica AKA Silicic Major constituent of
continental crust
Basaltic Composition Dark silicates & Ca-rich
feldspar Low in SiO2
~45% AKA Mafic
From magnesium & ferric (iron)
Higher density than felsic rocks
Major constituent of ocean floor & many islands
Basaltic Composition Dark silicates & Ca-rich
feldspar Low in SiO2
~45% AKA Mafic
From magnesium & ferric (iron)
Higher density than felsic rocks
Major constituent of ocean floor & many islands
Other CompositionsOther Compositions
Intermediate Contains ~25%
dark silicates AKA Andesite &
rhyotlie Associated with
dome-building & explosive eruptions
Associated with subduction zones
Intermediate Contains ~25%
dark silicates AKA Andesite &
rhyotlie Associated with
dome-building & explosive eruptions
Associated with subduction zones
Ultramafic Rare composition Rich in Mg & Fe Poor in SiO2
~40% Composed entirely
of ferromagnesian silicates
e.g. olivine & pyroxene
Peridotite & komatiite
Ultramafic Rare composition Rich in Mg & Fe Poor in SiO2
~40% Composed entirely
of ferromagnesian silicates
e.g. olivine & pyroxene
Peridotite & komatiite
Silica Indicates CompositionSilica Indicates Composition
Exhibits considerable range in crustal rocks 40% - 73% SiO2
Influences magma behavior Granitic magmas
More SiO2 = more viscous => more explosive
Basaltic magmas Less SiO2 = more runny => more like fluid
Exhibits considerable range in crustal rocks 40% - 73% SiO2
Influences magma behavior Granitic magmas
More SiO2 = more viscous => more explosive
Basaltic magmas Less SiO2 = more runny => more like fluid
Igneous Rock ContinuumIgneous Rock Continuum
Silica Indicates CompositionSilica Indicates Composition
Exhibits considerable range in crustal rocks 40% - 73% SiO2
Influences magma behavior Granitic magmas
More SiO2 = more viscous => more explosive
Basaltic magmas Less SiO2 = more runny => more like fluid
Exhibits considerable range in crustal rocks 40% - 73% SiO2
Influences magma behavior Granitic magmas
More SiO2 = more viscous => more explosive
Basaltic magmas Less SiO2 = more runny => more like fluid
Naming Igneous RocksNaming Igneous Rocks
Silicic Composition
Silicic Composition
>25% Quartz ~40% Feldspar Intrusive = Granite
Very abundant Associated w/
mountain building Extrusive =
Rhyolite May contain glass &
vesicles Less common than
granite
>25% Quartz ~40% Feldspar Intrusive = Granite
Very abundant Associated w/
mountain building Extrusive =
Rhyolite May contain glass &
vesicles Less common than
granite
Glassy Composition
Glassy Composition
Obsidian Dark-colored glassy
rock Forms when SiO2-rich
lava cools quickly Pumice
Forms when large amounts of gas escape
Frothy texture Usually found with
obsidian
Obsidian Dark-colored glassy
rock Forms when SiO2-rich
lava cools quickly Pumice
Forms when large amounts of gas escape
Frothy texture Usually found with
obsidian
Intermediate CompositionIntermediate Composition
60%-67% SiO2
Intrusive = Diorite Phaneritic
Extrusive = Andesite Named for Andes
Mts. Aphanitic /
Porphyritic
60%-67% SiO2
Intrusive = Diorite Phaneritic
Extrusive = Andesite Named for Andes
Mts. Aphanitic /
Porphyritic
Basaltic CompositionBasaltic Composition
45%-50% SiO2
Composed of pyroxene & Ca-rich feldspar
Intrusive = Gabbro
Extrusive = Basalt Most common
extrusive rock on Earth
45%-50% SiO2
Composed of pyroxene & Ca-rich feldspar
Intrusive = Gabbro
Extrusive = Basalt Most common
extrusive rock on Earth
Porphyritic RocksPorphyritic Rocks Composed of
fragments ejucted during eruption
Varieties: Ash Flow = loose
agglomeration of ash-sized particles from fallout in ash cloud
Tuff = lithified ash layer
Welded tuff = hot enough to fuse
Volcanic Breccia = particles larger than ash, usually angular & poorly sorted
Composed of fragments ejucted during eruption
Varieties: Ash Flow = loose
agglomeration of ash-sized particles from fallout in ash cloud
Tuff = lithified ash layer
Welded tuff = hot enough to fuse
Volcanic Breccia = particles larger than ash, usually angular & poorly sorted
Origin of MagmaOrigin of Magma
Highly debated topic Formation of Magma from Solid Rock
Role of Heat Role of Volatiles Role of Pressure
Highly debated topic Formation of Magma from Solid Rock
Role of Heat Role of Volatiles Role of Pressure
Evolution of Magmas
Evolution of Magmas
Fractional Crystallization
Crystal Settling Magmatic
differentiation Assimilation Magma Mixing
Fractional Crystallization
Crystal Settling Magmatic
differentiation Assimilation Magma Mixing
Bowen’s Reaction SeriesBowen’s Reaction Series
Assimilation & Magma Mixing
Assimilation & Magma Mixing