Transcript GEOL3025, Section 096 Lecture #6 28 August 2007

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