Download Igneous Rocks - MSU Billings

The Rock Cycle
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A rock is a naturally formed,
consolidated material usually
composed of grains of one or
more minerals
The rock cycle shows how one
type of rocky material gets
transformed into another
– Representation of how rocks are
formed, broken down, and
processed in response to
changing conditions
– Processes may involve
interactions of geosphere with
hydrosphere, atmosphere and/or
biosphere
– Arrows indicate possible process
paths within the cycle
Imagine the first rock and the cycles that it has
been through.
Igneous Rocks
• Form from Magma
– Hot, partially molten mixture of solid
liquid and gas
– Mineral crystals form in the magma
making a crystal “slush”
– Gases - H2O, CO2, etc. - are dissolved in
the magma
– Magma is less dense than solid rock
Igneous Rocks
• Magma vs. Lava
– Magma is molten rock beneath the surface
– Lava is molten rock that has reached the
surface
– Magma solidifies to form intrusive igneous
rocks
– Lava solidifies to form extrusive igneous
rocks
Igneous Rocks
• Composition varies widely
–Oxygen plus major elements
–Generally a silica (SiO2) melt
–Silica and water content control viscosity
–Silica content is used in classification
Mafic Magmas
• Silica content of ~ 50%
• High concentrations of Fe, Mg and Ca
• High temperature of molten magma
– 1000o to 1200oC
• Major minerals
– Olivine
– Pyroxene
- Ca Plagioclase
Silicic Magma
• Silica content of 65-77%
• High concentrations of Al, Na and K
• Lower temperature magmas
– Less than 850oC
• Major minerals
– Feldspars
– Quartz
- Micas
Magma Viscosity
• Controlled by silica and water and temperature
• As magma cools, silica tetrahedron form links
– Similar to polymers - e.g., nylon
• Increasing linkages
– Higher silica & lower temp
• Linkages increase viscosity
Note: this is just like oils, fats and other organic compounds
used in the household
Magma Viscosity
• Water and volatiles influence viscosity
like a big malted milkshake
• H2O and CO2 make up 90+% of dissolved gases in
magmas
• Typical range of dissolved gases is 0.1 to 5%
– Up to 15% is possible
– High H2O content prevents silica linkages
– High volatile content may cause explosive eruptions
what’s true for humans is true for Mother Nature
Occurrence of Igneous Rocks
• Found globally
• Formed in discrete geologic settings
– Convergent plate margins
• Intrusions and overlying volcanoes
– Divergent plate margins
• Mid-ocean ridge basalts and continental
rifts
– Mantle plumes (very mafic)
Figure 4.2. Distribution of igneous rocks in
North America
Igneous Textures
• Texture - the size, shape and relationship of
mineral crystals in the rock
• Reflects cooling history of the magma or
lava
• Slow cooling rate
• Fast cooling rate
• Very fast cooling rate
>> Big crystals
>> Small crystals
>> glass
Glassy Texture
• Very rapid cooling - quenched
– Volcanic glass (obsidian)
– Conchoidal fracture
• No apparent crystals
– embryonic crystals may be present or
devitrification may have begun
• Dark color from low concentrations of Fe
(a little goes a long way) and size of ‘crystalline
unit’- generally silicic composition
Figure 4.3A. Glassy texture in obsidian
Crystalline Textures
• Crystal growth requires time for ions
to migrate - form minerals
• Slow rate of cooling provides time for
crystal growth
• Crystals grow until melt is quenched or
is completely solidified
Aphanitic Texture
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Fine grained texture
Few crystals visible in hand specimen
Relatively rapid rate of cooling
Vesicles may be formed by gases
trapped in cooling magma
Figure 4.3B. Aphanitic texture in rhyolite
Phaneritic Texture
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Coarse grained texture
Relatively slow rate of cooling
Equigranular, interlocking crystals
Slow cooling = crystallization at depth
Pegmatites - very coarse grained
texture
Figure 4.3C. Phaneritic texture in granite
Porphyritic Texture
• Well formed crystals (phenocrysts)
• Fine grained matrix (groundmass)
• Complex cooling history
– Initial stage of slow cooling
• Large, well formed crystals form
– Later stage of rapid cooling
• Remaining magma crystallizes more rapidly
Pyroclastic Texture
• Produced by explosive volcanic eruptions
• May appear porphyritic with visible crystals
– Crystals show breakage or distortion
• Matrix may be dominated by glassy fragments
– Fragments also show distortion
– Hot fragments may “weld” together
Figure 4.3D. Pyroclastic texture
Classification of Igneous Rocks
–Texture
• Aphanitic
• Phanaritic
–Composition
• Silicic
• Intermediate
• Mafic
• Ultramafic
Combination of Texture and Composition
produces rock name
Figure 4.4. Classification of common igneous rocks
Extrusive Rock Bodies
Volcanic
• Form of extrusive bodies influenced by
magma properties
– Composition
• Silica content
– Viscosity
• Volatile content
• Temperature
Basaltic Eruptions
• Low Silica + High T = Low Viscosity
• Produce
– Lava Flows - Pahoehoe or Aa
– Flood basalts
– Fissure eruptions
– Spatter cones
– Shield Volcanoes
– Pillow lavas
Aa flow
Pahoehoe flow
Figures 4.6 A & B
Devil’s Tower; a volcanic neck, a feeder pipe
Shiprock, New Mexico; a volcanic neck
Rhumski, Cameroon; a volcanic neck
Sill; parallels layers in the country rock
Dike; cuts across layers in the country rock
Half Dome; part of the Sierra Nevada batholith
Beginnings of a
spatter cone
Large cinder cone
Flood basalts with several thick and
thin layers. Each layer represents a separate eruption.
pillow lavas
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/pillow_lava.html
Intermediate & Silicic Eruptions
• Higher Silica + Lower T = Higher Viscosity
– Composite or Stratovolcanos
– Lava Domes
– Ash Flow Calderas
Formation of Volcanic Domes
Mt Fuji: Stratovolcano
Mt. St. Helen's prior to 1980 eruption,
a classic stratovolcano
http://www.youtube.com/watch?v=bgRnVhbfIKQ
Process of formation of ash flow caldera
- e.g., Crater Lake, Oregon or the super
Caldera of Yellowstone
Size comparison of various volcanic features
Intrusive Rock Bodies
Plutonic
• Less dense magmas rise through the
crust
• Rising magmas slowly cool
– Viscosity increases
– Density increases
• Intrusions form as magma solidifies
beneath the surface
Intrusive Rock Bodies
• Intrusions are classified by their size, shape
and relative age
• Large intrusions
• Batholiths
• Stocks
• Small intrusions
• Dikes
• Sills
• Laccoliths
Figure 4.18. Types of magmatic intrusions
Origin of Magmas
• Solid rock is at equilibrium with its
surrounding
• Changes in the surroundings may cause
solid rock
magma
– Raising T
– Lowering P
– Changing composition
Origin of Magmas
• Lowering P
– Mantle convection moves deep mantle rocks
upwards
• Raising T
– Hot mafic magma intrudes into the crust
• Changing composition
– Adding small amounts of water
Magma Differentiation
• Magmas, and the resulting igneous rocks,
show a wide range of compositions
• Source Rock
– variations cause major and minor
variations in the magma
• Magma Mixing
• Assimilation
Figure 4.21. Processes of magma differentiation
Magma Differentiation
• Partial Melting
– Individual minerals within source rock
melt at different T
– Resulting magma is enriched in many
elements, especially SiO2
– Resulting magma is less dense than
source rock
Magma Differentiation
• Fractional Crystallization
– Individual minerals precipitate from
magma at different T
• Bowen’s Reaction Series
– Remaining magma is enriched in many
elements, especially SiO2
– Remaining magma may migrate
Bowen’s Reaction Series
Figure 4.22. Bowen’s reaction series
Classification of Igneous Rocks
1000 C
Solidifying Temperature
500 C
Increasing Grain Size
Silica
Content
low
Minerals
Andesite
intermediate
feldspar,
amphibole,
pyroxene, biotite
mica
Diorite
Rhyolite
high
feldspar, quartz,
muscovite mica,
& amphibole
Granite
Present (in order of
abundance)
pyroxene,
olivine, feldspar,
& amphibole
Plutonic
Rocks
Gabbro
Lighter Color
Volcanic
Rocks
Basalt
Plate Tectonic Setting of Igneous Rocks
• Divergent Plate Boundaries
– Partial melting of mantle produces basaltic
magma
• Convergent Plate Boundaries
– Subduction and partial melting of wet basalt,
sediments and the surrounding mantle
– Andesitic and rhyolitic magma generated
through fractional crystallization
– Ascending magma may assimilate lower crustal
material
Plate Tectonic Setting of Igneous Rocks
• Mantle Plumes
– Partial melting of rising plumes of solid mantle
material
– Distinctive basaltic magma is produced
– Rising magma may produce
• Intraplate island chains
• Flood basalt
• Basalt plateaus and rhyolitic calderas
Rock types and tectonic setting
Coarse grained igneous rock
Fine grained igneous rock
Pegmatite:
Very coarse grained igneous rock
Porphyritic igneous rock:
Big xtals in a fine grain matrix