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1
G-100 Lectures 6 & 7Introduction to Rocks
Focus on: Igneous Rocks
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What are rocks?
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Three Rock Types
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5
Igneous rocks
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Sedimentary rocks
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Metamorphic rocks
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The Rock Cycleriftingdivergent marginsedimentation
rifting continuesocean spreadingmagmatic activity
subsidencesedimentation
erosion, streamssedimentationsedimentary basins
ocean basin closescontinental collisionmountains → metamorphism
subduction of ocean crustmagma formsvolcanoes
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• Classification of Igneous Rocks– Texture– Mineral & Chemical Composition
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MAGMA• Molten Rock
– with dissolved gas: H2O + CO2
• Generated at depth• Eruption if magma reaches surface• If doesn’t reach surface, solidifies underground
–Intrudes country rock–Intrusive contact
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12
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How does magma form?
• Partial melting of rock at depth
• Source of heat?– Geothermal gradient
• Rate of Temperature increase with depth
• Average 30 oC/km depth• Not same everywhere
• Tells us that heat is being conducted to the surface of the earth from the interior
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• Factors that control melting temperatures–Pressure
• melting points of minerals increase with pressure– This is why increasing temperature along the geotherm
alone fails to melt crustal rocks
• reduction in pressure can therefore induce melting
–Water added under pressure• lowers melting point of minerals
How does magma form?
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Where does all that basalt come from, dad?
??
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• Magmatic differentiation–Mirror image of partial melting
• Crystal formation over a range in temperature– First to melt are also first to crystallize
• Fractional crystallization– As crystallization proceeds, crystals are separated from the
melt, thus the melt evolves away from the composition of the crystals removed
• Bowen’s reaction series, a model of crystallization:
How does magma evolve?
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How does magma evolve?
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Basa
ltic
in
tru
sio
n2
45
–2
75
m (
80
0–
90
0 f
t)
SandstoneBasalt
Mostly sodium-richplagioclase feldspar;no olivine
Calcium-rich plagioclase feldspar and pyroxene; no olivine
Olivine
Sandstone
Basalt
From: Paul Olsen web pages LDEO
Palisades Sill, New Jerseynature, matched by experiments
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Palisadesintrusion
Olivinecrystals
~1200°C
Magma withcomposition A
Olivine crystallizesfirst.
Olivine
Plagioclasefeldspar
Pyroxene
Magma withcomposition B
Pyroxene and plagioclasefeldsparcrystallize.
Olivine
Plagioclasefeldspar
Pyroxene
Magma withcomposition C
A gradient ofpyroxene andfeldspar is established.
~600°C
Plagioclasefeldspar
Magma withcomposition C
Plagioclase feldspar continues to crystallize.
Palisades Sill, New Jerseynature, matched by experiments
20
Magmachamber B
basalt
Magmachamber A
rhyolite MixingMixing of the basaltic and rhyolitic magmas results in andesitic magma.
How does magma evolve?
Basaltic inclusions in granite, Basaltic inclusions in granite, AdamelloAdamello batholithbatholith, Italy, Italy
Quenched inclusions
Qcf 3 hostdacite
Basaltic Basaltic inclusions inclusions (grey) in (grey) in glassy glassy dacitedacitelava flow lava flow (black), (black), San Pedro San Pedro Volcano, Volcano, Chilean Chilean AndesAndes
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Countryrock
Volcano
Lava flow Ash falls and pyroclasts
Volcanic neck withradiating dikes
StockStock
SillSillDik
eD
ike
BatholithBatholith
PlutonPluton
SillSill
SillSill Dik
eD
ike
Dik
eD
ike
Dikes cut across layersof country rock…
…but sills areparallel to them.
Batholiths are the largest forms of plutons, covering at least 100 km2.
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Sill
A sill parallelscountry rock layers.
DikeDike
A dike cuts acrosslayers.
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Chalten (Cerro Fitzroy) plutonPatagonian BatholithSouthern Andes, Argentina
A dike cuts acrosslayers.
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Cordilleran BatholithsNorth and South America
GraniteBatholithsof the North and South American Continental Margins
El Chaltenfrom Winter (2001)
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Tectonic/Dynamic setting of magmatism
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ISLAND ARCPLATE SUBDUCTION
Mafic to intermediateintrusives (plutonism)Mafic to intermediateextrusives (volcanism)Island arc
volcano
OceaniclithosphereOceaniclithosphere
Island arc subduction zone
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PLATEDIVERGENCE
Basaltic extrusivesBasaltic intrusivesMid-ocean ridge
Partial meltingOf upper mantlePartial melting
Of upper mantle
Rising magmaRising magma
Divergent plate boundary-spreading center
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HOT-SPOTVOLCANISM
Basaltic extrusivesBasaltic intrusivesHot-spot volcano
Mantle plume(hot spot)Mantle plume(hot spot)
MantleMantle
Hot-spot volcanism-upwelling mantle plume
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Oceaniccrust
Moho
Mantle
Spreadingcenter
Pillow lavaNewer, thinnersediments
Older, thickersediments
Peridotite layerPeridotite layer
Sheeted dikesin basaltSheeted dikesin basalt
GabbroGabbro
Decompression melting at mid ocean ridge
Hot mantle rises, decompresses,and melts.
A thin dike intrudesForming sheeted dikes, Occasionally erupts,spilling lava in “pillows.”
Sediments are deposited on the spreading seafloor.
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Trench
Water remains trappedas the pressure and temperature increase.
Subducting oceanic crust carries sediments with it.
The trapped water is released as the temperature increases,…
…causing the sedimentary rocks to meltat lower temperatures.
The waterand molten sediments melt parts of the overlying plate.
Andesitic magma erupts, forms arc volcanoes
Oceanic sediments
Oceanic crust basalt
Oceanic mantlelithosphere
Asthenosphere
Sedimentgrains
Water
Magma chamber
H2O H2OH2O
Magma ascends, interacts withlithosphere, possibly mixing with existing magma
Fluid induced melting in subduction zone