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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

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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