September 27, 2011

Agenda: SILENCE YOUR CELL PHONESILENCE YOUR CELL PHONE• Exams complete – hand backExams complete – hand back• Hand in Igneous rock QRHand in Igneous rock QR• Volcanoes, weathering, sedimentary rocksVolcanoes, weathering, sedimentary rocks QR’s onlineQR’s online

Geology – Chapter 3 – Igneous Rocks

Magma, Igneous Rocks,Magma, Igneous Rocks,

and and

Intrusive ActivityIntrusive Activity

Magma, Igneous Rocks,Magma, Igneous Rocks,

and and

Intrusive ActivityIntrusive Activity

IgneousIgneousRocksRocks

Solid rockSolid rock

PressurePressure 150 mi below earth’s surface150 mi below earth’s surface partial melting of solid rock partial melting of solid rock

Solid RockSolid Rock Solid RockSolid Rock Solid Rock Solid Rock

Droplets rise throughDroplets rise throughthe mantle and collectthe mantle and collectto form magma poolsto form magma pools

Magma poolMagma pool

SurfaceSurface

Cooled solid rockCooled solid rock

Extrusive Ig-rocksLava

Intrusive Ig-rocks

Magma: The parent material for all rocksMagma: The parent material for all rocksThere are three distinct components of magma:There are three distinct components of magma:

Liquid portion Liquid portion referred to as referred to as “melt”“melt”• “ “liquid rock” – low densityliquid rock” – low density• composed of: Si, O, Al, Fe, Ca, Mg, Na, Kcomposed of: Si, O, Al, Fe, Ca, Mg, Na, K

Solid portion • crystallized portions of the melt as magma cools – precipitating various silicate minerals.

Gaseous portionGaseous portion• “ “volatiles” created from vaporizingvolatiles” created from vaporizing materials at the Earth’s surfacematerials at the Earth’s surface (H(H22O, COO, CO22 and SO and SO22))

MagmaMagmaTypical temperature depending on chemical composition:Typical temperature depending on chemical composition:

70070000C (1,292C (1,29200F) – 1300F) – 130000C (2,372C (2,37200F) F)

High in Si,O

High in Fe,Mg

Melting solid rock into magmaMelting solid rock into magma

Rising temperatures within a solid (tightly packed Rising temperatures within a solid (tightly packed atoms)atoms)creates “vibrating ions” eventually breaking thecreates “vibrating ions” eventually breaking thechemical bonds and causing ions to roll over eachchemical bonds and causing ions to roll over eachother ----- producing a liquid.other ----- producing a liquid.

How would the process of crystallization work?How would the process of crystallization work?

Intrusive RocksIntrusive Rocks

Extrusive RocksExtrusive Rocks

Intrusive Rocks

•Solidified below the Earth’s surface

• Plutonic rocks – “the God of lower world”

• magma cooled slowly allowing xl growth

• coarse-grained textures (can see grains)

• large granite mountains • Sierra Nevada Mountains•Mt Rushmore

Extrusive RocksExtrusive Rocks

• Solidified above the Earth’s surfaceSolidified above the Earth’s surface

• Volcanic rocks – “the roman God of fire”Volcanic rocks – “the roman God of fire”

• magma cooled fast allowing microscopicmagma cooled fast allowing microscopic xl growth (can’t see grains)xl growth (can’t see grains)

• fine-grained textures (can’t see grains)fine-grained textures (can’t see grains)

• Hawaiian Volcanoes, Cascade RangeHawaiian Volcanoes, Cascade Range

visible visible grainsgrains

non-visible grains

When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock????????When observing “frozen magma” (an igneous rock), how does a geologist know if the rock is an Extrusive or Intrusive igneous rock????????

Checking the crystal size - does the rock contain visible crystals or are crystals non-existent?Checking the crystal size - does the rock contain visible crystals or are crystals non-existent?

(4) Factors that affect crystal size in an igneous rockThe rate at which magma cools

•slow cooling – visible minerals•rapid cooling – nonviable minerals

The amount of silica present•the more silica – more visible minerals

The amount of dissolved gases•abundant dissolved gases – larger crystals

The amount of space for xl growth

(4) Factors that affect crystal size in an igneous rockThe rate at which magma cools

•slow cooling – visible minerals•rapid cooling – nonviable minerals

The amount of silica present•the more silica – more visible minerals

The amount of dissolved gases•abundant dissolved gases – larger crystals

The amount of space for xl growth

Igneous Rock TexturesIgneous Rock Textures

Slow cooling of magmaSlow cooling of magma

Phaneritic – xl grains can be seen as an interlocking “mosaic” mass of xls coarse-grained texture

Phaneritic – xl grains can be seen as an interlocking “mosaic” mass of xls coarse-grained texture

Types of igneous rock texturesTypes of igneous rock textures

InterlockinggrainsInterlockinggrains

Pegmatitic – xl grains are greater than 1 cm results from late stage magma cooling- charged with volatiles

Pegmatitic – xl grains are greater than 1 cm results from late stage magma cooling- charged with volatiles

Slow cooling of magmaSlow cooling of magma

Large GrainsLarge Grains

What do you observe ?What do you observe ?

No minerals visible xl grains are to small to see with an unaided eye

No minerals visible xl grains are to small to see with an unaided eye

Fast cooling magmaFast cooling magma

Glassy – xl grains are “frozen” before xlation. Produced from high silica viscous rich magmas that are rapidly quenched

Glassy – xl grains are “frozen” before xlation. Produced from high silica viscous rich magmas that are rapidly quenched

Fast cooling magmaFast cooling magma

Porphyritic – two distinct xl sizesPorphyritic – two distinct xl sizes

PhenocrystsPhenocrystsGroundmassGroundmass

(same mineral)

Fast cooling of magmaFast cooling of magma

Pyroclastic – fragmented texture, fragments incorporated into molten lava as it solidifies Tuffacous (tuff) – composed of fragments

Pyroclastic – fragmented texture, fragments incorporated into molten lava as it solidifies Tuffacous (tuff) – composed of fragments

Fragments of other rock piecesFragments of other rock pieces(“different pieces other rock)(“different pieces other rock)

Igneous Rock Textures -

Igneous Rock Composition – What’s in your rock?Composition is based on the “mineral make-up”

• dominantly composed of silicates (Si and O)• Al, Ca, Na, K, Mg and Fe

Ferromagnesian composition (Mafic)• high in Fe, Mg• low in Si, O• Dark colored rocks

PyroxenePyroxene

AmphiboleAmphibole

BiotiteBiotite

Ca-plagioclaseCa-plagioclase

orthoclaseorthoclaseMuscoviteMuscovite QuartzQuartz

Non-ferromagnesian (felsic)Non-ferromagnesian (felsic)• high in Si, Ohigh in Si, O• low in Fe, Mglow in Fe, Mg• light colored rockslight colored rocks

Classification of Igneous RocksIgneous rocks are classified based on TEXTURE and

COMPOSTION

TE

XT

UR

E

ph

ane

ritica

ph

an

itic

COMPOSITIONFelsic (light color)

High silicaMafic (dark color)

Low silicaIntermediate color

Granite Diorite Gabbro

Rhyolite Andesite Basalt

Ig rocks have the same chemistry but different textural characteristicsdue to the type of geologic environment the rock forms

The Origin of Magma

•How can magma form from “solid rock ?”How can magma form from “solid rock ?”

•Why is one magma Why is one magma maficmafic and others and others felsicfelsic??

•Why do volcanoes erupt basaltic lava, and Why do volcanoes erupt basaltic lava, and continent volcanoes erupt andesitic/rhyolitic lava ?continent volcanoes erupt andesitic/rhyolitic lava ?

• most magma originates in the upper mantle• greatest quantities form at divergent boundaries• small amounts of magma at subduction zones

Questions:Questions:

Increasing Temperature:Increasing Temperature:

• increasing the temperature within the earthincreasing the temperature within the earth begins to melt solid rockbegins to melt solid rock

Temperatures within the Earth increase as a Temperatures within the Earth increase as a function of depth ---- Geothermal Gradientfunction of depth ---- Geothermal Gradient

Temperature inside the earthTemperature inside the earth

00 500500 10001000 15001500 20002000

De

pth

(km

)D

ep

th (

km)

100100

200200

300300

400400

5,0005,000

10,00010,000

15,00015,000

Pre

ssu

re (

mpa

)P

ress

ure

(m

pa)

GeothermalGeothermalgradientgradient

• the rate at whichthe rate at which temperature increasestemperature increases with depthwith depth

Continent gradientContinent gradient

• In thicker crust,In thicker crust, gradient increases.gradient increases.

• average 7average 7ooC/km rateC/km rate

• temperature increasestemperature increases gentlygently

Oceanic gradientOceanic gradient

• Below the ocean floor,Below the ocean floor, temperature increasestemperature increases rapidly.rapidly.

• average 13average 1300C/kmC/km

So, why is the mantle still considered solid?• Pressure increases with depth – raising the rocks melting points

Decompression Melting:• ascending mantle rock moves into lower pressure zones which lowers rock melting points ------- generating voluminous magma

• Remember: most magma occurs along spreading ridges.

Addition of volatiles (water and gases)• addition of water lowers the rocks melting point

• “wet” rock drives the melting points downward

• mantle rock (peridotite) melting points lower by 1000C/.1% water!!

Magma is generated in three ways:Magma is generated in three ways:• Increase in temperature causing rocks to exceedIncrease in temperature causing rocks to exceed their melting points ---- melting occurstheir melting points ---- melting occurs

• Decrease pressure (decompression melting), rocksDecrease pressure (decompression melting), rocks ascend to low pressure zonesascend to low pressure zones

• Introduction of volatiles (principally water) lowersIntroduction of volatiles (principally water) lowers rock melting point.rock melting point.

Evolving MagmasEvolving Magmas: : • A variety of igneous rocks = wide variety of magmasA variety of igneous rocks = wide variety of magmas

• observations of many igneous compositions fromobservations of many igneous compositions from volcanoesvolcanoes

• magma – the parent material of all igneous rocksmagma – the parent material of all igneous rocks

• investigated by N. L. Bowen (famous geologist)investigated by N. L. Bowen (famous geologist) (1887-1956) --- Bowen’s Reaction Series(1887-1956) --- Bowen’s Reaction Series

Magmatic DifferentiationMagmatic Differentiation

• the formation of many kinds of igneous rocks fromthe formation of many kinds of igneous rocks from a single magmaa single magma

FeFe

Simple exampleSimple example

FeFeFeFe

FeFe MgMgMgMg

MgMg

MgMg

SiOSiO22

SiOSiO22

SiOSiO22

SiOSiO22

SiOSiO22

SiOSiO22

SiOSiO22

SiOSiO22

Liquid magmaLiquid magma Part liquid/solidPart liquid/solid

FeSiOFeSiO22

FeSiOFeSiO22MgSiOMgSiO22

FeSiOFeSiO22 MgSiOMgSiO22

FeSiOFeSiO22MgSiOMgSiO22

solidsolid

liquidliquid

SiOSiO22

SiOSiO22SiOSiO22

SiOSiO22

CoolingCooling

How has the liquid magma changed composition?How has the liquid magma changed composition?

As the liquid magma begins to cool, various minerals precipitate asAs the liquid magma begins to cool, various minerals precipitate assolids and become separated from the liquid melt. This separationsolids and become separated from the liquid melt. This separationof various chemistries changes the composition of the original magma.of various chemistries changes the composition of the original magma.

3333

Changing the composition of magmas:

Magmatic Differentiation:• separating the xlized minerals from the melt changing the overall magma composition

Assimilation and Mixing Magma:• mixing magmas with various compositions

Magmatic Differentiation

MixingMagmas

Changing the magma from a basaltic composition to a graniticcomposition

Composition of magmas – explained by P.T.The Earth- One big chemistry set!

Basaltic composition

Granitic composition

Basalt magma:

Assimilating oceanic plate material (basalt + basalt = basalt magma) Rocks concentrated in mafic minerals – formed high on BRS Low in Silica

Granitic magma:

Assimilating oceanic plate material (basalt + continental = granitic magma)

Rocks concentrated in felsic minerals – formed low on BRS High in Silica

What types of features areWhat types of features areformedformed

when magmawhen magmacools below the surface?cools below the surface?

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Intrusive Igneous Rock ActivityIgneous rocks solidifying below the surface can be classified as:

• tabular or massive : (“table-tops” or irregular”)

• discordant: cuts across other rock bodies

• concordant : parallels other rock bodies

Tabular intrusive bodiesTabular intrusive bodiesforming below the earth’sforming below the earth’ssurfacesurface

DikesDikes

SillSill

TabularTabular

BatholithBatholith

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BatholithBatholithBatholithBatholith

LoccolithLoccolithLoccolithLoccolith

DikeDikeDikeDike

SillSillSillSill

StockStockStockStock

Intrusive BodiesIntrusive Bodies

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Intrusive Bodies:Intrusive Bodies:

Batholith: intrusive body GREATER than 40 mi2

Stock: intrusive body LESS than 40 mi2

Dike: intrusive body cutting across strata (discordant)

Sill: intrusive/extrusive body parallel to strata (concordant)

Laccolith: “mushroom-shaped” intrusive body forming a dome-like structure

Sierra Nevada BatholithSierra Nevada BatholithGranite/DioriteGranite/Diorite

Melting magma rises andMelting magma rises andmixes with continentalmixes with continentalmaterial (high SiOmaterial (high SiO22) and) andsolidifies beneath the solidifies beneath the surface.surface.

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I I I I intrusiveintrusiveintrusiveintrusive1.1.Given the block diagram below, describeGiven the block diagram below, describe the following plutonic (intrusive) typethe following plutonic (intrusive) type features:features:

rocks.rocks.