3 Pats Rock Cycle

7/30/2019 3 Pats Rock Cycle

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Rocks and RockRocks and Rock--FormingForming

ProcessesProcesses

Smith & Pun, Chapter 3Smith & Pun, Chapter 3

3.4 How are the rock classes3.4 How are the rock classes

related to one another?related to one another?

The Rock Cycle

Processes link

typesPlate tectonics is

driving force

3.13.1 How and where do rocks form?How and where do rocks form?

If we look closely at the loose materials undIf we look closely at the loose materials und

our feet what might we expect to find?our feet what might we expect to find?

Consider a beach along a sandstone cliff.Consider a beach along a sandstone cliff.

Fig 3.2a


Bits of shell; calcite or aragonite

Small glassy

grains of quartz Some few dark

grains of magnetite

If we look closely we see

minerals of various sorts

Fig 3.2b


But what else can we tell from observing?

The quartz

grains are

small, and

very round

Even the

dark grains

are round-

ish, if small

and the shell fragments, while flat, have rounded edges. What

processes are involved? Watch the waves moving the sand

Fig 3.2b


So, we know for some rocksSo, we know for some rocksThey are made from bits of other rocks and/orThey are made from bits of other rocks and/or

organically derived mineralorganically derived mineral--like matterlike matter

(shells)(shells)

An abrasive process moves and reshapes theAn abrasive process moves and reshapes the

fragmentsfragments

Fragments of older rock consolidate into newFragments of older rock consolidate into new

rock with a mineral cementrock with a mineral cement


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Water coming from a spring atop a rock. The

entombed trees bear evidence that a rock-forming

process is ongoing...The rock around the spring

itself appears to be made of

layered crystalline materialwith sponge-like holes.

Fig 3.3


So, we now know that some other rocks:So, we now know that some other rocks:

Form by precipitation of minerals from waterForm by precipitation of minerals from water

These minerals cement together other looseThese minerals cement together other loose

sediments, ORsediments, OR

These precipitating mineralsThese precipitating minerals intergrowintergrow

3.13.1 How and where do rocks form?How and where do rocks form?At a volcano lava quickly

freezes into rock. That rock

(bottom left) is made of very small

mineral crystals. Other rocks

coughed up from deep within the

volcano (bottom right) show

mosaics oflarger crystals and

banded structures.

Fig 3.4


Rocks around a volcano form by much differentRocks around a volcano form by much different

processes than in the previous examinations ofprocesses than in the previous examinations of

sediments and sedimentary rockssediments and sedimentary rocks

Minerals in these rocks have angular shapesMinerals in these rocks have angular shapes

Must have crystallized in place from molten rockMust have crystallized in place from molten rock

Both the spring and volcano have intergrownBoth the spring and volcano have intergrown

crystals, but the origins of crystals are differentcrystals, but the origins of crystals are different


Most natural rocks areMost natural rocks are aggregates of mineralaggregates of mineralgrainsgrains..

Many rocks originate from observableMany rocks originate from observable

processes on or near Earthprocesses on or near Earths surface.s surface.

The presence of rocks that are not related toThe presence of rocks that are not related to

surface processes suggests a relation tosurface processes suggests a relation to

internal processes.internal processes.

3.2 Can rocks be classified according3.2 Can rocks be classified according

to the processes that form them?to the processes that form them?

ClassificationClassification is a central theme of scienceis a central theme of scienceOur observations ofOur observations ofexternal processesexternal processes atat

EarthEarths surface show that active surfaces surface show that active surface

processes produce minerals and hence rocksprocesses produce minerals and hence rocks

Internal processesInternal processes observations of rocksobservations of rocks

unassociated with surface processes must beunassociated with surface processes must be

related to some other group of phenomenarelated to some other group of phenomena


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The three classes of rocks: sedimentary, igneous, and

metamorphic

Rock CycleRock Cycle

Proportions of Rock Types onProportions of Rock Types on

the Earththe Earth

Soil, sediment,

weathered rock,

regolith

gneousgneous

RocksRocks

IdentificaIdentifica

tiontion



Igneous rocksIgneous rocks

Fig 3.6

Form from moltenForm from molten

magmamagma

Volcanic rocks formVolcanic rocks form

above ground =above ground =

extrusiveextrusive

Plutonic form belowPlutonic form below

ground =ground = intrusiveintrusive


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Layered extrusiveLayered extrusive

rocksrocks

Massive plutonic rockMassive plutonic rockbodiesbodies

Fig 3.7

on:on:CompositionComposition

What minerals makeWhat minerals makeup the rock?up the rock?

TextureTexture

What is the shape,What is the shape,size and orientationsize and orientationof the mineral grainsof the mineral grainsthat make up thethat make up therock?rock?

Major Difference:Major Difference:Crystalline (below)Crystalline (below)vs. fragmental (left)vs. fragmental (left)

Igneous CompositionIgneous CompositionMafic

Felsic

Richer in Magnesium and Iron

(Fe) and poorer in silica;

example, basalt.

Richer in feldspar and quartz,

and thus richer in silica;

example, rhyolite.

Igneous TexturesIgneous Textures -- CrystallineCrystalline

Coarse Grained Fine Grained

Aka: phaneritic Aka: aphanitic


Porphyritic

Phenocrysts

Groundmass


Glassy

Vesicular


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Igneous TexturesIgneous Textures volcaniclasticvolcaniclastic

or fragmentalor fragmental

Made of rock fragments

rather than crystals

Igneous RocksIgneous Rocks

GraniteDioriteGabbro

RhyoliteAndesiteBasalt Dacite

Granodiorite

Mafic Intermediate Felsic

Fine

Coar

se

Increasing silica

Ultramafic RockUltramafic Rock

-- DuniteDuniteGrain Size:Grain Size:

Plutons and VolcanoesPlutons and Volcanoes

Fine

Grained

Coarse

Grained

Name this Igneous FeatureName this Igneous Feature

Igneous Dike

Figure 4.7aFigure 4.7a


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Figure 4.7bFigure 4.7b Figure 4.8bFigure 4.8b

Figure 4.10bFigure 4.10b

Plutonic StructuresPlutonic Structures

Inclusions: Xenoliths

Source: William E. Ferguson

Why do we find Plutons at theWhy do we find Plutons at the

Surface?Surface?

Sierra Nevada Mountains Batholith, California

Uplift & Erosion


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Intermission: QuizIntermission: uizWhat Can you say about theWhat Can you say about the

history of this rock?history of this rock?

Basalt

Magma produced by

partial melting of the

mantle that erupted

from a volcano at adivergent zone or hot

spot.

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nterm ss on: u znterm ss on: u z

What Can you say about theWhat Can you say about the


Gabbro

Magma produced by

partial melting of

mantle that cooledunderground (plutonic)

at a divergent zone or

hot spot.




Granite

Magma produced by


continental crust that

cooled underground

(plutonic) at a

subduction zone or

continental hot spot.




Andesite

Magma produced by


mantle, ocean crust &

continental crust, that

erupted from a volcano

at a subduction zone.



Sedimentary RocksSedimentary Rocks

Formed byFormed by depositiondeposition andand

precipitationprecipitation of materialsof materials

coming from the breakdowncoming from the breakdown

of older rocksof older rocks

Weathering breaks downWeathering breaks down

and/or dissolves parts of aand/or dissolves parts of a

rockrock

Dissolved ionsDissolved ions

Clastic sedimentClastic sediment

Fig 3.5



Fragments formFragments form clasticclastic sedimentary rocksedimentary rock

Dissolved ions formDissolved ions form chemicalchemical sedimentary rockssedimentary rocksPrecipitatesPrecipitates such as halitesuch as halite

Generally carbonates, halides, some oxides also areGenerally carbonates, halides, some oxides also are

the cement for clastic materialsthe cement for clastic materials

LithificationLithification sediments of either typesediments of either type

accumulate in layers, compress under their ownaccumulate in layers, compress under their own

mass and/or what buries them, and, withmass and/or what buries them, and, with

cements, form a hardened masscements, form a hardened mass


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Metamorphic rocksMetamorphic rocks

rocks that have changedrocks that have changed

Increased temperature (butIncreased temperature (but

not melted)not melted)

Increased pressureIncreased pressure

Presence of hot fluidsPresence of hot fluids

(chemical reactions)(chemical reactions)

Changes to:Changes to:

Shape and/or orientation ofShape and/or orientation of

crystalscrystals

The mineralsThe minerals

Fig 3.8



Three classes of metamorphic rocksThree classes of metamorphic rocks

RegionalRegional metamorphic rocks exmetamorphic rocks ex = schist &= schist &

gneissgneiss

PressurePressure--drivendriven ; occur across vast regions =>ccur across vast regions =>convergentconvergent plate boundariesplate boundaries

ContactContact metamorphic rocksmetamorphic rocks--exex = hornfels= hornfels

Thermally driven and common along boundaries ofThermally driven and common along boundaries of

igneous intrusionsigneous intrusions or under lava flow zonesor under lava flow zones

HydrothermalHydrothermal metamorphic rocksmetamorphic rocks

Driven byDriven by hothot--fluid chemical changesfluid chemical changes

Often associated with previous typesOften associated with previous types



Classic regional metamorphic structures in Scotland

Note bands, but also that they are folded

back on one another, attesting to the

pressure that drives such change.



Igneous rocksIgneous rocks crystallize as intergrown mineral massescrystallize as intergrown mineral massesfrom thefrom the molten statemolten state in extrusive or intrusive fashionin extrusive or intrusive fashion

SedimentarySedimentary rocks derive from weathering ofrocks derive from weathering ofpreviously existing rocks as particles or dissolved ions.previously existing rocks as particles or dissolved ions.Clastic sed. rocks form from the fragments, andClastic sed. rocks form from the fragments, andchemicalchemical sedsed rocks from the dissolved portion...rocks from the dissolved portion...

MetamorphicMetamorphic rocks form by the reaction of preexistingrocks form by the reaction of preexistingrocks in the presence of heat, pressure, fluids, orrocks in the presence of heat, pressure, fluids, or

combinations thereof, producing new minerals andcombinations thereof, producing new minerals and

rocks.rocks.

Rocks can be classifiedRocks can be classified descriptivelydescriptively ororgeneticallygenetically.. TheThe

three classes of rocks are genetic categories...three classes of rocks are genetic categories...

3.3 How do we know3.3 How do we know how tohow to

determine rock origins?determine rock origins?

How do geologists use observations of rocks toHow do geologists use observations of rocks toinfer process?infer process?

TheThe 33--classclass ggeneticenetic grouping isgrouping is newnew (only ~200(only ~200

yr oldyr old----Prior to the current genetic groups, therePrior to the current genetic groups, there

were competing thoughts)were competing thoughts)

DescriptiveDescriptive classes require only keen observationclasses require only keen observation

and sufficient adjectives.and sufficient adjectives.



Neptunism:Abraham Werner

(17491817) believed that all rocksderived from processes of chemical

precipitation.

Concluded that the bottom layers of

Earths rocks were least soluble and

thus first to precipitate (primitive

rocks), and that they were overlain

by stratified rocks and finally

washed deposits of loose

materials. Volcanic activity was

caused by coal fires underground

melting rock overhead.

Fig 3.10


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determine rock origins?determine rock origins?Vulcanism (or plutonist)Vulcanism (or plutonist) the belief that most rocksthe belief that most rocks

were formed by igneouswere formed by igneous--like origins.like origins.

Championed byChampioned by James HuttonJames Hutton (1726(17261797)1797)

Noted that veins ofNoted that veins ofPrimitive RockPrimitive Rock extended up into theextended up into the

Stratified FormationsStratified Formations and that the boundaries of contactand that the boundaries of contact

with the latter werewith the latter were baked.baked.

Posited that layered rocks were mainly lava flows of somePosited that layered rocks were mainly lava flows of some

kind.kind.

One of Huttons sketches



Neptunists, led by A. Werner, asserted that nearlyNeptunists, led by A. Werner, asserted that nearly

all rocks came from precipitates. Agreed with aall rocks came from precipitates. Agreed with a

view by many at the time, that Earth wasview by many at the time, that Earth was

internally coldinternally cold

VulcanistsVulcanists, following Hutton, used careful, following Hutton, used careful

observation to refute Neptunistsobservation to refute Neptunists claims byclaims by

demonstrating the presence of igneous rocks anddemonstrating the presence of igneous rocks and

showing evidence of Earthshowing evidence of Earths internal heat...s internal heat...

So how did geologists resolve these competing views?So how did geologists resolve these competing views?

Competing views met in the middleCompeting views met in the middle ----Hypothesized andHypothesized and

tested the observations they madetested the observations they made



Metamorphic rocks are preexisting rocks thatMetamorphic rocks are preexisting rocks that

have changed.have changed.

Igneous rocks are made from any rocks thatIgneous rocks are made from any rocks that

have melted and recrystallized.have melted and recrystallized.

Sedimentary rocks come from weathered bits ofSedimentary rocks come from weathered bits of

other preexisting rocks.other preexisting rocks.

Thus, all three rock types are connected in some manner.Thus, all three rock types are connected in some manner.



The Rock Cycle

Processes link

typesPlate tectonics is

driving force

3 Pats Rock Cycle

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