3.4 How are the rock classes Rocks and Rock-Forming Processes · 1 Rocks and Rock-Forming Processes...

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Rocks and RockRocks and Rock--Forming Forming ProcessesProcesses

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

3.4 How are the rock classes 3.4 How are the rock classes related to one another?related to one another?

The Rock Cycle

Processes link types

Plate 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 undeIf we look closely at the loose materials undeour 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 rocks

They are made from bits of other rocks and/or They are made from bits of other rocks and/or organically derived mineralorganically derived mineral--like matter like matter (shells)(shells)……

An abrasive process moves and reshapes the An abrasive process moves and reshapes the fragmentsfragments……

Fragments of older rock consolidate into new Fragments of older rock consolidate into new rock with a mineral cementrock with a mineral cement

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3.13.1 How and where do rocks form?How and where do rocks form?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 material with sponge-like holes.

Fig 3.3

3.13.1 How and where do rocks form?How and where do rocks form?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 loose These minerals cement together other loose sediments, ORsediments, OR……

These precipitating minerals These 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 smallmineral crystals. Other rocks coughed up from deep within the volcano (bottom right) show mosaics of larger crystals and banded structures.

Fig 3.4


Rocks around a volcano form by much different Rocks around a volcano form by much different processes than in the previous examinations of processes 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 intergrown Both the spring and volcano have intergrown crystals, but the origins of crystals are differentcrystals, but the origins of crystals are different


Most natural rocks are Most natural rocks are aggregates of mineral aggregates of mineral grainsgrains..Many rocks originate from observable Many rocks originate from observable processes on or near Earthprocesses on or near Earth’’s surface.s surface.The presence of rocks that are not related to The presence of rocks that are not related to surface processes suggests a relation to surface processes suggests a relation to internal processes.internal processes.

3.2 Can rocks be classified according 3.2 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 of Our observations of external processesexternal processes at at EarthEarth’’s surface show that active surface s surface show that active surface processes produce minerals and hence rocksprocesses produce minerals and hence rocksInternal processesInternal processes –– observations of rocks observations of rocks unassociated with surface processes must be unassociated with surface processes must be related to some other group of phenomena related 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 on Proportions of Rock Types on the Earththe Earth

Soil, sediment,weathered rock,regolith

Igneous Igneous RocksRocks

IdentificaIdentificationtion


Igneous rocksIgneous rocks

Fig 3.6

Form from molten Form from molten magmamagma……

Volcanic rocks form Volcanic rocks form above ground = above ground = extrusiveextrusive……

Plutonic form below Plutonic form below ground = ground = intrusiveintrusive

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Layered extrusive Layered extrusive rocksrocks

Massive plutonic rock Massive plutonic rock bodiesbodies

Fig 3.7

Rock Identification is based Rock Identification is based on:on:CompositionComposition

What minerals make What minerals make up the rock?up the rock?

TextureTextureWhat is the shape, What is the shape, size and orientation size and orientation of the mineral grains of the mineral grains that make up the that make up the rock?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 Textures Igneous Textures -- CrystallineCrystalline

Coarse Grained Fine GrainedAka: phaneritic Aka: aphanitic

Igneous Textures Igneous Textures -- CrystallineCrystallinePorphyritic

Phenocrysts

Groundmass

Igneous Textures Igneous Textures -- CrystallineCrystallineGlassy

Vesicular

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Igneous Textures Igneous Textures –– volcaniclastic volcaniclastic or fragmentalor fragmental

Made of rock fragmentsrather than crystals

Igneous RocksIgneous Rocks

GraniteDioriteGabbro

RhyoliteAndesiteBasalt Dacite

Granodiorite

Mafic Intermediate Felsic

Fine

Coa

rse

Increasing silica

Ultramafic RockUltramafic Rock-- DuniteDunite

Grain Size:Grain Size:Plutons and VolcanoesPlutons and Volcanoes

FineGrained

CoarseGrained

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 the Why do we find Plutons at the Surface?Surface?

Sierra Nevada Mountains Batholith, California

Uplift & Erosion

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Intermission: QuizIntermission: QuizWhat Can you say about the What 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 a

divergent zone or hot spot.

http://itc.gsw.edu/faculty/tweiland/basalt.jpg



Gabbro

Magma produced by partial melting of mantle that cooled

underground (plutonic)at a divergent zone or

hot spot.



Granite

Magma produced by partial melting of the continental crust that cooled underground

(plutonic) at a subduction zone or

continental hot spot.



Andesite

Magma produced by partial melting of the mantle, ocean crust & continental crust, that

erupted from a volcano at a subduction zone.


Sedimentary RocksSedimentary Rocks–– Formed by Formed by depositiondeposition and and

precipitationprecipitation of materials of materials coming from the breakdown coming from the breakdown of older rocksof older rocks

–– Weathering breaks down Weathering breaks down and/or dissolves parts of a and/or dissolves parts of a rockrock

Dissolved ionsDissolved ionsClastic sediment Clastic sediment

Fig 3.5


Fragments form Fragments form clasticclastic sedimentary rock sedimentary rock

Dissolved ions form Dissolved ions form chemicalchemical sedimentary rockssedimentary rocks••Precipitates Precipitates –– such as halitesuch as halite••Generally carbonates, halides, some oxides also are Generally carbonates, halides, some oxides also are the cement for clastic materialsthe cement for clastic materials

LithificationLithification –– sediments of either type sediments of either type accumulate in layers, compress under their own accumulate in layers, compress under their own mass and/or what buries them, and, with mass and/or what buries them, and, with cements, form a hardened masscements, form a hardened mass

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Metamorphic rocks Metamorphic rocks ––rocks that have changedrocks that have changed–– Increased temperature (but Increased temperature (but

not melted)not melted)–– Increased pressureIncreased pressure–– Presence of hot fluids Presence of hot fluids

(chemical reactions)(chemical reactions)Changes to:Changes to:–– Shape and/or orientation of Shape and/or orientation of

crystalscrystals–– The mineralsThe minerals

Fig 3.8


Three classes of metamorphic rocksThree classes of metamorphic rocks

RegionalRegional metamorphic rocks ex metamorphic rocks ex = schist & = schist & gneissgneiss

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

ContactContact metamorphic rocksmetamorphic rocks--ex ex = hornfels= hornfelsThermally driven and common along boundaries of Thermally driven and common along boundaries of igneous intrusionsigneous intrusions or under lava flow zonesor under lava flow zones

HydrothermalHydrothermal metamorphic rocksmetamorphic rocksDriven by Driven by hothot--fluid chemical changesfluid chemical changesOften associated with previous typesOften associated with previous types


Classic regional metamorphic structures in ScotlandNote 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 masses crystallize as intergrown mineral masses from the from the molten statemolten state in extrusive or intrusive fashionin extrusive or intrusive fashion……

Sedimentary Sedimentary rocks derive from weathering of rocks derive from weathering of previously existing rocks as particles or dissolved ions. previously existing rocks as particles or dissolved ions. Clastic sed. rocks form from the fragments, and Clastic sed. rocks form from the fragments, and chemical chemical sedsed rocks from the dissolved portion...rocks from the dissolved portion...MetamorphicMetamorphic rocks form by the reaction of preexisting rocks form by the reaction of preexisting rocks in the presence of heat, pressure, fluids, or rocks in the presence of heat, pressure, fluids, or combinations thereof, producing new minerals and combinations thereof, producing new minerals and rocks.rocks.

Rocks can be classified Rocks can be classified descriptivelydescriptively or or geneticallygenetically.. The The three classes of rocks are genetic categories...three classes of rocks are genetic categories...

3.3 How do we know 3.3 How do we know …… how to how to determine rock origins?determine rock origins?

How do geologists use observations of rocks to How do geologists use observations of rocks to infer process?infer process?–– The The ““33--classclass”” ggeneticenetic grouping is grouping is ““newnew”” (only ~200 (only ~200

yr oldyr old----Prior to the current genetic groups, there Prior to the current genetic groups, there were competing thoughts)were competing thoughts)

–– DescriptiveDescriptive classes require only keen observation classes require only keen observation and sufficient adjectives.and sufficient adjectives.


Neptunism: Abraham Werner(1749–1817) believed that all rocks derived from processes of chemical precipitation.

Concluded that the bottom layers of Earth’s 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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Vulcanism (or plutonist) Vulcanism (or plutonist) –– the belief that most rocks the belief that most rocks were formed by igneouswere formed by igneous--like origins. like origins. –– Championed by Championed by James HuttonJames Hutton (1726(1726––1797)1797)–– Noted that veins of Noted that veins of ““Primitive RockPrimitive Rock”” extended up into the extended up into the

““Stratified FormationsStratified Formations”” and that the boundaries of contact and that the boundaries of contact with the latter were with the latter were ““baked.baked.””

–– Posited that layered rocks were mainly lava flows of some Posited that layered rocks were mainly lava flows of some kind.kind.

One of Hutton’s sketches


Neptunists, led by A. Werner, asserted that nearly Neptunists, led by A. Werner, asserted that nearly all rocks came from precipitates. Agreed with a all rocks came from precipitates. Agreed with a view by many at the time, that Earth was view 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 by claims by demonstrating the presence of igneous rocks and demonstrating the presence of igneous rocks and showing evidence of Earthshowing evidence of Earth’’s 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 middle Competing views met in the middle ----Hypothesized and Hypothesized and tested the observations they madetested the observations they made……


Metamorphic rocks are preexisting rocks that Metamorphic rocks are preexisting rocks that have changed.have changed.

Igneous rocks are made from any rocks that Igneous rocks are made from any rocks that have melted and recrystallized.have melted and recrystallized.

Sedimentary rocks come from weathered bits of Sedimentary 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 types

Plate tectonics is driving force

3.4 How are the rock classes Rocks and Rock-Forming Processes · 1 Rocks and Rock-Forming Processes...

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