chapter 22: minerals, rocks, and volcanoesmtweb.mtsu.edu/nchong/psci1030-chap022-minerals, rocks,...

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Copyright © Houghton Mifflin Company. All rights reserved. 21 | 1

Chapter 22: Minerals, Rocks,and Volcanoes

• Homework: All questions on the“Multiple-Choice” and the odd-numbered questions on “Exercises”sections at the end of the chapter.


Geology

• Geology – the study of the the composition,structure, physical/chemical processes, andhistory of the Earth

– Geology is also related to the study of planets andmoons of our solar system

• The basic concepts of geology are introducedand briefly discussed in the next four chapterso that we may better understand thephysical nature of the world that we live in.

Intro


Minerals

• Mineral – a naturally occurring, inorganic,crystalline (solid) substance consisting of oneor more chemical elements in fairly specificproportions with a distinctive set of physicalproperties

• Minerals are around us everywhere – someare quite valuable (diamond, sapphires,emeralds) other minerals are very common(calcite, quartz.)

• Mineralogy – the study of minerals

Section 21.1 Copyright © Houghton Mifflin Company. All rights reserved. 21 | 4

Minerals

• We also speak of certain rock-types (ores) asbeing rich in minerals.

– For example an iron ore, gold ore, or copper ore

– Rocks classified as ores have a commerciallyvaluable amount of a some type of element ormineral.

• In many cases, mineral names have sometype of historical connotation and reflect thename of a geographic locality or a person’sname.

Section 21.1


Mineral Composition

• Eight chemical elements are the majorconstituents of most of the minerals.

• These elements are the most abundantelements in the Earth’s crust and include O,Si, Al, Fe, Ca, Na, K, and Mg.

• Over 2000 minerals have been identified inthe Earth’s crust.

• Only about 20 of these minerals are common.

• Less than 10 minerals account for more than90% of the Earth’s crust by mass.


Relative Abundance by Mass ofElements in the Earth’s Crust

• Only twoelements, O& Si, accountfor 74% ofthe elements(by mass) inthe Earth’scrust.

Section 21.1

Section 21.1

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Mineral Groups – the Silicates

• Most minerals that form crustal rockshave significant amounts of oxygen andsilicon.

• The mineral quartz (silicon dioxide orsilica) is entirely composed of O & Si,and has the chemical formula, SiO2.

• Quartz and many other minerals thatcontain O & Si are called silicates.

• Silicates are the most abundant familyof rock-forming minerals in the crust.


Silicates

• Silicate structure is based on a network ofSiO4

-4 tetrahedra.

• These basic building blocks of the silicatesare called the silicon-oxygen tetrahedra.– Covalent bonding occurs within the

tetrahedron.

• A significant amount of variation occurs inthe structural arrangement of thesetetrahedra.

• The silicon-oxygen tetrahedra may existindependently (no sharing of O) or theymay share oxygens in various ways.

Section 21.1


The Silicon-OxygenTetrahedron

• Basic BuildingBlock of theSilicateMineral Family


Silicates

• In addition to O & Si, most silicate mineralsalso contain Al and one or more otherelements.

• The various silicate minerals result from thedifferent structural arrangement of the silicon-oxygen tetrahedron building blocks and thedifferent metal ions within these structures.

• The silicon-oxygen tetrahedra may bearranged independently, in single chains, indouble chains, as continuous sheets, and asa 3-dimensional network.

Section 21.1


MolecularStructures ofSeveral CommonSilicate Minerals


Feldspars – Most Common Silicate Minerals

• Feldspar group - a group of structurally-related (3-dimension network) minerals,that as a set, are the most abundantminerals in the Earth’s crust

• There are two basic types of feldspars:

• Plagioclase feldspars – O, Si, Al, andCa or Na

• Potassium feldspars – O, Si, Al, and K

• All silicates have Si and O in theirformulas.

Section 21.1

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Nonsilicate Minerals

• All of the other minerals in the crust areconsidered “nonsilicate.”

• Nonsilicate minerals comprise less than10% of the minerals in the crust.

• Carbonates, oxides, and sulfides arethe most common nonsilicate minerals.

• Pure elements such as gold and silver,and some gemstones (diamond, ruby,sapphire) are also nonsilicates.


Carbonates

• Carbonate minerals form when the carbonateion (CO3

2-) bonds with certain metal ions.

– Ca, Mg, Mn, Fe, and others may bond with CO32-

• CaCO3, calcite, is a very common carbonatemineral.

– CaCO3 readily reacts (dissolves) in acidic water.

• Calcite is a very common mineral componentof the the rock limestone.

– Limestones will dissolve over time in slightly acidicground waters, contributing to cave formation.

Section 21.1


Oxides & Sulfides

• Oxide minerals form through the bonding of Oions with metallic ions.

– Fe2O3, SnO2, and UO2 are all oxide ores for theirrespective metallic ion.

• Sulfides minerals form through the bonding ofS ions with metallic ions.

– Fe, Pb, Cu, Zn, and others may bond with S2-.

– FeS2 is pyrite, commonly called “fool’s gold.”

– CuFeS2, PbS, ZnS are all sulfide ores for theirrespective metallic ion.


Identification of Minerals

• Mineral classification is based on bothphysical and chemical properties.

• This is advantageous because there aresome minerals that have the same chemicalformula but different molecular structures.

• For example the two minerals graphite anddiamond are both made of pure C but theyare dramatically different minerals.– Pure diamond is very hard, clear, and crystalline.

– Pure graphite is soft and black (dry lubricant andpencil lead.)

Section 21.1


Identification of Minerals

• Certainly minerals can be identified by carefuland costly chemical analysis.

• Many minerals can be easily identified bytaking advantage of the distinctive physicalproperties that each exhibits.

• These physical properties are easily learnedand well known by any serious rock andmineral collector.

– A particular physical property that helps ID onemineral may not help with another, so the key is tofind which property ID’s which mineral.


Physical Properties – Crystal Form

• Crystal form – the outward form of a crystal isa visible representation of its internalmolecular arrangement

– Many minerals have characteristic crystal forms.

– If crystallization occurs in unrestricted space, withjust the right components, and just the right P & Tconditions a perfect crystal forms. This is very rare.

• Usually an aggregate of crystals form withnone exhibiting a perfect crystal form.

Section 21.1

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Halite(NaCl)

Galena(PbS)Pyrite

(FeS2)

Fluorite(CaF2)

Each of these minerals has an orderlyinternal atomic structure that forms a cube.

Copyright © Bobby H. Bammel. All rights reserved


Physical Properties – Hardness & Cleavage

• Hardness – a mineral’s resistance toscratching or abrasion

– Hardness is relative to other minerals orsubstances.

• Cleavage – the tendency of someminerals to break along distinct planes

– These planes represent planes in thecrystal’s structure where the bonds areweakest.

Section 21.1


MohsHardnessScaleFriedrich Mohs(1773-1839)


Physical Properties – Fracture & Color

• Fracture – irregular or random breakage of amineral– Minerals that do not cleave have no planes of

weakness in their structure, therefore do not breakin a uniform manner.

• Color – the property of reflecting particularlight wavelength– Although this is probably the first property noticed,

it is usually not reliable.

– Quartz, for example, occurs in many colors,including clear, milky, smoky, yellow, purple, red,orange, pink.

Section 21.1


Physical Properties – Streak & Luster

• Streak – the color of the mineral in powderform– Although an intact mineral sample my exhibit

several colors, the streak color is always thesame.

• Luster – how a mineral’s surface reflects light– Minerals may exhibit either metallic or nonmetallic

luster.

– Metallic lusters appear like polished metal.

– Nonmetallic lusters can be vitreous (glassy),adamantine (diamond), pearly (opal), greasy(talc), or earthy/dull (clay.)


Hematite – red brown streak

Source: Breck P. Kent

Section 21.1

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Physical Properties – Specific Gravity

• Specific gravity – ratio of a mineral’s weight tothe weight of an equal volume of pure water

• Many minerals have specific gravitiesbetween 3-4 (in other words, 3 to 4 times asheavy as water.)

• Some minerals have a significantly higherspecific gravity.

– Galena (PbS) has a specific gravity of 7.6.

– Pure gold (Au) has a specific gravity of 19.3.


Rocks

• Rock – a natural, solid, cohesiveaggregate of one or more minerals

• Different types of rocks comprise thevast majority of the Earth’s crust.

• In general, when we look at a rock wedo not see the individual minerals.

• Rocks can be divided into three majortypes as a result of the way theyoriginated.

Section 21.2


Igneous Rocks

• Igneous rock – a type of rock formed from amolten material that has cooled and solidified

• Magma – molten rock material that originatesdeep within the Earth

– Rocks that solidify from a magma, beneath theEarth’s surface, are called “intrusive” igneousrocks.

• Lava – molten rock material that reaches theEarth’s surface due to a volcanic eruption

– Rocks that solidify from lava, at the surface, arecalled “extrusive” igneous rocks.


Enchanted Rock, TX – Solidified Magma


Section 21.2


Kalapana Flow, HI – Solidified Lava



Sedimentary Rocks

• Sedimentary rock – rocks that form at orvery near the surface of the Earth due tocompaction and cementation of sediments

• The sediments that comprise sedimentaryrocks come from three general sources:

1) Rock fragments due to the erosion of preexisting(older) rocks

2) Minerals chemically precipitated from solution

3) Plants or animal remains (fossils)

Section 21.2

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Grand Canyon, AZ –Sedimentary Rock Layers



Metamorphic Rocks

• Metamorphic rock – forms by thealteration of a preexisting rock due tothe effects of pressure, hightemperature, and/or a chemical change

• Metamorphism generally occurs wellbelow the surface of the Earth but atshallower depths and temperatures thanwould cause the rock to melt.

Section 21.2


El Paso, TX Contorted Metamorphic Rocks



Uniformitarianism

• Beginning with the Scottish physician/scientistJames Hutton, scientists started to realizethat ancient rocks were formed the same wayas modern rocks.

– Since they were formed the same way, they canbe interpreted similarly.

• Unifomitarianism – geologic processesoccurring today operated similarly in the pastand can therefore be used to explain pastgeologic events

– “The present is the key to the past”

Section 21.2


Hutton and the Rock Cycle

• James Hutton recognized that rocks inthe Earth’s crust were continuouslybeing formed, broken down, and thenre-formed.– These are the same processes that are

responsible for the three types of rocks –igneous, sedimentary, and metamorphic.

• This model of the continuous cycling ofthe crustal rock material is called therock cycle.


The RockCycle

Section 21.2

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Hutton and Geologic Time

• Hutton also recognized that the processesthat form and breakdown the different rock-types take enormous amounts of time.

– Thus he hypothesized the Earth to be very old.

• Central to the science of geology are thefollowing three concepts:

– The principle of uniformitarianism

– The rock cycle

– The recognition that the Earth is very old


Formation of Igneous Rocks

• Igneous rocks form from the solidificationof molten material that generallyoriginates far beneath the surface of theEarth.

• Magma – molten material beneath theEarth’s surface

• Lava – molten material a the Earth’ssurface– The term “lava” is also used to describe the

resulting and cooled igneous rock.

Section 21.3


Igneous Rocks

• Most geologists think that the first rocks onEarth were formed as the outer crust slowlysolidified billions of years ago.

• Therefore, the first rocks formed wereigneous.

• Since these initial rocks were formed,geologic processes have modified, covered,or eroded most of these ancient materials.

• It is estimated that approximately 80% of theEarth’s crust is comprised of igneous rocks.


Types of Igneous Rocks

• Igneous rocks can be divided into twobasic categories:– Extrusive igneous – igneous rocks that

cool at the surface of the Earth, generallydue to some type of volcanic eruption

– Intrusive igneous – igneous rocks thatcooled somewhere beneath the surface ofthe Earth• Intrusive igneous rocks only appear at the

Earth’s surface due to erosion of the overlyingrocks or due to some type of uplift.

Section 21.3


Plate Tectonics

• The prediction of individual volcanic eruptionsis generally not possible.

• However we are aware of specific trendswhere volcanic eruptions typically do occur.

• Most active volcanoes are located alonglinear zones, particularly along the margins ofthe Pacific Ocean.

– The so-called “ring of fire”

• The theory of plate tectonics can explain whyvolcanoes occur where they do.


Plate Tectonics

• According to the Theory of Plate Tectonicsthe solid outermost shell of the Earth is calledthe lithosphere.– The lithosphere is separated into several large

and small fragments, called plates.

• The rigid lithosphere rests or “floats” on asemimolten layer called the asthenosphere.

• We will study plate tectonics in greater detailin the next chapter.

• According to plate tectonics the lithosphericplates slowly move over the semimoltenasthenosphere.

Section 21.3

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Most Volcanoes and Earthquakes occur along PlateBoundaries in thePacific Ocean – “Ring of Fire”


Plate Boundaries

• The lithospheric plates interact witheach other in three basic ways:

• Convergent boundary – two platesmove towards each other

• Divergent boundary – two plates moveaway from each other

• Transform boundary – two plates slidepast each other

• The vast majority of the volcanoes onEarth occur at convergent boundaries.

Section 21.3


Convergent Boundaries

• When both converging plates consist ofoceanic crust one plate will bend andslide slowly underneath the other – aprocess called subduction.

• During this process of subduction, anenormous amount of friction isgenerated, resulting in the melting ofrocks close to the subduction zone.

• The new magma rises to the surfaceand forms a volcanic island arc.


Subduction forms a Volcanic Island ArcThe islands of Japan are a result of subduction.

Section 21.3


Convergent Boundaries

• Another type of convergent boundary existswhen an oceanic plate converges with acontinental plate.

• The oceanic plate is more dense andtherefore is subducted beneath thecontinental plate.– Once again friction between the two plates will

melt nearby rocks.

• The Andes and Cascade Mountains are bothvolcanic mountain ranges formed by theconvergence of an oceanic & continentalplate.


Andes Mountains, South America


Section 21.3

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Igneous Rock Texture and Composition

• Recall, there are two types of igneous rocks:

– Intrusive – cool slowly below Earth’s surface

– Extrusive – cool quickly at the Earth’s surface

• Igneous rocks are classified according to twocriteria:

– Texture – generally refers to the size of themineral grains (crystals) in the igneous rock

– Mineral Composition – refers to the specific typeof mineral crystals in the igneous rock


Texture and Cooling Rate

• The texture (grain or crystal size) of anigneous rock is determined primarily by thecooling rate of the magma or lava.

• If a magma cools very slowly, deep within theEarth, large crystals can form.

• If lava cools rapidly at the surface, largecrystals do not have sufficient time to form.

• If lava cools extremely rapidly (in water orice), no crystals form, resulting in volcanicglass.

Section 21.3


Mt. Rushmore, South DakotaThis rock originally cooled slowly at great depth.



Mount Rushmore, South Dakota – GraniteVisible Crystals – Slow Cooling


Section 21.3


Effects of Cooling on the Textures ofIgneous Rocks


Composition and the Color ofIgneous Rocks

• In general, the color of an igneous rock givesa clue as to its composition.

• The amount of silica (SiO2) within an igneousrock can be used to classify it according tocomposition.

• If the magma/lava is high in silica, mineralsform with abundant Si, Na, & K. Theseminerals are usually light in color.

• When the magma/lava is low is silica,minerals rich in Fe, Mg, and Ca form that aredark in color.

Section 21.3

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Stone Mountain, Georgia. Light ColoredGranite – High in Silica



Dark Volcanic Basalt – Low in Silica


Section 21.3


Density and Composition

• The dark low-silica rocks are more densethan the light colored high-silica rocks.

• Continental plates are composed of lessdense high-silica rocks and therefore standhigh about sea level.

• Oceanic plates are composed of more denselow-silica rocks and therefore rest lower,usually below sea level.

• Andesite is the name given to many rocks ofmedium-silica content.


Common Igneous RocksOrganized by Composition

Section 21.3


Plutons

• Pluton – a large body of intrusiveigneous rock formed below the Earth’ssurface by solidification of magma

• Plutons are classified according to twocriteria:

– Size (exposed aerial extent) and shape ofthe intrusive rock body

– Relationship of the intrusive rock body tothe surrounding rock that they penetrate


Pluton

• A pluton is said to be discordant if it cutsacross the grain or layering of thesurrounding rock.– Batholiths and dikes are discordant

igneous bodies.

• A pluton is said to be concordant if it isparallel to the grain or layering of thesurrounding rock.– Sills and laccoliths are concordant igneous

bodies.

Section 21.4

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Plutonic Bodies


Batholiths

• Batholith – large exposed discordantintrusive igneous body– By definition, a batholith must have an

exposed area of at least 103 km2.

• They originally crystallize slowly at greatdepths below the surface of the Earth.

• Batholiths only become exposed at theEarth’s surface when powerful forcespush them up or when a very deepcanyon is carved by erosion.

Section 21.4


Yosemite National Park, CAPart of the Sierra Nevada Batholith



Dikes

• Dike – a discordant intrusive igneousbody formed from magma that filled avertical or near-vertical fracture

• Since most fractures are narrow, theinfilling intrusive igneous materialhardens into a thin vertical sheet ofigneous rock.

• The size and extent of individual dikesis quite variable, depending on thedimensions of the fractures they fill.

Section 21.4


Volcanic Dike Exposed in ColoradoThe dike is more resistant to weathering.



Sills & Laccoliths

• Sills are similar to dikes except they areconcordant to existing layering.

– Sills are injected between layers ofpreexisting rock.

• Laccoliths are also concordant, and areinjected between layers of preexistingrock.

• Laccoliths cause a noticeable blisteringof the overlying rock layers.

Section 21.4

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Green Mountain, WY – LaccolithThe preexisting layers of rock are blistered up.



Volcanoes

• Volcano – a hill or mountain formed bythe accumulation of lava and volcanicrock fragments ejected through a vent inthe Earth’s surface

• Three basic products are ejected fromactive volcanoes:

– Gas

– Lava

– Solid rocks

Section 21.4


Products of Volcanic Eruptions

• Gases are generally expelled from a volcanoduring its entire life cycle .

– Mainly H2O with lesser amounts of CO2 and H2S

• Lava is extruded from volcanoes in varyingamounts and varying viscosities.

• Some volcanoes also emit large volumes ofsolid material, collectively known as tephra.

– Tephra is expelled in almost any size.

– Tephra includes material that is initially ejectedmolten and hits the ground as a solid.


Tephra EmissionsActive Volcano near Huehuetenango, Guatamala


Section 21.4


Volcano – Eruptive Style

• Volcanoes display two basic eruptive styles:explosive and peaceful (non-explosive)

• The viscosity of the magma largelydetermines the eruptive style of a particularvolcano.

• Low viscosity magma can flow easily andtherefore is characterized by peacefuleruptions.

• High viscosity magma has difficulty flowingand only moves when subjected to greatpressure.


Magma Viscosity

• Magma viscosity, in turn, is dependent on twofactors: temperature and silica content

• The higher the temperature the lower theviscosity. (flows easier)

– In general magmas that originate deep within theEarth have higher temperatures.

• The higher the silica content the higher theviscosity. (more difficult to flow)

– In general magmas that originate at shallowdepths have a higher silica content.

Section 21.4

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Peaceful Eruptive Style

• Peaceful eruptions are involve basalticmagmas.

• Basaltic magmas originate deep withinthe Earth and therefore are very hot.

• Basaltic magmas also have a relativelylow silica content.

• Due to the combination of hightemperature and low silica content,basaltic magmas flow easily, resulting inpeaceful eruptions.


Basaltic Hawaiian Lava – Peaceful Eruption


Section 21.4


Basaltic Eruptions

• Most basaltic eruptions occur in two distinctsettings:

– Along the length of a divergent plate boundary

– In isolated areas, as a lithospheric plate movesover an unusually hot and stationary zone in theupper mantle, called a “hot spot” or mantle plume

• Hot spots appear at the surface as a line ofactive, dormant, and extinct volcanoes.

– The Hawaiian Islands and Emperor Seamountsare a continuous series of volcanoes formed overthe past 70 million years.


Hawaiian Islands &Emperor Seamount Chain

• Rising magma from a stationary “hot spot” in theasthenosphere forms volcanoes as the Pacific platemoves slowly over the hot spot.

Section 21.4


Explosive Eruptive Style

• Explosive eruptions generally occur alongsubduction zones and involve silica-richmagma.

• Magmas that originate within a subductionzone are not deep, and are therefore coolerin temperature.

• These magmas are also higher in silicacontent.

• Due to the combination of ‘low’ temperatureand high silica content, these magmas arevery viscous, resulting in explosive eruptions.


Volcanic Features High in the AndesMountains Due to Subduction Beneath

South America


Section 21.4

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Explosive Eruption - Mt. Saint HelensDue to Subduction Beneath North America



Volcanic Structures

• Molten volcanic material that reaches thesurface of the Earth creates a number ofdramatic surface expressions.

• The type of volcanic feature formed is largelydependent on the composition andtemperature of the lava extruded.

• Specific and common volcanic structuresinclude flood basalts, shield volcanoes,stratovolcanoes, cinder cones, and calderas.

Section 21.4


Fissure Eruptions and Flood Basalts

• Fissure eruptions take place when largevolumes of lava are extruded from longfractures along the surface of the Earth.

• Significant portions of Washington, Oregon,and Idaho are covered with a series ofancient flood basalts called the ColumbiaPlateau.

• The basaltic lava that formed these layerswas extremely fluid (high temperature & lowsilica content), eventually covering an area of576,000 km2.– Average thickness of 150 meters


Columbia Plateau Flood Basalts, Oregon


Section 21.4


Shield Volcanoes

• Shield volcanoes form from lava that isnot quite as fluid as the lava that formedflood basalts.

• The Hawaiian volcanoes are the classicexamples of shield volcanoes.

• Shield volcanoes have very gentleslopes and were formed by repeatedbasaltic flows.

• Eruptions are generally frequent but notparticularly violent.


Kilauea Crater, Hawaii- Shield VolcanoNote the very gentle slope away from the crater.


Section 21.4

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Stratovolcanoes

• Stratovolcanoes are composed of alternatinglayers of medium- and high-silica contentlava and tephra.

• These volcanoes generally have violenteruptions, although they usually erupt muchless frequently than shield volcanoes.

• Most of the majestic solitary mountains of theworld are stratovolcanoes.

– For example, Mount Fuji, Mount Shasta, Mt. St.Helens, and Mount Hood are all stratovolcanoes


Mt. Shasta, CA – Stratovolcano


Section 21.4


Cinder Cones

• Some volcanic features are relativelysmall.

• Cinder cones are built when a volcaniceruption is particularly high in gascontent, and thus emits primarily tephra.

• Cinder cones rarely exceed 300 m inheight.


Recent Volcanic Cinder Cone to the right ofan older ‘grown-over’ and eroded Cinder

Cone


Section 21.4


Caldera

• Caldera – a large, roughly circular,steep-walled depression at thevolcano’s summit, formed when the roofof the magma chamber collapses afterall the volcanic material within has beenemitted

• Crater Lake in southern Oregonoccupies a caldera formed by thecollapse of the magma chamber of aonce active stratovolcano.


Crater Lake occupies a Caldera at the topof Mt. Mazama


Section 21.4

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Historic Eruptions

• Mount Saint Helens 1980 – a stratovolcanolocated in southern Washington state thathad been dormant since 1857– This eruption devastated more than 400 km2 of

area and resulted in the death of more than 60people.

• Mount Pinatubo 1991 – located in thePhilippines– Propelled ash into the atmosphere to a height of

over 24 km

– Destroyed thousands of acres of cropland andforced the abandonment of Clark Air Force base


Devastation Due to 1980 Mt. St. Helen EruptionOver 8 km from the volcano’s summit


Section 21.4


Why do People Continue to live in theShadow of a Dangerous Volcano?

• An eruption by Mt. Vesuvius in A.D. 79buried 2000 residents of Pompeii.

• Yet the large and growing Italian city ofNaples lies only a few kilometers fromMt. Vesuvius.

• Soils derived from volcanic ash are richand fertile, resulting in an extremelyproductive agricultural area.


Sediments & Sedimentary Rocks

• Particles that are eroded from one placeare later deposited as sedimentssomewhere else, usually along rivers, inlakes, at the shore, or on the seafloor.

• Hutton also observed layered rocks inthe highlands that were composed ofcemented sand and other rockfragments. He called thesesedimentary rocks.


Sedimentary Rocks

• Hutton also realized that most sedimentswere deposited in horizontal layers, or strata.

• Older layers are converted to sedimentaryrock as they are compacted by the weight ofthe overlying and successively youngerlayers.

• Later, some of these sedimentary layers arepushed up by powerful forces within the Earthto form mountain ranges.

• The process continues as the newly upliftedareas are affected by weathering processes.


Importance of Sedimentary Rocks

• Although sedimentary rocks only comprise5% of the Earth’s crust, they are exposedover approximately 75% of the Earth’ssurface.– The Earth’s crust is dominated by igneous and

metamorphic rocks with sedimentary rocks servingas a thin veneer of only a few kilometers inthickness.

• Many of the necessities of modern life comedirectly from sedimentary rocks.– Petroleum, coal, some metal deposits, and many

materials used in the construction industry comefrom sedimentary rocks.

Section 21.5

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Sedimentary Rocks - Landforms

• Some of the most picturesque spots onEarth are composed of sedimentaryrocks.

• Sedimentary rocks occur in a number ofvarieties and colors.

• The Colorado Plateau of thesouthwestern U.S. offers a particularlyoutstanding display of sedimentaryrocks and their erosional products.


The Grand Canyon – Erosion ofSedimentary Rocks


Section 21.5


Origins of Sedimentary Rocks

• Lithification – the process of transformingsediments into sedimentary rocks

• Compaction and cementation of theloose sediments are the dominantlyresponsible for lithification.

• Calcium carbonate (CaCO3), silica(SiO2), and iron oxides that are dissolvedin groudwater serve as the dominantcementing agents.


Sedimentary Rock Classification

• Sedimentary rocks can be classifiedaccording to the origin of theirconstituents.

• There are two main types of sediments:detrital and chemical.

• Detrital sediments originate from thesolid fragments (detritus) that erodefrom preexisting rocks.

• Chemical sediments originate asdissolved minerals in solution.

Section 21.5


Detrital Sedimentary Rocks

• Detrital sedimentary rocks are classifiedaccording to the grain size of its components.

• Shale – composed of very fine-grainedparticles that were initially mud

• Sandstone – composed of sand-sizedparticles

• Conglomerate – composed of roundedpebbles

• Breccia – composed of angular pebbles

• Detrital rock particles are generally heldtogether by CaCO3 or SiO2.


Classification of Detrital Sedimentary Rocks

Section 21.5

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Chemical Sedimentary Rocks

• Chemical sedimentary rocks are dividedinto two basic types: organic andinorganic.

• In both cases the chemical sedimentsare composed of minerals that weretransported in solution to their eventualdeposition site.


Types of Chemical Sedimentary Rocks

Section 21.5


Organic Chemical Sedimentary Rocks

• Organic sediments are formed largelyby the action of organisms that removethe minerals out of solution.

• For example, many large andmicroscopic organisms in the oceanextract CaCO3 out of seawater toconstruct skeletal and shell material.

• As these organisms die, their hard partscome to rest on the bottom andeventually lithify into organic limestone.


Organic Limestone, full of marine fossils,Everglades, Florida


Section 21.5


Coal

• Although coal is not formed fromminerals carried in solution, it is stillclassified as an organic sedimentaryrock.

• Coal is formed from the lithified remainsof ancient plants.


Inorganic Chemical Sedimentary Rocks

• Inorganic chemical sedimentary rocksare commonly formed by theevaporation of water.

• As the water evaporates, dissolvedminerals in solution will precipitate out.

• Examples of inorganic chemicalsedimentary rocks include halite (HClrock salt), gypsum (CaSO4), andvarious cave deposits (CaCO3).

Section 21.5

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Sedimentary Rock Characteristics

• Many inherent characteristics of sedimentaryrocks are used by geologists to gain a betterunderstanding of the conditions under whichthese rocks were deposited.

• Characteristics such as rock color, grainrounding, grain sorting, bedding, fossilcontent, ripple marks, mud cracks, footprints,and even raindrop prints within the rockreveal the rock’s origin and history.


Rock Color Can be Quite Varied

• Gray shades - common in sedimentary rocks– This color indicates that the rock was deposited in

shallow, well aerated marine waters.

• Shades of yellow, brown, and red indicatethat the rock was deposited on land, abovesea level in the presence of abundantoxygen.– These bright shades indicate an iron oxide.

• Dark gray to black rocks contain abundantamounts of organic carbon.– Accumulated in stagnant, oxygen-poor areas

Section 21.5


Bright Colors Indicate Terrestrial Deposition



Grain Rounding and Size

• Angular grains indicate that these sedimentswere not transported very far.

• Rounded grains indicate great transportdistances and vigorous water/wind action thatwore away the sharp edges.

• Large grains indicate that strong currentsdeposited the sediments.

• Small grains indicate that a very quietenvironment existed when the sedimentswere deposited.

Section 21.5


Sorting

• Sorting is the degree to which sediments areseparated according to size.

• Well-sorted sediments are comprised ofgrains all about the same size.

– Well-sorted sediments have been transportedgreat distances over long periods of time.

• Poorly-sorted sediments are comprised ofgrains a vastly different sizes.

– Poorly-sorted sediments indicate a shorttransportation distance and/or time.


Glacial Moraine, WYGlacial deposits are typically poorly-sorted


Section 21.5

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20


Bedding/Stratification

• Bedding – layering of the rock that developsas the sediment is deposited

• Bedding may be very obvious or subtle. Itmay be very thick (massive) or it may be thin.

• Most bedding is horizontal, since mostsediments come to rest due to gravity.

• In some instances, where strong water/windcurrents are present, bedding is tilted. Thistype of bedding is called cross-bedding.


Cross-beddingwithin a SandstoneZion National Park,Utah


Section 21.5


Fossils

• Perhaps the most distinctive andinteresting feature of many sedimentaryrocks is the fossils it may contain.

• Fossil – the remains, an imprint, or anytype of trace of an ancient organism,preserved in the rock record

• Later in Chapter 24 we will discuss indetail the use of fossils in thedetermination of geologic age.


Dinosaur tracks andfossilized dinosaurbones are bothconsidered fossils.


Section 21.5


Metamorphic Rocks

• As we have already learned, igneous rocksform by the crystallization of magma thatgenerally forms deep within the Earth.

• Sedimentary rocks are deposited at or nearthe surface of the Earth.

• Metamorphic rocks form in conditions belowwhere sedimentary rocks form and abovewhere igneous rocks form.

– Metamorphic rocks form where pressure andtemperature conditions lie between the other tworock types.


Metamorphism

• Metamorphism – conditions within the Earththat result in the changing of the structureand mineral content of a solid rock withoutmelting it

– Any type of rock may be metamorphosed -igneous, sedimentary, or even metamorphic.

• Heat, pressure, and chemically active fluidsare the ‘agents’ of metamorphism.

– Heat and pressure break some of the mineralbonds in the original mineral, with the fluidsserving to allow movement of the ions.

Section 21.6

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21


Parent Rocks

• Parent rock – the original rock beforemetamorphism takes place

• In addition to heat, pressure, and fluid activity,the composition of the parent rock helpsdetermine what metamorphic rock forms.

• The overall process of metamorphism canchange both the texture (crystal size) and themineral composition of the parent rock.

– Or metamorphism could only change one of thesecharacteristics of the parent rock


Classification of Some CommonMetamorphic Rocks

Section 21.6


Parent Rock Metamorphic Rock

• In general, if the parent rock only containsone mineral. The metamorphic rock will alsobe composed of that same mineral.

• For example limestone (parent rock) willmetamorphose into marble.

– Both of these rocks are composed of CaCO3.

• When limestone is metamorphosed intomarble, the ‘structure’ of it is changed but notthe chemical composition.


Fossiliferous Limestone & MarbleAs the limestone recrystallizes the fossils are destroyed

Fossiliferous Limestone

Marble

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Bo

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Section 21.6



• The change from sandstone to quartziteis another example of themetamorphism of a rock containing onlyone mineral.– Both sandstone and quartzite are

composed of SiO2.

• When sufficient heat and pressure areapplied to a sandstone, the grains fusetogether to form the very resistant rockquartzite.



• If the parent rock is composed of severalminerals, the process of metamorphism willcreate a new suite of different minerals.

– These new minerals are formed in response to theescalated heat and pressure conditions.

• Shale is a very fine-grained mixture of severalminerals: quartz, clay, mica, and chlorite.

• Depending on the degree of metamorphism,shale will be progressively changed into slate,schist, or gneiss.

Section 21.6

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22


Effects of Increasing Temperature andPressure on the Metamorphism of Shale


Types of Metamorphism

• Three basics types of metamorphism arerecognized: contact, shear, and regional.

• Contact metamorphism occurs when magmacomes into direct contact with the parent rock– Changes in the parent rock are primarily due to

very high temperatures but not increasedpressures.

• Immediately next to the magma, intensemetamorphism results in the formation ofcoarse-grained crystals.– Moving away from the magma, the resulting rock

becomes progressively finer-grained.

• A contact metamorphic zone occurs when aparent rock is intruded by molten magma.

Section 21.6



• Shear metamorphism results from theintense pressures that exists alongactive fault zones, where rock unitsslide past each other.

• Mechanical deformation andrecrystallization of the minerals resultfrom the heat, pressure, and movementof fluids as rock units slide or shear pastone another.



• Regional metamorphism affectsextremely large areas and is caused bya combination of both high temperatureand high pressure.

• Most areas that are affected by regionalmetamorphism are areas undergoingintense deformation due to mountainbuilding processes.– Convergent plate boundaries are common

zones of regional metamorphism.

Section 21.6


Foliation

• The enormous pressures that accompanyregional metamorphism cause the newlyformed mineral grains to align themselves ina distinctly parallel arrangement.

• Foliation – the prominent layering in ametamorphic rock

• Only rocks that are metamorphosed underintense pressure will exhibit foliation.

– Metamorphic rocks that form almost entirely due tohigh temperatures will not have foliation.


Foliation

• The progressive patterns of foliation thatdevelop in a metamorphosed shale serve asa good example of foliation types.

• Slaty cleavage develops when a shaleundergoes mild metamorphism.

• Schists are a foliated metamorphic rock withlarger, visible grains (crystals.)

• Gneisses form under intense regionalmetamorphism and display banding.– The more intense the metamorphism, the larger

the crystal but less distinct the foliation.

Section 21.6

chapter 22: minerals, rocks, and volcanoesmtweb.mtsu.edu/nchong/psci1030-chap022-minerals, rocks,...

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