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Our solar system has 1 star (our Our solar system has 1 star (our sun); the galaxy has hundred’s of sun); the galaxy has hundred’s of

millions; the universe millions; the universe encompasses all the galaxiesencompasses all the galaxies

Structure of EarthStructure of Earth

Planets derived from material circling Planets derived from material circling early sun (star)early sun (star)

Particles in solar nebula clumped to form Particles in solar nebula clumped to form planetesimalsplanetesimals

Planetesimals collided to form larger planets Planetesimals collided to form larger planets by accretionsby accretions

Fractionation of material among inner rocky Fractionation of material among inner rocky and outer gassy planetsand outer gassy planets

Age of planetsAge of planets

Oldest rocks on earth ~4.4 by oldOldest rocks on earth ~4.4 by old– Dating zirconDating zircon

Planet is older ~4.6 byPlanet is older ~4.6 by Time since crystallization ~age of planetsTime since crystallization ~age of planets Solar system formed 10 – 100 my earlierSolar system formed 10 – 100 my earlier

Composition of Early EarthComposition of Early Earth Earth is layeredEarth is layered

– Liquid outer core and solid inner core – Fe & NiLiquid outer core and solid inner core – Fe & Ni Mantle – silicateMantle – silicate

– Crust – continental & oceanicCrust – continental & oceanic Planetary formationPlanetary formation

– Initial accretion – homogeneous; non-gravitational; weak van Initial accretion – homogeneous; non-gravitational; weak van der Waal’s binding (planetesimals R=1-10 km)der Waal’s binding (planetesimals R=1-10 km)

– Then gravitational attraction – accretions (protoplanets)Then gravitational attraction – accretions (protoplanets)– Major and large collisions – major accretionsMajor and large collisions – major accretions– Really large collisions – melting (allowing Ni and Fe to Really large collisions – melting (allowing Ni and Fe to

separate); magma formationseparate); magma formation– Collisions also brought water and other volatilesCollisions also brought water and other volatiles

Chemical composition of EarthChemical composition of Earth

Evidence of melting, chemical fractionation Evidence of melting, chemical fractionation and separationand separation– Assume composition of early Earth = Assume composition of early Earth =

composition of chondritic meteoritecomposition of chondritic meteorite– Mantle depleted in FeMantle depleted in Fe– Separation of Ni/Fe core during meltingSeparation of Ni/Fe core during melting

Composition of the early Earth (prior to segregation)

Separation of the Fe/Ni core during melting(mantle depleted in these elements)

Crustal formation

Formation of the crustFormation of the crust

Continental versus oceanic crustContinental versus oceanic crust Continental crustContinental crust

– Repeated recycling and partial melting of mantle material Repeated recycling and partial melting of mantle material and oceanic crustand oceanic crust

– Repeated heating, cooling, subsidence, burial, and melting Repeated heating, cooling, subsidence, burial, and melting leads to distillation/segregation of lighter granitic material leads to distillation/segregation of lighter granitic material from heavier oceanic crust & mantle; further chemical from heavier oceanic crust & mantle; further chemical separation of elementsseparation of elements

– As old as 3.8 byAs old as 3.8 by Oceanic crust Oceanic crust

– Young (100 my) – controlled by plate tectonicsYoung (100 my) – controlled by plate tectonics– More denseMore dense

Bowen's reaction series demonstrates how the cooling and crystallization of a primary magma of basaltic composition can change from basaltic to andesitic to rhyolitic, through reactions between mineral grains and magma followed by separation of mineral grains and magma.

~20

-70

km

Oceanic (basalt) crust ( = 2.8 g/cm3)

} ~2-5 km

The Oceans ( = 1 g/cm3)

Mantle ( = 3.3 g/cm3)

Continental (granitic) crust( = 2.7 g/cm3)

SegregationSegregation

Separation of Ni/Fe core during meltingSeparation of Ni/Fe core during melting Crust formed from partial melting of the Crust formed from partial melting of the

mantlemantle Crustal material enriched in Na, Si, and AlCrustal material enriched in Na, Si, and Al Depleted in MgDepleted in Mg Further fractionation formed continental Further fractionation formed continental

(granite) and oceanic (basalt) crust(granite) and oceanic (basalt) crust

Period of major accretion (first 10-30 my)

Period of heavy bombardment

Major accretionMajor accretion– Once though to be 100 Once though to be 100

mymy– Recent thought is planet Recent thought is planet

cooled quicklycooled quickly– Water begins to Water begins to

accumulate on Earth’s accumulate on Earth’s surfacesurface

– Began forming crustal Began forming crustal materialmaterial

Heavy bombardmentHeavy bombardment– 500-700 my500-700 my– Continued to bring Continued to bring

material and volatiles (and material and volatiles (and water) to earthwater) to earth

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Importance of the moonImportance of the moon

TidesTides Gravitational attraction of moon & sun on earth’s Gravitational attraction of moon & sun on earth’s

bulge causes precession of earth’s orbitbulge causes precession of earth’s orbit– Role in Milankovitch cycles (glacial cycles)Role in Milankovitch cycles (glacial cycles)

Tends to stabilize tilt of the earthTends to stabilize tilt of the earth– Earth’s axis at an angle relative to plane of earth’s orbitEarth’s axis at an angle relative to plane of earth’s orbit– Causes seasonalityCauses seasonality– Tilt of axis varies between 21.8Tilt of axis varies between 21.8oo and 24.4 and 24.4oo

– Without moon, tilt would vary by a greater amountWithout moon, tilt would vary by a greater amount Up to 85Up to 85oo

Wreak havoc with climate due to extreme seasonalityWreak havoc with climate due to extreme seasonality

Formation of the moonFormation of the moon

Lots of theories – implausible or statistically Lots of theories – implausible or statistically unlikelyunlikely– CaptureCapture– Fission – spinning of earth ejected moonFission – spinning of earth ejected moon– Binary accretion – Earth and moon formed side by sideBinary accretion – Earth and moon formed side by side

Likely a collision (unlikely, but plausible)Likely a collision (unlikely, but plausible)– Debris reassembled in orbit around earthDebris reassembled in orbit around earth– Analysis of moon rocks compared with earth rocksAnalysis of moon rocks compared with earth rocks

Formation of the moonFormation of the moon

Early in Earth’s history (>4 bybp)Early in Earth’s history (>4 bybp)– Moon formed 30-50 my after solar systemMoon formed 30-50 my after solar system

Formed during accretionFormed during accretion– Impact with a nearly fully-formed Earth?Impact with a nearly fully-formed Earth?– Impact led to termination of accretion?Impact led to termination of accretion?

Impact may have affected earth’s rotationsImpact may have affected earth’s rotations– Caused the axial tilt?Caused the axial tilt?– Therefore contributed to seasonality and glacial Therefore contributed to seasonality and glacial

cycles?cycles?

History of the moonHistory of the moon

Before 4 bybpBefore 4 bybp– Moon formed from hot debris after collision then Moon formed from hot debris after collision then

solidifiedsolidified– Formation of small coreFormation of small core

The next billion yearsThe next billion years– Volcanic activity formed the moon’s crustVolcanic activity formed the moon’s crust– Some similarity to earthSome similarity to earth

Maria (dark seas)basalt-like[3.1-3.9 bybp]

Highlandsgranite-like (anorthosite)[> 4 bybp] – more like continental crust

Moon structureMoon structure Both maria and highlands are oldBoth maria and highlands are old

– Maria 3.1-3.9 by; lava flows into giant impact cratersMaria 3.1-3.9 by; lava flows into giant impact craters– Highlands > 4 byHighlands > 4 by

Little or no evidence of tectonic activity in the last 3 byLittle or no evidence of tectonic activity in the last 3 by– Small size allowed internal heat to escape w/o mantle Small size allowed internal heat to escape w/o mantle

convectionconvection– Moon’s surface pock-marked by comet and asteroid Moon’s surface pock-marked by comet and asteroid

impactsimpacts– No evidence of plate tectonics or other landscape forming No evidence of plate tectonics or other landscape forming

processesprocesses Moon has no atmosphere or oceansMoon has no atmosphere or oceans

– Size to small to gravitationally retain gas and volatilesSize to small to gravitationally retain gas and volatiles

Back to the Earth’s structureBack to the Earth’s structure

Earth is layeredEarth is layered Heat did not escapeHeat did not escape Recycling, reheating, remelting, Recycling, reheating, remelting,

recrystallizationrecrystallization Density stratifiedDensity stratified

Most simply

The earth is layered & density stratified

1. Crust – cold, rigid, thin

2. Mantle – warmer, moredense; outer partrigid and innerpart plastic (deformable)

3. Outer core –transition zone then thick liquidzone

4. Inner core –solid but warm,very dense, rich in magnetic materials (Ni, Fe)

How do we know this?How do we know this?

All we see is the crust!All we see is the crust! Deepest drill-hole – 12,063 m (7.5 miles)Deepest drill-hole – 12,063 m (7.5 miles)

– Still crustalStill crustal Deepest ocean drilling – 2 km (1.2 miles)Deepest ocean drilling – 2 km (1.2 miles)

– Still crustalStill crustal Studies of the earth’s orbit – gave an idea of Studies of the earth’s orbit – gave an idea of

massmass– Surface rocks predicted lower total mass if the Surface rocks predicted lower total mass if the

earth were homogeneousearth were homogeneous

Mohorovicic “Moho” discontinuityMohorovicic “Moho” discontinuity

Density discontinuity – P waves arrived at seismic Density discontinuity – P waves arrived at seismic station before they should have in an station before they should have in an homogeneous earthhomogeneous earth

Boundary between the crust and mantleBoundary between the crust and mantle Discovered by Croatian geophysicist based on Discovered by Croatian geophysicist based on

observations of seismic waves generated by observations of seismic waves generated by earthquakes.earthquakes.

Fun fact – there was an effort to drill a “Mohole” Fun fact – there was an effort to drill a “Mohole” but failed due to lack of $$ and technologybut failed due to lack of $$ and technology

Evidence for layeringEvidence for layering Mainly we know depend on seismologyMainly we know depend on seismology Seismic waves generated from earthquakesSeismic waves generated from earthquakes

– ““Primary” P-waves (compression waves; longitudnally propagated Primary” P-waves (compression waves; longitudnally propagated waves; oscillate in same direction as movement like sound waves)waves; oscillate in same direction as movement like sound waves)

– ““Secondary” S-waves (transverse waves; horizontally propagated; Secondary” S-waves (transverse waves; horizontally propagated; oscillate perpendicular to movement like water waves)oscillate perpendicular to movement like water waves)

1900 – identified P & S waves on a seismograph 1900 – identified P & S waves on a seismograph (Oldham)(Oldham)– Waves were passing through the earth faster than predictedWaves were passing through the earth faster than predicted

Wave speed increases with increasing density!Wave speed increases with increasing density!

– Waves were being refracted (bent so they changed direction)Waves were being refracted (bent so they changed direction)– Hypothesized that there were areas of Earth with different densitiesHypothesized that there were areas of Earth with different densities

1906 – no S-waves passed through the earth1906 – no S-waves passed through the earth– Shadow zone – no S-wavesShadow zone – no S-waves– P-waves took longer than expectedP-waves took longer than expected

•We can detect these waves independently

•They behavedifferently passingthrough differentmedia

Why are thesewaves important?

Prediction of earthquake waves passing through a homogeneous planet.

Prediction of earthquake waves passing through a planet of regularly changing density.

Point of origin of seismic source.

What S waves do around liquidouter core (do not penetrate).

P-wave

shadow zone

P-wave

shadow zone

142o

What P waves do in & around liquid outer core (bend)

142o

Sharp increase in P-wave velocity at Moho

SeismologySeismology

Changes in travel time and path tell us Changes in travel time and path tell us about the earth’s structureabout the earth’s structure– Refraction of waves led to discovery of earth’s Refraction of waves led to discovery of earth’s

core and Mohocore and Moho– Travel time of waves led to discovery of layersTravel time of waves led to discovery of layers

Now we use changes in travel time and path Now we use changes in travel time and path tell us about location of disturbances tell us about location of disturbances (earthquakes or bombs)(earthquakes or bombs)

Earth’s functional layersEarth’s functional layers Crust – we know most about it; continental crust is less denseCrust – we know most about it; continental crust is less dense Moho – a density discontinuity that separates crust from the Moho – a density discontinuity that separates crust from the

mantlemantle– Depth varies under continents and oceansDepth varies under continents and oceans– First thought that this was layer where crust moved relative First thought that this was layer where crust moved relative

to earth’s interior BUT, outer layer of mantle moves with to earth’s interior BUT, outer layer of mantle moves with crust!crust!

Lithosphere – crust plus rigid mantle (not totally rigid but, Lithosphere – crust plus rigid mantle (not totally rigid but, movements cause things like earthquakes and volcanoesmovements cause things like earthquakes and volcanoes

Asthenosphere – plastic layer of mantle; lithosphere floats on Asthenosphere – plastic layer of mantle; lithosphere floats on asthenosphereasthenosphere

Mantle includes part of lithosphere, asthenosphere and solid Mantle includes part of lithosphere, asthenosphere and solid mesospheremesosphere

Chemical compositionof layers:• Crust – lightweight (0.4% mass/1% volume of earth) – ocean crust (basalt – O, Si, Mg & Fe) is denser than continental crust (granite – O, Si, Al)

•Mantle – denser (68% mass/83% volume of earth) - Si, O, Fe & Mg

•Core – densest (31.5% mass/16% volume of earth) - mainly Fe & Ni with some Si, S and heavy elements

TABLE ITABLE I   

Typical Densities of Earth MaterialsTypical Densities of Earth Materials   

SubstanceSubstance Density*Density*

Sea WaterSea Water 1.021.02

LimestoneLimestone 2.68-2.76**2.68-2.76**

GraniteGranite 2.64-2.76**2.64-2.76**

SandstoneSandstone 2.14-2.36**2.14-2.36**

SlateSlate 2.6-3.3**2.6-3.3**

BasaltBasalt 2.4-3.1**2.4-3.1**

Average Density of ContinentsAverage Density of Continents 2.72.7

Average Density of SiMa (Mantle Material)Average Density of SiMa (Mantle Material) 3.33.3

* Actual densities vary slightly, depending * Actual densities vary slightly, depending on chemical composition.on chemical composition.   

(** Source: Handbook of Chemistry and (** Source: Handbook of Chemistry and Physics)Physics)   

Physical responses

Lower mantle

Core 2900 – 6370 km ~3400 Dense, viscous liquidSolid inner core

Classifying layers By composition

Isostatic equilibrium and reboundIsostatic equilibrium and rebound This concept helps us understand the This concept helps us understand the

“floating” of lithosphere on asthenosphere“floating” of lithosphere on asthenosphere

IsostacyIsostacy

Ocean basins and continents “float” on Ocean basins and continents “float” on asthenosphere at equilibrium so that total pressure asthenosphere at equilibrium so that total pressure at depth in mantle is everywhere the same.at depth in mantle is everywhere the same.

Depending on density, things will float at a certain Depending on density, things will float at a certain height and displace a different amount of waterheight and displace a different amount of water

Most mass is below the surface, what sticks out of Most mass is below the surface, what sticks out of the fluid is supported by bouyancy of displaced the fluid is supported by bouyancy of displaced fluid below the surfacefluid below the surface

Examples – icebergs, ships, blocks of wood of Examples – icebergs, ships, blocks of wood of different densities in waterdifferent densities in water

What does this mean?What does this mean?

Mountains have roots that are deeper than surface Mountains have roots that are deeper than surface expressionexpression

As erosion removes mass from the top of a As erosion removes mass from the top of a mountain, the roots shrink upward or the mountain, the roots shrink upward or the asthenosphere “rebounds”asthenosphere “rebounds”

Example: younger (higher) Rockies have deeper Example: younger (higher) Rockies have deeper roots than older Appalaciansroots than older Appalacians

Example: continental rebound from glaciers (Great Example: continental rebound from glaciers (Great Lakes & Long Island Sound examples); sea level Lakes & Long Island Sound examples); sea level decreases even though more water!decreases even though more water!

Next upNext up

Mantle convectionMantle convection Plate tectonics (Chapter 7)Plate tectonics (Chapter 7)