Structure of EarthStructure of Earth

Chapter 2

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) – seebook.

142o

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!

Take home pointsTake home points

Layers of the earth – density stratificationLayers of the earth – density stratification How do we know earth’s structure – How do we know earth’s structure –

seismology and the role of S and P wavesseismology and the role of S and P waves Moho, lithosphere and asthenosphereMoho, lithosphere and asthenosphere Isostacy; Isostatic equilibriumIsostacy; Isostatic equilibrium