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Formation de la Terre
Abundances in the
Universe/Crust
Mg Al Si
Fe
Pb
Be
Melting Temperature
Goldschmidt Classification
Chondrite
Atmophile (N, He...)
Lithophile (Si, Mg, Ca, Al, K...)
Siderophile (Fe, Ni...)
L’interieur de la Terre:
proprietes radiales
Geophysical Observations
Seismics:
- 1st order spherical
- lower BC
- material Prop.
Nature of the core 1)
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Mass of the Earth M : 6. 1024 kg
Radius of the Earth R : 6371 km, of the core Rn=3470 km
Average density rhobar : 5.5 g/cm3
Inertia I : 0.33 MR2
The average inertia is larger that the density of crustal silicates (around
2.8 g/cm3) and mantle silicates (around 3.2 g/cm3). The deep Earth is thus
denser
The inertia is lower than that of an homogeneous Earth (2/5MR2). The
deep Earth is thus denser
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Exercise : compute the mass, average density and inertia of a two layer
sphere. Estimate the Earth’s mantle and core densities
solution : rho1 = 12.7 kg m3, rho2=4.2 kg m3. The core mass is 37% of
Earth’s mass.
Nature of the core 2)
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The composition of sample mantle rocks is typically
(Mg0.9 Fe0.1)2 Si O4 Olivine
Solid solution of Forsterite Mg2 Si O4 & Fayalite Fe2 Si O4
In chondrites and solar wind a molar ratio Fe/Mg=.84 is observed
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What is the mass of the iron core? (The molar masses of Mg, Fe, Si, O are 24.3, 56, 28
et 16).
The total number of Fe moles is
NFe=Mnoyau/mfe+0.2 Mmanteau/mOl
The total number of Mg moles is
NMg=1.8 Mmanteau/mol
as NFe/NMg is .84 the core mass is 33 % of the Earth’s
mass
Average Composition Of
the Oceanic Crust
Average Composition of the
Continental Crust
Weight %
SiO2
50.0
Al2O3
16.0
CaO
11.3
FeO
MgO
Weight%
SiO2
66.0
Al2O3
15.5
FeO
4.6
CaO
3.7
K2O
3.5
Na2O
3.2
MgO
2.4
TiO2
0.6
P2O5
0.2
10.0
7.7
Na2O
2.8
TiO2
1.5
K2O
0.15
P2O5
0.12
Molar %
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The total Earth’s composition is similar to that
of the refractory part of the Universe
The total Earth’s composition is similar to
chondrites (non-differenciated meteorites)
The siderophile elements went into the Core
The lithophile elements went into the
crust+mantle
The most incompatible elements are in the
crust (oceanic but mostly continental)
Stratification of the Earth:
Composition
R=6371 km
7 km oceans
crust
basalt
30 km continents
crust
granite
Mantle
pyrolite
R=3500 km
Core
Iron (+Nickel +O,S,Si…)
Stratification of the Earth:
Upper mantle
Mineralogy
410 km depth olivine+pyroxene+garnet
520 km depth olivine beta
(wadsleyite)+garnet+pyroxene
610 km depth olivine gamma
(ringwoodite)+garnet+pyroxene
Lower mantle
oxides magnesiowustite
Mg-Fe perovskite
Ca-perovskite
•Molecular clouds
contraction (0.1-1 ma)
•Disk formation
•Condensation
•Coagulation of dust
•Up to km-size
Orion Nebula
Chondrite
4566 Ma
Ca-AlInclusions
4563 Ma
Chondrules
t0+5-15
Iron
meteorites
t0+30
Earth Core
t0+50
Moon formed
t0+100
End of
accretion
Datation: ex U/Pb sur zircons ZrSiO4 (accepte U rejete Pb)
= stable 183W
= radioactif 182Hf
= radiogénique
182W
Half-life 9 ma
W « siderophile » Hf « lithophile »
Core segregation after Hf decay
Same 182/183W ratio in the
mantle and in
undifferenciated objets
W « siderophile » Hf « lithophile »
Cor formation before Hf decay
Larger 182/183 W ratio in
the mantle than in
undifferenciated objets
Une planete active
Des mouvements horizontaux…
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La tectonique des plaques
sur le million d’annee
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La tectonique des
plaques sur l’annee
Distance
Pac-Fr
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Observations of plate motion
Modèle de la tectonique des plaques
Des mvts verticaux…
Finland
Hudson
Scotland
Bretagne
Estimates of the viscosity:
Observation of heat flux :
Using Earth heat flow, in how much time
can I boil the water for my coffee cup??
t~108 s = 3.5 ans
Can we expore the mantle
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KTB, Kola
Hotspots:
seem too:
- exist over long time
- being stationary
- independent of the
plate motion
- are geochemically
distinced
Stratification of the Earth:
Rheology
Lithosphere ~100 km
~1026 Pa s
solid
Asthenosphere
~1019-22 Pa s
solid
« real asthenosphere » 1019-21 Pa s
solid
1021-22 Pa s
solid
deep mantle
D ’’
<<1021 Pa s ? solid
Outer core
10-3 Pa s
Inner core
?
liquid
Not so solid
Liquide
Solide
Liquide
Solide
Stratification of the Earth:
Temperature
Clapeyron slopes of phase transformations
Olivine-wadsleyite
Ringwoodite-perovskites+oxides 1870 K
Liquid iron/solid iron 5000 K
Modelisation…
Seismic tomography
On a smaller scale:
From Grant/Van der Hilst
La convection