Earth History
GEOL 2110
Lecture 11
Origin and Early Evolution of the Earth
Part 2: Differentiation of the Earth’s Spheres
Major Concepts
• Early Earth became strongly heated by gravitational
condensation, radioactive heating of short-lived isotopes, and
impacting of asteroids; this resulted in the differentiation of the
dense iron core and the light crust from the primordial mantle
• Partial melting of the earth’s ultramafic mantle created the
chemically distinctive crust of mafic and felsic composition
• Outgassing of volatiles and photochemical dissociation during
early crust formation created an early hydrosphere and
atmosphere that quickly evolved to create the dominant
chemistry we observe today
• The evolution of the biosphere has subsequently modified the
atmosphere’s composition by enriching it in oxygen
• The chemical balance of the earth’s spheres have a great
capacity to buffer pertubations to the system
Layers of the Earth
OCEANIC
CRUST
CONTINENTAL
CRUST
SiO2
47%
56%
Al2O3
16%
18%
FeO
13%
9%
MgO
10%
3%
CaO
10%
4%
Na2O
2%
5.5%
K2 O
0.7%
2.5%
TiO2
1.1%
1.3%
P2O5
0.2%
0.7%
MANTLE
SiO2 – 45%
MgO – 37%
CORE
Fe – 86%
S – 10%
Ni – 4%
FeO – 8%
Al2O3 – 4%
CaO – 3%
others – 3%
---Mohorovicic
Discontinuity
= chondritic
meteorites
Compositional
Layers
Structural
Layers
Igneous Mineralogy of the Earth’s Crust
Continental Crust
Ocean Crust
Mantle
Oceanic Crust
Distillation of the Mantle by Partial Melting
Magmatism at Mid-Ocean Ridges
Peridotite
Oceanic Crust
Distillation of the Mantle by Partial Melting
Incongruent
Melting –
mineral
phases don’t
melt in equal
proportions
Pl
Sp
Gt
Normal
Ocean
Geotherm
Upwelling
Geotherm
Komatiite
Oceanic Crust
Distillation of the Mantle by Partial Melting
Evidence in Solid Solution Minerals
OCEANIC
CRUST
MANTLE
SiO2
47%
45%
Al2O3
16%
4%
FeO
13%
8%
MgO
10%
37%
CaO
8%
3%
Na2O
76%
2.5% 24%
0.2%
K2 O
0.7%
0.1%
TiO2
1.1%
0.3 %
P2O5
0.2%
0.05%
94%
6%
Ni
200ppm
5000ppm
Cr
100ppm
7000ppm
Olivine/Pyroxene Basalt/
Gabbro
Na
Fe
Ca
Mg
Continental Crust
Distillation of Ocean Crust by Partial Melting
Evidence in Solid Solution Minerals
OCEANIC
CRUST
CONTINENTAL
CRUST
SiO2
47%
56%
Al2O3
16%
18%
FeO
13%
9%
MgO
10%
3%
CaO
10%
4%
Na2O
2%
5.5%
K2 O
0.7%
2.5%
TiO2
1.1%
1.3%
P2O5
0.2%
0.7%
Present-day Formation of Crust
A Double Distillation Process
Melting the Mantle Makes Mafic Magma – ALWAYS
Melting Crust Makes Intermediate to Felsic Magma
Crust Formation in Early Earth
Not likely that Plate Tectonics, as we know it today, existed in the
early Earth (>3.0Ga). Ocean crust too thick and too hot (plastic)
to behave rigidly. Probably overthickened in compression zones
causing partial melting of basal zones
Evidence of Early Continental Crust
4.3-4.4 Ga zircon grains found in
Sedimentary Rocks in Western Australia
Zircons are rare in mafic rocks but plentiful
in granitic rocks
Formation of the Hydrosphere and
Atmosphere
Evidence of Early Formation of the
Hydrosphere
Acta Gneiss of the Slave Province in northwestern Canada
4.03 Ga metamorphosed sedimentary rocks
Origin of Seawater and the Atmosphere
Volcanic Outgassing
Photochemical Evolution of the
Atmosphere
Prior to development of the
Ozone layer, ultaviolet
radiation caused
dissociation of water vapor
to create oxygen:
2H2O + UV rad2H2 (lost) + O2
Oxygen then transformed
methane and ammonia to
carbon dioxide and nitrogen
gas
CH4 + 2O2  CO2 + 2H2O
Biogentic Build-up of Oxygen in the
Atmosphere
Banded Iron-formation deposition
Fixing biogenic oxygen in chemical sediments
The Global Chemostat and Thermostat
The Earth’s Capacity to Buffer
Summary of Early Earth Events
Next Lecture
Theory of Plate Tectonics
Part 1:
Toward the Development of the Grand Paradigm
By SARAH
Quiz : Chapters 6 & 7