Transcript Origin and Early Evolution of Earth I

Earth History
GEOL 2110
Lecture 10
Origin and Early Evolution of the Earth
Part 1: Accretion of the Planets
Major Concepts
• The Earth and the other planets formed from a spinning
solar nebula that was graded in composition from heavier
elements nearer the sun, thus giving rise the inner rocky
planets and the outer icy planets
• Earth formed by accretion of increasingly larger particles;
moons may have formed by capture of large asteroids or
impact of other planetesimals
• Early Earth became strongly heated by gravitational
condensation, radioactive heating of short-lived isotopes,
and impacting of asteroids; this resulted in partial melting
and the differentiation of the iron core from the mantle
• Partial melting of the earth’s ultramafic mantle created the
chemically distinctive crust of mafic and felsic composition
Early Ideas about the Origin of the Earth
• Galileo (1564–1642) The Earth is not the center of the
universe
• Buffon (1779) – originally molten planets spalled off
the sun by passing comets
• Late 1800’s - accretion from a solar nebula
• Chamberlin & Moulton (1910) – Planetesimal
hypothesis – a passing star pulled solar gases away
from the sun to make the nebula. The gases and dust
gradually accumulated to small planetesimal bodies
that then accreted to each other by gravitational
attraction to grow to planet size.
The Solar Nebula Theory
VON WEIZSACKER'S NEBULAR
HYPOTHESIS (1944)
KUIPER'S PROTOPLANET
HYPOTHESIS (1951)
Spinning nebular cloud of gas
and dust flattens, contracts, and
heats up at its core. Hydrogen
fusion lights the sun.
Thermonuclear energy creates
eddies in the gas cloud which
starts the accretion process
ultimately leading to formation
of protoplanets.
Early Planetary Differentiation
Large Protoplanet Stage
Differentiated Planet
The Revised Solar Nebula Theory
In the thermal gradient of the solar nebula, different
elemental compounds would have condensed at different
temperatures. This explains the compositional progression
of the planets from the heavy element-rich inner planets and
light element-rich outer ones
The Revised Solar Nebula Theory
Sulfides
The Planets
FuN FaCtS
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Density
Rotation
Revolution
Kg/m3
per Earth Day
Earth Days
5420
5250
5520
3940
1314
690
1290
1640
58.6
243
1
1.03
0.41
0.44
0.72
0.67
88
225
356
687
4,329
10,680
30,685
60,190
Tilt
Temperature
degrees K
0
178
23.4
25 
3.1 
26.7 
97.9 
29.6 
452
726
281
225 
120 
88 
59 
48 
The Inner
Terrestrial
Planets
Mercury
Venus
Earth
Mars
Atmosphere
none
97% CO2
N, O2, H20, CO2
Thin CO2
Tectonics?
No, heavily cratered
Core
Fe core 70vol%
Volcanism Only
Earth-like
You betcha
liquid and solid Fe
Used to
Solid Fe
The Moon
No Atmosphere
No Core – bulk composition similar
to Earth’s mantle – Hmmm
Highlands (4.5-4.2 Ma) – cratered
pulverized anorthosite
Mare (3.8-3.2 Ma) - less cratered,
flood basasts formed by melting
due to meteor impact
Asteroids and Comets
Asteroids are essentially chunks of rock that measure in
size from a few feet to several miles in diameter. The
largest asteroid, Ceres, is about 590 miles (950
kilometers) wide. Like most asteroids, it lies in the
asteroid belt between Mars and Jupiter. Many
astronomers believe the belt is primordial material that
never glommed into a planet because of Jupiter's
gravitational pull. Other astronomers say the belt is a
planet that was broken apart during a collision.
Comets are balls of rock and ice that grow tails as they
approach the sun in the course of their highly elliptical
orbits. As comets heat up, gas and dust are expelled and
trail behind them. The sun illuminates this trail, causing it
to glow. Short-period comets come from the Kuiper belt
out beyond the orbit of Neptune and pass through the
inner solar system once or twice in a human lifetime.
Long-period comets come from the Oort Cloud, which
rings the outer reaches of the solar system, and pass near
the sun once every hundreds or thousands of years.
Earth
The Goldilocks
Planet
Average whole Earth density – 5.5 g/cm3
Average crustal density – 2.8 g/cm3
Geophysics – Imaging the Deep Earth
Geophysics – Imaging the Deep Earth
-Zone of Partial Melting
-Abrupt density
increases due to
mineral changes
Composition of the Earth
How Do We Know?
- Mantle xenoliths
- Ophiolite complexes
- Seismic data
- Density constraints
- Meteorites
- stony ~mantle
- iron ~core
- chondrites ~whole
earth
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
Chemical
Layers
Physical
Layers
Next Lecture
Origin and Early Evolution of the Earth
Part 2:
Differentiation of Earth’s Spheres