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Earth History GEOL 2110. Lecture 10 Origin and Early Evolution of the Earth Part 1: Accretion of the Planets. Major Concepts.

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Earth history geol 2110

Earth History GEOL 2110

Lecture 10

Origin and Early Evolution of the Earth

Part 1: Accretion of the Planets

Major concepts
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
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
The Solar Nebula Theory



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
Early Planetary Differentiation

Large Protoplanet Stage Differentiated Planet

The revised solar nebula theory
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 planets fun facts
The PlanetsFuNFaCtS

Density Rotation Revolution Tilt Temperature

Kg/m3 per Earth Day Earth Days degrees K

Mercury 5420 58.6 88 0 452

Venus 5250 243 225 178726

Earth 5520 1 356 23.4 281

Mars 3940 1.03 687 25  225 

Jupiter 1314 0.41 4,329 3.1  120 

Saturn690 0.44 10,680 26.7  88 

Uranus 1290 0.72 30,685 97.9  59 

Neptune 1640 0.67 60,190 29.6  48 

The inner terrestrial planets
The Inner Terrestrial Planets

AtmosphereTectonics? Core

Mercury none No, heavily cratered Fe core 70vol%

Venus 97% CO2 Volcanism Only Earth-like

Earth N, O2, H20, CO2 You betcha liquid and solid Fe

Mars Thin CO2 Used to Solid Fe

The moon
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 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.


The Goldilocks


Average whole Earth density – 5.5 g/cm3

Average crustal density – 2.8 g/cm3

Geophysics imaging the deep earth1
Geophysics – Imaging the Deep Earth

  • Zone of Partial Melting

  • Abupt density increases due to mineral changes

Composition of the earth
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

  • --Mohorovicic

  • Discontinuity



SiO2 47% 56%

Al2O3 16% 18%

FeO 13% 9%

MgO 10% 3%

CaO 10% 4%

Na2O 2% 5.5%

K2O 0.7% 2.5%

TiO2 1.1% 1.3%

P2O5 0.2% 0.7%


SiO2 – 45%

MgO – 37%

FeO – 8%

Al2O3 – 4%

CaO – 3%

others – 3%


Fe – 86%

S – 10%

Ni – 4%





= chondritic


Next lecture
Next Lecture

Origin and Early Evolution of the Earth

Part 2:

Differentiation of Earth’s Spheres