Earth History GEOL 2110

Lecture 10Origin 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 DifferentiationLarge Protoplanet Stage Differentiated Planet

The Revised Solar Nebula TheoryIn 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 PlanetsFuN FaCtS

Density Rotation Revolution Tilt Temperature Kg/m3 per Earth Day Earth Days degrees K

Mercury 5420 58.6 88 0 452Venus 5250 243 225 178 726Earth 5520 1 356 23.4 281Mars 3940 1.03 687 25 225 Jupiter 1314 0.41 4,329 3.1 120 Saturn 690 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

Atmosphere Tectonics? Core Mercury none No, heavily cratered Fe core 70vol%

Venus 97% CO2 Volcanism Only Earth-like

Earth N, O2, H20, CO2You betcha liquid and solid Fe

Mars Thin CO2 Used to Solid Fe

The Moon

No AtmosphereNo Core – bulk composition similar to Earth’s mantle – HmmmHighlands (4.5-4.2 Ma) – cratered

pulverized anorthositeMare (3.8-3.2 Ma) - less cratered, flood basasts formed by melting due to meteor impact

Asteroids and CometsAsteroids 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.

EarthThe GoldilocksPlanet

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 EarthHow Do We Know?- Mantle xenoliths- Ophiolite complexes- Seismic data- Density constraints- Meteorites- stony ~mantle- iron ~core- chondrites ~whole

earth

ChemicalLayers

Physical Layers

MANTLE

SiO2 – 45%

MgO – 37%

FeO – 8%

Al2O3 – 4%

CaO – 3%

others – 3%

COREFe – 86%S – 10%Ni – 4%

OCEANIC

CONTINENTAL

CRUST CRUST

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%

Layers of the Earth

= chondritic meteorites

---MohorovicicDiscontinuity

Next Lecture

Origin and Early Evolution of the EarthPart 2:

Differentiation of Earth’s Spheres