Origin of Solar System
Nebular Hypothesis

• Rotating dust cloud (nebulae)
• Rotation causes flattening
• Gravity causes contraction
• Rotation increases
• Material accumulates in center--protosun
• Compression cause T to increase to 106 °C—fusion begins
• Great explosion

• Origin of planets
• Gases condense
• Gravity causes them to coalesce into planetesimals
• Planetesimals coalesce and contract into planets

• The planets
• Terrestrial or inner planets
• Mercury, Venus, Earth, Mars
• loss of volatiles (H, He, H2O) by solar wind
• made of rock
• Jovian planets (4 of the 5 outer planets)
• Jupiter, Saturn, Neptune, Uranus
• mostly volatiles (H, He)
• Pluto
• anomalous composition--rock with frozen water and methane

• Homogenous planetesimal
• Earth heats up
• Accretion and compression (TÆ1000°C)
• Radioactive decay (TÆ2000°C)
• Iron melts--migrates to center
• Frictional heating as iron migrates
• Light materials float--crust
• Intermediate materials remain--mantle

Differentiation of Earth Continents, Oceans, and Atmosphere

• Continental crust forms from differentiation of primal crust
• Oceans and atmosphere
• Two hypotheses
• internal: degassing of Earth’s interior (volcanic gases)
• external: comet impacts add water CO2, and other gases
• Early atmosphere rich in H2, H2O, N2, CO2; poor in O2
• Photosynthesis begins: CO2 + H2O --> organic matter + O2

• How do we know the structure of Earth?
• Average density of Earth (5.5 gm/cm3)
• Denser than crust and mantle
• Composition of meteorites
• Seismic wave velocities
• Laboratory experiments
• Chemical stability
• Earth’s magnetic field

• Ocean Crust
• 3-15 km thick
• Basaltic rock
• Young (<180 Ma)
• Density ~ 3.0 g/cm³
• Continental Crust
• 35 km average thickness
• Granitic rock
• Old (up to 3.8 Ga)
• Density ~ 2.7 g/cm³
• Crust "floating" on "weak" mantle

The Mantle

• ~2900 km thick
• Comprises >82% of Earth’s volume
• Mg-Fe silicates (rock)
• Two main subdivisions:
• Upper mantle (upper 660 km)
• Lower mantle (660 to ~2900 km; "Mesosphere")

• Outer 660 km divided into two layers based on mechanical properties (Fig. 1.15)
• Lithosphere
• Rigid outer layer including crust and upper mantle
• Averages 100 km thick; thicker under continents
• Asthenosphere
• Weak, ductile layer under lithosphere
• Lower boundary about 660 km (entirely within mantle

• Outer Core
• ~2300 km thick
• Liquid Fe with Ni, S, O, and/or Si
• Magnetic field is evidence of flow
• Density ~ 11 g/cm³
• Inner Core
• ~1200 km thick
• Solid Fe with Ni, S, O, and/or Si
• Density ~13.5 g/cm³

• Continent
• Shield--Nucleus of continent composed of Precambrian rocks
• Continent-Ocean Transition
• Continental shelf--extension of continent
• Continental slope--transition to ocean basin

• Ocean basin--underlain by ocean crust
• Why do oceans overlie basaltic crust?
• Mid-ocean ridge
• Mountain belt encircling globe
• Ex: Mid-Atlantic Ridge, East Pacific Rise
• Deep-ocean trenches
• Elongate trough
• Ex: Peru-Chile trench