Origin of the Earth

Pamela J. W. Gore
Georgia Perimeter College

The Earth in Space

Origin of the Universe - Big Bang Hypothesis

Our Galaxy - Milky Way - spiral. Rotates once every 240 million years

Solar System

Sun - 70% H, 27% He, 3% heavier elements
Fusion reaction

Planets

Names?

Origin of the Solar System

Lines of evidence that must be considered:
  1. Planets revolve around sun in same direction - CCW
  2. Planets lie roughly within sun's equatorial plane (plane of sun's rotation)
  3. Planets rotate CCW, except for:
    1. Venus - slowly CW
    2. Uranus - on its side
    3. Pluto - on its side
  4. Moons go CCW around planets (few exceptions)
  5. Distribution of densities and compositions related to distance from sun
  6. Age - Moon and meteorites 4.6 by

Hypotheses

  1. Solar Nebula Hypothesis or Nebular Hypothesis - cold cloud of gas and dust, contracts. flattens, and rotates, 90% of mass to center; condensed, shrank, and heated by gravitational compression to form sun. Accretion of matter around central mass to form protoplanets (cold accretion). Solar wind drove lighter elements outward causing observed distribution of masses and densities.

  2. Cold accretion model - Earth was initially unsorted material; but now layered. Requires a process of differentiation. Heating and at least partial melting. Iron and nickel sink to form core. Less dense material forms mantle and lighter crust.

    Source(s) of heat for melting?

  3. Hot accretion model - Internal zonation of planets is a result of hot heterogeneous accretion. Hot solar nebula (over 1000 C). Initial crystallization of iron-rich materials forms planet cores. With continued cooling, lower density silicate materials crystallized.

Layering within the Earth

Seismic waves used to determine Earth's structure.

Primary Waves

Secondary Waves

Surface Waves - Rayleigh Waves and Love waves

Discontinuities

Shadow Zones

Earth's Crust

  1. Oceanic - 5 - 12 km thick. Layered. More dense (3.0 g/cm3). Basaltic.
    1. Sediment
    2. Basalt
    3. ?Metamorphosed basaltic mantle?

  2. Continental - avg. 35 km. Less dense (2.7 g/cm3). Floating on denser mantle rocks. Isostasy. Granitic. (Richer in Si, K; poorer in Fe, Mg, Ca).

    Origin of Continental Crust - Remelting of lava; Lighter components separated out. Chemical weathering forming sediments. Metamorphism to form granitic/gneissic rocks.

Origin of the Atmosphere

Volcanic outgassing

Comparison of gases with modern volcanic gases

Comparison with extraterrestrial planetary atmospheres

Origin of the oxygen

Origin of the Ocean

Steam from volcanic outgassing cools and condenses.

Precipitation

Waters accumulate in low places to form oceans

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This page created by Pamela J. W. Gore
Georgia Perimeter College
Clarkston, GA

Page created October 15, 1995
Modified November 4, 1996
Modified February 3, 1999
Last modified July 17, 1999