The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 8 - page 1

The Earliest Earth: 2,100,000,000 years of the Archean Eon

Pamela J. W. Gore
Georgia Perimeter College


    The Archean Eon is the oldest unit on the geologic time scale. It began 4.6 billion years ago and ended 2.5 billion years ago. It lasted for 2.1 billion years (2,100,000,000 years).

    Earth's oldest rocks are found in Canada. They are about 4.04 billion years old.

    But there are even older mineral grains. Sand-sized zircon grains in metamorphosed sedimentary rocks from Australia are 4.4 billion years old.

    There are no rocks on Earth that date back to 4.6 billion years ago because the Earth is geologically active, and the oldest rocks have been recycled by plate tectonics or by weathering and erosion as part of the rock cycle.

    Much of our knowledge of the Earth's earliest history comes from indirect evidence - meteorites.

    The Archean and Proterozoic Eons comprise the Precambrian, which spans 87% of the geologic time scale.

Archean and Protozoic Eras of the Precambrian span 87% of Earth's history
The Archean and Protozoic Eras of the Precambrian span 87% of Earth's history.
(The Cambrian Period began 542 million years ago,
and the geologic record began 4.04 billion years ago.)

The Earth in Space

Origin of the Universe - The Big Bang Theory

    The Earth is part of the Solar System, the Solar System is part of the Milky Way galaxy, and the Milky Way galaxy is part of the Universe.

    The story of the origin and history of the Earth requires that the origin and history of the Universe and Solar System must be considered.

Evidence to be considered when interpreting the history of the Universe:

  1. The galaxies are rapidly moving apart (Hubble's Law).
    This suggests that galaxies were closer together in the past.
    This was discovered in 1929 by Edwin P. Hubble.
  2. Observed temperature of the Universe today (background microwave radiation) 3 degrees above absolute zero.
  3. Present abundances of hydrogen and helium.


    The Universe is expanding.
    Everything began together at a point.
    A big explosion occurred, which astronomers call the Big Bang.
    This explosion caused everything in the Universe to begin moving rapidly apart.

How do we know the galaxies are moving apart?

    Red shift.
    In 1914, W.M. Slipher first noted that galaxies displayed the red shift.
    Their light is shifted toward the red (or long wavelength) end of the spectrum.

    Colors of the spectrum
    R O Y   G   B I V

    1. Red
    2. Orange
    3. Yellow
    4. Green
    5. Blue
    6. Indigo
    7. Violet

    Red = long wavelengths
    Blue = short wavelengths
    (Microwaves and radio waves are longer than visible
    Ultraviolet and X-rays are shorter than visible.)

    The spectrum of a star reveals:

    Light reaching us from distant receding galaxies has its absorption lines shifted toward the red end of the spectrum. This indicates that the galaxy is moving away from the Earth.
    The red shift indicates that the universe is expanding.

    A similar phenomen, except related to sound, is the Doppler Effect.

Calculations indicate that the Big Bang occurred 18-15 billion years ago.

The Big Bang marked the instantaneous creation of all matter in the Universe.

Variations on the Big Bang theory:

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

October 25, 2005