22.1 - Early Earth

							Chapter 22 – The
Precambrian Earth

     Notes 22.1 – Early Earth
The Age of Earth

• Early earth was hot, volcanically active,
  and had no continents
• Rocks did not exist
• Scientists know very little about Earth’s
  first 700 million years.
  Oldest existing rocks
• From Archean Eon
• Earliest life-forms were simple,
  unicellular organisms
  • Multicellular organisms did not appear
    until the end of the Proterozoic
Crustal Rock Evidence

• Absolute-age dating has revealed that
  the oldest crustal rocks are between
  3.96 and 3.8 billion years in age
• Earliest rocks were made of Zircon
  (ZrSiO4)
Zircon

•   Stable and common mineral
•   Can survive erosion and metamorphism
•   Often used to age-date old rocks
•   Zircon found in Precambrian rocks in
    Australia are dated to be at least 4.4
    billion years old
    • Earth must be at least that old
Solar System Evidence

• Scientists agree all parts of the solar
  system formed at the same time
  • Therefore Earth and meteorites are
    approximately the same age
• Meteorites = small fragments of orbiting
  bodies that have fallen on Earth’s surface
  • Meteorites have fallen all throughout Earth’s
    history
  • Most meteorites have been dated between
    4.7 & 4.5 billion years old
Solar System Evidence

• Moon Rocks = oldest samples have
  been dated at 4.45 billion years old
  • Scientists believe Moon formed after Earth
    when a massive solar system body collided
    with Earth
  • Based on all the evidence, scientists agree
    that Earth is about 4.56 billion years old.
Early Heat Sources

• Earth was extremely hot after it formed
• Three likely sources of heat
  • Earth’s gravitational contraction
  • Radioactivity
  • Bombardment by asteroids, meteorites,
    and other solar system bodies
Gravitational contraction

• Earth formed by the gradual accumulation
  of small, rocky bodies in orbit around the
  Sun
  • As Earth accumulated these small bodies, it
    grew in size and mass.
  • As Earth’s mass increased, gravity increased
  • Gravity increased caused Earth’s center to
    squeeze together with so much force that the
    pressure raised temperature
Radioactivity

• Radioactive decay generates heat
• There were more radioactive isotopes in
  early Earth
  • So more heat was generated
  • Making Earth hotter than it is today
Bombardment

• Asteroids = metallic or silica-rich objects
  between 1 km and 950 km in diameter
• For the first 500 to 700 million years of
  Earth’s history, asteroids struck Earth
  much more frequently than they do today.
  • The impacts generated a tremendous amount
    of thermal energy
  • The debris caused a blanketing effect, which
    prevented the newly generated heat from
    escaping to space
Earth Cooling

• Evidence suggests that Earth cooled
  enough for liquid water to form within its
  first 200 million years
• The cooling process continues today
• As much as half of Earth’s internal heat
  remains from Earth’s formation
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