Transcript Chapter 22 – The Precambrian 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
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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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