1. Chapter 22- The Precambrian Earth

2. Early Earth The Age of Earth
Earth is estimated to be 4.56 billion years old.
Precambrian time accounts for 90% of Earth’s history, and is divided into THREE eons:
Hadean – No signs of life
Archean – Prokaryotic organisms
Proterozoic – Eukaryotic organisms
Evidence of Earth’s Age
Crustal Rock – pg. 620
Solar System – pg. 621

3. Early Earth Early Earth’s Heat Sources – pgs. 621 & 622
Gravitational Contraction
Radioactivity
Bombardment
Earth’s Cooling
Took about 200 million years, for liquid water to form.
Cooling continues to this day.

4. Formation of the Crust & Continents
Differentiation
High density elements sank toward the center. – Graph pg. 624
Earliest Crust
Resembled oceanic crust, and was recycled. None exists today.
Continental Crust
Reactions with the early crust and water, created less dense material, which led to microcontinents.
Cratons were formed, the deepest parts of the crust… up to 200km thick.
Precambrian Shields – Areas where cratons are exposed, In North America, we have the Canadian Shield.

5. Formation of the Crust & Continents
Growth of the Continents
Mountain building
as microcontinents collided, mountains and mountain ranges were created.
Laurentia
Ancient North America, was created by collisions bya.
Rodenia
1st supercontinent. Positioned at the equator, with Laurentia at the center, was composed of 75% of all the continental crust.
Pangaea – 300 mya
Laurasia & Gondwanaland – 200 mya

6. Formation of the Atmosphere & Oceans
Formation of the Atmosphere – read pg. 628
Outgassing
Volcanic eruptions released large amounts of water vapor, carbon dioxide and Nitrogen into the atmosphere.
Oxygen in the Atmosphere
Volcanoes do not typically produce Oxygen, so how’d it get into our atmosphere?
1st Oxygen producers
Cyanobacteria – use photosynthesis and produce Oxygen.
These cyanobacteria clustered together by the billions and formed Stromatolites.
Evidence in Rocks – Iron Oxide presence.
Banded Iron
Red Beds
Importance of Oxygen
Animal Respiration
Ozone Layer

7. Formation of the Crust & Continents
Formation of the Oceans
Where did the water come from?
Outgassing
Bombardment
Earth cooled, liquid water formed, water cycle began… Rain!
Oceans and rivers formed, minerals dissolved and made the new oceans salty.
Water & Life
Liquid water made it possible for life to exist on our planet.

8. Early Life on Earth Origin of Life Primordial Soup
Miller’s Hypothesis – 1953
Read pg.633,634
How Life Might have Begun on Earth… 3 Ideas.
Copy table 22.1
Earth’s Surface
Deep Earth
Space
RNA World – read pg.635

9. Early Life on Earth Proterozoic Life
Precambrian was dominated by unicellular organisms
TWO types:
Prokaryotes – NO membrane bound nucleus, smaller.
Eukaryotes – membrane bound nucleus, larger.
Multicellular Organisms
Present around 630 mya.
Ediacaran biota