Monday Warm-Up
• Create vocab cards/flaps/whatever you
like for Chapter 3, lesson 2 words:
• Hadean eon
• Archean eon
• Protocontinent
• Proterozoic eon
• The Great Oxygenation Event/Oxygen
Catastrophe
Lesson 2: Ancient Earth
Earth’s Earliest History
• Scientists hypothesize that the solar system formed
when a nebula was pulled together by gravity.
• Gravity pulled the
particles of the nebula
together.
• The particles formed a
flat, rotating disk.
• Material in the center of
the disk formed the Sun.
Lesson 2: Ancient Earth
Earth’s Earliest History
• Remaining particles of the disk attracted each other,
forming the planets.
• Collisions of particles
produced thermal energy.
• The hot, soft materials of
ancient Earth flowed into
the shape of a sphere.
• Asteroids crashed into
Earth, making Earth even
hotter.
Lesson 2: Ancient Earth
POP QUIZ
1. How could studying asteroids and comets help
scientists to understand how the solar system
formed?
2. Describe the sequence of events in which the solar
system formed.
3. What is a nebula?
4. What effect did gravity have on Earth’s formation?
5. What was one source od thermal energy of early
Earth?
Lesson 2: Ancient Earth
The Hadean Eon
• The first 640 million years of Earth history are called
the Hadean eon.
• Earth was much hotter during the Hadean eon.
Scientists think that molten rock covered Earth’s
surface.
• Collisions with asteroids and radioactive decay
produced huge amounts of thermal energy.
• Eventually, fewer and fewer asteroids struck Earth.
(cooling)
• The radioactive elements on Earth decayed and
formed stable daughter elements, so less radioactive
material was present. (cooling)
Lesson 2: Ancient Earth
The Hadean Eon
• Small islands of solid rock
may have started floating
on Earth’s surface… but the
motion of the molten seas
or asteroid impacts would
have melted them… until…
• As Earth cooled, the molten
surface solidified to form an
ancient crust.
• Poisonous volcanic gases
formed Earth’s earliest
atmosphere when they
escaped from inside Earth.
Lesson 2: Ancient Earth
POP QUIZ
1. How did gravity affect Earth after its formation?
2. Why isn’t the earliest crust formed present on
Earth today?
3. If small islands of solid rock had formed, why
would they have not survived for long?
4. Do you think that the same amount of volcanic
gases are produced by volcanoes today as they
were during the Hadean eon?
5. How did Earth’s early atmosphere form?
Lesson 2: Ancient Earth
The Archean Eon
• During the Archean eon (the period of time that
occurred from about 4 to 2.5 billion years ago), Earth
had its first solid surface.
• Earth was cooler during the Archean eon, but still
produced twice as much thermal energy as present
Earth.
• The oldest rocks discovered are from this eon.
• Extensive volcanic activity formed Earth’s first
oceanic crust through convection currents.
• The first, small continents were the protocontinents.
Lesson 2: Ancient Earth
The Archean Eon
• During the Archean eon, the temperature in Earth’s
atmosphere dropped, causing the water vapor in the
air to condense.
• The water, which was made acidic by gases in the air,
fell as rain.
• By the time the low areas of the oceanic crust filled
with water, the new oceans were salty.
Lesson 2: Ancient Earth
The Archean Eon
• Fossil remains of unicellular bacteria and
cyanobacteria are the earliest evidence of life that
formed in the warm Archean oceans.
• Sometimes sticky strands of cyanobacteria trap
sediment from the ocean and form visible mounds
called stromatolites.
Lesson 2: Ancient Earth
POP QUIZ
1. What are convection currents? How do they form?
2. Would the rate of formation of oceanic crust have
been different from its present rate during the
Archean eon?
3. What are protocontinents?
4. What are stromalites? Why are stromatolite fossils
so rare?
5. What conditions made early Earth able to support
life?
6. How did Earth’s oceans form?
7. Why did the rain during the Archean eon dissolve
rocks more rapidly than it does today?
Lesson 2: Ancient Earth
The Proterozoic Eon
• The time after the Archean eon is known as the
Proterozoic eon.
• During the Proterozoic eon, oxygen was added to
the atmosphere.
• During the Great Oxygenation Event, or Oxygen
Catastrophe, atmospheric oxygen increased from
about 3% to 20%.
• The oxygen-rich atmosphere harmed organisms
adapted to the earlier lower oxygen levels.
• The Snowball Earth hypothesis states that Earth was
completely covered with ice at the end of the
Proterozoic eon.
Lesson 2: Ancient Earth
POP QUIZ
1. How were the convection currents in the mantle
affected by the overall change in Earth’s
temperature during the Proterozoic eon?
2. How long did the Proterozoic eon last?
3. What geologic evidence of the hypothesized
Snowball Earth period would you expect to find in
the fossil record?
4. How did changes in Earth’s environment affect the
evolution of life?
5. What happened during the Great Oxygenation
Event?
Lesson 2: Ancient Earth
The Proterozoic Eon
• Scientists hypothesize that the supercontinent
Rodinia formed during the Proterozoic eon, before
Pangea.
• Rodinia might have formed as separate continents
collided and stuck together.
• The first multicellular
organisms evolved in the
oceans at the end of the
Proterozoic eon
Lesson 2: Ancient Earth
POP QUIZ
1. Why do you think it took millions of years for a
supercontinent such as Rodinia to form?
2. What caused the continents to collide and stick
together?
3. What supercontinent existed before Pangea?
4. What was the dominant type of life during the
Proterozoic eon?
5. What important evolutionary change occurred at
the end of the Proterozoic eon?
6. Why were unicellular organisms dominant during
this eon, even though multicellular organisms also
evolved during this time?
Lesson 2 Review Time
• Do the Lesson 2 review (p. 94) in
your notebooks…
• We’ll review in 15 minutes.