Download 5 Early Earth

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Section
5
Early Earth
Early Earth
Reading Preview
Objectives
Key Concepts
After this lesson, students will be able to
G.4.5.1 State when Earth was formed.
G.4.5.2 Explain how Earth’s physical
features developed during Precambrian
Time.
G.4.5.3 Describe what early Precambrian
organisms were like.
• When did Earth form?
• How did Earth’s physical features
develop during Precambrian
Time?
• What were early Precambrian
organisms like?
Key Terms
• comet • continental drift
Target Reading Skill
Comparing and Contrasting Explain that
comparing and contrasting information
shows how ideas, facts, and events are similar
and different. The results of comparison can
have importance.
Answers
Possible answers:
Early Earth:
Atmosphere—Hydrogen and helium
Oceans—Earth’s surface is too hot. All water
evaporates into water vapor.
Continents—Less dense rock at surface
forms continents.
Later Precambrian Earth:
Atmosphere—Carbon dioxide, nitrogen,
and water vapor
Oceans—Earth cools, water vapor condenses,
and rain falls. Rain forms oceans.
Continents—Old continents break apart and
new continents form as a result of continental drift.
Target Reading Skill
Comparing and Contrasting As
you read, compare and contrast
early Earth with Earth later in
Precambrian Time by completing a
table like the one below.
Precambrian Earth
Feature
Atmosphere
Oceans
Continents
Think It Over
Making Models If you repeated Steps 2 and 3, what would
happen to the size of your “planet”? How is this model like the
early Earth? How is it different?
Your science class is going on a field trip, but this trip is a little
out of the ordinary. You’re going to travel back billions of years
to the earliest days on Earth. Then you will move forward
through time to the present. Enter the time machine and strap
yourself in. Take a deep breath—you’re off!
A dial on the dashboard shows the number of years before
the present. You stare at the dial—it reads 4.6 billion years.
You peer out the window as the time machine flies above the
planet. Earth looks a little strange. Where are the oceans?
Where are the continents? How will Earth change over the next
billions of years? You’ll answer these and other questions about
Earth’s history as you take this extraordinary trip.
Your journey starts at the beginning of Precambrian Time with
the formation of planet Earth. Scientists hypothesize that
Earth formed at the same time as the other planets and the
sun, roughly 4.6 billion years ago.
• Transparency G44
The Age of Earth How do scientists know the age of Earth?
Using radioactive dating, scientists have determined that the
oldest rocks ever found on Earth are about 4 billion years
old. But scientists think Earth formed even earlier than that.
Preteach
338
◆
L1
Characteristics of Earth
Show students a photo of Earth taken from
outer space by astronauts. Ask: What are
some of the features that you see on Earth?
(A typical answer will include cloud
formations, oceans, landmasses, or
continents.) Do you think that Earth had
any of these features when it first formed?
What were they? (Accept all reasonable
responses. Record them on the board to refer to
as students read through the section.)
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Later
Precambrian
Earth
1. Place a sheet of paper on top of a small magnet. The paper
represents outer space and the magnet models gravity.
2. Sprinkle a half teaspoon of iron filings along one end of the
paper to model the materials that formed Earth.
3. Gently blow through a straw for about 10 seconds from the
end of the paper with the iron filings toward the magnet.
CAUTION: Be sure the straw is pointed away from other
students.
4. Observe what happens to the iron filings.
The Planet Forms
Teaching Resources
Build Background
Knowledge
Early
Earth
How Could Planet Earth Form in Space?
L1
Skills Focus making models
Materials circular magnet, iron filings,
paper, straw
Time 10 minutes
Tips Caution students to blow gently
through the straw so as not to spread iron
filings all over. Be sure students do not
blow the iron filings in the direction of
other students.
Expected Outcome The iron filings will
gather around the poles of the magnet.
Think It Over The iron filings would
continue to gather in clumps around the
magnet. The model shows how early
materials in space were gathered in a ball by
the force of gravity to form Earth. In the
magnet model, the force is the attraction of
iron to the poles of the magnet.
sx05_TE_(nc8-8)c04G.fm Page 339 Thursday, June 2, 2005 1:16 PM
Instruct
The Planet Forms
For: Links on
Precambrian Earth
Visit: www.SciLinks.org
Web Code: scn-0745
Download a worksheet that will guide students’
review of Internet resources on Precambrian Earth.
Teach Key Concepts
According to this hypothesis, Earth and the moon are about
the same age. When Earth was very young, it collided with a
large object. The collision threw a large amount of material from
both bodies into orbit around Earth. This material combined to
form the moon. Scientists have dated moon rocks that were
brought to Earth by astronauts during the 1970s. Radioactive
dating shows that the oldest moon rocks are about 4.6 billion
years old. Scientists infer that Earth is also roughly 4.6 billion
years old—only a little older than those moon rocks.
Earth Takes Shape Scientists think that Earth began as a ball
of dust, rock, and ice in space. Gravity pulled this mass together.
As Earth grew larger, its gravity increased, pulling in dust, rock,
and ice nearby. As objects made of these materials struck Earth at
high speed, their kinetic energy was changed into thermal energy.
The energy from these collisions caused Earth’s temperature to rise until the planet was very hot. Scientists think that
Earth may have become so hot that it melted. Denser materials
sank toward the center, forming Earth’s dense, iron core. At the
same time, Earth continuously lost heat to the cold of space.
Less dense, molten material hardened to form Earth’s outer
layers—the solid crust and mantle.
As the growing Earth traveled around the sun, its gravity also
captured gases such as hydrogen and helium. But this first atmosphere was lost when the sun released a strong burst of particles.
These particles blew away Earth’s first atmosphere.
What force caused the materials that formed Earth
to come together?
FIGURE 16
Early Earth
This artist’s illustration shows Earth
shortly after the moon formed.
Notice the rocky objects from space
striking Earth, and the molten rock
flowing over the surface.
When Earth Formed
Focus Refer students to Figure 16.
Teach Ask: How was early Earth different
from planet Earth today? (Earth today is
made up of oceans and landmasses, and early
Earth was initially a mass of dust, rock, and ice
that eventually turned into a molten mass.)
When did Earth form? (About 4.6 billion
years ago) How do scientists know? (The
oldest moon rocks are 4.6 billion years old,
and scientists think that the moon formed
after Earth.)
Apply Explain that though Earth is older
than the moon, moon rocks are older than
Earth rocks. The oldest rocks on Earth have
been destroyed by processes such as plate
tectonics and erosion. These processes do
not occur on the moon. learning modality:
logical/mathematical
Independent Practice
L2
Teaching Resources
• Guided Reading and Study Worksheet:
Early Earth
For: Links on Precambrian Earth
Visit: www.SciLinks.org
Web Code: scn-0745
Chapter 8
◆
Student Edition on Audio CD
Help Students Read
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Differentiated Instruction
L1
English Learners/Beginning
Comprehension: Modified Cloze
Distribute a simplified paragraph about
the age of Earth and how it formed. For
example, “Scientists infer that Earth is
about 4.6 _____ years old. Earth began as
a mass of dust, rock, and ice in _____.
The mass was pulled together by _____.”
Provide students with a list of correct
answers to fill in the blanks. learning
L2
L2
English Learners/Intermediate
Comprehension: Key Concepts Ask
students to complete the Beginning activity
and then use the statements to write a
paragraph describing the formation of
the atmosphere, oceans, and continents.
learning modality: verbal
L1
Identifying Supporting Evidence See
the Content Refresher for guidelines on
identifying supporting evidence. Have
students create a graphic organizer for
evidence that supports the hypothesis about
how Earth formed.
Monitor Progress
L2
Writing Have students write a description of
how Earth was formed.
Answer
Gravity
modality: verbal
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Earth’s Surface Forms
Teach Key Concepts
L2
Development of Physical Features
Focus Ask volunteers to read the captions in
Figure 17 that describe the atmosphere.
Teach Ask: What factors contributed to
Earth’s second atmosphere? (Volcanic
eruptions and comets) How did oceans form?
(Water vapor condensed to form rain and
began to accumulate.) How were the
continents formed? (Earth’s rock cooled and
hardened to form large landmasses.)
Apply Ask: Could humans or other
animals have survived on early Earth after
oceans and continents formed? (No. They
could not have breathed the atmosphere
because it did not contain the oxygen they
need.)
FIGURE 17
Development of the Atmosphere
Earth soon lost its first atmosphere
(left) of hydrogen and helium.
Earth’s second atmosphere (right)
slowly developed the mixture of
gases—nitrogen, oxygen, carbon
dioxide, water vapor, and argon—
of the atmosphere today. As
oxygen levels increased, the
ozone layer also developed.
Comparing and Contrasting
Compare and contrast Earth’s first
and second atmospheres.
Earth’s Surface Forms
The Atmosphere After Earth lost its first atmosphere, a second atmosphere formed. This new atmosphere was made up
mostly of carbon dioxide, water vapor, and nitrogen. Volcanic
eruptions released carbon dioxide, water vapor, and other
gases from Earth’s interior. Collisions with comets added other
gases to the atmosphere. A comet is a ball of dust and ice that
orbits the sun. The ice in a comet consists of water and frozen
gases, including carbon dioxide.
Life Develops
L2
Early Precambrian Organisms
Focus Remind students that oxygen is a
product of photosynthesis.
Teach Ask: What did the first life forms
look like? (Single-celled; resembled presentday bacteria) How did they get energy?
(They used energy from the sun to make their
own food.) What was the effect
of the release of oxygen in the atmosphere?
(The amount of oxygen in the atmosphere
slowly increased.)
Apply Ask: If photosynthetic organisms
had not appeared during Precambrian
Time, what do you think Earth would be
like today? (Possible answers: The atmosphere
would contain little or no oxygen; life that
depends on oxygen as we know it would
probably not exist.) learning modality:
ind
rw
la
So
Watching early Earth from your time machine, you can see the
planet change as the years speed by. During the first several
hundred million years of Precambrian Time, an atmosphere,
oceans, and continents began to form.
learning modality: visual
Teach Key Concepts
First Atmosphere
Hydrogen and helium
blasted back into space
by impact of space debris
and particles from the sun
Calculating
Precambrian Time lasted
about 4 billion years. What
percentage is this of Earth’s
entire history of 4.6 billion
years? If the first continents
formed about 500 million
years after Earth itself
formed, what percentage of
Precambrian Time had
elapsed? (Hint: To review
percentages, see the Math
Review section in the Skills
Handbook.)
The Oceans At first, Earth’s surface was too hot for water to
remain as a liquid. All water evaporated into water vapor.
However, as Earth’s surface cooled, the water vapor began to
condense to form rain. Gradually, rainwater began to accumulate to form an ocean. Rain also began to erode Earth’s rocky
surface. Over time, the oceans affected the composition of the
atmosphere by absorbing much of the carbon dioxide.
The Continents During early Precambrian Time, more and
more of Earth’s rock cooled and hardened. Less than 500 million years after Earth’s formation, the less dense rock at the
surface formed large landmasses called continents.
Scientists have found that the continents move very slowly
over Earth’s surface because of forces inside Earth. This process
is called continental drift. The movement is very slow—only
a few centimeters per year. Over billions of years, Earth’s landmasses have repeatedly formed, broken apart, and then
crashed together again, forming new continents.
What is continental drift?
340 ◆
visual
Teaching Resources
• Transparency G45
L2
Skills Focus calculating
Materials calculators
Time 10 minutes
Tips Tell students to write out the zeroes
in the large numbers and then cancel them
before entering the numbers in their
calculators.
Expected Outcome Precambrian Time
lasted for 86% of Earth’s history. About
11% of Precambrian Time elapsed before
the development of the continents.
Extend Have students refer to the
geologic time scale and calculate the
percentage of Earth’s history for which
each era lasted. learning modality:
logical/mathematical
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Second Atmosphere
L2
Carbon dioxide, water vapor, and nitrogen
from volcanic eruptions and comet impacts
Comparing and Contrasting
Atmospheric Gases
Oxygen from bacteria
in the oceans
Ul
Ozone layer
Materials paper, colored pencils
Time 15 minutes
Focus Review early Earth’s atmosphere.
Teach Tell students that Earth’s early
atmosphere was about 92% carbon dioxide,
5% nitrogen, 0% oxygen, and 3% other
gases. Today’s atmosphere is about 78%
nitrogen, 21% oxygen, and 1% other gases,
including carbon dioxide. Have students
make circle graphs of each atmosphere and
discuss the differences.
Apply Ask: Why did the percentage of
oxygen change? (Early life forms gave off
oxygen as a waste product.) learning
Ozone layer gradually forms
as amount of oxygen increases
tra
vio
l e t l ig
ht
Life Develops
Scientists cannot pinpoint when or where life began on Earth.
But scientists have found fossils of single-celled organisms
in rocks that formed about 3.5 billion years ago. These
earliest life forms were probably similar to present-day
bacteria. Scientists hypothesize that all other forms of life on
Earth arose from these simple organisms.
About 2.5 billion years ago, many organisms began using
energy from the sun to make their own food. This process is
called photosynthesis. One waste product of photosynthesis is
oxygen. As organisms released oxygen into the air, the amount
of oxygen in the atmosphere slowly increased. Processes in the
atmosphere changed some of this oxygen into a form called
ozone. The atmosphere developed a layer rich in ozone that
blocked out the deadly ultraviolet rays of the sun. Shielded
from the sun’s ultraviolet rays, organisms could live on land.
modality: logical/mathematical
Monitor Progress
5
Section 5 Assessment
Target Reading Skill Comparing and
Contrasting Use the information in your table
3. a. Identifying What do scientists think were
the first organisms to evolve on Earth?
about early Earth to answer the questions below.
b. Predicting How would Earth’s atmosphere
Reviewing Key Concepts
1. a. Reviewing How long ago did Earth form?
b. Summarizing Summarize the process by
which scientists determined the age of Earth.
2. a. Listing What physical features formed during
Earth’s first several hundred million years?
b. Explaining How did volcanic eruptions and
comets change early Earth?
c. Relating Cause and Effect What caused
water erosion to begin on Earth’s surface?
be different if organisms capable of
photosynthesis had not evolved? Explain.
Web Site Plan a Web site for early Earth. To
plan your Web site, make a list of the topics
you will include. Make sketches of the screens
that visitors to the Web site will see. Then
write short descriptions for each topic.
Chapter 8
Writing Mode Description
Scoring Rubric
4 Exceeds criteria; plan includes
information about early Earth’s atmosphere,
oceans, continents, and life forms; sketches
and descriptions are accurate and
informative
3 Meets criteria but sketches and
descriptions are not imaginative
2 Includes only some criteria
1 Is inaccurate and incomplete
◆
341
L2
Answers
Figure 17 The first atmosphere contained
hydrogen and helium. The second
atmosphere contained nitrogen, oxygen,
carbon dioxide, water vapor, and argon.
The process by which the
continents move very slowly
over Earth’s surface because of forces inside
Earth
Assess
Reviewing Key Concepts
1. a. About 4.6 billion years ago
b. Radioactive dating of rocks from Earth
and the moon
2. a. Oceans, the atmosphere, and
continents b. They released carbon dioxide
and water vapor into the atmosphere.
c. Rain
3. a. Bacteria-like organisms b. It would
have little or no oxygen.
Reteach
L1
Summarize how oceans, the atmosphere,
and continents were formed.
Teaching Resources
• Section Summary: Early Earth
• Review and Reinforce: Early Earth
• Enrich: Early Earth
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