Earth’s Evolution through Geologic Time
22
Earth’s Evolution through Geologic Time opens with a presentation of the origin of Earth, including the
physical and chemical differentiation during the early evolution of Earth. Also examined are the formation of
Earth's primitive atmosphere and how it changed over time. The geologic history of the Precambrian is
presented along with a brief discussion of crustal evolution and the assembling of continents. The
supercontinent cycle is presented in some detail and the chapter concludes with an examination of the
significant geologic and biologic events that occurred during the Paleozoic, Mesozoic, and Cenozoic Eras.
Learning Objectives
After reading, studying, and discussing the chapter, students should be able to:
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Explain how the planets in the solar system originated.
Briefly discuss the physical and chemical differentiation that took place during the early evolution
of Earth.
Discuss how Earth's atmosphere formed by a process called outgassing.
Understand how Earth’s atmosphere evolved over time.
Briefly explain the geologic history of the Precambrian including crustal evolution and the
assembling of crustal fragments to form cratons.
Explain the supercontinent cycle, including associated climatic and sea level changes.
List the principal geologic events that took place during the Phanerozoic eon.
Summarize the history of life during the Paleozoic, Mesozoic, and Cenozoic eras.
Chapter Outline___________________________________________________________________
I.
Early evolution of Earth
A. Origin of planet Earth
1. Most researchers believe that Earth
and the other planets formed at
essentially the same time from the
same primordial material as the Sun
2. Nebular hypothesis
a. Solar system evolved from an
enormous rotating cloud called the
solar nebula
b. Nebula was composed mostly of
hydrogen and helium
c. About 5 billion years ago the
nebula began to contract
d.
Assumes a flat, disk shape with
the protosun (pre-Sun) at the
center
e. Inner planets begin to form from
metallic and rocky clumps of
substances with high melting
points
f. Larger outer planets began
forming from fragments with a
high percentage of ices – water,
carbon dioxide, ammonia, and
methane
B. Formation of Earth’s layered structure
1. As Earth formed, high-velocity
impacts caused the temperature to
increase and iron and nickel began to
melt and sink toward the center
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2.
Buoyant masses of molten rock rose
to the surface to produce a primitive
crust
3. Early chemical segregation
established the three basis divisions
of Earth’s interior
a. An iron-rich core
b. A thin primitive crust, and
c. The mantle between the core and
crust
4. A primitive atmosphere evolved as
gaseous materials escaped from
Earth’s interior
II. Earth's atmosphere evolves
A. Primitive atmosphere formed from
volcanic gases
1. A process called outgassing
2. Water vapor, carbon dioxide, nitrogen,
and several trace gases
3. Very little free oxygen
B. Water vapor condenses and forms
primitive oceans as Earth cools
C. Bacteria evolve
D. Plants evolve and photosynthesis produces
oxygen
E. Oxygen content in the atmosphere
increases
F. By about 4 billion years after Earth
formed, abundant ocean-dwelling
organisms that require oxygen existed
III. Earth's history
A. Precambrian era
1. 4.5 billion to 540 million years ago
2. 88 percent of Earth's history
3. Only sketchy knowledge
4. Most Precambrian rocks are devoid of
fossils
5. Precambrian rocks
a. Most are buried from view
b. Each continent has a "core area"
of Precambrian rocks called a
shield
c. Extensive iron ore deposits
d. Absent are fossil fuels
6. Precambrian fossils
a. Most common are stromatolites
1. Material deposited by algae
2. Common about 2 billion years
ago
b. Microfossils of bacteria and algae
have been found in chert
1. Southern Africa (3.1 billion
years of age)
2. Lake Superior area (1.7
billion years of age)
c. Plant fossils date from the middle
Precambrian
d. Animal fossils date from the late
Precambrian
e. Diverse and multicelled
organisms exist by the close of the
Precambrian
B. Paleozoic era
1. 540 million years ago to about 248
million years ago
2. First life forms with hard parts
3. Abundant Paleozoic fossils
4. Early Paleozoic history
a. Southern continent of
Gondwanaland exists
b. North America
1. A barren lowland
2. Seas move inland and recede
several times and shallow
marine basins evaporate
leaving rock salt and gypsum
deposits
3. Taconic orogeny, a mountain
building event, affects eastern
North America
5. Early Paleozoic life
a. Restricted to seas
b. Vertebrates had not yet evolved
c. Life consisted of several
invertebrate groups
1. Trilobites
2. Brachiopods
3. Cephalopods
d. First organisms with hard parts,
such as shells – perhaps for
protection
6. Late Paleozoic history
a. Supercontinent of Pangaea forms
b. Several mountain belts formed
during the movements of the
continents
Earth’s Evolution through Geologic Time
c. World's climate becomes very
seasonal, causing the dramatic
extinction of many species
7. Late Paleozoic life
a. Organisms diversified
dramatically
b. Land plants
c. Fishes evolve into two groups of
bony fish
1. Lung fish
2. Lobe-finned fish which
become the amphibians
d. Insects invade the land
e. Amphibians diversify rapidly
f. Extensive coal swamps develop
C. Mesozoic era
1. 248 million years ago to about 65
million years ago
2. Often called the "age of dinosaurs"
3. Mesozoic history
a. Begins with much of the world's
land above sea level
b. Seas invade western North
America
c. Breakup of Pangaea begins
forming the Atlantic ocean
d. North American plate began to
override the Pacific plate
e. Mountains of western North
America began forming
4. Mesozoic life
a. Survivors of the great Paleozoic
extinction
b. Gymnosperms become the
dominant trees
c. Reptiles (first true terrestrial
animals) readily adapt to the dry
Mesozoic climate
d. Reptiles have shell-covered eggs
that can be laid on the land
e. Dinosaurs dominate
f. One group of reptiles led to the
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birds
g. Many reptile groups, along with
many other animal groups,
become extinct at the close of the
Mesozoic
1. One hypothesis is that a large
asteroid or comet struck Earth
2. Another possibility is
extensive volcanism
D. Cenozoic era
1. 65 million years ago to the present
2. Often called the "age of mammals"
3. Smaller fraction of geologic time than
either the Paleozoic or the Mesozoic
4. North America
a. Most of the continent was above
sea level throughout the Cenozoic
era
b. Many events of mountain
building, volcanism, and
earthquakes in the West
c. Eastern North America
1. Stable with abundant marine
sedimentation
2. Eroded Appalachians were
raised by isostatic adjustments
d. Western North America
1. Building of the Rocky
Mountains was coming to an
end
2. Large region is uplifted
a. Basin and Range Province
formed
b. Re-elevates the Rockies
c. Rivers erode and form
gorges (e.g., Grand
Canyon and Black
Canyon
d. Volcanic activity is
common
1. Fissure eruptions form
the Columbia Plateau
2. Volcanoes form from
northern California to
the Canadian border
e.
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Coast Ranges form
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f.
Sierra Nevada
become fault-block
mountains
5. Cenozoic life
a. Mammals replace reptiles as the
dominant land animals
b. Angiosperms (flowering plants
with covered seeds) dominate the
plant world
1. Strongly influenced the
evolution of both birds and
mammals
2. Food source for both birds
and mammals
c. Two groups of mammals evolve
after the reptilian extinctions at
the close of the Mesozoic
1. Marsupials
2. Placentals
d. Mammals diversify quite rapidly
and some groups become very
large
1. e.g., Hornless rhinoceros,
which stood nearly 16 feet
high
2. Many large animals became
extinct
e. Humans evolve
Answers to the Review Questions
1. The metallic core of Earth is important to humans today because it supports our magnetic field, which
prevents cosmic rays (solar winds) from reaching the surface.
2. Hydrogen and helium most likely made up much of the early universe.
3. Supernova.
4. The theory for the origin of the solar system, called the nebular hypothesis, states that approximately 5
billion years ago the bodies of the solar system condensed from an enormous cloud. As the cloud
contracted and began to rotate, the protosun began forming. The protoplanets (planets in the making)
formed from material that had condensed and accreted inside the cloud. The inner planets, Mercury,
Venus, Earth, and Mars were unable to retain appreciable amounts of the lighter components of the
primordial cloud; while the outer planets (Jupiter, Saturn, Uranus, and Neptune) accumulated large
amounts of hydrogen and other light materials because of their much colder temperatures.
5. Outgassing refers to the process by which Earth’s early atmosphere was formed by gases, trapped in the
planet’s interior, erupted from volcanoes over millions of years. Outgassing continues today from
hundreds of active volcanoes worldwide.
6. Earth’s early atmosphere was composed mainly of water vapor and carbon dioxide.
7. The free oxygen in Earth's atmosphere today originated from green plants releasing oxygen during the
process of photosynthesis.
8. Water in the first oceans came from the water vapor erupted from volcanoes via outgassing.
Earth’s Evolution through Geologic Time
189
9. Carbon dioxide is highly soluble in water and it is removed from our atmosphere by the precipitation of
calcium carbonate to form the chemical sedimentary rock, limestone. Marine organisms, such as
foraminifera, remove calcium carbonate from seawater to make their shells and other hard parts.
10. Precambrian history is more difficult to decipher because of the lack of fossils, which hinders correlation
of rocks. Also, rocks of this great age are metamorphosed and deformed, extensively eroded, and
obscured by younger overlying strata.
11. Cratons are thoughts to form from the collision and accretion of various types of terranes. Such collisional
tectonics deforms and metamorphoses the sediments caught between the converging crustal fragments,
which shortens and thickens the developing crust. Partial melting in the deepest regions of these collision
zones generates silica-rich magmas that intrude the crustal rocks above.
12. Supercontinents are large landmasses that contain all, or nearly all, of the existing continents on Earth at
any given time. The supercontinent cycle is the idea that rifting and dispersal of one supercontinent is
followed by a long period during which the fragments are gradually reassembled into a new
supercontinent having a different configuration.
13. The movement of continents can produce global climate change because they change the patterns of
ocean currents and global wind patterns. As a result of such changes, the distribution of temperatures and
precipitation worldwide changed as well.
14. (a) Paleozoic; (b) Precambrian; (c) Cenozoic; (d) Precambrian; (e) Mesozoic; (f) Cenozoic; (g) Paleozoic;
(h) Paleozoic; (i) Mesozoic; (j) Precambrian; (k) Mesozoic; (l) Paleozoic; (m) Paleozoic; (n) Cenozoic;
(o) Paleozoic; (p) Mesozoic; (q) Precambrian; (r) Cenozoic
15. During the Cenozoic the eastern continental margin of North America was tectonically stable and the site
of abundant marine sedimentation. The western margin, on the other hand, was the leading edge of the
North American plate. As a result, plate interactions during the Cenozoic gave rise to many events of
mountain building, volcanism, and earthquakes in the West.
16. In order to move onto the land from the oceans plants had to develop a mechanism for obtaining water
and also a structure that allowed them to stay upright, despite gravity and wind.
17. The lobe-finned fish are thought to have developed the ability to stay out of water for longer periods of
time and eventually became the first amphibians.
18. Amphibians are not considered to be true land animals because they are born in the water and originally
they have gills and tails. Over time these features disappear as the amphibians become air-breathing
organisms with legs.
19. Unlike amphibians, reptiles have shell-covered eggs that can be laid on land. The appearance of shellcovered eggs in the Mesozoic eliminated a water-dwelling stage and it was an important evolutionary step
in the development of reptiles.
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Lecture outline, art-only, and animation PowerPoint presentations for each chapter of Earth,
9e are available on the Instructor’s Resource Center CD (0131566911).