1. Essentials of Geology, 9e
Earth History: A
Brief Summary
Chapter 19

2. Early evolution of Earth
Origin of planet Earth
Most researchers believe that Earth and the other planets formed at essentially the same time from the same primordial material as the Sun
Nebular hypothesis
Solar system evolved from an enormous rotating cloud called the solar nebula
Nebula was composed mostly of hydrogen and helium

3. Early evolution of Earth
Origin of planet Earth
Nebular hypothesis
About 5 billion years ago the nebula began to contract
Assumes a flat, disk shape with the protosun (pre-Sun) at the center
Inner planets begin to form from metallic and rocky clumps of substances with high melting points
Larger outer planets began forming from fragments with a high percentage of ices - water, carbon dioxide, methane and ammonia

4. Early evolution of Earth
Formation of Earth’s layered structure
As Earth formed, high-velocity impacts caused the temperature to increase and iron and nickel began to melt and sink toward the center
Buoyant masses of molten rock rose to the surface to produce a primitive crust

5. Early evolution of Earth
Formation of Earth’s layered structure
Early chemical segregation established the three basis divisions of Earth’s interior
an iron-rich core
a thin, primitive crust
the mantle between the core and crust
A primitive atmosphere evolved as gaseous materials escaped from Earth’s interior

6. Earth’s atmosphere evolves
Primitive atmosphere formed from volcanic gases
A process called outgassing
Water vapor, carbon dioxide, nitrogen, and several trace gases
Very little free oxygen
Water vapor condenses and forms primitive oceans as Earth cools

7. Earth’s atmosphere evolves
Bacteria evolve
Plants evolve and photosynthesis produces oxygen
Oxygen content in the atmosphere increases
By about 4 billion years after Earth formed, abundant ocean-dwelling organisms that require oxygen existed

8. Earth’s history Precambrian era 4.5 billion to 540 million years ago
88% of Earth's history
Only sketchy knowledge
Most Precambrian rocks are devoid of fossils

9. Earth’s history Precambrian era Precambrian rocks
Most are buried from view
Each continent has a "core area" of Precambrian rocks called a shield
Extensive iron ore deposits
Absent are fossil fuels

10. Precambrian shields
Figure 19.3

11. Earth’s history Precambrian era Precambrian fossils
Most common are stromatolites
Material deposited by algae
Common about 2 billion years ago
Microfossils of bacteria and algae have been found in chert
Southern Africa (3.1 billion years of age)
Lake Superior area (1.7 billion years of age)

12. Earth’s history Precambrian era Precambrian fossils
Plant fossils date from the middle Precambrian
Animal fossils date from the late Precambrian
Diverse and multicelled organisms exist by the close of the Precambrian

13. The geologic time scale
Figure 19.2

14. Earth’s history Paleozoic era
540 million years ago to about 248 million years ago
First life forms with hard parts
Abundant Paleozoic fossils
Early Paleozoic history
Southern continent of Gondwanaland exists

15. Earth’s history Paleozoic era Early Paleozoic history North America
A barren lowland
Seas move inland and recede several times and shallow marine basins evaporate leaving rock salt and gypsum deposits
Taconic orogeny, a mountain building event, affects eastern North America

16. Reconstruction of Earth in early Paleozoic time
Figure 19.9 B

17. Earth’s history Paleozoic era Early Paleozoic life Restricted to seas
Vertebrates had not yet evolved
Life consisted of several invertebrate groups
Trilobites
Brachiopods
Cephalopods
First organisms with hard parts, such as shells - perhaps for protection

18. A natural cast of a trilobite
Figure 19.7 A

19. Earth’s history Paleozoic era Late Paleozoic history
Supercontinent of Pangaea forms
Several mountain belts formed during the movements of the continents
World's climate becomes very seasonal, causing the dramatic extinction of many species

20. Formation of Pangaea in late Paleozoic time
Figure 19.9 D

21. Earth’s history Paleozoic era Late Paleozoic life
Organisms diversified dramatically
Land plants
Fishes evolve into two groups of bony fish
Lung fish
Lobe-finned fish which become the amphibians

22. Earth’s history Paleozoic era Late Paleozoic life
Insects invade the land
Amphibians diversify rapidly
Extensive coal swamps develop

23. Earth’s history Mesozoic era
248 million years ago to about 65 million years ago
Often called the "age of dinosaurs"
Mesozoic history
Begins with much of the world's land above sea level
Seas invade western North America

24. Earth’s history Mesozoic era Mesozoic history
Breakup of Pangaea begins forming the Atlantic ocean
North American plate began to override the Pacific plate
Mountains of western North America began forming

25. Reconstruction of Earth in the late Jurassic period
Figure B

26. Earth’s history Mesozoic era Mesozoic life
Survivors of the great Paleozoic extinction
Gymnosperms become the dominant trees
Reptiles (first true terrestrial animals) readily adapt to the dry Mesozoic climate
Reptiles have shell-covered eggs that can be laid on the land

27. Major reptile groups in the Mesozoic era
Figure 19.14

28. Earth’s history Mesozoic era Mesozoic life Dinosaurs dominate
One group of reptiles led to the birds
Many reptile groups, along with many other animal groups, become extinct at the close of the Mesozoic
One hypothesis is that a large asteroid or comet struck Earth
Another possibility is extensive volcanism

29. The Chicxulub impact site
Figure 19 B

30. Earth’s history Cenozoic era 65 million years ago to the present
Often called the "age of mammals"
Smaller fraction of geologic time than either the Paleozoic or the Mesozoic
North America
Most of the continent was above sea level throughout the Cenozoic era

31. Earth’s history Cenozoic era North America
Many events of mountain building, volcanism, and earthquakes in the West
Eastern North America
Stable with abundant marine sedimentation
Eroded Appalachians were raised by isostatic adjustments

32. Earth’s history Cenozoic era North America Western North America
Building of the Rocky Mountains was coming to an end
Large region is uplifted – Basin and Range province is formed, Rockies are re-elevated, rivers erode the Grand and Black canyons, and volcanic activity is abundant

33. Earth’s history Cenozoic era Cenozoic life
Mammals replace reptiles as the dominant land animals
Angiosperms (flowering plants with covered seeds) dominate the plant world
Strongly influenced the evolution of both birds and mammals
Food source for both birds and mammals

34. Earth’s history Cenozoic era Cenozoic life
Two groups of mammals evolve after the reptilian extinctions at the close of the Mesozoic
Marsupials
Placentals

35. Earth’s history Cenozoic era Cenozoic life
Mammals diversify quite rapidly and some groups become very large
e.g., Hornless rhinoceros, which stood nearly 16 feet high
Many large animals became extinct
Humans evolve

36. End of Chapter 19