Chapter 11
Earth History:
A Brief Summary
I. Origin of Earth
Theory called the nebular hypothesis
Bodies of the solar system evolved from an enormous cloud (nebula) composed of hydrogen and helium, with a small percentage of all the other heavier elements
Sequence of events within the nebula
Cloud began to contact about 5 billion years ago
Small concentrations from the nuclei of the planets
Protosun ( sun in the making) forms
Protoplanets accumulate more and more debris
Inner planets (Mercury through Mars) heat and lose their lighter components
Cold outer planets(Jupiter through Saturn) accumulated gases and other light materials
Partial melting of Earths early interior
Heat comes from the decay of radioactive materials and collisions
Iron and nickel sink to center
Lighter components rise toward the surface
Gaseous materials escape from the interior and form the primitive atmosphere
II Earths atmosphere
A. Primitive atmosphere formed from volcanic gases
A process called outgassing
Water vapor, carbon dioxide, nitrogen , and several trace gases
Very little free oxygen
Earths atmosphere continued
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 million years after Earth formed, abundant ocean-dwelling organisms that required oxygen existed
A. Precambrian Era
4.6 billion to 570 million years ago
87 % of Earths history
Only sketchy knowledge
Most Precambrian rocks devoid of fossils
Precambrian rocks
Most are buried from view
Each continent has a " core area" of Precambrian rocks called a shield
Contain extensive iron ore deposits
Absent are fossil fuels
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 age)
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
B. Paleozoic Era
570 million years ago to about 245 million years ago
First life forms with hard parts
Abundant Paleozoic history
Early Paleozoic history
Southern continent of Gondwanaland exists
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
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
Late Paleozoic history
Supercontinent of Pangaea forms
Several mountain belts formed during the movements of the continents
Worlds climate becomes very seasonal, causing the dramatic extinction of many species
Late Paleozoic life
Organisms diversified dramatically
Land plants develop
Fishes evolve into two groups of bony fish
Lung fish
Lobe-finned fish which become the amphibians
Insects invade the land
Amphibians diversity rapidly
Extensive coal swamps develop
C. Mesozoic Era
245 million years ago to 66 million years ago
Often called the "age of dinosaurs"
Mesozoic history
Begins with much of the worlds land above sea level
Seas invade western North America
Breakup of Pangaea begins forming the Atlantic ocean
North America plate began to override the Pacific plate
Mountains of western North America began forming
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
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
Cenozoic Era
1. 66 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
Cenozoic era continued
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
Stable with abundant marine sedimentation
Eroded Appalachians were raised by isostatic adjustments
Cenozoic era continued
d. Western North America
Building of the Rocky Mountains was coming to an end
Large region is uplifted
Basin and Range province formed
Re-elevates the Rockies
Rivers erode and form gorges (e.g., Grand Canyon)
Western North America continued
Volcanic activity is common
Fissure eruptions form the Columbia Plateau
Volcanoes form from northern California to Canadian border
Coast Ranges form
Sierra Nevada become fault-block mountains
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
Cenozoic life continued
Two groups of mammals evolve after the reptilian extinctions at the close of the Mesozoic
Marsupials
Placentals
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
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