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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 Earth’s 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 Earth’s 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


Earth’s 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 Earth’s 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




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

World’s 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 world’s 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



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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