Essentials of Geology, 9e

Earth History: A

Brief Summary

Chapter 19

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

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

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

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

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

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

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

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

Precambrian shields

Figure 19.3

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)

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

Figure 19.2

The geologic

time scale

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

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

Reconstruction of Earth in early Paleozoic time

Figure 19.9 B

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

A natural cast of a trilobite

Figure 19.7 A

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

Formation of Pangaea in late Paleozoic time

Figure 19.9 D

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

Earth’s history

Paleozoic era • Late Paleozoic life

Insects invade the land Amphibians diversify rapidly Extensive coal swamps develop

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

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

Reconstruction of Earth in the late Jurassic period

Figure 19.13 B

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

Major reptile groups in the Mesozoic era

Figure 19.14

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

The Chicxulub impact site

Figure 19 B

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

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

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

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

Earth’s history

Cenozoic era• Cenozoic life

Two groups of mammals evolve after the reptilian extinctions at the close of the Mesozoic

Marsupials Placentals

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

End of Chapter 19