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 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