Geologic Time

… grab from blackboard

 

Geologic time scale

  • Earth has very long history

    • Long af

    • Years and centuries are not very helpful

  • Scientists use geologic time scale for Earth's history

  • Geologic time scale

    • Record of how earth and its life forms have changed through time

    • For example: scale shows when life first appeared on Earth

  • In the geologic time scale

    • Time divided into bigger blocks than years or centuries

    • Scale begins when earth formed 4.6 billion years ago and goes to the present

 

Divisions of geologic time

  • Divided into eons, eras, periods, and epochs

  • Unlike divisions of time such as days or minutes, the divisions of the geologic scale have no fixed lengths

  • Instead they are based on changes or events recorded in rocks and fossils

  • Largest unit of time is EON

    • Extremely long, indefinite period of time

  • Earth's 4.6 billion year history is divided into Archean (preproterozic)

    • Precambrian time, Mesozoic, Cenozoic

    • The three eras are also called phanerozoic

 


 

 

We live in Cenozoic Era

  • Present day earth is in Cenozoic era and the Quaternary period in the Holocene epoch

  • Geologic time has not ended!!

  • WE ARE IN (THE AGE OF MAN)

    • Cenozoic Era

    • Quaternary Period

    • Holocene Epoch

 

  • Cambrian period: explosion of life

  • Mass extinctions

    • Triassic period

    • Cretaceous period

  • Himalayas rose 5.3 million yrs ago

 

Precambrian time

  • 4.6 million years ago

  • This period is most of earth's history

  • For nearly 4 billion years

 

Paleozoic era

  • 544 million years ago

  • All life lived in oceans

 

Mesozoic era

  • 345 to 65 million years ago

  • Age of reptiles and dinosaurs

  • Lived along with mammals, birds, and flowering plants

  • Reptiles were dominant

 

Cenozoic era

  • 65 mya to present day

  • First humans appeared in later part of Cenozoic era

  • New mammals and birds appeared while others became extinct

 

Fossils

  • Preserved impressions of life

  • Preservation can occur through several periods

 

Fossils help with relative dating

  • Principle of fossil succession

    • Assemblage of fossil species in a given sequence of sedimentary strata differs from that found in older sequences or in younger sequences (due to evolution)

    • A given species appears at a certain level and then disappears (goes extinct at a higher level)

 

Age dating

  • Based on isotopes

  • Parent mineral; daughter mineral

  • Initial # of parent = current # of parent + current # of daughter