Fossils and Geologic Time

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Fossils and Geologic Time
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The Fossil Record

• Provides evidence about the history of Earth
• Shows how groups of organisms changed over time

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The Fossil Record

• More than 99 of species that have lived on Earth
  are extinct
• A very small percentage became fossils

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How Fossils Form

• Most form in sedimentary rock
• Rock made from compacted particles of sand, silt,
  and clay
• Organism is buried by sediment shortly after
  dying
• More layers of sediment are laid down on top of
  the dead organism
• Compression of layers of sediment prevent decay
  of the dead organism

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Relative Dating

• Geologists noticed rock layers with certain
  fossils consistently appeared in the same
  vertical order
• Led to relative dating
• Determining the age of a fossil by comparing it
  with fossils found above and below it
• Does not provide an exact date for the fossil

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Radioactive Dating

• Using radioactive decay to assign absolute ages
  to rocks and fossils
• Scientists know exactly how long it takes
  radioactive isotopes to decay to other elements
• Also know approximately how much of a radioactive
  isotope rocks and living things contain when they
  form or are alive

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Radioactive Dating

• Measured using half lives
• Amount of time for half the amount of a
  radioactive isotope found in a rock or living
  thing to decay
• Different radioactive isotopes decay at different
  rates
• Examples Carbon-14 5,730 years
• Potassium-40 1.26 billion years

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Radioactive Dating

• Carbon-14
• All living things contain a lot of Carbon-12
• Also contain a small amount of Carbon-14
• When organisms die they stop taking in C-14 and
  it begins to decay into C-12

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Radioactive Dating

• Carbon-14
• After 5,730 years (one half life) half the C-14
  an organism contained when it was alive will be
  gone
• If an organism contained 20g C-14 at death, 5,730
  years later it will contain 10g C-14
• Half the C-14 is gone

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Radioactive Dating

• Carbon-14
• After 11,460 years (two half lives) half the C-14
  an organism contained at the end of the last half
  life will be gone
• If an organism contained 10g C-14 after one half
  life, 5,730 years later it will contain 5g C-14
• One quarter (half x half) the C-14 is gone

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Grams C-14 remaining
Number of half lives
Years
0 0
1
2
3
14
Grams C-14 remaining
Number of half lives
Years
0 0
1 5,730
2
3
15
Grams C-14 remaining
Number of half lives
Years
0 0
1 5,730
2 11,460
3
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Grams C-14 remaining
Number of half lives
Years
0 0
1 5,730
2 11,460
3 17,190
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Grams C-14 remaining
Number of half lives
Years
0 0 20g
1 5,730
2 11,460
3 17,190
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Grams C-14 remaining
Number of half lives
Years
0 0 20g
1 5,730 10g
2 11,460
3 17,190
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Grams C-14 remaining
Number of half lives
Years
0 0 20g
1 5,730 10g
2 11,460 5g
3 17,190
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Grams C-14 remaining
Number of half lives
Years
0 0 20g
1 5,730 10g
2 11,460 5g
3 17,190 2.5g
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Radioactive Dating

• Carbon-14
• Because its half life is relatively short it can
  only be used on samples younger than 60,000 years

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Radioactive Dating

• Potassium-40
• Much longer half life 1.26 billion years
• Used for oldest fossils and rocks
• Effective on samples up to 10 billion years old
• Earth is only 4.6 billion years old

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Geologic Time Scale

• Earths age is estimated at 4.6 billion years
• History of Earth is divided into major sections
• Eons (very long), eras (long), and periods
  (shorter)

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Geologic Time Scale

• Entire history of Earth is divided into two eons
• Precambrian Eon
• 3.5 billion years ago 550 million years ago
• Life began as single-celled prokaryotes
• Single-celled eukaryotes developed
• By the end multicellular life had developed

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Geologic Time Scale

• Entire history of Earth is divided into two eons
• Phanerozoic Eon
• 550 million years ago present
• Began with primitive multicellular marine life
• Divided into many eras

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Geologic Time Scale

• The Phanerozoic eon is divided into three eras
• Paleozoic Era
• 550 mya 245 mya
• Divided into 6 periods
• Began with all life living in seas
• Land organisms developed (forests, insects,
  reptiles)
• Ended with a mass extinction of 95 of life on
  the planet (End-Permian extinction)

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Geologic Time Scale

• The Phanerozoic eon is divided into three eras
• Mesozoic Era
• 245 mya 65 mya
• Divided into 3 periods
• Began with the few life forms that survived the
  End-Permian extinction
• Dinosaurs became dominant, first mammals, first
  birds, first flowering plants
• Ended with the mass extinction that killed the
  dinosaurs (called the K-T Extinction)

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Geologic Time Scale

• The Phanerozoic eon is divided into three eras
• Cenozoic Era
• 65 mya present
• Divided into 2 periods
• Began with the few life forms that survived the
  K-T extinction
• Mammals and insects became dominant, lots of
  flowering plants
• What will it end with? Current mass extinction?