A Trip Through Geologic Time

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A Trip Through Geologic Time
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The Fossil Record

• fossils tell the history of life on earth
• not all organisms form fossils
• conditions necessary for fossil formation
• protected from destruction and decay
• sediments bury it and become rock
• hard parts

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Types of Fossils

• petrified fossils mineralized copies of
  original bones
• minerals in water replace original matter
• parts turned to stone

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• molds and casts
• mold - cavity in the shape of an organism
• cast - minerals fill in the cavity

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• trace fossils
• footprints, tracks, trails, and burrows
• coprolites
• tell about what an organism ate
• unchanged fossils
• amber, tar pits

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Interpreting FossilsRelative Age Dating

• principle of superposition
• -younger rock on top of older rock
• -basis for most relative age dating of fossils

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Disruption of Layers

• folding faulting disrupt order of layers
• erosion removes layers

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• Rules for interpreting layers
• Rock layers are horizontal before they deform
• A fault or igneous dike did not exist when the
  layers formed, and is younger than the layers it
  cuts across

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Absolute Dating

• absolute dates expressed in years
• Radiometric dating compares amount of
  radioactive isotope present to the amount of
  decay present
• half-life time it takes for half of isotope to
  decay

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Isotope Half-Life (Years) Used to Date
Thorium-232 14 billion Very old rocks
Potassium-40 1,300 million Old rocks fossils in them
Carbon-14 5,730 Fossils less than 50,000 years old
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The Geologic Time Scale

• Geologic Eras
• Precambrian
• Paleozoic Era
• Mesozoic Era
• Cenozoic Era

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Precambrian Era The Dawn of Life

• 4600 mya - 540 mya
• Earths history begins
• Seas form from volcanic emissions comet
  impacts.
• Continents form grow.
• Oxygen builds up in atmosphere from
  photosynthesizing organisms (stromatolites)

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Paleozoic EraOld Life - Life Comes Ashore

• 540 mya - 248 mya
• Age of Invertebrates
• Began with Cambrian Explosion of new life
  forms, due to preceeding mass extinction.
• Abundant sea life. First fishes appear, first
  amphibians evolved from fishes, first reptiles
  evolved from amphibians.
• First terrestrial life (arthropods other
  invertebrates) invades from sea.
• Carboniferous swamps create coal deposits.
• Pangaea formed near end of era.

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Mesozoic Era Middle Life - Age of Dinosaurs

• 248 mya - 65 mya
• Era of many changes
• Dinosaurs, birds, mammals all evolved from
  reptiles. Mammals avoid competition
  with/predeation by dinosaurs by being nocturnal
  eating insects.
• Pangea broke up 200 mya.
• Flowering plants became dominant plant life.

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Cenozoic Era New Life - Age of Mammals

• 65 mya present
• Mammals dominated.
• Primates, human ancestors, modern humans
  evolved.
• Land bridge formed between N. S. America,
  disrupted ocean currents triggered Ice Ages -
  sheets of ice advance and recede several times.
• Alps and Himalaya Mountains form from collisions
  of continents.

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Check These Out!

• Geologic Time Machine
• Fossils, Rocks, and Time
• Ice Age Paleoecology
• USGS FAQ about Dinosaurs
• Carbon-14 Dating
• Paleontology Without Walls
• Dinosaur Images
• Paleomapping Project

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The End