The Geologic Time Scale

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The Geologic Time Scale

• A History of Earth and Life

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Geologic Time Scale

• Division of Earths history into blocks of time

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Ideas about time

• Processes Forces that are occurring now
  occurred in Earths past

But how do we tell how old things are
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1. Relative Dating

• Trying to determine which rock units formed
  first, second, third, so on
• Forming a sequence of timeno dates!

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Rules for Relative Dating

• Rocks on the bottom are older than rocks on top
  Law of Superposition

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2. Principle of Original Horizontality

• Layers of rock are generally
• deposited
• in a horizontal positions

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• 3. Rock that cuts through is younger than the
  rock it cuts
• Principle of Cross-Cutting

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Matching Rock Layers

• Correlation
• To make a time scale we have to be able to find
  similar rock in different areas

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Fossils also help

• What are fossils?
• Remains or traces of an organism that have been
  preserved

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Types of fossils

• 1. MOLD created when an organism is buried in
  sediments is dissolved by underground water

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Types of fossils

• 2. CAST Created when the hollow spaces of a
  mold are filled with mineral matter

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Types of fossils

• 3. CARBONIZATION occurs when an organism is
  buried under fine sediment. Pressure causes only
  a thin residue of carbon to be left.
• Best for preserving leaves delicate animals

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Types of fossils

• 4. Impression a replica of the surface that
  occurs when the carbon film is lost.

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Types of fossils

• 5. Amber Created when organisms are preserved
  in tree sap (resin)

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Types of fossils

• 6. Petrified Occurs when minerals from water
  replace the organism
• turned to stone

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2. Absolute Dating

• Radiometric Dating Radiocarbon Dating
• Radioactive decay is used to date fossils. This
  gives us a specific time that the organism died.

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So lets look at what this showed us
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Earth is about 4.6 billon years old
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Pre-Cambrian Era 4.6-540 mya
First prokaryotes (bacteria)

• 3.6 bya life appears - Single celled organisms

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Paleozoic Era (Cambrian Period) 540 mya

• Explosion of multi-cellular life in oceans

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Paleozoic Era (Ordovician Period) 490 mya

• Many new invertebrates in seas first land plants

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Paleozoic Era (Silurian Period) 443 mya
Fish dominant first insect fossils land
plants diversify
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Paleozoic Era (Devonian Period) 417 mya
Fish dominate seas First amphibians
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Paleozoic Era (Carboniferous Period) 354 mya
Amphibians and insects dominate land first
reptiles
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Paleozoic Era (Permian Period) 290 mya
Mammal -like reptiles
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Mass Extinction 248 mya
95 Marine species extinct 70 land species
extinct
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Mesozoic Era (Triassic Period) 248 mya
Small reptiles and mammals First dinosaurs
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Mesozoic Era (Jurassic Period) 206 mya
Dinosaurs dominant first birds appear
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Mesozoic Era (Cretaceous Period) 144 mya
Dinosaurs dominate first flowering plants
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Mass Extinction 65 mya
85 of all species extinct
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Cenozoic Era (Tertiary Period) 65 mya
Mammals and birds dominate
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Cenozoic Era (Quaternary Period) 1.8 mya - today
Mammals abundant Humans appear holla
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You know the history of life!