The Rock Record

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The Rock Record

• Chapter 17

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Relative Age

• Earth is 4.6 billion years old
• ? James Hutton
• Studying present reveals past and future
• Principle of Uniformitarianism
• Big word, easy concept
• Earth processes take a long time which made
  Earth older than 6,000 years.
• He was thinking --- millions of years!

3
Law of Superposition

• Layers of rock are called strata
• Strata are laid down according to certain
  principles
• Bottom rocks are oldest
• Top rocks are youngest
• Strata is deposited horizontally
• Particle size
• The boundary between layers is called a bedding
  plane
• These tell us the relative age of each of the
  rock layers as we are only comparing ages of
  rocks/layers

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Unconformities

• Unconformity - a break in the geologic record
• Deposition stops, erosion occurs followed by more
  deposition three types
• Nonconformity
• Angular Unconformity
• Disconformity
• Each will be discussed briefly in the following
  slides

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Nonconformity
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Angular Unconformity
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Disconformity
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Law of Crosscutting Relationships
Intrusion is younger than layers around it
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Absolute Age

• What makes a clock a clock?
• Clocks keep perfect time.
• We need a clock to determine a rock or stratas
  actual age
• Finding Earths absolute age has been endeavored
  by many scientists through many techniques for
  hundreds of years.

Niagara Falls is 10,000 years old
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Rates of Deposition and Erosion

• Niagara Falls erosion rate
• Glacial lakes deposition rate (varve count)

11
Radioactive Decay

• This is our clock
• Large, unstable nuclei break down at a constant
  rate
• U-238 is found in some rocks
• Alpha decay gives off one helium nuclei (2
  protons 2 neutrons)
• Losing 2 protons ? lose 2 atomic numbers
• Beta decay is an e- given off as one neutron
  breaks apart into its e- and p components
• Gaining 1 proton ? gain 1 atomic number

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Half-Life
Amount of time it takes for half of parent
material (mass) to decay into its daughter
elements
C-14 ? N-14
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Fossil Records
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Kinds of Fossils

• Preservation
• Mummification
• Amber
• Tar seeps
• Freezing
• Petrification
• Traces
• Tracks
• Imprints, molds and casts
• Coprolites (waste/dung) ?
• Gastroliths

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Index Fossils

• Help date rock layers
• Requirements
• Present in rock all over the world
• Clearly distinguishable from other fossils
• Lived during a short amount of geologic time
• Occur in large numbers within the rock layers

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A View of Earths Past

• Chapter 18

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Geologic Time Scale

• Geologic column represents a timeline in
  Earths history
• Layers are determined by fossils they contain,
  relative position and physical characteristics
• Index fossils can help determine absolute age
  without radiometric dating

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• Eras
• Periods
• Epochs
• The four geologic eras
• Precambrian (huge span of time!)
• Paleozoic
• Mesozoic
• Cenozoic

• Earth history is divided into units
• The units end or begin with major changes on
  Earth (ex extinction)
• Rocks found in a particular unit of time often
  have similar fossils (dominant life form)

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Earth History Calendar
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? To scale notice all the blue/purple
Blue/purple is all one era (Precambrian)
Shows periods - not to scale!
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Geologic History

• Not written in books but in the Earths rocks!
• Theory of evolution
• Charles Darwin 1859
• Survival of the fittest (natural selection)
• Adaptation survival ? reproduction
• Major climate/geologic changes lead to many
  species not being able to survive

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Precambrian Era

• Began 4.6 billion years ago
• Ended 540 million years ago
• Yep its 4 billions years long!
• 88 of Earth history
• Shield rocks
• ½ of all worlds mineral deposits
• Very few fossils
• Stromatolites
• Organisms had no hard body parts to fossilize
• Bacteria ruled!

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East Canadian Precambrian Shield
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Stromatolites Today
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Paleozoic Era

• Cambrian
• Ordovician
• Silurian
• Devonian
• Mississippian
• Pennsylvanian
• Permian

• Began 540 million years ago (with the evolution
  of the trilobite)
• Ended 248 million years ago w/ a mass extinction
  - including trilobite
• By the end of the era, Pangaea had formed
  causing climate change inland
• Abundant fossils
• 7 periods ?

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Mesozoic Era

• Began 248 million years ago
• Ended 65 million years ago with the extinction of
  the dinosaurs
• Pangaea broke up
• Climate during the era favored reptiles ?
  warm/humid
• Age of the Reptiles
• ? 3 periods
• List characteristics of each of the 3 periods of
  the Mesozoic

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Cenozoic Era

• Began 65 million years ago to present
• Major changes in climate (ice ages)
• Age of the Mammals
• 2 periods ?
• 7 epochs
• We live in the Holocene epoch of the Quaternary
  period.

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Cenozoic Events
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The tarsier (most common primate of early
Cenozoic Era) is related to the lemur.