• Principle of superposition
  
• Principle of fossil succession
  
• Not at all dependent on absolute ages
  
• Basis for the construction of the geologic time scale
  
• Phanerozoic Eon
  
  • 570 Ma to present
    
  • Time of abundant fossilized life
  
  
• Precambrian Time
  
  • Deep time prior to fossils

• How do we measure such an abstract thing?
  
• Physical measurements
  
  • Example: coral growth lines
    • Thick growth bands
      
      • Yearly increments of growth
      
      
    • Fine growth bands
      
      • Daily increments of growth
      
      
    • In the Devonian (410-360 Ma):
      
      • There were 400 days/year
        
      • Implies progressive slowing of Earth's rotation by tidal friction

Radiometric age dating

• Radioactive parents decay to non-radioactive daughter isotopes
  
• Example: a cooling magma solidifies by crystallization
  
  • A certain amount of Isotope A was distributed in the melt and included in a new mica crystal
    
  • From that time of crystallization forward, Isotope A decays to Isotope B
    
  • We measure the relative amounts of Isotope A and B the crystal

Decay processes

• Linear process of decay (e.g., hour glass)
  
• Non-linear process (e.g., radioactive decay)
  
  • There is a certain probability of decay per unit of time
  
  
• Types of radioactive decay
  
  • alpha = helium nucleus emission
    
  • beta = electron emission from spitting a neutron
    
  • gamma = high energy x-ray emission

• Chemical means can not be used to separate isotopes
  
  • But they can be separated by mass

• Filament
  
• Flight tube (arpeture)
  
• Magnet
  
• Collectors

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written by R. Behl.
Last changes: 1 Sept 2003