Relative dating

• Relative dating
  
  • Placing rocks and events in sequence
    
  • Principles and rules of
    
    • Principle of uniformitarianism – “The present is the key to the past”
      
    • Law of superposition – oldest rocks are on the bottom
      
    • Principle of original horizontality – sediment is deposited horizontally
      
    • Principle of original continuity – sediment is deposited in continuous sheets
      
    • Principle of cross-cutting relationships – younger feature cuts through an older feature
      
    • Principle of inclusions – inclusions (xenoliths) are older than the magmatic intrusion
      
    • Principle of “baked” contacts – Host rock that has been “baked” is older than the magmatic intrusion
      
    • Unconformities - A break in the rock record
      
    • Types of unconformities
      
      • Angular unconformity – tilted rocks are overlain by flat-lying rocks
        
      • Disconformity – erosion of strata leaves a “gap” in geologic time. Strata on either side are parallel
        
      • Nonconformity
        • Metamorphic or igneous rocks below
          
        • Younger sedimentary rocks above
          
      • Several unconformities are present in the Grand Canyon
        

Fossils: evidence of past life

• Types of fossils
  
  • Petrified – cavities and pores are filled with precipitated mineral matter
    
  • Formed by replacement – cell material is removed and replaced with mineral matter
    
  • Mold – shell or other structure is buried and then dissolved by underground water
    
  • Cast – hollow space of a mold is filled with mineral matter
    
  • Natural casts of shelled invertebrates
    
  • Carbonization – organic matter becomes a thin residue of carbon
    
  • Impression – replica of the fossil's surface preserved in fine-grained sediment
    
  • Preservation in amber – hardened resin of ancient trees surrounds an organism
    
  • Indirect evidence includes
    
    • Tracks
      
    • Burrows
      
    • Coprolites – fossil dung and stomach contents
      
    • Gastroliths – stomach stones used to grind food by some extinct reptiles
      
    • A dinosaur footprint
      
• Conditions favoring preservation
  
  • Rapid burial
    
  • Possession of hard parts
    
• Fossils and correlation
  
  • Principle of fossil succession
    
    • Fossils succeed one another in a definite and determinable order
      
    • Proposed by William Smith – late 1700s and early 1800s
      
  • Correlation of rock layers
    
    • Matching rocks of similar age in different regions
      
    • Often relies upon fossils
      
  • Index fossils
    
    • Widespread geographically
      
    • Existed for a short range of geologic time
      

Relative dating | Fossils: evidence of past life | Radioactivity and radiometric dating | Geologic time scale | Links | top | Classes Home

Radioactivity and radiometric dating

• Atomic structure reviewed
  
  • Nucleus
    
    • Protons – positively charged
      
    • Neutrons - Neutral charge
      
      • Protons and electrons combined
        
  • Orbiting the nucleus are electrons – negative electrical charges
    
  • Atomic number
    
    • An element's identifying number
      
    • Number of protons in the atom's nucleus
      
  • Mass number
    
    • Number of protons plus (added to) the number of neutrons in an atom's nucleus
      
  • Isotope
    
    • Variant of the same parent atom
      
    • Different number of neutrons and mass number
      
• Radioactivity
  
  • Spontaneous breaking apart (decay) of atomic nuclei
    
• Radioactive decay
  
  • Parent – an unstable isotope
    
  • Daughter products – isotopes formed from the decay of a parent
    
  • Types of radioactive decay
    
    • Alpha emission
      
    • Beta emission
      
    • Electron capture
      
• Radiometric dating
  
  • Half-life – the time for one-half of the radioactive nuclei to decay
    
    • Requires a closed system
      
    • Cross-checks are used for accuracy
      
    • Complex procedure
      
    • Yields numerical dates
  • The radioactive decay curve
    
  • Dating sedimentary strata using radiometric dating
    
  • Carbon-14 dating
    
    • Half-life of only 5730 years
      
    • Used to date very recent events
      
    • Carbon-14 produced in upper atmosphere
      
    • Useful tool
      

Geologic time scale

• Divides geologic history into units
  
• Originally created using relative dates
  
• Subdivisions
  
  • Eon
    
  • Era
    
  • Period
    
  • Epoch
    
• Difficulties in dating the time scale
  
  • Not all rocks are datable (sedimentary ages are rarely reliable)
    
  • Materials are often used to bracket events and arrive at ages
    

Relative dating | Fossils: evidence of past life | Radioactivity and radiometric dating | Geologic time scale | Links | top | Classes Home