California State University Long Beach

GEOL 300i; Earth Systems and Global Change
Lecture 3

Dr. Rick Behl

Fundamentals of Plate Tectonics

• I.E., strength & viscosity of layers


• Differs from the compositional structure
  
  • I.e., Core, Mantle, and Crust
  
  
• Lithosphere
  
  • ~100-150 km thick
    
  • Cool and rigid slab or plate
    
  • Composed of the combined crust and upper mantle
  
  
• Asthenosphere
  
  • Separated from lithosphere by the Low Velocity Layer in Mantle
    
  • From 100-~670 km deep
    
  • Weak, flows plastically under stress
  
  
• Mesosphere
  
  • >670 km depth
    
  • More rigid than asthenosphere
    
  • More fluid than lithosphere

Plate tectonics

• ~13 major rigid but moving lithospheric plates
  
• Unifying theory which explains age & movement of Earth's surface
  
• Plate boundaries identified by earthquakes


• Plate boundaries
  
  • Divergent/Constructional margins
    
    • Where plates move apart and create new lithosphere
      
    • Mid-ocean ridges or rises
    
    
  • Convergent/Destructive margins (3 types)
    
    • Where plates come together and destroy lithosphere
      
    • Subduction zones, narrow belts of seismic & volcanic activity
      
      • Ocean-continent
        
        • E.g., Peru-Chile, Cascadia
          
          • Andes volcanic arc
        
        
      • Ocean-ocean
        
        • E.g., Tonga, Marianas trench
        
        
      • Continent-continent
        
        • Builds mountain ranges & continental suture zone
          
          • E.g., Alps, Himalayas
    
    
  • Transform/Conservative margins
    
    • Boundaries where plates slide by one another
      
      • Continental settings
        
        • E.g., San Andreas Fault
        
        
      • Oceanic settings
        
        • E.g., Mendocino Fracture Zone

Continental drift and the evolution of ocean basins

• Wilson tectonic cycle (100-300 m.y.)
  
• As lithospheric plates and continents have traveled over the face of the Earth, separating and converging again and again, oceans have formed and disappeared, mountains have risen and eroded away.
  
  • Embryonic stage
    
    • Continental rifting
      
    • E.g., east African rift valleys
    
    
  • Young, narrow seas
    
    • Subsidence and initial generation of oceanic lithosphere
      
    • E.g., Red Sea
    
    
  • Mature oceans
    
    • Mid-Ocean Ridges and seafloor spreading
      
    • E.g., Atlantic Ocean
    
    
  • Declining stage
    
    • Subduction and spreading, but shrinking
      
    • E.g., Pacific Ocean
    
    
  • Terminal stage sea
    
    • Subduction only
      
    • Shrinking in area
      
    • E.g., Mediterranean Sea
    
    
  • Relict
    
    • Continental collision
      
    • Formation of mountanous suture zone
      
    • E.g., Himalayas, Appalachians

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