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Plate Motions-2



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  • What is the largest plate and smallest plate?
  • pp.10, Fowler: It is probable that if the thick, relatively low density continental material (the continental crustal density is approximately 2.8 10**3 kg m**-3) reaches a subduction zone, it may descend a short way, but, because the mantle density is so much greater (approximately 3.3 10**3 kg m**-3), the downwards motion does not continue. Instead, the subduction zone ceases to operate at that place and moves to a more favorable location.
  • Pp 6, Fowler: Along divergent boundaries, which are called accreting or constructive, plates are moving from each other. At such boundaries new plate material, derived from the mantle, is added to the lithosphere. The divergent plate boundary is represented by the mid-ocean ridge system, along the axis of which new material is produced.
  • Along convergent boundaries, which are also called consuming or destructive, plates approach each other. Most such boundaries are represented by the oceanic-trench, island-arc systems of subduction zones where one of the colliding plates descends into the mantle and is destroyed.
  • pp.6. Fowler: Along conservative boundaries, lithosphere is neither created nor destroyed. The plates move laterally relative to each other. These boundaries are represented by transform faults, of which the San Andreas Fault in California, U.S.A. S a famous example. Transform faults can be grouped into six basic classes. By far the most common transform fault is the ridge-ridge fault, which can range from a few kilometers to hundreds of kilometers in length. Some very long ridge-ridge faults occur in the Pacific, equatorial Atlantic and southern oceans. Adjacent plate move relative to each other at rates up to about 15 cm yr-1.


  • 1. Solid Earth Geophysics Ali Oncel [email_address] Department of Earth Sciences KFUPM Today’s class: Plate Motions Reading: Fowler Chapters 2-3
  • 2. OVERVIEW: PLATE TECTONICS Basic concept: The outermost layer ( LITHOSPHERE ) is divided in a small number of “rigid” plates in relative motion one respect to the other. It FLOATS on the weak ASTHONOSPHERE .
    • Basic assumption:
    • As a result of DISCOVERY, the plates are made up of both oceanic and continental material, whereas the ONLY oceanic parts of plate are created or destroyed. However, it is hard to understand why continental material usually is not destroyed at convergent plate boundaries.
    • Sea floor spreading at the mid ocean ridges produces only oceanic lithosphere.
    From: Fowler, the solid Earth
  • 3. How many plates? What is the largest plate and smallest plate?
  • 4. Ocean or continent?
    • Oceanic crust is primarily basalt
    • Continental crust is primarily granite
    • What are the differences between these 2 rock types?
    What are the criteria for deciding whether Earth’s crust any location is oceanic or continental in origin?
  • 5. Why does the Pacific plate subduct?
    • As new crust cools, it thickens and becomes denser.
    • Blue (cool) colors on this map show older crust, warm colors show younger crust .
    • Crustal Density : 2.8X10 3 kgm -3
    • Mantle Density : 3.3 X10 3 kgm -3
  • 6.
    Oceanic- Continental Plate Separation VOLCANOES AND EARTHQUAKES CONCENTRATE Divergent Boundaries
  • 7.
    Convergent Boundaries Ocean-Ocean Convergence Ocean-Continent Convergence A VOLCANIC BELT OF MOUNTAINS FORMS
  • 8. Transform-Fault Boundaries Mid-Ocean Ridge Transform Fault Continental Transform
  • 9. As plates move past each other... … creek beds are offset San Francisco Los Angeles San Andreas fault
  • 10. Seismicity Depth Distribution
    • Where are the deepest parts of the oceans?
    • W hy here?
    • Which ocean is surrounded by the most volcanic and earthquake activity? South America Japan Alaska Tonga
  • 11. Depth Cross-Sections South America Tonga
  • 12. Depth Cross-Sections Japan Alaska
  • 13. Basics
  • 14. Plate Motions on a Flat Earth
  • 15.  
  • 16. Question
    • Spreading center with half rate of 2 cm/yr between plates A and B. What is happening at the eastern boundary between plates A and B?
  • 17.