Gravity and Isostasy

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  1. 1. Ali Oncel [email_address] Department of Earth Sciences KFUPM Gravity and Isostasy Introduction to Geophysics Introduction to Geophysics-KFUPM
  2. 2. Previous Lecture
    • Gravity Units
      • Example 1: Mars's twin moons
      • Example 2: The Valley and Ridge Province (1-5 mgals)
    • Sensitivity: Density Distribution
      • Sensitive to changes in Mass (Density) changes
    • Densities of Geological Materials
    • On-Line Gravity Map Construction Tool
    • Color table for standard Gravity map
      • Example 1: North Anatolian Fault Zone in the sea of Marmara
      • Example 2: Ghawar region of Saudi Arabia
    Introduction to Geophysics-KFUPM
  3. 3. Parks and Plates,pp.27 2005 Robert J. Lillie Introduction to Geophysics-KFUPM
  4. 4. Parks and Plates 2005 Robert J. Lillie
  5. 5. Parks and Plates 2005 Robert J. Lillie Introduction to Geophysics-KFUPM
  6. 6. Introduction to Geophysics-KFUPM
  7. 7. Parks and Plates 2005 Robert J. Lillie Topography of the Ocean Floor Introduction to Geophysics-KFUPM 1977, Marie Thorpe
  8. 8. National Aeronautics and Space AdministrationTwo Forms of Isostatic Uplift pp.28 Parks and Plates2005 Robert J. Lillie Introduction to Geophysics-KFUPM Low density due to higher temperature
  9. 9. Two Forms of Isostatic Uplift Introduction to Geophysics-KFUPM
  10. 10. What is Isostatic Equilibrium?
    • Crust floats on Mantle
    • (hydrodynamic equilibrium)
    • Surface elevation is function of
    • underlying mass (thickness and density of crust)
    • Surface elevation is compensated by mass below it
    Introduction to Geophysics-KFUPM
  11. 11. Bouguer Anomaly
    • Slightly positive over oceans
    • Slightly negative over continents
    • Because a uniform density was used in InternationalGravity Formula
    Introduction to Geophysics-KFUPM 2.67 g/cm 3 3.3 g/cm 3 Mantle
  12. 12. Example from Arabia Introduction to Geophysics-KFUPM
  13. 13. Example from Arabia Data Source: Cornell University
    • Bouguer anomalies that are negative over high areas and positive over oceans.
    • Near sea level, the average is close to zero.
    • The negative Bouguer anomalies areas due to the excess of low density material at depth in mountainous regions when compared to rocks at depth in areas of low relief near sea level.
    Introduction to Geophysics-KFUPM
  14. 14. Homework, Due to May 17
    • Use the online tool for making a gravity map from the site .
    • Make your gravity map and compare changes in the gravity to changes through regions of major tectonics?What is the gravity through the areas:
    • a) Red Seab) Dead Sea Fault c) Iranian Sea d) Gulf of Aqaba e) Zagros Thrust Fold
    Introduction to Geophysics-KFUPM
  15. 15. The quiz of the week For the following equation, define the variables: FAA= g obs g th+FAC BA = FAA BC + TC Introduction to Geophysics-KFUPM