Engineering Geology - Chapter 17 - Earth's Interior

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Engineering Geology - Chapter 17 - Earth's Interior
2012-12-20 16:15:27
Engineergin Geology Geotechnical Civil Minnesota Earth Interior Anomaly Anomalies Gravity Magnetic Mohorovicic discontinuity seismic wave shadow zone

Chapter 17
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  1. Describe the differences between continental crust and oceanic crust. 
    • Continental crust: thick; Felsic composition
    • Oceanic crust: thin; Mafic composition
  2. What is a gravity anomaly, and what does it generally indicate about Earth’s interior. 
    • Gravity Anomaly: difference in gravity in a region than what is expected, even after correctiing for elevation differences.
    • Indicative of a difference in densities.
  3. Discuss seismic-wave shadow zones and what they indicate about Earth’s interior.
    Seismic waves from an earthquake travel through the entire world. However, there are specific areas on the opposite side of the world from an earthquake that do not receive these seismic waves. These areas are known as seismic-wave shadow zones.

    The size and pattern of these seismic-wave shadow zones for P-waves and S-waves have shown that the outer part of the core must be liquid, or behave like a liquid (because no S-waves are able to pass through at all), but the inner core must be solid (because the P-waves, which are able to pass through the liquid outer core, are bent in such a way that indicate an inner solid core. Seismic reflection)
  4. What is the Mohorovicic discontinuity?
    The crust is separated from the mantle by the Mohorovicic (or Moho) discontinuity (seismic waves speed up at this discontinuity indicating a change in composition - seismic refraction).

    The mantle, like the crust, is made of solid rock with only isolated pockets of magma. Higher seismic wave velocities of mantle rocks (as compared with crustal rocks) are indicative of denser, ultramafic composition for rocks in the mantle.
  5. What is a magnetic reversal? What is the evidence for magnetic reversals?
    • Magnetic reversals are the times when the poles of Earth’s magnetic field have switched.
    • Evidence for Earth’s magnetic reversals is recorded in magnetic minerals; these suggest that reversals have occurred many times, and apparently at random times.
  6. Felsic and mafic are terms used by some geologists to describe
    A. composition of continental and oceanic crust
    B. regions in the mantle
    C. behavior of earthquake waves
    A. composition of continental and oceanic crust
    (this multiple choice question has been scrambled)
  7. The boundary that separates the crust from the mantle is called the
    A. lithosphere
    B. Mohorovicic discontinuity
    C. none of the preceding
    D. asthenosphere
    B. Mohorovicic discontinuity
    (this multiple choice question has been scrambled)
  8. The core is probably composed mainly of
    A. iron
    B. oxygen
    C. sulfur
    D. silicon
    A. iron
    (this multiple choice question has been scrambled)
  9. The principle of continents being in a buoyant equilibrium is called
    A. subsidence
    B. rebound
    C. isostacy
    D. convection
    C. isostacy
    (this multiple choice question has been scrambled)
  10. A positive gravity anomaly indicates that
    A. tectonic forces are holding a region up out of isostatic equilibrium
    B. the land is sinking
    C. local mass deficiencies exist in the crust
    D. all of the preceding
    C. local mass deficiencies exist in the crust
    (this multiple choice question has been scrambled)
  11. A positive magnetic anomaly could indicate
    A. the presence of a granitic basement high
    B. an intrusion of gabbro
    C. all of the preceding
    D. a body of magnetic ore
    E. the magnetic field strength is higher than the regional average
    D. a body of magnetic ore
    (this multiple choice question has been scrambled)
  12. What is seismic reflection?
    Seismic reflection is the return of some waves to the ground surface after bouncing off a rock layer boundary. A sharp boundary between two materials of different densities will reflect seismic waves.
  13. What is seismic refraction?
    Seismic refraction is the bending of seismic waves as they pass from one material to another having different seismic wave velocities. At an interface, seismic (or sound or light) waves will bend toward the lower-velocity material.
  14. What is Isostasy? 
    Isostasy refers to the equilibrium of adjacent blocks of brittle crust “floating” on upper mantle. Thicker blocks of lower density crust have deeper “roots” and float higher (as mountains).
  15. What is Isostatic adjustment? 
    Isostatic adjustment refers to rising or sinking of crustal blocks to achieve isostatic balance.  (Example: Crustal Rebound)
  16. What is Crustal Rebound? 
    Crust will rise when a large mass is rapidly removed from the surface, as with the case of ice sheets at the end of ice ages. The rise of crust after ice sheet removal is called crustal rebound.
  17. What is a magnetic anonomaly, and what does it generally indicate about Earth's interior? 
    • Magnetic Anomolies: local increases or decreases in the Earth’s magnetic field strength.
    • Positive magnetic anomolies are indicative of hidden bodies of ore/magnetic rock