Class 22: Discussion Notes

Geophysics

Great Indian Survey

  • Survey findings
    • Survey of India had mismatch between surface and astronomical methods
    • Attributed to attraction of plum bobs by the Himalayas
    • Calculation indicated larger error should have been present – attributed to a mass deficiency under the mountains (Pratt, 1855)
  • Airy (1855)
    • Mass deficiency attributed to a “root” of lighter crust displacing denser underlying material
    • Crust interpreted to be 10 miles thick in most areas, but thicker root supports elevated areas
    • Iceberg-style bouyancy
    • Implies a dynamic crust
  • Pratt (1859)
    • Assumes a thick (800-1000 mile) rigid crust
    • Mass deficiency attributed to lateral changes in crustal density
    • Uniform compensation depth with same mass above it
    • Mountains held up by thicker but less dense crust

Fisher (1881)

  • First book on geophysics!
  • Crust
    • Argued that temperature gradient would require melting at relatively shallow depths
    • Density difference between crust (~ 2.7 gr/cm2) and entire earth (5.5 gr/cm2) requires a denser interior
    • Support Airy model of thin and dyanamic crust
  • Contraction
    • Possible volume reduction due to cooling is insufficient to accommodate the amount of shortening visible in mountains like the Appalachians and Alps.

Tectonic Models that try to address geophysical issues…

  • Reade (1886)
    • Solid earth throughout
    • Temperature “pulsations” lead to compression, and thus folds, thrust faults
    • Zones of heating are variable
    • Related to Herschel in using thermal shifts in crust
    • Exact opposite of Fisher
  • Dutton (1887)
    • Isostasy
    • Vertical tectonics
    • Followed rather directly from Fisher
      • Solid crust and fluid substratum are in equilibrium
      • Earth is inhomogeneous
  • Reyer (1888)
    • Thermal heating elevates the land
    • Thin sheets (nappes) slide off due to gravity
    • Airy isostasy rejuvenates the uplift
    • Ultimately cooling would lead to subsidence
  • Willis (1890s)
    • Contracting earth would not be directional in nature
    • Response (structure) should be depend on orientation of strata
    • Isostasy would have localized influences
    • Combination of some ideas of Dana and Dutton