Kelvin and Chamberlin

Kelvin

• One of two major physics-based attack on geological models (other one was Fisher arguments against contraction)
• Argues that age of the earth is limited – no perpetual motion machine
• Three arguments
  • Age of the sun
    • Sun has finite energy being lost through cooling (recall before radioactive decay and fusion known)
    • Must be cooling
    • Estimate that sun can only be 10-100 million years old; later shortened to 20-25 million years
  • Cooling of the earth
    • Earth starts as molten ball, then radiates heat energy
    • Liquid interior possible in this idea
    • Estimate that crust can only be 98-200 million years od
  • Tidal friction
    • Tidal friction will slow the Earth’s rotation (22 seconds/century)
    • Distance from sun slowly shortens
    • Estimate that earth can only be ~ 1 billion years old
• Over time, his estimates of age of the earth shortened through time from about 100 ma to 25 ma.
  • Disputed by geologists at the short end
  • Really problematic to paleontologists
• Problems
  • Assumes a homogeneous earth for cooling model
    • No layered structure
  • No convection allowed – only conduction through solid
  • No additional heat source allowed (as main sense at the time)
• Radioactivity
  • Basic assumption of no additional source of heat energy removed
  • Gave rise to another tectonic model (Joly – see next class)

Chamberlin

• Follows from the American tradition of the permanence of ocean versus continent tying back to Dana
• Another attempt to use cooling-driven contraction and isostasy
• Basic idea
  • Earth consolidates into pyramidal prisms
    • Oceans are underlain by denser material (basalt) with broad bases at the surface
    • Continents are less dense material (granite) with narrow truncated tops at surface
    • Each prism maintains an isostatic balance
  • Slow cooling causes stress to build up
    • Released as prisms shift so that oceans sink relative to continents
    • Causes pulsed deformation along boundaries of continents
    • Sea level drops (regression) are world-wide; slowly creep back as oceans fill
    • Normal erosion can cause some readjustments due to isostasy but deformation is pulsed.
• Widely adopted in the US. Reflects many aspects of American thinking about tectonics

Tectonics circa 1900-1910

• Many different tectonic ideas by the beginning of the 20th century.
  • Very hard to see which is most applicable!
• Here are some for the interval 1870-1910
• Cooling-driven compression
  • Suess and the nappists: one-sided compression
  • Heim: two-sided compression (original Glarus interpretation)
  • Dana: permanent ocean basins leveraging against continental blocks; geosynclines, borderlands and continental accretion
  • Haug: former huge continents with narrow geosynclines; relatively recent floundering of some continental areas to make oceans
• Isostacy-driven – modifications of Herschel
  • Reade: temperature pulsations disrupt equilibrium as heat expands rocks
  • Dutton: classic isostacy with equilibrium adjustments to erosion, intrusion, etc.
• Earth physics
  • Fisher: thin, mobile crust with isostacy; contraction inadequate to explains observed shortening
  • Kelvin: limits age of the earth
• Composite models
  • Reymer: nappes slide off isostacy-driven uplifts
  • Willis: large-scale cooling-driven compression with more localized isostacy
  • Chamberlin: Dana-style tectonics with addition of continent and ocean blocks that adjust due to isostacy and cause orogenies.
• Radioactivity-driven
  • Joly: deformation of marginal geosynclines with radioactive heating; continental accretion