Tectonics to 1852

Four tectonic models of the second quarter of the 19th century

• Von Buch (1820s-50s)
  • Two basic models for uplift
    • Craters of eruption: normal volcanoes
    • Craters of elevation: larger features
      • Accumulation of basalt under the sea floor
      • Up-arching of a “blister”
      • Bursts to form an ragged set of mountains
      • Need a line of these to form an elongate chain
  • Driving force is internal heat that produces magmas to accumulate; attributed to chemic reactions
  • Works in areas like:
    • Baltic uplift (early phase of initial doming
    • Canary Islands
    • Some German mountains
• Elie de Beaumont (1829-31 version)
  • Cooling earth creates stress in the near-surface
    • Links up with heat measurements and implied cooling
  • Stress release via catastrophic uplift along a “great circle” line
  • Sequence of uplifts, each along a different great circle
  • Uplifts can be dated in the adjacent sediment record by dating the unconformities.
  • Seems widely applicable to linear mountain chains: Alps, Pyrennes, Appalachians, Andes
  • In later versions (c. 1850), geometry predominated over geology
• Lyell (1830-33)
  • Two ways to make mountains:
    • Isolated volcanics (Etna)
    • Intrusion of subsurface magmas that elevate the surface.
      • Could also get magma moving away, leading to subsidence
      • Directly linked to earthquakes that he ties to igneous processes
      • Gradual changes linked to his physical process model
      • Sicily was one example
      • Calls up vertical crustal motions combined with horizontal shift of magmas
    • Not really explain linear chains
    • Ignores central heat and its implications of cooling
• Herschel/Babbage (1837)
  • Erosion of uplands
  • Deposition of sediment in adjacent ocean basins
  • Oceanic crust depresses under the sediment load
  • Leading to lateral flow of ductile layers from areas of loading to areas that  were eroded, promoting more uplift.
  • Subsurface horizontal flow in the earth; surface areas undergo vertical motion
  • Application
    • Works well where mountain ranges parallel ocean (Andes, even Baltic)
    • Not really fit Alps, Pyrennes, etc.
  • Driving forces are erosion and heat (reminding us of Hutton)
  • Not clear exactly how this relates to metamorphism
  • Not deal with central heat issue
  • Often added onto Lyell’s ideas

Issues

• Links to Stratigraphy
  • System in continental usage: referred to a set of strata with a similar structural trend that was bounded by unconformities (used by Sedgwick)
  • Unconformities used to date times of uplift
• Basic Differences
  • Rate of uplift: gradual versus virtually instantaneous
  • Role of Internal Heat in driving uplift
  • Whether forces were primarily horizontal or vertical
  • Resulting geometry of uplift
• Tempo of Earth History
  • Can already see differences in terms of tempo of change and continuity of processes.