ISOSTASY

1
ISOSTASY

• A Plate Tectonic Process of Equilibrium

2
Purpose

• To apply what we know and new information about
  how glacial rebound relates to erosion,
  deposition and glaciation to complete the MAP
  Activity on page 206-207

3
Concepts and Activities

• Earths structure (review)
• Plate Tectonics (review)
• Glacial rebound and isostasy
• MAP Activity pg. 206-207

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Earths Structure
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Plate Tectonic Theory

• Lithospheric plates (continental and oceanic)
  float on top of the upper mantle (red)
• The crust reacts to being loaded and unloaded
  because it floats on the mantle.
• Types of loading glaciation, sediment deposition
• Types of unloading receding glaciers, erosion

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Isostasy

• Isostasy is the vertical movement of the crust to
  attain buoyancy in the mantle.
• The height a block of wood floats in water
  depends on its density and thickness.
• The height of the earths crust also depends on
  its density and thickness.

• Wood is less dense than water.
• The earths crust is less dense than the
  underlying mantle
• Video

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Glacial Rebound
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Crustal Density

• Variations in elevation are due both to thickness
  and density.
• The continents stand high because continental
  crust is thick and light (felsic).
• The ocean basins are low because oceanic crust is
  thin and dense (mafic).

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Isostatic ReboundHudson Bay

• Isostatic rebound in North America since 6000 yrs
  before present (figure 11.18 pg. 207)
• Measurements suggest that the crust in the Hudson
  Bay region has another 100 meters still to rise.

10
Isostatic Rebound The Baltic Sea, an
epicontinental sea
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Epicontinental Seas

• Two of the largest epicontinental seas (seas
  extending deep into a continent) on earth are
  Hudson Bay and the Baltic Sea
• Both are dead center over areas of active
  isostatic uplift.
• In all likelihood, the crust in these regions is
  still depressed and has not finished rising, when
  uplift is complete both seas will mostly or
  entirely disappear.

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Raised Beaches

• Beaches are shoreline features, they must have
  formed at sea level.
• Finding a beach at a higher elevation than
  current sea level is evidence of crustal uplift.
• Similarly, finding marine fossils at elevations
  above sea level also indicates that the land has
  been uplifted.

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In Conclusion

• If material is removed from continental crust, by
  erosion, it will rise.
• If it is added, by glaciation, the crust will
  sink as the crust attempts to reach a state of
  equilibrium in the mantle.
• Does isostasy occur at an constant rate? 
• Why do eroded mountains keep rising? 
• Do melting glaciers have a similar effect on
  continental crust?
• Factors that influence isostasy are crust
  thickness, crust density, erosion rates, and
  glaciation rates.
• Could sea level effect isostasy?