ISOSTASY A Plate Tectonic Process of Equilibrium

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

Concepts and Activities Earths structure (review) Plate Tectonics (review) Glacial rebound and isostasy MAP Activity pg

Earth’s Structure

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

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 it’s density and thickness. The “height” of the earth’s crust also depends on it’s density and thickness.  Wood is less dense than water.  The earth’s crust is less dense than the underlying mantle  Video Video

Glacial Rebound:

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).

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

Isostatic Rebound: The Baltic Sea, an epicontinental sea

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.

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.

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?