Transcript epicontinental seas

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 206-207
Concepts and Activities
 Earths structure (review)
 Plate Tectonics (review)
 Glacial rebound and isostasy
 MAP Activity pg. 206-207
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 is less dense than
water.
The earth’s crust is less
dense than the
underlying mantle
Video
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.
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 11.18
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?