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More about Isostacy
tom.h.wilson
tom. [email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
Back to isostacy- The ideas we’ve been playing
around with must have occurred to Airy. You can
see the analogy between ice and water in his
conceptualization of mountain highlands being
compensated by deep mountain roots shown below.
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
A few more comments on Isostacy
Tom Wilson, Department of Geology and Geography
A
B
C
The product of density and thickness must
remain constant in the Pratt model.
At A 2.9 x 40 = 116
At B C x 42 = 116
At C C x 50 = 116
Tom Wilson, Department of Geology and Geography
C=2.76
C=2.32
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Physical Evidence for Isostacy
Japan Archipelago
North
American
Plate
Pacific Plate
Philippine
Sea Plate
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
The Earth’s gravitational field
In the red areas you weigh more and
in the blue areas you weigh less.
North
American
Plate
Pacific Plate
Philippine
Sea Plate
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
The gravity anomaly map shown here indicates that the mountainous region is associated with an
extensive negative gravity anomaly (deep blue colors). This large regional scale gravity anomaly
is believed to be associated with thickening of the crust beneath the area. The low density crustal
root compensates for the mass of extensive mountain ranges that cover this region. Isostatic
equilibrium is achieved through thickening of the low-density mountain root.
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Watts, 2001
Tom Wilson, Department of Geology and Geography
Watts, 2001
Tom Wilson, Department of Geology and Geography
Tom Wilson, Department of Geology and Geography
http://pubs.usgs.gov/imap/i-2364-h/right.pdf
Morgan, 1996 (WVU Option 2 Thesis)
Tom Wilson, Department of Geology and Geography
Morgan, 1996 (WVU Option 2 Thesis)
Tom Wilson, Department of Geology and Geography
Crustal thickness in WV Derived from Gravity Model Studies
Tom Wilson, Department of Geology and Geography
http://www.nasa.gov/mission_pages/MRO/multimedia/phillips-20080515.html
http://www.sciencedaily.com/releases/2008/04/080420114718.htm
Tom Wilson, Department of Geology and Geography
Surface topography represents an excess of mass that must be
compensated at depth by a deficit of mass with respect to the
surrounding region
See P. F. Ray http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect1/index.htm
Tom Wilson, Department of Geology and Geography
Consider the Mount Everest and tectonic thickening problems
handed out last time.
Tom Wilson, Department of Geology and Geography
Take Home (individual) Problem
A mountain range 4km high is in isostatic equilibrium.
(a) During a period of erosion, a 2 km thickness of
material is removed from the mountain. When the new
isostatic equilibrium is achieved, how high are the
mountains? (b) How high would they be if 10 km of
material were eroded away? (c) How much material
must be eroded to bring the mountains down to sea
level? (Use crustal and mantle densities of 2.8 and 3.3
gm/cm3.)
There are actually 4 parts to this problem - we must
first determine the starting equilibrium conditions
before doing solving for (a).
Tom Wilson, Department of Geology and Geography
The importance of Isostacy in geological
problems is not restricted to equilibrium
processes involving large mountain-beltscale masses. Isostacy also affects basin
evolution because the weight of sediment
deposited in a basin disrupts its
equilibrium and causes additional
subsidence to occur.
Isostacy is a dynamic geologic process
Tom Wilson, Department of Geology and Geography
Have a look at the take home isostacy
problem handed out today.
Complete reading of Chapters 3 and 4
Text problems 3.10 and 3.11 are due next Tuesday
We’ll take a quick look at computer
quadratics exercise and then move on
to Problem 3.11 (next Tuesday)
There will be a mid-term test next Thursday & on Tuesday we will
set aside some time for review.
Tom Wilson, Department of Geology and Geography