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Geol 351 - Geomath
Isostacy II - Wrap up isostacy and begin
working on the settling velocity lab
tom.h.wilson
tom. [email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography
Explanations for lowered gravity over
mountain belts
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
Other examples of isostatic computations
Tom Wilson, Department of Geology and Geography
Another possibility
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
Some expected differences in the mass
balance equations
Tom Wilson, Department of Geology and Geography
Island arc systems – isostacy in flux
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Topographic extremes
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
g ~0.6 cm/sec2
Pacific Plate
Philippine
Sea Plate
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Quaternary vertical uplift
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
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.
Total difference of about 0.1 cm/sec2 from
the Alpine region into the Japan Sea
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Schematic representation of subduction zone
The back-arc area in the Japan sea, however,
consists predominantly of oceanic crust.
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
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
Isostacy wrap-up
Any questions about the Mount Everest and tectonic thickening
problems returned today?
Tom Wilson, Department of Geology and Geography
Take Home Problem (due this Thursday)
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 solving part a.
Tom Wilson, Department of Geology and Geography
Remember you are redistributing the excess
crustal thickness (h) through time
The importance of Isostacy in geological problems is not
restricted to equilibrium processes involving large
mountain-belt-scale 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
Today and Thursday
Text problems 3.10 and 3.11 are due today.
The take-home isostacy problem is due this Thursday.
Let’s get started on the Settling Velocity lab
There will be a mid-term test on Thursday (February 27th) . We’ll have a
review session on Tuesday the 25th.
Note that mid-term exam will be in rm 325 Brooks
Tom Wilson, Department of Geology and Geography