Tom Wilson, Department of Geology and Geography tom.h.wilson tom. [email protected] Department of Geology and Geography West Virginia University Morgantown, WV More about Isostacy
Jan 02, 2016
Tom Wilson, Department of Geology and Geography
tom.h.wilsontom. [email protected]
Department of Geology and GeographyWest Virginia University
Morgantown, WV
More about Isostacy
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
At A 2.9 x 40 = 116
The product of density and thickness must remain constant in the Pratt model.
ACB
At B C x 42 = 116 C=2.76C=2.76
At C C x 50 = 116 C=2.32
Tom Wilson, Department of Geology and Geography
Japan ArchipelagoJapan Archipelago
Physical Evidence for Isostacy
Izu-
Bon
in A
rc
Izu-
Bon
in A
rc
Pacific PlatePacific Plate
Izu-B
on
in T
rench
Izu-B
on
in T
rench
Kuril Trench
Kuril Trench
Jap
an T
ren
ch
Jap
an T
ren
ch
Nankai Trough
Nankai Trough
North American Plate
North American Plate
Philippine Sea Plate
Philippine Sea Plate
Eurasian Plate
Eurasian Plate
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
The Earth’s gravitational field
In the red areas you weigh more and in the blue areas you weigh less.
Izu
-Bo
nin
Arc
Izu
-Bo
nin
Arc
Pacific PlatePacific Plate
Izu-B
on
in T
rench
Izu-B
on
in T
rench
Kuril Tre
nch
Kuril Tre
nch
Jap
an T
ren
ch
Jap
an T
ren
ch
Nankai Trough
Nankai Trough
North American Plate
North American Plate
Philippine Sea Plate
Philippine Sea PlateEurasian Plate
Eurasian Plate
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.
Geological Survey of Japan
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.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swfhttp://www.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swf
Tom Wilson, Department of Geology and Geography
http://www.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swf
Tom Wilson, Department of Geology and Geography
Seismically fast lithosphere thickens into the continental interior from the Atlantic margin
Rychert et al. (2005) Nature
Tom Wilson, Department of Geology and Geographyhttp://www.sciencedaily.com/releases/2008/04/080420114718.htm
http://www.nasa.gov/mission_pages/MRO/multimedia/phillips-20080515.html
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
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).
Take Home (individual) Problem
Tom Wilson, Department of Geology and Geography
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
Isostacy and the shoreline elevations of the ancient Lake Bonneville. Gilbert (1890) noticed that the shorelines near the center of the ancient lake were at higher elevation that those along the earlier periphery.
Similar observations are made for Lake Lahonton in Nevada (at right), where peripheral shorelines are located at lower elevation (~20 meters) than those toward the interior of the ancient lake basin
Caskey and Ramelli, 2004