Transcript No Slide Title
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
Tom Wilson, Department of Geology and Geography
Another possibility
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
Tom Wilson, Department of Geology and Geography
Topographic extremes
Japan Archipelago
North American Plate Pacific Plate Philippine Sea Plate Geological Survey of Japan
The Earth’s gravitational field In the red areas you weigh more and in the blue areas you weigh less.
g ~0.6 cm/sec 2
North American Plate Pacific Plate Philippine Sea Plate
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan
Quaternary vertical uplift
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. Total difference of about 0.1 cm/sec 2 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
Tom Wilson, Department of Geology and Geography Watts, 2001
Tom Wilson, Department of Geology and Geography Watts, 2001
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
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/cm 3 .)
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 27 th ) . We’ll have a review session on Tuesday the 25 th .
Note that mid-term exam will be in rm 325 Brooks Tom Wilson, Department of Geology and Geography