1. Geol Geomath
Isostacy II - Wrap up isostacy and begin working on the settling velocity lab
tom.h.wilson
tom.
Department of Geology and Geography
West Virginia University
Morgantown, WV
Tom Wilson, Department of Geology and Geography

2. 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.
Lid refers to the mantle part of the lithosphere; lid is the brittle part of the asthenosphere
Lithosphere includes the crust and upper part of the mantle (the mantle lid)
Asthenosphere is upper mantle
Tom Wilson, Department of Geology and Geography

3. Other examples of isostatic computations
Tom Wilson, Department of Geology and Geography

4. Another possibility
Tom Wilson, Department of Geology and Geography

5. C B A
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
C=2.76
At C C x 50 = 116
C=2.32
Tom Wilson, Department of Geology and Geography

6. Some expected differences in the mass balance equations
Tom Wilson, Department of Geology and Geography

7. Island arc systems – isostacy in flux
To illustrate the variety of quantitative applications brought to bear on geological problems
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

8. Topographic extremes Japan Archipelago Geological Survey of Japan
North American Plate
Kuril Trench
Pacific Plate
Eurasian Plate
Japan Trench
Nankai Trough
Izu-Bonin Arc
Izu-Bonin Trench
Philippine Sea Plate
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

9. In the red areas you weigh more and in the blue areas you weigh less.
The Earth’s gravitational field
North American Plate
In the red areas you weigh more and in the blue areas you weigh less.
Kuril Trench
g ~0.6 cm/sec2
Japan Trench
Pacific Plate
Nankai Trough
Eurasian Plate
Philippine Sea Plate
Izu-Bonin Trench
Izu-Bonin Arc
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

10. Quaternary vertical uplift
Geological Survey of Japan
Tom Wilson, Department of Geology and Geography

11. 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

12. 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

13. Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

14. Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

15. Geological Survey of Japan
Tom Wilson, Department of Geology and Geography
Geological Survey of Japan

16. Marquesas in the southern pacific - part of french polynesia
Canary offshore northern africa
Watts, 2001
Tom Wilson, Department of Geology and Geography

17. Onlapping suggests deposits originally sloping down onto ocean floor
Onlapping suggests deposits originally sloping down onto ocean floor. Off lapping suggests sediments deposited in a crustal depression surrounding the volcanic island is it continued to develop. The weight of the volcanic deposits began to deform the lithosphere elastically.
Watts, 2001
Tom Wilson, Department of Geology and Geography

18. Crustal Scale Modeling
Tom Wilson, Department of Geology and Geography

19. Crustal thickness in WV Derived from Gravity Model Studies
Tom Wilson, Department of Geology and Geography

20. http://www. nasa. gov/mission_pages/MRO/multimedia/phillips-20080515
Tom Wilson, Department of Geology and Geography

21. 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
Tom Wilson, Department of Geology and Geography

22. Isostacy wrap-up
Any questions about the Mount Everest and tectonic thickening problems returned today?
Tom Wilson, Department of Geology and Geography

23. 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

24. Isostacy is a dynamic geologic process.
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

25. Text problems 3.10 and 3.11 are due today.
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