Geol 351 - Geomath Isostacy II - Wrap up isostacy and begin working on the settling velocity lab tom.h.wilson tom. wilson@mail.wvu.edu 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. 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

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

Another possibility Tom Wilson, Department of Geology and Geography

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

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

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

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

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

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

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

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

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

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

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

Crustal Scale Modeling 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 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

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

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