tom.h.wilson tom. wilson@mail.wvu

Geology 351 - Geomath More about Isostacy 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

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. The Airy Hypothesis Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

A few more comments on Isostacy The Pratt Hypothesis Tom Wilson, Department of Geology and Geography

At B C x 42 = 116 C B A The product of density and thickness must remain constant in the Pratt model. At A 2.9 x 40 = 116 C=2.76 At C C x 50 = 116 C=2.32 Tom Wilson, Department of Geology and Geography

Tom Wilson, Department of Geology and Geography

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

Physical Evidence for Isostacy Japan Archipelago North American Plate Kuril Trench Pacific Plate Japan Trench Eurasian Plate Nankai Trough Izu-Bonin Arc Izu-Bonin Trench Philippine Sea Plate Tom Wilson, Department of Geology and Geography Geological Survey of Japan

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 Pacific Plate Japan Trench Nankai Trough Eurasian Plate Philippine Sea Plate Izu-Bonin Trench Izu-Bonin Arc 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. Tom Wilson, Department of Geology and Geography Geological Survey of Japan

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

Morgan, 1996 (WVU Option 2 Thesis) 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

Seismically fast lithosphere thickens into the continental interior from the Atlantic margin Rychert et al. (2005) Nature Tom Wilson, Department of Geology and Geography

http://www.nasa.gov/mission_pages/MRO/multimedia/phillips-20080515.htmlhttp://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

In-class problems Consider the Mount Everest and tectonic thickening problems handed out last time. Tom Wilson, Department of Geology and Geography

Take Home (individual) Problem 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). 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

Isostatic Rebound of Ancient Lake Shorelines 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 Tom Wilson, Department of Geology and Geography

Forthcoming ... Have a look at the take home isostacy problem handed out today. Complete reading of Chapters 3 and 4 For the remainder of the day We’ll take a quick look at quadratics and then move on to Problem 3.11 In Chapter 4 look over questions 4.7 and 4.10 next Thursday – the 18th Tom Wilson, Department of Geology and Geography

tom.h.wilson tom. wilson@mail.wvu