tom.h.wilson tom. Department of Geology and Geography West Virginia University Morgantown, WV More about Isostacy

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.

A few more comments on Isostacy

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

Geological Survey of Japan

Japan Archipelago Physical Evidence for Isostacy Izu-Bonin Arc Pacific Plate Izu-Bonin Trench Kuril Trench Japan Trench Nankai Trough North American Plate Philippine Sea Plate Eurasian 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. Izu-Bonin Arc Pacific Plate Izu-Bonin Trench Kuril Trench Japan Trench Nankai Trough North American Plate Philippine Sea Plate Eurasian Plate Geological Survey of Japan

Japan Archipelago Physical Evidence for Isostacy Izu-Bonin Arc Pacific Plate Izu-Bonin Trench Kuril Trench Japan Trench Nankai Trough North American Plate Philippine Sea Plate Eurasian Plate 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. Geological Survey of Japan

Watts, 2001

Morgan, 1996 (WVU Option 2 Thesis)

Crustal thickness in WV Derived from Gravity Model Studies

Seismically fast lithosphere thickens into the continental interior from the Atlantic margin Rychert et al. (2005) Nature

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

Consider the Mount Everest and tectonic thickening problems.

Take Home 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/cm 3.) There are actually 4 parts to this problem - we must first determine the starting equilibrium conditions before doing solving for (a).

The preceding questions emphasize the dynamic aspects of the problem. A more complete representation of the balance between root and mountain is shown below. Also refer to the EXCEL file on my shared directory.

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

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

Have a look at Problem 2.15 for Thursday (the 8 th ) and the take home isostacy problem handed out today. Complete reading of Chapters 3 and 4 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 15th