tom.h.wilson tom. [email protected]

Tom Wilson, Department of Geology and Geography

tom.h.wilsontom. [email protected]

Department of Geology and GeographyWest 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.

ACB

At B C x 42 = 116 C=2.76C=2.76

At C C x 50 = 116 C=2.32


Geological Survey of Japan


Japan ArchipelagoJapan Archipelago

Physical Evidence for Isostacy

Izu-

Bon

in A

rc

Izu-

Bon

in A

rc

Pacific PlatePacific Plate

Izu-B

on

in T

rench

Izu-B

on

in T

rench

Kuril Trench

Kuril Trench

Jap

an T

ren

ch

Jap

an T

ren

ch

Nankai Trough

Nankai Trough

North American Plate


Philippine Sea Plate


Eurasian Plate

Eurasian Plate



The Earth’s gravitational field

In the red areas you weigh more and in the blue areas you weigh less.

Izu

-Bo

nin

Arc

Izu

-Bo

nin

Arc

Pacific PlatePacific Plate

Izu-B

on

in T

rench

Izu-B

on

in T

rench

Kuril Tre

nch

Kuril Tre

nch

Jap

an T

ren

ch

Jap

an T

ren

ch

Nankai Trough

Nankai Trough




Philippine Sea PlateEurasian Plate

Eurasian Plate



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.



Watts, 2001


http://pubs.usgs.gov/imap/i-2364-h/right.pdf


Morgan, 1996 (WVU Option 2 Thesis)


Crustal thickness in WV Derived from Gravity Model Studies


http://www.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swfhttp://www.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swf


http://www.uky.edu/AS/Geology/howell/goodies/elearning/module06swf.swf


Seismically fast lithosphere thickens into the continental interior from the Atlantic margin

Rychert et al. (2005) Nature

Tom Wilson, Department of Geology and Geographyhttp://www.sciencedaily.com/releases/2008/04/080420114718.htm

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


Consider the Mount Everest and tectonic thickening problems handed out last time.


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).

Take Home (individual) Problem


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 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 18th

tom.h.wilson tom. [email protected]

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