Geol 351 - Geomath

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Geol 351 - Geomath. Isostacy II - Wrap up isostacy and begin working on the settling velocity lab. tom.h.wilson tom. [email protected] Department of Geology and Geography West Virginia University Morgantown, WV. Explanations for lowered gravity over mountain belts.

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Geol 351 - Geomath

Isostacy II - Wrap up isostacy and begin working on the settling velocity lab

tom.h.wilson

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.

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

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

Some expected differences in the mass balance equations

Tom Wilson, Department of Geology and Geography

Island arc systems – isostacy in flux

Tom Wilson, Department of Geology and Geography

Geological Survey of Japan

Topographic extremes

Japan Archipelago

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

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

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

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

Watts, 2001

Tom Wilson, Department of Geology and Geography

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.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

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

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

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