Transcript Geological Survey of Japan

More about Isostacy
tom.h.wilson
tom. [email protected]
Department of Geology and Geography
West Virginia University
Morgantown, WV
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
A
B
C
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
At C C x 50 = 116
C=2.76
C=2.32
Geological Survey of Japan
Physical Evidence for Isostacy
Japan Archipelago
North
American
Plate
Pacific Plate
Philippine
Sea 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.
North
American
Plate
Pacific Plate
Philippine
Sea Plate
Geological Survey of Japan
Physical Evidence for Isostacy
Japan Archipelago
North
American
Plate
Pacific Plate
Philippine
Sea Plate
Geological Survey of Japan
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
Geological Survey of Japan
Geological Survey of Japan
Geological Survey of Japan
Geological Survey of Japan
Watts, 2001
Watts, 2001
Watts, 2001
http://pubs.usgs.gov/imap/i-2364-h/right.pdf
Morgan, 1996 (WVU Option 2 Thesis)
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.swf
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 http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect1/index.htm
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/cm3.)
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.
Mountain Elevation &
Mountain Root (km)
Isostatic Response to Erosion
25
20
Root Extent (km)
15
10
Mountain Elevation(km)
5
0
0
5
10
15
20
Amount Eroded (km)
25
30
The importance of Isostacy in geological
problems is not restricted to equilibrium
processes involving large mountain-beltscale 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 8th) 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