Google "craton" and look for a discussion on "lithospheric root". Mountain areas have much thicker crust than non-mountain areas (up to 120 miles thick vs 60 miles. Referred to a mountain "roots" they can be as much as twice the non-mountain crustal areas. Oceanic crust areas, however, do not show these analogously thick areas. Evidence is seismic analysis. The theory is a bit complicated but a fairly good discussion can be found at http://en.wikipedia.org/wiki/Craton. The thickest part of the crust in the USA is in the Rocky Mountains/Western US. See the following :
This is Google's cache of http://quake.wr.usgs.gov/study/CrustalStructure/. It is a snapshot of the page as it appeared on Feb 2, 2010 10:22:19 GMT. The current page could have changed in the meantime.
The Earth's Crust
How Thick Is the Earth's Crust?
We present a new contour map of the thickness of the Earth's crust. We use a 10 km contour interval plus the 45 km contour. This contour map was created directly from the 5 deg. by 5 deg. gridded crustal model CRUST 5.1 (Mooney et al., 1998) plus complementary information. An initial contour map was created using the command "grdcontour" in GMT, and the resultant map was adjusted in Adobe Illustrator to honor individual point measurements and newly available information from Russia. The final contour map honors all available seismic refraction measurements for features with a dimension greater than 2 degrees. To a first approximation, the continents and their margins are outlined by the 30 km contour. That portion of the continental interior enclosed by the 40 km contour, and regions with crustal thickness of 45 to 50 km are found on all well surveyed continents (i.e., North and South America, Australia, and Eurasia). Continental crust with a thickness in excess of 50 km is exceedingly rare and accounts for less than 10% of the continental crust. These observations, now available on a global basis, provide important constraints on the evolution of the crust and sub-crustal lithosphere.
A Contour Map of the World's Crustal Thickness
This contour map of the thickness of the Earth's crust was developed from the CRUST 5.1 model. The contour interval is 10 km; we also include the 45 km contour for greater detail on the continents.
U.S. Department of the Interior | U.S. Geological Survey
Page URL: http://earthquake.usgs.gov/research/structure/crust/index.php
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Page Last Modified: August 14, 2009 16:17:45 UTC