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U.S. Geological Survey Crustal Studies Technical Letter Number 20

In cooperation with the Defense Advanced Research Projects Agency

Continental Crust

By L.C. Pakiser


Thumbnail of and link to report PDF (11.8 MB)Abstract

The structure of the Earth’s crust (the outer shell of the earth above the M-discontinuity) has been intensively studied in many places by use of geophysical methods. The velocity of seismic compressional waves in the crust and in the upper mantle varies from place to place in the conterminous United States. The average crust is thick in the eastern two-thirds of the United States, in which the crustal and upper-mantle velocities tend to be high. The average crust is thinner in the western one-third of the United States, in which these velocities tend to be low. The concept of eastern and western superprovinces can be used to classify these differences. Crustal and upper-mantle densities probably vary directly with compressional-wave velocity, leading to the conclusion that isostasy is accomplished by the variation in densities of crustal and upper-mantle rocks as well as in crustal thickness, and that there is no single, generally valid isostatic model. The nature of the M-discontinuity is still speculative.

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Contact Information, Earthquake Science Center, Menlo Park Science Center
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Menlo Park, California 94025

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Suggested citation:

Pakiser, L.C., 1964, Continental crust: U.S. Geological Survey Crustal Studies Technical Letter Number 20, 24 p. (Available at




Methods of study

Crustal models

Description of a continent

Isostasy and roots of mountains

Nature of the M-discontinuity


References cited

Additional references to seismic profiles in figure 1

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