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The average composition of the continental crust is more mafic than hitherto supposed. The conterminous United States can be divided, on the basis of seismic structure, into ten regions. The seven western and the three eastern regions can be termed western and eastern superprovinces. Seismic studies show that the crust is thinner and more silicic in tectonically active regions (western superprovince - average crustal thickness 34 km), than in stable regions (eastern superprovince - average crustal thickness 44 km). Mafic rocks are estimated to average 55% of the continental crust: 45% in the western and 59% in the eastern superprovince. These results express quantitatively the ideas expressed qualitatively by Pakiser and Zietz (1965). The computations of percentages of major oxides in the crust associate seismic velocities with rock compositions. ?? 1956.
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Composition and evolution of the continental crust as suggested by seismic observations