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Published January 27, 1984 | public
Journal Article

The Earth as a Planet: Paradigms and Paradoxes


The independent growth of the various branches of the earth sciences in the past two decades has led to a divergence of geophysical, geochemical, geological, and planetological models for the composition and evolution of a terrestrial planet. Evidence for differentiation and volcanism on small planets and a magma ocean on the moon contrasts with hypotheses for a mostly primitive, still undifferentiated, and homogeneous terrestrial mantle. In comparison with the moon, the earth has an extraordinarily thin crust. The geoid, which should reflect convection in the mantle, is apparently unrelated to the current distribution of continents and oceanic ridges. If the earth is deformable, the whole mantle should wander relative to the axis of rotation, but the implications of this are seldom discussed. The proposal of a mantle rich in olivine violates expectations based on evidence from extraterrestrial sources. These and other paradoxes force a reexamination of some long-held assumptions.

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© 1984 American Association for the Advancement of Science. I thank all participants at the 1982 workshop on the lithosphere sponsored by the National Research Council and Geodynamics Committee held in Austin, Texas, in March 1982. I thank particularly J. Maxwell and C. Drake, but they do not necessarily endorse the views in this article. I thank S. Grand, M. Walck, D. Helmberger, B. Hager, T. Tanimoto, J. Bass, and I. Nakanishi for permission to use results in advance of publication. Figure 3 was prepared by R. Clayton and B. Hager from observational results of I. Nakanishi and T. Tanimoto. Supported by NSF grant EAR811-5236 and NASA Geodynamics grant NSG-7610. Contribution No. 3921, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125.

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