Theory of the Earth / Теория Земли

The maturing of the Earth sciences has led to a fragmentation into subdisciplines which speak imperfectly to one another. Some of these subdisciplines are field geology, petrology, mineralogy, geochemistry, geodesy and seismology, and these in turn are split into even finer units. The science has also expanded to include the planets and even the cosmos. The practitioners in each of these fields tend to view the Earth in a completely different way. Discoveries in one field diffuse only slowly into the consciousness of a specialist in another. In spite of the fact that there is only one Earth, there are probably more Theories of the Earth than there are of astronomy, particle physics or cell biology where there arc uncountable samples of each object. Even where there is cross-talk among disciplines, it is usually as noisy as static. Too often, one discipline’s unproven assumptions or dogmas are treated as firm boundary conditions for a theoretician in a slightly overlapping area. The data of each subdiscipline are usually consistent with a range of hypotheses. The possibilities can be narrowed considerably as more and more diverse data are brought to bear on a particular problem. The questions of origin, composition and evolution of the Earth require input from astronomy, cosmochemistry, meteoritics, planetology, geology, petrology, mineralogy, crystallography, materials science and seismology, at a minimum. To a student of the Earth, these are artificial divisions, however necessary they are to make progress on a given front.Examples abound. A seismologist struggling with the meaning of velocity anomalies beneath various tectonic provinces, or in the vicinity of a deeply subducting slab, is apt to interpret seismic results in terms of temperature variations in a homogeneous, isotropic half-space or series of layers. However, the petrological aspects—variations in mineralogy, crystal orientation or partial melt content—are much more important. These, in turn, require knowledge of phase equilibria and material properties.An isotope geochemist, upon finding evidence for several ancient isolated reservoirs in the rocks and being generally aware of the geophysical evidence for a crust and a 650-km discontinuity, will tend to interpret the chemical data in terms of crustal contamination or recycling, a “normal” mantle source and a lower mantle source. This “standard” petrological model is a homogeneous pcridotite mantle which contains about 20 percent basalt, available as needed, to fuel the midocean ridges with uniform magmas. Exotic basalts are assumed to be from the core-mantle boundary or to have interacted with the cmst. The crust and shallow mantle may be inhomogencous, but the rest of the mantle is viewed as well homogenized by convection.The convection theoretician treats the mantle as a homogeneous fluid or as a two-layered system, with constant physical properties, driven by temperature-induced bouyancy, ignoring melting and phase changes and even pressure.In Theory of the Earth 1 attempt to assemble the bits and pieces from a variety of disciplines which are relevant to an understanding of the Earth. Rocks and magmas are our most direct source of information about the interior, but they are biased toward the properties of the crust and shallow mantle. Seismology is our best source of information about the deep interior; however, the interpretation of seismic data for purposes other than purely structural requires input from solid-state physics and experimental petrology. Although this is not a book about seismology, it uses seismology in a variety of ways.