Physical Processes in Earth and Environmental Sciences Phần 3

Chương 3Ví dụ, một bề mặt đại dương hiện tại mật độ 1 có thể được cho biết "cảm thấy" lực hấp dẫn giảm vì sự nổi tích cực tác dụng lên nó bằng cách nằm dưới nước môi trường xung quanh có mật độ cao hơn một chút.
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Physical Processes in Earth and Environmental Sciences Phần 3LEED-Ch-03.qxd 11/27/05 3:59 Page 54 54 Chapter 3 differing density does not “feel” the same gravitational that of the ambient lake or marine waters (Fig. 2.12); these attraction as it would if the ambient medium were not are termed turbidity currents (Section 4.12). there. For example, a surface ocean current of density 1 Motion due to buoyancy forces in thermal fluids is may be said to “feel” reduced gravity because of the posi- called convection (Section 4.20). This acts to redistribute tive buoyancy exerted on it by underlying ambient water heat energy. There is a serious complication here because of slightly higher density, 2. The expression for this buoyant convective motion is accompanied by volume reduced gravity, g , is g g( 2 1)/ 2. We noted earlier changes along pressure gradients that cause variations of that for the case of mineral matter, density m, in atmos- density. The rising material expands, becomes less dense, phere of density a, the effect is negligible, corresponding and has to do work against its surroundings (Section 3.4): to the case m a. this requires thermal energy to be used up and so cooling occurs. This has little effect on the temperature of the ambient material if the adiabatic condition applies: the net rate of outward heat transfer is considered negligible. 3.6.3 Natural reasons for buoyancy We have to ask how buoyant forces arise naturally. 3.6.4 Buoyancy in the solid Earth: The commonest cause in both atmosphere and ocean is Isostatic equilibrium density changes arising from temperature variations acting upon geographically separated air or water masses that then interact. For example, over the c.30 C variation in In the solid Earth, buoyancy forces are often due to near-surface air or water temperature from Pole to equa- density changes owing to compositional and structural tor, the density of air varies by c.11 percent and that of changes in rock or molten silicate liquids. For example, the seawater by c.0.6 percent. The former is appreciable, and density of molten basalt liquid is some 10 percent less than although the latter may seem trivial, it is sufficient to drive that of the asthenospheric mantle and so upward the entire oceanic circulation. It is helped of course by movement of the melt occurs under mid-ocean ridges variations in salinity from near zero for polar ice meltwater (Fig. 3.27). However we note that the density of magma is to very saline low-latitude waters concentrated by evapora- also sensitive to pressure changes in the upper 60 km or so tion, a maximum possible variation of some 4 percent. of the Earth’s mantle (Section 5.1). Density changes also arise when a bottom current picks up In general, on a broad scale, the crust and mantle are sufficient sediment so that its bulk density is greater than found to be in hydrostatic equilibrium with the less dense mountain range thickness of iceberg root hir = ri /(rw - ri) hmr ocean, rwrw ri ho o crustal equilibrium crust rc thickness of crustal root, hcr = rc / ( rw – rc ) antiroot har Moho thickness ...