Only the uppermost part of the crust is available for direct sampling at the surface or from boreholes. At greater depths within the crust, virtually all information about its composition and structure is indirect. Geologic studies of high grade metamorphic rocks that once resided at depths of 20-50 km and have been brought to the surface by subsequent tectonic activity provide some useful information (Miller & Paterson, 2001a; Clarke et al., 2005). Foreign rock fragments, or xenoliths, that are carried from great depths to the Earth's surface by fast-rising magmas (Rudnick, 1992) also provide samples of deep crustal material. In addition, much information about the crust has been derived from knowledge of the variation of seismic velocities with depth and how these correspond to experimental determinations of velocities measured over ranges of temperature and pressure consistent with crustal conditions. Pressure increases with depth at a rate of about 30 MPa km-1, mainly due to the lithostatic confining pressure of the overlying rocks, but also, in some regions, with a contribution from tectonic forces. Temperature increases at an average rate of about 25°C km-1, but decreases to about half this value at the Moho because of the presence of radioactive heat sources within the crust (Section 2.13). Collectively, the observations from both geologic and geophysical studies show that the continental crust is vertically stratified in terms of its chemical composition (Rudnick & Gao, 2003).
The variation of seismic velocities with depth (Section 2.2) results from a number of factors. The increase of pressure with depth causes a rapid increase in incompressibility, rigidity, and density over the topmost 5 km as pores and fractures are closed. Thereafter the increase of these parameters with pressure is balanced by the decrease resulting from thermal expansion with increasing temperature so that there is little further change in velocity with depth. Velocities change with chemical composition, and also with changes in mineralogy resulting from phase changes. Abrupt velocity discontinuities are usually caused by changes in chemical composition, while more grada-tional velocity boundaries are normally associated with phase changes that occur over a discrete vertical interval.
Models for the bulk chemical composition of the continental crust vary widely because of the difficulty of making such estimates. McLennan & Taylor (1996) pointed out that the flow of heat from the continental crust (Section 2.13) provides a constraint on the abundance of the heat producing elements, K, Th, and U, within it, and hence on the silica content of the crust. On this basis they argue that on average the continental crust has an andesitic or granodioritic composition with K2O no more than 1.5% by weight. This is less silicic than most previous estimates. The abundance of the heat producing elements, and other "incompatible" elements, in the continental crust is of great importance because the degree to which they are enriched in the crust reflects the extent to which they are depleted in the mantle.
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