Past theories of crustal construction suggested that the upper continental crust was made up of rocks of granitic composition. That this is not the case is evident from the widespread occurrence of large negative gravity anomalies over granite plutons. These anomalies demonstrate that the density of the plutons (about 2.67 Mg m-3) is some 0.10-0.15 Mg m-3 lower than the average value of the upper crust. The mean composition of the upper crust can be estimated, albeit with some uncertainty due to biasing, by determining the mean composition of a large number of samples collected worldwide and from analyses of sedimentary rocks that have sampled the crust naturally by the process of erosion (Taylor & Scott, 1985; Gao et al., 1998). This composition corresponds to a rock type between grano-diorite and diorite, and is characterized by a relatively high concentration of the heat-producing elements.
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