Continental crust andesite model

A particularly influential model for the average composition of the continental crust is the andesite model of Taylor and McLennan (1981, 1985). This model is based upon the observation that modern continental growth primarily takes place at a convergent margin. The model has two principal assumptions. First, that the continental crust could be divided into geochemically distinct upper and lower portions which are present in the proportions of 1 to 2. Second, that the bulk composition of the continental crust is andesitic. Given these premises and an estimated average composition for the upper continental crust, it was possible for Taylor and McLennan to calculate the composition of the lower continental crust, which they found to be basaltic. 

The paradox of modern mantle fluxes One of the most profound problems in understanding the origin of the Earth s continental crust is the discrepancy between the composition of the continental crust, which is andesitic, and the composition of the present-day flux across the Moho, which is basaltic. Two types of solution have been proposed. On the one hand it has been argued that the composition of the continental crust has been modified after it formed in order to adjust its composition from a basaltic protolith to andesite. Alternatively, the balance of fluxes across the Moho has changed over time so that modern processes provide an incomplete explanation for the time-averaged composition of the continental crust. These two models will be examined in turn. 

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