The petrogenesis of igneous rocks can be controlled significantly by the mixing of dissimilar magmas. Within the contexts of basaltic and silicic magma chambers and komatiite lava flows we identify circumstances in which the extent to which contrasting magmas are mixed by natural convection potentially controls their differentiation. To evaluate the amount of mixing in each context, we apply the experimental results from part 1 <Jellinek et al., this issue> of this study, in which we quantified the conditions under which fluids could be mixed by convection at large Rayleigh numbers (>1011). When our laboratory results are applied to basaltic magma chambers, we find that convection driven by compositionally buoyant magma released during floor crystallization or floor dissolution will produce partial to nearly complete mixing of the ascending fluid in chambers that are tens of meters to kilometers high, respectively. We also conclude that substantial floor melting (with extensive mixing) is expected only for basaltic chambers emplaced in the deep crust. During the turbulent flow of Archean komatiites, underlying sediments melted by forced convective heat transfer are predicted to have been mixed nearly completely into the overriding lavas. During the replenishment of silicic magma chambers by basaltic magmas we predict that the convection of buoyant silicic magma overrun by a spreading injection of denser basalt will cause little mixing. However, after emplacement, heat transfer from a basalt layer will gradually melt and mobilize its felsic floor, producing a small flux of buoyant felsic liquid that will be mixed extensively. ¿ 1999 American Geophysical Union