Boutilier and Keen <1999> showed that small scale convection may produce the observed thick igneous crust along volcanic passive margins provided that a non-linear rheology is adopted and the viscosity of the upper mantle is increased suddenly after 132 km of continental crust extension. The sudden viscosity increase is justified by the melt induced dewatering of the upper mantle rocks. Through this comparative study, it is shown that inclusion of the pressure of the newly formed igneous crust into the melting model, and/or adopting a more realistic viscosity law, significantly reduces the thickness of igneous crust produced by Boutilier and Keen's models. Furthermore, it is shown that melt migration would narrow the width of the thick igneous crust. Results of this study show that small scale convection may not play a significant role in the formation of the observed thick igneous crust along volcanic passive margins.