Final answer:
Rhyolitic lavas are more viscous than basaltic lavas due to their higher silica content and typically solidify into very finely crystalline rocks, whereas basaltic lavas, which contain more iron and magnesium, form the majority of the oceanic crust and solidify into fine- or coarse-grained rocks depending on the cooling rate.
Step-by-step explanation:
Rhyolitic (felsic) lavas are significantly different from basaltic (mafic) lavas in terms of composition and physical properties. Firstly, rhyolitic lavas contain more silica than basaltic lavas, which makes them much more viscous (thick and resistant to flow). On the other hand, basaltic lavas are less viscous due to a lower silica content and a higher proportion of iron and magnesium. Because of the high viscosity, rhyolitic lavas tend to cool and crystallize quickly, resulting in a very finely crystalline (aphanitic) texture as compared to the often larger crystals found in the slower cooling and less viscous basaltic lavas.
The composition of rhyolite typically includes a high percentage of silica-rich minerals such as quartz and alkaline feldspar. In contrast, basalt is rich in iron and magnesium-bearing minerals like pyroxene and olivine, reflecting its mafic nature. Basaltic lavas, forming the majority of the oceanic crust, typically solidify into fine-grained rocks like basalt when erupted or into coarser-grained rocks like gabbro when cooled more slowly beneath the surface.