The magma types vary from mafic magmas , which have relatively low silica and high Fe and Mg contents, to felsic magmas , which have relatively high silica and low Fe and Mg contents. Mafic magma will cool and crystallize to produce the volcanic rock basalt , whereas felsic magma will crystallize to produce dacite and rhyolite . Intermediate-composition magmas will crystallize to produce the rock andesite . Because the mafic rocks are enriched in Fe and Mg, they tend to be darker colored than the felsic rock types. The viscosity of a substance is a measure of its consistency.
Viscosity is defined as the ability of a substance to resist flow.
In a sense, viscosity is the inverse of fluidity . Cold molasses, for example, has a higher viscosity than water because it is less fluid. The effect of temperature on viscosity is intuitive. Like most liquids, the higher the temperature, the more fluid a substance becomes, thus lowering its viscosity. Composition plays an even greater role in determining a magma's viscosity.