Final answer:
A felsic magma's viscosity increases as it becomes richer in silicon, aluminum, sodium, and potassium due to the formation of more complex silicate structures.
Step-by-step explanation:
When a body of magma becomes more felsic, meaning that it is richer in elements such as silicon (Si), aluminum (Al), sodium (Na), and potassium (K), the magma's viscosity will increase. Felsic magma contains a higher proportion of silica, which forms chains and networks of silicate tetrahedra when the magma is in its liquid state.
Since these tetrahedral structures create more resistance to flow, the viscosity of the magma is increased. Molecules in felsic magma interact with each other more strongly than in mafic magma (which is richer in magnesium and iron), thus requiring more energy, or a higher temperature, to move past each other and flow. Therefore, as a final answer, in a two-line explanation: With the increasing felsic composition of magma, its viscosity increases due to the greater presence of complex silicate structures, which impedes the flow of the molten material.