Viscosity of magma increases with

• A. Decreasing temperature and low silica content
• B. Increasing temperature and low silica content
• C. Decreasing temperature and high silica content
• D. Increasing temperature and high silica content

Answer: (C)

Magmas flow less easily when their melt is highly polymerized and has less thermal energy to overcome that structure. Silica-rich melts form a network of Si-O-Si bonds that link together many tetrahedra, creating a strong, interconnected structure. This polymerized network resists deformation, so the melt behaves more like a solid and becomes very viscous. Temperature also matters: as temperature falls, molecular motion slows and the network becomes harder to break, further increasing viscosity. So the combination of decreasing temperature and high silica content yields the greatest resistance to flow, making the magma most viscous.