Which statement about magma viscosity is true?

Which statement about magma viscosity is true?

• A. Higher silica content magma is less viscous.
• B. Lower silica content magma is less viscous.
• C. Viscosity is independent of silica content.
• D. Viscosity increases only with gas content.

Answer: (B)

Magma viscosity is determined mainly by how polymerized the silica network is in the melt. More silica means more polymer chains that tangle and resist flow, making the magma thicker and more viscous. Less silica means fewer chains, so the melt flows more easily and is less viscous. This is why basaltic magmas (low silica) are relatively fluid and rhyolitic magmas (high silica) are very thick and sticky. The statement that lower silica content magma is less viscous aligns with this idea, while the other options either contradict the silica–viscosity relationship or treat gas content as the sole or primary factor.

Magma viscosity is determined mainly by how polymerized the silica network is in the melt. More silica means more polymer chains that tangle and resist flow, making the magma thicker and more viscous. Less silica means fewer chains, so the melt flows more easily and is less viscous. This is why basaltic magmas (low silica) are relatively fluid and rhyolitic magmas (high silica) are very thick and sticky. The statement that lower silica content magma is less viscous aligns with this idea, while the other options either contradict the silica–viscosity relationship or treat gas content as the sole or primary factor.