Silica

Silica is SiO2, the predominant molecular constituent of volcanic rocks and magmas. It tends to polymerize into molecular chains, increasing the viscosity of the magma. Basaltic magma having lower SiO2 is fairly fluid, but with increasing contents of SiO2, andesite, dacite, rhyodacite, and rhyolite magmas become progressively more viscous. The greater difficulty for dissolved gas to escape from more viscous magma makes higher-silica magmas generally more explosive