VOLCANOES

Volcanoes

The violence of a volcanic eruption is largely determined by the viscosity of the magma. Higher viscous magma resists flow and tends to be associated with violent eruptions. Magma tha is less viscous is associated with gentle volcanic eruptions.
There are many factors that affect the viscosity of magma, but none as obvious as temperature. Yet, the largest factor affecting viscosity is its silica content. Magmas with a silica content of about 50% produce basalts. Magmas with over a 70% silica content produce granites and are more viscous. The increase in viscosity is caused by the silicate structures aligning into chains before crystallization can occur.

Furthermore, the higher viscosity magmas can sometimes plug up volcanic vents causing volatile gases like water vapor and carbon dioxide to build to explosive levels increasing the violent nature of the volcano. In low viscosity basaltic lava flows, expanding gases escape with ease, but can propel lava hundreds of meters into the air.