What causes explosive vs. non-explosive eruptions?
• An explosive eruption is caused by viscous magma (stiff or thick, resists flow) with a high gas content (most of the gas is water). • The presence of gas is the driving force for explosion. Gas expands (boils away) as magma reaches surface, causing an explosion. • The silica content determines whether the magma is viscous or not. • In silica-rich magma, linking of silica-tetrahedra can actually occur in the liquid state, creating a high viscosity that is susceptible to an explosive eruption. • In silica-poor magma, there is little linking of silica tetrahedra and the lava can flow easily. • Non-explosive volcanoes form by melting of mantle rock, which contains little water and relatively low amounts of silica. The lava produced (basalt) is low in silica and usually has relatively little gas, therefore the eruptions are non-explosive. • Explosive volcanoes form by melting of crustal (& mantle) rock, which contains much more water and silica. Therefore, explosive eruptions of andesite
