The character of volcanic eruptions are largely controlled by the viscosity – “gooeyness” or resistance to flow – of the magma: Low viscosity fluids flow more easily than high viscosity fluids
Familiar fluids range in viscosity. Water has low viscosity (flows easily) while syrup and honey have greater viscosity. Heating reduces viscosity (warm syrup flows more easily than cold.)
(University of British Columbia)
Viscosity increases with increasing silica content due to silica chains
High viscosity lavas flow slowly and typically cover small areas. In contrast, low viscosity magmas flow more rapidly and form lava flows that cover thousands of square kilometers.
Low viscosity magmas allow gases to escape easily whereas gas pressures can build up in high viscosity magmas – resulting in violent eruptions
(Blowing through a straw, it’s easier to get water to bubble than a milk shake)