The nature of a volcanic eruption is determined by the temperature, viscosity and the amount of dissolved gases in the magma, according to San Diego State University. These factors determine whether an eruption is calm and nonviolent or explosive.
Viscosity, or resistance to flow, is one of the most important aspects of magma composition, as the University of Rhode Island states. Magmas with low viscosity, such as basaltic magmas, are more like fluids and allow gases to escape with relative ease. On the other hand, thick high-viscosity magmas allow gas pressure to build up under the surface, which can result in violent explosions.