Viscosity is important because it determines the type of eruption that will occur, and thus what shape or type of volcano will be formed by the eruptions. Gases are easily released from low viscosity substances. In more viscous magmas, gases are not released easily, and build up. When the pressure builds, a violent eruption can occur. Low viscosity lavas produce shield volcanoes, named after their appearance, which is much like a Roman shield.
High viscosity magmas tend to produce taller, steeper, more explosive volcanoes.
The eruptions consist of steaming gases that carry with them pyroclastic rocks, or material ejected from volcanoes as solid fragments. Stratovolcanoes are also produced by high viscosity magmas. The viscosity level makes the lava more brittle, and while some pyroclastic material is emitted, molten rock also escapes. This combination produces a higher structure. Stratovolcanoes are constructed from multiple eruptions of this nature, creating a layered and very high structure.