Higher viscosity means that the fluid does not flow well.
Low viscosity means the fluid flows easily.
Higher Si content results in higher viscosity. Moderately steep slopes, formed of alternating layers of lava flows and pyroclastic debris. Formed by lava with intermediate viscosity, having moderate Mg and Si content. Small, steep-sided volcanoes made up almost entirely of pyroclastic material and very little lava. Formed by magma with very high viscosity, with high Si and low Mg.