Is pyroclastic material likely to form from low-viscosity lava or high- viscosity lava?

Pyroclastic materials are clouds of ash, vapor, and lava fragments carried through the air. When this occurs during a volcanic eruption it produces low-viscosity lava. Viscosity refers to the thickness of materials and its resistance to flow. When pyroclastic materials sore through the air they are very hot and they move rapidly because vapor provides its buoyancy. Due to its general make-up, these materials are capable for destroying life and property within their paths and can extend miles away from the volcano.