What term refers to the fluid motion of volcanic materials during an eruption?

The term that refers to the fluid motion of volcanic materials during an eruption is "pyroclastic flow." This phenomenon occurs when volcanic materials, such as ash, pumice, and gases, are erupted explosively and flow down the slopes of a volcano at high speeds, often exceeding 100 kilometers per hour (62 mph). Pyroclastic flows are characterized by their density and the ability to travel rapidly over the ground, making them one of the most dangerous volcanic hazards.

In contrast, volcanic ash refers specifically to the small particles that are ejected into the atmosphere during an explosive eruption, but it does not encompass the dynamic flow aspect of the material. A lahar is a destructive mudflow composed of volcanic ash and water, typically triggered by melting snow and ice or heavy rainfall during or after an eruption, but it is a different flow type related to water rather than the direct volcanic gases and materials involved in a pyroclastic flow. Magma, on the other hand, is molten rock found beneath the Earth’s surface before it reaches the surface during an eruption; it is not a term used for the material once it has erupted and is in motion.