Final answer:
The cloud of hot gas and rock descending downslope is known as a pyroclastic flow, a high-temperature, high-speed mixture that can have devastating effects, as seen during the eruption of Mount Vesuvius.
Step-by-step explanation:
We call the cloud of hot gas and rock descending downslope a pyroclastic flow. Pyroclastic flows are rapid, hot mixtures of volcanic matter that include ash, rock fragments, and gases that surge down the sides of a volcano during an eruptive event. They can be extremely dangerous due to their high temperatures and speeds, capable of destroying nearly everything in their path.
Such phenomena were observed during historic volcanic events like the eruption of Mount Vesuvius. Pyroclastic flows occur when the structural integrity of volcanic material surrounding a vent can no longer be maintained, causing a collapse and the subsequent rapid flow of volcanic matter downslope. Understanding the dynamics of pyroclastic flows is crucial for volcanic hazard assessment and for the safety of populations living near active volcanoes.