Final answer:
A pyroclastic flow is formed when there are volatiles at very high critical temperatures, which contribute to the explosiveness of an eruption and the subsequent rush of hot gas-and-ash clouds.
Step-by-step explanation:
The condition for the formation of a pyroclastic flow is volatiles at very high critical temperature. Pyroclastic flows are extremely hot gas-and-ash clouds that can rush down the slopes of a volcano at high speeds, destroying nearly everything in their path. These flows can form when an erupting volcano ejects viscous magma near the surface, combined with water trapped in the fractured rock above the magma, under pressure with temperatures in excess of 180°C (under pressure), and a caprock trapping the pressurized steam. The existence of volatiles in the magma lowers its melting temperature, allowing for a more violent and explosive eruption that can produce pyroclastic flows.
Historical accounts like the eruption of Mount Vesuvius provide examples of devastating pyroclastic flows that incinerated or suffocated populations, and altered the landscapes. These flows are indicative of the powerful forces at work during a volcanic eruption and their potential for destruction.