https://www.usgs.gov/programs/VHP/pyroclastic-flows-move-fast-and-destroy-everything-their-pathPyroclastic flows contain a high-density mix of hot lava blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes, typically following valleys. Most pyroclastic flows consist of two parts: a lower (basal) flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
With rock fragments ranging in size from ash to boulders that travel across the ground at speeds typically greater than 80 km per hour (50 mph), pyroclastic flowsknock down, shatter, bury or carry away nearly all objects and structures in their path. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C (390-1300°F), can ignite fires and melt snow and ice.
Pyroclastic flows vary considerably in size and speed, but even relatively small flows that move less than 5 km (3 mi) from a volcano can destroy buildings, forests, and farmland. On the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases.
Pyroclastic flows generally follow valleys or other low-lying areas and, depending on the volume of rock debris carried by the flow, they can deposit layers of loose rock fragments to depths ranging from less than one meter to more than 200 m (up to about 700 ft).
A pyroclastic flow from Mount Unzen, which erupted Monday, June 3, 1991, killed 43 people, many of whom were too close to the volcano.
https://en.wikipedia.org/wiki/Mount_UnzenMount Unzen (雲仙岳, Unzen-dake) is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island.
Catherine Joséphine "Katia" Krafft (née Conrad; April 17, 1942 – June 3, 1991) and her husband, Maurice Paul Krafft (March 25, 1946 – June 3, 1991), were French volcanologists who died in a pyroclastic flow on Mount Unzen, in Japan, on June 3, 1991.
On June 3, 1991, at around 4pm local time, Mount Unzen erupted, forming pyroclastic flows that rushed down its slopes, killing 37 people, including the Kraffts and fellow volcanologist Harry Glicken.
https://en.wikipedia.org/wiki/Harry_GlickenThe Geological Survey hired Glicken in 1980 to monitor Mount St. Helens from a trailer on a ridge 5.7 miles northwest of the volcano. After working six consecutive days, he was relieved by his friend and mentor, David Johnston, the elder Glicken said.
Johnston was on duty at 8:32 a.m. May 18, 1980, when Mount St. Helens unleashed a catastrophic sideways blast of hot gas and ash. Moments before he died, Johnston yelled into his radio to Geological Survey officials: ″Vancouver, Vancouver, this is it 3/8″