On the margins ofВ pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hotВ ashВ and gases. 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. Reinforcement rods in concrete bend in direction of flow. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200вЊ and 700вЊ (390-1300вЏ), can ignite fires and melt snow and ice.Д«uilding remnant in Francisco Leon destroyed by pyroclastic surges and flows during eruption of El Chichon volcano in Mexico 1982. With rock fragments ranging in size fromВ ashВ to boulders that travel across the ground at speeds typically greater than 80 km per hour (50 mph),В pyroclastic flowsknock down, shatter, bury or carry away nearly all objects and structures in their path. Pyroclastic flows destroy nearly everything in their path

Collapse ofВ lavaВ domesВ or flows: The fronts of lava flows or domes can become so steep that they collapse due to gravitational force.

'Boiling over' from eruptiveВ vent: during anВ explosive eruption, material is erupted without forming a highВ plumeВ and rapidly moves down slope.

Collapse ofВ eruption column: during a highlyВ explosive eruption, the column ejected upwards into the atmosphere cools and can become too cool and dense to maintain upward momentum.

Pyroclastic flows form in different ways: MostВ pyroclastic flowsВ 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.В AshВ may fall from this cloud over a wide area downwind from theВ pyroclastic flow. They move at very high speed down volcanic slopes, typically following valleys. This means that the development of pyroclastic flows is controlled not only by the volume of lava and gravitational force, but also by the explosivity related to the pore gases in the lava.Pyroclastic flows contain a high-density mix of hotВ lavaВ blocks,В pumice,В ashВ and volcanic gas. The infrasonic energy is almost the same order as the seismic energy but the ratio of infrasonic to seismic energies increases for larger and more mobile pyroclastic flows. The excitation of infrasonic and seismic signals is affected by the topography of the mountain slope. The speed of pyroclastic flows is estimated as 10-30 m/s from the infrasonic records.

This suggests that the fragmentation of pyroclastics occurs mainly near the front of pyroclastic flows. The migrating source of infrasonic signals and probably seismic signals is inferred to be located near the front of pyroclastic flows by comparison with video images. The location of dome collapse and the path of pyroclastic flows can be identified and traced by a network of low-frequency microphones.

Some of the infrasonic signals show an obvious Doppler effect, indicating that the pyroclastic flows emit infrasonic signals during their propagation. This suggests that the seismic waves are generated by the collision of pyroclastics on the mountain slope and that the infrasonic waves are excited by small fractures of the dome and the fragmentation of pyroclastics. When the lava blocks fall onto the mountain slope and are fragmented, larger waves are excited. Small infrasonic and seismic waves are excited when the lava dome starts to collapse. Characteristic infrasonic and seismic signals were recorded corresponding to the collapse of lava blocks from the dome, the drop of blocks on the slope and the migration of pyroclastic flow on the mountain slope. The process of the collapse of the dacitic lava dome and the development of pyroclastic flows at Unzen volcano, Japan, were studied using infrasonic, seismic and video records.