One of the most deadly aspects of a volcano is the pyroclastic flow. This current of fast moving rock and molten lava can easily reach temperatures of over 1,800 degrees and travel at speeds over 100 km per hour. Contained within the fiery streams are toxic gases which maintain the liquefied state of the flow.
Apart from the initial dangers of the flow to anything or anyone in its path, it also produces damage to the environment. The hot debris that is left may generate forest fires, and the accumulation of lava and debris in waterways can cause damming, and consequently, flooding. The eruption of Krakatoa in 1883, not only killed over 40,000 people, the resulting flow traveled across water, reaching Sumatra, some thirty miles away. The extremely rare, caldera generated flow, actually has the capability to fill in entire valleys within hours. The last one of this magnitude happened 74,000 years ago in Indonesia.
The two varieties of pyroclastic flows are determined by the initial cause of the eruption, and what materials the flow contains. The nuee ardentes , or “glowing cloud” flow actually produces no cloud, but an avalanche of fiery material encased in ash particles, that gives it that appearance. French geologist Alfred LaCroix first named this type of flow that is produced from the collapse of the dome of the volcano. Examples of this in recent history was the eruption of Mt. Pelee in 1902 and the eruptions of Mt. Unzen in Japan between the years between 1991-1995.
Pumice flows are the faster of the two flows, produced by the collapse of the volcano tower. The speed of the resulting flow is increased by the fact that the it begins at a higher altitude, and as it moves downward, picks up more energy the further it travels. This flow, that is composed of ash matrix and pumice, gains strength and speed as it pushes particles of debris ahead of it, developing more fluidity. One volcanic eruption, producing pumice flows, can send flows down all sides of the volcano.
Ignimbrites are primarily pumice deposits from a pyroclastic flow. These are normally around fist size, and may harden immediately when the flow cools to produce an unwelded flow that contains the texture of the various pumice deposits, or a welded flow, that is smooth due to the heat and the weight of the materials.
Pyroclastic flows are one of the main hazards involved in a volcanic eruption. They are a force of nature that is unpredictable and virtually impossible to control.
"How Volcanoes Work - Pyroclastic Flows." SDSU - Department of Geological Sciences. Web. 20 Feb. 2011. <http://www.geology.sdsu.edu/how_volcanoes_work/Pyroflows.html>.