The term "pyroclastic''--derived from the Greek words pyro (fire) and klastos (broken)--describes materials formed by the fragmentation of magma and rock by explosive volcanic activity. Most volcanic ash is basically fine-grained pyroclastic material composed of tiny particles of explosively disintegrated old volcanic rock or new magma. Larger sized pyroclastic fragments are called lapilli, blocks, or bombs. Pyroclastic flows--sometimes called nuees ardentes (French for "glowing clouds")--are hot, often incandescent mixtures of volcanic fragments and gases that sweep along close to the ground. Depending on the volume of material, proportion of solids to gas, temperature, and slope gradient, the flows can travel at velocities as great as 450 miles an hour. Pyroclastic flows can be extremely destructive and deadly because of their high temperature and mobility. During the 1902 eruption of Mont Pelee (Martinique, West Indies), for example, a nuee ardente demolished the coastal city of St. Pierre, killing nearly 30,000 inhabitants.
Pyroclastic flows commonly are produced either by the fallback and downslope movement of fragments from an eruption column or by the direct frothing over at the vent of magma undergoing rapid gas loss. Volcanic froth so formed is called pumice. Pyroclastic flows originated in both ways at Mount St. Helens on May 18, but flows of mappable volume were of the latter type. The flows were entirely restricted to a small fan-shaped zone that flares northward from the summit crater.
Explosion pits were formed by "secondary" eruptions when the hot volcanic debris came into contact with water or moist ground. This picture also shows an eruption in progress (lower center) (Photograph by Daniel Dzurisin).
Pyroclastic flows were first directly observed shortly after noon, although they probably began to form a short time after the lateral blast. They continued to occur intermittently during the next 5 hours of strong eruptive activity. Eyewitness accounts indicated that the more voluminous pyroclastic flows originated by the upwelling of volcanic ejecta to heights below the rim of the crater, followed by lateral flow northward through the breach of the crater. One scientist likened this process to a "pot of oatmeal boiling over." Most of the rock in these flows was pumice. A few smaller pyroclastic flows were observed to form by gravitational collapse of parts of the high eruption column. The successive outpourings of pyroclastic material consisted mainly of new magmatic debris rather than fragments of preexisting volcanic rocks. The resulting deposits formed a fan-like pattern of overlapping sheets, tongues, and lobes. At least 17 separate pyroclastic flows occurred during the May 18 eruption, and their aggregate volume was about 0.05 cubic mile.
When temperature measurements could safely be made in the pyroclastic flows 2 weeks after they were erupted, the deposits ranged in temperature from about 570° to 785°F. As might be expected, when the hot material of the debris avalanche and the even hotter pyroclastic flows encountered bodies of water or moist ground, the water flashed explosively to steam; the resulting phreatic (steam-blast) explosions sent plumes of ash and steam as high as 1.2 miles above the ground. These "secondary" or "rootless" steam-blast eruptions formed many explosion pits on the northern margin of the pyroclastic flow deposits, at the south shore of Spirit Lake, and along the upper part of the North Fork of the Toutle River. These steam-blast explosions continued sporadically for weeks or months after the emplacement of pyroclastic flows, and at least one occurred about a year later, on May 16, 1981.
URL: <https://pubs.usgs.gov/gip/msh/pyroclastic.html>
Contact: John Watson
Last updated: 06.25.97
Ash eruption and fallout