DESCRIPTION:
Pyroclastic Flows and Pyroclastic Surges

MSH80_pyroclastic_flow_from_st_helens_crater_08-07-80.jpg
During the May 18, 1980 eruption, at least 17 separate pyroclastic flows descended the flanks of Mount St. Helens. Pyroclastic flows typically move at speeds of over 60 miles per hour (100 kilometers/hour) and reach temperatures of over 800 Degrees Fahrenheit (400 degrees Celsius). Photographed here, a pyroclastic flow from the August 7, 1980 eruption stretches from Mount St. Helens' crater to the valley floor below.
USGS Photograph taken on August 7, 1980, by Peter W. Lipman.
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Pyroclastic Flow: High-speed avalanches of hot ash, rock fragments, and gas move down the sides of a volcano during explosive eruptions or when the steep edge of a dome breaks apart and collapses. These pyroclastic flows, which can reach 1500 degrees F and move at 100-150 miles per hour, are capable of knocking down and burning everything in their paths.

Pyroclastic Surge: A more energetic and dilute mixture of searing gas and rock fragments is called a pyroclastic surge. Surges move easily up and over ridges; flows tend to follow valleys.

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 Pelée (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.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

Pyroclastic flow: Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles and hour). The term also can refer to the deposit so formed.

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from their source vents at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both, or from the collapse of vertical eruption columns of ash and larger rock fragments. Pyroclastic flows may also result from a laterally directed explosion, or the fall of hot rock debris from a dome or thick lava flow.

Rock fragments in pyroclastic flows range widely in grain size and consist of dense rock, pumice, or both. Individual pyroclastic flows, worldwide, range in length from less than one to more than 200 kilometers, cover areas from less than one to more than 20,000 square kilometers, and have volumes from less than 0.001 to more than 1000 cubic kilometers. Pumiceous pyroclastic flows with volumes of 1-10 cubic kilometers can reach distances of several tens of kilometers from a vent and travel downslope at speeds of 50 to more than 150 kilometers per hour, their velocity depending largely on their volume and on the steepness of slopes over which they travel. Pyroclastic flows and their deposits commonly contain rock debris and gases with temperatures of several hundred degrees Celsius. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of finer particles (ash cloud) that rises above the basal flow. Ash may fall from the cloud over a wide area downwind from the basal flow.

Pyroclastic surges are turbulent, low-density clouds of rock debris and air or other gases that move over the ground surface at high speeds. They typically hug the ground and depending on their density and speed, may or may not be controlled by the underlying topography. Pyroclastic surges are of two types: "hot" pyroclastic surges that consist of "dry" clouds of rock debris and gases that have temperatures appreciably above 100 degrees C, and "cold" pyroclastic surges, also called base surges, that consist of rock debris and steam or water at or below a temperature of 100 degrees C.

Both hot and cold pyroclastic surges damage or destroy structures and vegetation by impact of rock fragments moving at high speeds and may bury the ground surface with a layer of ash and coarser debris tens of centimeters or more thick. Because of their high temperatures, hot pyroclastic surges may start fires and kill or burn people and animals. Both types of surges can extend as far as 10 km from their source vents and devastate life and property within their paths. During an eruption of Mont Pelee on Martinique in 1902, a cloud of hot ash and gases swept into the town of St. Pierre at an estimated speed of 160 kilometers per hour or more. About 30,000 people died within minutes, most from inhalation of hot ash and gases. Pyroclastic surges have occurred at volcanoes in the Cascade Range in the past and can be expected to occur again. Future cold surges (base surges) are most likely to occur where magma can contact water at volcanic vents near lakes, those that have crater lakes, and at vents in areas with a shallow water table.

Pyroclastic flows and pyroclastic surges: Mixtures of hot rock fragments and gases can sweep away from their source vents at hurricane velocity. Pyroclastic flows are dense and most are confined to valleys around a volcano; the largest ones can travel tens or even hundreds of kilometers beyond a volcano. Pyroclastic surges are turbulent, low-density variants of pyroclastic flows. Some unusually rapid pyroclastic flows or surges originate from laterally directed explosions from a vent. Because of their high speed and high temperature, pyroclastic flows and surges kill or destroy virtually all that is in their path.

Base Surge: Turbulent, low-density cloud of rock debris and water and (or) steam that moves over the ground surface at high speed. Base surges are generated by explosions.

Mount Pelée in Martinique, West Indies, and Lassen Peak and Mono domes in California are examples of lava domes. An extremely destructive eruption accompanied the growth of a dome at Mount Pelée in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust. Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely through the burning city.

Mont Pelée Menu

The June 1912 eruption of Novarupta Volcano altered the Katmai area dramatically. Severe earthquakes rocked the area for a week before Novarupta exploded with cataclysmic force. Enormous quantities of hot, glowing pumice and ash were ejected from Novarupta and nearby fissures. This material flowed over the terrain, destroying all life in its path. Trees up slope were snapped off and carbonized by the blasts of hot wind and gas. For several days ash, pumice, and gas were ejected and a haze darkened the sky over most of the Northern Hemisphere. When it was over, more than 40 square miles of lush green land lay buried beneath volcanic deposits as much as 700 feet deep. At nearby Kodiak, for two days a person could not see a lantern held at arm's length. Acid rain caused clothes to disintegrate on clotheslines in distant Vancouver, Canada. The eruption was ten times more forceful than the 1980 eruption of Mount St. Helens. In the valleys of Knife Creek and the Ukak River, innumerable small holes and cracks developed in the volcanic ash deposits, permitting gas and steam from the heated groundwater to escape. It was an apparently unnamed valley whehn the 20th century's most dramatic volcanic episode took place. Robert Griggs, exploring the volcano's aftermath for the National Geographic Society in 1916, stared awestruck off Katmai Pass across the valley's roaring landscape riddled by thousands of steam vents. The Valley of Ten Thousand Smokes, Griggs named. it.

1912 Novarupta Eruption

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