Pyroclastic Flows

Pyroclastic flow, August, 1986, flowing down valley from St. Augustine volcano, Alaska. Photo by Maurice and Katia Krafft.

What is a pyroclastic flow?

Pyroclastic flows are heavier-than-air gas-particle emulsions that travel across the ground at velocities ranging from 10 m/sec to 300 m/sec. They can attain temperatures of over 1000 C. They range from high density flows that move down valleys and can move beneath water, to dilute flows that extend over mountains and can move across water. The term "pyroclastic surge" is a synonym for "dilute pyroclastic flow." As shown by lateral facies transitions in pyroclastic flow deposits, pyroclastic flows and surges commonly develop simultaneously from the same flow and evolve one from the other.

Quarry within pyroclastic deposits of Laacher See volcano, Germany. Pyroclastic flow deposits are the thick, massive beds. The pyroclastic surge deposits are the duned and cross bedded deposits. Field study with Professor Hans-Ulrich Schmincke and students in 1980.

Pyroclastic flows are one kind of sediment gravity flow. Sediment gravity flows are unique fluids because properties such as density and viscosity can change as they move, unlike fluids such as water or air within which density and viscosity change very little, if at all, during movement.

The driving force of a sediment gravity flow is the action of gravity upon the particles. Sediment motion moves the interstitial fluid (gas or liquid mixed with fine particles), and together they constitute the fluid mass. Particles with different settling velocities, however, behave differently within the mass. Very small particles tend to couple with the gas phase because of drag effects (Sparks, 1976), thereby greatly increasing the carrying capacity of the current. This particulate-gas-phase matrix may move in turbulent flow lines and entrain larger fragments, though fragments with the higher settling velocities tend to migrate toward the base of the flow thereby increasing fragment concentration and causing an increase in bulk density and an increase in effective viscosity (Fisher, 1966).

Origins of Pyroclastic Sediment Gravity flows

Pyroclastic flows and surges form in several ways: (1) gravitational collapse of a vertical eruption column (Sparks et al., 1978), (2) the "boiling-over" of a highly gas-charged magma from a crater (Taylor, 1958), (3) inclined blasts from the base of an emerging spine or dome (Lacroix, 1904), (4) lateral blasts following release of pressure caused by collapse of part of a volcano edifice (Bogoyavlenskaya et al., 1985; Siebert et al., 1987) (5) collapse of a growing dome (Mellors et al., 1988), (6) ash fountaining (Hoblitt, 1986), and (6) collapse from the front of a lava flow (Rose et al., 1977).

The collapse of vertical eruption columns to form pyroclastic flows was recognized at the 1929 eruption of Komagatake, Japan (Kozu,1934) and postulated from sedimentological data at St. Vincent, B.W.I. by Hay (1959). Using development of a base surge from a 1947 nuclear explosion at Bikini Atoll (south Pacific) as a model, column collapse ("bulk subsidence") was suggested as a cause of pyroclastic flows leading to development of ignimbrites by Fisher (1966), and the process of column collapse was described from a series of photographs showing the development of a surge at Capelinhos (Azores) (Waters and Fisher, 1971). The connection between column collapse and the origin of pyroclastic flow and surge deposits was quantitatively established by Sparks and Wilson (1976) and Sparks et al. (1978). Some pyroclastic flows and surges, however, originate without accompanying vertical eruption columns. For example, at Mount St. Helens on 22 July and 7 August, 1980, pyroclastic density currents began as fountains of gases and pyroclasts around the vent prior to development of a vertical eruption column.

Discovery of Pyroclastic Flows

One of the earliest photographs of a nuée ardente. Photograph taken at Mont Pelée, Martinique, on 16 December 1902 by A. Lacroix.

Volcanologists first became aware of pyroclastic flows in 1902 at Mont Pelée Martinique where 29,000 people died. LaCroix (1904) applied the name nuée ardente to the flows, pointing out the co-existence of a basal "glowing avalanche" (higher density basal flow) and a dilute upper lower density "glowing" cloud. The flow separated during flowage, the dense basal part forming a rheological mass that became essentially non-turbulent and followed the valley of Rivière Blanche, while the upper dilute part, being depleted in fragment population, remained turbulent and spread out fan-shaped across the terraces and hills at the foot of the volcano to overwhelm the town of St. Pierre.

Dense pyroclastic flows follow valleys; dilute pyroclastic flows are able to cross valleys, hills and mountains. They are able to surmount high mountain ranges, as illustrated by the Campanian Ignimbrite located in the Naples region of Italy.

Transport by Sediment Gravity Flow and Consequences

Particles within sediment gravity flows are supported by (1) turbulence, (2) fluids moving between particles (pressure variations; fluidization processes), (3) particle collisions, and (4) the strength of the fluid mass (Middleton and Hampton, 1976).

The flow dynamics within a single sediment gravity flow can change during movement by gravity separations of particles that continuously change particle concentration and therefore the density and viscosity properties. As a result of changing flow regime, sediment gravity flows exhibit (1) "flow transformations" where changes in the ratio of fluids to granular materials (density, viscosity) change turbulent to laminar flow behavior (Fisher, 1983, 1984), (2) "density stratification" (Valentine, 1987) because of gravitational settling, and (3) "decoupling" of a current into more than one current, each with different flow properties (Fisher, 1990, 1995).

As a pyroclastic flow moves along the ground, gravity pulls at each particle, causing larger, heavier ones to migrate downward faster than smaller, lighter ones despite random collisions with other particles that may jolt them upward or downward even as they move forward with the mass. As the heavier particles filter downward, the upper part of the current becomes increasingly depleted in particles (more dilute). The dilute part of the flow can remain turbulent and maintain particles in suspension and decouple from the lower part. The lower part of the current becomes increasingly dense and less turbulent to non-turbulent, and particles begin to deposit onto an aggrading surface. As particles continue to come to rest on the upward aggrading surface, a layer of ever-increasing thickness is deposited.

Differences in density, velocity and flow regimes between the nonturbulent and turbulent parts of a pyroclastic flow can result in a boundary separation between them. The upper part of a current can shear across and separate (decouple) from the lower part or vice versa. The two parts are then able to travel separate paths.

Decoupling of of pyroclastic currents into a lower part containing most of the solid fragmental mass (the "glowing avalanche") and an overriding expanding cloud of ash (the "glowing cloud"), together known as a nuée ardente, has been witnessed or attested by several workers (Anderson and Flett, 1903; LaCroix, 1904; Perret, 1937; Taylor, 1958; Smith, 1960; Davies et al., 1978; Nairn and Self, 1978). Decoupling of a single current can result in formation of pyroclastic flow and pyroclastic surge deposits from the same initial current.

Some Pyroclastic Flows of the 20th Century

Pyroclastic flows have been witnessed at: Soufrière, St. Vincent, 1902; Mont Pelée, 1902-3, 1929-30; Mayon Volcano, Philippines, 1968, 1983 and 1993; Mount St. Helens, Washington, 1980; El Chichón, Mexico, 1983; Kelut, Java, 1983; Mount Pinatubo, Philippines, 1991-93; and Mount Unzen Volcano, Japan, 1991-95. More than 30,000 people have died from direct engulfment by pyroclastic flows since the beginning of this century, most of them at the 1902 Mont Pelée eruption.


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Copyright (C) 1997, by Richard V. Fisher. All rights reserved.