Pyroclastic flows are one of the most dangerous of volcanic phenomenon. They are hurricanes of hot gases and volcanic particles. In the effort to catagorize dangerous volcanoes that have not been observed in eruption, it is important to recognise the deposits that pyroclastic flows produce on and around the source volcanoes.

There are two end-member kinds of pyroclastic sediment gravity flow deposits: (1) pyroclastic flow deposits that are relatively thick, poorly sorted, commonly containing abundant fine-grained ash in the matrix (<1/16 mm; >4 phi), and with crude or no internal bedding, and (2) pyroclastic surge deposits that are relatively thin, better sorted than flow deposits, with or without abundant matrix fines, and well bedded to cross bedded. Surge deposits may occur beneath or on top of pyroclastic flow deposits, or by themselves.

Pyroclastic flow deposits rich in pumice and glass shards are known as ignimbrite. Depending upon emplacement temperature, ignimbrites range from unconsolidated, to cemented by vapor phase minerals, to welded ignimbrites (welded tuff).

Welded Bishop Tuff (ignimbrite) in Owens Gorge north of the town of Bishop, California. Cooling joints in upper cooling unit form stately columns many meters high above the road from which the photograph was taken.

Pyroclastic flow deposits composed of mixtures of non-vesicular to partially or wholly vesicular, fine- to coarse-grained juvenile lithic particles, are known as block-and-ash flow deposits.

Pyroclastic sediment gravity flows can move rapidly for long distances, their deposits generally being much thicker in valleys than on ridges. Deposits from single flows range in volume from less than 0.1 km3 to over 3000 km3. Some pyroclastic flows of large volume are erupted at such high temperatures that they become welded. Structures caused by high temperature are discussed by Smith (1960), Ross and Smith (1961) and Fisher and Schmincke (1984).

Fragmental Components and Grain Size

Components of pyroclastic flow and surge deposits are juvenile pumice, bubble wall shards, phenocrysts and lithics, mixed with various amounts of accessory and accidental lithics and crystals. Large to intermediate volume flows are commonly composed of ash-sized glass shards, phenocrysts and lithic particles that enclose variable amounts of lapilli and blocks of pumice and juvenile to accidental lithic fragments. Small-volume pyroclastic sediment gravity flow deposits may be pumice-rich (e.g., afternoon eruption, 18 May 1980 Mount St. Helens, Wilson and Head, 1981), or primarily lithic if they are derived from domes (Perret, 1937; Mellors et al., 1988) or the collapse of fronts of dacitic lava flows (Rose et al., 1977).

Phenocrysts in pyroclastic sediment gravity flow deposits are generally more broken than those in lavas. Crystal abundance, ranging from near 0 to about 50 percent, is greater in the matrix than in enclosed pumice fragments. This, together with rounding of pumice and abundance of fines (particles <1\16 mm) is evidence that pumice is abraded during flowage. Large volumes of fine-grained ash are elutriated from such flows to form ash shards whose deposits may outdistance those of the flow by >1000 km. Most large-volume pyroclastic flow deposits are calcalkaline dacite to rhyolite, thus phenocrysts include euhedral, doubly terminated quartz, sanidine and plagioclase with minor amphibole, pyroxene, biotite, iron-titanium oxide crystals and minor zircon and sphene. In more alkaline rocks, anorthoclase is the dominant feldspar.

Pyroclastic Flow Deposits

Poor sorting, subtle grading or its absence, and poor or no bedding, characterise pyroclastic flow deposits. Although pyroclastic deposits tend to be massive, graded basal zones, discontinuous trains of large fragments, alternating coarse- to fine-grained layers, orientation of elongate or platy clasts, and color or composition changes may produce a crude layering. Slight differences in size of fragments in different layers give an irregular and indistinct stratification to some deposits. Flat fragments within pyroclastic flow deposits near their basal parts are commonly strongly oriented parallel to depositional surfaces or are imbricated, dipping up-flow (Schmincke et al., 1973; Mimura, 1984).

Pumice fragments tend to show inverse grading, the largest occuring at the top of a flow. Denser lithic fragments concentrate toward the base. Inverse grading of pumice is caused by buoyant rise during flow. Inverse grading of lithic fragments is likely due to shear effects at the lower boundary (Schmincke, 1967; Fisher and Mattinson, 1968; Sparks, 1976) (Fig.11). Maximum sizes of lithic and pumice fragments decrease with distance from source (Kuno et al., 1964).

Multiple grading (and bedding) may develop from separate flows of the same composition repeated at relatively short time intervals (Sparks, 1976), by topographic splitting of the flow front which advances around obstacles and reunites on their opposite side (Fisher, 1990), and mechanical segregation of different fragment sizes due to shearing within a high concentration flow (Fisher and Schmincke, 1984).

Pyroclastic Surge Deposits

Pyroclastic surge deposits are copmpositionally much like pyroclastic flow deposits, the main difference being that surges are commonly richer in crystals and lithics than flows and are better sorted). Pyroclastic surge deposits derived from ignimbrite or deposited during the same ignimbrite-forming eruption are low in lithic fragments (<5%), but those derived from domes may contain over 90% lithics (e.g. blast deposit of 18 May 1980 eruption of Mount St. Helens). Juvenile lithics are usually accompanied in the matrix by broken crystals with adhering matrix derived from the explosive disruption of the neck and dome of the volcano (Fisher and Heiken, 1982).

Pyroclastic surge deposits are thinly to thickly laminated and many have planar but slightly wavy bedded structures. Their most characteristic feature is wavy-, lenticular- or low angle cross bedding (Fisher and Schmincke, 1984), but in many instances they are thin and massive and some closely resemble dunes of medium sized sand (Sigurdsson et al., 1987). Many contain lenses of well-sorted and well-rounded pumice lapilli. In cases where deposits are planar and poor in fines similar to fallout deposits, they are distinguished from fallout because large fragments move into place during flow rather than impact from fall and thus do not form bedding sags.

Pyroclastic surge deposits are better sorted and more enriched in crystal and lithic fragments than pyroclastic flow deposits. Surge deposits have a more restricted grain size than pyroclastic flow deposits, lacking both the very fine and the coarse fractions. Sorting values of surges are intermediate between flows and fallout, but there is wide overlap.

Pyroclastic surges are relatively low-concentration (i.e., highly expanded) density currents with particles supported mainly by turbulence (Sparks, 1976) (rather than gas fluidization) where particle fall velocity is small relative to scale of turbulence. The surge is driven by expanding gas, by momentum of the particles, and by gravity, depending upon slope. Particle concentration increases within the lower part of the surge (i.e., the surge becomes density stratified, Valentine, 1987) and sedimentation takes place in a bedload region where particles may move by saltation and traction or by mass movement as high concentration sediment gravity flows. The bedload region of the surge may become detached from the surge and move independently according to slope direction and, if concentration is high enough, it may transform to a pyroclastic flow that moves down valleys and ponds in low areas (Fisher, 1990).

Pyroclastic Sediment Gravity Flow Facies

The interaction of pyroclastic sediment gravity flows with topographic irregularities (Freundt and Schmincke, 1985, 1986) are two important factors in the development of pyroclastic facies. Pyroclastic surges can override the sides of a valley and their deposits may mantle topography similar to fallout tephra, but unlike fallout tephra they are traceable into thicker pyroclastic flow deposits in valleys. This relationship can be interpreted in two different ways, both of which probably operate. One interpretation is that they are overbank deposits from pyroclastic flows that moved downvalley, overtopping their sides (Schumacher and Schmincke, 1990a), and another interpretation is that they developed from surges or flows that moved across the landscape leaving thin beds on the uplands, draining and coalescing in low places (Crowe and Fisher, 1973; Fisher, 1977, 1990; Hoblitt et al., 1981; Wilson, 1985; Fisher et al., 1987).

Topography profoundly affected distribution patterns, and thickness and grain sizes changes of the 18 May 1980 Mount St. Helens blast surge deposits (Fisher, 1990), indicated by the fact the blast surge travelled twice as far to the west where ridges parallel flow direction than to the east where ridges are at right angles to the flow direction. High ridges perpendicular to the surge were maximum roughness elements that greatly increased frictional drag. Greater velocity on ridge tops and lowered velocity on the lee sides resulted in large differences in thickness patterns. This same effect on a smaller scale occurred with small obstacles such as tree trunks and small gulleys.

To the west, north and northeast at Mount St. Helens, the turbulent top of the blast surge was many times higher than the mountain ridges that it crossed. Where the depositing volcaniclastic sediment was blocked or drained into separate valleys, "rootless" pyroclastic flows formed in valley bottoms and ponded within depressions. If the volume had been many times greater, the deposits in separate valleys would resemble a once-continuous sheet.

The mechanism of crossing mountainous barriers at Mount St. Helens may be applied to pumice-rich pyroclastic flows that are known to have crossed topographic barriers of considerable height. For example, the 22,000 yr B.P. Ito pyroclastic flow (Japan) travelled 70 km over topographic barriers as high as 600 m (Yokoyama, 1974). The Taupo Ignimbrite, although only ~30 km3 in volume, is spread out over a ~20,000 km3 area and mantles mountains up to 1500 m higher than the inferred vent as far as 45 km from the source (Wilson, 1985). The 33.5 ka Campanian ignimbrite, Phlegrean Field (Italy) (Paterne et al., 1988) travelled farther than 60 km from its inferred source near Pozzuoli, covered an area of about 7000 km2 and is 30-40 m thick in lowland areas (Barberi et al., 1978). It moved over many limestone peaks of 700 m and greater elevation, and in one place crops out at 900 m ~50 km from its inferred source. It is suggested that these pyroclastic flows were transported as expanded sediment gravity flows many times thicker than the highest peaks, and that topographic blocking and runoff resulted in pyroclastic flow deposits in valleys.

Calculations by Sparks et al. (1978) suggest that ground-hugging pyroclastic flows with velocities of 100 m/s are capable of overcoming barriers several hundred meters high. According to Sheridan (1979), the slope of the "energy-line" traces the potential flow head from the top of the gas-thrust region of an eruption column to the distal toe of a flow along the line of transport. Theoretically, a pyroclastic flow could surpass all topographic barriers that do not extend above the line. Although calculations permit the speculation that pyroclastic flows can move along the ground over high mountains as continuous, fast-moving, flowing pyroclastic sheets, observations at Mount St. Helens suggest that isolated pockets of pyroclastic flows found in separate basins across mountain ranges may also be emplaced from density stratified turbulent currents.

Origins of Pyroclastic Sediment Gravity Flows

Pyroclastic flows and surges can 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) explosive disruption from the front of a lava flow (Rose et al., 1977) (Fig. 14).

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.

Cited References

Barberi, F., Innocenti, F., Lirer, L. Munno, R. Pescatore, T. & Santacroce, R. (1978) The Campanian ignimbrite: A major prehistoric eruption in the Naples area (Italy). Bull. Volcanol. 41, 10-31.

Bogoyavlenskaya, G.E., Braitseva, O.A., Melekestsev, I.V., Kiriyanof, V.Yu. and Miller, C.D. (1985) Catastrophic eruptions of the directed-blast type at Mount St. Helens, Bezymianny and Shiveluch volcanoes. J. Geodynamics 3, 189-218.

Crowe, B.M. & Fisher, R.V. (1973) Sedimentary structures in base-surge deposits with special reference to cross-bedding, Ubehebe Craters, Death Valley, California. Bull. geol. Soc. Amer. 84, 663-682.

Davies, I.C., Querry, M.W. & Bonis, S.B., (1978) Glowing avalanches from the 1974 eruption of the volcano Fuego, Guatemala. Bull. geol. Soc. Amer. 89, 369-384.

Fisher, R.V. (1966) Mechanism of deposition from pyroclastic flows. Amer. J. Sci. 264, 287-298.

Fisher, R.V. (1977) Erosion by volcanic base-surge density currents: U-shaped channels. Bull. geol. Soc. Amer. 88, 1287-1297.

Fisher, R.V. (1983) Flow transformations in sediment gravity flows. Geology 11, 273-274.

Fisher, R.V. (1990) Transport and deposition of a pyroclastic surge across an area of high relief: The 18 May 1980 eruption of Mount St. Helens, Washington. Bull. geol. Soc. Amer. 102, in press.

Fisher, R.V. & Heiken, G. (1982) Mt. Pele, Martinique:May 8 and 20, 1902 pyroclastic flows and surges. J. Volcanol. geotherm. Res. 13, 339-371.

Fisher, R.V. & Schmincke, H.-U. (1984) Pyroclastic Rocks. Springer-Verlag, Berlin, 472 pp.

Fisher, R.V., Glicken, H.X. & Hoblitt, R.P. (1987) May 18, 1980, Mount St. Helens Deposits in South Coldwater Creek, Washington. J. geophys. Res. 92, 10,267-10,283.

Freundt, A. & Schmincke, H.-U. (1985) Hierarchy of facies of pyroclastic flow deposits generated by Laacher See-type eruptions. Geology 13, 278-281.

Freundt, A. & Schmincke, H.-U. (1986) Emplacement of small-volume pyroclastic flows at Laacher See (East-Eifel, Germany). Bull. Volcanol. 48, 39-59.

Hoblitt, R.P. (1986) Observations of eruptions, July 22 and August 7, 1980, at Mount St. Helens, Washington. US geol. Survey Prof. Paper 1335, 43 pp.

Hoblitt, R.P., Miller, C.D. & Valance, J.W. (1981) Origin and stratigraphy of the deposit produced by the May 18 directed blast. In Lipman, P.WE. and Mullineaux, D.R., eds., The 1980 eruptions of Mount St. Helens, Washington. U.S. geol. Survey Prof. Paper 1250, 401-419.

Kozu, S. (1934) The great activity of Komagatake in 1929. Tschermak's Mineral. Pet. Mitt. 45, 133-174.

Kuno, H., Ishikawa, T., Katsui, Y., Yagi, K., Yamasaki, M. and Taneda, S. (1964) Sorting of pumice and lithic fragments as a key to eruptive and emplacement mechanism. Jap. J. Geol. Geog. 35, 223-238.

Lacroix, A. (1904) La Montagne Pele et ses eruptions. Masson et Cie, Paris, 662 pp.

Lipman, P.W. & Mullineaux, eds. (1981). The 1980 eruptions of Mount St. Helens. U.S. geol. Survey Prof. Paper 1250, 844 pp. Lipman and Mullineaux, 1981

Lowe, D.R. (1982) Sediment gravity flows: II. Depositional models with special reference to the deposits of high density turbidity currents. J. sedimen. Petrol. 52, 279-297.

Mellors, R.A., Waitt, R.B. & Swanson, D.A. (1988) Generation of pyroclastic flows and surges by hot-rock avalanches from the dome of Mount St. Helens volcano, USA. Bull. Volcano. 50, 14-25.

Mimura, K. (1984) Imbrication, flow direction and possible source areas of the pumice-flow tuffs near Bend, Oregon, U.S.A. J. Volcanol. geotherm. Res. 21, 45-60.

Moore, J.G. (1967) Base surge in recent volcanic eruptions. Bull. Volcanol. 30, 337-363.

Paterne, M., Guichard, F. & Labeyrie, J. (1988) Explosive activity of the south Italian volcanoes during the past 80,000 years as determined by marine tephrochronology. J. Volcanol. Geotherm. Res., 34, 153-172.

Perret, F.A. (1937) The eruption of Mt. Pele 1929-1932. Carnegie Inst. Wash. Publ. 458, 126 pp.

Rose, W.I., Jr., Pearson, T. & Bonis, S. (1977) Nuee ardente eruption from the foot of a dacite lava flow, Santiaguito Volcano, Guatemala. Bull. Volcanol. 40, 1-16.

Ross, C.S. & Smith, R.L. (1961) Ash-flow tuffs; their origin, geologic relations and identification. U.S. geol. Survey Prof. Paper 366, 1-77.

Schmincke, H.-U., Fisher, R.V. & Waters, A.C. (1973) Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany. Sedimentology 20, 553-574.

Schumacher, R. & Schmincke, H.-U. (1990a) The lateral facies of ignimbrites at Laacher See volcano. Bull. Volcanol. 52, 271-285.

Sheridan, M.F. (1979) Emplacement of pyroclastic flows: A review. Sp. Paper geol. Soc. Amer. 180, 125-136.

Sheridan, M.F. & Updike, R.G., 1975. Sugarloaf Mountain tephra - - a Pleistocene rhyolitic deposit of base-surge origin. Bull. geol. Soc. Amer., 86, 571-581.

Siebert, L., Glicken, H. and Ui, T. (1987) Volcanic hazards from Bezymianny- and Bandai-type eruptions. Bull. Volcanol. 49, 435-459.

Sigurdsson, H., Carey, S.N. & Fisher, R.V. (1987) The 1982 eruptions of El Chichon volcano, Mexico (3): Physical properties of pyroclastic surges. Bull. Volcanol. 49, 467-488.

Sisson, T.W. (1982) Sedimentary characteristics of the airfall deposit produced by the major pyroclastic surge of May 18, 1980 at Mount St. Helens, Washington. Univ. Calif. Santa Barbara, M.A. Diss., 1-145.

Smith, R.L. (1960) Ash flows. Bull. geol. Soc. Amer. 71, 795-842.

Sparks, R.S.J. (1976) Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology 23, 147-188.

Sparks, R.S.J. & Wilson, L. (1976) A model for the formation of ignimbrite by gravitational column collapse. J. geol. Soc. London 132, 441-451.

Sparks, R.S.J. (1986) The dimensions and dynamics of eruption columns. Bull. Volcanol. 48, 3-15.

Sparks, R.S.J., Wilson, L. & Hulme, G. (1978) Theoretical modeling of the generation movement and emplacement of pyroclastic flows by column collapse. J. geophys. Res. 83, 1727-1739.

Suzuki, T., Katsui, Y. & Nakamura, T. (1973) Size distribution of the Tarumai Ta-b pumice-fall deposit. Bull. Volcanol. Soc. Jap. 18, 47-64.

Suzuki-Kamata, K. (1988) The ground layer of Ata pyroclastic flow deposits, southwestern Japan--evidence for the capture of lithic fragments. Bull. Volcanol. 50, 119-129.

Taylor, G.A. (1958) The 1951 eruption of Mount Lamington, Papua. Austr. Bur. Min. Resour. Geol. Geophys. Bull. 38, 1-117.

Tokunaga, T. and Yokoyama, S (1979) Mode of eruption and volcanic history of Jukaiyama Volcano, Nii-Jima. Geog. Rev. Jap. 52 111-125. (In Japanese)

Turbeville, B.N., Waresback, D.B. & Self, S. (1989) Lava-dome growth and explosive volcanism in the Jemez Mountains, New Mexico Evidence from the Plio-Pleistocene Puye alluvial fan. J. Volcanol. Geotherm. Res., 36, 267-291.

Valentine, G.A. (1987) Stratified flow in pyroclastic surges. Bull. Volcanol. 49, 616-630.

Valentine, G.A. & Wohletz, K.H. (1989) Numerical models of Plinian eruption columns and pyroclastic flows. J. geophys. Res. 94, 1867-1887.

Walker, G.P.L. (1984) Characteristics of dune-bedded pyroclastic surge bedsets. J. Volcanol. Geotherm. Res. 20, 281-296.

Waters, A.C. & Fisher, R.V. (1971) Base surges and their deposits: Capelinhos and Taal Volcanoes. J. geophys. Res. 76, 5596-5614.

Wilson, C.J.N. (1980) The role of fluidization in the emplacement of pyroclastic flows: an experimental approach. J. Volcanol. Geotherm. Res., 8, 231-249.

Wilson, C.J.N. (1984) The role of fluidization in the emplacement of pyroclastic flows 2: Experimental results and their interpretation. J. Volcanol. Geotherm. Res. 20, 55-84.

Wilson, C.J.N. (1985) The Taupo eruption, New Zealand II. The Taupo ignimbrite. Phil. Trans. R. Soc. Lond. A 314, 229-310.

Wilson, L. & Head, J.W., III (1981) Ascent and eruption of basaltic magma on the earth and moon. J. geophys. Res. 86, 2971-3001.

Wilson, L. & Walker, G.P.L., 1987. Explosive volcanic eruptions-VI. Ejecta dispersal in plinian eruptions: the control of eruption conditions and atmospheric properties. Geophys. J. R. astr. Soc. 89, 657-679.

Wilson, L., Sparks, R.S.J. & Walker, G.P.L. (1980) Explosive volcanic eruptions, IV. The control of magma properties and conduit geometry on eruption column behavior. Geophys. J. Roy. Astron. Soc. 63, 117-148.

Wilson, L., Sparks, R.S.J., Huang, T.C. & Watkins, N.D. (1978) The control of volcanic column eruption heights by eruption energetics and dynamics. J. Geophys. Res. 83, 1829-1836.

Wright, J.V. (1979) Formation, transport and deposition of ignimbrites and welded tuffs. Imperial College, London, Ph.D. diss., 451 pp.

Yokoyama, S. (1974) Mode of movement and emplacement of Ito pyroclastic flow from Aira Caldera, Japan. Sci. Reports of the Tokyo Kyoiku Daigaku, Sec. C (Geography, Geology and Mineralogy) 12, 17-62.

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