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