The role of fluidization in the emplacement of pyroclastic claws: An experimental approach

231

Journal of Volcanology and Geothermal Research, 8 (1980) 231-249 Elsevier Scientific Publishing Company, Amsterdam - Printed in Belgium

THE ROLE OF FLUIDIZATION IN THE EMPLACEMENT CLASTIC FLOWS: AN EXPERIMENTAL APPROACH

OF PYRO-

C.J.N. WILSON Geology Depkrtment, (Receive?

Imperial College, London SW7 2BP (Great Britain)

May 2, 1979; revised and accepted

March 6, 1980)

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

flows:

A series of experiments was carried out to review the process of fluidization for a number of particulate materials having various sorting and grain shape characteristics. The up (increasing gas velocity) curve on a gas velocity/bed-pressure drop plot for a poorly sorted mixture of irregularly shaped particles is divided into three sections; non-expanded, expanded, and segregating. These sections are used to define a threefold genetic classification of pyroclastic flows which can be directly linked to conditions within a semifluidized parent flow. Type 1 flows are ungraded and mostly result from hot-avalanche flows and pyroclastic flows formed of relatively dense material. Type 2 and type 3 flows are mainly pumiceous ignimbrites and are distinguished by expansion induced coarse-tail grading, coupled with segregation structures in the latter. The implications of this classification are discussed with reference to the flow regimes, deposition slope angles, crystal concentration and “fossil fumaroles” variously developed in the flows. The relevance of the source of fluidizing gas is discussed in relation to the zoning of flow types within a single flow unit and it is shown how this, and its attendant structures, can be used to estimate the relative importance of each gas source during and after flow emplacement.

INTRODUCTION

Pyroclastic flow eruptions are among the most violent forms of explosive volcanism. Because of their size and violence, pyroclastic flows are difficult to observe directly, and most of what can be known about them must be discovered from the resulting deposits. This particularly applies to those flows which are composed mainly of pumice, and form the rocks known as ash-flow tuffs or ignimbrites. Workers such as Smith (1960), Ross and Smith (1961), Sparks et al. (1973) and Sparks (1976) have developed a model of a pyro~last~~~ flow as a relatively dense, topographically controlled, poorly sorted, hot particulate mixture. Each flow is emplaced to form a single flow unit; commonly, a succession of flows form in an eruption and pro flow unit deposit (see Smith, 1960). Small, historic pyroclastic flows were observed to be a~~~rn~~~~e~ by an 0377-0273/80/0000-0000/$02.25

01980

Elsevier

Scientific

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overriding, voluminous, gas-rich dilute cloud, and often preceded by a hot or cold gas blast or surge. Sparks et al. (1973) and Sparks (1976) proposed that a sequence of rock types seen in numerous prehistoric pyroclastic flows could be correlated with these three “phases” of observed pyroclastic flows, and erected a “standard ignimbrite flow unit”. In this standard, unit, a lithic and crystal-rich horizon (layer 1) occurs at the base, being considered to be the deposit of the preceding surge. This is overlain by the flow itself, forming what is termed layer 2, and this by the deposit of the overriding dilute cloud, forming layer 3. The layer 2 is usually divisible into two parts, a layer 2a at the base, which is interpreted as the boundary layer to the flow, and a layer 2b which forms the bulk of the flow. Sparks (1976) postulated that pyro-. elastic flows (i.e. layer 2) are emplaced as partly fluidized bodies, in which large clasts are dispersed in a matrix of fluidized fine-grained material. The finest material in the flow is carried off by the gas flow to form the dilute cloud, and contribute towards the layer 3 deposit (Sparks and Walker, 1977). The variations in the appearance of layer 2 in the examples described by Sparks et al. (1973) and Sparks (1976) prompted the author to investigate experimentahy what role varying degrees of fluidization might have on the structures seen in prehistoric pyroclastic flows. This paper presents the results of this investigation and the theoretical deductions that can be made from them. Work in progress by the author is relating these deductions to a field example, the 1850-year-old Taupo Ignimbrite in New Zealand. Although most of the examples quoted are ignimbrites (i.e. the juvenile component is pumiceous), it is believed that the ideas presented here are applicable to all pyroclastic flows. The paper is in three parts. The first deals with the process of fluidization, its application to natural materials and the results of the fluidization experiments. The second relates the results to a proposed pyroclastic flow classification, and the third deals with a consideration of the effects of various gas sources. FLUIDIZATION

Fluidization can best be described (e.g. Davidson and Harrison, 1963, p. 1) asthecondition attained when a fluid (liquid or gas) is passed upwards through a bed of cohesionless particulate solids in which, at a certain superficial fluid velocity (Umf), the drag force across the bed exerted by the fluid is equal to the buoyant weight of the bed. The superficial fluid velocity (U) is obtained by dividing the volumetric~ flow rate of the fluidizing fluid by the cross-section area of the empty bed. At U > Umf, the behaviour of the system varies with the solid/fluid density ratio. Where the density ratio is low (e.g.~glass spheres in water), the bed expands evenly to accommodate the increased fluid flow. Where the density-ratio is high (e*g. glass spheres in air), the extra fluid is passed trough the bed in the form of bubbles. These bubbles are responsible for many of the mixing and gas an eat exchange phen ena which are

233

exploited in industrial applications. Normally Umf is measured from a fluidization plot of U versus APIH (the pressure drop across the bed per unit thickness of bed). This paper is only concerned with a gaseous fluidizing medium. The author’s experiments were carried out, using standard chemical engineering equipment and techniqu-es, in either a “2D” bed measuring 29.2 X 133 cm in cross-section by 100 cm high, using a cloth distributor, or a “3D” bed measuring 14 cm in inside diameter by 200 cm high, using a sintered ceramic distributor (Fig. 1). Both beds are constructed from “Perspex”, allowing observationof the bed during runs. The gas used was air at room temperature and pressure, regulated using rotameters, while the pressure drop across the bed was/measured using a water manometer.

Fig. 1. Schematic diagram of the fluidization rig. A high-pressure air supply (A) is regulated with valves ( V), through rotameters (R) and the distributor (D) into the fluidized bed (8%). During runs, the height of the bed (H) is recorded, together with the pressure drop (AP) on a water manometer ( WM).

Using narrow grain size cuts of ideally smooth and spherical particles, a fluidization plot shows two straight lines which intersect at, and define, Umf (Fig. 2a). The slope of the straight line for U G Umf can be predicted by reference to several published correlations (cf. Zenz and Othmer, 1960, Chapter 5), and this information has been used by other authors to obtain correlations of Umf versus particle and fluid characteristics, for example, the modified Ergun equation (Kunii and Levenspiel, 1969, p. 73; Sparks, 1976, p. 171). More commonly, some degree of hysteresis is evident between the up (increasing U) and down (decreasing U) curves on the fluidization plot. By fluidizing various materials, this hysteresis can be related to three main factors; changes in voidage during fluidization, the existence of a wide grain size distribution and an irregular particle shape. The first affects the curves mainly boded and below Vmf (Fig. 2b). The flu~d~zatio~ event between the up and down curves increases the voidage and this cause the lower AP/H values in the packed bed (U < CJTPlf)regime. The voidage &an e is a~~~~t~ated by v *

234

particle shapes, or density variations, or by introducing a spread of grain sizes. The effects of a wide grain size variation are seen mainly around and above “ Umf” (Fig. 2~). In such cases only a portion of the bed is fluidized when the bed weight is supported and the coarser fraction segregates out, accompanied by a drop in APIH. Only at higher gas velocities does the mixing caused by bubbling counteract the segregation; under these conditions, stable, full bed support is attained and the bed is fully fluidized. APIH

API H 160. /*<~-*-0-._,. 120. .e

'o/

./p

80.

(b)

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0' L.0

8.0

mf

U

12.0

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120

-.-0-e----0-*--0-

80

cc>

40 "Urn;'

FS

I 2.0

4.0

10.0

U 12.0

236

Fig. 2. Ftuidization plots for several materials. Gas velocities are in cm s”, AP/H in cm of water per metre of bed thickness. Filled circles define the up curve, circles the down curve. See text for the definitions of U, Umi, U,,, and AP/H. (a) Glass spheres, Md@ = 1.40, o 0.22. (b) Quartz sand, MdG = 1.68, o@ = 0.!2. (c) Glass spheres, Mdg = 2.29, ~6 = 0.89.’ FS marks the gas velocity at which full bed support was achieved. (d) Quartz sand, MdG = 1.28, CT@= 1.55.

With materials having both a wide grain size variation and an irregular shape, a plot is obtained (Fig. 2d) showing gross hysteresis. The grain size variation is so large that the material can never be fully fluidized in the strict sense; before the largest particles are supported, the smallest particles are being lost by elutriation. The boundary between materials (of uniform density and ideal shape characteristics) that can and cannot be fully fluidized is estimated thus. The maximum for Ut/lJmf (where Ut is the gas velocity that is equal to the terminal fall velocity for a single particle) for a given particle is about 100 (Kunii and Levenspiel, 1969, p. 73). Because Umf is proportional to the square of the particle size in most correlations (cf. Davidson and Harrison, 1963, p. 14; Kunii and Levenspiel, 1969, p. 73), then the ratio of particle diameters when U = Umf (larger particle) = Ut (smaller particle) is about 10. In the author’s experiments with sand mixes, elutriation of fines was first marked when Ut (calculated using the equations of Kunii and Levenspiel, 1969) for the finest 5% of the grain size distribution of the sample was exceeded. To be capable of full fluidization, the remaining 95% of the grain size population must not then exceed a grain size 10 times that of the coarsest particles in the 5% finest fraction of the sample, If a lognormal grain size distribution is assumed, then the boundary between materials that can and cannot be fully fluidized lies at a value of uG = 1. The Md, versus u@ (grain size parameters of Inman, 1952) plots for pyroclastic flows (Murai, 1961; Walker, 1971; Sparks, 1976) show that all examples yet published have a$ > 1.0. One important feature of fluidized systems, relevant to pyroclastic flows, is their rheology. Data in the chemical engineering literature (e.g. Schiigerl et al., 1961; Botterill and Bessant, 1976, and references therein) deal almost

236

exclusively with well-sorted systems ((I~< 1) at high gas velocities (U > Umf ). They show that, under such conditions, fluidized systems show a non-linear (i.e. non-newtonian) stress/strain rate behaviour, with a negligible yield strength(the term yield strength is used to denote the minimum stress required to cause strain in the material). For the purposes of this paper, the rheology of fluidized systems at U< Umf is considered as follows. At rest (U = 0), the bed behaves as a particulate material which, at a given depth in the bed, can support a certain differential stress before failure occurs, when the yield strength is exceeded. The yield strength (S,) will increase with depth as: So = I.lpd, (1) where P = tangent of the internal angle of friction, p = buoyant bulk density of the material, and d = depth in the bed. In a partly fluidized bed (0 < U d Umf),the passage of the gas results in the pressure drop across the bed, which reduces the yield strength at a given depth in the same way as pore fluid pressures acting in a soil. At U = Um~,the yield strength is effectively zero, whilst from O_< U < Umf the yield strength in general (S,) varies as: 8~ = (1 - U/Umf) So = (3. - U/Umf) ppd

(2)

(3) In materials with cr6 < 1, once full bed support is achieved, the rheology is probably similar to published industrial examples. For materials with o9 > 1, their rheology at high gas-flow rates is more complex as stable, full bed-support is never attained, that is, U/Umf inequations (2) and (3) is less than unity. From the author’s experiments on ignimbrite, and sand mixes, with ad, > 1, a distinctive picture has emerged of their fluidization behaviour which is used below as the basis for the proposed classification of pyroclastic flows. A typical up curve for such materials is shown in Fig, 3; the:up curve is used to represent the relatively random structure being modelled. The curve_is divisible into three sections; Section 1, where 0 < U < Uie(where Vieis defined here as the gas velocity at the onset of bed expansion), is where no expansion of the bed has occurred and the loss of fines by elutriation, except from the very surface layer of the bed, hisabsent due to the inability of fines to move through the packed bed structure. Section 2, where Uie< U < Ump (where Ump is defined here as the gas velocity at the maximum pressure drop that can be tolerated across the bed), is where, although superficially similar to section 1, the bed has partly fluidized and expanded to accommodate the gas flow; however, elutriation of fines is still minor in effect. Section 3, where U > Ump, is where the pressure drop across the bed exceeds the buoyant weight of the bed divided by the bed cross-sectional area; an instability sets in, and part of the gas flow is concentiated into discrete

237 API H

“ie”rnp 2.0

U 4.0

60

8.0

10.0

12.0

Fig. 3. Fluidization up-curve plot for ignimbrite fines (MdG = 1.22, CJ@= 1.75). See text for definition of Ui,; other symbols as in Fig. 2.

channels. This leads to a sharp reduction in AP/H which is accompanied by bubbling, the deposition of coarse/dense material in the channels (Fig. 4a), and the elutriation of fines, If pre-existing zones of higher permeability are present in the bed, the value of APIH at Ump may be less than equivalent to the buoyant weight of the bed. At higher gas velocities, bubble-induced circulation inhibits the formation of vertical channels, and the segregating material forms pods and layers (Fig. 4b) which gradually accumulate at the top and/or bottom of the bed (Fig. 4~). Fine/light particles migrate to the top of the bed, while coarse/dense particles sink to the base; in extreme cases the bed may become completely gas sorted. At the top of the bed, the segregation layer is finer grained, better sorted and light-material enriched compared with the bulk of the bed, and could in the field superficially resemble the layer 3 deposits of Sparks et al. (1973) and Sparks and Walker (1977). In materials having these fluidization properties, Ump is used instead of Umf the latter term now being meaningless. In such poorly sorted systems, it can be seen that the formation of segregation pods and channels has a marked effect on the rheology of the system at U > Ump. The diversion of much of the gas flow through the better sorted segregation channels means that the bed as a whole has a higher permeability, resulting in the continuing presence of a yield strength. The rheology of such a material in a pyroclastic flow is thus liable to be very complex; the author knows of no available published data on the subject. The coarse/dense material segregation features themselves are relatively well sorted and very permeable and thus have a high yield strength because U/Ump in them is low. This makes them very resistant to mechanical mixing and under laboratory conditions they are impossible to destroy. The fine/light material segregation features are difficult to because their lower density carries them to the bed surface and inhibits their r~~rnixi~~g.

238 .:

/.

-_ , i

j_.

i_ ,,,

,-

239

The preservation of structures characteristic of a higher degree of fluidization as the gas velocities decrease is a vital assumption in the following pyroelastic flow classification. Work by the author to test the ideas in this paper on some New Zealand ignimbrites has convinced him that this assumption is valid. A CLASSIFICATION

OF PYROCLASTIC

FLOWS

It is here proposed that the fluidization behaviour of all pyroclastic flows is typified by a fluidization plot similar to that in Fig. 3. The terms type 1, type 2Land type 3 are used to relate a pyroclastic flow to the corresponding section on the fluidization plot. Type 1 pyroclastic flows are defined as those in which 0 < U < Uie. They are characterised by the lack of expansion and the high yield strength which prevents any gravitation-induced grading, even of the largest clasts, and may also hinder the formation of a basal layer (layer 2a of Sparks et al., 1973). The low degree of fluidization means that other flow mechanisms must be considered in their emplacement (see, for example, Nairn and Self, 1978). By analogy with sedimentary high-concentration dispersions, some morphological features should be present on such flows. These are represented by the surface longitudinal and transverse ridges, block trains, levees and channels, and lobate flow fronts variously displayed by, for example, the hot-avalanche deposits of San Pedro (Francis et al., 1974), the “semi-inflated clast” Agatsuma pyroclastic flow of Asama (Aramaki, 1956), recent flows of Ngauruhoe (Nairn and Self, 1978) and the 1929 pumice flows of Komagatake (Kuno, 1941). From these examples it is evident that the morphology and structure of type 1 deposits need not differ from those of mudflows and debris flows and may be indistinguishable from them. Although deposits of this type, like mudflows, do sometimes show coarse-tail grading, the implication of this work is that this grading is caused by other processes, for example, by grain dispersive interactions (Johnson, 1970). Type 2 flows are defined as those in which Vi, < U G vmp. Although still possessing a yield strength, the expansion allows the distinguishing feature of coarse-tail grading (as described by Sparks, 1976) to develop. This may take the form of normal or reverse pumice grading (depending on the density relationship of clasts to matrix) and/or normal lithic grading. The common presence of a well-developed basal layer (layer 2a) in these flows deflation suggests a correlation with the expansion. The post-emplacement Fig. 4. A sequence of photographs, taken through the side wall of the “ZD” bed, to show the formation of segregation features in an ignimbrite sample (Md$ = 0.75, eo = 3.2). Scale bar = 26 cm. (a) Vertical segregation pipes propagate upwards from the base of the bed. (b) Irregular pods and areas of coarse/dense material appear, partly formed by the break-up and reworking of the pipes in photo (a). (c) The basal part of the bed is now largely formed of fines-poor coarse/dense material, while the upper part of the bed is fine grained and pumice rich.

240

of the flow results in a slightly concave upper surface. However, in the laboratory experiments, the change in bed thickness from vi, < U G UmP to U = 0 is less than that which can subsequently be induced by mechanical effects (i.e. shaking the apparatus). Many small to intermediate volume ignimbrites, such as those on Tenerife (Booth and Walker, in preparation; author’s own observations), are of this type. Type 3 pyroclastic flows are defined as those in which U > UmP. Like type 2 flows, they show coarse-tail grading but they are characterised by the presence also of segregation structures. These st&tures are lo&lised concentrati-ens of coarse/dense material which are strongly depleted in fines as a resul~f~~he_bi~~ga~~~lo_w~~~~~~t~rol them. They oscur__most commonly as lenticular, curvilinear, or cres&ntic masses of coarse pumice or dense-rock fragments (e.g. Yokoyama, 1974, fig. 19; Aramaki, 1963, fig. 201, as gas-jettype ‘(fs.~~l-fUmarotes” and so-metirnes as ~discrete.segregation layers. The laboratory experiments suggest that strongly deveioped .pumide- and/or lithicconcentratio-n Zones. which have sharp boundaries tie also- segregation bodies on a larger scale.:This is~bprne~~out by grain size studies on these features (autho!‘s research in progress). The laboratory expe&nentsaIso suggest that some of~the finegrained (layer 3, or co-ignimbrite air&l1 -ash) deposits overlying type 3 flows may not be of genuine air-fail origin (Sparks and Walker, 19771, but representfilze-grain&d segregation fayers.~Despite-their poor preservation potential (see -p. 245), close field examination Mayo make it possible to distinguish between a se@egation layer and the layer-3 pruper; for example, the layer 3 will show air-fall-style mantle bedding, whereas the segregation layer will behave as a flow, i.e., pond in topographic depressions. The Taupo Ignimbrite shows a segregation layer of this form at many localities. The features described above are regarded as indicators of the three flow types; now~consider the implications of the! three styles of fluidization behaviour on some general aspects of pyroclastic flows. (1) Fbw regimes. The features developed within the three flow types are common to flows of an extreme range of dimensions. This and the lack of features positively indicative of widespread turbulence (analogous to high Reynolds number turbulence in liquids) imply that once a pyroclastic flow has formed, its rheolugy is expressed mainly in the development of laminar and/or semi-rigid-plug flow. (Semi-rigid-plug flow is here defined as that prevailing in a body, through the thickness of which, shear strain rates, although not necessarily zero, are insufficient to cause shear-induced grading and are not -high=enough-~to significantly inhibit the sinking or floating of large clasts). The author concurs with the views of Sparks (1976) on the likelihood of laminar flow in manycases. It.& here suggested that turbulence normally on& occurs in two situations, one at the transition from the source material (e.g.. a call~ps’lng eruption column) to~the developed flow (p. 242), and the other at-the front oft&-moving flow (.p.~243). Although turbulence may also occur lo~&lly_elsewhere (e.g. eat a sharp break, in topography), it is believed t-hat the~preenee or ab&nce of turbulence (a@defined above) is of

241

secondary importance in determining the characteristics of the resulting deposits. (2) Deposition slope. Although most commonly found on slopes of < lo”, non-welded pyroclastic flows are occasionally found on slopes of 10-30”. Because of their higher yield strength, type 1 flows of a given thickness are inherently capable of adhering to steeper slopes than type 2 or type 3 flows, regardless of whether they are composed of pumice or dense rock. For a very poorly fluidized flow, where little of the weight of the flow is supported, the slope angle on which it can rest may approach the internal angle of shear for the non-fluidized material (typically 30-35” 5. (3) CrystaZ concentration. As documented by Hay (1959), Lipman (1967), Walker (1972) and Sparks and Walker (1977), ignimbrites show a remarkable concentration of free crystals relative to their inferred original magmatic abundance. In type 1 and type 2 flows, the lack of elutriation as indicated by the laboratory experiments implies that most of the crystal concentration must result from the loss of vitric ash either in or near the vent, in the sequence of magma vesiculation and flow formation, or at the flow front, rather than from the body of the flow during its emplacement. In type 3 flows, elutriation causes a general increase in crystal concentration by loss of fines as well as a local increase around segregation structures (e.g. Walker, 1972, tables 1 and 2). (4) ‘%ossil fumaroles”. This work implies that the gas-jet type of “fossil fumarole” (Walker, 1971,1972) need not always be a secondary or postemplacement feature, but can represent a primary gas-flow feature surviving from the moving flow, or reflect the gas-flow rates as the flow comes to rest. Once established though, these pipes may concentrate the post-emplacement gas flow and modify the primary structure. In flows of types 1 and 2, any juvenile gas would tend to pass evenly through the flow, giving the broad post-depositional zones of vapour phase alteration which are so common. All four aspects are amenable to testing in the field and by grain size analysis. PYROCLASTIC

FLOW

TYPES

AND GAS SOURCES

The three fluidization types have been considered above as ch~~cterisi~g the entire flow; the effects of the various gas sources are now considered as they modify this simple model. The gas sources are: (I) Internal gas sources: (a) gases released from juvenile clasts by ~~ff~s~~~~ (b) gases released from juvenile clasts by breakage and attrition. (2) External gas sources: (a) gas trapped at initial flow formation; (b) air incorporated at the front of th.e moving flow; (c) gases released by the cambustion of vegetation and by the heating of surface water, ~~~~~wate~ an fluids contained in plants. A gas source can be co~s~de~e~ as being ~ppr~~~rnate~y eon&a or instantaneous on the scale of an ~~~o~~~e, with respect to the t~~~k~es~ of the flow, length of the exposure, and time (Table 1.).

242 TABLE 1 Relationship between a gas source and the pyroclastic flow. A gas source is considered to be roughly constant (C), variable (V), or instantaneous (I) with respect to the thickness and lengkh of an exposure, and time .-__ -___ ____ Time Length Gas source Thickness _ 1 (a) 1 (b) 2 (a) 2 (b) 2 (c) --~-

V V C C C

~_ ___

__

c c V V V

V G I c V

___~~

Process l(a), gas released by diffusion from juvenile clasts (Sparks, 1978), is potentially the most important source of gasin pyroclastic flows. It characteristically produces a gas-flow rate that varies systematically with height in the flow. This upward increase in gas-flow rate could produce a flow consisting of a layer of one flow type overlying another, e.g. a type 3 top overlying a type 1 and/or type 2 base, so that different structures may be developed at various levels in the flow. Such an upward change in flow type may be a contributory factor in forming m_ultiple flow-unit ignimbrites; as the flows decelerate away from the vent, the more mobile flow top may detach itself and continue, to form a separate flow unit, The potential of this process merits its. close examination in the field. Process 1 (b), gas released by the breakage and attrition of juvenile clasts, is almost certainly present to some extent and may attain importance by localising intraflow zones of high shear (i.e. high attrition rates) by reducing the local effective “viscosity” of the flow. Its importance is limited by the amount of overpressure of gas within the clasts. The variable development of post-fragmentation vesiculation in many pyroclastic flow clasts (e.g. Aramaki, 1956, p. 211) suggests that the importance of gas release by attrition as opposed to diffusion may vary within a single flow. Process (2 (a), gas trapped at the time of initial flow formation although 1~~importawLt~nnsteee~~~o~~a~oe~ (eg. Mayon, Mooreand Melson, 1969 1, is of primary importance in “low-profile-vent” eruptions (e.g. the Acatlan Ignimbrite, Wright and Walker, 1977), where the flows are deduced to have been formed by the gravitational collapse of an eruption column. Gas entrapment can take place both during initial column collapse (Sparks and Wilson, 1976) and continuous column collapse (Sparks et al., 1978). In the former case, air entrapment is here postulated as the mechanism by which the vertical momentum of the column is converted to the subhorizontal mo~~tum of the-pyroelastlc flow, The eruption_ ~~ol~rn~~~o~la~s~sen m.asse as a dilute, t~b~ent g~/~~ti~Ie mixture. When this approaches the ground, the trapping and compression of -air by thermal and mechanical effects y-high gas flow: Pert of this gas may causes the g~~~~ti~n ~~~rnen

243

be ejected laterally, while the remainder is forced through the aggregating, concentrated gas/particle dispersion which forms the body of the flow, causing a short-lived period of violently agitated fluidization. If continuous column collapse ensues, the same process continues,though the only gas involved then is that expelled during the (dilute) eruption column to (concentrated) pyroclastic flow transition. During the agitation-fluidization event, heavy lithic blocks segregate out very rapidly and accumulate near the vent to produce a co-ignimbrite lag-fall deposit (Wright and Walker, 1977). At the sam,e time, large amounts of fines are generated and lost into the overlying dilute cloud, producing at least part of the crystal concentration seen in the 90~s. The time scale of the agitation fluidization event is believed to be very short (Sparks et al., 1978), at most a few tens of seconds, whereas the column may continue to collapse for much longer periods. There are several corollaries to this. The flows that appear (say from topographic considerations) to have formed by column collapse should display a higher crystal-enrichment factor (Walker, 1972) than equivalent deposits where gas entrapment was not important. They should also display a coignimbritelag-fall deposit, and possibly also near-vent surge deposits. Process 2 (b), fluidization by air entrapped at the moving flow front was first proposed by McTaggart (1960). However, his experiments were carried out under conditions that especially favoured air entrapment; the chutes down which his samples were accelerated apparently end above the ground surface (McTaggart, 1960, fig. 5a), giving the material some free flight. The role of air entrapment at the moving flow front is here re-examined in terms of the features it should produce. At the front of the moving flow (of types 1, 2 or 3), basal friction will cause an overhang which will act as a funnel for air (Fig. 5), in much the same way as a density current head takes in water (Allen, 1971). Under the flow, the compressed and heated air will expand and, as in initial column collapse, either pass upwards through the flow, or be ejected out at the flow front. The escaping gas and fines give the flow head its “billowing” or “sprouting” appearance as seen for example by Per-ret (1937) on some Mt. Pelee pyroelastic flows and attributed by him to this cause. Part of the trapped air passes up through the flow, causing violent agitation fluidization within the flow head. In this state, fines are lost to the overlying dilute cloud, and an as yet undocumented proportion of new fines generated by breakage and attrition. The rearward boundary of the agitated zone is marked by the point at which all the ingested air has expanded to the ambient conditions; this is also taken as the rearward boundary of the flow head. Corollaries of this flow head model are easily tested. The portions of flows that have undergone fluidization in the flow head should show an increase in crystal enrichment factor, pumice rounding (e.g. Williams, 1942), pumice density (due to the preferential attrition and elutriation of ligbter pumice~~ Md, and the proportions of lithics and free crystals. They should also show a decrease in pumice .~ax~~~~ size, a0 (due to the loss of fines during the

244

I

I I

FE%

’

ML,AZ FH

1

I

Fig.5. Schematic diagram of a pyroclastic flow head. DC denotes the dilute cloud, ML the mixing length of the ingested air, the end of which marks the rearward boundary of the agitated zone (AZ), and the flow head (FH), which merges into the flow body (FB). FS rePresents the streamlines of the flow-head material, GFP the gas flow paths, P the pumice ckts (P < matrix), L the lithic clasts (P > matrix), 5% the segregation bodies and GS the ground surge (layer I) deposits.

fluidization) and be generally non-welded, or show a decrease in welding intensity, when compared with the body of the flow. To complicate this, the inter-relationship between the body and head of a flow depends on the balance between “basal” and “internal” friction, which controls the flow velocity profile and thus the degree of lateral mixing. Two “end-member” aspects of lateral mixing are considered. When the velocity profile is relatively flat, there is a low degree of lateral mixing (Fig. 6). Thus, the resulting deposit should show the changes outlined above as the deposit is examined in more distal exposures. Where the velocity profile is such that shearing is more even through the flow thickness, there is a high degree of lateral mixing. In such flows, material is carried to the flow head, agitated and strongly fluidized, and the coarse/dense fraction deposited at the base of the flow, partly by a “caterpillar track” mechanism and partly by its density contrast with the matrix. If the flow velocity is low, a section will show the head deposit at the base, overlain by the flow body. If the flow velocity is high, ~muchof t-he body of the flow will continue outwards from the vent and the features deduced above will show mainly in nearer-vent sections, a result opposite to that where-there is a low degree of lateral mixing. Recent work (Walker et al., 1981; author’s own observations) suggests that the layer 1 deposits at the base of some ignimbrites are, pyroclastic flows distal extremities in fact, head deposits. Although low degree of lateral mixing should show are rarely documented, those wi

245

FB

,

FH

1

Fig. 6. Schematic diagrams to illustrate the contrasting effects of low and high degrees of lateral mixing on the flows. Part (a) shows a flow with a low degree of lateral mixing, and part (b) a flow with a high degree of lateral mixing. The left-hand side of each part shows a section through the flow head; the right-hand side, a map of the resulting deposit. VP denotes the flow velocity profile, while FB and Fli are the flow head and flow body, respectively. The numbers in the key represent: 1 = the vent position; 2 = near-vent (co-ignimbrite lag-fall) deposits; 3 = material that has undergone the agitation-fluidization event within the flow head; 4 = material in the flow head; 5 = material in the flow body; 6 = arrows indicating the relative rates of mass transfer across boundaries.

steeper flow fronts, possibly with an accumulation of large blocks (e.g. Nairn and Self, 1978). Those flows with a high degree of lateral mixing will show a flatter fiow front, where the deceleration of the flow causes the gas-richer upper parts of the flow to detach and continue as an overlapping series of subsidiary flows (e.g. Aramaki, 1963, p* 384). Process 2 (c) would show a slight time lag between the arrival of the flow and the production of gas. Only where a flow travels over abundant vegetation, or down a valley with a large stream, or over a lake, is enough gas likely to be available to influence the fluidization behaviour of the moving flow. If gas produced under a stationary flow, then some interesting consequences are possible. If the flow is standing on an appreciable slope, the gas may be capable of renewing portions of the flow and causing the slumping of secondary flows to lower levels. If on a gentle slope, type 2 or 3 flow structures may be formed within a flow, though only if welding (i.e. p icle cohesion) has not commenced~ If welding has dev~loped~ other st~~~~~~e~

246

may form, one example of this being the rootless vents described by Wright and Coward (1977). One example of superimposed flow-type structures is where thin type 1 or type 2 flow units in the Minoan Ignimbrite are intercalated with torrent deposits and show vertical segregation pipes (Bond and Sparks, 1976). The possibility that type 2 and type 3 structures seen in a flow may be secondary must be taken into account when attempting to use flow-type structures to interpret the emplacement history of a flow, In most cases, the vertical disposition of such features as gas-jet “fossil fumarole” pipes (e.g. if they cut an earlier or later deposit; cf. Guest and Jones, 1970), and the lateral control supplied by widespread sampling will serve to distinguish local, secondary fluctuations from the primary flow characteristics. The complex variety of structures shown by pyroclastic flows may thus be related in part to variations in the nature of the gas sources. The major difference between gas sources in groups (1) and (2) is that the former lead to changes in the flow characteristics in the vertical direction (i.e. produce changes throughthe flow thickness at a single exposure), whereas the latter are responsible for changes in the flow characteristics in the horizontal direction (i.e. cause a zonation about the vent of the various structures). CONCLUSIONS

From experimental studies, a threefold classification of pyroclastic flows in proposed, based on the differing structures shown, as they reflect differing degrees of fluidization (Fig. 7). The least fluidized (type 1) flows tend to be internally homogeneous, and increasing fluidization (in types 2 and 3) leads to an increasing departure from homogeneity. These departures are marked firstly by coarse-tail grading (in type 2 flows) and then by segregation bodies (in type 3 flows) which include gas-jet “fossil fumarole” pipes and coarse/dense clast concentration zones. Consideration of the possible sources of fluidizing gas shows that the three flow types may develop at various points within the flow, both during and after emplacement, and criteria for the recognition of a gas source are presented. It should be noted that type 1 flows cannot necessarily be distinguished from disintegrated landslides, rockfalls, debris flows and lahars purely on the basis of their internal structures (cf. Bond and Sparks, 1976, p.6). Even using the external morphology (e.g. surface structures, and the ratio of vertical drop to_horizontal distance travelled), the distinction may not be possible. That this applies to~pyroclastic flows having a variety of origins (see examples quoted on p. 239) suggests that more heat in itself is not a significant factor in theempiacement of type 1 flows. Because type 2 and type 3 flows are almost exclusively ignimbrites, this seems to imply that the nature of the juvenile material is important. However, the concomitant increase in flow volume on the flow type may be of more significance than the nature of the juvenile ki, 1963): material (see Aramaki and Yam To test the importance of fluidization, it is necessary to compare equal-

247

a

b

C

Fig. 7. Sketch to show the characteristic features of the three flow types (see text for details). (a) Type 1. Ungraded, homogeneous nature; surface ridging and block trains present; basal layer poorly developed or absent. (b) Type 2. Coarse-taiI grading (pumice (p < matrix )-rich towards top, lithic (p > matrix)-rich towards base), well-developed basal layer and a concave upper surface. (c) Type 3. Strongly developed coarse-tail grading with sharply bounded pumice and lithic concentration zones, segregation bodies and pipes, and a fine-grained pumice-rich segregation layer at the top of the flow.

volume deposits and then to see if the variation in flow type, internal structures and external morphology can be related to any variations in the properties of the juvenile clasts, or the influence of potential gas sources. Only then can any confident assessment be made of the role of fluidization in promoting such features as the apparent excessive mobility of pyroclastic flows. ACKNOWLEDGEBENTS

This work forms part of the author’s Ph.D. studies under the helpful supervision of Professor J. Sutton and Dr. G.P.L. Walker. Professor Janet Watson and Drs. B. Booth, D.L. Pyle, S. Self, R.S.J. Sparks, G.P.L. Walker and L. Wilson contributed greatly in discussions and criticisms of the manuscript. Dr. D.L. Pyle, C. Boyle and W. Meneer helped considerably in the tbeoretic~~ and practical aspects of fluidization engineering. The work was carried out while the author was in receipt of a N.E.R.C. research st~de~tsbi~ which is gratefully acknowledged.

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