TEXTURAL EVIDENCE FOR THE ORIGIN OF IGNIMBRITES

N. RAST

Despite the large number of recent papers on ignimbrites, their origin is still debatable. Textural features of ignimbrites are considered and certain deductions are made as to the origin of these rocks.

I. INTRODUCTION The word ignimbrite has been introduced into vulcanological literature by Marshall

(1932, 1935) for certain acid volcanic formations of New Zealand. These were previously assumed to be rhyolitic lavas, but following Marshall’s investigations are widely accepted as pyroclastic deposits, which have been produced as a result of particularly violent eruptions involving the production of turbulent fast-flowing fluidized systems (Reynolds, 1954) of gas and solid particles, such as are held to have been responsible for the great sand flow at Katmai, Alaska.

Lately the subject has been reviewed in several comprehensive papers (e.g., Ross and Smith, 1961; Vlodavetz, 1961; Weyl, 1961). In these publications essentially the Recent and Tertiary ignimbrites were dealt with. In addition, there are numerous articles in which ignimbrites have been reported in older geological formations from the Pre- Cambrian onwards. A particularly full bibliography has been prepared by Cook (1959). Most modem authors adopt Marshall’s term ignimbrite although with somewhat Wkrent connotations. There is, however, a growing body of opinion that the term should be used vulcanologicaIIy as a product of a particular kind of eruptive process rather than petro- graphically by reference to composition or a particular texture. The not infrequent occurrence of rocks in which particles of glass have been welded and distorted is especially under discussion. Beavon, Fitch and Rast (1960) point out that in the field a strict separa- tion of welded and non-welded varieties of these rocks is rather difficult. Ross and Smith (1961) come to the same conclusion and record the fact that ignimbrites of rhyolitic, rhyodacitic, dacitic and even andesitic compositions are known, while Van Bemmelen and Rutten (1955) report a rare basaltic ignimbrite from Iceland. Although glass shards are charateristic of most ignimbrites, usually pieces of pumice, some commonly broken crystals, few lithic fragments and some irresolvable interstitial material, presumed to have been very fine vitric dust, are found in ignimbrites. In specific cases the proportions of these constituents vary widely. Ross and Smith (1961) report ignimbrites in which the crystal content varies from 0.1 % to 60%. The Flinty Flow (“Flinty Rhyolite”) of Snowdonia (Beavon, Fitch and R a t , 1960) has often but a few pumice lumps, whereas certain varieties of Lower Rhyolitic Tuffs of Snowdonia (PI. 6 ~ ) are in the main composed of distorted lapilli of pumice. Proportions between the lithic fragments and the fine shards are not so extreme but even then up to 30% of lithic fragments can be found in

Lpool uanehr. Geol. J. Vol. 3, Pt. 1, 1962. 0 97

98 N. RAST certain welded breccias of the Pass of Llanberis, Snowdonia, while non-welded ignimbrites with large shards have much fine dust. Thus the adoption of the term ignimbrite for an acid rock possessing textural characters often occurring in “welded tufEs”, as is advocated by Steiner (1960), seems unreasonable. On the other hand, a substitution of the term ash-flow tuff for ignimbrite as is advocated by Ross and Smith (1961) seems somewhat cumbersome, and by virtue of the term ash-flow carries the implication of fine grained deposits, which is not always the case.

The rather chaotic nomenclature of ignimbrites, often bristling with local terms, is the result of the fact that despite much recent research the origin of these rocks is still not completely clear. This is due to the fact that the production of welded varieties has not been scientXcally observed in any recent eruption, although Vlodavetz (1961) suggests that such were formed during the 1783 eruption of Assam volcano in Japan. The absence of undoubted modem welded ignimbrite is one of the features which prompts Steiner (1960) to advance a hypothesis of formation of ignimbrites based on the idea that such rocks are products of two intimately interspersed, immiscible, magmatic liquids, while most Russian geologists claim that the distinction between ignimbrites and lavas is not entirely clear-cut and that there are so called tuffolavas which show features intermediate between ign’mbrites and ordinary lava (e.g., Shirinian, 1961). Yet a third group of geologists suggest that ignimbrites are products of peculiar frothy acid magmas (Kennedy, 1955; Beck and Robertson, 1955). In the present paper observations on the textural features and general relationships of ignimbrites are made the basis of a re-examination of the problem of their origin. Field relationships of mainly British ignimbrites as well as some 2,000 thin sections of ignimbrites and associated lavas and minor intrusives from Britain and abroad have been used for this purpose.

2. TEXTURALFlEATURES (a) Glass-shsirds and Pumice

Although Marshall (1932,1935) in his original descriptions of ignimbrites defined them by reference to the volcanic eruption which allegedly had produced them, he laid a great stress on the early post-depositional textural modifications of glass shards generally

PLATE 6 Magnification: x35 throughout

A. Welded lapilli-tuf€ from the Lower Rhyolitic Tuffs of Moel Hebog, Snowdonia (thin section No. R.M.S.1129).

B. A non-welded ignimbrite with large shards from the Lower Rhyolitic Tuffs of Cwm Carqpg. Snowdonia (thin section No. N.R.261).

C. Welded ignimbrite with small shards from Cwm Idwal, Snowdonia (thin section No. D.W.115).

D. Owharoite ignimbrite (New Zealand) with fragments of pumice showing Merent degree of compaction (thin section No. 12845).

(R.M.S. = R. M. Shackleton’s collection; D.W. = David Williams’ collection; N.R. = N. Rast’s collection.)

A

C

Lpool Manchr. Geol. Journal, Vol. 3 PLATE 6

B

D

TEXTURAL+ EVIDENCE FOR THE ORIGIN OF IGNIMBRITES 99

described by the word “welding”. Thus for many years the terms welded tuff and ignim- brite were used synonymously, and although Fenner as far back as 1948 adopted the term sillar for rocks contahhg generally undistorted but pneumatolytically agglutinated shards, and expressed the opinion that such rocks originate by the same type of eruptive process as true welded tuffs, most geologists until the last two or three years associated welding with ignimbrites. It now seems clear that while the so-called welding (eutaxitic texture) is a common feature of ignimbrites, as noticed by Rast, Fitch and Beavon (1959), Martin (1959) and others, it is by no means universal. They pointed out that witbin a single sheet of ignimbrite (single cooling unit of Smith, 1960), one can recognise a zone of welding situated usually near the bottom of the sheet; welding disappearing downwards and upwards. This is easilyunderstoodin terms of Marshall’s explanation of welding, which involves deformation and compaction of the glass particles, which have retained much heat, by the superincumbent load and as load decreases upwards so does the weld- ing. On the other hand, the glass shards immediately at the bottom of the sheet cooled too quickly and too far to have been affected. In the lower and central parts of the unit the combination of a considerable load and a fairly high temperature cause the particles of glass and pumice to get squashed, distorted and welded together, the overall process being not dissimilar to sintering. Furthermore, away from the source of eruption as the sheet gradually thins, the factor of load becomes inoperative, the welded zone disappears completely and the ignimbrite sheet remains wholly non-welded. The thinning of the Flinty Flow on the south-eastern flank of the Snowdon syncline is attributed to this effect.

Vertical and lateral variations within single sheets apart, the degree of welding and compaction varies from unit to unit. In Snowdonia where owing to folding and erosion, considerable thicknesses of ignimbrites can be examined, this feature is especially noticeable. For instance around Snowdon much of the Lower Rhyolitic Tuffs are non- welded, whereas the Pitt’s Head Flow and the Flinty Flow (Eeavon, Fitch and Rast, 1960) are strongly welded. While the initial temperature of the ignimbrite is probably the important factor here, the nature of retention of heat has to be considered. This is done further on in the present paper (p. 104) after a fuller examination of textural evidence.

Microscopically degrees of “welding” can be recognised. Ross and Smith (1961) use the terms welding and moulding to express the cohesion and distortion of fragments. It seems that both terms should be used more narrowly than they have been in the past. In fact many so called welded tuffs show moulding of shards against each other and crystal or Ethic fragments and not welding, which term should be applied only when those glass- shards that actually start coalescing together, a sense in which the term was used by Iddings (1899, pp. 405-406). Vlodavetz (1961, p. 21) makes a similar distinction. Under a microscope all stages of plastic deformation of glass fragments can be recognised starting from slight moulding of shards round lithic fragments or crystals, as observed by Ross and Smith (1961) in the specimens from the great sand flow of Katmai, continuing through the general distortion of shards seen in most ”welded tuffs” and ending in true welding recorded in certain obsidians. The iinal stage sometimes involves an extreme distortion of shards simulating true fluidal texture. Such a texture has been d e d parataxitic by Beavon, Fitch and Rast (1960, p. 604), but can be distinguished from the flow banding of l a w by the fact that in cross sections individual streaks, representing severely flattened shards, divide laterally and even when they are welded together they present a shredded appearance. It is not unlikely that this stage is transitional into secondary flow phenomena.

N. RAST - For instance, the Flinty Flow at Cwm Caregog near Snowdon shows a fine non-tectonic lineation and the thin sections cut parallel to this lineation show a stronger deformation of shards than those cut at right angles to the lineation, thus implying flowage. On the other hand, large-scale secondary flow phenomena such as reported by Rittman (1960) from the “rheoignimbrites” of Tuscany need a wider confirmation since the paraboloid shear zones and the associated breccias taken to be diagnostic of such phenomena can be recognised in the undoubted rhyolitic lava at Bylchau Terfp (3 miles south of Snowdon).

Recently Steiner (1960) has suggested that the distortion and apparent compaction of glass-shards can be explained by the exsolution of a volatile rich from a volatile poor magma in globules which are sub-spherical at the top of the sheet and are elongated towards the bottom. During the flow there is a supposed separation of the volatile rich magma resulting in the breakdown of the partitions between the globules and the produc- tion of cuspidate shard-like bodies at the top and of elongated streamlined shreds and bands at the bottom of the sheet. Steiner’s explanation ignores certain textural features, discussed below, which tell against his hypothesis.

The shape of individual glass shards in cross-sections varies from lunulate, representing walls of individual bubbles, to rod shaped (bicuspidate and sometimes tricuspidate), Y-shaped and rarer cross-shaped sections representing mutual walls of two, three and four bubbles respectively (Fig. 1). Moreover, there are recorded cases of complete, or nearly complete, individual unbroken glass bubbles (Fig. 1). In many ignimbrites all these varieties occur mixed together, although Y-shaped shards generally are most abundant. There are, however, reported cases where one of these shapes is almost exclusive (e.g., Ross and Smith, 1961, Fig. 17). Such a neat segregation of shards is difficult to understand on the hypothesis that they originated in a lava flow since if one accepts Steiner’s mechan- ism, one has to explain why the breakdown of very elongated bubbles in a lava flow gives principally either Y-shaped or rod-shaped bicuspidate fragments. If, however, it is assumed that the shards have originated at a volcanic centre or fissure from the breakdown of pumice, and have been subsequently transported and somewhat re-sorted by a nu& ardente (ash-flow) the same acuity does not arise, since the shapes of the bubbles in the pumice can vary from time.to time. Moreover, in the so-called pressure shadows neat the

TEXTURAL EVIDENCE FOR THE ORIGIN OF IGNIMBRITES 101

t lmm J

Fig. 2.Deflection of pumice below a crystal of plagioclase and of glass shards and groundmass above the same crystal. (Owharoite ignimbrite.)

phenocrysts the shards, whatever their original shape, are much less distorted than within a particular thin-section. This would be expected if flattening were responsible for the distortion of shards, but cannot be accounted for if the shards are assumed to be already distorted walls of flow-distended and contorted globules. The three-dimensional aspect of shards is important in this connection. Thin-sections oriented at right angles to each other suggest that even those shards which have rod-shaped cross-sections are in reality plates, and neither these nor the flattened Y-shaped shards show linear preferred orient- ation such as would be expected if such shards were produced in the process of flow rather than welding.

The size of glass shards in ignimbrites varies within wide limits from one sheet to another. Within one sheet and especially in individual thin-sections, there is generally a considerable degree of uniformity, although a few relatively large shards have been observed to be present in some thin-sections. Certainly a hundred-fold range in dimen- sions, mentioned by Ross and Smith (1961), is rather rare, although a ten-fold range is not uncommon. In non-welded ignimbrites there is a tendency for the shards to become finer towards the top of individual sheets, suggesting a certain amount of sorting.

Amongst the Snowdonian ignimbrites there is a systematic relationship between the size of shards and the degree of welding. Ignimbrites with large shards (PI. 6 ~ ) rarely show any appreciable welding, whereas the strongly welded rocks (Pl. 6c) are commonly composed of much smaller shards. This phenomenon has been checked against large numbers of thin-sections from areas outside Wales and seems to be of a general occur- rence. Thus at first sight it seems that small fragments suffer postdepositional distortion more easily than the larger fragments. This generalisation while applicable to the shards, does not apply to the pumice fragments which are commonly many hundred times more voluminous as shards, but are invariably much more strongly compacted and distorted. In fact completely undistorted pieces of pumice are relatively rare, but variations in degree of distortion and compaction can be observed from fragment to fragment in an individual thin-section (p1. 6 ~ ) or even within individual pumice fragments. This is especially obvious in those fragments of pumice which have been compacted against a phenocryst (Fig. 2). It is clear from the figure that this is a feature of compaction and not of flow, since in the latter case one would not expect a symmetrical distribution of the less

102 N. RAST

Imm I

Fig. 3.-Embayment of a fractured surface in a plagioclase feldspar crystal. (Cum-y-Glo.)

collapsed parts of the pumice fragment at either side of the strongly fiattened part adjacent to the phenocryst. Were the flattening a result of flow drag against the phenocryst, every part of the piece which had suffered flattening would have been strongly distorted. Flattened pumice and lava fragments are found not only in ignimbrites but also in a rock called ‘‘piperno’’ which has been described from the neighbourhood of Naples. In hand specimen this rock appears exactly like a pumice-bearing welded variety of ignimbrite with flattened cake-like dark masses of pumice embedded in a he-grained tuff in which the pyroclastic texture is usually absent since the whole deposit is autopneumatolysed (Rittman, 1960). However, fresh non-pneumatolysed blocks of this deposit show clearly that the matrix consists of welded shards and the larger fragments are flattened pieces of obsidian, which in places preserve the characteristic tubular texture of certain varieties of pumice. Piperno often forms thin layers which gradually merge into tuffs. Rittman (1960) suggests that the rock originates as a result of the rapid accumulation of intensely hot portions of lava thrown up by lava fountains. The portions of liquid lava (and pumice) on deposition immediately spread out and get welded to the previously deposited material. Since piperno and pipernoid rocks are generally restricted to the immediate vicinity of volcanic centres, such an explanation seems reasonable. In other words, these rocks are also welded tuffs, although not ignimbrites, since their transport involved fragment by fragment aerial projection and not an avalanche of a gas solid mixture. The detailed similarity of obsidian fragments in piperno and masses of collapsed pumice in ignimbrites leaves little doubt that gravity is responsible for their shapes in both types of deposit.

The size of pumice fragments in ignimbrites varies from less than a millimetre to several centimetres along the diameter. In most ignimbrites 1 to 10 cms. is the usual range of sizes. While it is easy to understand the relatively uniform grain size of the shards, such being conditioned by the general uniformity of gas bubbles at the time of frothing up of the liquid magma, a similar feature with respect to the pumice fragments at first seems puzzling. However, the examhation of the shapes of moderately flattened pieces of pumice shows that commonly, despite distortion, they show rectilinear outlines (cf. Martin, 1959, p. 407, Fig. 10). This suggests that at the eruptive centre the vesiculated magma develops a system of joints and that the subsequent explosive disruption separates fragments of pumice along such joint boundaries. These fragments are carried thereafter within the ash-flow and get deformed postdepositionally.

TEXTURAL EVIDENCE FOR THE ORIGIN OF IGNIMBRITES 103

(b) Interstitial Dust The glass shards, pumice fragments and the crystals of practically all non-welded

and moderately moulded and welded ignimbrites are separated from each other by ke- grained material usually referred to as dust. In thin-sections this interstitial material as a rule is of a different colour than the shards or the wall-structure of the pumice. Some investigators (Weyl, 1954; Steiner, 1960) have reported differences in composition between the shards and the interstitial dust. Steiner, in particular, refers to this dust as mesostasis and suggests that it represents a solidified immiscible liquid complementary to that from which the shards originated. In the tectonically deformed recrystallised ancient volcanic rocks the interstitial material is normally widely chloritized, whereas the shards are only partially chloritized, or are completely replaced by quartz and feldspar. Thus it seems likely that the compositional differences between the shards and the interstitial matter are widespread in many ignimbrites. In New Zealand ignimbrites the interstitial matter is full of fine dark irresolvable dusty substance, occasional brown mineral grains with anomalous polarization coloux‘s and crystals of iron ore. In ancient ignimbrites the iron ore is especially conspicuous. In either case the interstitial matter is much less homogen- eous than the shards and its totaI amount in the same sheet of ignimbrite varies from thin section to thin section. In some cases dispersed ghosts of minerals altered into diffuse aggregates of oreminerds and dusty substance can be recognised. The existence of these suggests that the so-ded mesostasis has undergone a profound alteration (also see Fitch, 1961). It seems reasonable to suggest that the alteration is caused by pneumatolysis and the relicts of crystals exist owing to their having been affected by the gases which were in circulation between the shards. In a thin section from Manuinui ignimbrite (New Zealand) this kind of alteration is seen to be associated with phenocrysts of pyroxene which has the dusty substance and the brown mineral which appears to be sphene concentrated around the phenocrysts. Even in these young ignimbrites the “mesostasis” has appreciably recrystallised. In older ignimbrites crystallisation proceeded so far that it is not possible to draw any definite conclusions about the origin of the “mesostasis”. However, there are no facts which contradict Marshall’s suggestion that it represents the fine volcanic dust.

(c) Crystals and lithic fragments Quartz, potash feldspars and acid plagioclase constitute the most common crystal

fragments in ignimbrites. In addition, fiagments and occasionally complete crystals of augite, hornblende, biotite, mica, cordierite and garnet can be observed in relatively young rocks. Older ignjmbrites are generally devoid of the ferromagnesian minerals, such having been converted to chlorite. Degree of fragmentation of the crystals varies from one ignimbrite sheet to another.

For instance, the Precambrian ignimbrites of North Wales have numerous practically unbroken crystals of quartz riddled with cavities which have narrow channe-like wnnec- tions with the groundmass, while in many Ordovician ignimbrites from Snowdonia the crystals are mainly acid plagioclase which are fragmental. Such crystals have commonly embayed margins, which are present in the fragmented and complete crystals alike (see also p. 102). The examination of welded and non-welded ignimbrites does not show any systematic variations in the degree of fragmentation of its crystals. This supports the pyrochstic origin of these rocks, since if as Steiner (1960) maintains the dif€erences in

104 N. RAST shard texture are a result of different modes of flow, one should expect systematic varia- tions depending on the texture of shards which surround the crystals. On the other hand, the crystals which occur inside the pumice fragments commonly show euhedral outlines (e.g., Waiotapu Ignimbrite, thin section 18250).* Presumably at the time of original formation of the pyroclastic mass such crystals were protected by the surrounding foam of pumice. 0x1 very rare occasions adjacent broken crystals have margins of similar form suggest-

ing that they originated from in situ breakdown of a larger crystal. Generally, however, this feature is absent. In fact the embayed fractured surfaces of crystals of feldspar (Fig. 3) imply that crystal fracturing has happened during the eruptive explosion and the embay- ment occurred later.

In addition to crystals fragments of older rocks (sometimes older ignimbrites) also occur quite frequently and are usually of small dimensions of a few millimetres to a few centimetres across. There are no recorded cases where such fragments show in sirtc breccia- tion such as frequently occurs in volcanic mudflows.

3. INFERENCES The textural data so far advanced seem to provide convincing evidence that ignimbrites

originate from ash-flows and that their characteristic textures can be explained in terms of load deformation and welding which follow the deposition of the gas-solid mass. Nevertheless, the subject still involves numerous problems which can be summarised under three headings.

1. The source of heat for welding is still debatable. It is necessary to discover whether the heat .is merely the original magmatic heat or has been enhanced by hydrothermal reactions in situ.

2. The mode of transport is not precisely known. The fluidisation hypothesis of Reynolds requires a constant large-scale escape of volatdes in order to ensure the friction- less mobility of the gas-solid system. Such escape of volatiles would necessitate appreciable cooling of the ash-flow even over a short period of time that it flows.

3. The nature of original eruption is not known. Following Williams’ (1942) suggestion many accept the association of ignimbrite producing eruptions with caldera collapse. Others, however, deny any such associations With central eruptions.

Textural features of ignimbrites provide some evidence towards the solution of these problems.

The heat problem-Following the observations of Fenner (1923) on Katmai and Kozu (1934) on Kamagatake it is known that ash-flows remain hot for a considerably longer time than deposits of ash-falls. Ross and Smith (1961) suggest that t h i s effect arises out of conservation of heat occasioned by the absence of dispersal of pyroclastic material leading to the contact with cold air and also due to the short duration of transport. They point out that evidence of pneumatolysk is generally wanting. Steiner (1960) mentions the existence of strongly embayed crystals of quartz, but Ross and Smith rightly indicate that such are also present in acid lavas, and similar embayments can be found in the quartz- phenocrysts of plug rhyolites. However, neither acid lavas nor fine-grained intrusions show

*Slide numbers, unless otherwise specified, are in the collection of the University of Liverpool.

TEXTURAL EVIDENCE FOR THE ORIGIN OF IGNIMBRITES

c Imm I

105

a Bottle-shaped embayments in a crystal of plagioclase feldspar from Manuinui ignirn- brite (thin section No. 17366).

Quartz crystal in a Re-Cambrian ignim- brite from N. Wales ~Cwm-y-Glo).

much corrosion of the feldspar-phenocrysts, while in virtually all ignimbrites this is a common feature. Moreover, not infrequently there is in situ alteration of crystals of mafic minerals as has been described. In some cases growth of new minerals (e.g., Martin, 1959) has been reported. The geometry of the cavities in certain quartz crystals suggest that they have been produced in situ rather than in original magma chamber. The cavities not un- commonly have bottle shaped cross-sections (Fig. 4) with MITOW necks and wide cavities within the crystal. Usually glass shards can be seen to have been forced in and now line the cavities. In certain cases individual glass-shards can be traced from the inside of the cavity into the matrix. If it is assumed that such a crystal has been thrown out of original magma and carried in the ash-flow it seems unlikely that throughout the transport the cavity was empty. In fact it is difficult to see how the viscous rhyolitic magma gets ejected from the cavity to provide space for the shards. Thus it appears that the cavity originated after the crystal came to rest, in which case its origin can be attributed to the hot pneumatolytic solvents. Despite all this, the problem of whether any heat is generated in situ or not remains with us. It is difIicult to understand welding or the absence of it from one ignim- brite sheet to the other since thick ignimbrites exist in which despite the thickness there is no appreciable welding. Thus, while in an individual sheet thickness is a controlling factor, it does not exercise an overall control. Vlodavetz (1961) suggests that heat is conserved by rapid overlay of earlier ash-flows by later ones. This is the same idea as is incorporated in Smith's (1960) suggestion that single units may be composed of several flows. Thus in discussing ignimbrites a clear distinction must be made between a sheet which is a single cooling unit and a flow which may be only a part of it. For instance the Pitt's Head "Flow" of Snowdonia appears now to be composed of several flows (Shackleton, 1958), but in central Snowdonia no planes of demarcation can be recognised with confidence and it appears as a single sheet. Ross and Smith (1961) point out that most of the heat in an ash- flow must be retained in the solid particles of glass. Here the fact that welding is generally more pronounced in ignimbrites with finer shards appears to be paradoxical. Moreover in the experimental welding of tuf€ particles conducted by Smith and his co-workers, this feature has not been noticed. It seems thus that the grain sue of shards has no infiuence on

106 N. R4ST the in situ conservation of heat. However, it will be pointed out that the grain-size during the transport is an important factor. As regards any possible in situ generation of heat the mechanism is at present obscure. Vlodavetz (1961) calculated the total enexgy of the reaction 2Fe0+40a->Fe,0, in ignimbrites and came to the conclusion that it is insufficient for any appreciable in situ rise of temperature since the total amount of iron in these rocks is small. Steiner (1960) quotes 25 analyses of New Zealand ignimbrites with total FeO+Fe,O,, being in most about 2-2-5% which certainly supports Vlodavetz’s deduction. The magnetite in the iguimbrites nevertheless could have an important role as a catalyst for reactions involving dissociation and association of water since spinellids are known to be strong catalysts in this respect. Such reactions so far have not been explored geologidy, but the presence of partially dissociated water in an ash-flow is a distinct possibility (see below), in which case a source for in situ heat will be available.

The transport problem.-McTaggart (1960) indicates that the fluidization hypothesis of Reynolds’, which is widely accepted as a basis for the frictionless transport of solid matter, in nu& ardentes is not by itself satisfactory as an explanation, since the total volume of volatiles present is insufficient for the transport of really large blocks encountered in the deposits of Mont Pel& or Souffri&re. Therefore, he suggests, that trappingofair is necessary to help the fluidized state. Thus he explains the observed revitilization of nub ardentes on dropping over cliffs. This, however, is unnecessary for most iguimbrites which have much smaller fragments and can be supported in a fluidized state. Herein may lie the dif€erence between the ash-flows which give rise to unsorted block deposits of Mont Pel& type and those that produce ignimbrites. The air is entrapped in the first kind of ash-flow with resulting crystallisation and relatively fast cooling, whereas the second type of ash-flow does not involve the rising of the entrapped air. But if so, then some mechanism is required to explain the maintenance of volatiles within the ash-flow. Perret (1937, p. 99) advances the theory that electrostatic charges are responsible for the absence of dispersal in flow. McTaggart (1960) points out that since the charge on solid particles will be the same the particles will tend to repel each other. This is so if the charge on the gas is ignored. How- ever, in a gas-solid system it is likely that the total electrostatic field will be such that part of the gas (mainly water) is dissociated and therefore electrically charged. Thus from one particle to the other there will be a variation in the charge of the volatiles analogous to the so-calied zeta potential in a colloidal system. In such a case the overall cohesion and at the same time the frictionless transport will be assured. Moreover if the volatiles and the solid particles are held together by electric charges then there will be a tendency for the gas-solid system to retain its volatiles, and since the heat is partitioned between the volatiIes and the solid, it will be also retained. By analogy with a colloidal system the optimum conditions will be obtained with very s m a l l particles and this may be the explanation for the better wdding of k e r grained ignimbrites. In other words, such rocks during transport retained the heat better than the coarser varieties. One point must be emphasised, that the shards rather than crystals or lithic fragments will be important in this process. Thus welding occurs irrespective of relatively wide variations in crystal content.

Mechanism oferuption.-Marshall(1935) thought that ignimbrites have been produced by eruption from fissures. Since then Williams (1942), numerous Japanese workers and Gorshkov (1959) have drawn attention to the association of ignimbrites and calderas, while Van Bemmelen (1948) pointed out that similar rocks are produced near graben. There do not seem to be any undoubted cases where a central eruption has produced a

TEXTURAL EVIDENCE FOR THE ORIGIN OF IGNIMBRITES 107

lmm I

Fig. 5.Tubular texture of a dyke rock (Lake District, thin section No. 23535). The better developed flow bands show spherulitic growths.

welded ignimbrite. Moreover, the presence of several flows in single cooling units suggests that, whatever the nature of eruptions, they had followed each other in rapid succession. In other words, it seems that ignimbrites have been produced by sudden vesiculation of very large reservoirs of magma resulting in a breakdown of the viscous glass into shards and of the overlying layers of already vesiculated pumice into fragments. These fragments during the transport do not undergo a further fragmentation-otherwise their rectilinear outlines would be destroyed-but are carried together with the glass shards. Thus although the magma has a stage involving a foam, such is almost immediately destroyed as the actual flow begins. Therefore, the idea that ignimbrites are “foam-lavas” is untenable, as is also the idea that there are tuffolavas which in terms of their mechanism of transport are inter- mediate between the ignimbrites and lavas. Petrov (1961, Fig. 7) published a photograph of a tuf€olava with vesicular texture. The same textures occur in certain acid intrusions (Fig. 5) implying that it is a texture associated with rather volatile-rich rhyolites. Gorshkov (1961) points out that the turbulent flow encountered in ignimbrites is radically Merent from the essentially laminar flow of rhyolitic l a w . Thus when Milanovsky and Koron- ovsky (1961) suggest that a complete transition from tuffolava to an acid dyke can be observed, the tdolava in question must either be an ignimbrite or a rhyolitic lava. It is, however, unlikely that ignimbrites will be found in association with simple dykes. To produce sufEcient amounts of solid fragmentary material for even an individual ash-flow a much more widespread magnetic instability is necessary, which can occur at a caldera or in association with a volcano-tectonic graben, where the resultant eruption releases huge quantities of gas out of solution producing an exceedingly unstable foam which fragments instantaneously and on reaching the surface becomes an ash-flow.

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BECK, A, C. and ROBERTSON, E. I., 1955. Geology and geophysics. In geothermal steam for power in

BBMMeLBN (VAN), R. W., 1949. The geOrogy of Indonesia. The Hague. a n d RIJT~EN, M. G., 1955. Table mountains of Northern Iceland. Leiden. Coo& E. F., 1959. Ignimbrite bibliography. Idaho Bur. Mines & Geol. Information Circulor 4, 1.

with nfcrence to Snowdonia. Lpool Man&. geol. J. 5,600.

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DEPARTMENT OF GEOLOGY, THE UNIVERSITY, LrvERwoL 3.