Distinguishing strongly rheomorphic tuffs from extensive silicic lavas

Bull Volcanol (1992) 54:171-186 

Distinguishing strongly rheomorphic tufts 

from extensive silicic lavas 

Christopher D Henry 1 and John A Wolff / 

1 Bureau of Economic Geology, University of Texas at Austin, Austin, TX 78713, USA 

2 Department of Geology, University of Texas at Arlington, UTA Box 19049, Arlington, TX 76019, USA 

Received August 23, 1990/Accepted October 3, 1991 

Abstract. High-temperature silicic volcanic rocks, in- 

cluding strongly rheomorphic tuffs and extensive silicic 

lavas, have recently been recognized to be abundant in 

the geologic record. However, their mechanisms of 

eruption and emplacement are still controversial, and 

traditional criteria used to distinguish conventional 

ash-flow tuffs from silicic lavas largely fail to distin- 

guish the high-temperature versions. We suggest the 

following criteria, ordered in decreasing ease of identif- 

ication, to distinguish strongly rheomorphic tuffs from 

extensive silicic lavas: (1) the character of basal depo- 

sits; (2) the nature of distal parts of flows; (3) the rela- 

tionship of units to pre-existing topography; and (4) the 

type of source. As a result of quenching against the 

ground, basal deposits best preserve primary features, 

can be observed in single outcrops, and do not require 

knowing the full extent of a unit. Lavas commonly de- 

velop basal breccias composed of a variety of textural 

types of the flow in a finer clastic matrix; such deposits 

are unique to lavas. Because the chilled base of an ash- 

flow tuff generally does not participate in secondary 

flow, primary pyroclastic features are best preserved 

there. Massive, flow-banded bases are more consistent 

with a lava than a pyroclastic origin. Lavas are thick to 

their margins and have steep, abrupt flow fronts. Ash- 

flow tuffs thin to no more than a few meters at their 

distal ends, where they generally do not show any sec- 

ondary flow features. Lavas are stopped by topographic 

barriers unless the flow is much thicker than the bar- 

rier. Ash-flow tuffs moving at even relatively slow velo- 

cities can climb over barriers much higher than the re- 

sulting deposit. Lavas dominantly erupt from fissures 

and maintain fairly uniform thicknesses throughout 

their extents. Tufts commonly erupt from calderas 

where they can pond to thicknesses many times those 

of their outflow deposits. These criteria may also prove 

effective in distinguishing extensive silicic lavas from a 

postulated rock type termed lava-like ignimbrite. The 

latter have characteristics of lavas except for great areal 

Offprint requests to: CD Henry 

Volc a' ology 

9 Springer-Verlag 1992 

extents, up to many tens of kilometers. These rocks 

have been interpreted as ash-flow tuffs that formed 

from low, boiling-over eruption columns, based almost 

entirely on their great extents and the belief that silicic 

lavas could not flow such distances. However, we inter- 

pret the best known examples of lava-like ignimbrites 

to be lavas. This interpretation should be tested 

through additional documentation of their characteris- 

tics and research on the boiling-over eruption mecha- 

nism and the kinds of deposits it can produce. Flow 

bands, flow folds, ramps, elongate vesicles, and proba- 

bly upper breccias occur in both lavas and strongly 

rheomorphic tufts and are therefore not diagnostic. Pu- 

mice and shards also occur in both tuffs and lavas, al- 

though they occur throughout ash-flow tufts and gener- 

ally only in marginal breccias of lavas. Dense welding, 

secondary flow, and intense alteration accompanying 

crystallization at high temperature commonly obliterate 

primary textures in both thick, rheomorphic tufts and 

thick lavas. High-temperature silicic volcanic rocks are 

dominantly associated with tholeiitic flood basalts. Ex- 

tensive silicic lavas could be appropriately termed 

flood rhyolites. 

Introduction 

The textures and structures developed in silicic ash- 

flow tuffs (ignimbrites), including densely welded tuffs, 

are sufficiently different from those of lavas that the 

two can be distinguished even in ancient deformed and 

metamorphosed terranes (Table 1). These contrasts ar- 

ise from the very different eruption and emplacement 

mechanisms of lavas and tufts. In contrast, high-tem- 

perature, strongly rheomorphic tuffs and extensive 

silicic lavas, recently recognized to be abundant in the 

geologic record (Table 2), undergo different and/or 

more complex eruption and emplacement processes 

that blur these distinctions. High-temperature, large- 

volume silicic lavas have areal extents and aspect ratios 

(mean thickness divided by the characteristic horizontal

172 

Table 1. Characteristics commonly used to distinguish rhyolite 

lavas from ash-flow tufts ~ 

Common in conventional Common in rhyolite lavas and 

ash-flow tufts in rheomorphic tufts 

Fiamme 

Eutaxitic texture 

Abundant lithic fragments 

Nonwelded tops, bottoms, 

sides 

Gradual thinning at edges 

Wide areal extent 

Large aspect ratio 

Glass shards 

Broken phenocrysts ranging 

widely in size 

Gas elutriation pipes 

Flow bands 

Ramp structures 

Elongate vesicles 

Autobreccias 

Vitrophyres at or near tops 

Areally restricted b 

Small aspect ratio b 

Similar discrimination characteristics were discussed by Bristow 

and Cleverly (1979), Cleverly and Bristow (1982), Bonnichsen and 

Kauffman (1987), Henry and others (1988, 1989), and Bristow 

(1989) 

Rheomorphic tufts may have dimensions similar to those of 

other ash-flow tufts 

dimension of the rock body; Walker et al. 1980) com- 

parable to those of tufts (Cas 1978; Bonnichsen and 

Kauffman 1987; Henry et al. 1989, 1990; Christiansen 

and Hildreth 1989). Strongly rheomorphic tufts un- 

dergo such intense episodes of coherent viscous flow 

that they develop flow bands, flow folds, breccia zones, 

Table 2. Occurrences of extensive silicic volcanic rocks of controversial origin 

and other features of lavas (Schmincke and Swanson 

1967; Chapin and Lowell 1979; Wolff and Wright 

1981; Orsi and Sheridan 1984; Bonnichsen and Kauff- 

man 1987; Henry et al. 1989). Because of this conver- 

gence of characteristics, the traditional criteria of Table 

1 largely fail to distinguish extensive silicic lavas from 

strongly rheomorphic tufts. Both rock types may be 

found in some volcanic provinces (e.g. SW Idaho, Bon- 

nichsen and Kauffman 1987; Trans-Pecos Texas, Henry 

et al. 1989), which can further hinder interpretation of 

individual units, inasmuch as investigators have tended 

to interpret all widespread silicic units within a particu- 

lar province as being of either one or the other type. 

Until recently, the existence of extensive silicic lavas 

was controversial. With rare exceptions, any extensive 

silicic volcanic unit was assumed to be an ignimbrite. 

The main purpose of this paper is to propose diagnostic 

criteria to distinguish between strongly rheomorphic 

tufts and widespread silicic lavas. 

High-temperature silicic magmas, emplaced as pyro- 

clastic deposits or as widespread lavas, are significant 

throughout the geologic record (Table 2). Recognition 

of these rocks is especially significant for interpretation 

of old, eroded, and deformed volcanic terranes; for ex- 

ample, facies models for ancient terrains take silicic 

lavas to be near-source rock bodies (Cas and Wright 

1987). The continuously exposed extent of the Oligo- 

cene, single-flow-unit Bracks Rhyolite of Trans-Pecos 

Texas is 55 km, with a probable source area near one 

end of that extent (Henry et al. 1988, 1989, 1990). Clear- 

Volcanic province Characteristics of silicic volcanic rocks References 

Basal breccia Upper breccia Distal thinning Temperature (~ C) 

Snake River, Idaho Yes Yes No a 850-1050 Ekren et al. 1984; Bon- 

nichsen and Kauffman 

1987; Manley 1990; Honjo 

et al. 1992 

Yellowstone Park Yes Yes No > 850 Christiansen and Hildreth 

1989 

Trans-Pecos Texas Yes Yes No a > 900 Parker and McDowell 1979; 

Franklin et al. 1987, 1989; 

Henry et al. 1988, 1989, 

1990 

Keweenawan, Minnesota Minor Yes No > 1000 Green 1989; Green and Fitz 

1989 

Parana, Brazil Minor Yes No 1000-1150 Bellieni et al. 1984; Petrini et 

al. 1989; Whittingham 1989 

Etendeka, Namibia Minor Yes No 1000-1100 Milner 1986; Milner et al. 

1992 

Lebombo (Karoo) Minor Yes No 900-1100 Bristow and Cleverly 1979; 

South Africa Cleverly and Bristow 1982; 

Bristow 1989 

Rooiberg (Bushveld) Minor Yes No 1100 Twist and French 1983; Twist 

South Africa et al. 1989; Twist and EI- 

ston 1989 

Australia Yes Yes No 1100 Cas 1978; Creaser and White 

1991 

Some definite rheomorphic tufts thin to a few meters at margins, where they show little or no secondary flow. Lavas in Idaho and Texas 

have steep, abrupt flow fronts

ly, a conventional facies interpretation of a sequence 

containing such units could be seriously in error. 

Emplacement mechanisms and magma rheology 

To identify criteria useful to distinguish among the var- 

ious types of extensive silicic volcanic rocks, it is neces- 

sary to consider eruption and emplacement mecha- 

nisms and what they imply about resulting deposits. 

Thus, we envisage three possible rock types. 

Rheomorphic welded tuff 

The unit is distributed over most or all of its extent as a 

gas-supported particulate flow and undergoes welding 

and secondary coherent viscous flow after depositon 

(Wolff and Wright 1981). Only pyroclastic flow (and 

not fall) deposits are likely to undergo rheomorphism 

over a wide area, and hence the term rheoignimbrite is 

synonymous in this context. 

Lava-like tuff or lava-like ignimbrite 

This postulated rock type may form from a weakly ex- 

plosive eruption and low column. Deformation of con- 

stituent clasts occurs during primary particulate flow 

(i.e. within-flow agglutionation; see Henry et al. 1989, 

for a fuller discussion of terminology). This is similar to 

the primary laminar viscous flow concept of Schmincke 

and Swanson (1967) or Chapin and Lowell (1979). We 

recognise this as a separate rock type because clast 

properties influence deposition of material from the 

primary flow, and because agglutination requires lower 

clast viscosities than welding and rheomorphism (Wolff 

1983; Wolff and Horn 1991). Rheomorphism is there- 

fore likely to be more intense in such a deposit (and 

recognition of a pyroclastic origin correspondingly 

more difficult) than in one where coherence was only 

achieved after deposition, i.e. during welding. 

Although agglutination is known to occur in the for- 

mation and deposition of some pyroclastic flows (Tri- 

gila and Walker 1986), we point out that the concept of 

lava-like tuffs arose following identification of rock 

bodies that have the outcrop and textural characteris- 

tics of lavas but the low-aspect ratios of ignimbrites 

(Bristow and Cleverly 1979; Cleverly and Bristow 1982; 

Ekren et al. 1984; Milner 1986; Milner et al. 1992), and 

not through recognition of a distinct depositional 

mechanism. Having examined the major examples of 

lava-like tuffs, we conclude that they are lavas (see 

Concluding Remarks). Also, the units described by 

Schmincke and Swanson (1967) and Chapin and Low- 

ell (1979) are unequivocally pyroclastic and, in the 

former case, the recognition of 'primary welding' struc- 

tures is questionable (Wolff and Wright 1981; Leat and 

Schmincke 1990). Our experience indicates that, with 

sufficient information, individual units can be unequi- 

vocally identified as either lavas or rheomorphic tufts, 

173 

and there may ultimately be no need for a category of 

lava-like tuffs. We recognize it in this discussion be- 

cause the possibility of a distinct depositional mecha- 

nism, involving agglutination, has some implications 

for the diagnostic criteria we wish to evaluate. 

Extensive silicic lavas 

These are emplaced over their great extents as coherent 

flows, even though eruptions may vary from continuous 

effusion to some mildly explosive 'fire fountaining' 

(e.g. Duffield 1990). 

The above definitions focus on how the magma is 

emplaced over its observed extent, rather than the form 

in which it emerges from the vent. The need for this 

approach becomes clear when one considers that a low 

pyroclastic fountain playing over a vent could, depend- 

ing on the grainsize and 'stickiness' of the clasts, pro- 

duce either a partly fluidized particulate flow or a spat- 

ter-fed lava. In general, we envisage that intensely rheo- 

morphic ignimbrites are erupted from thermally conser- 

vative, low pyroclastic fountains (collapsing eruption 

columns), whereas widespread silicic lavas are spatter 

fed and/or the products of direct effusion. 

The fundamental physical property controlling be- 

havior of magmas during surface emplacement is vis- 

cosity (and allied rheological parameters), itself de- 

pendent on temperature and chemical composition. 

Welding and rheomorphism of ignimbrites is controlled 

by clast viscosity and deposit thickness, which in turn 

controls internal load pressure and cooling rate. Many 

intensely rheomorphic ignimbrites are, however, only a 

few meters thick and must have cooled rapidly after 

emplacement. It has been recognized for some time 

(e.g. Schmincke 1974) that intense welding and rheo- 

morphism, and possibly also the effects of within-flow 

clast agglutination, are common in pantelleritic and 

comenditic ignimbrites. These magmas produce pyro- 

clasts with lower viscosities at emplacement than most 

silicic magma types, by virtue of higher temperatures 

and Na-, K-, Fe-, and F-rich compositions. It is clear 

from several recent studies (Ekren et al. 1984; Bon- 

nichsen and Kaufman 1987; Henry et al. 1988; Honjo 

et al. 1992) that many intensely rheomorphic ignim- 

brites in continental settings are not peralkaline, but 

had magmatic temperatures (850-1100~ higher, in 

many cases much higher, than common silicic rocks 

(Table 2). They are typically porphyritic and lack evi- 

dence for superheating; the high temperatures there- 

fore require that they had unusually low water contents 

prior to eruption. Calculated magmatic viscosities, us- 

ing the method of Shaw (1972), are in the range 103-106 

Pa. s for compositions reported in the literature, using 

temperatures (900-1100 ~ C) given by the respective au- 

thors, and allowing 0-2 wt% water. These values are 

several orders of magnitude less than the eruptive vis- 

cosities of 'conventional' volatile-rich silicic magmas, 

which have lower initial magmatic temperatures, degas 

during eruption, and may undergo up to a few hundred 

degrees of eruptive cooling (Sparks et al. 1978).

174 

Formation of extensive silicic lava flows is also fa- 

vored by low magmatic viscosity and therefore high 

temperature. Although extrusion rate is probably the 

most important control on lava-flow length (Walker 

1973; Pieri and Baloga 1986), maximum magma rise 

rates in the conduit (and hence extrusion rates at con- 

stant conduit width) are controlled by viscosity or yield 

strength (Wilson and Head 1981). Thus a low-viscosity 

magma is inherently more capable of forming extensive 

flows than a high-viscosity equivalent. 

As pointed out by Mahood (1984), unusually fluid 

felsic magmas only exhibit 'anomalous' behavior when 

compared with common magmas of similar silica con- 

tent. When compared instead with rheologically similar 

types (e.g. andesites), closely similar behavior is seen. 

Andesites exhibit the whole range of eruptive behavior 

seen among silicic magmas (both 'hot' and 'normal'); 

they may form extensive lavas or domes and coulees, 

and may erupt explosively to yield plinian fallout and 

pyroclastic flow deposits that show agglutination, weld- 

ing, and rheomorphism. 

The difference between effusive and explosive erup- 

tions are relatively well understood, allowing straight- 

forward analysis of most of the deposits they can pro- 

duce. In contrast, the eruptive mechanism (and even 

existence) of lava-like tuffs is not established. They may 

form from low, boiling-over type eruptive columns in 

which heat loss to the atmosphere is minimal (Sparks et 

al. 1978; Bristow and Cleverly 1979; Ekren et al. 1984; 

Wolff 1989). Such an eruptive style has been recognized 

for Cotopaxi, Ecuador (Wolf 1878), and Mount St. Hel- 

ens, USA (Rowley et al. 1981). However, these erup- 

tions produced small, nonwelded tuffs, decidedly un- 

like the postulated lava-like ruffs. Our lack of under- 

standing of the mechanisms involved in producing 

lava-like tufts greatly restricts our ability to propose 

distinguishing criteria for them. 

Distinguishing criteria 

The extent to which a rheomorphic tuff or lava-like tuff 

resembles a lava depends on the extent to which vis- 

cous flowage has obscured features diagnostic of pyro- 

elastic origin. Rheomorphic tuffs commonly preserve 

abundant evidence of their pyroclastic origin, and sec- 

ondary flow generally obscures such evidence only lo- 

cally. Tuffs that have undergone extensive secondary 

viscous flow closely resemble lavas, and lava-like tuffs 

may be indistinguishable. Hence, much of this discus- 

sion emphasizes features diagnostic of extensive silicic 

lavas. 

The following discussion is based largely on our 

work in Trans-Pecos Texas (Franklin et al. 1987, 1989; 

Henry et al. 1988, 1989, 1990) but also upon field exam- 

ination and published reports of extensive, high-tem- 

perature silicic volcanic rocks in Idaho (Bonnichsen 

and Kauffman 1987; Ekren et al. 1984), Yellowstone 

National Park (Christiansen and Hildreth 1989), Nami- 

bia (Milner 1986; Milner et al. 1992), and South Africa 

(Bristow and Cleverly 1979; Twist and French 1983), 

and published reports of many others. We are particu- 

larly indebted to earlier attempts by the above authors 

to provide distinguishing criteria. Field criteria, espe- 

cially those features most likely to be preserved in an- 

cient sequences, are emphasized. 

Basal deposits 

Basal deposits potentially provide the most diagnostic 

and most easily recognized characteristics with which 

to distinguish rheomorphic tufts from extensive silicic 

lavas. Basal deposits of tuffs and lavas are commonly 

markedly different and are well preserved due to 

quenching against the ground and immediate burial by 

the flow of which they are part. Also, they can be ob- 

served in single outcrops and do not require knowing 

the full extent of a unit. Unfortunately in many cases, 

such as much of the mid-Tertiary of the western USA, 

basal sections are poorly exposed or covered. 

The bases of lavas are characterized by breccias that 

develop as the chilled, plastic to brittle fragments of 

broken crust on the top of a flow are brought to the 

flow front and overridden by it (cf. 'caterpillar-track' 

motion of aa lavas). Under some conditions, a large 

proportion of fragments may also spall directly from 

the flow front (Cas and Wright 1987; Manley and Fink 

1987). Such breccias consist of clasts of a mix of lava 

textural types, including massive, vesicular, flow- 

banded and flow-folded, glassy, pumiceous, and devi- 

trifled (Fig. 1), all of which are exposed in flow tops 

and fronts (Christiansen and Lipman 1966; Fink 1983); 

large clasts reside in a matrix of more finely commin- 

uted material, which can include pumice and shards. 

Basal flow breccias can occur throughout the areal ex- 

tent of a flow, exhibit no systematic grain-size varia- 

tions (although sorting chracteristics may vary), may 

vary markedly in thickness over short lateral distances, 

and may locally be absent. Thickness varies because 

different amounts of material spall off the front as the 

flow progresses. Felsic lava breccias tend to thicken in 

valleys due to a tendency for increased breccia forma- 

tion when the lava is poised on a slope (C. R. Manley, 

personal communication, 1988). Because the breccia is 

a product of the particular emplacement mechanism, 

the occurrence of basal breceias throughout most of the 

extent of a unit argues that it flowed as a lava over its 

entire extent. 

Because clast deformation in a rheomorphic tuff (as 

defined above) only occurs after deposition, chilling of 

the base of a tuff should prevent it from participating in 

secondary flow (and to some extent in welding) and 

preserve its pyroclastic character. The basal flow unit in 

the Barrel Springs Formation in Texas, the most in- 

tensely and pervasively rheomorphic tuff known to us 

(Henry et al. 1989), provides a good example (Fig. 2). 

The upper 95 m of this 100-m-thick tuff is foliated, 

flow-banded, flow-folded, ramped, and even brec- 

ciated; the only indication of a pyroclastic origin is the 

presence of lithic fragments in low to moderate abun- 

dance. However, the 5-m-thick basal facies contains un-

equivocal evidence of the pyroclastic origin of the unit; 

it grades upward from poorly through densely welded 

to rheomorphic tuff, contains easily recognizable pu- 

mice (fiamme where densely welded), and has abun- 

dant crystal- and lithic-enriched gas elutriation pipes. 

Distinction between the base of a lava and a lava- 

like tuff may be more difficult. Within-flow agglutina- 

tion prior to deposition, and, presumably development 

of a significant temperature profile through the deposit, 

could obliterate most evidence of pyroclastic origin, 

even at the basal contact. Ekren and others (1984) sug- 

gested that subsequent rheomorphic flow could brec- 

ciate the base and that this process generated basal 

breccia in what they interpreted to be lava-like tuffs in 

southwestern Idaho (but interpreted by us as lavas). 

The process is essentially the same as brittle failure and 

brecciation due to intense shearing within the interior 

of a lava flow (Bonnichsen and Kauffman 1987; Man- 

ley and Fink 1987). However, such internal breccias are 

composed only of clasts from the immediately adjacent 

parts of the flow. Breccias resulting from this process 

acting on the base of a lava-like tuff should be distin- 

guishable from those of a lava flow. Clasts in the brec- 

cia could consist only of basal agglutinated and densely 

welded tuff and the lowest part of the rheomorphic 

portion. The mix of lithologies that can occur in a lava- 

flow breccia would not occur. A/so, such breccias, by 

analogy with those developed in the interior of lava- 

flow units, should be limited in extent and of sporadic 

occurrence. It is also possible that secondary flow of 

tuff beyond the limits of the initial pyroclastic flow 

deposition could generate a 'lava-flow' breccia, but 

such a breccia would be confined to the outer margins 

of the body. Not only would the distribution of such a 

breccia be unlike that of a true lava-flow breccia, but 

secondary flow is least likely in the thin, distal parts of 

an ignimbrite, which is also the part of the sheet with 

the least preservation potential. 

175 

Fig. 1. Basal breccia of extensive silicic lava, 

southwestern Idaho (Bonnichsen and Kauff- 

man 1987), formed by collapse of chilled flow 

front. Breccia clasts include a variety of lava 

textural types (massive, flow-banded and 

folded, vesicular, glassy, and devitrified) in a 

finely clastic matrix. Such a deposit is diagnos- 

tic of a lava-flow origin 

A basal coarse-clast accumulation layer of an ash- 

flow tuff (such as the ground layer of Walker et al. 

198I) might be difficult to distinguish from a basal 

breccia of a lava if the dominant 'lithics' were poorly 

vesiculated magma lumps rather than accidental frag- 

ments. However, in most cases, clast variety and shape 

serve to distinguish the two breccia types. Additionally, 

clasts in a basal accumulation layer should show a sys- 

tematic size variation related to distance from source 

and to the size of clasts (if discernible) in the overlying 

part of the unit, which would not be the case in a basal 

lava breccia. 

Basal breccias are locally absent in some silicic lav- 

as, particularly near source (e.g. Figure 9 of Bonnichsen 

and Kauffman 1987), and are rare and, where present, 

thin and discontinuous in many of the postulated lava- 

like tuffs (Table 2; Fig. 3; Milner et al. 1992; CDH, per- 

sonal inspection). The bases of these lava-like units are 

mostly flow-banded, flow-folded, and slightly vesicu- 

lar; they resemble bases of many rapidly emplaced, 

low-viscosity basaltic lavas (e.g. Figure 9b of Green 

1989). Therefore, absence of basal breccia is not evi- 

dence for either origin. We note that basal breccia is 

least common in units from those provinces with the 

highest reported magmatic temperatures (Table 2) and 

therefore the lowest probable viscosities. Whether or 

not flow-banded and folded, unbrecciated bases can 

also form from pyroclastic flows is not established. 

Also, prolonged heating by an overlying thick and hot 

flow interior may homogenize initial basal breccia (C. 

R. Manley, personal communication, 1990). 

The presence of some pyroclastic material at the 

base of a unit is not proof of either origin. Pyroclastic 

eruptions commonly precede lava emplacement in 

small silicic domes and flows (Fink 1983; Heiken and 

Wohletz 1987). The eruptions produce air-fall or surge 

deposits, which may contain large clasts of volatile- 

poor lava. Bonnichsen and Kauffman (1987) suggested

176 

~,. .:~ 

,.,'. On'.'~..~ 

3_: W"5 if.O: 

-~a W, .'t~.. o' ,~, 

"" "- b.* 

z~o~o 

~. :o.:r ~. 

r.O 

t,.. 

N 

Irregular top~ either eroded or primary top 

of rheomorphic tuff 

Glossy to devitrified rheomorphic flow breccia; 

blocks to 4m, commonly flow-bonded glass is 

hydrated portly to enlirely spherulitically de- 

vitrified matrix consisls of finer closts hy- 

drated 1o devtrified and commonly s lcified, 

alteration increases upward 

Massive, flow-bonded, devitrified zone within 

breccia 

Not exposed 

Massive, devitrified, flow-banded rheomorphic 

tuff containing lithics 

fleginning of brecciafion 

Beginning of flow folding 

Massive, devitrified, folioted rheomorphic 

tuff containing Iithics 

Basal ash-flow tuff; phenocrysts of alkali 

feldspar and clinopyroxene (5-10%); sparse 

dark pumice and lithics; gas-escape pipes; 

welding increases upward 

Fig. 2. Section through rheomorphic tuff of the Barrel Springs 

Formation, Trans-Pecos Texas, the most intensely rheomorphic 

tuff known to us. Basal 5 m of unit preserves abundant ash-flow 

features, including shards, pumice, lithic fragments, and gas elu- 

triation pipes. Above a narrow transition interval, the rock is fol- 

iated. Pumice and shards are obliterated. However, the rock is 

chemically and mineralogically identical to the tuff, and lithic 

fragments, identical to those in the tuff, are common. Upward, the 

rock is flow-banded, folded and brecciated. The upper half of the 

unit is a breccia identical to those of lava flows; clasts include a 

variety of textural types of the unit in a matrix of finer clasts 

that volcanic ash beneath two large-volume silicic lavas 

in Idaho represents near-vent fall deposits. Fusion of 

initial pyroclastic deposits by later lava (e.g. Chris- 

tiansen and Lipman 1966), particularly if the latter is 

massive at its base, could result in a very misleading 

apparent increase in degree of 'welding' upward to co- 

herent rock. Similar complications result when second- 

ary pyroclastic flows are generated at the front of an 

advancing lava, as described by Fink and Manley 

(1989). In this process, sudden collapse of the lava-flow 

front exposes the volatile-overpressured interior of the 

flow, with a resultant explosion producing a pyroclastic 

flow. Deposits from the pyroclastic flow may be over- 

ridden and fused by the moving lava (Fig. 4), resulting 

in an apparent welding/rheomorphism zonation. 

In turn, the absence of air-faU or surge deposits at 

the base of a unit is not evidence for a lava origin. Py- 

m 

5O 

40 

30 

20 

I0 

roclastic flows may move beyond the extent of any pre- 

cursor deposits, and in any case explosive eruptions of 

volatile-poor magmas commonly do not involve an ini- 

tial plinian phase (Sparks and Wilson 1976). 

Interiors and tops of units 

Interiors and tops of units provide the least useful in- 

formation to distinguish rheomorphic tuffs from exten- 

sive silicic lavas. The interior of a strongly rheomorphic 

tuff will appear most lava-like because secondary flow 

is most intense there. Flow bands, flow folds, ramp 

structures, and elongate vesicles are common in the up- 

per parts of many easily recognizable rheomorphic 

tuffs (Fig. 2; also, Chapin and Lowell 1979; Wolff and 

Wright 1981; Orsi and Sheridan 1984; Hargrove and 

Sheridan 1984; Parker 1986). Additionally, intense re- 

crystallization within the interior of a flow commonly 

disguises primary textures. 

Upper breccias, in contrast to basal breccias, are not 

diagnostic of emplacement mechanism, although they 

are undoubtedly less common in rheomorphic tuffs 

than in lavas. The only example of an upper breccia in 

a rheomorphic tuff known to us is in the strongly rheo- 

morphic flow unit in the Barrel Springs Formation (Fig. 

2; see also Henry et al. 1989). This unit contains an up- 

per breccia as much as 40 m thick that developed dur- 

ing extreme secondary flow. Upper breccias are also 

common in rocks interpreted to be lava-like tuffs (Table 

2; Ekren et al. 1984; Milner et al. 1992). 

Evidence of welding zonation, particularly including 

cooling breaks, or of upper nonwelded or poorly 

welded tuff, would be diagnostic of a pyroclastic origin. 

The only reported examples of cooling breaks in lava- 

like tuffs are from southwestern Idaho (Ekren et al. 

1984). However, we interpret most of those examples to 

be contacts between lava flows. A few others (see Fig- 

ure 20 of Ekren et al.) appear to represent discontinui- 

ties where cooling joints that propagated from the bot- 

tom and top of a flow meet in the middle (Bonnichsen 

and Kauffman 1987). One rheomorphic tuff of the Ke- 

weenawan province has a poorly welded top (J. C. 

Green, personal communication, 1990). Generally, up- 

per nonwelded material is unlikely to be preserved and 

could be confused with highly vesicular upper breccia 

produced by lava flows. 

Distal thinning and unit margins 

Strongly rheomorphic ruffs and extensive silicic lavas 

should be most easily distinguished in distal deposits. 

Ash-flow tuffs emplaced on flat topography thin grad- 

ually toward their margins unless they encounter a to- 

pographic barrier (Cas and Wright 1987), although lev- 

ees may be present at the margin. The most strongly 

rheomorphic tufts of Texas thin to as little as 2 m at 

their distal edges, where they show few or no secondary 

flow features (Figs. 5 and 6). In contrast, silicic lavas 

generally maintain considerable thicknesses throughout

(a) 

' "~ 9 "" _~__ " "' ~ / ~ ~'~lj 

(b) Proximal Distal 

~ Massive lava 

~"~""~.~n Basal breccia to lava 

" 9 _'7".'" 9 ~"." -- - 

~ '."~'-" "~" :-: f.'D. " :'" ::" ~"~ i'." ~ Pyroclastic flow deposit . ii..'~. ('!'.~" .~'~-I i'.: 

.~ :.':c 

r-~ ,* 9 Lava front breccia -. ~ .~. 6t::2 :i 

Fig. 4. a Diagram showing collapse of lava-flow front and explo- 

sion of exposed, volatile-oversaturated lava interior, to produce a 

secondary pyroclastic flow (from Fink and Manley 1989). b Two 

schematic sections depicting resulting relations, after the pyro- 

clastic flow deposit has been overridden by the advancing lava: 

'proximal' section applies to the vicinity of the lava-flow front 

where the secondary explosion occurred; 'distal' section applies 

to any location further along the lava path. Note that heat from 

the overriding lava may fuse and flatten underlying glassy clasts 

their distribution and have steep, abrupt flow fronts 

(Figs. 5 and 6; Table 2; see also Figure 23 of Bon- 

nichsen and Kauffman 1987). Well-preserved flow 

fronts in southwestern Idaho (Bonnichsen and Kauff- 

1 

177 

Fig. 3. Rare occurrence of basal breccia in unit 

3, rhyolitic lava-like tuff of the Jozini Forma- 

tion, Lebombo Monocline, South Africa (Bris- 

tow and Cleverly 1979). Base is massive on left 

half of photo. On right is thin breccia, 10 to 

40 cm thick, composed of vesicular clasts. 

Rhyolite overlies basalt 

man 1987), Yellowstone National Park (Christiansen 

and Hildreth 1989), and Texas (Henry et al. 1989) range 

from a minimum of 25 m to as much as 200 m high. 

Most lava-like ruffs remain thick throughout their dis- 

tribution (Table 2; Milner et al. 1992; CDH, personal 

inspection), which we consider to be consistent with a 

lava-flow origin. Erosion of distal parts of a rheomor- 

phic welded tuff could complicate interpretation if, for 

example, an abrupt transition existed between rheo- 

morphic and non-rheomorphic parts of a tuff. Both 

lavas and pyroclastic flow deposits may split into indi- 

vidual flow lobes at their margins. Although we know 

of no examples, it is also possible that the rheomorphic 

flow of a welded tuff beyond its original depositional 

extent could develop flow lobes and steep flow fronts. 

Aspect ratios 

The sheetlike geometry and low-aspect ratio of these 

silicic rock bodies have, in many cases, been the princi- 

pal features used to infer a pyroclastic origin (Bristow 

and Cleverly 1979; Cleverly and Bristow 1982; Ekren et 

al. 1984; Milner 1986; Henry et al. 1988; Twist et al. 

1989; Milner et al. 1992) and the root of controversy 

about emplacement mechanism (i.e. lava versus pyro- 

clastic flow). On this basis, aspect ratio has no value as 

a genetic indicator. Nevertheless, aspect ratios of well- 

documented lavas are larger than those of many pyro- 

clastic flow deposits, and, in particular, averaged thick- 

nesses of lavas are greater (Fig. 7). The range of thick- 

nesses of widespread silicic lavas is similar to that of 

conventional flows and domes (Fig. 7). Insufficient 

data exist to calculate aspect ratios for other possible 

lavas, but their considerable thicknesses suggest a simi- 

lar pattern. Comparison of the rheomorphic Buckshot 

Ignimbrite with the Bracks Rhyolite, an extensive silicic 

lava, illustrates these differences (Fig. 7). Both crop out 

over similar areas in Texas and were emplaced on fairly

178 

2~ ] ~ _ ~ Mitchell Mesa Rhyolite - conventional ignimbrite 

2~] Gomez Tuff - rheomorphic ignimbrite 

OJ~ 

0 J 

Silicic Lavas 

Bracks R h y o l i ~ 

/ Dorsey Creek Rhyolite Covered 

0 10 20 30 km 20X Vertical Exaggeration 

Fig. 6. Longitudinal sections through a conventional ignimbrite 

(the Mitchell Mesa Rhyolite), a strongly rheomorphic tuff (the 

Gomez Tuff), and two extensive siticic lavas (the Bracks Rhyolite 

and Dorsey Creek Rhyolite), illustrating marked differences in 

distal thinning and flow margins. The first three are in Trans- 

Pecos Texas (our data plus Parker 1986); the Dorsey Creek 

Rhyolite is in southwestern Idaho (data from Bonnichsen and 

Kauffman 1987 and B. Bonnichsen, personal communication 

1989). Sections of the two ignimbrites start from near caldera 

sources (where they are ponded to much greater thicknesses) at 

the left edge of diagram. Sections of the two lavas extend through 

entire exposures; source of the Bracks Rhyolite is probably be- 

neath the thick part (Henry et al. 1990) 

flal topography. The Buckshot Ignimbrite has an out- 

crop area of about 2000 km 2 and a maximum thickness 

of about 20 m. The Bracks Rhyolites covers 1000 km 2, 

has a maximum thickness of 120 m, and is nowhere less 

than 25 m thick (Henry et al. 1990). 

The only comprehensive data for lava-like tufts are 

for the Etendeka sequence of Namibia (Fig. 7; Milner 

et al. 1992). Exceptionally low aspect ratios (1:2100) 

calculated for these rocks are based on lateral extents 

of 130 km. However, this large extent assumes correla- 

tion of chemically similar rocks in two areas of outcrop 

50 km apart. If units in these areas are considered sepa- 

rately, their aspects ratios would be similar to those of 

the silicic lavas plotted in Fig. 7. Interpreting aspect ra- 

tios of older and highly eroded deposits is difficult 

I000, 

Fig. 5. Contrast in thickness of distal parts of 

the strongly rheomorphic Gomez Tuff (area 

outlined by trees on plateau) and an extensive 

silicic lava (massive cliffs) of the Star Moun- 

tain Formation in Trans-Pecos Texas. Tuff is 

approximately 2 m thick here and not rheo- 

morphie. Lava is about 70 m thick 

0a i ~b ~do 100o 

Diameter of a circle with an area equal to that of unit (km) 

Fig. 7. Aspect ratio plot of extensive silieic iavas (T, Texas, Sleep- 

ing Lion, Adobe Canyon, and Star Mountain flows, Henry et al. 

1989;Y, Yellowstone National Park, Pitehstone Plateau flow, 

Christiansen and Hildreth 1989; I, southwestern Idaho, Dorsey 

Creek and Sheep Creek flows, Bonnichsen and Kauffman 1987; 

A, Australia, unit B1, Cas 1978). E, the Lower Springbok unit of 

the Etendeka, Namibia, interpreted by Milner et al. (1992) as a 

rheoignimbrite. Extensive silicic lavas generally plot above or just 

within the field of high-aspect-ratio ignimbrites (i.e. the lavas have 

similar or higher ratios of thickness to extent). The Bracks Rhyol- 

ite (Br), an extensive silicie lava, and Buckshot Ignimbrite (Bu), a 

rheomorphic tuff, crop out over similar areas of Texas but have 

markedly different aspect ratios. Fields are FL, felsic lava; ML, 

marie lava; HI, large-aspect-ratio ignimbrite; and LI, small-as- 

pect-ratio ignimbrite 

(Christiansen and Hildreth 1989; Henry et al. 1989). 

There are several known examples of clusters of dis- 

crete but compositionally similar rhyolitic lava domes 

(Christiansen and Hildreht 1989; Henry et al. 1989; 

Duffield and Dalrymple 1990) that could easily be mis- 

identified as a single flow, particularly if flow margins 

have been removed by erosion.

Response to topography 

Ingnimbrites are produced from collapsing eruption 

columns (pyroclastic fountains) up to a few km in 

height (Smith 1960; Sparks and Wilson 1976; Sparks et 

al. 1978). The more violently emplaced flows (for pres- 

ent purposes, 'conventional' ignimbrites) have flow vel- 

ocities up to several hundred meters per second, high 

enough for them to surmount considerable topographic 

barriers (Sparks et al. 1978). High-temperature, rheo- 

morphic tuffs are inferred tO form from low, thermally 

conservative fountains and may flow much more slow- 

ly. However, even at flow velocities of 50 m/s, a pyro- 

clastic flow can rise more than 100 m (Sparks et al. 

1978) and thus should still be able to overtop modest 

barriers. Inflation of pyroclastic flows also allows them 

to be deposited on top of barriers higher than the final 

deposit thickness. In contrast, lavas can only flood bar- 

riers lower than the flow thickness; otherwise they must 

pond against them. Two examples from Texas illustrate 

this behavior. In Fig. 8, the rheomorphic tuff thins but 

continues over a 40-m-high lava flow front. In the sec- 

ond example, the rheomorphic Gomez Tuff overlies 

and thins on top of a rhyolite dome that is approxi- 

mately 100 m higher than the base of the tuff. A lava of 

the Adobe Canyon Formation terminated against the 

dome, even though the mantling Gomez Tuff had re- 

duced the topographic relief. 

Sources 

Large-volume ash-flow tuffs, including rheoignim- 

brites, are commonly associated with calderas inferred 

to contain their source vents. Rheomorphic tuffs in 

Texas can be traced back to source calderas where they 

pond to thicknesses many times greater than in outflow 

sheets. The strongly rheomorphic Barrel Springs tuff is 

no more than 100 m thick in outflow deposits, yet 

reaches a thickness of 800 m within its caldera (Hender- 

179 

Fig. 8. Strongly rheomorphic tuff of the Barrel 

Springs Formation in Texas illustrating re- 

sponse to topography. Tuff is 50 m thick at left, 

has a preserved pyroclastic base, and is flow 

banded and folded in the middle and upper 

parts. It flowed from left to fight over the 40- 

m-high flow front of a silicic lava. Above the 

lava, it is approximately 10 m thick, shows no 

rheomorphie features, and is obviously an ig- 

nimbrite. A silicic lava of comparable thickness 

encountering a similar topographic barrier 

would have ponded against it 

son 1987). The Gomez Tuff is as much as 450 m thick in 

the Buckhorn caldera (Parker 1986) but no more than 

100 m thick elsewhere. It may, however, be difficult to 

identify source calderas in regions of heavily dissected 

paleotopography, where ignimbrites may have ponded 

in large, steep-walled canyons. 

Few sources for large-volume silicic lavas have been 

identified, but, as with many conventional lavas, at 

least some appear to have erupted from fissures (Bon- 

nichsen and Kauffman 1987; Christiansen and Hildreth 

1989; Henry et al. 1989; Manley 1990). A feeder dike to 

a widespread silicic lava flow of the Star Mountain 

Formation in Texas is the best example of probable fis- 

sure eruption known to us (Fig. 9). The northern end of 

the 3-km-long dike disappears beneath and passes 

without interruption into the overlying flow; the dike 

and flow are petrographically and chemically indistin- 

guishable. Manley (1990) interpreted coarse tephra de- 

posits to flank a fissure vent for a lava of southwestern 

Idaho that had previously been interpreted as a lava- 

like tuff (Ekren et al. 1984). 

The sources of many other large-volume silicic lavas 

have not been identified (Cas 1978; Twist and French 

1983; Bonnichsen and Kauffman 1987; Green 1989; 

Green and Fitz 1989). These flows maintain nearly uni- 

form thicknesses throughout their outcrop and clearly 

do not pond within calderas. If fissure-fed, the sources 

are likely to be buried beneath the flows, as is the case 

for many small rhyolite domes (Fink and Pollard 1983) 

and flood basalts (Cas and Wright 1987). In the ab- 

sence of topographic control, elongation of outcrop 

may suggest eruption from a fissure. Linear joints that 

are associated with vents for some silicic domes (Fink 

and Pollar 1983) may also indicate an underlying fis- 

sure system. 

The sources of lava-like tuffs are also equivocal. 

Ekren et al. (1984) suggested the lack of calderas asso- 

ciated with lava-like tuffs of southwestern Idaho indi- 

cated the tuffs erupted from great depths. In contrast, 

Milner et al. (1992) suggest a possible caldera source

180 

for the Etendeka rocks. No sources have been identif- 

ied for lava-like tufts of the Jozini Formation (Lebom- 

bo) in South Africa (Bristow and Cleverly 1979). How- 

ever, a 10-m-thick dike, petrographically similar to the 

Jozini rhyolites, cuts one flow (J. W. Bristow, personal 

communication, 1990). Its relation to the rhyolites is 

uncertain. 

Recognition of distinctive sources seems, empirical- 

ly, to be a good indicator of origin. However, there is 

no clear reason why a voluminous, extensive lava 

should not be erupted from and pond within a ealdera, 

nor why large volumes of similarly volatile-poor silicic 

magma should not explosively erupt from fissures unre- 

lated to calderas. Indeed, many of the Yellowstone 

lavas are ponded whitin an existing caldera (Chris- 

tiansen and Hildreth 1989). The eruptive style of a hot 

pyroelastic flow fed from a low fountain is little differ- 

ent from that a spatter-fed lava; which type of rock 

body results may depend only on the proportion and 

retention of fine particles (enabling partial fluidisation 

of a pyroclastic flow) produced in the fountain (Wolff 

and Horn 1991). Much more work is required to prop- 

erly understand the eruptive mechanisms for these 

rocks and the nature and location within the crust of 

their source magma bodies. 

Petrooraphic criteria 

It is often assumed that petrographic features such as 

shards, pumice, lithic fragments, and broken pheno- 

crysts provide unequivocal evidence of pyroclastic ori- 

gin. The presence of obscure shard- and pumice-like 

features at the base or top of some rock units has been 

interpreted as evidence of pyroclastic origin for several 

lava-like tufts (Fig. 10; Bristow and Cleverly 1979; 

Ekren et al. 1984; Milner 1986; Milner et al. 1992). 

However, these features are poorly preserved or even 

lacking in many rheomorphic tufts due to secondary 

flow, recrystallization, and erosion. Also, lavas com- 

monly develop textures that can be confused with those 

Fig. 9. Feeder dike to extensive silicic lava of 

the Star Mountain Formation (background) in 

Texas. Dike is petrographically and chemically 

identical to lava. Flow bands in dike are verti- 

cal and turn abruptly horizontal at level of 

flow 

of pyroclastic flows (e.g. Manley and Fink 1987; Allen 

1988). Thus it is imperative to understand exactly how 

these features form and where they can occur and be 

preserved in high-temperature tuffs and lavas. 

Magma vesiculation and fragmentation are integral 

parts of the eruption of pyroclastic flows. Therefore, 

pumice and shards can occur throughout an ash-flow 

tuff. However, several factors work to minimize their 

abundance and preservation in high-temperature tufts. 

First, the inferred low volatile content and low explo- 

sivity of eruptions that produce high-temperature pyro- 

elastic flows should minimize the amount of vesicula- 

tion and breakage. The abundance of shards and pu- 

mice relative to dense magma lumps is probably much 

less than in low-temperatue, more volatile-rich, pyro- 

elastic flows. Additionally, the dense welding that com- 

monly occurs throughout high-temperature ash-flow 

tuffs, even those that do not undergo sceondary flow, 

tends to disguise primary pyroclastie texture. Finally, 

secondary flow and intense crystallization during devi- 

trification can completely obliterate pumice and 

shards. 

In the strictest sense, pumice and shards indicate 

only that a magma vesiculated and fragmented. Lavas 

vesiculate, in some examples throughout a flow, and 

brecciate at their margins. Manley and Fink (1987) de- 

scribe coarsely vesicular and finely vesicular pumice in 

unequivocal silicic lavas that are virtually indistinguish- 

able from pumice of pyroclastic flows. Highly vesicular, 

pumiceous clasts are common in upper and lower brec- 

cias of all silicic lavas; Fink (1983), Fink and Manley 

(1987), Manley and Fink (1987), and Sampson (1987) 

provide excellent photographs of pumiceous textures 

from conventional, small-volume silicic lavas. Attrition 

of this pumice can yield a matrix composed of glass 

shards. However, these breccias commonly contain a 

spectrum of textural types that additionally include 

massive, flow-banded, devitrified, or vitrophyric clasts 

(Fig. 1). This assemblage of lithologies is, in fact, diag- 

nostic of a lava flow despite the presence of pumice.

Lensoid features resembling flattened pumice 

(fiamme) can develop in silicic lavas, although the 

mechanisms by which they form are not completely un- 

derstood. One mechanism is plastic deformation and 

stretching, presumably at low strain rates, of clasts in 

internal breccia zones originally produced by brittle 

failure at high strain rates. Zones of 'pseudo-fiamme' 

formed in this way are not systematically distributed 

with respect to flow margins, unlike true fiamme in 

rheomorphic and lava-like tuffs, which should be most 

abundant around flow tops, bases, and margins. Incom- 

plete magma mixing can also produce fiamme-like tex- 

tures (see Fig. 6b in Sampson 1987). 

Alteration processes may also produce pseudo-pyro- 

clastic textures in lavas. Allen (1988) describes silicic 

lavas in Australia in which hydrothermal alteration of 

initially massive, glassy material with abundant perlitic 

cracks produced textures that closely mimicked shards. 

He also noted that granulation, including dismember- 

ment of phenocrysts, by tectonic processes accentuated 

this false pyroclastic appearance. Fiamme-like struc- 

tures also develop in weakly metamorphosed and tec- 

tonized silicic lavas (Cas and Wright 1987; Allen 1988). 

Although Allen's examples were exclusively in areas of 

significant hydrothermal alteration, similar textures 

were produced in Jozini Rhyolites by normal deuteric 

alteration (J. W. Bristow, personal cummunication, 

1990). 

We conclude from this discussion that the presence 

of pumice and shards throughout a flow is proof of a 

pyroclastic origin, but such preservation should be un- 

common in high-temperature, rheomorphic tuffs. The 

presence of pumice and shards only in breccias at the 

base or top of a flow is at least equally consistent with 

origin as a lava flow. It is precisely the lack of preserva- 

tion of these textures in either rheomorphic tuffs or ex- 

tensive silicic lavas that complicates interpetration. In 

fact, preservation of recognizable pumice and shards in 

a rheomorphic or lava-like tuff suggests that primary 

181 

Fig. 10. 'Streaky' texture near base of lava-like 

tuff, unit 6 of Jozini Formation, Lebombo 

Monocline, South Africa (Bristow and Cleverly 

1979). This texture may represent highly 

stretched pumice (fiamme) as a result of pri- 

mary laminar or secondary flow of an ash-flow 

tuff. See text for discussion 

laminar or secondary flow was, at most, minor. The py- 

roclastic origin of such a deposit should be obvious. 

The presence of lithic fragments, characteristic of 

conventional ash-flow tufts, is not diagnostic but has 

some utility in distinguishing rheomorphic tufts from 

extensive silicic lavas. Lithic fragments in pyroclastic 

flow deposits are derived from erosion of event walls 

during eruption (Wilson et al. 1980) or are picked up 

from the ground across which the flow travels. In the 

whole spectrum of explosive silicic eruptions, vent ero- 

sion is probably at a minimum for the weakly explosive 

eruptions of gas-poor magmas envisaged to generate 

strongly rheomorphic and lava-like tuffs. Lithic abun- 

dances should generally be, and indeed are, low in such 

tuffs (authors' observations). Most examples that we 

have studied have no more than 1-2% lithics, but one 

peralkaline rhyolitic rheomorphic tuff on Gran Canar- 

ia, Canary Islands, locally contains 5-10% lithics (JAW, 

personal inspection). Silicic lavas may also contain lit- 

hic fragments, for example the Quaternary Banco Bon- 

ito flow of the Jemez Mountains, New Mexico (Self et 

al. 1991). If, as suggested above, some widespread 

silicic lavas are spatter-fed from pyroclastic fountains 

similar to those that feed hot, intensely rheomorphic ig- 

nimbrites, then a similar lithic content might be ex- 

pected. Also, autoliths or magma inclusions, both of 

which can be common in lavas (Bacon 1986), must be 

distinguished from true foreign fragments. 

The location of lithic fragments or inclusion within a 

unit may also be helpful. Large lithics are typically con- 

centrated near the base of a pyroclastic-flow deposit, 

including individual flows within compound ignim- 

brites. Secondary flow may redistribute them slightly 

but should not disperse them. Distinct lenses of lithics 

within the rheomorphic Gomez Tuff parallel secondary 

flow foliation and probably mark internal flow boun- 

daries. In contrast, inclusions transported by a lava 

may be more randomly distributed through the interior 

of a flow. A quartz latite of the Etendeka province in

182 

Namibia (interpreted by Milner et al. 1992, as a rheoig- 

nimbrite) contains quartzite inclusions as much as 

50 cm in diameter scattered apparently at random 

throughout it. Margins of the inclusions are delicately 

cuspate (Fig. 11), indicating significant dissolution. The 

distribution and texture of these inclusions are better 

accounted for by transport in a lava rather than in a 

pyroclastic flow. 

Phenocryst morphology also provides equivocal evi- 

dence of origin. Broken phenocrysts are typical of 

many 'conventinal' tuffs, whereas euhedral phenocrysts 

and glomerocrysts are more common in lavas. Howev- 

er, phenocryst breakage during explosive eruption is 

presumably controlled by magmatic viscosity and vola- 

tile content; broken phenocrysts will be uncommon in 

eruptives produced from relatively hot, fluid, and gas- 

poor magmas. For example, liberated phenocrysts in 

mafic scoria accumulations are rarely broken. Observa- 

tions of rheomorphic silicic tuffs are also consistent 

with this conclusion. Broken phenocrysts are sparse in 

the rheomorhic Green Tuff of Pantelleria (JAW, per- 

sonal inspection); the strongly rheomorphic Barrel 

Springs tuff contains many complex glomerocrysts that 

could not have survived strongly explosive eruption 

(Henry et al. 1989). 

Contrasting crystallization textures that result from 

devitrification of glass versus primary crystallization of 

homogeneous liquid constitute promising but still 

poorly understood criteria (Twist and Elston 1989). 

Dendritic or swallowtail crystallites indicate rapid crys- 

tallization at high degrees of undercooling from super- 

heated melts (Lofgren 1980; Twist and Elston 1989). 

Such texture can only form from liquids, not by devi- 

trification, and thus may be more indicative of a lava- 

flow origin. Examples have been found in very old (2- 

Ga Bushveld Complex; Twist and French 1983) and 

young (37-Ma Bracks Rhyolite, Texas; Henry et al. 

1990) extensive silicic lavas. 

Compositional homogeneity 

Fig. 11. Quartzite inclusion in the Upper 

Springbok unit, Etendeka Formation, Namibia, 

interpreted as a rheoignimbrite (Milner et ai. 

1992). Inclusions up to 50 cm in diameter are 

widely dispersed through the interior of the 

flow and commonly have delicately cuspate 

margins, indicating considerable dissolution. 

See text for discussion 

Given our current understanding, chemical composi- 

tion is not a diagnostic feature. Nevertheless, many 

studies have established that pyroclastic deposits are 

commonly compositionally zoned, whereas lavas tend 

to be homogeneous. However, there are numerous ex- 

ceptions to both generalizations. Preservation of any 

compositional heterogeneity from magma chamber to 

surface eruptive unit depends on many factors, espe- 

cially the nature of the pre-eruptive magma body and 

the mechanics of extraction (Wolff et al. 1990, and ref- 

erences therein). Without wishing to propose composi- 

tional variability as a diagnostic criterion, we can state 

that individual extensive silicic lavas and units de- 

scribed as lava-like ignimbrites are strikingly homoge- 

neous (Ekren et al. 1984; Bonnichsen and Kauffman 

1987; Christiansen und Hildreth 1989; Henry et al. 

1989, 1990; Milner et al. 1992). These rocks include tra- 

chytes, trachydacites, dacites, low-silica rhyolites, and, 

more rarely, high-silica rhyolites, or a silica range of 

about 65 to 77%. Most commonly, they contain about 

68 to 70% SiO2 and can be mildly peraluminous to 

mildly peralkaline. The bimodal association of most ex- 

tensive silicic lavas or lava-like ignimbrites with tho- 

leiitic flood basalts suggests the silicic rocks represent 

deep crustal melts (Bellieni et al. 1984; Bonnichsen and 

Kauffman 1987; Green 1989; Wolff 1989). Large de- 

grees of partial melting potentially could generate large 

volumes of homogeneous magma. Examples of strongly 

compositionally zoned rheomorphic tuffs are the peral- 

kaline units of Pantelleria and Gran Canaria (Mahood 

and Hildreth 1986; Crisp and Spera 1987). 

The Bracks Rhyolite is a striking example of a ho- 

mogeneous lava (Henry et al. 1988, 1990). Fifteen sam- 

ples representing the complete lateral and vertical ex- 

tent of the flow contain about 69% SiO2 and are chemi- 

cally and mineralogically indistinguishable. Although 

concentrations of some of the more mobile major ox- 

ides, such as the alkalis, vary slightly as a result of mi-

nor post-emplacement alteration, concentrations of 

more immobile elements, including the rare-earth ele- 

ments, vary no more than analytical uncertainty. The 

abundance and compositions of phenocrysts (alkali 

feldspar, clinopyroxene, fayalite, and magnetite) are 

identical throughout the flow. 

Discussion 

Table 3 presents a revision of criteria for distinguishing 

rheomorphic tuffs from extensive silicic lavas. Several 

points must be emphasized. Many individual character- 

istics listed in Table 3, commonly thought of as diag- 

nostic of one rock type or the other, are found in both 

rheomorphic tuffs and extensive silicic lavas. They can- 

not be used to distinguish the two. Some characteristics 

require knowledge of the complete distribution of a 

volcanic unit, which can greatly complicate interpreta- 

tion where correlation is uncertain for any reason. Few 

if any features alone are truly diagnostic. All must be 

carefully evaluated in the overall context of the rock 

being examined. 

The features that seem most useful in distinguishing 

rheomorphic tufts and extensive silicic lavas are basal 

deposits, flow margins, response to topography, and, 

183 

where distribution is fully determined, source. The 

characteristics of basal deposits are probably most di- 

agnostic and most easily determined for many units. 

Basal deposits quench against the ground, are most 

likely to preserve primary features, and can be observed 

in single outcrops. Flow margins are also revealing. 

Silicic lavas typically maintain thicknesses of several 

tens of meters to their flow fronts. Ash-flow tuffs thin 

to at most a few meters and generally do not show see- 

ondary flow features at their margins. Tufts, even those 

generated from low eruption columns, can be deposited 

across topographic barriers. Lavas pond against bar- 

riers unless the flow is much thicker than the height of 

the barrier. If an ash-flow tuff tuff can be traced to a 

source caldera, a thick correlative intracaldera fill 

should be present. 

Distinguishing lava-like tuffs from other widespread 

silicic volcanic rocks is hampered by the lack of agreed- 

upon examples on which to base criteria. Based on our 

criteria, we suggest that most of the described examples 

in the literature are in fact lavas, and that the category 

of 'lava-like tuff' may not be valid. However, much 

more research is needed on possible eruption and em- 

placement mechanisms, especially the mechanics of 

low pyroclastic fountains, agglutination, and particle 

collisions within moving pyroclastic flows (Freundt and 

Schmincke 1990; Wolff and Horn 1991). 

Table 3. Proposed distinguishing characteristics of rheomorphic tuffs, lava-like tuffs, and extensive silicic lavas 

Characteristic Rheomorphic tuff Lava-like tuff Extensive silicic lava 

Wide areal extent Yes Yes 

Large aspect ratio Yes Yes 

Flow margins Distal thinning to poorly or nonwelded 

deposits, but dependent on topography 

Same as rheomorphic tuff? 

Response to topography Can climb over many barriers, dependent 

on gas content and height of eruption 

column 

Same as rheomorphic tuff? 

Source 

Commonly caldera; tuff can pond within 

caldera 

Same as rheomorphic tuff 

Welding zonation Yes, but can be obscured by rheomor- ? 

phism; high-T or peralkaline tuffs can 

be welded throughout 

Basal breccia No, but basal lithic concentration zone Same as rheomorphic tuff 

could appear similar 

Upper breccia Yes, in rare cases Same as rheomorphic tuff 

Flow bands Yes Yes 

Ramp structures Yes Yes 

Elongate vesicles Yes Yes 

Pumice Yes, but diagnostic only if throughout Unlikely to be perserved 

flow; commonly obscured in interior except possibly at top 

of flow 

Shards Yes, same as pumice Same as rheomorphic tuff 

Gas escape pipes Yes No? 

Broken phenocrysts with Can be present but breakage is probably Same as rheomorphic tuff 

wide size range minor in low-explosivity tuffs 

Lithic fragments Yes, but can be sparse in low-explosivity Same as rheomorphic tuff 

tufts 

Composition Commonly vertically zoned Same as rheomorphic tuff? 

Yes 

Yes 

Remain thick and massive or 

brecciated to flow margin 

Stopped by most barriers un- 

less much less than flow 

thickness 

Commonly dike/fissure; lava 

maintains uniform thickness 

throughout distribution 

No 

Yes, composed of mixed tex- 

tural types of the lava 

Yes 

Yes 

Yes 

Yes 

Yes, particularly in marginal 

breccias 

Yes, in marginal breccias 

No 

No, but could develop in 

breccia or in tectonically 

disrupted lavas 

Can contain inclusions 

Commonly homogeneous

184 

Recognition of extensive silicic lavas is new, and 

their distinction from rheomorphic tuffs is still to some 

extent controversial. The criteria presented here should 

be considered our best current understanding, which 

will certainly evolve as the rocks are more thoroughly 

studied. 

Concluding remarks 

High-temperature silicic volcanic rocks occur in many 

of the world's major volcanic provinces (Table 2). They 

include low-silica rhyolites, dacites, trachydacites, or 

traehytes that typically contain a low percentage of an- 

hydrous phenoerysts and range from mildly peralumi- 

nous to mildly peralkaline. Most are temporally and 

spatially associated with flood basalts (Table 2). Trans- 

Peeos Texas is exceptional, inasmuch as the surface ex- 

pression of marie magmatism is minor. However, pre- 

liminary geochemical data from this province suggest 

that the silicic magmas were produced by melting of 

anhydrus continental crust, on a regional scale, by mas- 

sive intrusion of marie magma into the lower crust 

(Wolff and Davidson 1990). This model has obvious 

appeal for other high-temperature silicic-magma prov- 

inces (Wolff 1989). Many more geochemical data are 

required on this class of rocks before this suggestion 

can be properly tested. 

The emplacement mechanisms for the rocks of Table 

2, whether turfs or lavas, are controversial. All have 

wide areal extents and low aspect ratios, and many 

have pumice- or shad-like structures in basal zones. 

However, we do not consider these features to be diag- 

nostic. Nevertheless, some units in each province have 

characteristics that we consider diagnostic of emplace- 

ment as lavas, particularly the occurrence, albeit minor 

in some cases, of basal breccia. Additionally, units in 

all these areas maintain large and relatively uniform 

thicknesses throughout their known distribution, al- 

though steep, abrupt flow margins are known only in 

the first three examples. Finally, rocks initially inter- 

preted as lava-like tuffs of the Lebombo and Rooiberg 

regions of South Africa, two provinces in which basal 

breceias are minor, have recently been reinterpreted as 

lavas (Twist et al. 1989). We emphasize that some of the 

original workers strongly disagree with our interpreta- 

tion that many of these are lavas. Both lavas and 

strongly rheomorphic tuffs may occur within individual 

provinces. Thus, the presence of one does not preclude 

the presence of the other. Nevertheless, we suggest that 

extensive silicic lavas are far more common than pre- 

viously recognized and that they are an integral part of 

most flood basalt provinces. The term "flood rhyolite' 

seems appropriate. 

Acknowledgements. We acknowledge the contributions of many 

geologists who generously gave us the opportunity to compare 

rocks in other areas with those in Texas, especially Bill Bon- 

nichsen (southwestern Idaho), Simon Milner, Andy Duncan, and 

Tony Ewart (Etendeka, Namibia), John Bristow (Lebombo, South 

Africa), and David Twist (Rooiberg Felsites, Bushveld Complex). 

Collaboration with Steve Self, Richard Franklin, Don Parker, and 

Jon Price greatly aided our research in Texas. The views of these 

people do not match ours in all cases. Discussions with Curtis 

Manley, and thorough reviews by A. Ewart, J. McPhie, S. C. Miln- 

er, and A. J. R. White are greatly appreciated. Research was sup- 

ported (CDH) by the COGEOMAP program of the US Geologi- 

cal Survey through Agreement No. 14-08-0001-A0516 and by 

Grant Gl144148 from the US Bureau of Mines to the Texas Min- 

ing and Mineral Resources Research Institute. 

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