Assembling an Ignimbrite: Mechanical and Thermal Building Blocks ...

Assembling an Ignimbrite: Mechanical and Thermal 

Building Blocks in the Bishop Tuff, California 

Colin J. N. Wilson and Wes Hildreth 1 

Institute of Geological and Nuclear Sciences, P.O. Box 30368, Lower Hutt 6315, New Zealand 

(e-mail: c.wilson@gns.cri.nz) 

ABSTRACT 

The building blocks that control ignimbrite physical characteristics are here defined as mechanical (flow units and 

flow packages) and thermal (cooling units). Ignimbrite construction from these mechanical and thermal building 

blocks is illustrated by a longitudinal profile of the Bishop Tuff in the Owens River Gorge, California. In the Bishop 

ignimbrite, flow unit boundaries can be defined only locally, but known or inferred multiple flow units combine to 

form eruptive packages showing imbricate, offlapping relationships such that each package is thickest successively 

farther from the source. In these packages, the presence or absence of flow unit boundaries does not necessarily reflect 

directly the presence or absence, respectively, of time breaks; thus massive ignimbrite need not be the product of 

continuous accumulation (progressive aggradation). The intrinsic thermal properties of each package (or parts thereof) 

have combined with local thicknesses of accumulated material to control the welding state, measured here by bulk 

densities. Four zones of maximum density/welding occur, three of which display imbricate, offlapping relationships 

away from the source, in concert with changes in package thicknesses. The ignimbrite is thus a single compound 

cooling unit in existing terminology, but minima in the density/welding profiles are not at chronostratigraphically 

significant horizons. Thus thermal descriptors, such as simple and compound cooling units, may not have timestratigraphic 

significance, and use of ignimbrite density/welding profiles to infer emplacement temperatures and 

timings is problematic. Development of the Bishop welding zones is, however, explicable by the thicknesses, emplacement 

temperatures, and imbricate distribution of the different ignimbrite packages. 

Introduction 

Ignimbrites (ash-flow tuffs) are an abundant and voluminous 

product of explosive volcanism, usually 

with evolved magma compositions, and generated 

by emplacement of concentrated pyroclastic density 

currents (pyroclastic flows, sensu stricto). Ignimbrites 

may attain volumes of 110 3 km 3 and can 

show a bewildering variety of morphologies, depositional 

facies, and lithological characteristics 

(Smith 1960a, 1960b; Ross and Smith 1961; Walker 

1983; Wilson 1986; Carey 1991; Druitt 1998; 

Freundt et al. 2000). These physical characteristics 

must be interpreted to gain insights into the timing 

and dynamics of ignimbrite emplacement, interrelationships 

with caldera collapse processes, and 

the ordering of evacuation from the parental 

magma chamber. In this article, we consider two 

elements of these physical characteristics that we 

Manuscript received July 25, 2002; accepted May 8, 2003. 

1 U.S. Geological Survey, Volcano Hazards Team, Mailstop 

910, 345 Middlefield Road, Menlo Park, California 94025, U.S.A. 

[The Journal of Geology, 2003, volume 111, p. 653–670] 2003 by The University of Chicago. All rights reserved. 0022-1376/2003/11106-0003$15.00 

653 

label the “building blocks” of an ignimbrite. The 

first is the mechanical structure of the deposit, that 

is, how it was constructed by the individual pyroclastic 

flows and how this is reflected in the overall 

geometry of any larger-scale emplacement packages 

and the deposit as a whole. The second is the 

thermal structure of the deposit, that is, how the 

retained heat of the bulk material varied within the 

ignimbrite and subsequently influenced its properties. 

In many (most?) ignimbrites, deciphering the 

mechanical structure is hampered by a lack of obvious 

internal marker horizons and/or by the effects 

of processes that reflect the thermal structure 

of the deposit (Smith 1960b; Ross and Smith 1961). 

If methods can be found to break down ignimbrites 

into their component parts, then quantitative reconstruction 

of the parental eruptive pulses would 

be possible for a much wider variety of prehistoric 

ignimbrites. Here we use part of the 0.76 Ma Bishop 

Tuff to show how an archetypical partly welded

654 C. J. N. WILSON AND W. HILDRETH 

ignimbrite was physically constructed and use the 

implications of the field data to address broader 

concepts in usage of the terms flow unit and cooling 

unit. 

Mechanical Building Blocks: Flow Units. The term 

“flow unit” was originally introduced to describe 

the body of material generated by the passage of a 

single pyroclastic flow (Smith 1960a; Sparks et al. 

1973). Flow units were envisaged to be separated 

from each other by some kind of boundary, indicative 

of a time break separating the individual 

flows, and to be typically decimeters to meters 

thick. Subsequent work has modified the concept 

of flow units in three ways. (1) It was recognized 

that different depositional facies or layers could be 

associated with the passage of a single pyroclastic 

flow (e.g., Sparks et al. 1973; Fisher 1979; Wilson 

and Walker 1982; Wilson 1985; Freundt and 

Schmincke 1986). (2) It was recognized that ignimbrites 

could be divided into simple (one flow unit) 

and compound (multiple flow unit) varieties, analogous 

to divisions long recognized in lava flows 

(Wright 1981; Walker 1983, cf. Walker 1970). (Note, 

however, that Smith [1960a] proposed that multiple 

flow units could also be generated from emplacement 

of a single pyroclastic flow by the superposition 

of bodies of material that had taken alternative 

travel routes; such flow units also could have developed 

within them some degree of depositional 

layering.) (3) A more controversial modification has 

arisen over the interpretation of ignimbrite sections 

where no flow unit boundaries are visible; are they 

the product of single flows or of layer-by-layer deposition 

(progressive aggradation: Fisher 1966; Branney 

and Kokelaar 1992)? Furthermore, if flow unit 

boundaries are taken as evidence for time breaks, 

can the absence of such boundaries be used to infer 

that deposition of ignimbrite was continuous (Branney 

and Kokelaar 1992), or could deposition have 

been punctuated yet leave no clear boundaries? 

Thermal Building Blocks: Cooling Units. A characteristic 

feature of many ignimbrites is that they 

are emplaced at residual temperatures such that the 

juvenile components (pumice and shards) have 

welded together (Marshall 1935; Gilbert 1938; 

Smith 1960b; Ross and Smith 1961). For a given 

juvenile composition, primary controls on the 

welding process are thought to be temperature on 

deposition and the load stress (Smith 1960b; Riehle 

1973), with a possibly significant control from any 

residual volatiles in the glass phase that would reduce 

the glass viscosity (e.g., Schmincke 1974). The 

welding may range from weak cohesion (sintering) 

caused by tacking across clast contacts, through the 

complete elimination of pore space with accom- 

panying flattening of pumices, and occasionally to 

the rheomorphic state where the tuff moves farther 

as a viscous mass akin to a lava flow. Because of 

the nature of our data, we discuss only nonrheomorphic 

welded tuffs because several lines of evidence 

(e.g., Chapin and Lowell 1979; Branney and 

Kokelaar 1992; Leat and Schmincke 1993; Freundt 

1999) indicate that the onset of significant welding 

during emplacement will lead to further complexities 

in the depositional structure of the tuff that 

are beyond the scope of this article. However, ignimbrites 

similar to the Bishop Tuff, where welding 

can be inferred to have largely or entirely occurred 

after emplacement, with in situ cooling rates and 

lithostatic loads as dominant controls (Branney and 

Kokelaar 1992), are abundant in the geological record 

and are amenable to the kind of documentation 

employed here. 

Welding is often accompanied and overprinted by 

devitrification, recrystallization, and alteration by 

a residual vapor phase exsolved from the glass 

phase. These processes tend to obscure any mechanical 

boundaries and make recognition of flow 

units more difficult. However, the overall thermal 

structure of an ignimbrite, as reflected in variations 

of welding state (closely matched, and most conveniently 

measured, by changes in bulk density) 

and any associated crystallization/alteration can be 

described quasi-independently of the flow-unit 

structure. Smith (1960a, 1960b) thus introduced 

the term “cooling unit” for a body of material (one 

or more flow units) that was emplaced such that it 

had undergone continuous cooling, that is, that the 

earliest material was still hot when the latest material 

was emplaced. A “simple cooling unit” was 

defined by Smith as a body of material that had 

accumulated sufficiently rapidly that it cooled as 

a single body and had correspondingly simple, idealized 

variations of density/welding, with or without 

overprinting by devitrification or vapor-phase 

crystallization, from base to top. In a simple cooling 

unit, the basal and top parts are least welded, the 

welding maximum lies within the lower half of the 

deposit, and there are no irregularities in welding 

intensity with height. A “compound cooling unit” 

was defined as any deposit that showed departure 

from the hypothetical simple cooling unit profile 

qualitatively illustrated by Smith (1960b) and quantitatively 

modeled by Riehle (1973). However, despite 

such departures, it was taken that the lower 

parts of a compound cooling unit had not cooled 

entirely before the upper parts were emplaced and 

the term “single cooling unit” could be applied. 

The term “composite sheet” was introduced by 

Smith (1960b) for those deposits in which there was

Journal of Geology ASSEMBLING AN IGNIMBRITE 655 

somewhere evidence of significant time breaks 

(e.g., erosion breaks, sediments, lava flows) between 

zones of maximum welding intensity, such 

that earlier parts of the deposit had effectively 

cooled completely before later parts were emplaced, 

but that such separate cooling units graded 

laterally into single, simple, or compound cooling 

units elsewhere. Because the characteristic timings 

of flow emplacement are observed or inferred to be 

significantly more rapid than the characteristic 

cooling times that would control welding in the 

resulting deposits (Riehle 1973), cooling units, 

whether simple or compound, are characteristically 

developed over greater vertical length scales than 

flow units, typically meters to hundreds of meters 

thick. 

Background to Data Sets. In this article, we consider 

the structure of an archetypical partly welded 

ignimbrite, forming part of the Bishop Tuff, in 

terms of the fundamental building blocks that created 

the deposit as now seen. We use data from the 

particularly well-exposed cross sections in the 

Owens River Gorge in the eastern part of the Bishop 

Tuff (fig. 1). These illustrate how the mechanical 

structure of the ignimbrite, coupled with consideration 

of its thermal state on emplacement, leads 

to complex patterns of rock properties. Detailed 

measurements at 10 sections along the gorge walls 

(fig. 1) were supplemented at other points by measurements 

of lithic sizes, abundances and lithologies, 

and observations of the welding state of the 

rocks that were subsequently calibrated to the density 

measurements (see below). Although, as mentioned 

previously, the thermal state of ignimbrites 

on emplacement is expressed by various states of 

welding compaction, devitrification, and vaporphase 

alteration and recrystallization, we adopt the 

view implicit in other studies (Ragan and Sheridan 

1972; Riehle 1973) that, in any given deposit, the 

degree of welding compaction is the single most 

important quantifiable parameter that reflects the 

emplacement temperature. In turn, we have 

adopted bulk density as the simplest way to quantify 

the welding state because of the ease with 

which measurements can be made (cf. flattening 

ratios of fiamme: Ragan and Sheridan 1972; Peterson 

1979). However, we acknowledge that our ap- 

Figure 1. a, Location map showing the eastern part of the Bishop ignimbrite and the positions of the 10 detailed 

sections along the Owens Gorge. b, Longitudinal section projected on the true left bank, down the line of the Owens 

Gorge, showing the topographic relief of the valley and the gorge walls, main ignimbrite packages (from Wilson and 

Hildreth 1997; see also fig. 5), and locations of the density sections. Note that the presence of Ig2Ec on the gorge 

rim between sections E and F shows that the thinning over the basement high of older rocks of Ig2Ea and Ig2Eb is 

a primary feature and not due to subsequent erosion.


Table 1. Summary Stratigraphy of the Bishop Tuff in the Area East of Long Valley Caldera Discussed in This Article 

Ignimbrite unit Fall unit(s) Notes (ignimbrite) 

Ig2Ec F9 185% rhyolite in lithic component 

Ig2Eb F9 30%–70% 

Ig2Ea Later F8–F9 !20% 

Onset of Glass Mountain rhyolite in lithic fraction; incoming of pyroxene-bearing pumice 

Ig1Eb F6–earlier F8 

Fall horizon (part F6) 

Lithic poorer 

Ig1Ea Later F2–F6 

Fall horizon (F1–earlier F2) 

Lithic richer 

Note. After Wilson and Hildreth (1997). 

proach will become inaccurate for deposits in 

which postemplacement diagenetic cementation or 

leaching has occurred, and fiamme measurements 

may then be the only route to mapping variations 

in welding intensity (Roberts and Siddans 1971; Peterson 

1979). 

Bishop Tuff Stratigraphic Background 

The 0.76 Ma Bishop Tuff consists of two components: 

a Plinian pumice fall deposit that occurs below, 

and interbedded within, a nonwelded to 

densely welded ignimbrite (Wilson and Hildreth 

1997; table 1). The fall deposit is exposed east of 

the source and is divided into nine units (F1–F9) 

correlated by bedding and grain-size characteristics 

and lithic components (Wilson and Hildreth 1997). 

The ignimbrite is well exposed along the walls of 

the Owens River valley, the Owens Gorge, and the 

Chalk Bluffs escarpment (fig. 1), and material ranging 

from nonwelded through densely welded is 

widely exposed (Sheridan 1970; Ragan and Sheridan 

1972; Sheridan and Ragan 1976). 

Our work (Wilson and Hildreth 1997) has developed 

ways of subdividing the Bishop ignimbrite 

into stratigraphically manageable units. Clearly defined 

flow unit boundaries are scarce in the ignimbrite 

in general, and so we introduced the concept 

of the eruptive package to encompass ignimbrite 

with broadly similar components and lithological 

characteristics (but independent of welding zonation), 

that was emplaced as multiple pulses or flows 

over a definable stratigraphic interval (table 1). In 

the Owens Gorge and other areas of the eastern 

Bishop Tuff, flow unit boundaries occur in three 

main circumstances: (1) in package Ig1Ea, where 

they are frequently demarcated by thin (centimeterto 

decimeter-thick) discontinuous-fall or hybridfall 

intercalations (fig. 2) or by truncations of degassing 

pipes (fig. 3); (2) in distal parts of Ig1Eb and 

Ig2, where they are demarcated by the tops of concentration 

zones of coarser pumice (fig. 4) or occur 

as thin individual flow units intercalated within 

fall deposits (Wilson and Hildreth 1997, their fig. 

9b); and (3) where continuous decimeter-thick to 

meter-thick intercalations of fall material occur, 

defining the boundaries between packages Ig1Ea 

and Ig1Eb, and Ig1Eb and Ig2Ea (Wilson and Hildreth 

1997, their figs. 5, 9, and 10). 

In the deposits discussed here, east of the caldera, 

earlier pyroxene-free ignimbrite (packages Ig1Ea, 

Ig1Eb) was emplaced synchronously with fall units 

F2 through all but uppermost F8, and later pyroxenebearing 

ignimbrite (Ig2Ea–c) was emplaced synchronously 

with uppermost F8 and F9 (table 1; Wilson 

and Hildreth 1997). Some minor diachroneity 

is present in the Ig1E–Ig2E contact as the onset of 

Glass Mountain–derived rhyolite lithics occurs 

slightly below the time break at the F8–F9 contact. 

Where Ig1E and Ig2E are in direct contact with no 

intervening fall material, the boundary between the 

two is placed at the onset of the Glass Mountain 

rhyolite lithics. The recognition that fall and flow 

deposits were coevally emplaced allowed us to infer 

overall timings for emplacement of two packages; 

ca. 25 h for Ig1Ea and ca. 36 h for Ig1Eb, and hence 

to relate average accumulation rates to the presence 

or otherwise of flow unit boundaries that would 

indicate episodicity in deposition. Variations of 

welding in the tuff are superficially straightforward, 

with one zone of dense welding being present in 

the upper to middle reaches of the Owens Gorge, 

two zones in the middle to lower reaches, then a 

single zone along the Chalk Bluffs escarpment. Because 

of these variations, the Bishop was previously 

regarded as containing two cooling units (Sheridan 

1968), which led Hildreth (1979) inappropriately to 

term it a composite sheet by the criteria of Smith 

(1960b). 

Sampling Strategies and Methods 

To quantify the mechanical structure of the ignimbrite, 

observations were made of any intercalated


Figure 2. a, Photograph of nonwelded to sintered Ig1Ea 

at 546607 (grid reference to 100 m in the universal transverse 

Mercator [UTM] metric grid) to show fall intercalations 

(arrowed) in Ig1Ea. Height of cliff is ∼27 m. b, 

Close-up view of fall-deposit intercalation at 546607. 

fall beds, flow unit boundaries, or stratification that 

could be interpreted to represent some kind of hiatus 

in deposition. With two exceptions (table 1), 

the intercalated fall beds are only centimeters to 

decimeters thick, are discontinuous, and cannot 

consistently be used to regionally subdivide the ignimbrite 

packages. In the ignimbrite, measurements 

were also made of the maximum lengths of 

lithic and pumice clasts or fiamme, and the five 

largest values were averaged. The abundance of 

lithics was measured by counting the number of 

lithic fragments 15 mm across per square meter, 

and their lithologic proportions tallied by counting 

rock types (Wilson and Hildreth 1997). 

To quantify the thermal structure of the ignimbrite, 

observations of the welding state were made 

at many localities, using the degree of induration, 

presence or absence of a eutaxitic texture, and the 

relative hardness of the rock. These observations 

were then calibrated against bulk density measurements 

(table 2) obtained from multiple sets of 

samples taken through the 10 detailed sections (A– 

J) along the Owens Gorge (fig. 1) and were used to 

generate a 2-D profile of welding intensity along 

the gorge wall. 

Density measurements were made on samples 

collected from representative blocks or cored from 

typical matrix material, avoiding any obvious local 

Figure 3. a, Photograph of flow-unit boundary (arrowed) 

in Ig1Ea at 521609, locally demarcated (b)bytruncated 

degassing segregation pipes.


Figure 4. Photographs of flow units in (a) Ig1Eb in the Owens Gorge at 632466, defined by the tops of swarms of 

fiamme marking former pumice concentration zones, and in (b) Ig2Ea at 790475, defined by pumice swarms. 

concentrations of lithic or pumice clasts that might 

skew the results. Lithic proportions in grain-size 

samples of representative ignimbrite from the 

Owens Gorge area are all !10 wt%, mostly !5 wt%, 

and so the effects of lithics on the sample density 

have been neglected. Densities of coherent (sintered 

to densely welded) ignimbrite were determined 

on representative 300–1800-g blocks, using 

water displacement techniques (Archimedes’ principle) 

to determine block volumes, with specimens 

previously being saturated in water to eliminate 

ingress of water during measurement. Ten determinations 

were made on each sample, and the resulting 

standard deviations on the density determinations 

ranged between 0.001 and 0.010 Mg/m 3 . 

Densities of nonwelded (sievable) ignimbrite were 

determined from single samples cored in the field 

using a carpenter’s brace and bit to excavate a cylindrical 

hole of measurable volume (137–296 cm 3 ). 

The excavated material was caught in a bag for subsequent 

drying and weighing to determine its density. 

At one point in section J, block and cored samples 

could be obtained from the same level 

(everywhere else, the ignimbrite was either too indurated 

to excavate with the brace and bit or too 

soft to be taken as a coherent block); density values 

are 1.262 0.002 (1 SD) Mg/m 3 and 1.240 Mg/m 3 

for block and core, respectively. The raw data set 

is available from the The Journal of Geology’s Data 

Depository free of charge upon request or as an Excel 

spreadsheet from C. J. N. Wilson. 

Mechanical Structure of the Bishop Ignimbrite 

Of the various parameters measured in the ignimbrite, 

variations in lithic sizes, numerical abundances, 

and lithologies show up the mechanical 

structure best (fig. 5). Coupled with observations of 

intercalations of the main fall deposit between 

Ig1Ea and Ig1Eb and between Ig1 and Ig 2, these 

data show the large-scale cross-sectional structure 

of the ignimbrite down the line of the Owens River 

(i.e., approximately parallel to the inferred paleoflow 

direction) to be a series of offlapping or imbricate 

lenses. Internal subdivision of Ig1Eb is discernible 

on the basis of lithic counts (fig. 5), 

suggesting that the imbricate structure is also paralleled 

by growth of the individual packages. Each 

main package in turn reaches farther from the 

source, with the exception of Ig2Ec, which is poorly 

developed in this area (Wilson and Hildreth 1997, 

their fig. 12). In a section at 830529 (grid reference 

to 100 m in the universal transverse Mercator 

[UTM] metric grid) on the slopes of the White


Table 2. Summary of Field Descriptive Terms Used for Welding Textures in the Bishop Ignimbrite 

Welding term Nature of deposit 

Density 

(Mg/m3 ) 

Densely welded Strong eutaxitic texture, dark color if glassy 1ca. 2.00 

Moderately welded Clear eutaxitic texture, still relatively soft 1.74–2.00 

Poorly welded Some pumice flattening, highly porous and soft 1.49–1.81 

Sintered Coherent; requires hammer to fracture; no eutaxitic texture 1.22–1.57 

Nonwelded Noncoherent; can be disaggregated between fingers 1.09–1.47 

Note. Correlated with density measurements reported in this article. 

Mountains east of the Owens Gorge area, a single 

centimeter-scale “swash”-like bed of Ig2Eb flow 

material occurs within thick F9 fall deposits, at a 

stratigraphic level that implies that package Ig2Eb 

traveled the farthest of all to the east of the source. 

The causes of this offlapping structure in the ignimbrite 

cannot be related simply to increasing velocities 

of the parental flows, since lithic sizes (and 

hence inferred velocities and flow-carrying capacities) 

in Ig2E are smaller than in Ig1Eb (fig. 5). Instead, 

two other factors were probably important. 

First, earlier flows of Ig1Ea and early Ig1Eb would 

have ponded in nearer-source low-lying areas, 

whereas later flows could travel over a smoother 

surface of subdued topographic relief. Second, the 

earlier flows are poorer in fine ash-grade material 

(fig. 6), regardless of distance from the source, and 

are thus likely to have deaerated more rapidly and 

consequently to have been less mobile (Roche et 

al. 2002). 

Changes in the parameters illustrated in figure 5 

serve to highlight two points. First, within Ig1Ea 

and Ig1Eb, there do not appear to be major hiatuses 

between material separated by flow-unit boundaries 

(apart from changes in pumice or fiamme abundance 

that define the boundary itself, e.g., fig. 4) or 

by fall-deposit intercalations. This implies that the 

overall changes in lithic sizes and abundances in 

Ig1E were controlled by gradual variations in the 

source area, such that flows separated by minutes 

to hours share broadly similar characteristics. Second, 

at the contact between Ig1E and either Ig2Ea 

or Ig2Eb, there are locally abrupt changes in measured 

parameters (especially the count percentage 

of rhyolite in the lithic fraction, Wilson and Hildreth 

1997) over distances of !0.5 m, such as at 

section G (figs. 1, 5), but with no sign of any discontinuity 

to reflect any time break or hiatus in 

ignimbrite deposition. This is so even though the 

presence farther down-valley of F9 fall deposits 

along this contact (Wilson and Hildreth 1997, their 

fig. 10) demonstrates unequivocally the presence of 

a significant time break. The possibility that ignimbrite 

deposition was continuous at section G 

but discontinuous down-valley is precluded by the 

presence of Ig2Ea down-valley between fall F9 and 

Ig2Eb (fig. 5); both F9 and Ig2Ea are absent at section 

G. Thus, in the material exposed in the Owens 

Gorge, some parts of the Bishop ignimbrite show 

that a demonstrable time break may not be accompanied 

by significant changes in the properties of 

the ignimbrite (first point above), whereas other 

parts show that clearly defined changes in ignimbrite 

properties may lack any evidence for a significant 

time break, even though such a break can 

be shown elsewhere to have occurred (second point 

above). 

Thermal Structure of the Bishop Ignimbrite 

Density values measured at each section are plotted 

in figures 7–10 as vertical profiles linked to the 

horizontal profile along the line of the Owens 

Gorge, with independently determined stratigraphic 

units and horizons plotted for comparison. 

The overall welding state of the ignimbrite along 

the walls of the gorge is shown in figure 11. We 

draw attention to five features from our data. 

1. We recognize four density, and hence welding, 

maxima (labeled Zones a through d, from oldest to 

youngest). Continuity of these zones between sections 

has been ensured by tracing the welding zones 

(and stratigraphic marker planes) along the wellexposed 

walls of the Owens River valley. The combination 

of stratigraphic controls and good exposure 

demonstrates that these four welding zones 

are not equally prominent at any one section, and 

that the ignimbrite hosting Zones b through d 

forms an imbricate, overlapping succession, such 

that the most prominent development of each 

package and its associated welding zone occurs successively 

farther from the source (fig. 11). Each 

welding zone is developed in a stratigraphically 

confined part of the ignimbrite: (a) Ig1Ea; (b) lower 

Ig1Eb; (c) upper Ig1Eb; (d) upper Ig2Eb, and thus the 

most heat-retentive (i.e., highest emplacement 

temperature) material was emplaced to form these 

parts of the ignimbrite at certain times. The development 

of welding then depends on the local 

thickness of each package and the development of


Figure 5. Series of longitudinal sections down the line of the Owens Gorge projected on the true left side to show 

the data used to define the ignimbrite packages. a, Maximum lithic size (average lengths of the five largest clasts) 

in millimeters. b, Numbers of 15-mm-long lithic clasts per square meter. c, The count percent of Glass Mountain 

rhyolite in the 15-mm lithic fraction (measurement techniques after Wilson and Hildreth 1997). The dotted line in 

b is a contour of 50 lithics/m 2 used to separate lower and upper parts of Ig1Eb and their associated welding zones 

(see fig. 11). Note that the approximate paleoflow direction is parallel to the gorge, and so distances down the gorge 

approximate to radial distances from the source. 

additional stresses due to subsequent syneruptive 

lithostatic load; for example, Zone c attains lower 

maximum densities in the upper Owens Gorge (fig. 

7, section A) because it was thinner and was buried 

under lesser thicknesses of Ig2E material there 

compared with the middle and lower reaches of the 

gorge. 

2. The upper Owens Gorge (sections A–C) is the 

only area where all four zones are exposed in proximity, 

and Zone b is the best developed. Zone a is 

represented by incipient pumice flattening and is 

exposed only over a limited area where downcutting 

to the east by the Owens River intersects a 

westward-thickening wedge of Ig1Ea material. 

Zones b and c, both in Ig1Eb, are separately demarcated 

in this area by at most a slight reduction 

in density (e.g., section A). However, the stratigraphic 

control given by variations in numerical 

abundances of lithic fragments (particularly the 50 

lithics/m 2 contour; fig. 5) show that the thick 

welded Ig1Eb deposits in the middle Owens Gorge 

(e.g., sections F and G) correlate only with Zone c 

and that material equivalent to Zone b in the upper 

parts of the gorge forms only a nonwelded to poorly 

welded basal zone to the ignimbrite down-valley 

from the prominent high in the basement rocks (fig. 

11). 

3. In the middle to lower Owens Gorge (sections 

G–I), there is a pronounced density minimum between 

the maxima of Zones c and d (figs. 7–9), but 

the location of this minimum is at a level of no 

special stratigraphic significance. The basal parts


Figure 6. Plot of the contents of !1/16-mm material 

versus the mean grain size (M z of Folk and Ward 1957) 

in sieved samples collected from the four main packages 

of Bishop ignimbrite along, or close to, the Owens Gorge. 

Samples represent both proximal and distal material in 

all packages and also bracket stratigraphically material 

that is representative of the densest-welded ignimbrite. 

Note the clear distinction in !1/16-mm ash contents between 

Ig1E and Ig2E samples. 

of Ig2Ea show an upward diminishing welding intensity 

at sections H (fig. 9) and I, yet at all sites 

in the Bishop Tuff where Ig2Ea overlies solely fall 

deposits, it is nonwelded. We infer that the emplacement 

temperature of Ig2Ea in itself was insufficient 

to cause welding, but where it was deposited 

on still-hot Ig1Eb, the residual heat from 

the underlying deposit was sufficient to cause 

“back-welding” of Ig2Ea under the lithostatic stress 

of later-deposited material. The density minimum 

in the middle Owens Gorge is poorly defined at 

sections F and G, largely because Zone d is less 

well developed than farther down-gorge as a consequence 

of the lesser overlying thickness of Ig2Eb. 

4. Wide fluctuations in density are seen in the 

nonwelded to sintered material of section J, and the 

lowest in situ densities are actually in sintered, not 

nonwelded, material (fig. 10). Grain size and component 

characteristics of the nonwelded material 

change very little, and so the density fluctuations 

are not due to variations in either dense (lithic, 

crystal) or fine-ash (!1/16 mm) components (fig. 

10). We interpret the density values to reflect the 

locking in by the sintering processes of bulk densities 

at a stage when compaction of the ignimbrite 

from its newly emplaced state was incomplete (i.e., 

while the deposit was stationary but still hot and 

somewhat expanded). We infer that nonwelded material 

could continue to compact locally by intergrain 

movements in response to load stresses (probably 

augmented by earthquake-induced ground 

shaking in this tectonically active area; Pinter 

1995), whereas the structural framework induced 

by sintering could resist further compaction. Thus 

the mathematically convenient, extrapolated density 

value of 1.0 Mg/m 3 adopted by Ragan and Sheridan 

(1972) and Sheridan and Ragan (1976) for nonwelded 

material to calculate compactional strain 

in ignimbrites and infer the position of the original 

ignimbrite surface is not supported by our data. 

5. An important aspect of interpreting the welding 

and thermal history of an ignimbrite is identifying 

whether devitrification and vapor-phase 

crystallization follow on from the welding compaction, 

or interrupt and terminate it. In several 

places in the sections measured, there are sharp 

changes from vitric to nonvitric tuff, and samples 

collected in proximity from each type of material 

show no major differences in density (fig. 12). 

We thus infer that welding densification was effectively 

completed before devitrification and/or 

vapor-phase crystallization and that the latter two 

processes did not act to prematurely halt densification 

(cf. item 4, above). Since the densities of the 

minerals created by devitrification are greater than 

those for the vitric phase, a lack of systematically 

increased density in devitrified material must 

therefore be accompanied by some increase in the 

microscale porosity of the rock. 

Three prominent zones of vapor-phase crystallization 

are also present in parts of the Bishop ignimbrite 

described here: (1) sandwiched between 

the most strongly welded portions of Zones b and 

c in the upper Owens Gorge, (2) sandwiched between 

the most strongly welded portions of Zones 

c and d in the middle to lower Owens Gorge (fig. 

10; Holt and Taylor 1998), and (3) above Zone d 

in the lower Owens Gorge. In similar fashion to 

the zones of denser welding that they overlie, each 

vapor-phase crystallization zone becomes successively 

most prominent in sections along the horizontal 

profile of the Owens Gorge. Parallel to variations 

in the welding profiles, vapor-phase 

crystallization is prominent in Ig2Ea and lower 

Ig2Eb only where they overlie welded and devitrified 

Ig1Eb (i.e., Zone c; fig. 9); in contrast, where 

they overlie vitric welded ignimbrite or fall deposits, 

no significant vapor-phase crystallization is 

seen (fig. 10). Thus the regional distribution of areas 

of intense fumarolic alteration reported by Sheridan 

(1970) closely reflect the alteration of Ig2E

Figure 7. Density profiles through the Bishop ignimbrite at seven of the 10 sections (A–F, I) shown in figure 1. For 

sections G, H, and J, see figures 8–10, respectively. In each profile, filled circles represent data from nonwelded 

(sievable) tuff, and filled squares, data from coherent blocks of tuff.


Figure 8. View of east wall of the Owens Gorge at grid reference 624490 showing position of section G, selected 

data on densities and lithic lithologies, and positions of welding zones and package boundaries. Symbols are as in 

figure 7. 

above largely buried fans of Ig1Eb (welding Zone c) 

that accumulated down the paleodrainages of the 

Owens Gorge and Chidago Canyon (Wilson and 

Hildreth 1997, their fig. 6). 

Discussion 

Here we review usage of the terms “flow unit” and 

“cooling unit” in the light of our Bishop Tuff data. 

In doing this, we emphasize a key factor, namely, 

that although these terms remain useful for descriptive 

purposes, their quantitative application in 

interpreting emplacement histories of ignimbrites 

is problematic in several important respects. To use 

either term in a nonsimplistic way requires recognition 

and acknowledgment of implicit complexities 

that have been widely ignored. 

Mechanical Building Blocks: Flow Units. Recognition 

of flow units in ignimbrites fundamentally 

depends on recognizing flow-unit boundaries, 

which in turn indicate (to some extent) a hiatus in 

deposition. Note, however, that the inverse does 

not hold, that is, not all discontinuities need represent 

flow-unit boundaries. Some discontinuities 

may arise, for example, through irregular rates of 

deposition at the base of a single short-lived flow 

(e.g., layering in the Taupo ignimbrite veneer deposit; 

Wilson 1985). The nature of flow-unit boundaries 

can be highly variable, including the presence 

of material of contrasting depositional mechanisms 

(especially fall material, that would have taken a 

finite time to accumulate; fig. 2; Wilson and Hildreth 

1997), the presence of an inverse-graded layer 

2a (Sparks et al. 1973), or truncation of such features 

as degassing pipes or pumice concentration 

zones in the underlying material (figs. 3, 4). From 

this, two corollaries arise. 

First, whether or not a flow-unit boundary is seen 

depends primarily on the state of the earlierdeposited 

material. If the earlier material was unconsolidated, 

then shearing at the base of the later 

flow may cause mixing of the earlier and newly 

depositing material to obscure any contact; unless 

there is some contrast in properties (e.g., grain size 

or composition), no flow-unit boundary and hence 

no evidence for a hiatus may be present. Processes 

that would enhance consolidation of the substrate 

include the following, each of which may act on 

different time scales: the presence of abundant 

coarse clasts that would supply a rigid framework 

(e.g., fig. 4a); degassing and compaction of nonwelded 

material; welding-induced cohesion; or deposition 

of fall material. The second corollary follows 

from this, that there is a spectrum of possible 

time breaks, reflected by boundaries of some sort, 

within the course of emplacement of any ignimbrite, 

from seconds up to (conceivably as long as) 

months. Thus, depending on local circumstances,


Figure 9. View of east wall of the Owens Gorge at grid reference 633468 showing position of section H, selected 

data on densities and lithic lithologies, and positions of welding zones and package boundaries. See Wilson and 

Hildreth (1997, their fig. 10) for a closer view of the fall bed contact from this section. Symbols are as in figure 7. 

a hiatus between separate flow units (identified as 

such here by differing characteristics) may or may 

not result in a visible flow-unit boundary. 

Fisher (1966) and Branney and Kokelaar (1992, 

1997) suggested that thick ignimbrite sequences 

with no apparent flow-unit boundaries may have accumulated 

progressively in a continuous fashion 

rather than being the products of a single flow or of 

piecemeal accumulation from a number of discrete 

flows. Our observations and data for the Bishop Tuff 

imply, however, that ignimbrite with no visible 

flow-unit boundaries can result from emplacement 

of multiple flows separated by time breaks. In other 

words, a lack of flow-unit boundaries is not diagnostic 

of continuous, progressive aggradation. The 

clearest example of this is at section G, where Ig2Eb 

overlies Ig1Eb with no visible boundary in the field, 

the contact being demarcated to !0.5 m by the incoming 

upward of Glass Mountain–derived rhyolite 

lithic fragments (figs. 5, 8). Elsewhere in the Bishop 

ignimbrite (e.g., sections H and I, fig. 10; also Wilson 

and Hildreth 1997, their fig. 10), fall material 

and Ig2Ea were deposited at this horizon, indicating 

a time gap of hours to tens of hours between ignimbrite 

packages Ig1Eb and Ig2Eb. We thus conclude 

that the presence of a flow-unit boundary 

does indicate a hiatus in deposition but that the 

reverse is not necessarily true, and that a lack of 

flow-unit boundaries, in the absence of other evidence, 

cannot be taken to indicate that accumu- 

lation was continuous (cf. Branney and Kokelaar 

1997). 

Thermal Building Blocks: Cooling Units. In the definitions 

and illustrative examples of Smith (1960b, 

especially pl. 20), it is implicit that the primary 

means whereby a simple cooling unit is recognized 

is the ideality of its patterns of zonation in welding, 

devitrification, and vapor-phase recrystallization. 

The term “compound cooling unit” is then automatically 

applied to any deposit that shows “pronounced 

deviations” from this pattern (Smith 

1960b, p. 157) but can still be considered to have 

accumulated rapidly enough for the whole rock 

body to cool as one (i.e., to be a single cooling unit). 

However, these definitions, as well as the concept 

of a composite sheet, are problematic in several 

respects: 

1. As the perceived zonations for a single cooling 

unit can vary so widely (Smith 1960a, 1960b), the 

criterion for “pronounced deviations” and hence 

the simple versus compound distinction can be 

given only in qualitative terms. 

2. Even given a simple welding and alteration 

profile, turning such a description into interpretation 

of the time-temperature relationships of the 

deposited material is not always possible. For example, 

several workers have inferred (e.g., Christiansen 

1979) or assumed (Riehle 1973; Riehle et 

al. 1995) that a simple cooling unit is effectively 

isothermal on emplacement, consistent with rapid


Figure 10. View of Chalk Bluffs at grid reference 733421. Section J was measured from the fall deposit up to the 

“cryptic ledge” (a zone of barely sintered material that marks the approximate contact between Ig2Ea and Ig2Eb) on 

this face and above this level in a slip scar 100 m farther east. Selected grain size and componentry data (the latter 

measured down to 1/16 mm, with !1/16 mm material assumed to be vitric ash) from material taken at the same 

point as the density samples are shown to compare with the density variations. Symbols are as in figure 7. 

accumulation and simple patterns of welding and 

alteration. If, however, a simple cooling unit is built 

up progressively (whether continuously or not), 

then the emplacement temperatures may have deviated 

significantly from uniform during accumulation. 

For example, the basal and topmost materials 

may have been cooler and the interior hotter, 

or increasing magma temperatures in a zoned package 

may influence emplacement temperatures, yet 

the welding and alteration zonation could, in principle, 

be simple in the sense used by Smith. 

3. The distinction between single and multiple 

cooling units is valid in principle but may be problematic 

to apply in practice. We label the Bishop 

ignimbrite in the Owens Gorge a single cooling 

unit because of evidence from the density profiles 

that the earlier parts of the ignimbrite (Ig1E) were 

still hot when later parts (Ig2E) accumulated. For 

example, welding Zone c in Ig1Eb was still hot 

enough when buried by Ig2Ea and/or Ig2Eb to have 

caused welding in the immediately overlying Ig2 

material (sections G–I; figs. 7–9) even though that


Figure 11. Longitudinal section down the line of the Owens Gorge projected on the true left side showing the 

overall welding structure of the ignimbrite. Density data from the 10 profiles (figs. 7–10) were combined with sparse 

point measurements and then matched to field observations at numerous sites along the gorge walls using the 

calibration between density, rock texture, and welding given in table 2. The dotted line is the 50 lithics/m 2 contour 

derived from lithic numerical abundance data (fig. 5b); note how this contour serves to demarcate welding Zone b 

(best developed at sections A and B) and Zone c (best developed down-gorge from section E), with an overlap around 

sections C and D. 

material was insufficiently hot to weld where it 

overlay a cold substrate (section J; fig. 10). We thus 

disagree with Sheridan (1968), who considered that 

there were two cooling units in the same area as 

sections G–I, implying that the Bishop ignimbrite 

is a composite sheet. In general, we suggest from 

our data that only in circumstances where the earlier 

unit was effectively “cold,” that is, unable to 

influence the welding profile or alteration state of 

the later unit, might the density minimum between 

two zones of greater welding actually coincide 

with the chronostratigraphic boundary between 

the two bodies of material. Only under such 

circumstances could the two bodies then be considered 

as separate cooling units. In the case of the 

Bishop Tuff, lateral changes in welding down the 

Owens Gorge from areas with only one maximum 

in welding intensity (e.g., section E) to those with 

two welding maxima do not reflect splitting of 

welding zones (from “simple” to “compound”) or 

of cooling units (from one to two), but instead result 

from differing development of separate welding 

maxima as successive packages of host material become 

thickest in turn down-gorge. 

Thus, in documenting ignimbrites even as young 

and clearly exposed as the Bishop, problems arise 

with the quantitative interpretation of the cooling 

unit concept. Although the original concepts have 

demonstrated their value as a descriptive framework 

(e.g., Christiansen 1979, and references 

therein), interpretative application of the concept 

to infer the timings and temperatures of accumulating 

ignimbrite material is fraught with problems. 

These arise primarily because the welding intensity 

(and any consequent devitrification and vaporphase 

alteration) depends on several properties of 

the deposited material, including variations in (1) 

emplacement temperatures (themselves complexly 

related to eruption composition, heat losses during 

transport, and admixture of lithic material), (2) juvenile 

compositions, (3) residual volatile contents, 

and (4) load stresses. However, even when such factors 

as juvenile composition and residual volatiles 

are accounted for (or are assumed to be uniform) 

in a particular deposit, a given density/welding profile 

has a nonunique relationship to its emplacement 

temperature profile because the individual 

mechanical building blocks (flow units or emplacement 

pulses) and their associated temperatures cannot 

necessarily be uniquely defined, even in an ostensibly 

simple cooling unit. 

Furthermore, although the descriptive distinc-


Figure 12. Plot of density data from five pairs of samples 

from across vitric to nonvitric boundaries in portions 

of the ignimbrite that show minimal background gradients 

in density. We use these data to infer that densification 

of the ignimbrite by welding processes acting on 

the viscous vitric component had largely or completely 

ceased by the time crystallization due to devitrification 

or vapor-phase alteration occurred. 

tion between simple and compound cooling units 

is in principle straightforwardly based on the patterns 

of welding and crystallization in the rock 

mass, interpreting the genetic and chronologic significance 

of a compound cooling unit profile may 

be complex. First, and most straightforwardly, compound 

cooling profiles that show only one welding 

maximum that occurs toward the top of the ignimbrite 

(e.g., section J in the Bishop; fig. 10) are often 

interpreted as due to the superposition of increasingly 

hotter material with no perceived time breaks 

(Lipman et al. 1966; Branney and Kokelaar 1997). 

However, although the welding zonation is compound 

(according to Smith 1960b), the emplacement 

dynamics of the deposit may not differ in any 

respect from those of a simple cooling unit, other 

than in the emplaced material becoming hotter 

with time. Second, compound cooling units that 

have more than one welding maximum are widely 

interpreted as reflecting emplacement at intervals 

of time sufficient to allow partial cooling of the 

earlier material (Christiansen 1979, p. 29; Riehle et 

al. 1995), unless evidence for other sources of cool- 

ing, particularly the incorporation of cold lithic material, 

is present (Lipman et al. 1989, p. 340). 

From our work, we infer that there are only two 

extended breaks in emplacement of the Bishop ignimbrite, 

represented by the F6 fall material separating 

Ig1Ea and Ig1Eb (sections A–C), and the F8– 

F9 fall material between Ig1Eb and Ig2Ea (sections 

H and I). Using fall-accumulation rate estimates 

(Wilson and Hildreth 1997), these time breaks are 

estimated as 5–10 h for the former and 3 h for the 

latter (with an additional but short period represented 

by the settling out of ash and trivial erosion 

[at one site only] at the level of the F8–F9 contact). 

In Ig1Ea, other thin fall horizons and flow-unit 

boundaries are seen (fig. 2), but the time breaks are 

by inference !3 h. None of these time breaks in 

themselves is adequate to permit significant cooling 

of the ignimbrite packages (Riehle 1973), and 

no discontinuity in density is seen across any of 

these depositional breaks. Thus the whole accumulation 

of Bishop ignimbrite in this sector represents 

a single emplacement unit from the perspective 

of its thermal history; that is, it is a single, 

compound cooling unit (Smith 1960b). 

However, the density minima in sections A–J, 

although defining compound cooling, do not coincide 

with or represent time breaks at all. If one 

accepts that the prime controls on welding intensity 

in the interior of the Bishop ignimbrite are emplacement 

temperatures and load stresses, and that 

load stresses diminish systematically upward, the 

closely spaced vertical density variations must then 

reflect differing emplacement temperatures with 

time. For example, the lower part of Ig2Eb and all 

of Ig2Ea were emplaced at significantly cooler temperatures, 

in the latter case insufficient to induce 

welding where overlying a fall-deposit substrate. 

Our data imply that models presented by Riehle 

(1973) and Riehle et al. (1995) for inferring the emplacement 

temperature of simple or compound, 

single or multiple cooling units are thus limited in 

two respects. First, as discussed earlier, a given density 

profile in a real deposit (even an apparently 

simple cooling unit) can result from a number of 

possible combinations of increments of material (or 

flow units) with nonuniform temperatures. To 

match a real-world density profile of a simple cooling 

unit with a given inferred emplacement temperature 

thus requires an assumption of isothermal 

emplacement. Second, in a single compound cooling 

unit such as the Bishop ignimbrite, we can show 

that the position of the density minimum between 

two zones of more strongly welded ignimbrite does 

not coincide with any time break, such breaks elsewhere 

being unequivocally flagged by interbedded


fall material (e.g., sections H and I) or inferred from 

sharp changes in ignimbrite lithology (e.g., sections 

F and G). Thus, defining individual bodies of material 

for the purposes of thermal modeling by using 

the density minimum between them is inherently 

flawed for single cooling units and will lead to artificial 

results. 

Our work suggests also that the use of the term 

“composite sheet,” as introduced by Smith, may 

also be problematic, for two reasons. First, it is unclear 

whether the composite sheet is meant to apply 

to material from a single eruption, or could be 

from widely separated eruptions, or both. For example, 

the Huckleberry Ridge Tuff is described by 

Christiansen (1979, 2001) as a composite sheet 

composed of three members, each forming separate 

cooling units in distal areas yet merging to yield a 

single compound cooling unit in proximal areas. 

The whole assemblage was envisaged as being 

erupted in “hours to days,” that is, over a time scale 

insufficient for significant cooling. On the other 

hand, features such as lava flows or sediments were 

suggested by Smith (1960b) as possibly present between 

separate cooling units in a composite sheet, 

with the implication of substantial time intervals. 

Under such circumstances it is difficult to envisage 

the earlier and later cooling units as being part 

of what could be seen as a single eruption. Thus 

the grounds for delineating single versus multiple 

compound cooling units, and hence identification 

of a composite sheet in a form that permits genetic 

interpretation, are unclear. Second, the lateral 

changes from simple to compound, or from single 

to multiple cooling units are generally inferred or 

assumed to represent the same body of material, in 

order for comparisons to be made. However, the 

imbricate structure of the packages in the Bishop 

ignimbrite along the Owens Gorge show that this 

inference or assumption is false in this case, and 

that the apparent lateral change of one into two (or 

simple into compound) cooling units can be an artifact 

of welding development in separate, imbricate 

packages of ignimbrite. Thus, while application 

of the term “composite sheet” remains valid 

as a descriptive term for an ignimbrite that shows 

differing development of cooling units in different 

areas, we suggest that any genetic interpretation of 

that development requires further information 

about relationships between the ignimbrite material 

in those areas. 

Conclusions 

The Bishop ignimbrite exposed in a longitudinal 

profile down the Owens Gorge shows four large- 

scale imbricate depositional lenses (packages Ig1Ea, 

Ig1Eb, Ig2Ea, and Ig2Eb of Wilson and Hildreth 

1997), each of which is the product of numerous 

individual flow units or pulses of material. The 

clearest-defined flow-unit boundaries reflect intervals 

of ignimbrite nondeposition represented by 

thin fall deposits. Such intervals are inferred to 

have lasted for at least several hours and as long as 

10–15 h. However, in most such cases, and in other 

examples in which partings occur between flow 

units, the physical characteristics (e.g., grain size, 

maximum lithic sizes, lithic lithologies) of the ignimbrite 

do not change significantly across such 

boundaries. Conversely, some time breaks of hours 

to tens of hours are demonstrably not accompanied 

by any visible flow-unit boundary or parting. Our 

data thus show that material with no visible flowunit 

boundaries may reflect quite different emplacement 

histories. It may be difficult to show 

whether such material accumulated as a single flow 

unit, or gradationally by progressive aggradation, or 

in punctuated fashion from a series of individual 

flows without consideration of other information. 

In the Bishop Tuff, the presence of time breaks in 

deposition of massive ignimbrite can be demonstrated 

from sharp changes in lithic lithologies (e.g., 

between Ig1Eb and Ig2Eb at section G; fig. 8) between 

units that elsewhere can be shown to be separated 

by periods of fall and ignimbrite deposition. 

Progressive aggradation may sometimes occur 

(Branney and Kokelaar 1992), but our Bishop data 

show that discrete flow units are the norm in building 

multiflow packages that may or may not retain 

evidence for partings between flows. 

The Bishop ignimbrite in the Owens Gorge displays 

four zones of maximum density (and correlated 

welding) that collectively form a single, compound 

cooling unit. Each depositional package was 

emplaced at temperatures that varied in time during 

the eruption. Maximum welding occurred 

where the deposited material was initially hottest, 

and the zones of minimum welding reflect the presence 

of cooler-emplaced material and not time 

breaks when cooling occurred. Thus the distinction 

drawn by Smith (1960b) between simple and compound 

cooling units in ignimbrites cannot necessarily 

be interpreted solely in terms of the postemplacement 

cooling history. The intensities of the 

welding maxima vary with local load stresses (i.e., 

thicknesses of the flow packages) and are therefore 

greatest in the middle to lower Owens Gorge and 

are unrelated to proximity to the source. In the 

most distal section, the onset of sintering locked 

in a lower bulk density than that which nonwelded 

material could attain by compaction in the 0.76


million years since the eruption. Models that determine 

ignimbrite densities at the top or bottom 

of flows by linear extrapolation of welded-tuff densities 

in the central part of the flow are thus likely 

to be in error. We suggest that density values for 

nonwelded tuff at the tops and bottoms of ignimbrite 

units should be separately determined. 

Existing definitions of the terms “flow unit” and 

“cooling unit” remain valid in theory, are valuable 

tools for descriptive purposes, and represent the basic 

building blocks for constructing ignimbrites. 

However, different kinds of data are required, in 

addition to identification of flow packages and density 

profiles, to constrain the episodicity and original 

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Assembling an Ignimbrite: Mechanical and Thermal Building Blocks ...

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