01 branney et al 2008 bv.pdf

Bull Volcanol (2008) 70:293–314 

DOI 10.1007/s00445-007-0140-7 

RESEARCH ARTICLE 

‘Snake River (SR)-type’ volcanism at the Yellowstone 

hotspot track: distinctive products from unusual, 

high-temperature silicic super-eruptions 

M. J. Branney & B. Bonnichsen & G. D. M. Andrews & 

B. Ellis & T. L. Barry & M. McCurry 

Received: 26 April 2005 /Accepted: 8 March 2007 / Published online: 20 June 2007 

# Springer-Verlag 2007 

Abstract A new category of large-scale volcanism, here 

termed Snake River (SR)-type volcanism, is defined with 

reference to a distinctive volcanic facies association 

displayed by Miocene rocks in the central Snake River 

Plain area of southern Idaho and northern Nevada, USA. 

The facies association contrasts with those typical of silicic 

volcanism elsewhere and records unusual, voluminous and 

particularly environmentally devastating styles of eruption 

This paper constitutes part of a special issue dedicated to Bill 

Bonnichsen on the petrogenesis and volcanology of anorogenic 

rhyolites. 

Editorial responsibility: W Leeman 

Electronic supplementary material The online version of this article 

(doi:10.1007/s00445-007-0140-7) contains supplementary material, 

which is available to authorized users. 

M. J. Branney (*) : G. D. M. Andrews : B. Ellis 

Department of Geology, University of Leicester, 

University Road, 

Leicester LE1 7RH England, UK 

e-mail: mjb26@le.ac.uk 

B. Bonnichsen 

Idaho Geological Survey, University of Idaho, 

Moscow, ID 83844-3014, USA 

T. L. Barry 

Volcano Dynamics Group, Open University, 

Milton Keynes, 

MK7 6AA England, UK 

M. McCurry 

Department of Geosciences, Idaho State University, 

Pocatello, ID 83209-8072, USA 

G. D. M. Andrews 

Volcanology Lab. & MDRU, EOS, 

University of British Columbia, 

Vancouver, BC, Canada 

that remain poorly understood. It includes: (1) largevolume, 

lithic-poor rhyolitic ignimbrites with scarce pumice 

lapilli; (2) extensive, parallel-laminated, medium to coarsegrained 

ashfall deposits with large cuspate shards, crystals 

and a paucity of pumice lapilli; many are fused to black 

vitrophyre; (3) unusually extensive, large-volume rhyolite 

lavas; (4) unusually intense welding, rheomorphism, and 

widespread development of lava-like facies in the ignimbrites; 

(5) extensive, fines-rich ash deposits with abundant 

ash aggregates (pellets and accretionary lapilli); (6) the 

ashfall layers and ignimbrites contain abundant clasts of 

dense obsidian and vitrophyre; (7) a bimodal association 

between the rhyolitic rocks and numerous, coalescing lowprofile 

basalt lava shields; and (8) widespread evidence of 

emplacement in lacustrine-alluvial environments, as 

revealed by intercalated lake sediments, ignimbrite peperites, 

rhyolitic and basaltic hyaloclastites, basalt pillow-lava 

deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial 

sands and palaeosols. Many rhyolitic eruptions were high 

mass-flux, large volume and explosive (VEI 6–8), and 

involved H2O-poor, low-δ 18 O, metaluminous rhyolite magmas 

with unusually low viscosities, partly due to high 

magmatic temperatures (900–1,050°C). SR-type volcanism 

contrasts with silicic volcanism at many other volcanic 

fields, where the fall deposits are typically Plinian with 

pumice lapilli, the ignimbrites are low to medium grade 

(non-welded to eutaxitic) with abundant pumice lapilli or 

fiamme, and the rhyolite extrusions are small volume silicic 

domes and coulées. SR-type volcanism seems to have 

occurred at numerous times in Earth history, because 

elements of the facies association occur within some other 

volcanic fields, including Trans-Pecos Texas, Etendeka- 

Paraná, Lebombo, the English Lake District, the Proterozoic 

Keewanawan volcanics of Minnesota and the Yardea 

Dacite of Australia.

294 Bull Volcanol (2008) 70:293–314 

Keywords Snake River . Yellowstone . Intraplate . 

Hot-spot . Ignimbrite . Welded tuff . Rheomorphic . 

Super-eruption 

Introduction 

In this paper we define a new category of large-scale silicic 

volcanism that is distinct in character to the more wellknown 

pumice-rich, Plinian/ignimbrite type of silicic 

volcanism that is described extensively in the literature 

(e.g. Cas and Wright 1987). The distinctive category of 

volcanism has occurred at several times in Earth history, but 

is under-represented in the literature, possibly because the 

unusual, and particularly environmentally devastating, 

eruption styles are not well understood. To highlight its 

character, we describe the best-preserved example known to 

us, which serves as a defining ‘type’ example, and where 

further study would be richly rewarded. We then contrast 

SR-type volcanism with silicic volcanism elsewhere, 

discuss the enigmatic eruption styles and, finally, list other 

volcanic provinces where SR-type volcanism may be 

represented. 

Large-volume silicic eruptions involving 10s to 

1,000s km 3 of magma are the most catastrophic form of 

volcanism and can cause abrupt regional obliteration and 

climatic perturbation (Sparks et al. 2005). Typically, they 

Fig. 1 The bimodal Columbia 

River–Yellowstone volcanic 

province of northwest USA 

showing the type area of SR-type 

volcanism as defined in this paper 

(rectangle) and the location of 

Miocene–Pleistocene Lake Idaho 

(vertical hachure; SE limits yet to 

be defined). Northern Nevada 

rhyolites after Pierce et al. (2002). 

Rhyolitic calderas: Y, Yellowstone 

caldera field; N, Newberry; B, 

Burns; M, McDermitt. Approximate 

outlines of inferred, largely 

concealed eruptive centres: OH, 

Owyhee-Humboldt; BJ, Bruneau- 

Jarbidge; TF, Twin Falls; P, Picabo; 

H, Heise, after Bonnichsen 

et al. (1989) andMorganand 

McIntosh (2005); ages given in 

Ma. Locations of outflow ignimbrite–ashfall 

successions: R, Rogerson 

Graben; C, Cassia Hills; 

BH, Bennett Hills; T, Trapper 

Creek–Goose Creek Basin. Dark 

grey: extent of Columbia River– 

Oregon plateau basalts (including 

Steens Mountain, Malheur Gorge 

and Picture Gorge basalts). MR, 

Magic Reservoir centre 

involve (1) Plinian explosive eruptions that produce fallout 

layers of pumice lapilli with subordinate lithic clasts, 

accompanied by (2) emplacement of voluminous pumice 

lapilli-bearing ignimbrites, followed by (3) extrusion of 

relatively small-volume domes and coulées of degassed 

silicic lava. However, the silicic eruption products in some 

volcanic fields differ significantly from this general pattern. 

The youngest, and best-preserved, example of this is found 

in the central part of the Snake River Plain in southern 

central Idaho and northernmost Nevada, north-west USA. 

There, Miocene rhyolitic volcanism was large-scale and 

catastrophic but the assemblage of volcanic deposits is 

markedly different to the familiar pumice-rich assemblage 

conventionally associated with large Plinian and ignimbrite 

eruptions. Unusual characteristics include a paucity of 

pumice lapilli, a predominance of particularly intenselywelded 

rheomorphic ignimbrites, layers of parallel-stratified 

coarse ash in which large cuspate shards are commonly 

clearly visible in the field, and the presence of exceptionally 

large-volume, low-aspect ratio rhyolite lavas. We recognise 

that this association of facies represents a distinctive 

category of volcanism, and propose the name ‘Snake 

River-type’ (SR-type) volcanism, after the c.13 Ma– 

c.8 Ma succession in the central part of the Snake River 

Plain (Fig. 1), where it is best represented. Elements of the 

same facies association occur in other parts of the world 

and suggest that SR-type volcanism has punctuated Earth

Bull Volcanol (2008) 70:293–314 295 

history; during such events, the eruption of large volumes 

of silicic ash at extremely high temperatures and at high 

eruptive mass flux rates is likely to have had a profound 

environmental impact. 

Regional setting 

Miocene silicic rocks of south central Idaho and northernmost 

Nevada (Fig. 1) form part of the bimodal (basalt– 

rhyolite) Columbia River–Yellowstone volcanic province of 

northwest USA (Fig. 1). The volcanic activity generally 

youngs north-eastwards towards the most recent activity at 

Yellowstone (Fig. 1) and is thought to relate to the passage 

of the North American continent over a stationary hot-spot 

(e.g. refs in Pierce et al. 2002). An overall northeastyounging 

of silicic eruptive centres in southern Idaho is 

thought to reflect diachronous injection of basaltic magma 

into the lithosphere, causing the generation of lithospheric 

melts (e.g. Leeman 1982a) and/or the partial re-melting of 

earlier injected material (refs in Christiansen and McCurry 

2007). The chemistry of the rhyolitic rocks across the 

province broadly reflects the nature of the lithosphere from 

which the magma derives, i.e. peralkaline to metaluminous 

older rhyolites in the west where the lithosphere comprises 

accreted oceanic terrane material, to younger metaluminous 

rhyolites of the central and eastern parts of the Snake River 

Plain, where lithosphere is part of the North American 

craton (e.g. Wright et al. 2002). This transition in 

lithospheric affinity is reflected in the Sr, Nd, Pb and O 

isotopes (e.g. Bennett and DePaolo 1987; Wooden and 

Mueller 1988; Leeman et al. 1992). The voluminous central 

Snake River Plain rhyolitic rocks that are the subject of this 

paper formed during a major regional ignimbrite ‘flare-up’ 

that heralded continental rifting, with formation of the western 

Snake River rift (Fig. 1). They are typical of intracontinental 

A-type granitic melts; metaluminous and relatively anhydrous, 

with elevated HFSE concentrations and high halogen 

concentrations (e.g. Christiansen and McCurry 2007). 

The Columbia River–Yellowstone volcanic province 

records diverse eruptive styles, ranging from the effusion 

of extensive basaltic lava fields, low-profile basaltic shields, 

to silicic extrusion and magmatic and phreatomagmatic 

explosivity. Features characteristic of SR-type volcanism 

occur in several parts of the province, but are best 

developed in the Miocene succession in the type area 

(depicted in Fig. 1). This location is the original type area 

for the ‘Idavada Volcanic Group’ of Malde and Powers 

(1962) and it is where the term ‘super-eruptions’ was first 

coined (Bonnichsen 2000 BBC Horizon); super-eruptions 

are catastrophic environmentally devastating eruptions 

>300 km 3 (Sparks et al. 2005). The rhyolitic (70–76% 

SiO2) eruptions are inferred to have occurred from broad 

eruptive centres (e.g. Bruneau-Jarbidge and Twin Falls 

centres; Fig. 1), which are largely concealed beneath 

Pliocene–Pleistocene basalt lavas, so much of our understanding 

derives from well exposed and deeply dissected 

outflow successions in massifs to the north and south of the 

Snake River Plain, such as around Jarbidge, the Cassia 

Mountains and the Bennett Hills (Fig. 1; e.g. Bonnichsen 

and Citron 1982; Honjo et al. 1992; McCurry et al. 1996; 

Andrews et al. 2007; Bonnichsen et al. 2007). The 

stratigraphy and petrology of the individual volcanic 

successions are described elsewhere (previous refs), and 

the present paper presents a synthesis of the physical 

volcanology of the area. Field locations are given as grid 

references (prefixed GR) using 11 T zone UTM coordinates. 

Pliocene silicic volcanic rocks, 2–5 m.y. younger than 

those described in this paper, at the Magic Reservoir 

eruptive centre (MR on Fig. 1) to the north of the hotspot 

track in the central Snake River Plain are not entirely SRtype 

in that they exhibit features such as rhyolite domes and 

Plinian pumice fallout layers (Honjo and Leeman 1987; 

Leeman 1982b; Leeman 2004). 

Snake River-type eruptive activity has never been 

observed and so must be inferred from the deposit record. 

Therefore, we will describe the volcanic facies that make up 

the distinctive SR-type volcanism facies-association. Some 

of the features are unusual or have unusual facets, whilst 

others are fairly common in silicic volcanism elsewhere. 

The distinctiveness of SR-type volcanism lies in the relative 

proportions of the facies; for example, in SR-type ignimbrites 

lava-like facies predominate, yet in many other 

volcanic fields lava-like facies are, typically subordinate, 

if present at all. 

SR-type ignimbrites 

We estimate that there are ≥40 extensive welded rhyolitic 

ignimbrite sheets in the central Snake River Plain area. 

Ignimbrite successions dominate the SR-type volcanism 

landscape and give rise to widespread trap-topography and 

terracing of multiple cliffs along the walls of river canyons 

and escarpments (Appendix Fig. 1). Individual escarpments 

expose as many as ten ignimbrites, and bases of many of these 

successions are not exposed (e.g. Bonnichsen and Citron 

1982; Honjo et al. 1992; Perkins et al. 1995; McCurry et al. 

1996; Cathey and Nash 2004; Andrews et al. 2007). 

Size and morphology 

Ignimbrite outflow sheets are up to 200 m thick and include 

both simple and compound cooling units (e.g. the Grey’s 

Landing ignimbrite and Cougar Point Tuff VII, respective-


ly). Several, but not all, of the ignimbrites have low aspectratios 

(Fig. 2): they trace laterally for tens of kilometres, 

thickening locally into basins (e.g. Rogerson Graben) and 

tapering to less than 3 m thick across palaeotopographic 

highs (Fig. 3). This makes estimation of eruption volumes 

difficult: in addition, mapping of individual outflow sheets 

remains poorly advanced, and thicknesses and extents of 

proximal ignimbrite within eruptive centres in the axis of 

the Snake River Plain are obscured by younger basalt lavas. 

Current understanding is that the ignimbrite eruptions were 

similar in scale to those elsewhere in the volcanic province, 

with eruption volumes spanning three orders of magnitude, 

from tens to thousands of km 3 (VEI 6–8). For example, the 

Sand Springs ignimbrite (Andrews et al. 2007) has an 

estimated volume of 10 km 3 , whereas the ignimbrites of 

Jackpot, Big Bluff and Cougar Point Tuff XIII may exceed 

1,000 km 3 (Bonnichsen and Citron 1982; Andrews et al. 

2007). 

Pyroclast populations 

Snake River-type ignimbrites are commonly true tuffs (e.g. 

the Backwaters Member; Andrews et al. 2007). This is in 

contrast to ignimbrites elsewhere, which are commonly 

-3 

10 

thickness of unit t (km) 

0 

10 

-1 

10 

-2 

10 

-3 

10 

eruption 

volume 

10 km 

10 km 

-3 3 

10 km 

-2 3 

-4 3 

-1 

10 

-2 

10 

10 km 

-1 3 

Intermediate lavas 

n = 239 

-1 

10 

Basalt lavas 

n = 479 

aspect 

ratio 

0 

t/d = 1:10 

Rhyolite lavas 

n = 176 

0 

10 

10 km 

1 3 

2 

area covered by unit A (km ) 

Fig. 2 Dimensions and aspect-ratios of SR-type ignimbrites and SRtype 

rhyolite lavas compared with those from other volcanic fields 

(data for non-SR-type volcanics adapted from Walker 1983; Cas and 

Wright 1987). SR-type ignimbrites (black spots) span the range from 

high to low aspect-ratio type. The field of SR-type rhyolite lavas 

(dashed line) is quite distinct from that of non-SR-type rhyolite lavas: 

SR-type lavas have similar aspect ratios to many basalt lavas (not 

extensive flood basalts, which are not indicated), and they occupy a 

10 km 

0 

10 

0 3 

1 

10 

10 km 

composed mostly of massive lapilli-tuff (or lapilli-ash). 

Pumice lapilli are mostly absent (Fig. 5), as are fiamme 

formed from welded pumice lapilli. A few ignimbrites 

contain pumice lapilli locally. Sieving of rare, non-welded 

parts of massive ignimbrites reveals grainsizes of φ>2, and 

sorting values of σφ


Fig. 3 Characteristic features of 

SR-type rhyolite lavas (top) and 

SR-type ignimbrites (bottom) 

contrasted with those typical of 

other volcanic fields. SR-type 

lavas (note vertical exaggeration) 

are much more voluminous, 

more extensive, and have 

lower aspect ratios than non-SRtype 

silicic lavas. SR-type 

ignimbrites are extremely highgrade, 

commonly lack pumice 

lapilli (or fiamme), with intense 

welding, rheomorphism and development 

of lava-like facies. 

They are distinguishable from 

lavas by their tapering margins, 

extensive zones containing pervasive 

vitroclastic textures, and 

the absence of extensive basal 

autobreccia and blunt, lobate 

terminations 

SR-type rhyolite lava 

~ 300 m 

silicic dome 

100 m 

feeder 

dyke 

large gas cavities 

autobreccia 

juvenile, others may be derived from underlying welded 

ignimbrites or lavas. 

Most SR-type ignimbrites are massive, and some show 

some vertical zoning with respect to phenocryst abundance 

and/or composition (Wright et al. 2002; Andrews et al. 

2 km 

proximal 

tephra 

columnar jointing 

basal 

vitrophyre 

30 km 

flow interior flow margin 

lithoidal 

rhyolite 

silicic coulee 

upper 

vitrophyre 

100 m 

autobreccia 

‘Typical’ non-SR-type rhyolite lava (e.g. Italy, Jemez Mtns, La Primavera) 

SR-type ignimbrite 

~ 100 m 

local 

peperites 

autobreccia 

columnar 

jointing 

medium-scale 

rheomorphic folds 

basal 

vitrophyre 

100 km 

lithoidal 

rhyolite 

‘Typical’ non-SR-type ignimbrite (e.g. Italy, Jemez Mtns, Pinatubo) 

~ 100 m 

eutaxitic lapilli-tuff 

10 km 

pumice-rich lapilli-tuff 

sheet 

joints flow 

lobe 

autobreccia 

5 km 

upper 

vitrophyre 

sub-horizontal 

welding and small-scale 

rheomorphic fabric 

splay-and-fade 

stratification 

~ 30 m 

feather-edge vitrophyre 

2007). These characteristics indicate deposition from 

sustained density currents whose lower parts were a 

granular fluid (‘granular fluid-based pyroclastic density 

currents’ of Branney and Kokelaar 2002). Some contain 

moulds of trees (e.g. Greys Landing Ignimbrite at Monu- 

5 m 

5 m


Fig. 4 Grain size and sorting 

characteristics of SR-type 

ignimbrites, compared to those 

from other volcanic provinces. 

Shaded fields from Walker 

(1971); percentages cited are 

from 300 ignimbrites; sorting 

is described in terms of a 

standard deviation, 

σφ ¼ ðφ84 φ16Þ=2. SR-type ignimbrites are 

generally better sorted than 

ignimbrites from other volcanic 

provinces 

(less well 

sorted) 

(no grainsize 

variation) 

ment Canyon, Twin Falls County). A few non-welded 

zones also include horizons, traceable for several kilometres, 

of well-developed tractional cross-stratification, 

indicative of deposition from currents in which clast 

Fig. 5 Non-welded facies of SR-type ignimbrites, showing the 

absence of pumice lapilli and lithic lapilli, and the presence of 

abundant small vitric chips supported in a massive, fine ash matrix. a 

Typical SR-type non-welded massive ignimbrite with small vitric 

grains; Sand Springs ignimbrite, US Highway 93. b Jackpot 6 

ignimbrite also contains coarser vitric fragments; from US Highway 

93 south of Jackpot, Nevada. Rule shows centimetres 

sorting (84%-16%) / 2 (phi) 

6 

5 

4 

3 

2 

1 

99% 

96% 

0 

-10 -8 -6 -4 -2 0 2 4 6 

(finer grained) 

median diameter (phi) 

SR-type ignimbrites of the central SRP 

Sand Springs Ignimbrite 

Grey's Landing Ignimbrite 

Dry Gulch 1 Ignimbrite 

Steer Basin ignimbrite 

Cougar Point Tuff 15 

Cougar Point Tuff 11b 

Cougar Point Tuff 11a 

Eastern SRP ignimbrites 

Tuff of Kilgore 

Tuff of Wolverine Creek 

concentrations were too low to cause significant grain 

interactions or dampen turbulence even within basal parts 

(‘fully dilute’ pyroclastic density currents of Branney and 

Kokelaar 2002), and they contain accretionary lapilli with 

concentric laminations as well as cored accretionary lapilli 

in which the concentric laminations enclose a central vitric 

fragment (Fig. 8a and b). 

Vitroclastic textures reveal that most of the juvenile shards 

are relatively thick, platy and cuspate, bubble-wall types 

(Fig. 6a). In addition there are rare occurrences of subspherical, 

globule-shaped shards that appear to be more 

mafic than the surrounding cuspate shards (Fig. 6b). These 

are non-flattened despite the eutaxitic nature of the surrounding 

cuspate vitroclastic matrix. This suggests that, as with 

achneliths in basaltic fire-fountaining eruptions, the globuleshaped 

shards were liquid at the time of fragmentation, yet 

solidified prior to deposition, possibly as a result of higher 

solidification temperatures than those of the surrounding 

welded rhyolitic shards. In many SR-type ignimbrites, 

however, welding and rheomorphism has pervasively transposed 

and obliterated the vitroclastic textures (Fig. 6c). 

Unusually high welding intensities, and contact-thermal 

effects 

Most Miocene ignimbrites in the central Snake River Plain 

range from high-grade (sensu Walker 1983) to extremely 

high-grade (sensu Branney and Kokelaar 1992). Welding is 

intense—typically off the top of the welding intensity scale 

proposed by Quane and Russell (2005)—and it extends 

right to the base of many of the ignimbrites. Contactthermal 

effects extend to as much as 5 m beneath some 

ignimbrites: underlying stratified ash layers are fused to 

black vitrophyre (Fig. 7a) and substrate palaeosols are 

intensely baked to red-brown terracotta and have developed 

columnar joints by cooling-contraction (Fig. 7a). Another 

unusual characteristic of the welding in SR-type ignimbrites 

is that it sometimes extends to, or close to, the preserved


Fig. 6 Typical microscopic textures of Snake River-type ignimbrites. 

a SEM image of non-welded bi- and tri-cuspate bubble-wall type 

shards. Base of Cougar Point Tuff Member XV, Murphy Hot Springs. 

b Welded cuspate shards form a eutaxitic fabric, with rare, nondeformed 

mafic globule-shaped shards (white arrows). Ignimbrite 

from the Mount Bennett Hills (Tuff of Fir Grove GR: 663660 478290; 

photomicrogram of thin section in PPL; scale as in A). c Intense 

welding and rheomorphism has transposed and attenuated former 

vitroclastic textures, producing a flow-lamination similar to that seen 

in silicic lavas (the flow lamination grades into vitroclastic tuff near 

the base and top of the ignimbrite: not shown). Isoclinal fold pair 

indicates top-to-left rheomorphic shear. Basal vitrophyre of the Grey’s 

Landing ignimbrite (Rogerson Formation); Grey’s Landing, Twin 

Falls County, Idaho 

Fig. 7 Field characteristics of SR-type ignimbrites. Columnar-jointed 

baked palaeosol (behind notebook) and fused parallel-laminated 

ashfall, beneath the massive basal vitrophyre (top) of the Greys 

Landing ignimbrite. Arrow indicates base of ignimbrite. Scale is 

10 cm. Backwaters, Idaho, GR:685154 4659673. b Isoclinal folding in 

flow-laminated, lava-like facies of the Greys Landing Ignimbrite 

(location and scale as in a). c Open rheomorphic fold with 

subhorizontal fold axis in upper part of ignimbrite folds steep isoclinal 

folds (similar to those in b). Steer basin ignimbrite. Cassia Mountains. 

Scale 5 m high 

tops of units, with fusing of overlying stratified ash layers 

(e.g. layer of fused accretionary lapilli - bearing stratified 

tuff overlying Jackpot 5; Fig. 8b). 

Pervasive and intense rheomorphism 

The intense welding of SR-type ignimbrites is commonly 

associated with intense rheomorphic deformation. Perva-


Fig. 8 Ash aggregates in SR-type tephras. a Concentric-laminated 

accretionary lapilli from a non-welded, cross-stratified lapilli-tuff, 

deposited from a long-runout, fully dilute pyroclastic density current. 

Note abundant angular vitric fragments (black), some coated in fine 

ash. Jackpot Member 6; US Highway 93, south of Jackpot, Nevada. b 

Welded accretionary lapilli, some with vitric cores (black). The 

accretionary lapilli are framework supported in some layers, and 

supported in fine tuff vitrophyre matrix (dark, bottom) in others. 

Jackpot 6, Nevada. c Framework-supported ash coated pellets, some 

with vitric cores (left) in ashfall deposits. Lower part of Cougar Point 

Tuff XV at Murphy Hot Springs, Idaho. Scale in cm 

sive development of elongation lineations, including 

stretched vesicles associated with pervasive flow-lamination, 

and open to isoclinal small- to medium-scale folds 

(Fig. 7b, c), including oblique folds and sheath folds, 

indicate that the ignimbrites underwent agglutination and 

rapid ductile shear whilst still hot (Branney et al. 2004). In 

ignimbrites elsewhere, welding is commonly thought to 

post-date ignimbrite emplacement (‘load welding’ Freundt 

1999), but in SR-type ignimbrites, both the welding and the 

onset of rheomorphic deformation are thought to have 

occurred very rapidly, during deposition (Branney and 

Kokelaar 1992). Shear directions recorded by fold-axes 

and elongation lineations in some SR-type ignimbrites vary 

with height through an individual unit, and are thought to 

indicate changing shear directions within a diachronous 

rheomorphic shear-zone that ascended through the aggrading 

agglutinate while deposition occurred from the base of 

an over-riding, sustained and shifting pyroclastic density 

current (Branney et al. 2004; Andrews and Branney 2005; 

Andrews 2006). Top surfaces of several of the ignimbrites 

have been folded into tight ogive-like medium-scale folds, 

commonly associated with high-angle thrusts: this indicates 

late-stage rheomorphic flow, similar to that seen elsewhere 

in silicic lavas and strongly peralkaline ignimbrites (e.g. 

Sumner and Branney 2002). Intense welding and rheomorphism 

in some of the ignimbrites persists even where 

the ignimbrite thins to less than 5 m. 

Lava-like lithofacies 

An unusual characterisitc of SR-type ignimbrites is the 

widespread development of lava-like facies. We use ‘lavalike’ 

as a non-genetic term (after Branney and Kokelaar 

1992) for a lithofacies that looks like lava; it may be 

massive or flow-banded (e.g. Fig. 6c) but does not exhibit 

vitroclastic or eutaxitic texture (e.g. Fig. 6a, b). In most 

volcanic fields lava-like facies constitute a rare or restricted 

facies of rheomorphic ignimbrites. However, in SR-type 

volcanism lava-like facies dominate entire ignimbrite sheets 

(e.g. House Creek, Grey’s Landing, Big Bluff, Jackpot 1–5 

and Castleford Crossing ignimbrites and Cougar Point 

Tuffs 11 and 13; Bonnichsen et al. 2007). Lava-like facies 

of SR-type ignimbrites are indistinguishable from true lava 

in both hand specimen and in thin section, and their 

pyroclastic origin can be inferred only from the field 

relations: for example, rheomorphic ignimbrites generally 

do not have widespread basal autobreccias, whereas basal 

autobreccias characterise most rhyolite lavas; also, the lavalike 

facies in some ignimbrites grade laterally and/or 

vertically into less intensely welded, unequivocal vitroclastic 

tuff (Bonnichsen and Kauffman 1987; Branney et al. 

1992; Henry and Wolff 1992). A characteristic problem of 

SR-type volcanism is that in cases where critical field 

relations (e.g. basal contacts and distal feather-edges) are 

not exposed, it can be almost impossible to determine 

whether some predominantly lava-like units are rheomorphic 

ignimbrites or true lavas. Although lava-like facies 

occur within ignimbrites of some strongly peralkaline 

volcanic fields (Sumner and Branney 2002), they are a

Bull Volcanol (2008) 70:293–314 301 

relatively subordinate facies of ignimbrites within most 

metaluminous rhyolitic provinces, with the exception of 

some thick, caldera-filling ignimbrites (e.g. Batchelor 

caldera, Lipman 1984; Scafell caldera, Branney et al. 1992). 

Other characteristics of the ignimbrites 

SR-type ignimbrites form cooling units with upper and 

lower vitrophyres, each as much as 5 m thick, enclosing a 

lithoidal (microcrystalline) central zone, which may be as 

thick as 150 m (Bonnichsen and Citron 1982). Where the 

ignimbrites thin over topography, the vitrophyres merge to 

form a single vitrophyre. They characteristically contain 

laterally extensive zones of spherulites and lithophysae, 

ranging from a few millimetres up to 50 cm in diameter, 

close to the contact with the central lithoidal zone. The 

spherulites are typically spheroidal, but are oblate where 

their growth was influenced by the presence of a pronounced 

welding foliation, and are prolate where they grew 

in vitrophyre with a strong rheomorphic elongation lineation. 

Primary porosity has disappeared in intensely welded 

facies, but upper parts of many of the ignimbrites variously 

contain stretched and spherical vesicles that indicate 

continued vesiculation during and after the final stages of 

rheomorphic deformation. Some of the ignimbrites have 

developed scoriaceous or coarsely pumiceous zones in 

upper parts (e.g. in Cougar Point Tuff 11; Bonnichsen and 

Citron 1982: Tuff of Dry Gulch at Rock Creek, GR:720159 

4698838), in which the walls of the elongated vesicles have 

become so attenuated that the normally black vitrophyre 

takes on a golden brown colour, similar to that seen in 

basaltic reticulite. Clearly, in SR-type ignimbrites density 

cannot be used as a proxy for welding intensity. However, 

most SR-type rheomorphic ignimbrites are denser and 

much less vesicular than is typical of rheomorphic 

ignimbrites elsewhere, such as the eutaxitic Wall Mountain 

Tuff of Colorado (Chapin and Lowell 1979) and the Green 

Tuff of Pantelleria (Mahood 1984). 

Flow banding, or lamination, is common in the ignimbrite 

vitrophyres and lithoidal zones. Some of it may have 

originated from healed fractures, former vesicular zones 

and other textural heterogeneities that have become 

transposed and attenuated during ductile shear, as in flowbanded 

lavas, except that in the case of SR-type ignimbrites 

it must have developed at the surface by rheomorphism 

after deposition and agglutination, rather than within a 

magma-filled eruption conduit. Close-spaced (1–2 cm) 

straight or curved ‘sheeting joints’ (Bonnichsen and Citron 

1982) are abundant within the lithoidal zones. Their 

orientation is usually developed sub-parallel to the flow 

banding, apart from at tight fold hinges, and they are 

probably developed during static devitrification, mimetic 

after the anisotropy of the flow banding. 

SR-type volcanism is characterised by a general paucity 

of low-grade (non-welded) and moderate-grade (partly 

welded) ignimbrites with extensive zones of non-welded 

lapilli-ash, sillar, and non-rheomorphic eutaxitic lapilli-tuff. 

This is in marked contrast with many other ignimbrite 

fields, such as the Jemez Mountains of New Mexico, Taupo 

Volcanic Zone in New Zealand, and the Central Mexican 

Volcanic Belt, where those facies dominate. Non-welded 

ignimbrites do occur in the central Snake River Plain, such 

as the ash-pellet—bearing ignimbrites in the Cassia Hills 

(Fig. 1; see later section), but are not widely reported: this 

may in part be due to burial by talus and slope-wash from 

overlying, cliff-forming welded units, and partly because of 

bioturbation and reworking (e.g. unit below the Three 

Creek Rhyolite of Bonnichsen and Citron 1982). Loose, 

non-welded ignimbrites would be more prone to erosion and 

reworking, and may have contributed to the numerous layers 

of rhyolitic volcaniclastic sands (see later section). However 

the paucity of eutaxitic and moderately welded facies cannot 

be so easily explained in this way. Several other common 

features typical of ignimbrites elsewhere, such as coarse-tail 

grading, internal layering and diffuse stratification, and 

lithic-rich and pumice-rich lenses (Branney and Kokelaar 

2002) also have not been reported in the region. 

Extensive layers of parallel-stratified rhyolitic ash 

Thin-bedded to laminated, grey to white rhyolitic ash is a 

common facies of SR-type volcanism (Fig. 9a) and forms 

layers ≤5 m thick. The stratification is formed of sharp and 

gradational changes in grainsize; millimetre-scale normal 

and reverse, vertical distribution grading is common, with 

no obvious overall asymmetry or cyclicity. Individual beds, 


Fig. 9 Typical SR-type ashfall 

deposits. a Parallel-laminated 

and thin-bedded fine to coarse 

ash, showing laterally persistent 

stratification. Nat-Soo-Pah, 

Twin Falls County, Idaho. Metre 

ruler. b Fused, largely parallelstratified 

tuff of dominantly 

ashfall origin, with layers of 

abundant fused framework-supported 

small (


Fig. 10 Typical vitroclastic textures of SR-type ashfall deposits. a 

SEM image of cuspate, bubble-wall shards. Basal ashfall of Cougar 

Point Tuff Member XV, Murphy Hot Springs, Owyhee County, Idaho. 

b SEM image of bubble-wall shards from an ashfall fall deposit 

overlying the Grey’s Landing ignimbrite, at Salmon Dam, Twin Falls 

County, Idaho 

Bonnichsen 1982; Ekren et al. 1984; Bonnichsen and 

Kauffman 1987; Bonnichsen et al. 1989; Henry and Wolff 

1992) and we summarise only their salient features. 

At least eight rhyolite lavas from the Bruneau-Jarbidge 

eruptive centre exceed 10 km 3 each: the Dorsey Creek 

Rhyolite (Appendix Fig. 3) exceeds 75 km 3 and is over 

40 km long, and the Sheep Creek Rhyolite exceeds 200 km 3 

(Bonnichsen 1982). Large rhyolite lavas are present also at 

the inferred Twin Falls eruptive centre (e.g. Shoshone Falls 

Rhyolite, Balanced Rock Rhyolite, Bonnichsen et al. 1989), 

and in the Juniper Mountains of SW Idaho (e.g. the Badlands 

Rhyolite, Manley 1996; Manley and McIntosh 2002). Small 

rhyolite domes and coulées are not characteristic of SR-type 

volcanism, although the possibility that some lie concealed 

within subsided eruptive centres cannot be excluded. 

Aspect ratios of SR-type rhyolite lavas are unlike those of 

viscous rhyolite lavas elsewhere (Fig. 2). They are sufficiently 

low to coincide with those of many basalt lavas (Fig. 2). 

This is reflected in their long distance run-outs from their 

inferred source locations. The unusual dimensions led to 

early interpretations that they were ignimbrites (Ekren et al. 

1984), but they are now thought to be lavas on the basis of 

several criteria, including the presence of widespread basal 

autobreccias and abrupt, thick, stubby lobate terminations 

with thick talus aprons (e.g. see Bonnichsen and Kauffman 

1987; Henry and Wolff 1992; Manley and McIntosh 2002). 

In contrast, basal autobreccias in rheomorphic ignimbrites 

are rare and restricted to locations where rheomorphic flow 

has carried the hot agglutinate beyond the original extent of 

pyroclastic deposition (e.g. Sumner and Branney 2002). 

The lavas are blocky with thick (>5 m), variously vitric 

and coarsely pumiceous lower, marginal and upper carapace 

autobreccias (Bonnichsen 1982). Hot-state shear of 

basal, marginal, and internal coarsely pumiceous autobreccia 

during flowage has caused localised fusing and ductile 

shear, with the development of vitroclastic textures that can 

superficially resemble welded pyroclastic textures (Manley 

1996) as is common in viscous blocky lavas elsewhere (e.g. 

Iddings 1889; Pichler 1981; Sparks et al. 1993). Thick, 

central zones of the lavas (Appendix Fig. 3) are lithoidal 

(microcrystalline), massive or flow-banded, and dominated 

by steep columnar jointing, and low-angle close-spaced 

sheeting joints that in places form intersecting sets, 

producing ‘pencil-type’ jointing (Bonnichsen 1982). Spherulites 

and lithophysae are common near the base of the 

lithoidal zone. Some of the lavas, particularly to the west of 

the central Snake River Plain, exhibit agglutinate textures 

with spatter rags that indicate a clastogenic origin by some 

form of rhyolitic fire fountaining (e.g. Juniper Mountains; 

Manley and McIntosh 2002), although the majority of the 

rhyolite lavas are not visibly clastogenic. 

Lacustrine-alluvial facies 

The facies association that characterises SR-type volcanism 

in south central Idaho and northern Nevada includes 

widespread evidence for the presence of surface water at 

the time of the eruptions. 

Aqueously deposited volcaniclastic sediments 

Numerous thin layers of aqueously deposited volcaniclastic 

sediments are intercalated with ignimbrites across the central 

and western Snake River Plain (Appendix Fig. 4a, b). Alluvial 

and lacustrine facies are represented, with graded beds, 

parallel-laminated silts, ripple cross-lamination, scours, and 

abundant soft-sediment deformation, loading and dewatering 

structures. Many of the parallel-stratified ash layers appear to 

have been deposited directly into shallow standing water as 

there are local gradations into facies with minor truncations 

and scours that indicate reworking by gentle aqueous currents 

or wind, and into rippled sands of the same composition (e.g. 

Perkins et al. 1995). Lacustrine sediments occur between the


Cougar Point Tuffs around Jarbidge; within the Rogerson 

Formation near Jackpot and in the Rogerson Graben; and 

between ignimbrites dated at 13.7 to 8.6 Ma in the Shoshone 

Basin, Cassia Hills and Goose Creek Basin (Fig. 1; e.g. 

Hildebrand and Newman 1985; Perkins et al. 1995). 

Some of the lacustrine intervals within the Miocene 

ignimbrite-dominated successions north and south of the 

central Snake River Plain (Appendix Fig. 4a, b) may reflect 

somewhat isolated and/or short-lived bodies of water, as 

they form thin (100 m) masses of dominantly vitrophyric, 

clast-supported breccia, and chalcedony, jasper and opal 

locally fills fractures and other cavities in the lavas, with 

extensive groundmass silicification to their deep interiors. 

Basaltic pillow lava–hyaloclastite deltas 

Basalt lavas in central southern Idaho include hydrated 

basalts (water-affected basalts or ‘WAB’ of Bonnichsen and


Fig. 11 Ignimbrite peperites from the central Snake River Plain, 

Idaho. a Peperite at the base of an ignimbrite (‘Picabo Tuff’), just 

north of the plain. Basal vitrophyre (dark) is finely brecciated and 

loaded into wet silt (pale). b Ignimbrite vitrophyre peperite at the base 

of the Wooden Shoe Tuff, Rock Creek Canyon, Cassia Mountains. 

Note pervasive injection of disaggregated silt (pale) into fractures in 

the glassy welded ignimbrite. c Detail of peperite showing clouds of 

admixed angular hyaloclasts and silt. Location as for b 

Godchaux 2002), pillow lavas and hyaloclastites, including 

prograding pillow-hyaloclastite deltas (Appendix Fig. 4d; 

Shervais et al. 2005). Evidence for the interaction of the 

basalt lavas with lake water, streams and associated ground 

water has been widely documented (Jenks and Bonnichsen 

1989, refs in Bonnichsen and Godchaux 2002). 

Phreatomagmatic tuffs 

Several basaltic tuff cones and tuff rings dating back to 

Miocene times occur within the region of former Lake 

Idaho (Godchaux and Bonnichsen 2002). The tuffs contain 

abundant ash pellets and accretionary lapilli, exhibit wet, 

soft-state deformation and are associated with aqueously 

reworked facies. Both Surtseyan and Taalian (maar-forming; 

Kokelaar 1986) eruption styles are represented and 

indicate that the erupting magma interacted explosively 

with, respectively, lake water and groundwater. There are 

few equivalent examples of rhyolitic phreatomagmatic 

centres because the rhyolitic source vents are buried in the 

interior of the central Snake River Plain. However, the 

numerous layers of fine vitric ash containing abundant ash 

aggregates that occur within the ignimbrite successions may 

derive from water-enhanced explosivity. For example, 

white, fine ash - rich ashfall layers and associated nonwelded 

ignimbrites containing abundant coated ash pellets 

occur in eruption units in the Cassia Hills and Trapper 

Creek (Fig. 1). The subaerial vent area of the Wilson Creek 

ignimbrite is exposed in the western Snake River Plain, and 

proximal water-altered phreatomagmatic tuffs contain 

abundant angular chips of older rock types indicating 

explosive fragmentation of near-surface, water-bearing 

rocks by rising rhyolite magma prior to the main ignimbrite 

eruption into Lake Idaho (Ekren et al. 1984; Godchaux and 

Bonnichsen 2002; Bonnichsen et al. 2007). 

Associated basaltic volcanism 

The rhyolitic rocks are associated with basalts, in the form 

of scattered tuff rings and coalescing low-profile shield 

volcanoes with gentle concave slopes (1–2° flanks steepening 

to 5° near summits) and sometimes spatter ramparts 

surrounding Hawaiian-type summit collapse craters (e.g. 

Shervais et al. 2005). Some of the basalt lavas are fed by 

fissures along rift-zones. The basaltic volcanism is transitional 

between typical (i.e. steeper) shield volcanoes and 

true flood basalts, and has been termed ‘basaltic plains style 

volcanism’ to distinguish it from those end-members 

(Greeley 1977, 1982). Most of the visible basalt forms a 

cap overlying the Miocene ignimbrites and lavas across the 

width of the plain. Because of limited incision, it not known 

when basaltic volcanism began in the central Snake River 

Plain and whether any early basalt effusion occurred 

associated with emplacement of the inferred mid-crustal 

mafic intrusion (Rogers et al. 2002). 

Discussion 

This section considers the eruptive processes, the nature of 

the eruptive centres, and other examples worldwide where 

SR-type volcanism may have occurred.


SR-type eruptive processes 

Snake River-type eruptions were voluminous and extraordinarily 

environmentally devastating, with large-scale 

explosivity (VEI 6–8) during which vast glassy ignimbrites 

were fused across the landscape, and accompanied by 

widespread atmospheric dispersal of vitric ash. The 

extensive, predominantly massive and intensely welded 

nature of the ignimbrites is best reconciled with hightemperature, 

sustained, granular fluid-based pyroclastic 

density currents with run-out distances exceeding 100 km 

that were fed from high mass-flux pyroclastic fountaining 

eruptions (e.g. Bursik and Woods 1996; Branney and 

Kokelaar 2002). High eruptive mass flux, possibly via 

fissures associated with caldera subsidence, would have 

minimised cooling during transport (e.g. Branney et al. 

1992; Freundt 1999). Non-welded bases of a few of the 

ignimbrites may be the legacy of entrainment and mixing of 

atmospheric air into leading parts of the currents during 

early waxing stages of eruptions while the currents initially 

advanced across the landscape. Such flow-front cooling 

seems to have been minor in cases where the ignimbrites 

are welded right to their bases. However, it is doubtful that 

such ignimbrites were ever initially isothermal as is 

commonly assumed in welding models (refs in Russell and 

Quane 2005): phenocryst geothermometry indicates that 

initial thermal gradients may have existed in the ignimbrites 

even before they started cooling (Andrews et al. 2007). 

Gradual vertical changes in the trend of rheomorphic 

structures (elongation lineations and sheathfold hinges) 

within some of the most intensely welded ignimbrites 

indicate that pyroclasts in these examples had viscosities 

sufficiently low to enable them to agglutinate and shear 

even during deposition (Branney et al. 2004). The abundance 

of lava-like facies indicates that coalescence of hot 

pyroclasts, with obliteration of the clast outlines (Branney 

and Kokelaar 1992) was common: such rapid welding 

requires unusually low viscosities compared to those 

required for post-depositional welding (‘load welding’ of 

Freundt 1999) which is thought to characterise ignimbrites 

elsewhere (e.g. Ross and Smith 1961). The hot deposits 

retained a sufficiently low viscosity to spread gravitationally 

across low (≤5°) topographic slopes. Exsolution of 

volatiles continued, with the growth and rheomorphic 

attenuation of ellipsoidal vesicles and in some cases 

development of golden, thick-walled coarse pumiceous 

zones in upper parts of ignimbrites; such frothy lava-like 

facies are generally rare in ignimbrites elsewhere. 

Low viscosity of pyroclasts in some rheomorphic 

ignimbrites elsewhere is attributable to strongly peralkaline 

chemistries in which alkalis act as network-modifiers 

disrupting silicate polymerisation (Mahood 1984). The 

globular shards discovered in the (metaluminous) SR-type 

ignimbrites are found also in some strongly peralkaline 

rheomorphic ignimbrites (e.g. Johnson 1968; Sumner and 

Branney 2002) and indicate pyroclast viscosities sufficiently 

low for clast shapes to be controlled by surface-tension 

prior to chilling during transport (Branney and Kokelaar 

1992), just as in droplet-shaped pyroclasts from Hawaiian 

fire-fountaining eruptions (Pele’s tears or ‘achneliths’ of 

Walker and Croasdale 1972). However, the SR-type 

eruptions are not peralkaline and the low viscosities must 

result from high emplacement temperatures and possibly 

retention of some dissolved volatiles. This is consistent 

with high magmatic temperatures of 830–1,050°C estimated 

from pyroxenes, feldspars and oxide phases in SRignimbrites, 

and estimates that at least some of the rhyolite 

magmas had high fluorine contents (e.g. Honjo et al. 1992; 

Cathey and Nash 2004; Christiansen and McCurry 2007). 

Elsewhere, unusually low rhyolite viscosities indicated by 

rocks thought to record rhyolitic fountaining and agglutination 

have also been attributed to high fluorine contents 

(Duffield 1990). The high mass-flux eruptions may have 

minimised both cooling and the exsolution of certain 

volatile species during eruption and emplacement. 

The absences in SR-type ignimbrites of features like lithic 

and pumice dunes, bedding and pumice concentration zones, 

may be because agglutination of sticky particles limited 

granular segregation. However, absence of such features also 

characterises the rare non-welded facies, and so may simply 

reflect the paucity of pumice and lithic lapilli. Paucity of 

lithic lapilli is a characteristic of extremely high-grade 

ignimbrites elsewhere (e.g. Branney et al. 1992) and may 

reflect minimal erosion and entrainment from the vent 

margins and from the land surface by the pyroclastic current 

which, instead, tended to plaster surfaces with hot agglutinate. 

The general absence of pumice lapilli and fiamme is 

difficult to explain (fiamme are common in many rheomorphic 

ignimbrites elsewhere) and points to some facet of the 

fragmentation process, in which cuspate shards were 

generated from the vesiculating magma more readily than 

were pumiceous blocks and lapilli. Such a fragmentation 

mechanism is not understood. The morphology and large 

size of the cuspate shards in both the ignimbrites and the 

ashfall layers indicates that bubble sizes were larger than is 

typical of rhyolitic micro-vesicular pumice elsewhere; this 

may be the result of more rapid diffusion rates of volatiles 

within the rhyolite magmas due to their unusually high 

temperature and low viscosity. Fiamme present locally in 

some of the rheomorphic ignimbrites (e.g. Cougar Point Tuff 

XI) appear to be lenticular zones of late-vesiculated welded 

tuff matrix rather than former pumice clasts. 

The stratified ashfall deposits with their widespread 

distal correlatives have great value in long-distance, 

terrestrial stratigraphy and tephrochronology (Perkins and 

Nash 2002) and record widespread atmospheric dispersal of


large volumes of vitric ash, and is associated with major 

mammalian death assemblages, c.1,400 km to the west of 

the eruptive sources (Voorhies and Thomasson 1979; Rose 

et al. 2003). The widespread dispersal would have been 

favoured by the combination of high eruptive temperatures 

with high eruptive mass-fluxes. It may have occurred via 

vent-derived convective ash plumes, and/or from phoenix 

clouds (co-ignimbrite ash plumes) that lofted from the 

large, hot pyroclastic density currents. Many of the ash 

layers are coarser-grained than is typical of co-ignimbrite 

ashes elsewhere (cf. Walker 1981; Smith and Houghton 

1995), even distally (see Rose et al. 2003). This could 

reflect the unusually large shard-sizes produced by SR-type 

explosivity coupled with enhanced elutriation and lofting as 

a result of the large thermal input by the exceptionally hot 

pyroclastic currents. However, although some fine vitric 

ash layers overlie several of the ignimbrites, a significant 

proportion of the stratified ash occurs beneath ignimbrites 

of the same eruption-unit; such ashes could only be of coignimbrite 

origin if the leading edges of the pyroclastic 

density currents advanced rather slowly across a landscape 

that was already being mantled in co-ignimbrite ash from 

earlier stages of the eruption. The stratified ash layers 

beneath the ignimbrites could, alternatively, derive from 

Plinian explosivity. However, they are finer grained than is 

typical for Plinian deposits of a similar thickness: Plinian 

deposits thicker than a metre or so are typically composed 

of predominantly lapilli-sized pumice clasts (e.g. at Vesuvius, 

Valles, Santorini); because they tend to thin out as 

they become more fine grained distally, a Plinian layer 

more than a couple of metres thick of only ash sized 

particles would represent an unusually large eruption. 

Alternatively, the lack of pumice lapilli may be a facet of 

the fragmentation mechanism. One possibility is that the 

explosive fragmentation of the vesiculating magma was 

enhanced by interaction with meteoric water. This would be 

consistent with the widespread evidence for lacustrine 

conditions: Snake River Plain volcanic rocks are important 

aquifers, and one might expect that water occupied 

fractures beneath lakewater, for example in areas of 

proximal volcanic subsidence. The abundance of ash pellets 

and accretionary lapilli in the rhyolitic ash also are 

consistent with the involvement of external water. Such 

ash aggregates form commonly (but not exclusively) during 

phreatomagmatic eruptions, or where there is abundant 

moisture in ash plumes such as is generated by surface 

evaporation where a hot density current flows across 

standing water. The parallel bedding and lamination 

suggests unsteady, pulsatory fallout, generated either at 

the vent or by convective instabilities within an umbrella 

cloud (e.g. Carey et al. 1988; Branney 1991). Pulsatory 

explosivity is also a characteristic of some phreatomagmatic 

eruptions, and can arise from intermittent access of 

water to the erupting magma. Some of the ash-pelletbearing 

ash deposits are very fine grained and have a 

distinctly phreatomagmatic character (e.g. units beneath the 

Tuff of Wooden Shoe Butte, Cassia Mountains). 

However, the granulometry of many of the stratified ash 

layers in the Snake River Plain is not typical of phreatomagmatic 

ash. The ash layers are relatively well-sorted and 

medium to coarse-grained, whereas phreatoplinian ashes 

(e.g. Hatepe and Rotongiao ashes of New Zealand; Walker 

1981) tend to be finer grained and less well-sorted. The 

cuspate shape of the shards indicates that the magma was 

coarsely vesicular at the time of fragmentation, and so the 

explosive fragmentation was probably driven primarily by 

volatile exsolution. Given that the absence of pumice 

blocks and lapilli characterises the ignimbrites as well as 

the fallout deposits, reconciling the involvement of large 

volumes of meteoric water with the very high welding 

intensity of the ignimbrites presents something of a 

paradox. However, intensely welded pyroclastic rocks do 

occur in association with non-welded, ash-aggregate bearing 

deposits at some flooded caldera volcanoes, like Taal in 

the Philippines (Torres et al. 1995) and Scafell in England 

(Branney and Kokelaar 1995). 

The abundant obsidian and vitrophyre fragments in the 

ignimbrites and ashfall deposits may have been entrained at 

walls of eruption conduits cutting dense proximal obsidian 

and vitrophyre. The absence of other lithic lithologies (e.g. 

basalts, basement) suggests that the explosivity occurred at 

shallow-levels in eruptive centres that at the time were 

characterised by thick accumulations of glassy rhyolitic 

products, possibly in a lacustrine setting. Incorporation and 

fragmentation of these lithologies may have been facilitated 

by groundwater within fractures in the glassy rocks flashing 

to steam. Similar angular glass chips are reported from 

Plinian (Brown and Branney 2004) and phreatoplinian 

(Smith and Houghton 1995) fallout deposits elsewhere, and 

in the latter case a shallow obsidian source at the floor of a 

caldera lake was invoked. 

A feature of the SR-type rhyolite lavas and ignimbrites 

of the central part of the Snake River Plain is that they have 

remarkably low δ 18 O values (−1.4 to 3.8‰; Boroughs et al. 

2005) compared to non-SR-type rhyolites elsewhere in the 

province. The low values have been ascribed to large-scale, 

shallow melting of Idaho batholith basement that had been 

extensively hydrothermally altered during the Eocene 

(Boroughs et al. 2005). Intriguingly, however, the low 

δ 18 O values coincide with the SR-type volcanism, with its 

association with former lakes: further exploration of the 

possible role of sub-lacustrine groundwater or hydrothermal 

waters in fractures associated with rifting and/or caldera 

formation might be illuminating. 

The SR-type rhyolite lavas were emplaced onto low 

slopes. Emplacement predominantly by ductile effusion


rather than brittle and semi-brittle extrusion is indicated by 

the complex refolded folds and the large ratio of nonbrecciated 

to brecciated lava. Their large sizes and runnout 

distances, and low aspect-ratios (Fig. 2) indicate that the 

lava flows were erupted at far higher mass-flux rates, and 

with significantly lower viscosities than is typical for 

rhyolite spines, domes or coulées. As with other examples 

(e.g. Bracks Rhyolite of Trans-Pecos Texas; Henry et al. 

1990) the aspect ratios of the silicic lavas are consistent 

with estimated high magmatic temperatures and anhydrous 

compositions. However, it is not clear what drove such 

large volumes of magma so rapidly to the surface. 

Caldera subsidence in the Snake River plain 

Explosive eruptions larger than a few km 3 generally 

produce calderas, and it is likely that the larger SR-type 

eruptions were caldera-forming. Silicic calderas are best 

identified by thick intra-caldera ignimbrite together with 

caldera-collapse breccias, and overlying caldera lake sediments, 

rhyolite domes, and associated shallow intrusions, 

hydrothermal alteration, and contemporary faulting (e.g. 

Lipman 1984; Branney 1995). Where these are not 

exposed, the presence of a caldera may be inferred from 

an extant topographic rim, together with proximally-coarsening 

lithic breccias in outflow ignimbrites (e.g. Druitt and 

Sparks 1982) and thick, coarse pumice fall layers. Most of 

these features have not been recorded in the central Snake 

River Plain, so the location and size of any calderas in the 

region is tentative. For these reasons, Bonnichsen (1982) 

inferred the presence of broad ‘eruptive centres’ (Fig. 1) that 

were considered to have been discrete topographic depressions 

1,000s km 2 containing lakes and extensive rhyolite 

lavas, prior to burial by younger basalt lavas. 

Source locations of most of the ignimbrites are imprecisely 

known and cannot be inferred from thickness 

variations of outflow sheets. Spatial distributions of 

azimuth orientations of rheomorphic lineations, folds and 

kinematic indicators can help locate an eruptive source 

(Branney et al. 2004). For example, lineations in the upper 

tuff of McMullen Creek, Cassia Hills (Fig. 1) form a fanshaped 

pattern that may indicate northerly source (McCurry 

et al. 1996). However, rheomorphic transport directions are 

likely to be influenced by local topographic slopes rather 

than an overall transport direction from source (e.g. lower 

units of the tuffs of McMullen Creek; McCurry et al. 1996). 

It has been proposed that local successions of outflow 

ignimbrites derive from a common eruptive centre, for 

example that all nine Cougar Point Tuffs and associated 

lavas derive from the Bruneau-Jarbidge eruptive centre, and 

that the ignimbrites of the Cassia Mountains derive from 

the inferred Twin Falls eruptive centre (Fig. 1). In this 

scenario, a small number of distinct and spaced eruptive 

centres (Fig. 1) each underwent numerous large eruptions. 

It is equally possible, however, that a larger number of 

subsidence structures formed, each accompanied by emplacement 

of just one large ignimbrite. In this scenario, the 

large number of ignimbrites would suggest that the central 

and eastern Snake River Plain is a complex of numerous 

partly overlapping calderas, rather like the Olympic rings, 

in which the locus of subsidence migrated overall northeastwards. 

This is consistent with our understanding of the 

younger calderas around Yellowstone, which are shingled, 

and where each caldera subsided during a single ignimbrite 

eruption (Morgan and McIntosh 2005). The diameters of 

some of these calderas (Blacktail, Kilgore, Huckleberry 

Ridge, and Yellowstone) are thought to be similar to the 

width of the Snake River Plain. In detail, the temporal trend 

is likely to have been rather more complex than a simple 

north-eastward migration (e.g. a large ignimbrite flare-up 

occurred 11.7–10.0 Ma from a broad region of the central 

Snake River Plain, and temporal overlap of eruptions in 

widely separate areas occurred during the subsequent ∼4 

million years; Bonnichsen et al. 2007). 

The low topography of the Snake River Plain relative to 

the adjacent massifs is associated with marked downwarping 

of ignimbrites and basement towards the axis of 

the plain. This structure has been interpreted variously as 

due to SW–NE extension, thermal contraction following the 

volcanism, or loading by large mid-crustal mafic intrusions 

(discussion in Rogers et al. 2002). The structurally-defined 

subsidence exceeds 4.5 km and, in some locations, 8.5 km 

(Rogers et al. 2002). We propose that this includes a 

component of caldera subsidence; the central and eastern 

Snake River Plain may thus be regarded as a volcanotectonic 

depression formed by successive, partly overlapping 

caldera subsidence events in addition to later 

thermal adjustments and flexural loading by a mid-crustal 

mafic intrusion (Rogers et al. 2002). The inward dip of the 

basement and ignimbrite sheets towards the axis of the 

plain may include a component of caldera-related downwarping 

(‘downsag’ of Walker 1984; Branney 1995) 

peripheral strata that steepen with proximity to the caldera 

margins is a common feature of caldera subsidence 

(Branney and Kokelaar 1992; Branney 1995; Roche et al. 

2000; Kokelaar and Moore 2006), e.g. the inward tilting 

around the 18 km diameter Namibian Messum Complex 

and Goboboseb mountains, interpreted as due to caldera 

subsidence associated with very large-volume silicic eruptions 

(Ewart et al. 1998). 

Given the thickness of the outflow ignimbrite successions, 

we expect proximal ignimbrite accumulations >2 km 

thick under the axis of the central Snake River Plain. These 

thicknesses are not known (base not seen) but in the eastern 

Snake River Plain seismic refraction data suggest rhyolitic 

rocks are c. 2.5 km thick (Sparlin et al. 1982) and borehole


INEL-1 penetrated 2.39 km of rhyolitic ignimbrite without 

reaching its base; this may represent a caldera fill (Doherty 

et al. 1979). 

Other possible occurrences of SR-type silicic volcanism 

Features of SR-type volcanism also occur elsewhere in the 

Proterozoic and Phanerozoic record. Most, though not all, 

examples involve large-volume eruptions of intracontinental, 

high-temperature and relatively anhydrous, potassic and 

high-Fe, metaluminous rhyolites and dacites (or latites, e.g. 

Marsh et al. 2001) in a bimodal association with voluminous 

basaltic magmatism. In many cases, the physical 

volcanology of the deposits, particularly of the non-welded 

tephras, is poorly understood. 

Elsewhere along the Yellowstone hot-spot track 

Styles of volcanism vary along the Yellowstone hot-spot 

track. The variation reflects temporal changes in both 

chemistry and magmatic temperatures of the magmas 

generated as the hot-spot encountered differences within 

the overriding continental lithosphere (Perkins and Nash 

2002). Amongst the resultant great diversity of facies 

within the province, some successions outside the area 

described in this paper (Fig. 1) also exhibit SR-type 

characteristics. In the eastern Snake River Plain, several 

rhyolitic ignimbrites are high to extremely high-grade, 

rheomorphic, locally lava-like, and associated with parallel-stratified 

ashes and aqueously deposited volcaniclastic 

sands silts (e.g. Walcott Tuff, Blacktail Creek Tuff, Tuff of 

Kilgore, and associated units; Morgan and McIntosh 

2005). Other ignimbrites, however, are more transitional 

in character and exhibit some but not all of the 

characteristic features of SR-type volcanism (e.g. the of 

Wolverine Creek; Morgan and McIntosh 2005). Wellknown 

ignimbrites from the Yellowstone area have both 

welded and non-welded parts, eutaxitic fabrics, and restricted 

rheomorphic, lava-like facies. They contain abundant pumice 

clasts and were erupted at lower temperatures than typical SRtype 

ignimbrites. Some Quaternary rhyolites in the volcanic 

province (Magic Reservoir centre, Leeman 2004; Big Butte, 

Cedar Butte and related rhyolite domes further east, 

McCurry et al. 1999) contrast with the Miocene volcanic 

rocks described in this paper, and are not of SR-type. The 

13.8–12 Ma Owyhee-Humboldt eruptive centre (Fig. 1) 

produced widely dispersed ash, intensely rheomorphic 

ignimbrites some with lava-like facies, sheet-like rhyolite 

lavas, rhyolite lavas inferred to have been fountain-fed 

(clastogenic) and ascribed to eruptions of low-viscosity, hot 

and relatively anhydrous rhyolite magma (Ekren et al. 1982; 

Bonnichsen and Kauffman 1987; Manley1995; Manleyand 

McIntosh 2002). 

Eocene–Oligocene rhyolites of Trans-Pecos Texas 

Associations of extensive, low-aspect ratio silicic lavas with 

extremely high-grade rheomorphic ignimbrites, including 

some lava-like units of equivocal origin occur in Trans- 

Pecos Texas (Henry et al. 1988, 1989; HenryandWolff 

1992) and have characteristics of SR-type volcanism. The 

38 Ma–32 Ma mafic to silicic volcanic rocks are related to 

subduction beneath the North American continent, and 

include caldera-related, large volume, predominantly 

alkalic low-silica rhyolites. Intense welding and rheomorphism 

of ignimbrites (e.g. Buckshot ignimbrite, 

Gomez Tuff and in the Barrel Springs and Sleeping Lion 

formations, Henry et al. 1989), and the low aspect-ratios 

of lavas (Fig. 2), such as the 25–120 m thick Bracks 

Rhyolite, which covers 1,000 km 2 and flowed ≤35 km 

from source, have been ascribed to eruptive temperatures 

≥900°C (Henry et al. 1990). Other ignimbrites exhibit 

fiamme and lithic clasts, and are less typically SR-type. 

Cretaceous Etendeka-Paraná volcanic province, Namibia, 

Angola and South America 

Vast silicic sheets of the early Cretaceous Etendeka 

volcanic provinces of Namibia and Angola, and the 

southern Paraná of South America (Marsh et al. 2001; 

Garland et al. 1995; Milner et al. 1995; Kirstein et al. 2001) 

have features of SR-type volcanism. Individual sheets in 

Namibia are 70–250 m thick, exceed 25,000 km 2 (Milner et 

al. 1995; Jerram 2002), and have estimated magmatic 

temperatures exceeding 1,000°C. They have been interpreted 

as predominantly lava-like ignimbrites, although few 

unequivocal pyroclastic features are preserved and some 

have basal breccias (Milner et al. 1992; Ewart et al. 1998). 

Some units contain non-flattened globules, some larger than 

those in the Snake River ignimbrites. Little has been 

published on the fallout tephras, and few of the eruptive 

centres are well exposed, with the exception of the Messum 

Complex (Ewart et al. 1998). 

Middle Proterozoic Keweenawan volcanics, Minnesota 

Low-aspect ratio rhyolite lavas and extremely high-grade 

rheomorphic ignimbrites of SR-type have been described in 

the Middle Proterozoic Keweenawan plateau volcanics of 

Minnesota (Green 1989; Green and Fitz 1993). They reach 

250 m in thickness and trace for 40 km, with volumes of 

100–400 km 3 . Some are lava-like and their pyroclastic 

versus effusive origin is equivocal. Less is known about the 

non-welded pyroclastic units, although one 20 cm-thick 

layer of pumice lapilli is reported beneath a lava. Former 

surface water is indicated by thin units of fluvio-deltaic 

sediments and basaltic pillow lavas. Temperatures of c.


900°C have been invoked for the rhyolites, which are 

thought to have derived from crustal melting during 

continental rifting (Vervoort and Green 1997). 

Jurassic Karoo volcanic province, southern Africa 

Rhyolite sheets with some characteristics of SR-type 

volcanism occur within the Lebombo monocline of southern 

Africa. The rhyolitic eruptions were large, with a 

combined volume of 35,000 km 3 (Cleverly et al. 1984). 

Individual rhyolite sheets are ≤60 km, ≤200 m thick, and have 

been interpreted as extremely high-grade, largely lava-like 

rheomorphic ignimbrites (Cleverly 1979). They have low 

δ 18 O values (c. 5.6 ‰) ascribed to deep penetration and 

extensive circulation of meteoric water facilitated by brittle 

fracturing accompanying continental rifting (Harris and 

Erlank 1992). 

Middle Proterozoic Yardea Dacite, south Australia 

The Yardea Dacite (67% SiO2) of south Australia is as much 

as 250 m thick and covers an area of 12,000 km 2 (Creaser 

and White 1991). It is not known whether or not it is a single 

eruptive unit, but it shows mostly lava-like characteristics 

and may be either rheomorphic ignimbrite or unusually 

extensive lava. It is thought to have been erupted from a hot 

(>1,000°C) felsic magma that was relatively poor in H2O. 

Unlike most SR-type rhyolites, however, it locally contains 

abundant lithic fragments, and the magmas are slightly more 

mafic and crystal rich. Little is published on the volcanology 

of associated facies (Creaser and White 1991). 

Ordovician Scafell caldera volcano England, UK 

Associations of intensely rheomorphic ignimbrites, parallelstratified 

ashes and lacustrine volcaniclastic sediments occur 

in Ordovician extensional continental arc successions of the 

English Lake District (Millward 2004; Branney 2006). Some 

of the rheomorphic ignimbrites, such as the Bad Step Tuff in 

Scafell caldera are of SR-type, with extensive lava-like 

facies, rheomorphic folding, sheeting joints, zones of 

lithophysae zones, and upper autobreccias (Branney et al. 

1992). Intercalated thin layers of stratified tuffs lack pumice 

lapilli. Some have ash pellets, trace for long distances, and 

resemble ashfall layers of the central Snake River Plain. 

Some are interpreted as recording very large-volume 

phreatoplinian ashfalls that blanketed a subaerial landscape 

and, proximally, fell into subsiding syn-eruptive lakes 

(Branney 1991). The rheomorphic ignimbrite eruptions are 

thought to have been characterized by high temperature, high 

mass-flux pyroclastic fountaining generating hot density 

currents, and the intercalated non-welded stratified units are 

thought to record intermittent explosive interaction with 

ground and lake water. Differences with SR-type volcanism 

include the presence of lava domes and coulees, an 

intracaldera setting, and ignimbrites that contain pumice 

and fiamme (Branney and Kokelaar 1995). 

Silurian Glencoe caldera volcano, Scotland, UK 

The Lower, Middle and Upper Etive Rhyolites of Glencoe 

caldera volcano, Scotland, are each 100–150 m thick, 

predominantly lava-like, intensely rheomorphic ignimbrites 

with contorted flow lamination and upper autobreccias, and 

they occur interstratified with fluvial and lacustrine sediments 

and thin layers of stratified rhyolitic ash containing 

ash aggregates (Kokelaar and Moore 2006). They are 

overlain by more conventional eutaxitic ignimbrites. They 

are high-K rhyolites with 74–77% silica and thought to 

represent crustal melts erupted in a regional volcanic flareup 

in a transtensional continental setting. 

Most examples of volcanic successions that exhibit SRtype 

features record large-volume, high mass-flux eruptions 

of hot, relatively anhydrous, high-K, metaluminous rhyolite, 

at continental large igneous provinces (LIPs). With 

some exceptions, relatively little is known about their 

physical volcanology; for example, the nature and dispersal 

characteristics of non-welded tephras—such studies have 

been hindered by the vast scale of the phenomena. Clearly, 

not all silicic volcanism at continental LIPs is of SR-type. 

For example, large-volume Oligocene silicic pyroclastic 

eruptions in Yemen–Ethiopia bimodal LIP produced low- to 

moderate-grade welded ignimbrites in which pumice lapilli 

and fiamme are abundant (e.g. Peate et al. 2005). 

Conclusion: a summary of the characteristic features 

of SR-type volcanism 

The characteristic features of SR-type volcanism as 

exhibited by the type example, the Miocene rocks of 

south-central Idaho and northernmost Nevada, are listed (1– 

18 and Appendix Table 1). (1) A bimodal association of 

basalts (‘basaltic plains-style’ volcanism) with H2O-poor, 

metaluminous rhyolites (70–77% SiO2) that have elevated 

contents of HFSE and halogens. (2) Large, regionally 

devastating explosive rhyolitic eruptions (VEI 6–8). (3) 

Voluminous, high to low aspect-ratio rhyolitic ignimbrites. 

(4) Numerous ashfall layers with parallel thin-bedding and 

lamination; extensive (to 1,000s of km) atmospheric 

dispersal of ash. (5) Unusually large-volume, extensive 

rhyolite lavas with low aspect-ratios. Domes and coulées 

are not characteristic. (6) A paucity of pumice lapilli in the 

ignimbrites (or, in welded ignimbrites, of fiamme after 

pumice lapilli) and an absence of pumice lenses and 

pumice-concentration zones. (7) The ignimbrites are gen-


erally lithic-poor and lack proximal lithic breccias. (8) 

Small angular glassy fragments are abundant both in many 

ashfall layers and ignimbrites. (9) The ignimbrites are true 

tuffs rather than lapilli-tuffs, and they are unusually well 

sorted (σ φ


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