Ash-Flow Tuffs: Their Origin 

Geologic Relations 

and Identification 

By CLARENCE S. ROSS and ROBERT L. SMITH 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 366 

A study of the emplacement, by flowage, of hot 

gas-etnitting volcanic ash; its induration 

by wetding and crystallization, and criteria 

for recognizing the resulting rock 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1961 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director 

/ 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington 25, D.C. 
Abstract --------------------------------------------------------------------------------

In trod ucti on --------------------------------------------------------------------------

Acknowledgments ------------------------------------------------------------------

Nomenclature ------------------------------------------------------------------------· 

Glossary of terms ________________________________ ______________________________ ___ _ 

Development of concepts ____ __________________ _______________________________ _ 

Historic eruptions of ash-flow tuffs ___ ________ _________________ _____ _ 

Pelee -------------------------------------------------------------------------------- Valley of Ten Thousand Smokes ____ ____ __________________ ___ __ _ 

Cotopaxi -------------------------------------------------------------------------

Source vents for ash-flow tuffs _________________________________________ _ 

Recognition of ash-flow tuffs ________________ __________________ __ ________ _ _ 

Field characteristics __ ____ _________ ______ ________________________________ _ 

Pyroclastic character--------------------------------------------

~~f:::e:sf --~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Layering ---------------------------------------------------------------- Areal extent ___________________________________________________________ _ 

Gen tie dips ___________________________________________________________ _ _ 

Welding and deformation of pumice _____ ____________ _ 

Fused tuffs contrasted with w~lded tuffs ______ _ _ 

Devitrification and vapor-phase minerals _______ _ 

Jointing ------------------------------------------------------------------ Erosion forms ______________________________________________ _________ _ 

Fossil fumaroles _______________________________________________ _ 

CONTENTS 

Pag~ 

1 

1 

2 

2 

8 

15 

15 

16 

16 

16 

18 

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18 

19 

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20 

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23 

24 

26 

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28 

29 

30 

Recognition of ash-flow tuffs-Continued 

Microscopic characteristics __________ _______________________________ _ 

Pyroclastic character-------------------------------------------- Pumice fragments _________________________________________________ _ 

Welding, distortion, and stretching ____ _____________ _ 

Phenocrysts and foreign materials ___________________ _ 

Devitrifica tion -----------------------------------------------------

Physical chemistry ----------------------------------------------------------------

Heat --------------------------------------------------------------------------------

Viscosity ______ ----------------------------------------------------------------- Vola tile components ________ ____ _________________________________________ _ 

Welding --------------------------------------------------------------------------

D evi trifica tion --------------------------------------------------------------

Physical properties ______________________________________________________________ _ 

Indices of refraction ___________________________________________________ _ 

Specific gravity --------------------------------------------------------------

Porosity --------------------------------------------------------------------------

Ash-flow materials of intermediate composition ___________ _ 

Andesitic ash flows of Costa Rica ______________________ _____ _ 

Distribution in time _____________________________________________________________ _ 

Mesozoic __________________________________ ----------------------------------------

paleozoic ------------------------------------------------------------------------

Precambrian ------------------------------------------------------------------

Selected references ___________________ ____________________________________________ _ 

Index --------------------------------------------------------------------------

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ILLUSTRATIONS 

Page 

FRONTISPIECE. Oxidation and reduction in glassy welded tuffs. 

FIGURE 1. Welded tuff from Battleship Rock, Jemez Springs quadrangle, New Mexico______________ __ ______ ____________ _________ _______ 4 

2. A "small typical nuee ardente" of Feb. 8, 1930, at Pelee ------------------------------------------------------------------------------------ 6 

3. Battleship Rock, Valles Mountains, N.Mex., showing columnar structure characteristic of many ashfl. ow tuffs ____ ______ _____ _ ___ _________ __________________ _______________ ____ ___ ___ ___ __ ____ __________ ______________________ ___________ _______ ______ ______ ______________________ __ 19 

4. Tuff section on the eastern wall of Colle Caiion, near its junction with Peralta Canyon, Valles Mountains, N. M ex -------------------------------------------------------------------------------------------------------------------------------------------------------------- 19 

5. Welded-tuff scarp in Caiion Media Dia, Valles Mountains, N.Mex., showing columnar joining in thoroughly wei ded tuffs ---------------------------------------------------------------------------------------------------------------------------------------------------- 2 0 

6. Ash-flow tuff scarp below the Puye Pueblo ruins, Valles Mountains, N.Mex ---------------------------------------------------- 20 

7. Ash-flow tuff scarp of Capulin Canyon, Valles Mountains, N.Mex ------------------------------------------------------------------------ 23 

8. Ash-flow tuff scarp, Valles Mountains, N.Mex., showing level surface of an ash flow and even bedding 

in a series of ash flows _____________ ___ __ ______________________ __________ __________________________________ ______________________ __________________________________ __ 24 

9. Hand specimen of welded tuff with conchoidal fracture, superficially resembling a typical obsidian ___ 25 

10. Photomicrograph of thin section of same obsidianlike welded tuff of figure 9 showing very complete welding --------------------------------------------------------------------------------------------------------------------------,----------------------------------------------------- 25 

11. Ash-flow tuff from 4 miles north of Thumb Ranger Station, Yellowstone National Park.___________________ __ __________ 27 

12. Hand specimen collected from a 10- to 20-meter thick layer of glassy welded tuff, Guerrero, Mexico _______ 29 

13. Reverse side of the specimen shown in figu're 12, showing pyroclastic (eutaxitic) structure ______________________ __ 29 

14. Scarp in Wildcat Canyon, Valles Mountains, N.Mex., representing slightly welded to nonwelded part of 

a single ash flow______________________ __________________ ____________________________________________________ __ _____________________ _______________________________________ 30 

15. Porosity of Battleship Rock ash-flow tuff, Valles Mountains, N.Mex ------------------------------------------------------------------ 47 

16. Photomicrograph of glassy ash-flow tuff from South Sumatra____ ________ ________________________________________________________ ______ 56 

17. Photomicrograph of ash-flow tuff from the Ammon quadrangle, southeastern Idaho ------------------------------------- 56 

18. Photomicrograph of glassy ash-flow tuff from the Ammon quadrangle, southeastern Idaho, showing slight 

warping of the shards----------------------------------------------------------------------------------------------------------------------------------------------- 56 

19. Photomicrograph of ash-flow tuff from Iceland, showing no distortion of the structures ----------------------------- 56 

20. Photomicrograph of a sh-flow tuff showing alinement of shards due to orientation during emplacement by 

flowage ------------------------------------------------------------------------------------------------------------------------------------------------------------------------ 57 

21. Photomicrograph of fragment of an older welded tuff enclosed ~ a younger tuff materiaL__________________________ 57 

22. Photomicrograph of glassy welded tuff from South Sumatra showing sharply angular shard fragments _ 57 

23. Photomicrograph of glassy ash-flow tuff from Barley Canyon, Valles Mountains, N.Mex., showing only 

slight welding____ __________________ __________________________________________________________________ __ __ ____ ______________ ___ _____________________________________________ 57 

24. Photomicrograph of glassy ash-flow tuff from Arequipa region, Peru. showing; wide range in the size 

of the shards ______________________________________________________________________________________________________________ __ ________________ __________________________ ______ 58 

25. Photomicrograph of glassy welded tuff from Arequipa region, Peru, showing lack of characteristic shard 

forms due to disruption during flowage or explosive violence____ ____ __ ____________ __________________ ____ ______________ __________ 58 

26. Photomicrograph of glassy welded tuff from Lake Toba region, North Sumatra, with compressed shards 

molded against the phenocrysts------------------------------------------------------------------------------------------------------------------------------ 58 

27. Photomicrograph of glassy ash-flow tuff from Lake Toba region, North Sumatra, with only slight 

welding and no distortion of the shards______________________________ ___________ _______ __________________ _____________ _________________________ ____ __ 58 

28. Photomicrograph of welded tuff from Saguache County, Colo., showing incipient devitrification ____________ _ 59 

29. Photomicrograph of welded tuff of the Rattlesnake formation from central Oregon, showing only slight 

distortion, and partial d evi trifica ti on -------------------------------------------------------------------------------------------------------------------- 59 

30. Photomicrograph of obsidianlike welded tuff from the Valles Mountains, N.Mex., showing vague retention 

of tuff structure_______ _________________________________________________________ ____ ___________ ___________________ ____________ __ ____________________________________ ______ 60 

31. Photomicrograph of glassy welded tuff from the Valles Mountains, N.Mex.~ showing evidence of plasticity of the glass during welding and extreme distortion________________ ________________________________________________________ 60 
CONTENTS 

32. Photomicrograph of welded tuff from the Iron Springs area, Utah, showing slightly impaired shard 

structure due to thorough welding and strong compression followed by devitrification ___________________________ _ 

33. Photomicrograph of welded tuff from the Iron Springs area, Utah, which has remained glassy following distortion-----------------------------------------------------------------------------------------------------------"----------------------------------------------------

34. Photomicrograph of fine-grained glassy welded tuff showing extreme compression and molding against a 

phenocryst, simulating flow structure of an extrusive r hyolite --------------------------------------------------------------------

35. Photomicrograph of glassy welded tuff from southeastern Idaho, with compressed shards but good retention of the structure ________________________________________ ------------------------------------------------------------------------------------------------------

36. Photomicrograph of glassy welded t uff showing marked simulation of flow structure due to molding of 

the shards against the phenocrysts ____ _____________________________________________________ __ ____________________________________ __________________ _____ _ 

37. Photomicrograph of glassy flow-banded rock from an intrusive dike, Lake County, Colo ____ ____________________ ___ _ 

38. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing well-developed parallel ar- rangement of the shards ___________________________________________ ___________ __________________ ______ ____ ___________________________ __________________________ ___ __ _ 

39. Photomicrograph of glassy welded tuff from Lake Toba, northern Sumatra, showing collapse of pumice 

fragments with a retention of pumice structure --------------------------------------------------------------------------------------------

40. Photomicrograph of devitrified welded tuff from Iron Springs, Utah, that has undergone very marked 

compression, but retains recognizable shard structure __ ____________________ ____________________ _________________________________________ _ 

41. Photomicrograph of glassy welded tuff from same unit as specimen shown in figure 40, showing elimination, by stretching, of all r ecognizable shard structure ____ __ ________________________________________________ _________________ _ 

42. Hand specimen f r om Battleship Rock, N.Mex., shown in figure 3, showing nonwelded, uncollapsed pumice 

fragments that stand in relief on t he weathered surface ------------------------------------------------------------------------------

43. Photomicrograph of thin section from specimen shown in figure 42, showing several noncollapsed glassy 

pumice fragments ___ ______________________________________________________________________________________________ ____________________________________ _______________ ____ _ 

44. Photomicrograph of glassy welded t uff from the Western United States, showing thoroughly welded 

shard structure and complete collapse of pumice --------------------------------------------------------------------------------------------

45. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing complete collapse of pumice, but retention of pumice structure -----------------------------------------------------------------------------------------------------------------

46. Photomicr ograph of glassy welded tuff from Kuinimi, Kaga, J apan, showing collapse of pumice structure 

and development of per li tic fractures ----------------------------------- --------------------------------------------------------------------------------

47. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing distortion and molding of 

p la sti c p urn ice f ragmen ts ·----------.. ------------------------------------------------------------------------------------------------------------------------------

48. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing great distortion and molding of pumice against phenocrysts.------------------------------------------------------------------------------------------------------------------------

49. Hand specimen from the Ammon quadrangle, southeastern Idaho, showing lithophysal cavities that developed after complete welding and distortion of the shard structure ____ ________________________ __ ___________________ __ _ 

50. Photomicrograph of thin section from specimen shown in figure 49, showing distortion of tuff structure ___ __ _ 

51. Photomicrograph of thin section from specimen taken from same horizon as that shown in figure 49, 

but collected 8 or 10 miles farther southeast __________________________________________ __________________ _________ _______ __ ______ _________________ _ 

52. Hand specimen of welded tuff from southeastern Idaho showing vesicular cavities which developed after 

deposition and complete welding of the tuff----------------------------------------------------------------------------------------------------------

53. Photomicrograph of thin section from specimen shown in figure 52, showing a spherulite that has grown 

out from a plagioclase phenocryst acting as a nucleus ________ ___________ ______________________________________________________________ _ 

54. Photomicrograph of part of a very large spherulite developed in welded tuff showing three concentric 

zones -----------------------------------------------.. ------------------------------------------------------------------------------------------------------------------------------

55, Photomicrograph of specimen from the Jemez Springs quadrangle, Valles Mountains, N.Mex., showing 

complete destruction of shard structure by crystallization __________ ________________________ ____________________________________ ___ _ 

56. Same area shown in figure 55 but under crossed nicols, illustrating coarse-grained intergrowth of feldspar and cristobalite ____ ____ __ ____ __ __ _ .. ______________ __________ ________ ____________________________ ____________ ______ ____ _______________________ _________________ .. __ _ 

57. Photomicrograph of specimen from the Valles Mountains, N.Mex., showing devitrification of pumice and 

destruction of structure ____________ _ ,________________________________ ____________ ____________________________ __________________ ____ __ ____________ ______ ____ _______ _ 

58. Same area shown in figure 57, illustrating tendency for coarse-grained devitrification products to develop in the pumice f ragmen ts ____ ______________________ ------------------------------------------------------------------------------------------------------

59. Photomicrograph of welded tuff that has undergone welding, extreme distortion, and devitrificat1on of 

the shard structures ____ ________________________________________ ____ ___ ______________ __________________________ ____ ____ ___ _________________________________________ _ _ 

60. Same area as illustrated in figure 59, showing a coarse-grained radial aggregate of feldspar and cristobalite laths developed independently of the original shard structure ---------------------------------------------· ----------

61. Photomicrograph of welded tuff from southeastern Idaho, showing devitrified shards and partial destruction of pumice structure ___ ___________________________________________ ____________ _____________________________________________ ___ _____________________ __ _ 

62. Photomicrograph of welded tuff from the Jemez Springs quadrangle, Valles Mountains, N.Mex., showing 

complete destruction of original structure by development of coarse-grained devitrification products. ___ _ 

63. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing a very large spherulite in 

which radial aggregates of feldspar and cristobalite are well represented ____ ___ ___ _______ ____ __ ___________ ___ _ 

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VI CONTENTS 

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64. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing the development of groups 

of s ph eru 1 i te s ______________________ ___ __ ________________________ ________________________________ _______ ________ ------------------------------------------____________ ______ 6 9 

65. Photomicrograph of welded tuff from the Lake Toba region, Sumatra, showing spherulitic areas____________ 69 

66. Photomicrograph of welded tuff from the Valles Mountains, N.Mex., showing an open cavity in which 

platelike crystals of tridymite have developed____ ________________________________________ __________________________________ ______ _________________ 69 

67. Polished section through a so-called thunder egg, a type of hollow spherulite which commonly develops in 

glassy volcanic rocks ____ ____ __ ___________________ c_________ _________ ___ ___ ______ ___ __________________________________________________________________________________ 70 

68. Photomicrograph of the outer rim material of figure 67, showing section perpendicular to the plane of 

deposition -------------------------------------------------------------------------------------------------------------------------------------------------------------------- 70 

69. Photomicrograph of the outer rim material of figure 67, showing section parallel to the plane of deposition ------------------------------------------------------------------------------------------------------------------------------------------------------------------·-------------- 70 

70. Photomicrograph of welded tuff from southeastern Idaho, showing parallel structure produced by molding of shards against each other--------------------------------------------------------------------------------------------------------------------------- 7 0 

71. Photomicrograph of tuff from the Valles Mountains, N.Mex., showing welding and devitrification, but 

without distortion of the shard structure ___ __ _______________________________________________________________ __________ __ ______________________________ 71 

72. Photomicrograph of specimen shown in figure 71, under higher magnification, illustrating axiolitic structure -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 71 

73. Photomicrograph of devitrified welded tuff from the Valles Mountains, N.Mex., showing molding of the 

shards· around a feldspar phenocryst __________ _______________________________ ________________________________ ________________________________ _____ ___ ___ 71 

74. Photom~rograph of devitrified welded tuff from the Western United States showing uncommonly perfect axiolitic structure ·-------- -------------------------------------------------·------------------------------------------------------------------------------------ 71 

75. Photomicrograph of welded tuff from Australia, showing partial loss of structure by devitrification_______ _ 72 

76. Photomicrograph of devitrified welded tuff from Hungary, showing obscured shard structures________________ 72 

77. Photomicrograph of devitrified welded ,tuff from Korea, showing dim retention of shard structure___ 72 

78. Photomicrograph of welded tuff from Sweden, showing local preservation of shard structure .___________________ 72 

79. Photomicrograph of a welded tuff of intermediate composition from Russian Armenia, showing slight 

welding but distortion of pumice _________ ____________________________ _____________________________________________________________________________________ 73 

80. Photomicrograph of a welded tuff of intermediate composition from Russian Armenia, showing moderate welding and warping of shards ____________________ ------------------------------------------------------------------------------------------------------ 7 3 

81. Photomicrograph of andesitic welded tuff from Japan, showing lineation produced by thorough compaction ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 73 

82. Photomicrograph of welded tuff from Calaveras County, Calif., illustrating the type biotite-augite latite 

described by Ransome________________________________________________________________________________________________________________________________________________ 73 

83. Photomicrograph of welded tuff from Argentina described by Bonorino as andesite, showing section 

parallel to the plane of deposition_____ ______________________________ ________________________________________________ ______________________________________ 7 4 

84. Photomicrograph of same specimen shown in figure 83, showing section perpendicular to the plane of 

deposition __ _____ _______ ____ __________________________ ----------------------------------------------------------------------------------------------------------------------- 7 4 

85. Hand specimen of andesitic welded tuff from Costa Rica------------------------------------------------------------------------------------- 74 

86. Photomicrograph of section from specimen shown in figure 85, showing destruction of pumice structure 

by welding -------------------------------------------------------------------------------------------------------------------------------------------------------------------- 7 4 

87. Photographic reproduction of a colored drawing by Iddings_______________________________________________________________________________ 75 

88. Photomicrograph of one of Iddings' thin sections of rocks from Yellowstone National Park__________________________ 75 

89. Copy of a photomicrograph of Iddings' "welded pumice with axiolites"------------------------------------------------------------ 75 

90. Photomicrograph of specimen from the Valles Mountains, N.Mex., studied by Iddings_____________________________ 75 

91. Photomicrograph of welded tuff from Yellowstone National Park studied by Iddings, showing typical 

large inclusion of andesite __________ c----------------------------------------------------------------------------------------------------------------------------- 76 

92. Photomicrograph of welded tuff from Yellowstone National Park, studied by Iddings, showing incipient 

d evi trifica tion and thorough wei ding--------------------------------------------------------------------------------------------------------------------- 7 6 

93. Photomicrograph of glassy welded tuff from Wadsworth, Nev., studied by Zirkel, showing section diagonal to the plane of deposition __________________________ ,_________________________________________________________ __________________________________________ __ 76 

94. Photomicrograph of welded tuff from West Humbolt, Nev., studied by Zirkel, showing extreme flattening and stretching of shard structures------------------------------------------------------------------------------------------------------------------ 7 6 

95. Photographic reproduction of a drawing presented by Zirkel, showing axiolitic structure__________________________ 77 

96. Photomicrograph of ash-flow tuff from the Valley of Ten Thousand Smokes, showing little induration___ 77 

97. Photomicrograph of ·welded tuff from New Zealand, representing materials which were the basis for 

Marshall's outstanding contribution to an understanding of ash-flow tuffs___ _________________________________________ 77 

98. Photomicrograph of welded tuff from Morocco, described by Bouladon and J ouravsky________________________________ 77 
ASH-FLOW TUFFS: THEIR ORIGIN, GEOLOGIC RELATIONS~ AND IDENTIFICATION 

By CLARENCE S. ROSS and ROBERT L. SMITH 

ABSTRACT 

Pyroclastic materials, which are interpreted as having 

been deposited by flowage as a suspension of ash in volcanic 

gas, are becoming widely recognized as major geologic episodes. These may be unconsolidated, indurated by partial 

welding, or welded into a compact rock. Many students are 

working on these materials and the interest in them is so 

widespread that need for a coordinated treatise on them has 

developed. This report deals with the history of the concept 

of their origin; gives detailed descriptions of their character 

and mode of occurrence; gives criteria for their recognition; 

and considers their distribution and consolidation. 

The terminology to describe ash flows and the reasons for 

avoiding the invention of new specific names, or the redefining of old ones, are stated. The terms used are given 

and defined, so far as possible, by quotations from authoritative sources. The use of descriptive phrases, where generally accepted specific names are unavailable, is the usage 

adopted. 

The evolution of geologic concepts which led eventually 

to an understanding of the distribution and, in some deposits, the welding of tuffs is traced. Prior to about 1900, 

geologists argued as to whether these materials represented 

peculiar volcanic lava flows, or were pyroclastic in origin. 

The development of a better understanding is traced through 

the discussion of geologists who reported on the great 

eruptions of Pelee, Soufriere, and the Valley of Ten Thousand Smokes, and by other geologists who studied analogous 

ancient eruptions. 

Recognition of ash-flow materials involves both field observations and laboratory studies. These are presented in 

some detail, and lead to the conclusion that the ash-flow 

materials may be recognized by either of these approaches, 

but for some occurrences both are required. The field relations that are presented for evaluation are: extent, thickness, 

the relation of overload to welding, jointing, erosional forms, 

inclusions of alien materials or pumice fragments, devitrification, and the relations to source centers. Experience with 

distinctive features commonly allows identification of ashflow deposits even at a distance of many miles. 

Laboratory studies that require description and evaluation include chemical analyses, the relation of composition 

to other features, porosity and its relation to overload and 

welding, and the temperature of welding. Microscopic 

studies reveal a large number of variables that demand consideration and discussion of their relative value in identification. These include a variety of primary glassy shard 

structures, the greatly varied modification of the shards by 

compression, welding, and devitrification. These factors may 

leave clearly significant evidence of origin, or this may be 

obscured in varying degree. Experience will allow identification even in the presence of severe modifications, but the 

development of uncommonly coarse devitrification products 

may destroy significant structures. The significant characteristics are discussed, and anomalous features which may 

lead to misinterpretation are discussed and evaluated by the 

use of photographs. 

The minerals in these tuffs comprise the primary minerals 

(phenocrysts), those resulting from devitrification after emplacement, and those formed in the presence of a vapor 

phase. The bearing of these on the preeruption, emplacement, and subsequent mineral development is traced. 

The wide distribution of ash flows, and the immense volume of the materials represented by some deposits pose 

many problems in volcanology. The mechanism of emplacement and physical chemistry of the deposits are considered. 

The source of heat required for welding has long posed a 

problem to geologists, but a summation of all the geologic 

and physical factors leads to the conclusion that no exothermic heat source is required. The inherent heat and its 

conservation by physical relations seems to provide ample 

heat for welding. The physical chemistry of the welding 

process is presented. 

Typical occurrences of ash-flow deposits, several representative hand specimens, and a wide range of welded and 

nonwelded tuffs as seen under the microscope, are illustrated, and discussed where clarification of their significance 

is required. 

INTRODUCTION 

The study of volcanism as an observable geologic 

process, and of volcanic rocks, readily available at 

the earth's surface, provides a direct approach to 

the understanding of igneous processes and rocks. 

Consequently, a great deal of geologic work has been 

concerned with volcanism and its products, but more 

emphasis has been given to the lavas or flow rocks 

than to the fragmental rocks. One reason for this 

is that the pyroclastic rocks are a transitional 

group, composed of lava fragments, but laid down 

generally like sediments. These rocks have many of 

the characteristics of sedimentary deposits. Silicic 

pyroclastic rocks are, in many regions of the world, 

more widespread than other types, therefore an 

understanding of their origin is of major importance. Among these, the group known variously as 

ash flows, welded tuffs, glowing avalanche deposits, 

incandescent tuff flows, ignimbrites, and others, has 

1 
2 ASH-FLOW TUFFS 

been the subject of intensive study by many geologists, and several excellent papers describing specific 

occurrences have been published. 

Throughout the literature, however, an ever-increasing diversity and duplication in terminology 

has been used to describe ash-flow materials, and 

to designate different origins, owing in part to the 

development of criteria for recognition, and in part 

to the evolution of ideas on their origin. For several years, the authors have been engaged in a detailed study of the Valles Mountains region of New 

Mexico where ash-flow deposits reaching a maximum thickness of nearly 1,000 feet cover more than 

350 square miles. The area is dissected by many 

canyons where pyroclastic rocks and sections are 

well exposed. The experience gained in this study, 

supplemented by observations in many other areas 

of the Western United States, especially the Basin 

and Range province, and in Mexico, New Zealand, 

and Iceland, has led the authors to believe that the 

time is long overdue for a consideration and evaluation of the nomenclature, characteristics, and origin 

of ash-flow tuffs. Many ash-flow tuffs have not 

been mapped as such in the past because they were 

not recognized, and many areas previously mapped 

as lava flows are now known to be ash-flow tuffs. 

Other areas will doubtless be found after more detailed laboratory study and areal mapping. The 

data acquired in these studies will have more meaning if time is taken now to unravel the threads of 

the problem, define the terms, sort out the ideas, and 

reach a state, however temporary, from which further progress can be made. 

This report is concerned with the results of observations of pyroclastic eruptions as reported in 

the literature by earlier writers. Geologic and 

petrologic studies of several great eruptions were 

made and correlated with physical and chemiGal 

data. The report also gives the nomenclature of 

pyroclastic materials, and describes the genetic history of ash-flow tuffs, that is, eruption, transportation, consolidation (including welding), and 

devitrification. Because the varied and distinguished characteristics can be more adequately and 

precisely described by illustrations than by words, 

the report contains definitive photographs of hand 

specimens and thin sections. 

ACKNOWLEDGMENTS 

Geologists who have observed and studied volcanism and volcanic rocks in many parts of the world 

have contributed generously during the preparation 

of this report, and this help, either as criticism, 

suggestions, advice, or specimens supplied, is here 

appreciatively acknowledged. Some contributions 

should be mentioned particularly, and first among 

these is the collection of 800 thin sections assembled 

by F. Zirkel for his petrographic studies that accompanied the geologic report on the exploration of 

the 40the parallel published in 1876. It is historically valuable as one of the earliest collections of 

volcanic rocks from the Western United States. 

The fine collections at the U.S. National Museum 

were available through the kindness of the late 

W. F. Foshag, curator of geology. 

For many areas throughout the world that are 

classic localities for pyroclastic volcanic rocks, the 

authors have had the benefit of personal discussions 

with geologists familiar with the areas, or of specimens, and personal thanks are extended especially 

to Ivan Wilson, for ash-flow tuffs from Baja California; J. Hoover Mackin, for specimens from the 

Iron Springs area of southwestern Utah; W. H. 

Bryan, for a collection of tuffs from the Brisbane 

area, Queensland, Australia; the late C. N. Fenner, 

a pioneer in the study of ash flows, for specimens 

from Peru; F. G. Bonorino, for andesitic welded 

tuffs from Argentina; J. Bouladon, for ignimbrites 

from Morocco; S. Hjelmqvist, for ignimbrites from 

central Sweden; C. G. Johnson, H. L. Foster, H. 

Kuno, and T. Matumoto, for specimens from Japan; 

J. Westerveld, for discussions of Sumatran welded 

tuffs, and the late C. P. A. Zeylmans van Emmichoven, for specimens from the Toba region of 

North Sumatra, which were collected in 1892 by 

N. Wing Easton, and for specimens from the West 

Pasoemah region, South Sumatra, collected in 1933 

by M usper and W esterveld. 

NOMENCLATURE 

Early in the authors' study of the Valles Mountains region, it was evident that the terminology of 

pyroclastic materials was inadequate. For some 

materials or processes, there was no term; for 

others, there were several. As the study and understanding of pyroclastic rocks had progressed 

over the years, new terms had been suggested, old 

terms had not been completely discarded, terms in 

use for certain ideas or substances had been extended in their meaning, and others were no longer 

applicable in their original definition. As a result, 

an extended examination of the terms reported in 

the literature has been made to face the problem of 

the confusion of terminology that is the natural 

result of work carried on in different parts of the 
GLOSSARY OF TERMS 3 

world on a subject that lends itself so easily to 

differences in description and interpretation. 

Two alternatives were open-either to bring together the terms in the literature, with their definitions, show how they had been used originally and 

evolved, and modify them where necessary by descriptive words and phrases-or to devise an entirely new systematic nomenclature which would 

require discarding some terms, redefining others, 

and coining new ones. No set of terms will encompass all the variables that are found in a highly 

detailed description of any group of phenomena. 

If specific names are proposed, there will be a need 

for even finer splitting, and the making of still more 

names. The latter course was therefore not desirable particularly for the reason that Wentworth 

and Williams (1932, p. 44) have put so succinctly, 

However, no great acquaintance with geologic literature is 

needed to be aware that only a small fraction of the terms 

which are proposed are destined to come into general use. 

The majority are only likely to add to the already burdensome list. Experience has shown that complete, systematic, 

novel series of terms to cover a particular field are rarely 

adopted. 

The use of modifying words and phrases, usually 

requiring only 1 or 2 words to describe a variation 

from the norm, offers a flexibility not possible with 

specific terms. Thus it was decided to use the existing nomenclature and modify it by descriptive adjectives and phrases, a course that has wide 

acceptance and that gives others the opportunity to 

adapt terms to their own problems and circumstances. However, no system of nomenclature, no 

matter how well devised, can obviate the need for 

detailed description wherever that will give a more 

accurate picture. 

Among the contributors to the development of the 

nomenclature of pyroclastic rocks are Blyth (1940), 

MacGregor (1955), Pirsson (1915), and Wentworth 

and Williams (1932). 

The following glossary of terms gives the derivation and current usage of the term together with the 

authors' comments on interpretation or suggested 

usage. 

GLOSSARY OF TERMS 

Ash fall.-Deposition of volcanic ash directly 

from the air, generally, but not always, resulting in 

a stratified deposit showing crude to very complete 

sorting of its component parts. Unconsolidated 

deposits are called ash; consolidated deposits are 

called tuff. 

The term "ash fall" has been used by Griggs 

(1922, p. 25), Fenner (1923, p. 26-28), Kozu (1934, 

552858- 61- 2 

p. 136), and MacGregor (1952, p. 70). Some writers use the term "airborne" to describe their material. 

Ash flow.-A turbulent mixture of gas and pyroclastic materials of high temperature, ejected explosively from a crater or fissure, that travels 

swiftly down the slopes of a volcano or along the 

ground surface. The solid material in an ash flow, 

although unsorted, is dominantly of particles of 

ash size (less than 4 mm in diameter) but generally 

contains different amounts of lapilli and blocks. 

Fisher (1954, p. 74), Taylor (1954, p. 86), and 

MacGregor (1955), among others, use the term 

"ash flow." 

The flowage principle is used in industry for the 

transportation of solids suspended in a gas, and is 

known as fluidization (Work and others, 1949). 

Reynolds (1954) has applied the concept to the dispersal of ash-flow materials. 

Ash-flow tu[f.-The consolidated deposits of volcanic ash resulting from an ash flow are called ashflow tuff. Ash flow is here used as an adjective to 

indicate the mechanism of dispersal, and tuff indicates the state and size of the material. Ash-flow 

tuff is an inclusive, general term for consolidated 

ash-flow beds that may or may not be either completely or partly welded. 

Ash, volcanic.-Wentworth and Williams (1932, 

p. 45) define ash as "uncemented pyroclastic material consisting of fragments mostly under 4 mm 

in diameter." Blyth (1940, p. 148) described ash 

(volcanic ash) as "unconsolidated pyroclastic material consisting mainly of fragments less than 4 

mm in size." He adds, "The use of the term 'ash' 

to connote a consolidated pyroclastic deposit is not 

to be recommended ; * * * the word tuff should be 

used in such cases." (See Pyroclastic materials.) 

Aso lava.-Indurated pyroclastic materials surrounding the Aso caldera of Kyushu, Japan. Matumoto (1943, p. 6) states, "The name Aso lava was 

originally given to the obsidian-like agglomeritic, 

eutaxitic, or welded mud lava * * *." 

Aso lava was originally described by Iki (1899). 

The origin of this and many other deposits of similar materials is now well known in Japan and most 

of these deposits are described under the terms 

"welded tuffs" and "pumice flows." 

Avalanche, volcanic.-The sudden avalanching of 

large amounts of any volcanic material down the 

slopes of a volcano. The term has been used by 

many authors, among them Lacroix (1904, p. 350) 

who described the material in the avalanche at 

Mount Pelee as being composed of earth and blocks, 
4 ASH-FLOW TUFFS 

and by Anderson and Flett (1903) who described an 

avalanche of dust, sand, stones, and burnt timber at 

the 1902 eruption of La Soufriere on St. Vincent. 

Some volcanic avalanches have been described as 

glowing avalanches (Williams and Meyer-Abich, 

1955, p. 33) where the material in them is red hot 

and contains gas. Other avalanches, that occur 

where large quantities of water are involved, are 

known as lahars. 

Axiolite, axiolitic.-Zirkel (1876, p. 173, 176) described "linear aggregates of axially grouped fibers" 

as axiolites, and illustrated them in plate 7, figure 

4 of that report (reproduced in fig. 95 of this report). Iddings (1899, p. 419) has given a detailed 

description of "axiolitic structure" as follows: 

In certain kinds of rhyolite apparently composed of welded 

glass fragments, there is a microspherulitic growth which 

bears a definite relation to the form of the supposed glass 

fragments. The feldspar fibers are in groups which are approximately normal to the outline of the fragments and 

radiate inward. Where the fragment had a rudely triangular shape the central part often attained a greater 

degree of crystallization than the margin, and sometimes 

consists of distinct crystals * * *. 

Marshall ( 1935, p. 23) has described axiolitic structures under the term "pectinate structure." 

Blocks.-Blyth (1940, p. 147) defines blocks as 

follows: "Fragments of cognate or accidental material, larger than lapilli and usually angular, which 

have been erupted in a solid or nearly solid state." 

(See Pyroclastic materials.) 

Block and ash flow.-See Ash flow and Pumice 

flow. 

Dust clouds.-Airborne pyroclastic material of 

dust size (see Pyroclastic materials, also Dust, volcanic) that characterize explosive volcanic eruptions. 

The dust clouds associated with ash flows are not 

basically different from glowing clouds, glutwolken, 

and nuees ardentes (see Nuee ardente) although 

these terms emphasize the glow that is normally 

reflected from the underlying incandescent ash flow. 

Dust, volcanic.-Wentworth and Williams (1932, 

p. 47) have defined volcanic dust as follows: "Pyroclastic detritus consisting mostly of particles less 

than 1~ mm in diameter, that is, fine volcanic ash." 

Blyth (1940, p. 150) gives the dimension of dust as 

less than 0.05 mm. (See Pyroclastic materials.) 

Eutaxite, eutaxitic.-Fritsch and Reiss (1868, 

p. 414) proposed the name eutaxite for a volcanic 

rock composed of ejected fragments of different 

colors, and texture as follows : "The different fractions in general lie beside one another as streaks, 

bands, and lenses in seemingly well ordered distribution." 

This type of material is illustrated in figures 1 

and 11. Piperno seems to be a similar rock but may, 

however, differ slightly by having larger lenses of 

glass known as fiamme. 

Fiamme.-The Italian name used to describe black 

glassy inclusions in piperno and which have a cross 

section shaped like tongues of flame. These are 

often several centimeters in length, but may range 

from microscopic -size to several feet in length. 

Zavaritsky (1947, p. 11) has proposed to include 

all such glass lenticles (collapsed pumice fragments) under the term "fiamme." Matumoto (1943, 

pl. 24) illustrates "obsidian spindles" in the "Aso 

lava" from the Aso caldera of Kyushu, Japan. 

Fluidization.-See Ash flow. 

Glowing avalanche.-See Avalanche, volcanic. 

Glowing clouds.-See Dust cloud, also Nuee ardente. 

Glutwolken.-See Dust cloud, also Nuee ardente. 

FIGURE 1.-A specimen from the Battleship Rock tuf.: (fig. 3) northnortheast of Jemez Springs, N.Mex. This represents the thoroughly 

welded part, and figures 42 and 43 represent specimens from the nonwelded basal part of the same flow. In the nonwelded tuff the pumice 

contains about 70 percent of pore space, while in this specimen there 

has been complete collapse and welding with the elimination of all pore 

space. The black lenses of collapsed pumice in a fine-grained groundmass represent the structure known as piperno. 

lgnimbrite.-Marshall (1935, p. 4-10) proposed 

the name "ignimbrite" (literally fiery rain cloud 

rock). He defined ignimbrites (1935, p. 1) as follows: 

* * * they are thought to have been deposited from immense 

clouds or showers of intensely heated, but generally minute 

fragments of volcanic magma. The temperature of these 

fragments is thought to have been so high that they were 

viscous and adhered together after they reached the ground. 
GLOSSARY OF TERMS 5 

Elsewhere he states (1935, p. 38), 

Ignimbrite is used as a name for a tuffaceous rock of acid 

composition that has been formed from a 'nuee ardente 

Katmaieene' in the nomenclature suggested by A. Lacroix. 

Thus Marshall visualized ignimbrites as a rock of 

"acid composition" formed by the fall from a cloud 

of hot viscous material. The term "ignimbrite" has 

been used by Bonorino (1944), Bouladon and 

Jouravsky (1955, p. 25), Hjelmqvist (1956), and 

others. 

Incandescent tuff fiow.-Fenner (1948a, p. 879) 

described the "tuff flows" of the Arequipa region of 

Peru as "incandescent tuff flows," and stated, "The 

tuff deposits are the result of a series of fragmental 

outbursts of rhyolitic lava, similar to that which 

occurred in the Valley of Ten Thousand Smokes in 

Alaska." He also stated (1948a, p. 882), "An especially important feature * * * is the ability of 

such tuff flows to spread widely over level or gently 

inclined surfaces." Jenks and Goldich (1956, p. 

156) use the term "tuff flows" for the same deposits. 

"Tuff" is not a suitable term to describe unconsolidated material (see Tuff; also Ash, volcanic) 

during flowage. Only after emplacement and consolidation should the material properly be called 

"tuff." 

Lahar.-A term used in Indonesia to designate a 

volcanic mudflow. The term has been used by Curtis (1954, p. 458) "* * * for any volcanic breccia 

with a matrix of tuffaceous aspect which came to 

rest as a single unit and was originally mobilized 

by addition of water, gravity alone being the motivating force." Lahars may originate in different 

ways such as mobilization of rain-soaked debris on 

volcanic slopes (cold) or by eruption through crater 

lakes (hot) (van Bemmelen, 1949, p. 191). Hot 

lahars may also be initiated by nuees .ardentes entering streams. 

Lapilli.-Wentworth and Williams (1932, p. 33) 

state, "According to most writers they [the lapilli 

fragments] may consist either of juvenile lava, still 

liquid or plastic when ejected, or of broken rock of 

any sort from the walls of the vent or from the bed 

rock; that is, they may be essential, accessory or accidental ejecta." Blyth (1940, p. 147) describes 

lapilli as "Cognate or accidental ejecta ranging 

mainly from 32 to 4 mm in size." (See Pyroclastic 

materials.) 

The lapilli of ash-flow tuffs are commonly pumice 

fragments, as are most incluoed blocks, but accidental rock fragments may also be present. 

Mudfiow, volcanic.-Volcanic mudflows or lahars 

are not basically different from nonvolcanic mudflows except for the volcanic origin of their materials. Non volcanic mudflows or "mud spates" 

have been described by Rickmers (1913, p. 195) as 

follows: 

It is not dry nor is there much water, but the whole mass 

appears like a rapid flush of mud, although frequently the 

rock waste is so rough as to suggest that it is properly mud. 

Enormous boulders will float in this thick porridge. 

The term "mudflow" was also applied to ash flows 

before they were fully understood. Thus Marsters 

(1912) called the ash flows of Peru "mudflows" and 

Griggs (1917, pl. 2) in his early studies of the volcanic deposits of the Valley of Ten Thousand 

Smokes called this material "mudflow." This was 

later called "sand flow" by Griggs and "tuff" deposit 

by Fenner (1923). 

Mud lava.-The term "mud lava" has been used 

to describe some materials in Japan which are now 

known to be ash-flow materials. Matumoto (1943, 

pl. 30) referred to welded mud lava. (See Aso 

la~'a.) 

Nonwelded tufj.-The term "nonwelded tuff" will 

be applied to those ash flows or parts of ash flows 

that have not become welded. The mode of deposition and consequent cooling of ash-fall materials 

precludes welding, in contrast with ash-flow materials which may or may not become welded. Thus 

there should be no confusion, and it seems unnecessary to use the term "nonwelded ash-flow" materials 

each time they are mentioned. That is, "nonwelded 

tuffs" are always to be understood as the corollary 

of welded tuffs. 

An alternative would be to extend the term 

"sillar" (see Sillar) to include all nonwelded tuffs. 

Fenner (1948a, p. 883) proposed this name for the 

poorly indurated tuffs of Peru, but did not suggest 

its use as a general term for these materials. Also, 

some of the tuffs of Peru that were included as sillars seem to have undergone slight welding. However, one of the principal characteristics of the 

Peruvian tuffs is induration by vapor-phase crystallization. This was recognized by Fenner and it may 

be that this term could be more appropriately used 

for tuffs indurated by this process than as a general term for nonwelded tuffs. The term "sillar" 

has been used by Barksdale (1951) and Williams 

(1952, p. 76) for nonwelded ash-flow tuffs. 

Nuee ardente.-Lacroix (1903a, p. 442-443) proposed the term "nuee ardente" to describe the 

previously unrecognized type of volcanism that 

characterized the eruptions of Soufriere and Peiee 

in 1902. Lacroix (1904, p. 350) describes the eruption at Pelee as follows: 
6 ASH-FLOW TUFFS 

A nuee ardente is made up of an emulsion of solid materials 

in a mixture of water vapor and gas at high temperature, 

about the nature of which I shall not refer. The shape and 

dimensions which it presents at the moment of its emission 

from the carapace of the dome, show that it has been subjected to an enormous pressure, for after a few seconds, it 

occupies a volume greater by many thousand times that 

which it had at its inception. 

He also says, 

At the base [of a nuee ardente] is found a zone at very 

high temperature, in which the solid materials predominate 

(blocks of all dimensions, very small fragments, fine cinders) ; each of these pieces, or the solid particles of which it 

is formed, radiate heat, and must be surrounded by an 

atmosphere of gas and vapors, extremely compressed at the 

beginning, but expanding rapidly; it is this atmosphere 

which prevents the solid particles from touching one 

another, maintaining the mass in a state of mobility which 

allows it to flow over the slope almost in the manner of a 

liquid. 

The term "nuee ardente" is well established as the 

name for a type of volcanic eruption, a usage with 

which we are not concerned (see Pelean eruptions). 

We are, however, concerned with an understanding 

of the mechanism of deposition of welded tuffs. 

The second part of the quotation from Lacroix 

shows that he clearly recognized the dual character 

of the nuee ardente-the overriding dust clouds, and 

the dense basal part. The full significance of the 

concept of Lacroix has not always been understood. 

Perret (1937, p. 5) pointed out the common failure to recognize the dual character of nuees ardentes in the following statement: 

The term nuee ardente for want of something more precisely descriptive, has been quite generally adopted. 'Nuees 

ardentes' as well as 'Glutwolken' and 'Nuages dens es' all 

refer in too restricted a sense to the cloud aspect of the 

phenomenon, losing sight entirely of the source of energy 

within the avalanche, the inherently active mass of denser 

material, hidden, all but completely, by the magnificently 

spectacular convolutions of vapor and ash * * *. 

Again (1937, p. 20) in describing the eruptions of 

January 5, 1930, he states, 

A strong west wind * ':' * moved the ash cloud above while 

the heavy mass of the avalanche beneath was quite unaffected. The dust cloud * * * was not only checked, but 

forced backward even beyond the point of issue. 

Figure 2 shows the dual character of a Pelean nuee 

ardente. 

Fenner ( 1948a, p. 881) made the following comment: 

The terms 'nuees ardentes' and 'glowing clouds' were applied to these phenomena, but though these clouds are spectacular and terrifying, to call the whole group of actual 

manifestations 'nuees ardentes' tends to place the emphasis 

FIGURE 2.-A "small typical nuee ardente" of Feb. 8, 1930, at Pelee, illustrated by Perret ( 1937, fig. 16). The arrows indicate the ash-flow part 

of the nuee ardente which has sped in advance of the overriding dust 

clouds. 

on the relatively superficial clouds that rise from the plunging mass rather than on the mass itself * * ':'. 

Notwithstanding the well-established recognition 

(beginning with Lacroix) of the dense basal part 

of an ash flow as the effective medium of transportation, the strictly airborne concept has found its way 

into some geologic interpretations, and beclouded 

the whole approach. The geologist must study and 

interpret the deposits formed by the dense underlying part of the eruptions, and is in need of clearly 

distinguishing terms. This means that, as MacGregor (1955, p. 10) has pointed out, "* * * the 

classification of nuee ardente eruptions should be 

quite distinct from the classification of the products 

produced by such eruptions [and including the 

mechanism that emplaces these products]." 

In summary, nuee ardente has come to have two 

distinct usages; one for a special type of volcanic 

eruption, and the other for the clouds that ordinarily 

accompany these eruptions-glowing cloud being the 

commonly used English equivalent. Usage has not 

always differentiated between the clouds themselves 

and the dense ash- or block- and ash-transporting 

basal part. If so used, this basal part would constitute the noncloud portion of a glowing cloud, 

which may not even be glowing. In general, it 

glows only by reflecting the incandescent underlying ash flow. 

Obsidian spindle.-See Fiamme. 

Pectinate.-See Axiolite. 

Pelean eruption.-This term has been used to describe certain types of eruptions resulting in ashflow or avalanche deposits, and similar terms have 
GLOSSARY OF TERMS 7 

been proposed for several other types of volcanic 

eruptions. The classification of types of eruption 

has been outlined by Lacroix (1930), Fenner (1937, 

p. 236), Perret (1937, p. 3-5), and Williams 

(1941b). A detailed classification has been presented by MacGregor (1952, tables 1 and 2). These 

terms are widely used in descriptions of the eruptions of active volcanoes, but have also been applied 

to older ones where the character of the eruption 

could be verified. 

Piperno.-A rock first described from the Phlegrean Fields in Italy is locally known as piperno. 

It is characterized by conspicuous lenses of glass 

(fiamme). "Pipernoid" is a phase of the same rock 

in which a similar structure is revealed under the 

microscope. 

Dell'Erba (1892), and later Zambonini (1919, 

p. 72), concluded that piperno was a tuff, and was 

deposited at a high temperature. Rocks of this 

texture are illustrated in figure 1. 

Pumice.-Webster's New International Dictionary (1933, p. 1735) defines pumice as follows: "A 

highly vesicular volcanic glass produced by the extravasation [more properly exsolution from the 

glass] of water vapor at a high temperature as 

lava comes to the surface: hardened volcanic glass 

froth." Wentworth and Williams (1932, p. 39) 

quote Lacroix (1930, p. 437) as follows: 

The glass is not only puffed up, but spongy, drawn out, 

sometimes fibrous or filamentous; the cavities are extremely 

abundant and their form varies according to the composition. The types with elongated, tubular parallel vesicles 

have a fibrous or silky aspect; these are characteristic of 

rhyolite pumice, whereas the cavities are more or less spherical in trachytic and phonolitic pumice. 

The term "pumice" implies no specific size of materials and a pumice structure is retained by some 

ash.:.size materials. In general, however, explosive 

eruption has disrupted the original vesiculated 

magma into individual segments of the cell walls, 

and so the distinctive pumice structure has been 

destroyed to form glass shards. The material which 

results is known as volcanic ash. (See Ash, volcanic.) If the retention of pumice structure is 

conspicuous the material could be called a pumiceous ash. 

. Pumice fall.-Deposition of pumice directly from 

the air. (See Ash fall and Tephra.) 

Pumice flow.-Flows with a conspicuous proportion of pumice fragments of lapilli size ( 4 to 32 

mm) or block size ( >32 mm) have been called 

pumice flows. (See Pyroclastic 1naterials.) Tsuya 

(1930) applied the term "pumice flows" to the rnaterials formed by the 1929 eruption of the Komagatake volcano in Hokkaido, Japan. Other Japanese 

geologists, including Kozu (1934, p. 136) and Kuno 

(1941, p. 148), have described pumice flows. As 

used by the Japanese geologists "pumice flow" is 

synonymous with "ash flow." 

Flows characterized by coarser material have 

been called lapilli flows, and block flows. Williams 

(1942, p. 79) referred to the Crater Lake "glowing 

avalanches" as "pumice flows" and "scoria flows." 

Perret (1937, p. 5) applied the term "block and 

ash flow" to some materials at Peh~e. 

Pumiceous ash.-See Pumice. 

Pyroclastic.-Wentworth and Williams (1932, p. 

24) state, "Pyroclastic is an adjective commonly 

applied to rocks produced by explosive or aerial 

ejection of material from a volcanic vent." The 

noun form pyroclasts is defined by Holmes (1920 

p. 193) as "A general term for fragmental deposits 

of volcanic ejectamenta, including volcanic conglomerates, agglomerates, tuffs, and ashes." (See 

Tephra.) 

Pyroclastic materials.-The following size classification of volcanic fragmental materials is given 

by Wentworth and Williams (1932), and by Blyth 

(1940) : 

Size classification of volcanic fragmental materials, in 

millimeters 

Name Wentworth and Williams (1932) Blyth (1940) 

Block 

Lapilli 

Coarse ash 

Fine ash 

Volcanic dust 

>32 

Mostly 32 to 4 

4 to 14 

J<14 

>32 

32 to 4 

4 to 0.5 

{ 

0.5 to 0.05 

<0.05 

Wentworth and Williams, and Blyth are virtually 

in agreement, except that Blyth preferred decimal 

fractions, using the figure approximating the value 

of the common fractions used by Wentworth and 

Williams. 

Sand .flow.-Griggs (1922, p. 253-254) describes 

the "great hot sand flow" as follows: 

The bulk of the deposit is composed of fine fragments, many 

of them dust-like, but there are included numerous lumps of 

pumice which in places make up a considerable fraction of 

the whole. There is no trace of stratification of the materials, except where they were obviously subject to secondary readjustments after deposition * * *. At the foot 

of the valley we found clear-cut and positive evidence of the 

manner of the tuff making it certain that the sand of which 

it was composed must have flowed down the valley like a 

viscous liquid. 
8 ASH-FLOW TUFFS 

Griggs (1922, p. 261) also referred to the tuff as 

"the incandescent sand flow." 

Scoria fiow.-See Pumice flow. 

Sillar.-Fenner (1948a, p. 883) in describing the 

"incandescent tuff flows" of Peru writes, 

For those [tuffs] in which induration is primarily the result 

of recrystallization, and for those in which the fragments 

have little cohesion, another term is desirable [that is, other 

than welded tuffs]. The local term 'sillar' (pronounced 

seelyar), commonly used in the Arequipa region, has been 

applied in the present paper. 

Tephra.-Thorarinsson (1955, p. 12) proposed 

"the term tephra as a collective term for all clastic 

volcanic material transported from a crater through 

the air * * *." 

Tuff.-Wentworth and Williams (1932, p. 50) 

define tuff as "indurated pyroclastic rocks of grain 

generally finer than 4 mm, that is, the indurated 

equivalent of volcanic ash or dust." (See Blyth, 

1940, p. 153; also Ash, volcanic, and Incandescent 

tuff flows.) 

Tuff lava.-Abich ( 1882, p. 39) described the 

rocks of the Alaquez massif, Russian Armenia, as 

"tuflava," and Zavaritsky (1947, p. 12) has used 

the same term and shown that the materials are 

characteristic welded tuffs. 

Tuff, cTystal.-In describing crystal tuffs Pirsson 

(1915, p. 199) states, 

Tuffs composed entirely of crystals, must be very rare * * *. 

On the other hand, crystals of minerals, the kinds depending largely on the nature of the magma, either perfect in 

form or more or less fragmental, are found in nearly all 

tuffs; and when they become a dominating or striking feature of them the rocks are referred to as crystal tuffs. 

Crystals or crystal fragments characterize almost 

all ash-flow tuffs and range from a trace to more 

than 50 percent of the rock in some tuffs. 

Tuff, lithic.-In describing lithic tuffs Pirsson 

(1915, p. 201) states that "The essential feature of 

tuffs of this class is the presence in them of a striking or dominating degree, of fragments of previously formed rocks * * *. These fragments may be 

holocrystalline or partly glassy." 

Rock fragments are present in nearly all ash-flow 

tuffs but rarely exceed a few percent of the rock. 

Tuff, vitric.-A name proposed by Pirsson (1915, 

p. 194) for "Tuffs produced by the sudden and 

violent explosion of a more or less viscous 

magma * * *. The explosion of gas and rupturing 

began in a liquid medium; the resulting product 

falls as a rigid glass." 

In general, ash-flow tuffs are dominantly vitric 

or derived from originally vitric material. As Pirsson states, such material would fall from the air as 

a rigid glass (ash-fall tuffs). However, where 

flowage occurs, the glass may retain plasticity until 

after deposition. 

Tuff, welded.-Mansfield and Ross (1935, p. 308, 

321) described 

welded volcanic tuffs-that is, those in which individual 

fragments had remained plastic enough to become partly or 

wholly welded * * *. In a few specimens the original forms 

are unmodified; in others there is flattening, but without 

obliteration of characteristic ash structure; and in a few, 

extreme flattening and slight flowage has almost obscured 

the original structure. 

The term "welded tuff" is self explanatory, and 

has been widely used by many geologists including 

Gilbert (1938, p. 1851), Westerveld (1942), Williams (1942, p. 60), and Enlows (1955). Bodenhausen (1955, p. 44) used the French form of the 

same term (tufs cineritiques soudes). 

Welded pumice.-Collapsed pumice has been called 

welded pumice by Iddings ( 1899) . 

The materials called welded pumice by Iddings are 

discussed in detail on pages 11-12 of this report. 

The full relationships of these materials were not 

fully understood by Iddings, but the subject of 

welding interested him greatly, and he returned to 

it several times. Iddings is widely recognized as 

discovering the phenomenon of welding, and of having proposed the term "welded pumice," although 

the term "welded" was used long before by Zirkel 

(1876, p. 267). 

DEVELOPMENT OF CONCEPTS 

Present-day concepts of the origin of ash-flow 

tuffs and of processes which lead to welding in these 

rocks have developed through a series of disconnected observations and events over a long period. 

An attempt is made here to integrate a series of 

early interpretations, key studies, and important 

natural events into a chronology showing the general evolution of thought regarding these rocks. 

A review of the literature indicates that welded 

tuffs have caused much speculation and difference 

of opinion among geologists at least as far back as 

the late 1860's. Several workers have been confronted with rocks that showed most of the characteristics of pyroclastic materials and yet, in thelight of existing knowledge, could only be explained 

as lava flows. There was at times actual controversy, one man claiming the rock to be a tuff, and 

the other one calling it a peculiar type of lava. 

Many rocks have been described that show peculiar "flow" structures or "ash" structures, or what 
DEVELOPMENT OF CONCEPTS 9 

has been considered normal "rhyolitic" structure. 

Many of these rocks, from their descriptions, illustrations, or by reexamination, are now known to 

be welded tuffs. The writers believe that the attempts of some of the early workers to interpret 

these rna terials are an essential part of the historical 

background of welded tuffs. Thus the following 

review will include quotations from the older works 

as well as discussions of the papers dealing directly 

with welded and nonwelded ash-flow tuffs, and the 

relations of these rocks to volcanic eruptions of 

"Pelean" and "Katmaian" type. Special emphasis 

is given those older works where reexamination by 

the authors or by other workers has shown what is 

probably the true nature of the rocks described. 

Fritsch and Riess (1868), working on Tenerife, 

one of the Canary Islands, described rocks that they 

named eutaxites. These rocks, which to the authors' knowledge have not been reexamined, seem 

almost certainly to have been welded tuffs or at 

least welded clastic rocks. . This seems to be one 

of the first recorded examples of an attempt to 

understand the origin of such rocks and is important 

because from this work comes the term "eutaxitic." 

This term has since been used by many authors in 

different ways but has come to be applied most 

frequently to the structure seen in many welded 

tuffs (fig. 1). For further discussion of this term 

see page 4. 

During 1867-73, those engaged in the U.S. Geological Exploration of the 40th Parallel, under the 

leadership of Clarence King, made extensive collections of rocks in several of the Western States. 

These rocks were studied petrographically by Zirkel ( 1876) and the field studies were discussed by 

King ( 1878) . Of the more than 2,500 thin sections 

studied by Zirkel, the authors do not know how 

many were of rhyolitic rocks. However, more than 

800 of these thin sections are now in the collections 

of the U.S. National Museum. Reexamination of 

these sections by the authors has revealed that 

nearly 200 are of welded rhyolitic tuffs, most of 

which came from the 40th parallel in Nevada 

(figs. 93 and 94). Many of Zirkel's descriptions 

and illustrations (see reproduction in fig. 95) are 

clearly those of welded tuffs. It is doubted that 

Zirkel observed anything unusual about the origin 

of these rocks, but he did observe many textures 

that were new to him. 

Zirkel ( 1876, p. 163) says of the rhyolites in 

general, 

Of all the rocks, these rhyolites most excel in variety and 

diversity of microscopical structure; and since better facilities for investigation than had ever before been enjoyed, 

were furnished by the extraordinary number of occurrences 

at hand, it is highly probable that the following pages will 

be found to explain all, or nearly all, the most characteristic 

types of which the rhyolitic structure is capable. Particular 

attention has been paid to these interesting varieties, examples of which will doubtless be found in studying the 

comparatively unknown rhyolites of other countries. 

Zirkel has given one of the most remarkable accounts of variations in texture in welded tuffs ever 

presented, but he was unaware of their true origin. 

From Zirkel's work has also come the term "axiolitic" which he applied to a certain type of devitrification in glass shards. This structure is common in certain welded tuffs and is discussed on 

pages 4 and 37, and illustrated in figures 71-74. 

King (1878) gives detailed megascopic descriptions and accounts of the field occurrences of the 

rocks that had been studied petrographically by 

Zirkel. Several welded tuffs are discussed under 

the classification of breccias and cemented breccias 

by King. The reader is profoundly impressed by 

these accounts and by the widespread distribution 

of these rocks in the Western United States, especially in Nevada. 

Dell'Erba (1892), in a report on the origin of the 

much discussed piperno of Italy, concluded that this 

rock was a tuff rather than a lava as had been 

supposed by other workers. He also concluded that 

it had been hot at the time of its deposition. Piperno 

contains many lenticular fragments of black glass 

known locally as fiamme and Dell'Erba thought 

that it was these fragments that supplied the heat, 

the effects of which could be seen in certain peculiar 

characteristics of the tuff. Dell'Erba deserves 

great credit for recognizing that this rock had a 

pyroclastic origin. 

Several studies of the Italian piperno, particularly 

those of Zambonini (seep. 12 of this report) and of 

Zavaritsky (see p. 14), suggest that these lenticular 

glass fragments are collapsed and strongly welded 

pieces of pumice (fig. 1). 

Turner (1894), in discussing some of the rhyolites of the western slope of the Sierra Nevada in 

California, says, 

The rhyolite flows evidently followed the old river channels 

to a remarkable extent. The exact nature of these flows is 

not in all cases determined. They have been considered as 

tuffs or mud flows, but in thin section some specimens show 

trains of spherulites and other evidences of having been 

molten lava. 

Reexamination of similar and probably related 

rhyolites on the western slope of the Sierra Nevada 
10 ASH-FLOW TUFFS 

by several workers indicates that these rocks can 

now be regarded as welded tuffs. 

Ransome's (1898) study of a series of volcanic 

rocks of the western slope of the Sierra Nevada in 

California, provides the most outstanding example 

of careful consideration, indecision, and wrong conclusion caused by a welded tuff, that has yet come 

to the attention of the authors. Among the volcanic rocks Ransome discovered one that he named 

"biotite-augite latite." He seems to have had difficulty classifying it from the very beginning, and 

says of his rock (1898, p. 27), 

* * * the biotite-augite latite has characteristics that render 

its distinction from a tuff not always easy. The fact that 

it has a wider distribution than the two undoubted massive 

flows, also suggests that it may have had a different origin. 

But the field evidence does not support the view that it is 

in any sense a water-deposited tuff. It is entirely devoid of 

the horizontal bedding characteristic of the known tuffs of 

the region, and on the other hand it possesses the more or 

less uneven upper and lower surfaces, and the ability to lie 

upon perceptible slopes, which are characteristics of true 

lava streams. It also resembles lavas in being coarsely 

columnar in structure, although, as has been shown this 

structure may also occur in undoubted tuffs. The basal contact of the biotite-augite latite was nowhere seen sufficiently 

well exposed to give a decisive answer to the question 

whether it is tuffaceous or massive in character. 

The next quotation (Ransome, 1898, p. 42-44) is 

long, but is so pertinent to an understanding of the 

problems that faced earlier students of these rocks 

that the authors believe it should be included. This 

quotation not only indicates the views of some of the 

earlier European geologists, but shows the intensity 

of Ransome's thinking on the controversial nature 

of some tuffs and lavas. 

The general microstructure of the biotite-augite latite appears to approach very closely to the eutaxitic structure as 

the term was first employed by Fritsch and Reiss (1868), 

who applied it to certain facies of phonolites, andesites, and 

trachytes possessing some resemblances to clastic rocks and 

inclosing undoubted clastic fragments. They distinguished 

agglomerate lava, in which the structure was supposed to 

be due to partial refusion of clastic material, from piperno, 

in which portions of the magma in different stages of 

crystallization are brought into juxtaposition by the motion 

of the flow: Common usage, however, has somewhat altered 

the original application of the term eutaxitic, so that it frequently implies merely a fluidal banding brought about by 

alternating streaks of microfelsitic and clear glass (Zirkel 

1893), or is even used as 'a general name for banded volcanic rocks' (Kemp 1896). That such can hardly have been 

the earlier application of the word appears from the discussion that has centered about the original piperno of Pianura 

in the Phlegraean Fields, near Naples, a rock that Fritsch 

and Reiss took as the second type of eutaxitic structure, and 

which appears to resemble in some ways the biotite-augite 

latite of California. Luigi dell'Erba ( 1892) considers this 

rock a tuff, but the greater number of petrographers have 

regarded it as a lava flow. Zirkel ( 1893) refers to it as a 

sanidine trachyte with eutaxitic structure, remarking that 

Luigi Dell'Erba's view is not even probable, while Rosenbusch ( 1896) cites it as an example of his Ponza type of 

the trachyte family. Such a discussion could scarcely have 

arisen were the structure concerned a simple banding such 

as is observed in many vitrophyres, and which no petrographer could for a moment regard as indicating a tuffaceous 

ongm. Kuch (1892), following the terminology of Reiss, 

describes both kinds of eutaxitic structure, the agglomerate 

lava and the piperno, as occurring in the dacites and andesites of Colombia. The latter form is particularly abundant, 

and Kuch remarks that it is at times impossible to distinguish the two varieties, as the piperno often contains included fragments of other andesites. Wickmann (1897), in 

describing an augite-mica-andesite from the Indian Archipelago having a structure apparently nearly identical with 

that of the biotite-augite latite of the preceding pages, refers to it as having piperno structure; and there seems to 

be little doubt that this latter name, as used by the petrographers cited, expresses accurately the structure of the 

second flow of the Sierra Nevada latites. 

When, on the other hand, a thin section of the last-named 

rock is compared with that of a lava having typical flow 

banding-as, for example, the beautiful vitrophyre of San 

Lugano, in the Tyrol, or the pitchstone-vitrophyre occurring 

between Lake Lugano and Lago Magiore, both of which, 

according to Rosen busch ( 1896) , often show eutaxitic facies 

-the chief difference seems to lie in a greater irregularity 

and brecciation of the bands in the latite, and in the presence of included rock fragments. 

In a later paragraph Ransome (1898, p. 44), says, 

As an historical illustration of the difficulty that sometimes 

exists in distinguishing a tuff from a massive lava may be 

recited the case of the so-called 'peperino' near Viterbo, in 

Italy, which, according to Washington (1896), is probably 

a tuff, although it possesses some apparent flow structures, 

and has been by earlier investigators frequently designated 

a massive rock. 

As a result of reading the writings of Ransome 

on the biotite-augite latite and a group of associated 

rhyolites, Smith reexamined these rocks in the field 

in 1950 and concluded that they were welded tuffs. 

The biotite-augite latite shown in figure 82 is 

from an exposure near McKays, Calaveras County, 

Calif. This rock is a fine example of a crystal-rich, 

glassy, welded tuff and after examining it in the 

field and laboratory it is not difficult to understand 

Ransome's uncertainty about it. He finally concluded it was a lava because at that time no volcanic mechanism was known that might produce a 

rock intermediate between a lava and a tuff. 

Part of the group of rhyolites studied by Ransome are located near Altaville and Vallecito, Calaveras County, Calif., and are within an area recently 

mapped by Lorin Clark (oral communication, 1950) 

of the U.S. Geological Survey. These rocks have 
DEVELOPMENT OF CONCEPTS 11 

been recognized by Clark as welded tuffs, and are 

probably closely related to the rhyolites discussed 

by Turner (1894), and the welded tuffs mentioned 

by Curtis (1954, p. 453-454). 

The studies of Diller and Patton (1902) of the 

volcanic rocks of the Crater Lake region give us 

one more example of the uncertainties of nomenclature attached to these rocks. In writing of the 

Wineglass dacite flow Diller says, 

This peculiar tuffaceous dacite occurring along much of the 

crest of the rim all belongs to one flow, which spread as a 

uniformly thin sheet over that portion of the base of Mount 

Mazama. It is altogether unlike the other flows of dacite 

and appears to be intermediate between them and tuff. 

Reexamination of this same dacite was made by Williams ( 1942) and shown by him to be a welded tuff. 

The first major contributions to the understanding of welded tuffs were the works of Iddings 

(1885-86, 1899, 1909), an outgrowth of his studies 

of the volcanic rocks of Yellowstone National Park 

(figs. 87-89, 91 and 92). He must be credited with 

developing the concept of welding of pyroclastic 

materials as well as being the first to use the term 

"welded pumice." His first use of the term "welded" 

seems to have been in connection with a specimen 

from the rhyolite flow of Obsidian Cliff. The following quotation is important only because it seems 

to have been Iddings' first recorded use of the word 

"welded," and his concepts of welding of hot clastic 

rocks apparently developed from that time. Iddings 

says (1885-86, p. 274), in reference to colored bands 

in obsidian, 

In some the colored streaks are in broad, thin bands, either 

straight or twisted, according to the last movements of the 

viscous glass. In others, they are in the most delicate 

threads, alternating with streaks of black grains running 

continuously through the rock, though sometimes interrupted 

by streaked patches of other character or appearing as 

though the rock had been broken into fragments and welded 

together again (pl. 16, fig. 2). 

This figure is reproduced in this report as figures 

87 and 88. 

This rock from the Obsidian Cliff rhyolite flow is 

not a welded tuff, but rather a welded breccia which 

is not uncommon on the margins of rhyolite domes 

and flows or in sheared zones in rhyolite flows. 

Such welded breccias may often superficially resemble coarse-grained welded tuffs, especially if 

they are composed of pumiceous fragments. In a 

more detailed study of the rhyolitic rocks from 

Yellowstone National Park, Iddings (1899, p. 403-

404) says in relation to pumice, "In some instances 

it is evident, from the confusedly twisted and 

curved arrangement of the glass fibers and films, 

that the inflated glass mass settled back upon itself, 

or collapsed, after the escape of much of the gas." 

To explain this interpretation of his observations 

he says, "When we remember the enormous extent 

of many of the streams of rhyolite in this region, 

we may easily imagine the formation of pumice 

over the surface of an intensely heated area of lava, 

thus permitting its subsequent welding." 

These statements were made with reference to 

collapsed or deformed pumices, rather than to the 

typical shard structures of welded tuffs. However, 

Iddings states ( 1899, p. 405-406), 

In numerous cases a pumiceous character is entirely wanting. The mass is a compact glass, but it consists of irregularly shaped streaks and patches of different color. 

These twist and curve about one another and appear like a 

perfectly welded mass of strips or ribbons and irregular 

fragments of variously colored glass. In some cases their 

shape closely resembles that of fragments of pumice pressed 

together and welded (pl. 50, figs. 1, 2, and 3). In others it 

appears as though such fragments had been drawn out and 

twisted by a movement of the mass (pl. 51, fig. 2). Undoubtedly this has been the case, but it is doubtful whether 

all the streaked and variegated glasses have passed through 

the process of inflation, collapse, and welding with subsequent flow. However, the distinctly outlined and strongly 

contrasted streaks and ribbons of variously colored glass, 

are with difficulty explained in any other manner. 

Although Iddings recognized the phenomenon of 

the welding together of vitric fragments, he apparently was uncertain as to the causes of welding 

and the origin of the resulting rocks. This is understandable since Iddings' accounts show that he 

was dealing with three processes of welding. These 

processes are: (a) the brecciation of glass in shear 

zones, or at the base of rhyolite flows and the subsequent welding of the resulting breccia; (b) the 

formation of pumice on the surface of lava flows and 

the deformation or collapse of such pumice due to 

the heat from the flow and its subsequent movement. 

Although some of Iddings' "welded pumice" was 

formed in this manner probably many of the rocks 

examined by Iddings and believed by him to have 

formed in such a way were true welded tuffs; (c) 

the explosion of material from a vent into the air, 

the fall of the material while still hot back into the 

vent, and its subsequent welding, followed by extrusion from the vent in the manner of a lava. Concerning the third process, Iddings (1909, p. 331) 

says, 

When exploded fragments of molten magma, large or small, 

fall together in a still heated condition, as may readily 

happen within the crater of a volcano or in the mouth of a 

fissure, they may be plastic enough to weld together into a 

more or less compact, coherent mass. This may subse-
12 ASH-FLOW TUFFS 

quently flow like other lava, and is known as FLOW-BRECCIA. 

* * * [1909, p. 333] The same operation may result in the 

welding of exploded pumice, or of collapsed pumice that was 

highly inflated. The distinction between this and a flow 

breccia is chiefly in the size of the welded fragments. 

The explanations given by Iddings are inadequate 

to explain the process of welding involved in the formation of welded ash-flow tuffs but they are important historically. 

Twenty-two of the original thin sections studied 

by Iddings are still available and some of the best 

of these are shown in the frontispiece, figures 88, 

91, and 92. Although more than 50 years old, they 

are very well preserved. It is very fortunate that 

the thin sections, which were the basis for Iddings' 

recognition of the fact of welding together of fragments of glassy pumice and ash and for his proposal 

of' the name "welded pumice," are still available. 

The eruptions in May 1902, of Mount Pelee on the 

Island of Martinique and of La Soufriere on the 

Island of St. Vincent, marked a turning point in the 

understanding of the mode of formation of many 

of the world's deposits of pyroclastic rocks. The 

observations made at Sufriere and Pelee by Anderson and Flett (1903), and at Pelee by Lacroix 

(1904), and at a later date by Perret (1937), gave 

to the science of volcanology an understanding of 

a new type of volcanic eruption, and formed the 

basis for the present-day concepts of the origin of 

ash-flow materials, including welded tuffs. The 

studies of Anderson and Flett, Lacroix, and Perret 

are discussed on pages 15-16 in this report. 

The pyroclastic deposits formed by the eruptions 

of Pelee and Soufriere are characterized by their 

complete lack of sorting and at Pelee, at least in 

part, by the widely variable dimensions of the constituent blocks, fragments, and dust. Such deposits 

came to be known as Pelean tuffs and in the years 

following the eruptions of 1902 several writers described tuff deposits from other parts of the world 

and attributed their origin to eruptions of Pelean 

type. Dakyns and Greenly (1905) seem to have 

been the first to further develop the concept in a 

paper on the "felsitic slates" of Snowdon, Wales. 

In an earlier paper Dakyns (1900) had recognized 

that these rocks were clastic and concluded that 

they were "felsite tuffs." Greenly, inspired by the 

account of Anderson and Flett on the eruptions of 

Pelee and Soufriere, sought to reconcile certain unexplained features of the Snowdon rocks with the 

observed features of the ejecta of Pelee and Soufriere. In so doing, he concluded that the Snowdon 

felsites were due mostly to eruptions of Pelean type. 

Greenly says of these rocks (Dakyns and Greenly, 

1905, p. 548), "* * * in the felsitic slates of Snowdon we have a Pelean deposit of the Ordovician 

period." 

Lacroix (1906, p. 1020-1022) attributed a Pelean 

origin to a group of ancient volcanic rocks of the 

Auvergne region in France. 

Zambonini (1919) reviews the problem of the 

origin of the Italian piperno and concludes, as had 

Dell'Erba, that it is a tuff and that it was deposited 

in a hot condition as shown by the occurrence of 

several late-stage minerals, some of which contain 

fluorine. He also concludes that it probably originated as a "Pelean cloud" deposit. 

Although the eruptions of Mount Pelee and Soufriere and the observations by Anderson and Flett, 

Lacroix, and the later studies by Perret were of 

fundamental importance in establishing a new type 

of volcanic action, and especially a new mechanism 

for the transportation of pyroclastic materials, it 

was the formation in 1912 of the "sand flow" in 

the Valley of Ten Thousand Smokes, Alaska, accompanying the eruption of Mount Katmai, that 

provided the necessary link for our present understanding of the origin of ash- and pumice-flow materials in general. 

The first account of the discovery of the Valley 

of Ten Thousand Smokes and its now famous deposit of volcanic ejecta, by members of a National 

Geographic Society Expedition was by Griggs 

(1917, p. 12-68). In a series of papers Griggs 

(1917, 1918a, b, c, 1919, 1921, 1922) discussed other 

phenomena of the region and the deposit of volcanic 

tuff which he termed the "great hot mudflow" 

(Griggs, 1918b). 

Fenner (1920) and his associates, especially E. 

G. Zies of the Carnegie Institution of Washington 

Geophysical Laboratory, noted the effects of heat as 

shown by the so-called mudflow, but were unable to 

reconcile the observed heat effects with hot waterbearing mud and concluded that the deposit, while 

rich in gas, was virtually dry and called it the 

"great sand flow" (fig. 96). Fenner (1923) finally 

concluded that the sand flow was deposited as a 

mixture of gas and ash and was similar in many 

respects to the deposits formed by the eruptions of 

Pelee and Soufriere. However, he noted a difference in the materials deposited and therefore considered them to be the result of a type of, or 

modified, Pelean eruption. This conclusion has had 

a marked influence on nearly all the geologists who 

have subsequently worked with ash-flow pyroclastic 

materials because, in comparing most welded tuffs 
DEVELOPMENT OF CONCEPTS 13 

and their nonwelded equivalents with the deposits of 

Pelee and the Valley of Ten Thousand Smokes, a 

greater similarity is seen with the sand flow than 

with the Pelean deposits. 

As an example of this comparison Williams 

(1927), in a detailed study of the rocks of Snowdon, 

Wales, discusses the origin of some rhyolites that 

Dakyns and Greenly had previously concluded were 

tuffs formed from an eruption of Pelean type. Williams noted that the tuffs were composed largely of 

glass fragments and glass dust rather than crystals, 

which are more typical of Pelean tuffs, and concluded that while they were probably erupted in a 

manner somewhat analogous to the Pelean ejecta, 

they resembled more strongly the rhyolite tuffs of 

the Katmai area. 

Moore (1934) discusses the probability of a nuee 

ardente origin for the "older" pumice of Crater 

Lake, Ore., as opposed to an origin of direct fall 

from the air as shown by the "younger" pumice. 

His conclusions are based on several differences between the two types, especially with regard to size 

and sorting. The younger pumice is more uniform in size and is better sorted, whereas the older 

pumice is widely variable in size and shows no 

bedding or sorting. 

The work of Marshall (1932, 1935) on the vast 

deposits of rhyolitic tuffs on the North Island of 

New Zealand is a milestone in volcanic geology. 

Marshall proposed the name "ignimbrite" to include 

a large group of rhyolitic rocks that cover approximately 10,000 square miles of the North Island and 

that he regarded as formed by an eruptive process 

similar to that proposed by Fenner for the sand 

flow of the Valley of Ten Thousand Smokes. 

One of Marshall's greatest contributions was his 

correlation of field studies with petrographic 

studies. He noted welding in tuffs as observed by 

Iddings and he was the first to recognize the importance of devitrification in obscuring the fragmental character of these glassy pyroclastic rocks 

(fig. 97). 

Richards and Bryan (1934) in discussing the 

origin of the Brisbane tuff (fig. 75) of Queensland, 

South Australia conclude that 

* * * the combination of tuffaceous and non-tuffaceous characters presented by the Massive Tuff could be most readily 

explained as due to an enormous eruption of the incandescent avalanche type * * * 

and that 

The Massive Type of the Brisbane Tuff presents many features closely analogous with those of the Ignimbrites of 

New Zealand and has also much in common with the Hot 

Sand Flow of Alaska. 

As did Marshall, these writers related petrographic 

to field characteristics and recognized materials 

closely resembling the welded pumice of Iddings. 

At about the same time and without knowledge 

of the work being done in New Zealand and Australia, Mansfield and Ross (1935) described a group 

of rocks from southeastern Idaho for which they 

used the term "welded tuff" (figs. 9, 10, 17, 18, 49-

54, 61, 70). These tuffs were compared to the 

"welded pumice" of Iddings and are possibly related to the vast fields of Yellowstone rhyolite, much 

of which is now known to be welded tuff. These 

writers concluded that the Idaho tuffs have many 

features in common with the Katmai sand flow and 

may have had a similar origin. 

The integration of field studies with petrographic 

studies, as in the works of Marshall, Richards and 

Bryan, and Mansfield and Ross, established a direct 

relation between welding in pyroclastic materials 

and an origin of some of these materials from a 

particular type of volcanic eruption. It provided a 

correlation between some of the welded pumice of 

Iddings and volcanic eruptions of a type exemplified 

by Pelee, Soufriere, and the Valley of Ten Thousand 

Smokes. This relation seems true although no welding was observed in the deposits at Pelee, Soufriere, 

or at Katmai by the early workers there. The present authors have examined material from Katmai 

and have found evidence of slight plasticity and 

possibly very slight local welding of some of the 

shard fragments. 

Although in 1935 there was much yet to be 

learned about welded and nonwelded ash-flow tuffs, 

the outstanding features of these rocks had been 

presented in the above discussed papers on the New 

Zealand ignimbrites, the Brisbane tuff, and the 

welded tuffs of southeastern Idaho. There followed 

a series of papers by several authors describing new 

localities for welded tuffs and presenting detailed 

studies of tuffs from specific areas. 

Fenner (1937) proposed the term "Katmaian 

type of eruption" to apply to eruptions that formed 

deposits having the characteristics of the sand flow 

of the Valley of Ten Thousand Smokes. He suggested that several of the pyroclastic deposits of 

Yellowstone National Park had such an origin. 

Gilbert (1938) described a group of welded tuffs 

from eastern California. These tuffs comprise the 

Bishop tuff that covers 400 square miles and has an 

average thickness of 500 feet. Gilbert's paper is 
14 ASH-FLOW TUFFS 

an excellent treatment of the subject of welded tuffs 

and probably is one of the best in its organization 

and presentation of the facts, both field and petrographic. 

Stearns, Crandall, and Steward (1938) mention 

welded tuffs in the Snake River Plains area, and the 

Mud Lake region, Idaho. Anderson and Russell 

(1939, p. 243-247), in describing the Tertiary formations of northern California, were the first to 

use a recognized ash-flow deposit as a stratigraphic 

marker. The "N omlaki tuff," they believe, has 

"covered an area of at least 2,000 square miles." 

This study is valuable as it points out the tremendous potential use of ash-flow tuffs for stratigraphic 

correlation. Feitler (1940) briefly described glassy 

welded tuffs from Bare Mountain, Nev. Kuno 

(1941) discusses the different characteristics of deposits formed by "pumice flows" and those formed 

by "ejected pumice" as observed in the deposits 

formed by the eruption of the volcano Komagatake 

in 1929. He also compares these pumice-flow deposits with Pleistocene and Recent pumice deposits 

of some of the other Japanese volcanoes and finds 

that they have very similar features. Ross (1941) 

found that the well-known "thunder eggs" ( chalcedony-filled spherulites) from central Oregon 

formed in a welded tuff (figs. 67-69). 

Williams (1941a) in his excellent paper on calderas discusses welded and nonwelded ash-flow 

tuffs and their common association with collapse 

calderas of the Krakatau type. In his study of the 

geology of Crater Lake National Park, Williams 

(1942) describes both welded and nonwelded ashflow tuffs and discusses their origin from "glowing 

avalanches." Westerveld (1942) describes "welded 

rhyolitic tuffs" from South Sumatra and discusses 

their origin, chemistry, and similarity to similar 

deposits in other parts of the world (figs. 16 and 22). 

Matumoto (1943) describes in some detail the 

so-called Aso lava which forms vast deposits around 

the Aso caldera. He says, "* * * the name Aso 

lava was originally given to the obsidian-bearing, 

piperno-like, agglomeratic, eutaxitic, or welded mud 

lava found along the top of the somma." In an 

elaborate classification, he subdivides the Aso lava 

into several types. The paper is well illustrated 

with photographs of polished specimens of the Aso 

lavas and photomicrographs of thin sections (fig. 

81). Bonorino (1944) describes andesitic welded 

tuffs from Argentina (figs. 83 and 84). 

Zavaritsky (1946, 1947) adequately explains 

many hitherto anomalous features of the Armenian 

"tuff lavas" as compatible in origin with an eruption 

of Katmai type. Following Fenner's usage, Zavaritsky (1947) classifies as deposits of the Katmai type 

several of the better known ash-flow deposits. His 

list includes the New Zealand ignimbrites, Bishop 

tuff of Gilbert in California, Crater Lake welded 

tuffs and pumice flows, Japanese "Aso lavas" and 

the classical Italian piperno. Following a review of 

the main features of the above-named deposits he 

describes the principal physical features of the 

Armenian tuffs and "tuff lavas" (figs. 79 and 80). 

Westerveld (1947) concludes that the vast deposits of "acid" volcanic rocks around Lake Toba, 

Sumatra are welded rhyolitic tuffs (figs. 26, 27, 39, 

and 65). These rocks cover 20,000 to 30,000 square 

kilometers, and have an estimated volume of 1,500 

to 2,000 cubic kilometers. 

Fenner (1948a), in describing the "incandescent 

tuff flow" deposits in the Arequipa region, southern 

Peru, concludes that these materials have had an 

origin similar to the tuff deposit of the Valley of 

Ten Thousand Smokes, Alaska. The areal extent of 

these deposits is not known, but Fenner believes 

that these deposits rank high among the world's 

greatest known examples of "incandescent tuff." 

Fenner found that much of this tuff is composed of 

incoherent constituents and his studies indicated 

that the indurated varieties were due to recrystallization of shards and pumice fragments and the 

growth of secondary vapor-phase minerals. He 

thought that welding was not an important process 

of induration in these particular rocks (see p. 8, 28 

of this report). He thus decided that the term 

"welded tuff" was not satisfactory as a name for 

these materials and instead used the locally applied 

name "sillar" (figs. 24 and 25). 

Boyd and Kennedy (1951) were the first workers 

to approach the problem of welding in tuffs from a 

purely experimental standpoint. They found that 

by heating pulverized volcanic glass in a bomb they 

could effect welding. From this they reached conclusions as to the temperature of formation of 

welded tuffs (see p. 42 of this report) . 

Other noteworthy occurrences of welded tuffs that 

have been described or briefly mentioned in the 

literature in recent years are as follows: 

Recent descriptions of welded tuffs 

Location R eference 

United States: 

Little Hatchet Mountains, 

N .Mex ____ _____ ________________________ Lasky ( 194 7) . 
HISTORIC ERUPTIONS OF ASH-FLOW TUFFS 15 

Recent descriptions of welded tuffs-Continued 

Location Reference 

United States-Continued 

Iron Springs district, Mackin and Nelson (1950) ; 

Utah 1----------------------------------- Mackin (1952). 

Rattlesnake formation, 

Oregon 2 _____________________________ Wilkinson (1950). 

Lewis and Clark County, 

Mont ___ ______________________________ Barksdale (1951). 

Sierra Nevada, Calif _______ ____ _ Hudson (1951). 

Yellowstone National 

Park, Wyo __________________________ Boyd (1954). 

Lake Valley quadrangle, 

southwestern New 

Mexico _________________________________ Jicha ( 19 54) . 

Blake Range, New Mexico_Kuellmer (1954). 

Chiricahua Mountains, 

Ariz ______________________________ ______ Enlows ( 1955). 

Basement rocks of Texas 

and southeastern New 

Mexico _______________________________ Fla wn ( 19 56) . 

Dragoon Mountains, Ariz ____ Gilluly (1956). 

Other countries: 

Near Parra!, Chihuahua1 

Mexico _______________________________ ___ Wilson and Rocha (1948). 

Baja California, Mexico ______ Wilson and Veytia (1949). 

Islands of the Gulf of 

California and neighboring land areas, 

Mexico ______ ___ ___________________ ______ Anderson ( 1950). 

H unter-Karuah district, 

New South Wales _____ ______ Osborne (1950). 

Sikhote-alin Mountains, 

eastern Siberia ___ ______________ Solovev ( 1950). 

Costa Rica 3 ____________ ___ _____________ Williams (1952). 

Eastern Iceland~----------------- Dearnly (1954); Tryggvason 

and White (1955). 

English Lake district, 

northern England ____ __ ____ Oliver (1954). 

Balsam Chain, Salvador ______ Weyl (1954). 

Precambrian of southern 

Morocco 5-----------------------Bouladon and J ouravsky 

( 1954, 1955). 

Asia Minor ___________________________ Westerveld ( 1955). 

Salvador _____________________________ Williams and Meyer-Abich 

(1955). 

Northern Japan ___________________ Ishikawa and Min a to ( 1955) ; 

Yagi (1956). 

Corsica _______________ _______ _______________ Bodenhausen. 

Oita Prefecture, Japan ____ __ Ishii and others (1956). 

Peru ------------------------------------Jenks and Goldich ( 1956) . 

Japan -------------------------------------- Ma tumoto and others ( 1956) . Sudbury, Canada __________________ Thompson and Williams 

(1956). 

Precambrian of Sweden 6 ____ Hjelmqvist (1956). 

1 Illustrated in figures 32, 33, 40, and 41 of this report. 2 Illustr ated in figure 29 of this report. 

a Illustrated in figures 85 and 86 of this report. 

4 Illustrated in figure 19 of this report. 

5 Illustrated in figure 98 of this report. 

6 Illustrated in figure 78 of this report. 

HISTORIC ERUPTIONS OF ASH-FLOW TUFFS 

A study of the mechanisms that produce ash-flow 

tuffs must begin with the available records of direct 

observations of volcanic eruptions. These eruptions 

are rare and minor events compared with those in 

some earlier geologic periods, however, such eruptions assume great importance, and the following 

section will present some of the conclusions of the 

few geologists who have had the good fortune to 

make direct observations. 

PELEE 

The great eruption at Pelee on May 8, 1902, on 

the Island of Martinique, is recognized as having 

initiated an understanding of nm~es ardentes and 

the related dispersal of pyroclastic materials. The 

great initial eruption was not directly observed by 

volcanologists but a few of them promptly began 

investigations and gathered valuable information 

from residents, and the long continued observations 

on later eruptions built up an outstanding fund of 

information. Anderson and Flett, Lacroix, and 

Perret are those who contributed most to an understanding of the eruptions at Pelee. 

In discussing the factors that give nuees ardentes 

the properties of flowing, Anderson and Flett (1903, 

p. 508-509) say, 

As this turbulent mixture of expanding gases and fine dust 

pours down the surface of the mountain, the small solid 

grains are unable at first to rest on the ground, even when 

they may have sunk to the base of the cloud, and they are 

swept up again and borne along till they reach some sheltered hollow or the violence of the expansive force lessens 

and the turmoil diminishes. 

The fundamental question remains to be discussed-what 

is the source of the energy which drives the cloud along? 

To this we believe there is only one answer-the motive 

power is supplied by the weight of the mass. It is in a condition comparable to that of a heavy and mobile fluid which 

has been elevated by the volcanic forces and poised on the 

edge of the crater and proceeds to flow downward in obedience to the law of gravitation. 

The foregoing interpretation of ash-flow mechanisms is evidently a synthesis of observations of the 

deposits at Soufriere, and, in particular, the later 

and lesser eruptions at Pelee. 

The nuees ardentes eruptions at Pelee were described in great detail by Lacroix (quoted on p. 6 

of this report). Lacroix (1904, p. 354) granted 

the effect of gravity, but emphasized the great importance of an initial eruption and the violent projection of the debris that resulted from this explosion. The directed effect of the blast was the 

result of its coming from the base of the incipient 
16 ASH-FLOW TUFFS 

spine, whose influence has been discussed by MacGregor (1952, p. 54). 

Continuous observations of eruptions at Pelee 

were made by Perret from 1929 to 1932, and resulted in valuable contributions to the understanding of volcanism and, in particular, nuees ardentes 

of which he observed several hundred examples 

(fig. 2) . Perret (1937, p. 22-23) says, 

First of all it should be realized that the horizontal movement, three kilometers in three minutes, is due to an avalanche of an exceedingly dense mass of hot, highly gascharged and constantly gas emitting, fragmental lava, much 

of it finely divided, extraordinarily mobile, and practically 

frictionless, because each particle is separated from its 

neighbors by a cushion of compressed gas. For this reason 

too its onward rush is almost noiseless. To the end of its 

course the 'auto-explosivity' continues * * *. 

Elsewhere Perret (1937, p. 87) says, 

At the moment of explosion liquid mas.ses of lava, instead 

of being hurled high into the air to form bombs are converted more or less completely into ash by the rapid discharge of gas * * * 

VALLEY OF TEN THOUSAND SMOKES 

An important advance in our understanding of 

the eruptive processes that give rise to ash flows, 

was a result of studies in the Katmai region of 

Alaska, where the eruptions more nearly resembled 

the great ones of the past, than did those at Pelee. 

However, this eruption occurred in June 1912, and 

the area was first visited by Robert Griggs in 1915; 

thus no one directly observed the "sand flow" of 

that eruption. However, significant relations were 

observable and some of the later phenomena of the 

eruption were still active. 

A series of papers resulting from several successive expeditions to the Katmai region of Alaska, 

under the auspices of the National Geographic 

Society, by Griggs, and by Fenner and Zies of the 

Carnegie Institution of Washington Geophysical 

Laboratory, constitute outstanding contributions to 

the understanding of ash flows. 

In discussing the relationship Griggs (1922, p. 

253-254) says, 

Surrounded as it is by high and rugged mountains, the most 

striking feature of the conformation of the Valley of Ten 

Thousand Smokes is the flatness of its floor. One could ride 

a bicycle for miles along its smooth surface * * *. Altogether it occupies an area of 53 square miles (137 sq 

km) * * *. The bulk of the deposit is composed of fine 

fragments, many of them dust-like, but there are included 

numerous lumps of pumice, which in places make up a considerable fraction of the whole. There is no trace of stratification of the materials, except where they were obviously 

subject to secondary readjustments after deposition. 

In discussing the "sand flow" Fenner (1923, p. 

72) says, 

I would, therefore, stress the remarkable character imparted 

to the dust and gas mixture by a continuous evolution of 

gas, which must not only have eliminated almost completely 

the contact friction of the particles, but also have tended to 

drive them apart somewhat forcibly and caused the mass to 

spread almost as freely as a true liquid. 

Naturally, gravity would act upon a mixture that was 

undergoing the assumed reactions just as it would act upon 

a liquid and would direct its course down the slope of the 

Valley, but the important fact to be emphasized is the manner in which internal friction of the mass was eliminated, 

and it seems to me that only in some way as that suggested 

would it be possible for this to be accomplished. 

COTOPAXI 

Fenner (1923, p. 65) pointed out that although 

the mechanism of an ash flow was not understood 

at that time, Wolf has given an excellent description 

of such a flow in his discussion of an eruption of 

Cotopaxi, Ecuador. 

In the discussion Wolf (1878, p. 131-132) says, 

About 10 o'clock in the morning, while strong subterranean 

detonations were heard * * * the crater of Cotopaxi boiled 

over [tibersprudelte] with fluid, glowing lava, and this precipitated itself with furious velocity down the declivities of 

the cone. I chose the words 'boiled over' intentionally, because they indicated best the manner and form in which the 

effusion of lava occurred in this extraordinary outbreak 

'

1

' * * the mountain as it suddenly came to effervescence 

[ ebullicion] and threw out a 'black mass' smoking and 

steaming over all parts of the crater edge simultaneously * * *. It is indeed one of the singular features of 

this eruption that the lava poured out of the crater not in 

one or several streams, but symmetrically in all directions, 

over its lowest edge as over its highest points. On that account the floods were so general around the mountain * * *. 

Very many phenomena indicate that the new lava must have 

possessed a very high temperature at its exit from the 

crater and must have been almost as fluid as water. Its 

expulsion occurred suddenly with a fearful welling-up of the 

fluid, glowing masses; for only thus is it explicable that in 

a quarter, or at most, a half an hour, such a fabulous 

amount of lava was delivered * * * and that it flowed out 

over the highest crater edges like the foam from a boiling 

over rice pot. 

One of the significant observations by Wolf is the 

"boiling over like foam." Not knowing the identity 

or fluidity of an ash flow, Wolf believed that excessively high temperature was necessary. 

SOURCE VENTS FOR ASH-FLOW TUFFS 

No eruption of ash flows, that has produced 

welded tuffs, has ever been directly observed. Thus 

the nature of volcanic vents that have erupted ash 

flows in which welding has occurred has long been 

a subject of speculation. Lacking direct observa-
SOURCE VENTS FOR ASH-FLOW TUFFS 17 

tion, analogy must be drawn among the observed 

eruptions of nonwelded ash flows, pumice flows, and 

other types of deposits formed by certain historic 

outbursts of volcanoes such as Pelee, Soufriere, 

Komagatake, Merapi, and more recently Lamington 

and Hibok-Hibok. These are the nuee ardente, 

Pelean, ladu, glowing avalanche, and other deposits 

of various writers. 

The vents that gave rise to these historic eruptions are immediately divisible into two types, those 

characterized by open craters, and those superimposed on volcanic domes. Eruptions from domes 

seem to follow more than one pattern. The specific 

characteristics and origins of these eruptions have 

been a subject for controversy, and they have been 

discussed and classified by several authors: Lacroix 

(1930), Escher (1933a, b), Kemmerling 1(1932), 

Grandjean (1931), MacGregor (1946, 1952), van 

Bemmelen (1949), and Weyl (1954b). 

A lengthy discussion of these difference, is not 

pertinent to the main topic of this report. Briefly, 

they concern the directed-blast hypothesis of Lacroix, the simple, as well as explosive, avalanching 

of the sides of the volcanic dome, and uptions from fissures and subsidiary craters in the dome. 

These processes all seem to produce the sb-called 

nuee ardente phenomenon, but the pyroclastjc products vary greatly in size range and com~osition. 

The products of greatest significance to students of 

ash-flow tuffs are those that consist primarily of 

new magma. This condition is met in certain eruptions from the domes and, to a greater delree, in 

eruptions from open craters. Deposits form~d from 

dome eruptions seem to be restricted in volume and 

distribution, the latter being dependent on the location of the vent or vents. Eruptions frofu open 

craters, being less restricted, tend to prodbce deposits with a more symmetrical distribution !around 

the crater, with probably greater volume and 

greater areal extent. I 

As mentioned above, eruptions of both types have 

been observed and others have taken place \ in historic time, but in no known instance has eit~er type 

produced a welded ash-flow tuff. Therefore vents 

that produced deposits of welded ash-flow tuff remain to be discussed. 

Small deposits of welded tuff (covering I a few 

square miles) are probably not uncommon and they 

may have been derived from vents such as were discussed above. A small deposit of welded *hyolite 

tuff (less than 10 square miles) in the Valle~ Mountains, N.Mex., probably came from a small crater 

that was later filled and covered by a large rhyolite 

flow. However, a dominant proportion of welded 

tuffs are found in ash-flow sheets of great areal 

extent. The great extent and tremendous volume of 

some of these deposits are incompatible with eruptions from single domes or single craters. A third 

type of vent, the fissure type, offers an adequate 

explanation. Fissure vents have been assumed by 

van Bemmelen (1949), Westerveld (1952), MacGregor (1952), and others. 

The field evidence for fissures as a source for 

ash-flow tuffs is meager and indirect evidence is 

most suggestive. 

Williams (1952, p. 155) states, "There is no way 

of telling whether the voluminous Costa Rican avalanches issued from the summit vents of Poas and 

Barba or from :fissures on the flanks." 

Wilbur Burbank, of the U.S. Geological Survey, 

believes the rhyolite dikes he studied in the Independence Pass region of Colorado were feeders 

for the extensive welded tuffs closely associated with 

them. A suite of these rocks made available by 

Burbank to the authors was studied, and a thin 

section cut from one of the dikes is illustrated in 

figure 37. This shows intricate flow structures 

sweeping around the abundant phenocrysts. There 

is no sign of vesiculation or of the collapse of bubble 

fractures. 

Other evidence of the existence of fissure feeders 

is less direct, but still convincing. This indirect 

evidence is found in the large collapse structures 

so commonly associated with extensive pyroclastic 

deposits, especially ash-flow tuffs. The large size 

of some of these deposits precludes their origin from 

a single vent ; they were probably erupted from a 

series of vents or from fissures. The linear characteristics of some of these collapse structures clearly 

indicate control by regional faults or rift zones and 

strongly suggest that the ash flows were erupted 

along these linear zones of weakness. 

The relation between voluminous pyroclastic eruptions and certain collapse calderas has been discussed by Williams (1941a) and van Bemmelen 

(1939, 1949). 

Some welded--tuff deposits, especially the more 

andesitic ones, are traceable to definite volcanic 

centers, but here their specific point of emission is 

generally obscured by a collapse caldera. Where 

the caldera is smaller the eruptions probably came 

from a single crater, whereas in a larger more complex caldera the eruptions came from a group of 

craters or fissures, possibly ring fractures. In the 
18 ASH-FLOW TUFFS 

Valles Mountains, N.Mex., the welded tuffs came 

from an area now occupied by a large caldera 14 

miles in diameter. Within this caldera there is a 

ring of rhyolitic centers about 8 miles in diameter 

that formed after the caldera. This arrangement 

of centers within the area of caldera collapse surely 

represents a ring fracture and the eruptions that 

produced the welded-tuff deposits may have been 

localized along the same fracture. A similar origin 

from a ring dike was postulated for the rhyolitic 

pyroclastic rocks on the Island of Mull (Richey and 

Thomas, 1930, p. 370). 

In the Western United States and Mexico, especially in the Basin and Range province, there are 

vast deposits of welded tuffs that have not been 

correlated with any known centers. These deposits 

occupy very large areas on the ranges but are covered with immense thicknesses of fill in the intervening trenches, and offer many challenging problems. 

RECOGNITION OF ASH-FLOW TUFFS 

The recognition of ash-flow tuffs has long presented a difficult problem because of a lack of knowledge concerning their mode of deposition and the 

reliability and limitations of field and laboratory 

criteria. As has been discussed on page 13, a real 

understanding of the geologic relations of these 

rocks did not come until the early and middle 1930's, 

resulting from the correlation of field and laboratory 

studies with information gathered from earlier observations of significant volcanic eruptions. Although more and more geologists are becoming 

familiar with these rocks, and the actual existence 

of such rocks is recognized by most geologists, many 

workers are still confronted with problems of recognition. The discussions in the following pages together with the illustrations may help clarify some, 

if not most, of these problems. 

Nonwelded ash-flow tuffs are often confused with 

tuffs of other origins and welded tuffs are often confused with lava flows. This is especially true of the 

devitrified tuffs. The ash flows of a more heterogeneous nature may be confused with lahars and 

transitions may exist between one and the other. 

Normal vitric tuffs are sometimes fused in contact 

with lava flows or shallow intrusions and these may 

be mistaken for welded ash-flow tuffs. 

The following section is divided into two main 

parts. The first part pertains to field characteristics or those features of ash-flow tuffs readily 

discernible in the field. The second part is a treatment of microscopic characteristics. 

The sections on field characteristics, microscopic 

characteristics, and the one on physical chemistry 

must all necessarily involve a certain amount of 

recapitulation, but an effort has been made to keep 

this at a minimum. 

FIELD CHARACTERISTICS 

PYROCLASTIC CHARACTER 

The pyroclastic nature of ash-flow tuffs is generally discernible in the field. In unaltered nonwelded tuffs there is no problem because the 

presence of unconsolidated pumice fragments, ash, 

volcanic dust, and the ubiquitous foreign rock fragments would indicate pyroclastic origin for any 

tuff. These features are discernible through milder 

types of induration and alteration, and become obscure only when advanced stages of welding and 

devitrification have been reached. However, even 

in advanced and extreme examples of welding and 

de vitrification a careful search will generally reveal 

rock fragments which, even if not diagnostic in 

themselves, should cast doubt on the rock being a 

lava, and together with other features to be discussed, should give the correct answer. 

The most important single criterion for recognition of the pyroclastic nature of ash-flow tuffs in 

the field seems to be the presence of pumice fragments. These pumice fragments occur in nearly 

all ash-flow tuffs and commonly persist in some 

form through extreme conditions of welding and 

vapor-phase mineralization, and are generally obscured only when extreme devitrification follows 

extreme welding. In very fine grained tuffs the 

pumice fragments are too small or too few to be of 

much aid in field studies and other field or microscopic criteria must be used. A few crystal-rich 

tuffs also show few pumice fragments. Success in 

the use of pumice fragments as a criterion for pyroclastic character of a welded tuff depends on a 

familiarity with the extremely diverse appearance 

of pumice fragments under different conditions of 

welding, devitrification, and vapor-phase crystallization. This knowledge comes with the study of many 

diverse types of welded tuff. 

In some localities all the above mentioned conditions may be observed in a single rock outcrop because of varying degrees of compaction, welding, 

and devitrification in different parts of a single ash 
RECOGNITION OF ASH-FLOW TUFFS 19 

flow. Where this occurs the relation between seemingly different structures becomes obvious, and 

their dissimilarities become understandable. 

Ash flows of the type exemplified by Battleship 

Rock in the Valles Mountains, N.Mex., present an 

opportunity for studying pumice relations under 

varying conditions of welding and compaction. 

Here a single sheet is nonwelded at the base and 

top, whereas a central part shows marked welding. 

The character of the tuff scarp is shown in figure 

3; the highly pumiceous character of a hand specimen from near the base of the unit is shown in figure 42; and a thin section of the same material is 

shown in figure 43. A hand specimen of a thoroughly welded part of the same unit is shown in 

figure 1. Here the pumice fragments have completely collapsed, and welding has eliminated much 

of their internal structure. 

Were it not for a complete transition from typical 

pumice to a material resembling blebs of glass, the 

origins of the black lenses shown in figure 1 would 

not be recognizable. The characteristics of pumice 

that has undergone several modifications are discussed in some detail on page 33. However, 

nearly all lenticular elements in a welded tuff ( unmetamorphosed) that are observable in the hand 

specimen or the outcrop, probably are, or were, 

pumice fragments. The one exception to this is in 

those rare welded tuffs that have developed lithophysae. lVIost lithophysae are nearly spherical, but 

FIGURE 3.-Scarp locally known a s Battleship Rock in Canon de San 

Diego, 5 miles north of Jemez Springs, Valles Mountains, N.Mex., 

showing columnar structure characteristic of many ash-flow tuffs. A 

single sheet is represented here. The incoherent nonwelded top has 

been removed by erosion and the non welded base is concealed. Near 

the lowest visible part of the scarp (about one-third above the base ) is 

a zone of maximum w elding. The erratic jointing near the base was 

probably due to uncommon cooling conditions as this material descended 

a canyon, whose wall was not far to the left and so presented a lateral 

cooling surface. Mater ial from the nonwelded base of this unit is illustrated in figure 42, and the thoroughly welded central part in figure 1. 

a few may be flattened and thus appear lenticular in 

cross section. 

DEGREE OF SORTING 

A principal characteristic of ash-flow tuffs is their 

common occurrence in thick units (tens of feet) of 

typically nonsorted or nonbedded materials. This 

characteristic is in direct contrast to ash-fall tuff deposits of comparable thickness, in which pronounced 

bedding is nearly always present, as shown in figure 4. 

Ash-flow tuffs commonly show a wide range in 

size and relative amounts of constituent materials. 

However, the dominant material is generally ash 

or fine ash size ( < 4 mm) although some types are 

composed predominantly of pumice lapilli or blocks 

of different sizes, and for these the term "pumice 

flow" may be more suitable than ash flow. Dust 

or fine ash-size material is nearly always present. 

All gradations exist between those deposits that 

consist primarily of ash and dust and those that are 

predominantly pumice lapilli or blocks, although the 

ash-size types seem to be the most common. Included accidental rock fragments, that commonly 

add to the heterogeneous appearance of the ash-flow 

tuffs, may range from microscopic size to large 

boulders, but are most often 1 inch or less in diameter. They ar e generally present in amounts less 

than 5 percent of the whole, but the authors have 

observed as n1uch as 20 percent in some ash-flow 

tuffs and this figure is probably much less than the 

maximum amount. 

FIGURE 4.-Tuff section on the eastern wall of Colle Canon, near its junction with Peralta Canyon, Valles Mountains, N.Mex. The lower part of 

the section is composed of material with the distinct bedding characteristic of ash-fall tuffs. In the ash-flow materials above this bedding is 

absent. The tendency for poorly welded or nonwelded tuff·s to form 

conical erosion forms is shown in the light-colored ash flows. The darkcolored material above is also part of an ash flow. 
20 ASH-FLOW TUFFS 

THICKNESS 

Some ash flows are only a few feet thick, but in 

general they are many feet thick. Thus Macdonald 

and Alcaraz (1956, p. 174-175), in discussing the 

1951 eruptions of Hibok-Hibok, Philippine Islands, 

state that the avalanche materials were 100 to 150 

feet thick on the higher slopes of the mountain, but 

thinned toward the coast, and at Baylao had an average thickness of "60 centimeters." In the Valles 

Mountains, N.Mex., single flows range from a few 

feet to 300 feet or more in thickness. Ash flows 

shown in figures 5 and 6 range from 50 to 200 

feet in thickness. 

FIGUr.E 5.-Welded-tuff scarp in Canon Media Dia, Valles Mountains, 

N.Mex., showing columnar jointing in thoroughly welded tuffs. The 

visible part of the scarp represents the densely welded part of an ashflow unit about 300 feet thick. Unexposed parts, less thoroughly welded 

at the base, would add nearly 150 feet. Note level top. 

FIGURE 6.-Ash-flow tuff scarp be;ow the Puye Pueblo ruins, Valles Mountains, N.Mex. The lower bench represents the thoroughly welded part of 

an ash flow ; the debris-covered slope below represents the poorly welded 

or nonwelded part of the same flow. The upper cliff is the welded part 

of a later ash flow, and below it another poorly welded part of that 

flow. Columnar jointing ne:~.r the top and middle of the section. 

Gilbert (1938, p. 1849) states, "In this section, 

the Bishop tuff is composed of several members. 

Some are less than 100 feet in thickness, while 

others reach nearly 200 feet in thickness." In dis~ 

cussing the thickness of the tuff deposit in the 

Valley of Ten Thousand Smokes, Fenner (1923, 

p. 33) says, "* * * it seems indeed quite probable 

that in the upper valley the tuff may attain a depth 

of several hundred feet in places." The base is 

nowhere exposed in this area. Marshall (1935, p. 6) 

finds that the New Zealand ignimbrites range from 

about 60 to about 500 feet in thickness, but states, 

"* * * in general it appears that formations of the 

ignimbrites are not often thicker than 100 feet." 

Of those welded-tuff deposits visited by the authors, 

single units 200 to 300 feet thick are common and 

units 500 feet thick are probably not rare. In Yellowstone National Park, and the San Juan Moun~ 

tains of Colorado, greater thicknesses can be found. 

The thickness of an ash flow depends on the volume 

of material erupted and the type of topography over 

which it is emplaced; over gentle surfaces it will 

tend to spread laterally and form thinner units. If 

confined to canyons they will flow farther from 

the source than those flowin,g over a plain or plateau. 

If confined to a topographic basin the ash flows will 

no doubt be thicker. The foregoing statements 

assume that the initial volume is the same in each 

situation. There is no reason to doubt that units 

more than 1,000 feet thick may be found. 

LAYERING 

Different zones of single flow units have com~ 

monly undergone various degrees of consolidation, 

some parts remaining unconsolidated, others slightly 

or thoroughly welded. Columnar structures have 

formed in some zones and very often these zones 

show different colors or different shades of the 

same color, with shades of brown and gray occur~ 

ring most frequently. These features tend to give a 

layered appearance to many ash-flow tuff units, 

which is commonly mistaken for the bedding of 

several flows. This effect of "layering" is further 

accentuated by weathering in fresh rocks and, no 

doubt, by metamorphism in older rocks. Many tuffs 

show a case-hardening effect on the surface, due no 

doubt to the release of silica from the ash and its 

redeposition as opal or chalcedony by evaporation 

at the surface. This thin siliceous coating, having 

erratic distribution, causes inequalities in the 

erosion patterns, especially in the nonwelded parts 

of the ash flows. These are vulnerable to wind 

erosion that pockmarks their surfaces, commonly 
"'' 

RECOGNITION OF ASH-FLOW TUFFS 21 

with deep holes. These pitted nonwelded tuffs contrast sharply with the more firmly welded parts 

that react differently to wind and other erosive 

agents. The net effect is to increase the illusion 

of layering. 

In many ash flows, especially those in which the 

welded zone is in the central part of the flow, the 

transition from soft rock to hard rock commonly 

results in the formation of benches or ledges along 

this zone. These benches are often covered with 

rubble that accumulates from falling blocks. The 

harder zone commonly forms vertical cliffs because 

it has developed a more systematic pattern of prismatic joints than the softer underlying material, as 

shown in figure 6. Also, these softer zones may 

support vegetation; thus it is easily seen why these 

contrasting types of material have been mistaken 

for separate flow units and even separate rock 

types. They commonly have a different mineralogy, 

color, texture, joint pattern, and erosion pattern; 

in short, they look different and are so interpreted. 

The transition zone may be surprisingly narrow and 

only a careful examination reveals that it really is 

a transition and not a contact. 

In the immediate vicinity of volcanic vents some 

ash- and pumice-fall tuffs and breccias may tend to 

form thick beds of a heterogeneous or nonsorted 

nature, and these may be confused with tuffs of 

ash-flow origin. However, if it is possible to trace 

these beds laterally from their vents, their true 

nature will become manifest with the gradual appearance of graded bedding. The distance from the 

vent at which bedding will appear will depend on 

many factors and so it is not possible to give an 

arbitrary figure. However, within the limits of 

the authors' experience, bedding will be clearly 

shown within a few miles from a vent and more 

often much closer. Ash-flow tuffs have rarely been 

traced directly to a specific vent so that only under 

uncommon circumstances should vent tuffs be confused with them. 

Volcanic mudflows (lahars) and ash flows have 

not always been distinguished. However, lahars are 

typically nonsorted, but tend to be more heterogeneous in composition. They commonly contain a high 

percentage of rock fragments of boulder and cobble 

size and, although they may be emplaced in a hot 

condition, they rarely show any lasting effects of 

heat. On the contrary, ash-flow tuffs commonly 

show some indication of having been hot, such as 

devitrification, vapor-phase minerals, or welding and 

compaction of pumice shards. 

In general, ash-flow tuffs occur typically without 

bedding within a single flow unit. However, as in 

all other geological phenomena there are exceptions 

to the rule. Many ash-flow units are underlain by 

ash- or pumice-fall beds. Normally there is a clear 

contact between the fall tuffs and the flow tuffs, 

but in others the fall and the flow are transitional 

(fig. 4). The bedding in the ash or pumice becomes 

more and more obscure upward until it merges imperceptibly with the nonsorted flow tuff above. 

Probably the best explanation for this gradual 

transition is that the nonsorted tuff immediately 

above and transitional with the bedded tuff is also 

ash-fall material that was deposited in such large 

volume by eruptions of increasing intensity that 

gravity sorting was inhibited. The ash fall was 

then followed by ash flows of the same composition 

and the contacts between the nonsorted fall tuff and 

the flow tuff are not discernible. Where unconsolidated material is overridden by an ash flow, there 

may be an incorporation of ash-fall material in the 

ash flow. 

Some ash flows incorporate rubble of different 

sorts from the terrain over which they travel. This 

material, especially when large boulders are present, 

may be concentrated in lenses or linear zones within 

the ash flow. These are commonly near the base, 

but may occur well up in the flow and give an impression of bedding. 

A real sorting can take place near the distal ends 

of ash flows and this commonly results in the partial 

separation of pumice fragments and blocks. The 

coarser pumice seems to concentrate on the tops, 

bottoms, and sides of flow units. In the Valles 

Mountains, alternating layers, in single flow units, 

of thin pumice zones and typical nonsorted ash-flow 

tuff have been attributed to the overlapping of lobes 

or surges of an ash flow near its distal end. The 

entire flow near the end of its course was probably 

separating pumice and perhaps crystals. Thus lobes 

of the flow, having top and bottom concentrations of 

pumice and overriding one another, would leave a 

final product showing a distinct bedding. This bedding is traceable in some localities for several miles 

back from the ends of the flows and gradually 

merges with typical nonsorted ash flows. 

Probably a great many ash flows would show 

these features. However, as the ends of the flows 

are in general thinner and have lost more heat 

during emplacement than have the thicker parts of 

the ash sheet, they show less intense welding and 

vapor-phase crystallization or none at all and are 
22 ASH-FLOW TUFFS 

more readily eroded, and thus are less often preserved in older deposits. 

A significant paper on sorting during flowage is 

by Kuno (1941) who contrasts the distinctive features of "pumice flow" and "ejected pumice" deposits, especially those formed by the 1929 eruptions 

of Komagatake, Hokkaido, Japan. These "pumice 

flow" deposits show a definite lenticular banding 

caused by accumulations of larger pumice fragments into longitudinal ridges at the tops of, and 

within the flows, and formed parallel to the direction of flow. Kuno (1941, p. 147) attributes these 

to "* * * differential movements of a number of 

streams within each pumice flow, overlapping one 

another or merging the one into the other." He 

further states, "Obviously, the differential movements are the result of differences in the grain-size 

of the components that formed the pumice flows." 

The pumice flows of Komagatake described by 

Kuno are small compared to many prehistoric ash 

flows. The Osidasi-zawa pumice flow ends 4 kilometers from the crater and reaches a maximum thickness of only 4 meters. However, these pumice flows 

illustrate a process that could well be expected in 

any pyroclastic deposit of similar origin. Any 

differences would be of degree rather than kind and 

would depend on the expectable variables of size 

range, thickness of flow, terrain, distance from 

source, speed of flow, and probably others. 

Another form of overlapping of parts of the same 

ash flow may take place in terrains characterized 

by canyons. Ash erupted from a high mountain 

source may flow down adjoining canyons of different 

lengths. The flow traveling down the shorter canyon reaches its destination first and is overridden 

minutes later by the flow in the longer canyon. 

This process can give a compound effect in the final 

product and may make proper interpretation of the 

welding and vapor-phase crystallization processes 

difficult. Such a process can be demonstrated in the 

Valles Mountains and is probably not uncommon 

where ash flows have been erupted in mountainous 

regions where the valleys or canyons open on a 

common plain or plateau. 

In many ash flows probably some lateral sorting 

occurs, although there is generally no apparent variation except thinning over long distances. In some 

deposits the size of shard and pumice fragments 

decreases away from the source area, but this is 

less conspicuous than might be expected. The lessening momentum of an ash flow near its distal end 

may result in a greater accumulation of foreign 

materials near the base. Similarly crystals may 

concentrate near the base of units; this process has 

been suggested by some writers to explain crystal 

accumulations at the base of some ash-flow tuffs. 

An alternate explanation for some basal accumulation of crystals is well illustrated by an occurrence 

of tuffs in Media Dia Canyon in the southwestern 

slopes of the Valles Mountains, N.Mex. Here in an 

800-foot section of ash flows, one flow has a nonwelded base and overlies another flow from which 

a non welded top has been stripped (locally) by 

erosion. Several feet of tuff, showing a heavy concentration of crystals in a matrix of ash and pumice, 

occurs at the base of the upper flow. This mixture of 

crystals, ash and pumice, contains pumice typical of 

the upper flow and crystals derived from the underlying flow as is clearly shown by the presence of 

large quantities of chatoyant sanidine crystals which 

do not occur in the upper flow but are characteristic 

of the lower flow. Weathering of the tuffs in the 

Valles Mountains commonly produces a pavement of 

crystal fragments on the tops of the exposed surfaces. These accumulations have locally been incorporated in subsequent ash flows. The contact 

between 2 ash-flow tuff units is not always easily 

discernible, especially if the materials of the 2 flows 

are similar. In the specific occurrence described 

above the disconformity was marked by several 

angular blocks of dense welded tuff that were clearly 

derived from the underlying unit and, no doubt, 

littered the surface of that unit prior to the deposition of the upper ash flow. Without these blocks 

to mark it, the contact could easily have been missed 

even with good exposures. This would have been 

especially true had the base of the upper unit been 

welded. Comparable examples must be very common in many occurrences of ash-flow tuff. 

AREAL EXTENT 

The areal extent of an ash-flow unit depends primarily on the volume of ash erupted and type of 

terrain over which it is emplaced. Many workers 

engaged in the study of welded tuffs have described 

their great lateral extent. In some of these areas 

the lack of detailed examination has failed to show 

whether similarity of character represents wide 

extent of groups of flows or of single flows. Some 

ash flows, however, have distinctive characteristics 

which make it possible to identify single flows that 

have spread for long distances. In the Valles Mountains a single flow characterized by uncommonly 

complete welding and abundant chatoyant sanidine 

feldspar phenocrysts is traceable for about 15 miles 

west from the eruptive center. In the same region 
RECOGNITION OF ASH-FLOW TUFFS 23 

other flows are known to have traveled at least 20 

miles from the center of eruption. Griggs (1922, 

p. 253), and Fenner (1937, p. 236) described the 

"sand flow" of the Valley of Ten Thousand Smokes 

as extending for 14 miles with a slope of 2 to 3 

percent. According to Williams (1942) pumice 

flows from Mount Mazama reached a maximum distance of 35 miles from their source. Matumoto 

(1943, p. 4) states that the so-called Aso lava extends as far as 100 kilometers (62 miles) from the 

center of Aso caldera. 

Anderson and Russell (1939) have traced the 

N omlaki tuff member of the Tuscan formation for 

a distance of about 50 miles. The N omlaki tuff 

member is partly welded over a large part of this 

distance, which has allowed correlation of the Tertiary sedimentary section across the northern part 

of the Sacramento Valley in California. This study 

is very significant in pointing out the usefulness of 

ash-flow tuffs in the correlation of geologic sections. 

These distances are probably not excessive for ash 

and pumice flows, and may be exceeded when some 

of the world's great ash-flow sheets are studied in 

detail. 

Less is known about the maximum areal distribution of single ash-flow units, but the quaquaversal 

distribution around a crater or along a fissure, of 

material from a single eruption could conceivably 

cover several thousand square miles. 

The large ash-flow sheet deposits, such as those 

of New Zealand, Sumatra, Asia Minor, and others, 

cover thousands of square miles and are made up of 

many flows from different centers and different 

eruptions. 

The uniformity of these tuff sheets over large 

areas and distances is an important criterion for 

their recognition. This uniformity in both welded 

and nonwelded types is not to be found in either 

ash-fall tuffs or flow rocks of silicic composition 

and are rarely found in flow rocks of intermediate 

types. 

GENTLE DIPS 

Ash flows tend to have very even upper surfaces 

and very low angles of dip except where deformation has modified their original attitude. When 

erupted upon uneven topography the ash flows show 

evidence of having flowed around obstacles and 

down drainage channels. They may therefore have 

a very uneven base and nearly level top in contrast 

to the blanketing of topography as with ash-fall 

tuffs (fig. 7). Neither do normal ash-flow tuffs 

show steep primary dips such as exhibited by many 

lava flows. 

The very even surface of the "sand flow" of the 

Valley of Ten Thousand Smokes has been described 

and illustr ated by Griggs (1922, p. 257). He remarks that a bicycle could have been ridden over 

much of the surface. In discussing the New Zealand ignimbrites Marshall (1935, p. 4) states, 

"Physiographically it may be said that within this 

district the outcrops of the ignimbrite rocks can 

generally be recognized in the field, even before they 

are closely approached. Usually they have a surface 

that is approximately level; * * * ." The even upper 

surface of ash flows has also been described by Gilbert (1938, p. 1837) for the Bishop tuff of California, and by Zavaritsky (1947, p. 11) for ash 

flows of Russian Armenia. They are well shown 

by the Crater Lake "pumice and scoria" flows and 

by the ash-flow tuff deposits of the Valles Mountains, N.Mex. (figs. 6 and 8). The nearly level 

tops have also been observed by the authors in other 

parts of New Mexico, Arizona, Utah, Nevada, Colorado, Oregon, and Wyoming. 

FIGURE 7.-Ash-fiow tuff scarp of Capulin Canyon, Valles Mountains, N.Mex. 

Three ash fl ow units are shown in this figure, the upper two are welded 

tuffs, and t h e lower light-color ed one is a nonwelded tuff. The columnar 

structure commonly developed in welded tuffs is well shown in the two 

upper beds. The level surface is characteristic of the top of an a sh 

flow. 

In deformed rocks an evenness of surface may be 

reflected in the continuity of outcrop over long distances or persistent recurrence of the same bed in 

fault blocks. This even upper surface is complementary to the lack of a scoriaceous surface that 

Marshall (1935, p. 7) cites as an important differ-
24 ASH-FLOW TUFFS 

FIGURE 8.-Ash-flow tuff scarp, Valles Mountains, N.Mex.; illustrates the 

level surface of an ash flow and even bedding in a series of a sh flows. 

The dark upper part is thoroughly welded. The light-colored part is 

nonwelded. The peak in the background is a quartz latite volcano, 

formed before deposition of the tuff. 

ence between the New Zealand ignimbrites and true 

rhyolitic lava flows. 

WELDING AND DEFORMATION OF PUMICE 

Many ash flows contain a zone of welding which 

is easily distinguished in the field. With others it 

may be very difficult or impossible to detect welding 

from field examination alone. The boundary between welded and nonwelded material is always a 

transitional zone of indeterminate character and 

the point at which incipient welding begins cannot 

be located with any high degree of accuracy, even 

though the transition may be only a few feet thick. 

The welded zone may occur at the base of the flow 

or occupy a zone within the flow unit and show 

transitions above and below into unconsolidated 

nonwelded tuffs. The position of the zone of welding depends on several factors that are discussed on 

page 4 7. All ash flows that completed their cooling 

history without burial by other ash flows, or other 

materials, probably had nonwelded tops which may 

or may not have been removed by erosion. However, in some areas the emplacement of flows in 

rapid succession results in a complex of flows welded 

in their entirety, especially near the source areas. 

Compaction and flattening of the glassy components normally accompanies welding, although 

specimens obtained in the transition zones from 

welded to nonwelded material may at times show a 

high degree of welding without obvious deformation 

of the glassy parts (fig. 28). 

The most striking field characteristic that marks 

the transition from nonwelded to welded material 

is the change in appearance of the pumice fragments. Welding and compaction together, without 

devitrification and vapor-phase crystallization, cause 

the pumice to change in shape and color. Fresh 

glassy pumice normally ranges from white or gray 

to some shade of brown, depending on its chemical 

composition. However, during flattening and welding the pumice darkens until in the strongly welded 

zones it becomes black and obsidianlike. These 

black glass lenses have heretofore been commonly 

mistaken for, and referred to as "clots" or "shreds" 

or "pasty glass fragments." They are the "fiamme" 

of the Italian piperno (Zambonini, 1919, p. 66-68; 

Zavaritsky, 1947, p. 12), and the "obsidian spindles" 

of the "Aso lavas" (Matumoto, 1943, p. 7). It is 

this flattening of pumice fragments and other constituents by load compaction that imparts a foliate 

( eutaxitic) structure to most welded tuffs. Viewed 

normal to the plane of foliation the collapsed pumice 

fragments appear as discs or irregularly shaped 

flattened plates; viewed parallel to the plane of 

foliation they are lenticular. The difference in the 

two planes is illustrated in figures 83 and 84. 

Extreme conditions of welding may produce a 

zone of dense obsidianlike glass. This may be 

homogeneous or it may range into a "porphyritic" 

glass with a high percentage of crystals or rock 

fragments, depending on the makeup of the magma 

at the time of its eruption. Examination with a 

hand lens of glasses formed by extreme welding 

will usually reveal their fragmental nature. Even 

in those rocks with the outward appearance of a 

normal, nearly black obsidian, the collapsed pumice 

is commonly a more intense black than the material 

comprising the groundmass shards. This contrast 

is heightened by wetting. The welded tuff from 

southeastern Idaho, illustrated in figure 9, would 

undoubtedly be mistaken for a normal obsidian. 

However, where water or oil is applied to a ground 

or otherwise smooth surface, examination under a 

low-power lens reveals a fine-grained ghostlike 

structure that represents dim, but definitely recognizable, slightly flattened lenses a fraction of a 

millimeter in length. They show a foliation in one 

plane, but no semblance of a flow structure is revealed. That is, the rock dimly retains a very fine 

grained fragmental structure that is more clearly 

revealed by microscopic study (fig. 10). 

The welded zones commonly differ in appearance 

from the nonwelded parts because of differences in 

color, density, porosity, jointing, and the secondary 

effects of weathering and erosion. These differences 

give the appearance of gross bedding within a single 
RECOGNITION OF ASH-FLOW TUFFS 25 

flow, and frequently lead to a misinterpretation of 

the number of flows involved. This is especially 

true in areas }Vhere exposures are poor and the 

rocks have been deformed. In most of the glassy 

welded tuffs studied by the authors, welding to a 

compact glass has not taken place. Instead, the 

2 em 

FIGURE !J.-Photograph of a hand specimen of welded tuff with conchoidal 

fracture, and superficially resembling a typical obsidian. However, a 

polished or smooth wet surface shows a very fine grained, but recognizable, pyroclastic structure when examined un:hr a low-power microscope. Collected by Ross and Smith from the Ammon quadrangle, 

southeastern Idaho. X 1. 

FIGURE 10.-Photomicrograph of thin section of glassy welded tuff illustrated in figure 9. Very complete welding has eliminated all pore space 

and the different shards are molded one against another with moderate 

compression. The shard structure is well retained. Compare the distortion with that shown in figure 36. 

rock has a blotched or streaky appearance due to 

the contrast in color between the darker collapsed 

glassy pumice fragments and the lighter matrix of 

glass shards and crystals (fig. 1). This type of 

glassy welded tuff is the most common and easiest 

to recognize. The size of the pumice fragments 

varies greatly from locality to locality as well as 

within the same rock specimen. 

In most welded tuffs, load compaction and flattening of the shard structure occurs without complete 

elimination of all pore space, but in some tuffs all 

pore space has been eliminated, and distortion may 

stop here, or the pumice and more rarely the shards, 

may be further squeezed and distorted. When this 

happens the pumice and fragments will show as 

greatly flattened and enlarged discs in the plane of 

foliation and as greatly elongated lenses in the other 

plane. The result is a disc or plate with a vertical 

dimension many times smaller than the other two 

dimensions. 

Most pumice fragments have primary shapes that 

range from those that are virtually equidimensional 

to those that may have their short dimension 2 or 3 

times smaller than their long dimension. During 

emplacement of the ash flow there will be a tendency 

for the pumice lapilli or blocks to orient with their 

short dimensions in the vertical plane. These 

pumice fragments may be compressed and pore 

space eliminated during welding by a reduction of 

vertical dimensions of as much as an order of 2 to 

6 times. Deformation during welding added to an 

original flat shape may give an end product that will 

have a vertical dimension perhaps 3 to 20 times 

smaller than the dimensions in the horizontal plane. 

Collapsed pumice fragments having a discordance 

in dimension of this order should be considered 

normal and are probably the result of simple elimination of pore space. However, some welded tuffs 

are characterized by pumice fragments that have 

vertical dimensions ranging from 20 to 60 times 

smaller than their other dimensions. These pumice 

fragments have been flattened beyond the point of 

simple loss of pore space, and there has been a 

stretching of the structure, which may be mistaken 

for the flow lines of a lava flow (figs. 40, 41). 

The deformation or flattening of pumice will result in a higher concentration of crystals per unit 

volume in welded zones than in nonwelded zones. 

In tuffs with a low initial concentration of crystals 

the difference in apparent content of crystals between nonwelded and welded zones will not be 

obvious, but in those tuffs whose initial crystal concentration was high the difference may be striking. 
26 ASH-FLOW TUFFS 

The ratio of pumice fragments to shards will have 

a significant influence on the amount of crystal enrichment per unit volume of tuff in welded zones, 

because of the difference in porosity between the 

pumice fragments and packed shards. Thus other 

factors being equal, the ash flow with the highest 

volume percentage of pumice will have the highest 

potential for crystal concentration per unit volume 

during welding. 

Some densely welded tuffs have been found to 

occur in very thin units of a few feet to a few tens 

of feet in thickness. No detailed studies of such 

rocks have been published. Probably some very 

thin tuffs are products of fusion and are more properly classified with the fused tuffs discussed on page 

26. These thin tuffs are sometimes found at the 

base of a normal welded-tuff unit that has a strongly 

welded bottom where there was enough heat to cause 

fusion of a few inches or a few feet of underlying 

glassy ash or pumice. This material may be either 

of ash-fall or ash-flow origin. The prime requisite 

is, of course, fragmented glass. A 2-foot thick bed 

of this origin has been briefly described by Enlows 

(1955). This process of fusion of underlying ash 

could lead, in some instances, to misinterpretation 

of the structure of an ash-flow tuff deposit. As 

already discussed some ash-flow units have a truly 

nonwelded top and bottom with the zone of maximum welding somewhere above the base. Fusion 

of the top of a nonwelded unit by the overlying 

welded base of another flow of the same general 

appearance could produce a zone of dual origin that 

would appear to be a single flow with a nonwelded 

top and bottom. This process, together with the 

added effects of devitrification and vapor-phase activity, could produce some real complications in a 

sequence of ash-flow tuffs. 

A second type of thin welded-tuff unit is of true 

ash-flow origin; the only reasonable explanation for 

flows of this type seems to be that they were emplaced at temperatures high enough to induce complete welding without load being an important factor in the elimination of pore space. Some examples 

of this type of welded tuff form dense obsidianlike 

glass (fig. 10) without showing the high degree of 

flattening of shards as is commonly shown in the 

basal glass of thick welded-ash flows. Other examples develop lithophysae and spherulites in the 

glass and still others develop gas pockets in pore 

spaces where welding has not been complete. Examples of the latter type are usually devitrified. 

The complete lack, or rarity of phenocrysts is a 

further suggestion of a high temperature of emplacement. 

FUSED TUFFS CONTRASTED WITH WELDED TUFFS 

Rocks identical with or closely resembling welded 

tuffs and of the same composition may be formed by 

fusion under conditions quite different from the 

process which is known as welding. This fusion 

may occur: (a) at the contacts between vitric tuff 

and rhyolitic domes or dikes; (b) in the granulated 

breccias at the base of rhyolitic flows; and (c) in 

the brecciated zones in the glassy parts of flows and 

domes. The first process is common in areas of 

intense rhyolitic or dacitic volcanism. The second 

and third are comparatively unimportant and their 

mode of derivation obvious. 

DEVITRIFICATION AND VAPOR-PHASE MINERALS 

The postdeposition mineral changes that commonly affect ash-flow tuffs are devitrification and 

the development of vapor-phase minerals, which 

are best shown by microscopic studies ( p. 36-38) . 

Devitrification and vapor-phase mineralization 

are distinct episodes, even though they are related 

processes in the cooling history of many ash-flow 

tuffs. The distinction made by the authors is that 

in devitrification the formation of crystals takes 

place within the boundaries of the glass shards or 

glass mass. In vapor-phase crystallization the formation of crystals takes place in open spaces under 

the influence of a vapor phase. 

Devitrification has obliterated the original glassy 

character in most welded tuffs, but in a few the 

pumice fragments have -been devitrified, while the 

groundmass shards have remained glassy. Devitrification imposed on zones of intense welding and 

flattening of the structure results in a rock that 

closely resembles a flow lava. However, silicic flow 

rocks nearly always show flow banding. This means 

that if linear elements (exclusive of horizontal jointing) in unmetamorphosed rock extend over several 

feet, the rock almost surely is not a welded tuff. An 

unusual exception is a pumice-block flow or tuff containing large pumice blocks that have undergone 

extreme flattening and stretching. Even in these 

exceptional occurrences the linear elements are lenslike without great continuity. 

Devitrification as previously defined simply turns 

pumice fragments into crystalline material, and the 

pumice structure may be preserved in its entirety. 

However, the texture of recrystallized pumice fragments is greatly influenced by the degree of welding, 

or the extent of flattening, as there is a tendency 
RECOGNITION OF ASH-FLOW TUFFS 27 

for the pumice vesicle walls to coalesce during welding. The vesicles of the pumice become smaller and 

fewer and this may result in the complete obliteration of pumice structure. The formation of devitrification or vapor-phase minerals will tend to 

further obliterate pumice structure and only ghosts 

of this structure remain. Vapor-phase crystallization occurs in open spaces, and although it may 

happen in any of the pumice fragments, except 

where there has been complete collapse, it is most 

characteristically shown where there has been no 

collapse (fig. 11). Where vapor-phase crystals 

form, the pumice structure may be partly, or in 

some tuffs, wholly destroyed by the growth of discrete crystals and crystal aggregates. These show 

a coarser texture than the products of devitrification. Vapor-phase crystallization commonly results 

in cavities that are lined with or contain mesh works 

of crystals. The cavities may lack direct evidence 

of derivation from pumice areas, and may be mistaken for the lithophysal cavities and the vaporphase crystallization common to flow rocks. The 

presence of vapor-phase minerals in pyroclastic 

rocks is commonly a good criterion for ash-flow 

origin. Some ash flows have retained volatiles during welding, and these may build up vapor pressure 

so that cavities are developed, and in these typical 

lithophysal minerals will be found. 

Spherulites and lithophysae may form in some 

welded tuffs but they are less common than might 

be expected, perhaps because flow rocks tend to 

FIGURE H.-Photograph of ash-flow tuff from 4 miles north of Thumb 

Ranger Station, Yellowstone National Park, collected by Ross and 

Smith. The dark areas represent pumice fragments that have devitrified 

without collapse and in which vapor-phase minerals have formed. 

Tridymite formed in the cell walls giving them a very fragile texture 

that is readily broken down by weathering; this gives the tuff a deeply 

pitted surface. The groundmass is composed of typical shards. 

552858- 61---3 

retain a larger proportion of volatiles than do ashflow materials. True lithophysae do develop in 

some localities in great numbers (figs. 49 and 52) 

where they may so dominate the outcrop that all or 

nearly all the diagnostic features of the welded tuff 

as observed in the field may be obliterated. 

Typical round lithophysae are easily recognized 

and there should be no question as to origin. Hollow 

and expanded spherulites appear much the same as 

they do in normal lava flows and call for no additional discussion. However, in some rare occurrences of welded tuffs, lithophysae become flattened 

and thus more or less lenticular in cross section and 

resemble flattened miarolitic pumice fragments. The 

origin of these cavities may be difficult to determine, 

especially in older rocks or rocks that have undergone alteration or deformation. 

In many welded tuffs where vapor-phase minerals 

have formed in moderately to strongly flattened 

pumice fragments, pumice structure is not readily 

detected in the lenticular cross sections of the flattened pumice fragments. However, if the rock is 

split in the plane of flattening (foliation) the pumice 

fragments appear as discs or irregularly shaped 

plates, and ghost pumice structures are nearly 

always discernible. 

The vapor-phase minerals and minerals formed 

by devit:rification are discussed in detail on pages 

36-38. However, some of these minerals may be 

recognized in hand specimen and outcrop with the 

aid of a hand lens and so may be briefly mentioned 

here. The products of devitrification (as used in 

this report) are preponderantly cristobalite and 

feldspar and are too fine grained to be recognizable 

in the fLeld. The main products of vapor-phase 

crystallization are tridymite, alkalic feldspar, and 

cristobalite. Cristobalite that occurs as tiny white 

balls or crystal rosettes and alkalic feldspar predominate in lithophysae, while tridymite and alkalic 

feldspar predominate in the crystallized pumice and 

other porous areas of partly welded and nonwelded 

ash-flow tuffs. When these minerals are found in 

pyroclastic rocks it is an excellent indication that the 

rocks were of ash-flow origin. Here again exceptions 

may be found, but only rarely in such rocks as the 

fused tu1J:s that were discussed on page 26 and in 

other pyroclastic rocks that have undergone vaporphase alteration originating from sources outside the 

body of rock in question. Such occurrences are not 

common and should be readily resolved in the field. 

Some ash flows come to rest and remain in an 

entirely unconsolidated or nonwelded state. Others 
28 ASH-FLOW TUFFS 

may, depending on a number of factors, be partly 

or wholly welded and still remain in a glassy state. 

Other ash flows may be wholly or partly devitrified 

and may develop vapor-phase minerals within their 

porous parts or develop spherulites or lithophysae 

within their glass zones. Devitrification during 

cooling is usually accompanied by the formation of 

vapor-phase minerals in pore spaces in certain zones 

within the tuff unit. 

Sections of ash-flow tuff units that are welded to 

the base commonly have a basal glass zone. This 

may grade upward through a densely welded and 

devitrified zone to a less welded or nonwelded zone 

that is devitrified and contains vapor-phase minerals. Sections of units that have an unconsolidated 

base grading upward into a densely welded zone, 

that in turn grades upward into a nonwelded top, 

may have 2 zones of vapor-phase minerals, 1 above 

and 1 below the densely welded zone which may be 

either completely or partly devitrified. In thick 

flow units of this type the pumice and shards at the 

top and bottom of the unit (above and below the 

upper and lower zones of vapor-phase minerals respectively) may be unaffected by devitrification and 

vapor-phase activity. In those ash-flow tuffs that 

have developed two vapor-phase zones the lower 

zone is normally thinner than the upper. This fact 

is the result of the vapors having been squeezed out 

during compaction and welding and which tended to 

move upward, but were trapped below the densely 

welded zone and caused alterations similar or identical to those above but within a more restricted area. 

Ash flows that have a high volatile content, but 

without the load or retained heat adequate to cause 

welding, may still devitrify and develop vapor-phase 

minerals. Such ash flows produce deposits as described by Fenner (1948a, p. 884) in the Arequipa 

region, Peru, and which he termed "sillar." Thus 

we find the sillar-type ash flow forming complete 

units, or parts of units, that may also contain glassy 

or devitrified welded zones or both, as well as fresh 

unaltered pumice -and ash. Devitrified densely 

welded zones in many ash flows will be found to 

grade laterally into the sillar vapor-phase mineral 

zone. Examples of this type of transition are very 

common in the Valles Mountains, N.Mex. 

As shown in the foregoing discussion there may 

be a great deal of variety in individual ash-flow tuff 

deposits. When weathering, metamorphism, and 

deformation are superimposed on these rocks their 

complete history may be exceedingly difficult or impossible to unravel. 

JOINTING 

Columnar jointing is a common feature of many 

welded tuffs. It has been described by Marshall 

(1935, p. 4) in the New Zealand tuffs; by Richards 

and Bryan (1934, p. 53) in the Brisbane tuff of 

Queensland, Australia; by Gilbert (1938, p. 1836) 

as being characteristic of the Bishop tuff of California; by Westerveld (1947, p. 19) in the Lake 

Toba region of North Sumatra; by Zavaritsky (1947, 

p. 9) in Russian Armenia, and by several other 

writers in more recent years. Fenner (1948a, pl. 2) 

presents an excellent illustration of jointing in the 

Arequipa region, Peru. 

Columnar structures are common and well developed in the welded tuffs and in those parts of ash 

flows (sillar) indurated by vapor-phase minerals in 

the Valles Mountains, N.Mex. (figs. 5 and 7). Normally they do not occur in the noncrystalline nonwelded parts of the ash-flow units. No detailed 

studies of columnar structures in welded tuffs have 

been published, and there is little or no data on the 

details of spacing and attitude of joints or size of 

individual columns. Observations of the authors in 

the Valles Mountains indicate that joint spacings 

may range from a few inches to many feet. The 

more closely spaced joints are usually found in the 

zones of most intense welding. In any given locality 

the joints seem in general to be uniformly spaced, 

although the spacing may vary greatly over several 

miles of outcrop. Spacing is controlled by several 

factors such as rate of cooling, thickness, degree of 

welding, and others. 

Vertical jointing represents the conspicuous type, 

but departures from the vertical are not rare. Some 

welded tuffs have developed fan jointing, while 

others have distorted vertical joints that give rise 

to bent or warped columns. These features are not 

common and are probably related to local deviations 

in the cooling surface. Fan jointing has been illustrated by Richards and Bryan (1934, pl. 6, fig. 2) in 

the Brisbane tuff of Australia. Fan jointing is well 

shown in the tuff scarp known as Battleship Rock 

in the Valles Mountains, N.Mex. (fig. 3). The 

local relations indicate that this tuff was emplaced 

against canyon walls, and the abnormal plane of 

cooling resulted in the fanlike group of curved 

columns. 

Bent columns are known from several localities in 

the Valles Mountains, N.Mex., and in some of these 

localities their relation to buried topographic highs 

seems well established, but in other localities their 

origin has not been definitely determined. Boden-
RECOGNITION OF ASH-FLOW TUFFS 29 

hausen (1955, p. 47, fig. 10) shows an excellent occurrence of nearly horizontal columnar structure 

on the shore near the Island of Scandola, Corsica. 

An interesting type of jointing is represented by 

an obsidianlike welded tuff from near Taxco, State 

of Guerrero, Mexico, which has been studied by Carl 

Fries, Jr. (written communication, 1958), of the 

U.S. Geological Survey (figs. 12 and 13). This 

glassy layer is 10 to 20 meters thick and extends 

at least 10 kilometers. The jointing has produced 

a large proportion of plates about 1 inch in thickness, and some are as much as 2 feet square. 

FIGURE 12.-Photograph of a specimen collected by Carl Fries, Jr., U.S. 

Geological Survey, from a 10- to 20-meter-thick layer of welded tuff, at 

km 153 of Taxco Highway, State of Guerrero, Mexico. The photograph 

shows the obsidianlike character of this glassy welded tuff. At this 

locality jointing has resulted in a large proportion of platelike slabs 

that are commonly about 1 inch thick and cover as much as 2 square 

feet. For this reason the slabs are quarried and widely used in place 

of tile as flooring material in this part of Mexico. The joint surface 

is represented in the central part of the surface of this figure; the 

opposite joint surface is shown in figure 13. 

Many welded tuffs show a horizontal platy jointing in or near the zone of maximum compaction. 

This platy structure is accentuated by weathering 

and should not be confused with the platy structure 

that is commonly developed by weathering along 

the planes of foliation in zones of partial to complete welding and compaction, and where devitrification has furthered the inequalities of hardness inherent in the eutaxitic structure of these rocks. 

This type of horizontal platy jointing has not been 

studied in detail but it seems to the authors that 

it marks the zone of maximum flattening; however, 

in a few rocks it is best developed a little below that 

FIGURE 13.-The reverse side of the specimen shown in figure 12 where 

there has been weathering along a joint crack that has brought out the 

eutaxitic structure. The lighter colored groundmass shows a finegrained pryoclastic structure. The darker areas represent completely 

collapsed pumice fragments. The pumice character is shown by the 

fraylike ends of many of the pumice areas. 

zone. In some welded tuffs the vertical joints do 

not extend below this zone of horizontal joints. In 

others they continue downward through the horizontal joints to die out at the top of a nonwelded tuff 

that forms the base of the unit. 

Unlike the joint pattern seen in lava flows that 

often consists of the well known 5- or 6- (ideally) 

sided columns, many examples of welded tuffs have 

columns that are roughly to symmetrically rectangular, and some are square. In general these are 

tensional cooling joints, but the reason for the wide 

occurrence of this pattern appearing so commonly 

in welded tuffs is not clear. It has been observed 

in many localities in the Western United States, 

and many illustrations in the literature of welded 

tuffs from other parts of the world indicate that the 

joint pattern exists elsewhere. However, Bodenhausen (1955, p. 47, fig. 11) has illustrated columnar jointing in welded tuffs of Corsica that show 

closely spaced columns, a large proportion of which 

have 5 or 6 sides. 

EROSION FORMS 

The wide variations in texture and physical composition, and different degrees of hardness of ashflow tuffs will obviously produce a diversity of 

erosion forms. A complete discussion of these 

would fill many pages and does not seem justified 

here becam~e of their questionable value as an aid 
30 ASH-FLOW TUFFS 

in the recognition of welded tuffs. Locally a specific 

type of erosion form may aid in the correlation of 

flows, or may be very diagnostic in distinguishing 

ash-flow tuffs from rocks of other origins, but each 

locality must be evaluated by a study of its own 

peculiarities. 

A few specific examples of locally diagnostic 

forms should be cited. In the Valles Mountains, 

N.Mex., certain erosion forms are very characteristic and allow recognition of ash-flow tuffs at 

great distance. Foremost among these are conical 

shaped pinnacles (figs. 4, 14) that are locally 

known as tent rocks because of their resemblance 

to conical tents or tepees. There is a tendency for 

the formation of these in nearly all the nonwelded 

parts of the ash-flow tuff deposits but they are particularly outstanding in two members of the Bandelier formation. The Bandelier formation occupies 

an area of about 400 square miles and at least one 

of the "tent rock" members is present over most of 

this area. Some of the "tent rocks" are cones only 

a few feet high while others are 100 or more feet 

high (fig. 14). There are all gradations in size 

and of varying degrees of perfection in symmetry. 

Some cones are capped by large boulders that were 

rafted from the ground surface traversed by the 

ash flow during its emplacement. Some of these 

boulders weighing as much as several hundred 

pounds tend to retard erosion and produce bouldercapped pinnacles. Not rarely this results in whole 

colonies of pinnacles capped by boulders of varying 

size. 

Another diagnostic feature in the Valles Mountains ash-flow tuffs is a magnified "swiss cheese" 

effect produced by wind erosion. This is common 

in zones of porous poorly welded tuff (fig. 14) and 

nonwelded tuff that has been subjected to vaporphase activity, and to a lesser extent in the unaltered nonwelded zones. In the porous poorly 

welded zones where vapor-phase minerals have 

formed these minerals tend to be weakly coherent 

and thus they are very easily broken and vulnerable 

to the agents of erosion. This is especially true 

where these minerals occupy former pumice fragments. Here erosion forms pits that are further 

enlarged until some reach cavelike proportions. 

Many outcrops of ash-flow tuffs in the Valles Mountains have casehardened surfaces. Erosion may 

start at inequalities in these surfaces especially 

where pumice fragments occur. This gives wind 

and rain access to the poorly consolidated interior, 

and conspicuous cavities develop. Some of these 

cavities were enlarged and used as living quarters 

FIGURE 14.-Scarp in Wildcat Canyon, l lh miles southwest of La Cueva, 

Valles Mountains, N.Mex. This represents the slightly welded to nonwelded part of a single ash flow, about 400 feet thick. The tuff at the 

top of the scarp is poorly welded, and an unknown amount of nonwelded tuff has been eroded from the surface. In nonwelded and poorly 

welded tuffs, erosion localized along vertical joints tends to produce 

conical pinnacles. 

by the early Pueblo Indian inhabitants of the region. 

In a recent paper on the welded tuffs of Chiricahua National Monument, Ariz., Enlows (1955, p. 

1229) says, "The unusual and spectacular rock formations for which the Monument is famous * * * result from weathering guided by well-developed, vertical joints and the coarse eutaxitic structure." 

This type of weathering, which is controlled by the 

foliate or eutaxitic structures of welded 'tuffs, is 

well illustrated in Enlows' paper and is mentioned 

here because it is a common feature of many welded 

tuffs. Weathered eutaxitic foliation may appear 

in some welded tuffs like bedding in sedimentary or 

ash-fall tuffs or like flow banding in some lavas, 

and may make field determinations of some welded 

tuffs open to question. 

A few hard compact devitrified crystal-rich 

welded tuffs may superficially resemble intrusive 

rocks. These welded tuffs are usually dacitic or 

quartz latitic in composition. One occurrence seen 

by the authors in southern Nevada contained a large 

amount of biotite and resembled a granitic rock, 

even to weathering by exfoliation into rounded 

knobs typical of exfoliated granite. 

FOSSIL FUMAROLES 

Williams (1942, p. 86) in his study of the deposits 

of pumice and scoria flows surrounding Crater 

Lake, describes features which he concludes are 

"fossil fumaroles." He says, 
MICROSCOPIC CHARACTERISTICS 31 

Brown, pink, and white streaks cut the gray scoria where the 

gases rose to the surface. Some of the spires are hollow 

inside and have irregular openings at the top. The largest 

of these tubular spires is 8 feet across * * *. On Sand 

Creek, as many as 150 'fossil fumaroles' may be counted in 

a distance of 11-h miles along the canyon walls. 

Cementation of the materials surrounding these 

fumaroles takes place through the deposition of 

"iron oxides, kaolin, and opal." Erosion of the 

deposits may leave the indurated fumaroles standing as "spectacular columns and spires." 

Similar features have not been reported from any 

other ancient ash-flow tuff deposit which seems odd. 

The nature of the ash-flow concept as it is understood at the present time implies that a· deposit of 

this material would give off gases for a long time 

after its emplacement. Just how long may be seen 

from Kozu's (1934) study of the pumice flows from 

the 1929 eruption of Komagatake and from the 

studies of the sand flow of the Valley of Ten Thousand Smokes. These deposits were still giving off 

vapors years after they were formed (see p. 41). 

These gases should leave some trace of their passage through the unconsolidated upper parts of an 

ash-flow deposit. In the Valles Mountains, N.Mex., 

the tops of ash-flow deposits are preserved in many 

localities, but examination of these by the authors 

has failed to reveal "fossil fumaroles" although 

some show mottling. The explanation for the apparent absence of fumaroles is not clear. Because 

nearly all these deposits have a zone which contains 

vapor-phase minerals it is obvious that the vapors 

were there. Normally this zone does not extend to 

the top of the flow unit. The top still contains 

glassy ash and pumice and is, for the most part, 

still friable. It is in this part of the flow that the 

fumaroles should occur. It is all the more peculiar 

that fumaroles are not apparent in the Valles Mountains deposits when it is considered that the vapor 

phase was more effective here in converting glass 

into crystalline materials than in the Crater Lake 

deposits. There was apparently very little devitrification and growth of vapor-phase minerals in the 

Crater Lake tuffs. This fact plus lack of welding 

would indicate a much lower temperature of emplacement and probably a lower volatile content 

than the Valles Mountains deposits. 

Williams found that the signs of fumarolic activity disappeared about 10 miles from the summit 

of the former Mount Mazama. Thus distance from 

source might explain the absence of visible signs 

of fumaroles in other tuff deposits, but this should 

only be true in those that show no other evidence 

of vapor-phase activity. Many ash-flow tuff deposits show signs of this activity throughout their 

entire outcrop area, and this may be 20 miles or 

more. 

Fumaroles in more mafic ash flows should be more 

colorful because of the higher iron content yielding 

more iron oxide as a staining or cementing agent. 

If more colorful they would be more obvious. This 

might, in part, account for the differences between 

the Crater Lake flows and the Valles Mountains 

flows, as the Valles Mountains ash-flow tuffs contain over 20 percent more silica than the "scoria 

flows" of Crater Lake and about 10 percent more 

silica than the dacitic pumice flows. Nearly all the 

Valles Mountains ash-flow tuffs contain less than 2 

percent total iron, and some contain less than 11/2 

percent, while the Crater Lake rocks range from 

21;2 to more than 7 percent total iron. That is, the 

iron is inversely proportional to the silica. However, even if iron-oxide coloration was less obvious, 

fumarolic activity in the tops of the Valles Mountains ash flows should have caused induration because of the release and redeposition of silica and 

perhaps the development of clay minerals. The only 

explanation that seems to be left is that the difference in age of the deposits may be great enough to 

have allowed changes in the appearance of the Valles 

Mountains ash-:-flow tuffs, obscuring features that 

were never too obvious in the first place. This age 

difference is not very great as the Valles Mountains 

ash flows were formed in middle or late Pleistocene 

time and are so fresh in appearance that some of 

them could not be distinguished in hand specimens 

from Katmai "sand flow" material. 

In summary, the "fossil fumaroles" can be reasonably expected to occur in the upper nonwelded 

parts of ash-flow tuffs, but they have not been reported from any prehistoric deposits of these rocks 

except the Crater Lake dacitic pumice and mafic 

scoria flows. Why they have not been found in 

relatively young deposits that were certainly potential producers of such features is not clear. It may 

be that in rhyolitic ash-flow tuffs these fumaroles 

were never very obvious features and that only very 

careful scrutiny of prehistoric deposits will reveal 

their presence, if any trace remains. 

MICROSCOPIC CHARACTERISTICS 

The field characteristics of ash-flow tuffs have 

been presented in the preceding section and are 

usually adequate for identification. The characteristics observable under the microscope are in gen-
32 ASH-FLOW TUFFS 

eral also an adequate rneans of identification, but 

some ash-flow tuffs are best studied by coordination 

of both methods. The results of microscopic studies 

will be presented in this section and coordinated 

with field studies. Characteristics observable under 

the microscope also present important information 

about genetic history and must be considered in 

connection with the following section on physical 

chemistry. 

The various physical characteristics of ash-flow 

materials are shown in the illustrations, and discussion in the text will constantly refer to these. However, to minimize so far as possible the need for 

readers turning from the illustrations to text, detailed descriptions have been prepared to accompany the figures. 

Each of the genetic episodes-formation of a 

glassy magma, vesiculation, eruption, explosive disruption, transportation, emplacement, distortion, 

degrees of welding, devitrification, and vapor-phase 

mineralization-have imposed their distinctive effects on the resulting tuffs. The geologist may be 

called upon to differentiate and evaluate these effects 

and, so far as possible, to determine the genetic 

history presented. The following section describes 

these characteristics in some detail, emphasizes 

those which seem most significant for purposes of 

identification, and discusses the situations where 

ambiguities may arise. 

PYROCLASTIC CHARACTER 

The origin of both ash-fall and ash-flow materials 

through the vesiculation and explosive disruption of 

glass has given these materials physical forms that 

are not greatly influenced by the chemical differences in the magma from which they are formed. 

They vary within certain limits in successive eruptions, even from the same centers, but otherwise 

seem to be basically the same from region to region 

and have not varied through geologic time. The 

same shard forms seem to characterize both ashflow and ash-fall tuffs. However, the subsequent 

history of these materials is so varied that distinctive differences are developed. The most important 

of these are the absence of sorting, and the presence 

of welding and devitrification in ash-flow tuffs. 

Tuff fragments vary in physical form, and these 

may, for convenience, be divided into types, but 

there are transitions from one type to another. 

One widely occurring type is derived from a vesiculated glass characterized by . roughly globular 

bubbles as shown in figure 18. This material when 

explosively disrupted produces curved plates, as 

shown in figure 16, that represent fragments of the 

walls of these bubbles; cusp- and lune-shaped fragments as shown in figure 22; and different forms 

representing the interstitial glass between several 

bubbles. These may be tricuspidate fragments 

bounded by arcs of circles, Y-shaped fragments 

formed in the same way and illustrated in figure 20, 

and which seem to be one of the most common characteristics of glassy tuff materials. More rarely, 

forms which represent cross sections of undisrupted 

spherical bubbles have been observed (figs. 18 and 

29). The original plastic glass contained bubbles 

of different size, shape, and thickness of walls, and 

so there are many variations to the forms mentioned. 

A slight modification of the type of ash derived 

from globular bubbles was the result of fracture of 

the walls of elongate bubbles which gave a large 

proportion of more nearly flat, but commonly 

slightly curved plates (figs. 17, 20, and 28), among 

the resulting shards. With these shards are 

U-shaped or much-elongated Y-shaped forms. 

The pumice type ordinarily occurs with other 

types, and more rarely characterizes an entire occurrence. This material is made up of a fine aggregate of cells that are commonly tubular, but 

rarely are nearly circular. Pumice tuffs are illustrated in figures 1, 11-13, and 42-48. 

During any single eruption the chemical cornposition of the parent magma and its gas content 

was about uniform; and at the time of vesiculation, 

the temperature and the viscosity of the glass varied 

but little. For these reasons, eruption and vesiculation of the magma tended to produce shards showing marked uniformity during a single episode, except for associated dust and the pumice fragments 

to be discussed later. On the other hand, successive 

eruptions even from the same vent may differ 

greatly in the types of the shards produced. 

Some ash flows are made up of nearly uniformly 

sized shards, as illustrated in figures 28 and 71. 

More frequently there is a range in dimensions of 

a hundredfold or more. In other occurrences ash 

shards are enclosed in a matrix of dustlike materials 

as illustrated in figures 24 and 25. In some flows 

this dustlike material is no doubt due to attrition 

during flowage. However, in some tuffs delicate 

cusplike shards and bubble walls have not been 

marred during flowage, even where enclosed in 

dustlike material as shown in figure 18. The transportation of the shards while enveloped in a film of 

gas would tend to isolate each shard from its neighbors, and reduce attrition to a minimum. More-
MICROSCOPIC CHARACTERISTICS 33 

over the shards were at a high temperature, and 

were still viscous, although viscosity was normally 

very high. For this reason the hot shards were very 

much less fragile than they would have been if cold. 

Rhyolitic glass has such a high viscosity that it may 

undergo either plastic yield or fracture, depending 

on the physical conditions. 

PUMICE FRAGMENTS 

The very common occurrence of pumice fragments 

as observable in the field has been described in the 

preceding section but studieB under the microscope 

show the presence of pumice fragments at least of 

ash size in almost all ash-flow tuffs examined. Some 

flows are made up dominantly or wholly of pumice, 

and are best described as pumice flows. The character and modifications of pumice are most clearly 

observable where it occurs as fragments large 

enough to be studied in the field, and so conclusions 

based on field studies need not be repeated here. 

However, some relations are clarified by microscopic 

studies and other significant ones are revealed. 

The pumice fragments which are normally associated with shard materials, and which they 

greatly exceed in size, are illlustrated in figures 42-

48. Most commonly pumice fragments are made 

up of greatly elongated tubular pore spaces which 

give the specimen a fibrous structure, but more 

rarely they are made up of nearly equidimensional 

roughly spherical bubbles. Very porous pumice 

fragments are illustrated in figures 42-44. 

Pumice fragments in which there has been no 

collapse are shown in figures 42 and 43. The collapse of the cell structure in glassy pumice is illustrated in figures 45-48, and in figure 13. Crumpling 

and collapse of pumice lying across the plane of 

compaction and pumice compressed against phenocrysts are shown in figures ·i 7 and 48. There may 

also be collapse of the pumice and such thorough 

rewelding that all internal evidence of pumice structure is lost as illustrated in figure 45. In the hand 

specimen collapsed pumice may appear as dark 

blebs of glass as illustrated in figure 1, and which 

have been discussed in the section on field characteristics. 

Pumice may lose its glassy character either by 

vapor-phase alteration or by devitrification (figs. 

55-58) . The difference in the significance of these 

two processes have been discussed on page 44 and 

their effect on pumice fragments described. The 

effects of vapor-phase alteration are especially well 

shown by the field relations. The occurrence of 

the vapor-phase minerals-feldspar and tridymite 

-is shown in figure 66. 

Pumice fragments are subject to the same devitrification effects as are shards and the resulting 

minerals-feldspar and cristobalite-are the same. 

However, pumice fragments tend to be more readily 

de vitrified, and to develop a much coarser grained 

aggregate of devitrification products than the associated shard materials. In a few specimens the 

pumice has become devitrified while the shards have 

remained glassy. The development of the coarsegrained aggregate commonly results in the complete 

destruction of structure within the pumice as illustrated in figures 57, 58, 61, and 62. 

WELDING, DISTORTION, AND STRETCHING 

Welding and the accompanying distortion of pyroclastic materials is one of the outstanding characteristics of ash-flow materials. Therefore, the 

structures formed during welding and criteria for 

their recognition, even after extreme distortion, 

present one of the outstanding problems in the study 

of ash flows. This includes the recognition of the 

progressive modifications of these structures as a 

result of increasing distortion. 

Welding and distortion can be observed both in 

the field and under the microscope for many occurrences, but in others can be determined only under 

the microscope. Even where clearly observable in 

the field, relations are clarified by microscopic 

studies. 

All ash-flow materials are derived from glassy, 

vesiculated materials with a wide size range, but 

in general are dominantly of ash size, and all subsequent changes which these materials may have undergone, have merely been imposed on these primary 

vesiculated glass structures. However great may 

be the effect of subsequent changes, they rarely 

completely obscure the pyroclastic origin of the 

materials. The genetic history of welded tuffs has 

given them very definite textural characteristics 

that are expressed in what has been termed "fabric." Iddings (1909, p. 194) has defined fabric as 

follows: "The fabric, or pattern, of a rock as shown 

on a surface or in a section, is that factor of its 

texture which is dependent on the relative sizes of 

its component crystals, [or glass fragments] on 

their shapes and arrangements with respect to one 

another." This definition, as modified to include 

glassy materials, effectively outlines the microscopic 

characteristics which must be considered in a description of welded tuffs. 
34 ASH-FLOW TUFFS 

Welded tuffs usually have a foliate structure 

(eutaxitic) due to a parallel arrangement of originally flattened plates, or to compaction and flattening of glass shards and pumice fragments into more 

or less platelike units. The planar arrangement of 

these platelike units imparts a foliation which has, 

in many occurrences, been mistaken for lineation 

due to flowage. Welded tuffs may, however, show 

a marked lineation resulting from stretching and 

perhaps very rarely to incipient flowage of the 

welded, but still plastic material (figs. 40, 41, and 

51). 

The fabrics developed during welding depend on: 

the primary form of the shards; the degree of their 

plasticity-a function of chemical composition and 

temperature; the compressive stresses acting upon 

them; and the rate of cooling. Fabric is shown 

most clearly in the welded tuffs that have remained 

glassy, but some of the devitrified tuffs also show 

very well the effects of distortion of the fabric. 

Varying degrees of distortion are shown in the 

frontispiece, figures 9, 10, and 24-41. 

In some ash-flow materials the shards are in contact with one another only at their points but without observable distortion. Commonly such tuffs 

have remained glassy, but more rarely even poorly 

consolidated tuffs have been devitrified as illustrated 

in figure 17. In some tuffs, especially poorly consolidated ones, it may be difficult to determine 

whether incipient welding has preceded devitrification. However, many tuffs show marked distortion 

of the structures that has preceded devitrification as 

illustrated in figures 51, 59, 60, and 68-70; in these, 

welding definitely preceded devitrification. 

The degree of distortion in welded tuffs ranges 

from those in which it is slight (figs. 20-29) to 

those in which flattening and stretching produced 

a close resemblance to flow rocks (figs. 30, 36, 40, 

41, and 59). In many tuffs evidence of distortion 

may be observed only where the shards are molded 

against sharp corners of a phenocryst, or where 

they have been squeezed between two adjacent 

phenocrysts. In other occurrences there is slight 

distortion throughout the specimen, and very 

marked warping around phenocrysts (figs. 31 and 

34). Most welded tuffs which have undergone compaction show the greatest distortion perpendicular 

to the plane of deposition and much less distortion 

in those cut parallel to that plane, as illustrated in 

figures 68, 69 and 83 and 84. 

With extreme distortion welded tuffs may come 

to resemble flow rocks. A welded tuff with marked 

stretching and distortion of the structure is shown 

in figure 36, and a flow rock with distorted flow 

lines in figure 37. The two illustrations showed 

marked similarity, but close comparisons show certain dissimilarities. although these may not be great 

enough for definitely differentiating them. The 

discontinuity of the flow lines in the welded tuff 

(fig. 36), may be contrasted with the more continuous flow lines as shown in figure 37. 

In the greatly flattened tuff structures, groups of 

lines derived from distinct pumice fragments may 

butt against each other, or meet at an angle. Compressed Y- or U-shaped shards resemble flow lines 

which have been folded back upon themselves (figs. 

51 and 94). Spherical bubbles have been compressed into very fiat lenses as shown in figure 40. 

In some specimens the zones in which the original 

tuff structure has become obscure will alternate with 

other zones in which the structure is clearly recognizable. 

J. Hoover Mackin studied and brought to the 

authors' attention the welded tuff from near Iron 

Springs, southwestern Utah (figs. 40 and 41). The 

specimen shown in figure 41 has undergone such 

extreme stretching that it closely resembles a flow 

rock with lineation. The specimen shown in figure 

40 is from the same ash flow, but from a locality 

where the stretching was not quite as thorough, 

and the ash structure, although extremely compressed is still discernible. The two are otherwise 

different in that the specimen represented in figure 

41 is glassy whereas that in figure 40 has undergone devitrification. 

Solovev (1950, p. 219) in his description of the 

"ignimbrites" of the Sikhote-alin volcanic region 

lying between the Amur River and the Sea of 

Japan, describes stretching of the tuff structures 

as follows: 

A mass of fragments settled during the movement and the 

particles-at least a certain portion of them-were compressed because of the high heat and also the pressure from 

the accumulating overlying pyroclastic material, and were 

welded in some places, especially in the lower horizons of 

the column. Such masses, existing in a viscous state, acquired in some localities a fluid character (and could, possibly in some sections even become displaced somewhat to 

the sides under the weight of the overlying accumulation of 

fragments). Naturally, this condition could not have failed 

to be reflected in the structure of the rocks. In a number of 

places the pyroclastic character of the rocks was more or 

less completely effaced or even completely destroyed * * * a 

pyroclastic structure is clearly expressed at a number of 

points, whereas at other points it does not express itself, or 

its traces are completely destroyed. In the latter case, the 

main masses not infrequently acquire a fluid character * * * 
MICROSCOPIC CHARACTERISTICS 35 

all transitions are present, so that these singular rocks 

have in some places the features of tuffs, and in others the 

features of lavas. 

PHENOCRYSTS AND FOREIGN MATERIALS 

Crystalline materials in ash-flow tuffs, other than 

the devitrification products, are of two types-those 

which represent phenocrysts which developed as a 

part of the genetic history of the rocks, and those 

rock and mineral fragments which are alien to it. 

Only very rarely have tuffs been observed which 

were almost free from phenocrysts, and very few 

which were without at least a few alien inclusions. 

The proportion of observed phenocrysts ranged 

from a fraction of 1 percent to nearly 60 percent. 

The obsidianlike welded tuff from southeastern 

Idaho illustrated in figures 9 and 10 contained only 

about 0.1 percent of phenocrysts, and the tuff from 

McKays, Calif., illustrated in figure 82, is uncommonly rich in phenocrysts. The tuffs from Sumatra 

(fig. 26) are high in phenocrysts, whereas those 

from Peru (figs. 24, 25) are low. 

The rhyolitic welded tuffs are always characterized by feldspar phenocrysts, and commonly contain 

quartz. Biotite occurs in small amounts in many 

rhyolitic tuffs, but becomes abundant in some quartz 

latite, and rhyodacite tuffs. Magnetite is sparse 

but rarely, if ever, totally absent. Hornblende is 

present in a few tuffs. Augite is abundant in some 

andesitic tuffs, but is more rare in rhyolitic ones. 

The feldspars of the welded-tuff deposits of the 

Valles Mountains are almost wholly sanidine, and 

sanidine that shows a brilliant blue chatoyancy 

characterizes at least two welded-tuff units. Orthoclase or microcline is rare and where present is no 

doubt alien, probably being derived from invaded 

rocks. 

The essential feldspar grains are sharply euhedral 

in outline in a few tuffs, but more commonly they 

are subhedral. Some show one side with a crystal 

face, and elsewhere show irregular or fractured 

edges. Other feldspar grains are rounded or irregularly embayed as shown in figures 19, 34, and 38. 

In some tuffs, as in most of those from Sumatra, 

there has been extreme fracturing, so that most of 

the grains are sharply angular in outline (fig. 26). 

Quartz phenocrysts may be sharply euhedral as 

shown in figure 76, but more often show rounding 

and embayment as illustrated in figure 31. Anthills 

are abundant in tuff areas in the Valles Mountains, 

N.Mex., and in these, quartz grains with lesser 

amounts of sanidine feldspar are dominant components. The quartz grains are about 2 mm in 

552858~61 4 

diameter and are euhedral or partly euhedral in 

outline and all show evidence of the hexagonal symmetry of high-temperature quartz. 

A great variety of materials derived from rocks 

through which the tuff-forming magmas made their 

way to the surface, or that were picked up from the 

surface by the overriding flow, have been observed 

by the authors in the course of studies of many 

hundreds of thin sections of welded tuffs from 

different regions of the world. Alien inclusions are 

rarely absent and include granite fragments, occasionally fragments of metamorphic rocks, sedimentary rock material, and volcanic minerals and 

rock fragments that clearly differ in origin from 

those genetically related to the tuff. Some of these 

materials also occur as rock fragments large enough 

to be recognizable in the field or in hand specimens, 

but those of large size and those observable only 

under the microscope seem not to differ in character. 

Most commonly these foreign materials are those of 

andesitic rocks; andesitic minerals and rock fragments are so ubiquitous in tuffs from the United 

States and other countries that their absence seems 

to be a rare exception. Andesitic eruptions have 

commonly preceded explosive rhyolitic ones, a longrecognized, almost normal geologic sequence, and 

so an abundance of andesitic materials in welded 

tuffs is to be expected. In general these rock or 

mineral fragments are fresh and unaltered. Augite 

and rare biotite and hornblende are normally perfectly fresh except where subjected to vapor-phase 

activity. In one specimen examined, olivine in an 

included basalt fragment was nearly fresh. Very 

rarely there seems to have been partial fusion of an 

included fragment, probably representing material 

picked up at uncommon depths. 

Many tuffs show a wide range in the proportion 

of anorthite in the plagioclase crystals, and the 

more calcic ones are alien materials. The origin 

of the alien plagioclase is evidently the same as the 

commonly associated andesite rock fragments. The 

more calcic grains have tended to be out of equilibrium with their host rock and suffered alteration or re-solution. Biotite when present may be 

partly altered with the development of magnetite 

along its borders. Hornblende may be of a normal 

greenish color, or not rarely red brown. In some 

rocks both types of hornblende occur. Another type 

of alien material represents rock fragments that 

have been picked up from the surface by the overriding ash flow, and that are conspicuous near the 

base of some flows. Some of the smaller fragments 

are observable only under the microscope, but most 
36 ASH-FLOW TUFFS 

of them are so large that field studies show their 

relations best. 

DEVITRIFICATION 

A large proportion of ash-flow tuffs have undergone devitrification, and this process has been superimposed upon fabric resulting from the welding and 

distortion of the pyroclastic material. These successive events modify, in many occurrences profoundly, 

the primary glass shard structures. However, 

evidence of origin is retained in most devitrified 

tuffs. 

The physical characteristics of devitrified tuffs 

are closely related to the identity of minerals 

formed, therefore these will be discussed before 

further consideration of textures. The slender 

parallel intergrowths of the devitrification products 

(axiolitic structures) vary greatly in size, but are 

normally too fine grained to be identifiable under 

the microscope. The shards illustrated in figure 

28 are marked by light-colored borders which, under crossed nicols, show a very slight grayness, but 

no resolution of mineral aggregate, even with very 

high magnification. In nearly all specimens however, the materials are of such a size that under 

the microscope they have a rough surface, indicating a parallel aggregate of minerals of greatly 

differing indices of refraction, as shown in figures 

71-74, or as in the spherulites illustrated in figures 

63-65. The mineral aggregates within the shards 

can not be definitely identified by means of the 

microscope. However, many tests of devitrified 

glass shards and pumice fragments of rhyolitic ashflow tuffs from many localities have been made by 

means of X-rays, and these have invariably shown 

that the products are cristobalite and feldspar. 

Before the development of X-ray diffraction 

methods for the study of very fine grained materials, the identification of minerals of devitrification in volcanic glasses was difficult, or impossible. 

Fenner (1948a, p. 883) states that small pores between the shards of tuffs from the Valley of Ten 

Thousand Smokes and from Yellowstone National 

Park contained tridymite and orthoclase, and that 

the same secondary minerals occur in tuffs from the 

Arequipa region of Peru. Fenner's identification of 

the tridymite in cavities is clearly correct, but he 

did not specifically consider the devitrification material in dense glass. 

Marshall (1935, p. 23, 24) described the development of very slender needlelike structures that 

develop in glass shards-the material that he called 

"pectenate" but that Zirkel (1876, p. 173, 17 4) had 

previously named axiolites. Marshall also recognized feebly birefringent tablets which he believed 

to be tridymite, but did not distinguish between the 

two materials. The one was probably cristobalite, 

and the other undoubtedly tridymite as he believed. 

Mansfield and Ross (1935, p. 321) also concluded 

that the devitrification products in welded tuffs 

from southeastern Idaho were tridymite and feldspar. Zirkel (1876, p. 165-168) reported tridymite 

in a rock from the Truckee Canyon region in western Nevada, and restudy of his thin sections by the 

authors has shown that this rock was a welded tuff. 

Zavaritsky (1947, p. 12) reported cristobalite and 

tridymite from a welded tuff from Russian Armenia. 

Thus tridymite was recognized, but the very fine 

grained intergrowths within the shards remained 

unidentifiable until the application of X-ray 

methods. 

Tuffs from Katmai and Peru collected by Fenner, 

those from Idaho collected by Ross and Smith, and 

those from New Zealand collected by Ross were 

examined by means of X-ray, and in all these the 

characteristic devitrification product was cristobalite; and subsequent examination of many devitrified 

tuffs have shown cristobalite. The reason for concluding that devitrified tuffs were characterized by 

tridymite seems obvious. Tridymite developing in 

cavities occurs as crystals large enough to be readily 

identified, but the cristobalite cannot be so identified, 

although the microscope shows that the intergrowths contain a mineral of very low index of refraction. The presence of two distinct silica minerals seemed improbable, and it seemed a logical 

assumption that tridymite, the mineral that was directly observable, was representative of the other 

product of devitrification. 

In general the cristobalite-feldspar intergrowths 

that have developed entirely within a shard are 

compact, and without observable pore space. This 

is confirmed by the specific gravity of materials 

such as those illustrated in figure 68, which is similar to that of crystalline flow rocks of like composition. 

Devitrification tends to be more prevalent in tuffs 

that have undergone compaction and welding, but 

very porous ones may also be devi trified as in the 

tuff shown in figure 17. The development of crystals during devitrification and vapor-phase crystallization has commonly brought about a hardening 

or induration of the tuff without the intervention 

of welding. Fenner (1948a, p. 884) has discussed 

such tuffs from Arequipa region of Peru and called 

the resulting material "sillar." 
MICROSCOPIC CHARACTERISTICS 37 

The aggregate of cristobalite and feldspar that 

develops during devitrification may assume very 

different patterns and these merit some discussion. 

In figure 51, the tuff structure is greatly compressed, 

but is still fully recognizable, despite the development of devitrification products whose pattern 

transects that of the shards. In figures 59 and 77 

compression and devitrification have marred, but 

not entirely obscured, the tuff structures, as shown 

under normal illumination. On the other hand, the 

same area under crossed nicols (fig. 60) shows only 

a coarse-grained crystal aggregate with the tuff 

structure entirely obscured. 

The most characteristic and widespread type of 

devitrification product is that where the growth of 

the replacing feldspar and cristobalite began at the 

grain boundary, and the waves of crystal growth 

met at a central zone of discontinuity that forms a 

dark line, as illustrated in figures 71-7 4. This represents the axiolitic structure of Zirkel ( 1876, p. 

166, 167) and is illustrated in the reproduction of 

a drawing by Zirkel (fig. 95). In this type of 

devitrification each of the structural units is a 

replica of a glassy shard unit. This type of devitrification has a minimum effect in modifying shard 

structure as is shown in figures 71-74. This axiolitic structure is widespread in devitrified tuffs and 

is even discernible in the Precambrian welded tuff 

from Morocco (fig. 98). Axiolitic structure has 

been observed only in ash-flow tuffs, and seems to 

provide excellent criteria for distinguishing between 

ash-flow and ash-fall tuffs. Possibly it could develop in fused tuffs (see p. 26) but has not been 

observed. 

In some tuffs, devitrification began only after 

thorough welding and compaction, and the feldsparcristobalite intergrowths show continuity across 

large areas of tuff structure. This may or may not 

have important effects in obscuring the original 

shard structure. In figures 69 and 70, this development of feldspar and cristobalite has not markedly 

obscured the tuff structures. 

In figure 51, the tuff structure is less well defined, 

but this is due to extreme stretching during welding, rather than to the development of devitrification products. In figure 60, a very coarse grained 

parallel aggregate of feldspar and cristobalite is 

shown under crossed nicols. However, under normal illumination of the same area, the tuff structure, although greatly modified during welding, is 

still recognizable. 

In pumice grains in which there has been complete collapse, with elimination of pore space, but 

perhaps some re-solution of volatiles, there is a 

strong tendency for the development of an uncommonly coarse grained aggregate of feldspar and 

cristobalite. In figures 57, 58, and 61 and 62, the 

structure in pumice grains has been almost completely destroyed, but has been retained in the enclosing shards. 

The most severe destruction of tuff structures 

seems to be the result of the development of spherulites, with their radial aggregates of feldspar and 

cristobalite. In figures 55 and 56, radial groups 

have destroyed the structure over most of the area 

illustrated, but small lenslike areas retain tuff 

structures. 

Complete destruction of tuff structure is illustrated in figures 63-65. In figure 63 a very large 

spherulite is illustrated on the right and a plumose 

structure has developed. In figure 64 the concentric structure characteristic of many spherulites is 

illustrated. These spherulitic structures developed 

in welded tuffs differ in no way from those occurring in many rhyolitic extrusive rocks. The identity 

of such rocks may be determined from geologic relations, or inclusions of materials of clastic origin 

may indicate ash-flow origin. On the other hand 

the origin of rhyolitic rocks with strongly marked 

spherulitic structure may be indeterminable. Not 

all spherulites cause destruction of pyroclastic structure, as shown in figures 54, 67-69, and 92. The 

studies of many hundreds of thin sections from 

different parts of the world indicate that most ashflow tuffs are readily recognizable by means of 

thin-section studies. However, the foregoing discussion of devitri'fication structures indicates that 

a few may have lost all identifiable tuff char~ 

acteristics. 

Marshall (1935, p. 31-32) considered the possibility that devitrification could destroy all direct 

evidence of original ash structure and says, 

Obviously the question arises as to whether the rocks of 

some of the areas of spherulitic rhyolite, of relatively coarse 

texture, in which no remnant of ignimbrite origin is evident, 

have actually been formed in this way. 

This suggestion is strengthened by 

The frequent occurrence of spherulitic rocks in beds which 

are almost horizontal, without a scoriaceous surface, without 

obsidian selvages, with a columnar structure, and with a 

relatively crumbly nature * * *. 

Studies of ash-flow tuffs from other localities raises 

the same question posed by Marshall. 

Lithophysae seem to be much less common in 

welded tuffs than in the corresponding extrusive 
38 ASH-FLOW TUFFS 

rocks, but differ in no principal character. However, very abundant lithophysal cavities are especially conspicuous in one widely occurring welded 

tuff in southeastern Idaho (figs. 49 and 52). These 

cavities are the result of the release of volatiles 

during devitrification and suggest that this particular ash flow had retained an uncommonly large 

proportion of volatiles in solution in the glass. The 

so-called thunder egg shown in figure 67, represents 

an uncommonly large lithophysa which had developed in a welded tuff from Oregon. The inner 

cavity was subsequently filled with chalcedony. 

PHYSICAL CHEMISTRY 

The concept of the emplacement of pyroclastic 

materials by flowage, and in many occurrences their 

subsequent welding into a compact rock with many 

of the relations of lava flows, is so remarkable that 

the full recognition of this mode of origin was accepted but slowly. The seeming improbabilities of 

such a mechanism are so real that a very detailed 

consideration of the physical and chemical factors 

involved in ash flow, welding, and devitrification 

seems to be imperative. 

A consideration of the mechanisms of the formation of welded tuffs and related materials includes 

only a part of the broader concept of igneous activity. Several factors involved in volcanism, such 

as the ultimate source of igneous materials and the 

mechanism of their migration from that source, 

will probably long remain controversial. A study 

of ash-flow tuffs is concerned with one phase of 

volcanism, that seems to be a late and superficial 

phase of igneous activity. However this late activity makes important relations available to direct 

observation, but others that occur in depths are not 

so observable. 

A fuller understanding will be desirable of eruptive mechanisms in general, the volatiles that characterize them, the heat relations in an ash flow, and 

the flow mechanism of plastic ash shards suspended 

in an atmosphere of red-hot turbulent gas-the 

latter a unique flow medium. However, a synthesis 

of direct observation, geologic studies, thermodynamics, physical chemistry, and geophysical experiment seems to provide the basis for an attempt to 

understand the principal factors involved in the 

eruptions, vesiculation, explosive disruption, flowage, emplacement, welding and devitrification processes, and vapor-phase reactions that have produced 

welded tuffs. 

Not all geologists are agreed about the relative 

importance of differentiation in the formation of 

the different magma types. However, concentration of volatiles accompanying progressive stages 

of differentiation and crystallization seems to be 

well established. The later products of differentiation, commonly contain a concentration of volatiles, 

and so provide the necessary mechanism for explosive, pyroclastic eruptions. As volatile-laden 

magmas rise to regions of diminished pressure these 

volatiles tend to escape, and commonly escape explosively. Variations in volatile content and pressures on the system at the time of eruptive breakthrough probably determine both the violence of the 

eruption and its nature, whether ash flow or ash fall. 

The physicochemical factors controlling the eruption and later history of welded tuffs, nonwelded 

tuffs and related pyroclastic materials are: (a) The 

chemical composition of the magma from which they 

were formed; (b) the proportion and character of 

the volatiles originally dissolved in the magma; (c) 

the temperature at the time of eruption and at successively later stages, until all reactions cease; and 

(d) the physical and chemical reactions at successive 

stages in the genetic history of the tuffs. Such physical relations as thickness of the tuff deposit and the 

hydrostatic pressures developed in the system, total 

volume, slope on which they were deposited, and the 

tuff textures that affect retention of heat and volatiles are also essential factors in the formation of 

ash-flow deposits. The eruptions of the magma, its 

vesiculation, flowage as a turbulent mixture of ash 

and gas, welding after coming to rest, and final 

devitrification are all functions of the above relations. 

The initial difference between ash-fall and ashflow eruptions is probably only the relative gascontrolled violence of the eruptions. However, this 

is the basis for fundamentally different posteruption processes. The ash-fall material has been 

blasted high into the air where its magmatic volatiles were largely dissipated and where it was 

quickly chilled and its magmatic history brought to 

an end. In contrast the ash-flow tuffs have been 

emplaced by mechanisms that conserved heat and 

at least some of the volatile constituents. Thus a 

series of physical and chemical relationships was 

initiated which have left a partial geologic record. 

This section will explore the physical and chemical 

factors that seem deducible from that record. This 

means that the study of the origin of ash-flow tuffs 

is a geologic problem, buttressed by available direct observation, experiment, and chemistry and 

theories of mechanics and physics. 
PHYSICAL CHEMISTRY 39 

HEAT 

A source of heat adequate for welding of ash-flow 

tuffs after eruption and travel for long distances 

has presented a major problem for geologists. 

Transportation through the air was long the only 

recognized mechanism for dispersal of ash from 

volcanic eruptions. However, this mechanism would 

involve such a loss of heat, that welding after wide 

dispersal seemed to be precluded, and this deterred 

geologists from accepting the welding of tuffs. The 

wide application of the term "nuees ardentes" and 

its equivalent "glowing clouds" to describe the dispersal mechanism has tended to perpetuate the concept of cloud-borne materials in contrast with the 

ash-flow mechanism of dispersal. A dispersal mechanism compatible with conservation of heat became 

understandable only when ash flowage, rather than 

cloud dispersal, came to be recognized. However, 

the entire problem of heat conservation during the 

formation and transportation and emplacement of 

ash-flow materials remains to be considered. 

Many measurements of the temperature of 

magmas at the time of extrusion have been made. 

Some of these have been summarized by Daly (1933, 

p. 68) and indicates a range from about 1,050°C to 

nearly 1,200°C. Perret (1950, p. 55) has described 

temperatures of 1,0oooc to 1,150oC at Vesuvius. A 

large proportion of the recorded temperature measurements were made at basaltic or andesitic centers, 

and much less is known about the temperature in 

rhyolitic centers. However, the experimental work 

of Goranson (1932, p. 234) showed that a dry granite would be completely molten at a temperature of 

1,050°C. He (1932, p. 229) found that at 1,500 

bars pressure and with 3.8 percent H2 0, the granite 

became completely molten in 3 hours at 900°C. 

Bowen and Tuttle (1953, p. 50) found that granite 

melts at about 720oC under 1,000 atmospheres pressure of water, and at about 640°C under 5,000 atmospheres. Ingerson (1955, p. 346) remarks 

"These figures are much more in accord with field 

and mineralogical observations on granitic intrusions than are anhydrous melting temperatures." 

Explosive rhyolitic magmas are erupted under a 

sudden release of pressure, and so the foregoing 

figures do not directly apply to their temperature or 

volatile content. A knowledge is desirable of the 

temperature and pressure at which quartz phenocrysts began to form, volatile content of the magma, 

and the pressure and depth at which vesiculation 

began. However, the temperature of a rhyolite 

magma was much lower than that of dry fusion of 

granite; a temperature of about 900°C, or perhaps 

even less seems probable. Clearly it was below 

1,ooooc. 

Some geologists have postulated reactions subsequent to initial eruption that supplied exothermic 

heat, and of these, Fenner (1950, p. 600-601) believes that exothermic reactions developed during 

the exsolution of volatiles from the molten magma 

are a fundamental factor in welded-tuff formation. 

He says, 

We are now prepared to consider what happens when fugitive constituents are given an opportunity to escape * * *. 

In these events each individual reaction between the infinitesimal units is accompanied by a minute heat effect positive 

or negative, but the system as a whole is so immensely complicated, and there is so little information in the reactions 

of separate constituents, that no calculation of the net heat 

effect is possible. The evolution of heat might be very 

great * * *. Or, on the other hand, the net effect in the 

system as a whole might be negative. 

Fenner further considered field evidence of exothermic reactions, but this was based on relations 

in Katmai crater, rather than those of the "sand 

flow" in the Valley of Ten Thousand Smokes. The 

tuff of the "sand flow" shows no indications of solution of phenocrysts or of included andesitic rock 

fragments and so there is no evidence of the superheat that Fenner believed was indicated at Katmai. 

The study of ash-flow tuffs from many other locations has shown that magmas from which welded 

tuffs are derived reached the surface with different, 

but in general with important amounts of phenocrysts. Feldspar phenocrysts may be euhedral, or 

sharply angular, and fractured crystals rarely show 

the effect of rounding by re-solution. Quartz commonly shows embayment but in no greater degree 

than in extrusive lavas of the same composition; 

this embayment probably occurred before eruption. 

Augite grains and rock fragments derived ·from 

andesitic rocks rarely show significant reactions 

with an enclosing rhyolitic magma. Calcic feldspar grains derived from the rocks traversed by the 

magma on its way to the surface, seem in some occurrences to have been out of equilibrium with the 

magma, and show evidence of alteration or resolution. 

The relations between included crystal grains and 

the magma are not primarily different from those 

observed in extrusive rocks. Thus, in general, tuffs 

show no evidence of superheat, or for any addition 

of a substantial amount of heat subsequent to eruption. The original heat of the magma seems to be 
40 ASH-FLOW TUFFS 

all that is involved in the observable relations. A 

few welded tuffs are uncommonly low in phenocrysts and may have been uncommonly hot. The 

adequate heat for welding within an ash flow must 

be sought in connection with other physical and 

chemical relations that tended to conserve this 

initial heat supply. 

VISCOSITY 

The viscosity of glasses with a large proportion 

of Si02 and Al 2 0 3 is so high that it has an important 

effect on the mechanism of explosive ash-flow eruptions, and especially on welding and distortion of 

shards. Viscosity data for silicate melts have been 

discussed by Bowen (1934). Experiments on an 

albite glass indicated a viscosity of about 108 poises 

at 1,150°C. The viscosity of a rhyolite glass, although somewhat higher, would be of a similar order. An andesite glass with 51.00 percent Si02 had 

a viscosity of about 3.1 X 104 poises at 1,200°C. 

Macdonald (1954, p. 172), by basing his calcu-lations 

on the rate of flow, arrived at a viscosity of 2 X 104 

poises for the 1950 Honoku flow at Mauna Loa, but 

thought that the figure was too great. These figures 

indicate the very high viscosity of rhyolitic magmas 

and the much lower viscosity of andesitic and 

basaltic ones. No quantitative data are available on 

the effect on viscosity of variations in temperature 

or the addition of H2 0. However, the experiments 

of Shepherd on the vesiculation of glass give some 

roughly qualitative information about the effect of 

volatiles on viscosity. Shepherd (1938, p. 340-341) 

states, 

For these 'wet' glasses (and the same would be true of the 

mother liquor of a crystallizing magma) a rapid change in 

the apparent viscosity is caused by relatively insignificant 

concentrations of volatiles. When a block of No. 5 [1.25 

percent volatiles] reaches the vesiculating temperature it 

flows, i.e., froths, around and through small obstacles, or 

in a tube it froths all over the inside. But once volatiles 

are out, the septa and shreds of glass remain indefinitely 

rigid. It requires much time and a much higher temperature before the mass begins to gather itself together again. 

This quick shift from fluid to rigid has obvious importance 

both for volcanology and petrology. -

Shepherd further states (1938, p. 343), 

The above-cited experiments on the relations of volatile content to viscosity have an important bearing on the mechanism of extrusion and lava flows. In the interval where 

vesiculation occurs the glass is notably fluid, the expanding 

pumice can froth around and between obstacles. But once 

fully expanded, that is to say, once the volatiles have been 

liberated, the foam becomes rigid. 

VOLATILE COMPONENTS 

Direct knowledge of the amount, volume, and the 

chemical character of the volatiles taking part in an 

explosive volcanic eruption is lacking, and only inadequate indirect observations are available. Collections have been made from secondary vents, but 

it is not known how well they represent original 

composition and they tell little about the original 

proportion of volatiles. Moreover, most such collections have been made from basaltic or andesitic 

lavas, and very much less is known about volatiles 

from rhyolitic ones. 

Volatile constituents are essential in the entire 

sequence of events resulting in welded tuffs. They 

probably take part in the rise of the magma into 

the explosive eruptive vents, and they are most 

obviously important in the vesiculation and eruption of this magma. They are part of the gas-ash 

mixture that constitutes the ash flow and they are 

partly released from solution in the glass shards 

during flowage. Some volatiles have remained in 

solution until after the ash flow came to rest where 

they were a factor in lowering the temperature of 

welding, and promoted devitrification and the formation of vapor-phase minerals in open cavities. 

Some of the primary volatiles probably remained in 

solution in the glassy parts of ash flows. 

The studies of volatiles in obsidian and associated 

perlite by Ross and Smith (1955) have indicated 

that most glassy volcanic flow rocks which have any 

degree of permeability have undergone postdepositional hydration. The same detailed comparisons 

have been possible with glassy ash-flow tuffs, but 

the high permeability of nonwelded ash-flow tuffs 

indicates that they too have undergone hydration. 

This is confirmed by the similar water content in 

the two types of materials, that is, of pumice and 

obsidianlike welded tuff. Only a few welded tuffs 

studied by the authors seem to have escaped hydration. One of these, a specimen from southeastern 

Idaho (figs. 9 and 10), contained 0.15 percent 

H2 0, while another part of the same flow contained 

3.06 percent H2 0. These relations indicate that 

pristine water characterized the one specimen while 

the other had undergone hydration. The retention 

of even 0.15 percent H2 0 is significant because this 

has been retained throughout all the volatile-controlled episodes that preceded cooling. Moreover, 

the physical effect of volatiles is greatly disproportional to their amount. 
PHYSICAL CHEMISTRY 41 

WELDING 

Not all ash-flow materials have become welded, 

and others are welded only in some part of the flow. 

However, welding is one of the most significant 

features of ash-flow materials, and probably the 

one which has caused the most misunderstanding 

of ash-flow mechanisms. For this reason welding 

merits a detailed discussion. 

The factors to be considered in welding are as 

follows: (a) The initial heat of the magma; (b) 

dispersal by flowage; (c) insulation in thick ash 

flows; (d) the effect of volatiles. 

Ash flows must come to rest with substantial 

amounts of heat. The dispersal by flowage, and 

the speed of movement are necessary factors in the 

conservation of the heat which is required for welding after ash flows come to rest. No estimate, not 

even a vague one, of the speed of dispersal of the 

great ash flows of the past is possible, but the 

speed of some of the minor ones have been estimated 

as follows: Eleven to 26.6 meters per second, or 60 

miles per hour at Pelee (Lacroix, 1904, p. 203) ; 33 

meters per second, or 7 4 miles per hour at Pelee 

(Perret, 1937, p. 96) ; 100 miles per hour at Mount 

Lamington (Taylor, 1954, p. 86). 

The great flows that spread for many miles probably moved even faster. Thus, movement is so fast 

that the time for cooling during flowage is short. 

Rock materials are very poor conductors of heat, 

and would be still poorer with substantial amounts 

of pore space as in poorly consolidated materials. 

Thus, tuffs tend to remain hot long after they come 

to rest. In describing the eruptions at Pelee, Perret 

(1950, p. 95) says, 

Below the surface weeks after the passage of a nuee, a 

walking stick was burned at a depth of thirty centimeters 

or less * * *. So different was the more fully air borne ash 

which reached St. Pierre in the great nuee of May 8, 1902, 

that although its temperature on arrival is believed to be at 

least 800 °C * * *. Negroes entered the ruined town in the 

afternoon only eight or nine hours later. 

Kozu (1934, p. 143) has given the results of temperature measurements on ash-flow materials of the 

eruption at Komagatake, Japan, of 1929. At a 

depth of only 40 em, and 8 to 11 days after the 

eruption, the temperature at 6 different places 

ranged from 510 ° to 310 oC and about 1 year later 

the temperature ranged from 210 oC down to 28 °C 

at the same depths. A temperature of 510oC was 

probably too low to allow welding, but perhaps not 

excessively low; and had it been a thick flow, welding temperatures would probably have existed in 

some deeper part of the flow, and persisted for a 

long time. 

The reduction of the viscosity of glass by the 

presence of small but significant amounts of volatiles discussed in the preceding section would be a 

factor in promoting the distortion and compaction 

which normally accompanies the welding of tuffs. 

Many pumice fragments in welded tuffs show 

evidence of lower viscosity and greater distortion 

than the associated ash materials. Figure 47 illustrates a pumice area which was so plastic that it 

was distorted against an area of more rigid welded 

tuff. Figure 48 shows strong distortion of a pumice 

area. The complete elimination of porosity in many 

pumice fragments indicates the same relation, and 

that the vapor-phase material in the original pumice 

pores had been eliminated. 

The pumice was originally inflated by the exsolution of its magmatic volatiles. The hot highly 

viscous glass walls of the pumice had a strength that 

a cold brittle glass would not have had. Therefore 

the vesicular pores of the viscous glass were probably capable of retaining at least part of the original 

volatiles. Experiments show that the volatiles involved in the expansion of a pumice are about 0.1 to 

0.3 percent and some part of these would remain 

trapped. When this pumice collapsed probably some 

part of these trapped volatiles would go back into 

solution in the glass of the pumice. This suggests 

a possible explanation of the lowered viscosity of 

pumice fragments, as pointed out by Shepherd 

(1938, p. 343) when he says, "A mass of foam 

trapped and subjected to heat and water at sufficient 

heat and pressure will, of course, collapse and become fluid again." His study indicates that even 0.1 

percent of H2 0 would have an appreciable effect on 

viscosity of a glass. The relations in the pumice 

differed from those in the associated ash in which 

the volatiles escaped into intergranular spaces when 

not trapped as they were in the pumice vesicles. 

Several geologists have studied the temperature of 

welding in ash-flow materials. Marshall (1935, 

p. 21) believed that because tridymite, with a minimum stability temperature of 870 oC, was present in 

the New Zealand ignimbrites, this established that 

temperature as the minimum for their emplacement. 

He did not understand that both tridymite and 

cristobalite normally form as metastable minerals. 

Sosman (1927, p. 786) states, 

It might appear at first thought that the occurrence of one 

of the high temperature forms of silica in a rock is evidence 

that the rock was about 1470oC (cristobalite) or above 
42 ASH-FLOW TUFFS 

870oC (tridymite). But the tendency shown by silica to 

deposit in metastable form spoils this part of the argument. 

However. Marshall (1935, p. 22) noted that quartz 

was present and preceded tridymite in time of formation, and he was at a loss to explain this relation. 

Gilbert (1938, p. 1856) based his conclusions on 

temperatures in the Bishop tuff on the temperature 

of the conversion of part of the green hornblende 

to basaltic hornblende. This caused him to state, 

"The only safe conclusion is that the temperature 

of emplacement could not have been very much 

higher than 750°C, for if it had been, all the hornblende should have been converted to basaltic hornblende." 

Shepherd ( 1938, p. 342) reports experiments on 

the fusion of rhyolitic glass from Cerro No Agua 

and states. "But for anything one would call flow 

in the field sense, temperatures above 600 °C are 

needed * * * ." 

Boyd and Kennedy (1951, p. 327) experimented 

on the welding of rhyolitic pumice from Mono 

Craters and state that "The pumice was found to 

weld at a temperature ranging from 775 o to 900°C." 

Because the glass of ash-flow materials probably 

contained at least 0.15 to 0.3 percent of H20, the 

welding temperature would probably have been 

somewhat lower than this. 

A report on the investigations of the Carnegie 

Institution of Washington Geophysical Laboratory 

for 1953-54 cites the studies of Boyd (1954, p. 139) 

at Yellowstone National Park. Boyd concludes, 

A thermodynamic analysis of the eruptive process [of avalanches of fragmented viscous lava] suggested by field evidence has been undertaken. The temperature interval 

between the original magma temperature and the welding 

temperature of the erupted tuff will have a maximum value 

of lOOoC; in the average case it will be less. 

The amount of heat lost during emplacement of a tuff 

avalanche will depend primarily on the thickness of the tuff 

flow, the manner in which the flow moves, and the time 

elapsed during emplacement. Reasonable value for all these 

quantities can be found from either field evidence or from 

data available on nuees ardentes. Assuming a collapsed thickness of 30 meters, an average time of emplacement of one 

hour, and turbulent flow in the moving avalanche, the loss of 

heat during emplacement will reduce the temperature by 

less than 10°C-a value well within the estimated limits. 

These figures are reasonable approximations for 

ideal ash-flow conditions, but many ash flows are 

probably not ideal. That is, the ash flows have been 

subject to mixing with air and their distal ends 

have come to rest at much lower temperature than 

the initial eruption temperature. These differences 

in temperature may be very real, as can be readily 

seen by the changes in amount and degree of welding 

that take place in many single ash-flow sheets between source and distal end. 

Recent experimental work by Smith and Irving 

Friedman (written communication, 1956) has 

shown that welding of glassy rhyolitic ash and 

pumice can take place at temperatures at least as 

low as about 580°C. These e:xperiments were 

carried out under a water-vapor pressure of about 

300 psi (pounds per square inch) and a mechanical 

load of about 500 psi. They believe that pressure 

conditions of this order of magnitude could exist in 

ash flows having a precollapsed thickness of about 

800 feet and a density of 1.5, or about 1,200 feet and 

a density of 1.0. 

The physical relations within an ash flow may be 

restated as follows: 

1. Rhyolitic tuffs are derived from a magma with 

an initial temperature not greater than 

1,0oooc, and probably below 900°C. Andesitic magmas probably reach a maximum of 

about 1,150°C. 

2. The viscosity, especially of rhyolitic magmas, 

is high but not so high as to inhibit collapse 

and welding of shard materials under load. 

3. Volatiles, at least in minor amounts, are recorded in most welded tuffs. These reduce 

viscosity and lower the temperatures of welding. 

Relations that bear on heat conservation are as 

follows: 

1. Pyroclastic materials commonly escape to the 

surface without superheat, and show no evidence of later acquiring substantial amounts 

of heat. 

2. Gas phenomena accompanying ash-flow eruptions are powerful and the volumes of gas are 

large, but the mass is small compared with 

that of the ash. 

3. Because the mass of gases that escaped during 

flowage is small their escape does not remove 

large amounts of heat from the system. 

4. Ash flows move with great rapidity and the time 

effect on cooling is small. 

5. The mechanism of dispersal is that of a gasshard mixture of marked density and it rides 

close to the ground. The materials are not 

airborne as in ash falls. 

6. Ash-flow materials are very poor conductors of 

heat. 

These relations seem to indicate that the magmatic heat inherent in an ash flow is effectively 

conserved and adequate for welding. 
PHYSICAL CHEMISTRY 43 

The physical chemistry of the welding process is 

complex and the relative effects of the variables are 

difficult to evaluate. However, the relations in 

welded tuffs are similar to those known as sintering 

by the glass technologist, and some of the research 

on glass sintering helps the understanding of welding in volcanic ash. 

The relations in sintering have been outlined by 

Eitel (1954, p. 1047) and are summarized as follows: 

A minimum temperature must be attained for the 

initiation of the sintering reactions and atomic 

bridging. The mobility of the atoms or atomic 

groups must have attained a minimum intensity 

that is sufficient for a beginning of place exchange. 

The inference seems to be that the atoms and molecules must vibrate so intensely around their structural equilibrium positions that a large number of 

them are able to exchange places with surrounding 

atoms or molecules. 

Eitel (1956) published a "free translation" of a 

series of papers by Carl Kroger (1952-55), and in 

a supplemental discussion further amplified the 

mechanisms of sintering, which are abstracted as 

follows: 

The reactions in the solid system composed of 

individual units depend on the following factors: 

1. A necessary threshold mobility of the atomic 

and molecular particles in the solids, along their 

boundary zones. 

2. The temperature of a molecular exchange of 

place, under the action of thermal energy, is a function of fusion points. In a glass system this implies 

an adequate reduction of viscosity, which is dependent on temperatures and volatile content. 

3. Sintering (welding) involves a real "creeping 

or flow factor." That is, if two units have an intimate contact, particles from both solid parts will 

diffuse along the boundary surfaces. 

The rate of displacement will depend on the inner 

mobility (yield value viscosity) of the material, the 

surface tension of the material surrounding the 

medium, and the hydrostatic pressure under which 

the material is standing. 

Mobility of the particle material at the elevated 

temperature may mean a partial flow in a liquid 

phase. 

The above factors that control sintering apply 

primarily to crystalline materials, whereas in the 

welding of tuffs the materials are noncrystalline 

(glassy). The materials are also plastic, although 

the viscosity is so high that they will fracture under 

sudden stress. That is, glassy materials have properties of solids such as to make these factors of 

sintering applicable to an interpretation of the welding of tuffs. The glassy character would seem to 

simplify rather than complicate this relation. 

The factors that control welding are: (a) The 

presence of volatiles; (b) pressure within the system; (c) intimate contact of the individual particles; (d) threshold mobility within the system 

(adequate temperature); (e) inner mobility (yield 

value viscosity and surface tension) ; and (f) creeping or flow factor. 

These factors are interrelated and not entirely 

independent. The foregoing outline may now be 

specifically applied to the consolidation and welding 

of tuffs. 

The tuffs come to rest in a heated condition, with 

welding initiated at about 650° to 700°C, and perhaps reaching about 900 °C in the hotter tuffs. This 

provides them with a plastic yield, although the 

viscosity is high in glasses of rhyolitic composition. 

They contain small amounts of dissolved volatiles 

that have an important effect in reducing viscosity. 

After coming to rest the tuffs are under a static 

load, depending on the thickness of the over lying 

material. Porous glassy tuffs are exceedingly poor 

conductors of heat and remain hot probably for 

years. 

Shards and other glassy fragments, as formed 

by explosive eruption, are sharply angular and 

would come to rest touching one another only at 

points, and with a large proportion of interspaces. 

Under load, and with adequate initial mobility, there 

would be plastic yield accompanied by varying degrees of distortion and accommodation of grain to 

grain. This plastic yield would, with time, increase 

the areas of contact between individual particles, 

and under optimum conditions there may be nearly 

complete elimination of pore space. In the presence of volatiles some of the particles would be 

adsorbed on the grain surfaces even at temperatures 

of 700 °C. The effect of all these factors would tend 

to promote a bridging effect and a transfer of materials across surfaces of intimate contact. 

In the system here postulated volatile components 

were present, both as a vapor phase and in solution 

in the glass. The vapor phase may have been a 

factor in some transfer of materials, but it did not 

initiate any new phase (crystalline). A crystalline 

phase did not develop until after welding. 

Where viscosity is low and where there is contact 

only at points, welding may be confined to these 

small areas and recognition of welding under the 

microscope will be difficult. On the other hand, a 
44 ASH-FLOW TUFFS 

combination of adequate pressure and other factors 

promoting initial mobility may combine to permit 

thorough accommodation of grain to grain, and . 

result in the welding of tuffs into a compact obsidianlike glass. 

The glass film that forms on the contact between 

two glass shards might be thought of as a secondary 

glass derived from the original glass. The intershard films are too thin to study in any detail, but 

differ slightly from the original or primary glass. 

In the glassy pumice from Argentina (figs. 83 and 

84) the intershard film is colorless, while the original shards are brown. The lighter color is due in 

part to the nontransfer of pigmenting dust material, 

and indicates a slight difference in the composition 

of the two glasses. This does not seem, however, 

to constitute any discontinuity of phases. 

The relations shown in figures 59 and 60, and in 

figures 67-70, are especially significant. Here there 

was almost complete elimination of pore space so 

that shards had been brought into complete contact 

with one another previous to devitrification. The 

continuity of the glass structure had become so complete that crystal growth of feldspar and cristobalite 

took place directly across the zone of union between 

original shard boundaries. The originally fragmental glass has become a structurally homogeneous medium, that presented no impediment to 

crystal growth across the former zone of discontinuity. This would seem to represent welding into 

a single glass phase previous to the growth of crystals across the former zone of contact. 

The obsidianlike tuff illustrated in figures 9 and 

10 has remained glassy, but the establishment of 

complete physical continuity seems evident. The 

collapsed pumice fragments have lost all trace of 

the original pumice structure (fig. 45) and here a 

complete continuity of structure has been established. 

DEVITRIFICATION 

Not all ash-flow tuffs have undergone devitrification; many are devitrified only in part, and have 

remained glassy at the top and base. Nevertheless, 

devitrification is one of the most distinctive characteristics of ash-flow tuffs, hence the physical and 

chemical factors involved in devitrification are of 

especial interest. The factors controlling devitrification are: (a) Chemical composition of the tuff; 

(b) chemical composition of the accessory volatiles; 

(c) rate of cooling; (d) temperature of devitrification; (e) identity of the minerals formed; and (f) 

stability relations of these minerals. 

The chemical composition of the tuff may be 

closely determined, although it may have been very 

slightly modified during devitrification in the presence of volatiles. 

The amount and chemical composition of the 

volatiles retained in a glass would probably not be 

greatly different from that discussed in connection 

with welding. This similarity indicates that the 

volatiles were present in the glass to the extent of 

not more than a few tenths percent. However, 

even this proportion would reduce viscosity and promote crystallization. The presence of volatiles during devitrification is indicated in figures 49, 52, and 

67. In these specimens devitrification was related 

to the formation of vesicular gas cavities. 

Rhyolitic tuffs seem to be more commonly devitrified than andesitic tuffs. The reason for this 

is not clear because high-silica extrusive flows show 

a much greater tendency to remain glassy than do 

andesitic ones. The difference may be due to a 

more effective retention of volatiles in high-silica 

glass shards than in andesitic shards. 

The temperature of devitrification has a wide 

range, probably beginning immediately after welding and continuing to some problematical minimum 

temperature. However, once crystallization starts, 

it probably proceeds rapidly. 

X-ray examination of ash-flow tuffs has shown 

that the devitrification products of dense shards are 

a fine-grained intergrowth of cristobalite and sanidine feldspar. On the other hand, tridymite, and 

more rarely cristobalite, have formed in vesicles 

and uncollapsed pumice fragments. The factors 

that determine whether cristobalite or tridymite 

develop are of interest. Where compact homogeneous glass is devitrified there is a diffusion of ions 

within the glass over very small distances, but there 

seems to be no vapor-phase transfer, although volatiles are in solution in the glass and help to promote 

devitrification. The same minerals form in collapsed pumice where free vapors do not collect. 

On the other hand, where the pumice does not 

collapse or where vesicles form, tridymite and feldspar form in the presence of vapors. There is 

vapor-phase transfer of materials at least for short 

distances. The formation of tridymite in a gas 

cavity is illustrated in figure 66. In rhyolitic 

welded tuffs tridymite and feldspar are the typical 

vapor-phase minerals, but more rarely biotite, 

amphiboles, and zeolites may form. 
PHYSICAL PROPERTIES 45 

The metastable character of cristobalite and 

tridymite means that these minerals tend to invert 

to quartz, the stable allomorph of Si02 • The geologic conditions that allow tridymite or cristobalite 

to persist, or that may promote inversion to the 

stable form are not accurately known. 

Cristobalite and tridymite seem to have resisted 

inversion to quartz in all the very abundant ashflow tuffs of younger Tertiary and Quaternary age 

that have been available for study, the only exception being those which have undergone alteration. 

Tuffs of Mesozoic age are less well known, and 

cristobalite has not been definitely recognized in 

tuffs of Paleozoic or older age. Upper Carboniferous tuffs from Corsica described by Bodenhausen 

(1955, illus. 21, 22) show evidence of fine-grained 

quartz. A specimen of welded tuff of Precambrian 

age from Sweden described by Hjelmqvist (1956) 

is characterized by quartz (examination by the 

authors). Welded tuffs of Precambrian age from 

the Anti-Atlas region of Africa have been described 

by Bouladon and Jouravsky (1954, p. 37-48), and 

samples have been made available for study. The 

shards illustrated in figure 98 still retain typical 

axiolitic structures, but these now contain only 

quartz and feldspar. 

PHYSICAL PROPERTIES 

INDICES OF REFRACTION 

George (1924) has provided useful data for determining the approximate silica content of a volcanic glass by means of indices of refraction. The 

study of the effects of water in glasses by Ross and 

Smith (1955, p. 1080-1085) included their optical 

properties, and the effect of water in raising the 

indices of refraction. This showed that the presence of 1 percent of water would lower the indicated 

Si02 content (compared to a water-free glass) by 

about 2 percent. 

Ash-flow tuffs commonly contain foreign material 

and these may give an incorrect indication of the 

character of the primary eruptive magma. Bonorino (1944) has described "ignimbrites" from near 

Malargue Mendoza, Argentina, and says, "The crystals are acid to basic plagioclase, and clinopyroxene, 

the two Ia tter often associ a ted as to suggest their 

common origin from a basaltic rock." A restudy 

of this material submitted by Bonorino (figs. 83 and 

84) has indicated that the calcic plagioclase and 

clinopyroxene did have a common origin, but that 

these materials seem to have been derived from an 

invaded rock, probably an andesitic one. The index 

of refraction of the glass fraction (1.508 to 1.512) 

indicates that the eruptive material of the glass 

shards was much more silicic than the composition 

of the rock as a whole. For this reason the silica 

content of the glassy groundmass part of this rock 

was determined by the U.S. Geological Survey. The 

Si02 content of 67.20 percent indicated that the 

rock, without inclusions, was probably dacitic. 

A·n important variable affecting the index of refraction of a glass is the state of oxidation of the 

iron. The total iron content of volcanic glasses may 

be as much as 7 percent or more, and in unaltered 

glasses is present in both the ferrous and ferric 

state. Such iron in solution in a glass gives it 

varying intensities of dull-brown tints, except in 

very low iron rhyolites, in which the glass may be 

nearly colorless. 

The collapsed pumice fragments shown in figure 

1 appear black in the hand specimen. Under the 

microscope this glass is nearly colorless while the 

groundmass glass (red in the hand specimen) is 

slightly brown. The colorless glass contains magnetite microlites abundant enough to permit a perfect 

separation from the nonmagnetic groundmass. The 

presence of magnetite microlites in the colorless 

glass is also observable under the microscope. In 

this glass the iron that was originally in solution 

was reduced to magnetite and segregated into distinct granules by reactions that took place during 

the collapse and welding of the pumice fragments. 

The elimination of part of the iron from solution 

in the glass and its collection as discrete microlites 

of magnetite has lowered the index of refraction of 

the colorless glass below that of the brown glass. 

The different states of oxidation of iron, and the 

resulting changes in a glass are shown in the frontispiece where reproduction in color depicts relations 

that black and white illustrations could not reveal. 

The iron in the glassy tuff illustrated in the top 

photograph was oxidized and colloidally dispersed, 

as suggested by Iddings (1899, p. 407), probably 

during emplacement in the presence of oxygen from 

the air. A later change in the environment resulted 

in the partial reduction of the ferric iron, and its 

segregation as finely disseminated dustlike magnetite. Thus iron was removed from colloidal dispersion in the glass, leaving it nearly colorless, with an 

index of refraction abnormally low for a glass of 

this composition. 

The dull-brown glass in the central part of the 

two large shards shown in the bottom photomicrograph seems to retain the normal ferrous-ferric iron 

relations of a glass of that composition; the index 
46 ASH-FLOW TUFFS 

of refraction is intermediate, and characteristic of 

a glass of that composition. On the other hand, 

oxidation of most of the iron to Fe2 0 3 , and its colloidal dispersion in the glass, has raised the index 

of refraction above that of the normal brown glass. 

As may be seen from the foregoing discussion, 

several factors may have a marked effect on the 

indices of refraction of a glass, and caution should 

be exercised in determining the composition of a 

glass from its index of refraction. The glass fraction may differ in composition from the overall 

composition of the rock because of the presence of 

phenocrysts and the inclusion of alien minerals and 

rock fragments. The glass of tuffs has usually 

undergone hydration, which increases the index of 

refraction. If the glass has any appreciable proportion of iron the different states of oxidation will 

have a marked effect on the indices of refraction. 

The phenocrysts of a volcanic rock may give an 

incorrect indication of the chemical composition 

as pointed out by Rittman (1952, p. 76) who states, 

The phenocrysts by themselves give very often a wrong idea 

of the real composition of the whole rock, because quite 

different kinds of minerals than those forming the phenocrysts may be present in the optically indeterminable microcrystalline groundmass, or may be potentially present in the 

glass. For example, many dacites, or even rhyodacites have 

been called andesites because their contents in quartz and 

sanidine occult in the groundmass were not taken in account. 

Rittman shows the necessity of extreme reservation 

in assigning an andesitic character to a tuff on the 

basis of observable minerals. The rock in bulk may 

be much more silicic than suggested. Andesitic 

rock and mineral grains are so ubiquitous in welded 

tuffs throughout the world that their complete 

absence is rare. Where they occur in rhyolitic 

welded tuffs, their alien character is obvious; but 

in ash-flow tuffs of intermediate composition they 

may lead to incorrect identifications. 

SPECIFIC GRAVITY 

Most welded tuffs retain at least some pore space, 

and only a few specimens permit a determination 

of the true specific gravity. However, the specific 

gravity of four virtually pore-free rhyolitic welded 

tuffs is given as follows: 

Specific gravity of dense rhyolitic tuffs 

Character of material Locality 

Specific 

gravity 

Glassy welded tuff _______________ Valles Mountains, N.Mex __ 2.38 

Obsidianlike welded tuff _____ Southeastern Idaho ______________ 2.34 

The same tuff hydrated ________ Southeastern Idaho ______________ 2.40 

Dense devitrified tuff ____________ Qregon ------------------------------- 2.47 

The small difference in the specific gravity of 

glass (2.34, 2.38) and of dense devitrified tuff 

(2.47) might seem surprising, but this is due to the 

large proportion of cristobalite (specific gravity 

2.32) in the tuff. 

POROSITY 

Ash-flow tuffs range from nonwelded, unconsolidated materials, to thoroughly welded tuffs with a 

density equal to that of the corresponding normal 

glass or crystalline rock. This range in porosity is 

illustrated in columnar sections of tuff deposits 

from three localities. 

Marshall (1935, p. 29) gives porosities in a typical occurrence of the New Zealand ignimbrites. 

Fourteen nearly equally spaced samples were collected from a 250-foot cliff at Te Toki Point, Lake 

Taupo, New Zealand. 

Porosity of acid rocks, Taupo-Rotorua, New Zealand, 

volcanic district 

Sample Porosity (percent) Sample Porosity (percent) 

1__ ____________________________ 28.6 

2 ____________________________ __ 25.2 

3------------------------------2 3 .4 

4 _________ ___ _________________ _ 22.0 

5------------------------------20.0 

6 ______________________________ 19 .8 

7 ______________________________ 23 .1 

8 ____________________ ____ ______ 18. 4 

9------------------------------ 9.2 

10------------------------------10.7 

11 -------------~----- ------ ---- 9. 9 

12 ______ _______________________ 8.4 

13 ------------------------------ 8.0 

14 _________ _____________________ 3.4 

Marshall notes that samples 5 and 9 contained 

surface-deposited secondary silica. 

Gilbert (1938, p. 1843) gives the following table 

of densities of a characteristic section of the Bishop 

welded tuff of California : 

Porosity of Bishop tuff 

Vertical distance 

from top of tuff 

(feet) 

Porosity 

(percent) 

0 

175 ± 

250 ± 

300 ± 

400 ± 

2 5 -------------------------------------------------

2 5 --------------------------------------------------

2 5--------------------------------------------------

2 5 --------------------------------------------------

37 

17 

7.9 

4.6 

3.6 

Specific 

gravity 

1.35 

1.9 

2.2 

2.26 

2.35 

The welded tuff at Battleship Rock, Valles Mountains, N.Mex., was studied by the authors, and is 

shown in figures 42 and 43. 
ASH-FLOW MATERIALS OF INTERMEDIATE COMPOSITION 47 

Porosity of Battleship Rock, N.Mex., ash-flow tuff 

Distance below Porosity Distance below Porosit y 

Sa'rnple the top (feet) (percent) Sample the top (feet) (percent) 

L_______________ 0 69 8 ________________ 175 24 

2 ________________ 30 58 9 ________________ 185 33 

3 ________________ 50 51 10 ________________ 200 48 

4 ________________ 75 33 11_ _______________ 205 51 

5 ____ ____ ________ 105 29 12 ________________ 215 59 

6 _______________ 130 25.5 13 ________________ 255 70 

7 _______________ 145 24 

Only a single unit is represented at the Battleship 

Rock locality and exposures present an excellent 

opportunity for detailed study. The porosity of the 

Battleship Rock ash-flow unit ranges from about 

70 percent near the top, to 24 percent, about onethird the distance above the base, and then back to 

70 percent at the base. In another locality, a part 

of the same ash-flow unit had undergone complete 

welding and elimination of pore space. Its specific 

gravity was 2.36 and this value was used in calculating the porosity of the section studied in detail. 

The variations in porosity are shown in the curve 

in figure 15. 

Tuff units with maximum welding at or near the 

base, like those described by Marshall and by Gilbert, have been reported by others engaged in the 

study of ash-flow materials. The position of maxi250 

Iw 

v-v-- w 200 LL 

z 

w-

(/) 

5,! 

( 7. 

<( 

CD 150 

w 

> 

0 

CD 

<( 

w 100 0 

I-

<( 

z 8~ 

mum load is commonly believed to be the reason 

for this position of maximum welding. On the 

other hand, the position of the greatest conservation 

of heat is also a factor in effecting maximum welding. For this reason many ash flows show relations 

similar to those in the Battleship Rock ash-flow 

tuff. Here maximum welding occurs about onethird of the distance to the top of the unit or about 

80 feet above the base (fig. 3). 

Most of the ash-flow tuffs of the Valles Mountains show similar relations. This same relation 

has been observed in the Rattlesnake formation of 

Oregon (T. P. Thayer, oral communication, 1950), 

and has been reported in Salvador by Weyl (1954b) 

and in some of the A so flows of Japan (Williams, 

1941b, p. 279-280). 

ASH-FLOW MATERIALS OF INTERMEDIATE 

COMPOSITION 

The preceding discussion has applied to ash-flow 

materials in general. However, in the Western 

United States these are dominantly rhyolites, 

dacites, quartz latites, or rhyodacites in composition, 

and for this reason a large proportion of the tuffs 

discussed have been rhyolitic in composition. The 

physical characteristics of rhyolitic and andesitic 

ash-flow materials are mostly the same, but the 

andesitic ones differ in some details and merit a 

~ 

;v v 

-o.L_ 

~ .J2 

(/) 

0 

50 

0 

0 10 20 30 

~ 

~ 

~ 13 

40 50 60 

POROSITY, IN PERCENT 

70 

FIGURE 15.- Porosity of Battleship Rock ash-flow tuff, Valles Mountains, N. Mex. 

80 90 100 
48 ASH-FLOW TUFFS 

brief discussion. The basaltic types of ash-flow materials seem to be comparatively rare, therefore 

they are considered together with those of intermediate composition. Ash flows of intermediate 

type have been described under a variety of names, 

including andesite, quartz andesite, latite, some socalled dacite, and in some regions, trachyte. 

Ash flows of intermediate composition are dominant types in several regions, and have been described from Japan, Costa Rica, El Salvador, 

Argentina, Russian Armenia, and they occur locally 

in the Western United States. Specimens from 

the above localities, except El Salvador, are illustrated in figures 79-86. 

Basaltic "scoria" flow materials have been described by Williams (1942) from Crater Lake, Ore., 

where they overlie dacitic "pumice flows." These 

are related to the eruptions that formed the caldera 

of Crater Lake. 

Ash-flow tuffs of intermediate composition seem 

to be dominantly glassy, and in those available for 

study devitrification has been rare. The tuffs from 

Costa Rica, Argentina, Russian Armenia, and most 

of those from Japan, are glassy. The "schmelztuffe" from El Salvador described by Weyl (1954b) 

are glassy. An examination of figures 79-86 seems 

to indicate that the glass shards of intermediate 

composition tend to be platelike. In several of the 

figures the shards are almost shredlike. Few of 

them show forms derived from the shattering of 

rounded bubbles. 

The disparity between the mineral grains recognizable under the microscope and the composition 

of the groundmass material has been described for 

the so-called andesite welded tuffs from Argentina 

(p. 45). The same disparity is strongly suggested 

by microscopic studies of other welded tuffs of intermediate type. 

ANDESITIC ASH FLOWS OF COSTA RICA 

A description presented by Williams ( 1952, p. 

173-176) is the most detailed available of a typically 

andesitic welded tuff. His petrographic description 

of this rock is a valuable contribution to an understanding of andesitic welded tuffs, so will be quoted 

in some detail. One of the specimens (figs. 85 and 

86, this report) described by ~~illiams was collected 

by Gabriel Dengo, transmitted to the authors, and 

later divided with Williams to supplement his own 

collections. In his description Williams says, 

One of Senor Dengo's specimens {s a magnificent example of 

a thoroughly welded tuff. The unaided eye sees abundant 

frayed disks of black glass, up to an inch in length and onequarter as thick, set in an almost fluidal base of pale-gray 

glass shards, generally smaller, but equally drawn out and 

sinuous. Peppered throughout, and serving as nuclei around 

which the glassy lenses curve, are varicolored chips of 

andesite, up to half an inch in diameter, and smaller but 

more numerous phenocrysts, mainly of feldspar. A Rosiwal 

analysis shows that crystals total about 15 percent of the 

tuff. Among them, basic andesine-acid labradorite (12 percent) occurs in phenocrysts up to 2 mm long; smaller 

prisms of pale green diopsidic augite (2V = 55°) comprise 

2 percent of the whole, and hypersthene forms less than 1 

percent. Hornblende, biotite, and olivine are lacking. Lithic 

fragments of varitextured pyroxene andesite amount to 5 

percent. The remainder of the tuff is made up of sinuous, 

tightly adpressed shards of clear glass varying in color 

from pale lilac and buff to deep brown, the darker ones 

corresponding to the black lenses seen by the unaided eye. 

None of the glass is devitrified, but some of the minute 

pores between the shards carry spheroids of cristobalite. 

Especially noteworthy is the wide range in the refractive 

indices of the glassy particles. In the palest wisps the index is as 1.510 ± .002; in the darkest, it increases to approximately 1.550. Most of the glass varies in index from 

1.530 to 1.540. Even more remarkable is the fact that although most of the individual shards are composed of 

uniformly colored glass, some reveal a fine banding of pale 

and dark glass of notably different indices. 

Williams ( 1952, p. 175) also states that another 

tuff in the area consists of 10 percent andesite fragments. In discussing the different types of glass in 

the tuff Williams says (1952, p. 176), 

* * * in the Costa Rica tuffs the individual shards are 

usually of one color or another. This confused mingling of 

minute varicolored shards of widely differing refractive 

indices, presents a difficult problem for which a satisfactory 

answer has yet to be found. It cannot be attributed to 

successive eruptions of different magmas; on the contrary, 

magma of heterogeneous character seems to have effervesced 

simultaneously from the feeding vents. There is no sign here 

of solution or even dismemberment of the included lithic 

fragments * * *. If the heterogeneity of the Costa Rica 

tuffs resulted from contamination, the process must have 

taken place at depth and ·proceeded so far as to leave no 

indubitable evidence. 

DISTRIBUTION IN TIME 

Some of the notably large ash-flow deposits of 

the world, and others that have been most important 

in the development of an understanding of them, 

have been mentioned (p. 14, 23). A compilation of 

known localities would provide useful references for 

several lines of geologic study, but is not within 

the scope of this paper. Moreover, papers reporting 

newly discovered occurrences of welded tuffs, or 

the extension of previously recognized areas, are 

appearing in ever-increasing numbers, and any list 

of known localities would soon be out of date. On 

the other hand, some consideration of distribution 
DISTRIBUTION IN TIME 49 

of welded tuffs in time seems to have significance, 

and for this reason a discussion of known preTertiary occurrences of welded tuffs will be helpful. 

Volcanic activity has persisted throughout geologic time, and probably welded tuffs have always 

been involved in that activity. A compilation of all 

recognized occurrences would show a very great 

preponderance of those in Tertiary and Pleistocene 

time with a remarkable decrease in their number in 

older rocks. No doubt their recognition will be 

precluded in many of these older rocks, but there 

is increasing evidence that detailed studies, backed 

by a knowledge of significant characteristics, will 

reveal a welded-tuff origin in many rocks that might 

otherwise be misinterpreted. The increasing interest in the geologic occurrence of welded tuffs has 

led to the recognition of several Precambrian occurrences within the past few years. 

One of the major objectives of this report has 

been to present the characteristics of welded tuffs, 

as seen in those rocks in which there has been little 

or no postdepositional modification, and to trace 

significant characteristics into tuffs whose origin 

has been obscured by different changes. The authors feel that an intimate knowledge of significant 

features will lead to identification of increasing 

numbers of welded-tuff occurrences in older rocks. 

MESOZOIC 

The Brisbane tuff of Australia of Triassic age has 

been described by Richards and Bryan (1934, p. 

51). A suite of specimens of the Brisbane tuff was 

received through the courtesy of Bryan and one of 

these is shown in figure 75. A great thickness of 

welded tuffs of Triassic age occurs in Inyo County, 

Calif., and has been studied by Ward Smith, U.S. 

Geological Survey (written communication, 1950). 

A Cretaceous age is assigned to glassy welded tuffs 

from Montana, which were studied by Barksdale 

(1951). Welded tuffs studied by Solovev (1950, p. 

211) from the southern Sikhote-alin region of Siberia are said to be Late Cretaceous and early Tertiary in age. 

PALEOZOIC 

Tuff deposits of "Pelean" origin have been described from the Ordovician of Snowdon in Wales 

by Dakyns and Greenly (1905), and by Williams 

(1927). Oliver (1954) has described and given 

excellent illustrations of "welded tuffs" in the Lake 

district of England and in Wales. 

Bodenhausen (1955) has given detailed descriptions of welded tuffs from the lower Carboniferous 

of Corsica. His illustrations present many of the 

relations that characterize devitrified welded tuffs. 

Some of these show a large proportion of lenslike 

areas with coarse-grained devitrification products, 

clearly representing areas that were originally 

pumice. 

Ross (1958) has studied volcanic materials that 

occur in drill cores from a deep well ( 4,000 feet) in 

Clinch County, southeastern Georgia. One horizon 

in the well is characterized by welded-tuff fragments representing materials derived from some 

densely welded and devitrified tuff body. Above 

that is about 35 feet of material which was originally a glassy welded tuff that has been replaced by 

laumontite. Applin (1951, p. 9) concludes that 

these tuffs are probably of early Paleozoic age. Volcanic materials are widespread in southeastern 

Georgia and Florida, and welded tuffs will probably 

be recognizable in other localities of the region. 

Weyl (1954b, p. 28, 29) has considered the possibility of welded tuffs in central Europe and says, 

The wide distribution and large quantity of younger welded 

tuffs pose the question of whether such rocks are not in 

existence also in the volcanic series of central Europe, but 

have escaped identification so far. Judging by the magmatic-tectonic occurrence of the younger welded tuffs and 

their chemistry, similar formations would be expected first 

of all in the region of an acid subsequent volcanism. Thus, 

they could be perhaps encountered in the system of sandstones, shales, and conglomerates of the lower Permian 

(Rotliegendes) in the central mountains of Germany. [von 

Gaertner called attention to tuffs of this system in the 

Forest of Thuringia] * * *. The porphyritic tuffs of the 'Rotliegendes' system include 

rocks, the description of which-mostly written in older 

times-fit completely the character of welded tuffs. 

Weyl also calls attention to the streaked texture, 

coarse bedding, platelike jointing, and poorly delineated stratification in the tuffs of the Sturmheide 

porphyry and Kickelhahn porphyry. He has also 

mentioned the porphyry tuffs of the Black Forest 

and Oden Forest that suggest derivation from flows. 

Nuees ardentes were mentioned in connection with 

the origin of Trass in Brohltal. 

PRECAMBRIAN 

Rocks of Precambrian age that retain evidence of 

welded-tuff origin are of special significance. For 

this reason Bouladon and Jouravsky (1954, 1955) 

have made an outstanding contribution in presenting the first clear-cut evidence of ignimbrites of 

Precambrian age from the Anti-Atlas region of 

Morocco. The descriptions and illustrations presented by these geologists show good welded-tuff 

structures. Through the courtesy of Bouladon the 

authors received an excellent suite of these rocks. 
50 ASH-FLOW TUFFS 

The feldspar phenocrysts have been altered, but the 

shard structures show uncommonly good preservation. Traces of the parallel growth of de vi trification structure in the individual shards (axiolitic 

structure) are still observable. A photomicrograph 

of one of these specimens is illustrated in figure 98. 

Ignimbrites of Precambrian age have been described from the Dalarna region of central Sweden 

by Hjelmqvist (1956). His illustrations present 

excellent examples of welded-tuff structures. Several of these show marked distortion, but without 

loss of clear evidence of their true origin. The distorted welded tuff illustrated in figure 78 is derived 

from this same group of welded tuffs of Precambrian age (Hjelmqvist, written communication, 

1956). The recognition of the character of this 

specimen caused Ross to communicate with Prof. P. 

Geiger concerning the occurrence of welded tuffs in 

Sweden. (See footnote by Hjelmqvist, 1956, p. 9). 

Ross has identified welded tuffs from two deep 

wells in Texas, one, No. 4-B Masterson well, from 

near the Canadian River, Potter County, at a depth 

of 1,952 feet; and the other from the Phillips No. 

1-A Stevens well, Bailey County, Tex., at a depth of 

8,226 feet. A valuable paper on the basement rocks 

of Texas and southeast New Mexico has been published by P. T. Flawn (1956). This intimate knowledge of the basement rocks of Texas enabled Flawn 

to give the authors information about the geologic 

position of the aforementioned welded tuffs. He 

wrote (written communication, 1956) : 

I believe that the rhyolitic rocks penetrated in both these 

wells are late Precambrian age and part of an extensive 

late Precambrian volcanic terrane which I have called the 

Panhandle volcanic terrane. I suspect that in a rhyolitic 

volcanic terrane as extensive as this one you would expect 

to find welded tuffs * * *. 

Thompson and Williams (1956) have described 

glowing avalanche deposits of early Precambrian 

age in the Sudbury basin of Canada. 

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54 ASH-FLOW TUFFS 

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FIGURES 16-98 · 
56 ASH-FLOW TUFFS 

FIGURE 16.-Glassy ash-flow tuff from South Sumatra collected 

by Westerveld. Characterized by platelike, slightly distorted 

shards. Note the absence of fracturing of the slender 

branches of the large shard near the right. An unusually 

massive shard lies in the upper third of the figure. 

FIGURE 18.-Glassy ash-flow tuff from the Ammon quadrangle, 

southeastern Ic!aho, collected by Mansfield. The tuff is virtually nonwelded, but there has been slight warping of the 

shards. There is an uncommon number of unbroken 

nearly circular glass-bubble walls. There is a wide range 

in the size of the shards and interstitial dust is present. 

Phenocrysts are completely absent. 

FIGURE 17.-Ash-flow tuff from the Ammon quadrangle, southeastern Idaho, collected by Mansfield. Composed of large 

platelike shards, which are completely nonwelded and with 

a large proportion of pore space (small white areas). 

Very coarse grained devitrification products (feldspar and 

cristobalite) have developed. A large feldspar phenocryst 

is on the right margin. 

FIGURE 19.-An ash-flow tuff from Iceland, received from 

Tomas Tryggvason. There may be slight welding, but there 

is no distortion of the structures. A large euhedral plagioclase crystal at the bottom of the figure has been embayed 

on its right margin. The rounded area near the upper 

left corner is a pumice fragment with nearly round 

vesicles. Secondary alteration has resulted in the formation 

of a little bright-green celadonite. 

PHOTOMICROGRAPHS OF NONWELDED OR SLIGHTLY WELDED TUFFS 
ASH-FLOW TUFFS 

FIGURE 20.-Ash-flow tuff showing alignment of shards due to 

orientation during emplacement by flowage rather than to 

compression. Several Y -shaped shards are observable. At 

the extreme right is a large twinned plagioclase crystal, 

embayed along its left margin. 

FIGURE 22.-Glassy welded tuff from South Sumatra collected 

by W esterveld ( 1942) . This tuff is characterized by sharply 

angular shard fragments, many of which represent glass 

walls at the junction of several bubbles. The tuff is almost 

without phenocrysts. 

FIGURE 21.-The large central area represents a fragment of 

an older welded tuff enclosed in a younger tuff material. 

In the older tuff there is thorough welding and marked 

distortion of the shards. The younger tuff is poorly welded, 

but its shards show evidence of alinement. The small white 

areas represent pore spaces. 

FIGURE 23.-Glassy ash-flow tuff from Barley Canyon, Valles 

Mountains, N.Mex., collected by Ross and Smith. The tuff 

shows only slight welding, but platelike shards on the extreme right are bent against the fragment of quartz. A 

large irregular bubble wall on the left encloses a groundmass of tuff fragments. 

PHOTOMICROGRAPHS OF SLIGHTLY WELDED ASH-FLOW TUFFS 

57 
58 ASH-FLOW TUFFS 

FIGURE 24.-Glassy tuff with a wide range in the size of the 

shards and with much interstitial dust. Near the upper 

left is a biotite crystal, but the Arequipa tuffs are, for the 

most part, very low in phenocrysts. 

FIGURE 26.-Collected at Ka.mpong, Getang W. of Hoetambolon Samosier Peninsula, Lake Toba. A glassy welded tuff with 

compressed shards molded against the phenocrysts. Most of the feldspar and quartz occurs as fractural angular 

grains, but some show rounding by resorption. This speci- men illustrates an uncommonly crystal-rich welded tuff. Biotite (dark plates) is characteristic of the Sumatra tuffs. 

FIGURE 25.-Glassy welded tuff. Disruption during flowage or by explosive violence has resulted in a large proportion of fragments that lack characteristic shard forms, but a shard representing the glass walls between three bubbles lies in the upper part of the figure. The dark area is one of many fragments of andesitic rock. Very calcic plagioclase, also derived from alien invaded rocks, is present. 

FIGURE 27 .-Collected on the road to Sibolga near the waterfall Toroetoing, Lake Toba region, North Sumatra. A 

glassy tuff with only slight welding and no distortion of 

the shards. 

PHOTOMICROGRAPHS OF WELDED TUFFS FROM PERU AND SUMATRA 

Specimens for figures 24 and 25 are from the Arequipa region, Peru, collected by Fenner. These are the type of tuff called sillar by Fenner. 

Specimens for figures 26 and 27 are from the Lake Toba region, North Sumatra. Collected by N. Wing Easton in 1892 and part of a suite of 

specimens received from Zeiljlmans V. Emmichoven, and A. L. Simons of the East Indian Geological Survey. 
ASH-FLOW TUFFS 

FIGURE 28.-Welded tuff from 40 miles west of Crestone, Saguache County, Colo. The 

shards are largely glassy, but the narrow white margin marks incipient devitrification, 

whose materials are too fine to be resolved by the microscope. The shards are only 

slightly distorted, yet there has been slight plastic yield and very thorough accommodation of each shard- to its neighbor. Phenocrysts are sparse. 

FIGURE 29.-Tuff of the Rattlesnake formation from central Oregon, collected by William 

T. Pecora, U.S. Geological Survey. The individual shards show slight flattening, due to 

compression, but uncommonly perfect retention of the primary structure. Many of the 

unbroken glass bubbles are slightly distorted, and are now oval in outline. The shards 

are dominantly glassy, but local areas show partial devitrification. 

PHOTOMICROGRAPHS OF WELDED TUFFS SHOWING ONLY SLIGHT DISTORTION 

552858- 61--5 

59 
60 ASH-FLOW TUFFS 

FIGURE 30.~An obsidianlike welded tuff with perlitic cracks. 

There is only a vague retention of tuff structure, now represented by discontinuous horizontal lines, that suggest, but 

are not adequate proof of, welded tuff. However, the 

geologic relations confirm such a derivation. 

F IGURE 32.-From Leach Canyon near Iron Springs, Utah, 

collected by J . Hoover Mackin. Thorough welding and 

strong compression have only slightly impaired the shard 

structure. Devitrification has followed welding. A nearly 

euhedral biotite phenocryst lies in the upper left corner 

and one of plagioclase in the lower right. 

FIGURE 31.-From Jamarillo Creek. Glassy welded tuff showing evidence of plasticity of the glass during welding and 

extreme distortion. Glass material has been squeezed into 

a deep embayment in a quartz crystal through an opening 

about 0.2 mm in diameter. 

FIGURE 33.-Welded tuff from Granite Mountain near Iron 

Springs, Utah, collected by Ross and Smith. The distortion 

is similar to that in figure 32, but the specimen in figure 

32 is devitrified while this specimen has remained glassy. 

PHOTOMICROGRAPHS SHOWING DISTORTION OF WELDED-TUFF STRUCTURES 

Specimens for figures 30 and 31 from the Valles Mountains, N. Mex., were collected by Larsen, and Ross and Smith, respectively. 
ASH-FLOW TUFFS 

FIGURE 34.-A fine-grained glassy welded tuff showing extreme 

compression and molding against an embayed plagioclase 

phenocryst. The shard structure is recognizable, although 

it simulates the flow structure of an extrusive rhyolite. 

FIGURE 36.-Glassy welded tuff with extreme compression and 

distortion of the shards. These are molded against the 

phenocrysts until there is a marked simulation of flow structure, very similar to the specimen shown in figure 37. 

However, this specimen shows recognizable tuff structures 

at the extreme right, and a distinct discontinuity of the 

stretched shards that resemble flow lines. Represents a 

tuff with abundant phenocrysts. 

FIGURE 35.-Glassy welded tuff from southeastern Idaho, collected by Mansfield. Thoroughly welded tuff with marked 

compression of the shards, but with good retention of the 

structure. Note the flattened Y-shaped shard, upper left. 

FIGURE 37.-A glassy flow-banded rock from an intrusive dike, 

collected by Wilbur Burbank, U.S. Geological Survey, from 

the Twin Lakes tunnel, Lake County, Colo. Burbank believes that this represents a feeder for the immediately 

overlying welded tuffs of the area. There is no vesiculation 

and the flow lines, although nearly distorted, are more 

continuous than those in figure 36. 

PHOTOMICROGRAPHS SHOWING COMPARISON OF WELDED TUFF AND FLOW STRUCTURES 

61 
62 ASH-FLOW TUFFS 

FIGURE 38.-Photomicrograph of welded tuff from the Valles 

Mountains, N.Mex., collected by Ross and Smith. Welldeveloped parallel arrangement of the shards in a thoroughly welded tuff is shown. The shards were originally 

platelike and so the elongatiem is probably not due to 

stretching. 

FIGURE 40.-Photomicrograph of devitrified welded tuff that 

has undergone very marked compression, but retains recognizable shard structure. Just below the upper right corner 

there is a greatly flattened bubble (marked with an arrow) . 

To the right is a flattened Y -shaped shard. 

FIGURE 39.-Photomicrograph of glassy welded tuff from Lake 

Toba, northern Sumatra, between Pagaran Psang and 

Sekanan, along the road from Taroetoeng to Sibolga. Collected by N. Wing Easton, 1892. A thoroughly welded 

glassy tuff with large pumice fragments that have collapsed 

and all pore space eliminated, but with a retention of 

pumice structure. Dark areas represent biotite plates. 

FIGURE 41.-Photomicrograph of glassy specimen derived from 

the same welded-tuff horizon as specimen shown in figure 

40. Slightly greater stretching has eliminated all clearly 

recognizable shard structure, and the specimen resembles a 

flow rock. However, the lenses lack the continuity of 

typical flow rocks. Compare with figures 34 and 36. 

EXTREME DISTORTION OF THE WELDED-TUFF STRUCTURES 

Specimens for figures 40 and 41 were collected by J. Hoover Mackin from near Iron Springs, southeastern Utah. 
ASH-FLOW TUFFS 

FIGURE 42.-Photograph of a hand specimen showing nonwelded, uncollapsed pumice fragments that 

stand in relief on the weathered surface. The pumice retains about 70 percent of pore space. 

Compare with figure 1, which represents a completely welded horizon from the same flow. 

FIGURE 43.-Photomicrograph of thin section of the same tuff 

shown in hand specimen in figure 42. Several noncollapsed 

glassy pumice fragments are shown. The white areas are 

uncommonly large glass shards. This tuff contains a large 

proportion of alien rock fragments and crystal grains. 

FIGURE 44.-Photomicrograph of glassy welded tuff from unknown locality in the Western United States. On the right 

is a thoroughly welded shard structure. On the left is a 

large pumice fragment in which there has been complete 

collapse of the pore spaces and welding into a compact glass, 

but with retention of the pumice structure. Compare with 

the noncollapsed pumice shown in figure 43. 

PUMICE IN ASH-FLOW TUFFS 

Specimens shown in figures 42 and 43 are from the basal part of the Battleship Rock ash flow 5 miles north-northeast of Jemez Springs, N.Mex. 

63 
64 ASH-FLOW TUFFS 

FIGURE 45.-Specimen from the Valles Mountains, N.Mex., collected by Ross and Smith. The large light-colored area represents a pumice fragment in which there has been complete collapse and elimination of internal pumice struc- ture, but with traces of the original pumice structure re- tained in the taillike projection extending toward the 

lower right corner and molded against a feldspar grain. 

Compare with the frayed ends of collapsed pumice frag- ments in figure 13. 

FIGURE 47.-From Redondo Peak, Valles Mountains, N.Mex. In the lower left corner is an area of welded tuff with much flattened shard structure (diagonal). The pumice that formed the remainder of the figure was much more plastic than the welded shards, and was distorted and molded around the welded-tuff part. The original pore space of the pumice has been completely eliminated but the pumice structure is retained. 

FIGURE 46.-A glassy welded tuff from Kuinimi, Kaga, Japan, 

U.S. National Museum specimen 61728. The left two-thirds 

of the figure represents ash material, and the right third 

a pumice fragment. The tuff has been welded into a compact glass and subsequently perlitic fractures have developed. The collapsed pumice structure gives this fragment 

the appearance of a flow rock. 

FIGURE 4.8.-From Redondo Peak, Valles Mountains, N.Mex. 

A large area of pumice that has completely lost its original 

porosity and has been molded against the quartz phenocrysts and at the same time greatly distorted. 

PHOTOMICROGRAPHS SHOWING PLASTICITY OF SHARD AND PUMICE MATERIALS 
ASH-FLOW TUFFS 

FIGURE 49.-Hand specimen of a ledge-forming facies of the welded tuff that crops out 

over an extensive area 8 to 10 miles east-southeast of Idaho Falls, Idaho, in the Ammon 

quadrangle. This facies (6 to 8 ft thick) is everywhere characterized by abundant 

lithophysal cavities that developed after complete welding and distortion of the shard 

structures. In this specimen the lithophysae are 2 to 3 em in diameter. This bed is 

not recognizable as a welded tuff from field relations, and only by microscopic study 

is its true character revealed. 

FrGURE 50.- Photomicrograph of thin section cut from a dense 

area in the specimen represented in figure 49. The tuff 

structure is clearly evident but seems to have been distorted 

while still plastic by the development of the closely spaced 

vesicula r cavities shown in fig ure 49. 

FIGURE 51.-Photomicrograph of thin section of a specimen from the same horizon as figure 49, but collected 8 or 10 miles farther southeast. The vesicles are larger and more elongated (as much as 4 em) and much more widely spaced. The tuff structure has undergone extreme flattening, but is still recognizable. Note flattened shards that were originally 

Y shaped. The structure of devitrification products cuts across the shard structure. 

DISTORTION OF TUFF STRUCTURE 

65 
66 ASH-FLOW TUFFS 

1 2.25 cni 1 

FIGURE 52.-Photograph of a large hand specimen of a welded tuff. The dark areas are vesicular cavities which developed after 

deposition and complete welding of the tuff. Their presence indicates the ash retained volatiles in solution that were released 

after welding and during partial devitrification. Two circular areas near the lower left part of the figure show where lithophysae 

with concentric shell structure have broken out. X 2/ 3. 

FIGURE 53.-Photomicrograph of thin section cut from the specimen shown in figure 52. The rock is dominantly glassy, but the dark circular area represents a spherulite that has grown out from a plagioclase phenocryst acting as a nucleus. The feldspar-cristobalite intergrowth of the spherulite is not actually dark colored, but appears dark in the photograph because of the very strong dispersion of light resulting from the fine-grained intergrowth of materials with very different mean indices of refraction (K-Na feldspar= 1.524, cristobal- ite= 1.486). 

FIGURE 54.-Photomicrograph of part of a very large spherulite. The differences in the devitrification products have caused three distinct concentric zones. The inner zone seems almost black because of the very marked dispersal of transmitted light. Outside of that is a lighter zone where the ash-shard structure is observable, but it also shows in the third and outer zone. 

SPHERULITES AND LITHOPHYSAE SPECIMENS Collected by Mansfield in southeastern Idaho. 
ASH-FLOW TUFFS 

FIGURE 55.-Specimen collected half a mile south of Sulphur 

Springs, Jemez Springs quadrangle, Valles Mountains volcanic region, New Mexico, by Ross and Smith. The dark 

ar!!:_as retain tl'le shard structures, but in the lighter areas 

radial aggregates of feldspar and cristobalite have developed 

and completely destroyed the shard structure. The left 

third of the figure represents a large pumice fragment. 

FIGURE 57 .-A large pumice fragment imbedded in shard material in which the structure is still recognizable. The 

devitrification of the pumice grains has destroyed the 

original fibrous structure. Specimen collected by Ross and 

Smith, Valles Mountains, N.Mex. 

FIGURE 56.-The same area shown in figure 55, but under 

crossed nicols. This shows the coarse-grained intergrowth 

of feldspar and cristobalite in areas that have lost their 

shard structure (light) and the much finer structure in 

those areas where the structure has been retained (dark). 

FIGURE 58.-The same area shown in figure 57, but under 

crossed nicols. This shows the tendency, observable in 

almost all welded tuffs, for coarse-grained devitrification 

products (feldspar and cristobalite) to develop in the 

pumice fragments ; and much finer grained ones in the 

shards. 

PHOTOMICROGRAPHS SHOWING DEVITRIFICATION OF PUMICE 

552858- 61--6 

67 
68 ASH-FLOW TUFFS 

FIGURE 59.-Welded tuff that has undergone welding and 

extreme distortion of the still-recognizable shard structures. 

FIGURE 61.-Welded tuff from southeastern Idaho, collected 

by Mansfield. All the shards are devitrified, but the light 

area represents a pumice fragment in which coarser 

grained intergrowth of cristobalite and feldspar has developed and where the pumice structure has been partly 

destroyed. 

FIGURE 60.-Same area as illustrated in figure 59, but under 

crossed nicols. This shows a coarse-grained radial aggregate of feldspar and cristobalite laths which have developed independently of the original shard structure. 

FIGURE 62.-Welded tuff from Redondo Peak, Jemez Springs 

quadrangle, Valles Mountains, N.Mex., collected by Ross and Smith. Welding and extreme flattening of the ash structure is represented at the extreme right. In the central part of the figure (white) the development of uncommonly coarse grained devitrification products has completely destroyed the original structure. If the entire rock had undergone the same loss of structure, evidence of derivation from a tuff would be lost. 

PHOTOMICROGRAPHS SHOWING DESTRUCTION OF STRUCTURE BY DEVITRIFICATION 

Specimens for figures 59 and 60 were collected by Ross and Smith from the Valles Mountains, N.Mex. 
ASH-FLOW TUFFS 

FIGURE 63.-Welded tuff from the Valles Mountains, N.Mex., 

collected by Ross and Smith, showing a very large spherulite in which the radial aggregates of feldspar and cristohalite are well represented. At the left t he secondary 

minerals have developed a plumose structure. All direct 

evidence of tuff structure has been destroyed. 

FIGURE 65.-Welded tuff from the Lake Toba region, Sumatra, 

showing spherulitic areas developed in a welded tuff. 

FIGURE 64.-Welded tuff from the Valles Mountains, N.Mex., 

showing the development of groups of spherulites, with concentric banding due to variations in grain size of the 

aggregates of cristobalite and feldspar. 

FIGURE 66.-Welded tuff from the Valles Mountains, N.Mex., 

collected by Ross and Smith. An open cavity in a welded 

tuff in which platelike crystals of tridymite have developed. 

PHOTOMICROGRAPHS OF DEVITRIFICATION MATERIALS 

Figures 63, 64, and 65 illustrate the complete destruction of the original tuff structure. 

69 
70 ASH-FLOW TUFFS 

FIGURE 67.-Photograph of a polished section through a so- called thunder egg that represents a type of hollow spheru- lite which commonly develops in glassy volcanic rocks. Vola- tiles originally in solution in the glass collect at the center during the development of anhydrous minerals which form the gray border-the material shown in figures 68 and 69. The originally hollow central part was subsequently filled 

with chalcedony. X 1. 

FIGURE 69.-Photomicrograph of the outer rim material of figure 67. The section represented in this figure was cut parallel to the plane of deposition. Figure 68 and this figure shows the marked difference in flattening and distortion of the shard structure in the two planes ; in both planes the shard structures are well preserved. Compaction and weld- ing was complete before devitrification and the parallel 

growth of feldspar and cristobalite (diagonal structure in this figure) developed independently of the original shard structure. Compare with shards in figures 71-74, in which the development of feldspar and cristobalite started at the grain boundary of each shard. 

FIGURE 68.-Photomicrograph of the outer rim material of 

figure 67. The section represented in this fig ure was cut 

perpendicular to the plane of deposition. 

FIGURE 70.-Photomicrograph of welded tuff from southeastern 

Idaho, collected by Mansfield. The larger shards seem to 

have undergone very little distortion, but the others are 

molded against them giving a marked parallel structure. 

Welding was complete before devitrification and the feldsparcristobalite formed parallel aggregates cutting across the 

shard structure (horizontal) . 

DEVITRIFICATION CHARACTERISTICS 

Figures 67, 68, and 69 represent material from central Oregon, collected by Gen. J. S. Hatcher, U.S. Army, retired. 
ASH-FLOW TUFFS 

FIGURE 71.-Thin section of tuff showing welding and devitrification, but without distortion of the shard structure. 

FIGURE 73.-Devitrified welded tuff. Most of the thin section shows little distortion of the original shard structure, 

but at the right side of the figure there is marked molding 

of the shards around a feldspar phenocryst (white). 

FIGURE 72.-An area from thin section shown in figure 71 

under high magnification. The parallel intergrowth of feldspar and cristobalite ( axiolitic structure) is well represented. The large shard fragment represents the walls of 

several hubbies. 

FIGUHE 74.-Devitrified welded tuff from an unknown locality 

in the Western United States showing uncommonly perfect 

axiolitic structure. Although the devitrification products 

(feldspar and cristobalite) are uncommonly coarse grained, 

the original forms of the shards are perfectly retained. 

PHOTOMICROGRAPHS OF WELDED TUFFS SHOWING AXIOLITIC STRUCTURE 

71 

Specimens for figures 71, 72, and 73 are from the Valles Mountains, N.Mex.; collected by Ross and Smith. Axiolitic structure is the result of crystal 

growths, beginning at the shard boundaries, and meeting at a line of discontinuity (dark central line). The high relief is due to the large difference in index of refraction of the devitrification products-cristobalite and feldspar. Compare with figure 95 which is a reproduction of a sketch 

by Zirkel illustrating the structure he named "axiolitic." 
72 ASH-FLOW TUFFS 

FIGURE 75.- Welded tuff from Australia, collected by Richards 

and Bryan ( 1934 ). The shard structure is discernible, although it has been partly obscured by devitrification. Other 

specimens from the Brisbane tuffs show even greater loss of 

structure. 

FIGURE 77 .-Devitrified welded tuff from Pusan, Korea, collected by Allen Nicol, U.S. Geological Survey. The shard 

structure is only dimly retained, but axiolitic structures are 

obaervable at the left. 

FIGURE 76.-Devitrified welded tuff from Glashutte, Schermitz, 

Hungary; U.S. National Museum specimen 36270. The shard 

structures are obscured, but not entirely lost. An euhedral, 

high-temperature quartz phenocryst is near the bottom. 

FIGURE 78.-Thin section from an old German collection of 

slides, which was marked as representing a rock from Elfdalen Daloria, Sweden. Hjelmqvist states that these welded 

tuffs are sub-Jotnian (sub-Keweenawan) in age. The shard 

structure has undergone stretching so that parts of the thin 

section are indistinguishable from a flow rock, but, locally, 

shard structures are preserved. 

PHOTOMICROGRAPHS OF WELDED TUFFS FROM FOREIGN LOCALITIES 
ASH-FLOW TUFFS 

FIGURE 79.-A large glassy pumice fragment imbedded in shards 

(lower left). Welding is slight, but the pumice fragment 

seems to have undergone distortion. 

FIGURE 81.-Andesitic welded tuff from Aso caldera, Kyusyu, 

Japan, collected by C. G. Johnson, U.S. Geological Survey. 

A thoroughly welded tuff, with abundant andesitic phenocrysts and rock fragments. Augite crystals lie near the 

center and lower right corner ; an euhedral plagioclase crystal 

is near the lower left. 

FIGURE 80.-Fine-grained andesitic tuff. Welding is moderate 

but there has been warping and accommodation of shards to 

one another. The slide shows many rock fragments and 

crystals derived from an earlier andesitic rock. The large 

crystal (upper right) is augite. 

FIGURE 82.-Welded tuff from near McKays, Calaveras County, 

Calif., collected by Smith. Illustrates the type of biotiteaugite latite described by Ransome. The tuff structure is 

poorly preserved. This specimen represents a welded tuff 

uncommonly rich in crystal fragments. Sonie of these materials represent fragments derived from older andesitic rocks. 

PHOTOMICROGRAPHS OF WELDED TUFFS OF INTERMEDIATE COMPOSITION 

Figures 79 and 80 are from Erevan, Russian Armenia; U.S. National Museum specimen 52079. 

73 
74 ASH-FLOW TUFFS 

FIGURE 83.-Photomicrograph of specimen collected from the 

bank of the Malargue River at La Valenciana Mendoza, 

Argentina, by F. G. Bonorino, and described by him as 

andesite. The thin section was cut parallel to the plane of 

deposition. 

FIGURE 85.-Photograph of a hand specimen. The black areas 

represent the larger pumice fragments which have collapsed, 

and lost the original pumice structure. X 11;2. 

FIGURE 84.-Photomicrograph of a thin section cut from the 

same specimen as figure 83, but perpendicular to the plane 

of deposition. Note the greater flattening of the structure. 

FIGURE 86.-Photomicrograph of a thin sction of specimen 

shown in figure 85. The dark lenses represent pumice fragments in which the original pumice structure has been largely 

destroyed. 

ANDESITIC WELDED TUFFS 

The tuff shown in figures 83 and 84 is glassy and shows very complete welding and marked flattening of the shard structure. A large proportion of 

the crystal and rock fragments represent alien materials. The specimen for figures 85 and 86 illustrates andesitic welded tuffs from Costa Rica, 

collected by Gabriel Dengo, from the Nuestro Amo power plant, Virilla River. 
ASH-FLOW TUFFS 

FIGURE 87.-Photographic reproduction of a colored drawing by Iddings ( 1885-86, pl. 16, fig. 2). While this is not a 

welded tuff it is presented as a part of the development of 

Iddings' concept of welded pumice. 

FIGURE 89.-Photomicrograph slightly enlarged and copied from an illustration from Iddings' study ( 1899, pl. 50, fig. 100) of rhyolitic rocks from Yellowstone National Parle The illus- tration is also shown in another report by Iddings (1909, 

fig. 22, p. 332) and bears the title "Welded pumice with axiolites." This illustration is an uncommonly good representation of an ash-flow tuff that has undergone complete 

welding, some distortion of the shards, molding against the quartz phenocryst, and devitrification; all with good retention of the shard structure. 

FIGURE 88.-Photomicrograph of one of Iddings' thin sections 

of rocks from Yellowstone National Park. This evidently 

represents the same material shown in figure 87, but may not have been the same thin section used by Iddings for the drawing shown in figure 87. 

FIGURE 90.-Photomicrograph of a thin section from a group 

of specimens collected in the Valles Mountains, N.Mex., by 

Holmes and Powell and turned over to Iddings for study. 

The specimen from which this thin section was cut was described as "from the cliff overlooking Jemez Springs," and 

is from the same region now under study by the authors. 

ILLUSTRATIONS SHOWING SOME OF THE ORIGINAL MATERIAL DESCRIBED BY IDDINGS AS 

WELDED PUMICE 

75 

Figures 87 and 88 are from Obsidian Cliff, Yellowstone National Park, and represent some of the material called welded pumice by Iddings. 

This is evidently a brecciated and rewelded pumice from a rhyolitic lava flow. 
76 ASH-FLOW TUFFS 

FIGURE 91.-Welded tuff from Fall River basin, west slope of mountain north of the head of Conant Creek (Iddings, 

1899, p. 378; specimen 1958). Glassy welded tuff with a large inclusion of andesite that seems to be typical of the Yellowstone National Park welded tuffs. 

FIGURE 93.-Zirkel collection, specimen 22297; U.S. National 

Museum specimen 350. Locality described as "southeast 

from Wadsworth, Nevada." Glassy welded tuff. Near the 

left margin there is bleaching of the outer borders of the 

shards. The platelike character indicates that the thin section was cut diagonally to the plane of deposition. 

FIGURE 92.-Iddings' thin section 2150, not described by him. It belongs to a group from the northeastern corner of Yellowstone National Park. The dark part shows incipient 

devitrification and the light part is glassy. Thorough weld- ing and slight distortion of the shards is evident. 

FIGURE 94.-Zirkel collection, specimen 21834; U.S. National Museum specimen 499. Locality described as ''Mopung 

Hills, W. Humboldt, Nevada." The figure shows extreme flattening and stretching, but with good evidence of shard structures. The lenses show marked discontinuity, and in many places there are nearly parallel branches of a lens, due to flattening of a Y -shaped shard. Areas where very flat lenses are folded back upon themselves is a characteris- tic of much-compressed welded tuffs. 

PHOTOMICROGRAPHS OF WELDED TUFFS OF HISTORIC INTEREST 

Figures 91 and 92 are from Iddings' original set of thin sections of "pumice tuffs" from Yellowstone National Park. Figures 93 and 94 are reproduced from a very large collection of rocks studied by Zirkel as a part of the 40th Parallel Report ; a large proportion of this collection proved 

to be welded tuffs, and so these thin sections made about 80 years ago have now provided new evidence of the abundance of welded tuffs in the 

Basin and Range region. 
ASH-FLOW TUFFS 

FIGURE 95.-Photographic reproduction of a drawing pre- sented by Zirkel ( 1876, pl. 7, fig. 4). Zirkel did not recognize the full significance of the welded tuffs he ob- served, but this illustration shows that he recognized the essential petrographic characteristics. The parallel inter- growths of feldspar and cristobalite, for which he proposed the name " axiolitic" are well illustrated. Compare with these structures shown in figures 87- 90. 

FIGURE 97.-Photomicrograph of welded tuff from Taupiri 

Gap, Waikato River, North Island, New Zealand, collected by Ross. This represents the materials which were the basis for Marshall's (1935) outstanding contribution to au understanding of ash-flow tuffs. This tuff is welded and devitrified, and there has been dimming by devitrification, but little distortion of the tuff structure. The new Zea- land tuffs contain abundant phenocrysts of feldspar and quartz. 

FIGURE 96.-Photomicrograph of ash-flow tuff from the Valley of Ten Thousand Smokes, collected by Zies. Specimens of these tuffs available to the authors are nonwelded, but platelike shards which were bent against phenocrysts have been observed. Little induration is shown and the porosity is high. There is a wide range in size of materials, with dust, shards, and pumice fragments (extreme right) all present. A circular unbroken glass bubble lies near the center and another near the upper right corner. In the upper part of the figure there is a large irregularly shaped 

glass shard. 

FIGURE 98.-Photomicrograph of thin section of a welded and devitrified tuff cut• from one of several specimens submitted by Jean Bouladon. These specimens were collected by Bouladon and Jouravsky, and described by them (1954), from the Anti-Atlas region of Morocco. Their description 

represents the first recorded recognition of a welded tuff of Precambrian age. As can be seen from this thin sec- tion, the structure is remarkably good and traces of the axiolitic structure are recognizable. Compare with this structure in figures 71-74. 

CONTRIBUTIONS TO THE WELDED-TUFF CONCEPT 

77 
GEOLOGICAL SURVEY PROFESSIONAL PAPER 366 FRONTISPIECE 

Thin section showing oxidation and later partial 

reduction of the iron in a glassy welded tuff. 

Iddings (1899, pl. 50, fig. B) shows the same section in black and white. His belief that this was 

all originally red and that bleaching took place 

after deposition is confirmed by the presence of 

very fine grained microlites of magnetite in the 

light-colored area. From Yellowstone National 

Park. 

The duli brown cores of the large shards near 

the middle of this photomicrograph of glassy 

welded tuff represent the original character of the 

glass, and the red areas are a result of oxidation. 

The horizontal alinement of shards indicates 

marked compaction. USNM specimen 38771 

from Rio Blanco, 10 miles north-northwest of 

Guadalajara, Mexico. 

OXIDATION AND REDUCTION IN GLASSY WELDED TUFFS 
INDEX 81 

Tuff-Continued Tuff, welded-Continued 

welded, age -------------------------------------- 45, 48-50, 77 use of term ----------------------------------

Viscosity -------------------------------------------------------- 33, 40, 42 

8, 13, 14 Vitrophyre ------------------------------------------------------------------ 10 

Volatile content ____ 28, 31, 37, 38, 39, 40, 41, 44, 66 andesitic ---------------------------- 14, 17, 35, 73, 74 Tuff flows, incandescent ------------------------------------ 1, 13 

areal extent ------------------------------------------------ 29 Tuff lava, definition -------------------------------------------------- 8 W 

crystal content ---------------------------- 58, 60, 73 origin -------------------------------------------------------------------- 14 Welding, experimentally produced -------------------- 14 

definition ------------------------------------------------------ 8 Turner, H. W., quoted ---------------------------------------------- 9 fabrics developed ------------------------------------------------ 34 

erosion forms ---------------------------------------------- 30 Tuscan formation --------------------------------------------------------23 Welding processes ____________________ 11, 13, 18, 22, 24-26, 

glassy ________ ____ 14, 25, 28, 29, 33, 34, 35, 40, V 33-35, 41-44 

56, 57, 58, 60, 61, 62, 76 Wentworth, C. K. and Williams, Howe!, 

jointing ------------------------------------------------ 28, 29 Vallecito, Calif ---------------------------------------------------------- 10 quoted ------------------------------------ 3, 4, 5, 7 

mechanism of deposition ------------------ 6, ~9 Valles Mountains, N. Mex., Weyl, Richard, quoted -------------------------------------------- 49 

occurrence ___ _____ 14, 17, 18, 28, 29, 36, 38 ash-flow tuffs ____ 2, 17, 18, 19, 20, 21, Williams, Howe!, quoted ---------------------------------- 31, 48 

origin ------------------------ 9, 12, 14, 17, 18, 20, 22, 23, 28, 30, 31, 60, 62, 64, 67, 69, 75 Wineglass dacite flow ---------------------------------------------- 11 

33-35, 39, 41-44 Valley of Ten Thousand Smokes, pyrcolastic Y 

recent descriptions ---------------------------- 14-15 deposits --------------------- 5, 12, 13, 14, 16, Yellowstone National Park, volcanic 

recognition ____ 25, 26, 30, 65, 68, 69, 71, 77 20, 23, 31, 36, 39, 77 rocks __ __________ 11, 13, 20, 36, 42, 75, 76 reexamination -------------------------------- 9, 10, 11 Vents, volcanic -------------------------------------------- 16-18, 21 

rhyolitic -------------------------------------------------- 35, 75 volcanic, fissure type ---------------------------------------- 17 z 

study of -------------------------------------------- 1, 2, 13 on volcanic domes ------------------------------------ 17 Zeolites ------------------------------------------------------------------------ 44 

temperature of formation ---- 14, 26, 28, 39 open craters ------------------------------------------------ 17 Zirkel, Ferdinand, quoted -------------------------------- 4, 9 

thickness ---------------------------------------- 20, 26, 29 Vesiculation ------------------------------------------ 17, 39, 40, 61 Zoning, in flow units -------------------- 20- 22, 24, 26, 66 

U. S. GOVERNMENT PRINTING OFFICE: 1961 
A 

Page 

Agglomerate lava ------------ -------------------------------------------- 10 

Alaquez massif ------------------------------------------------------------ 8 

Alta ville, Calif ------------------------------------------------------------ 10 

Amphiboles ------------------------------------------------------------------ 44 

Anderson, Tempest, and Flett, J. S., quoted ---- 15 

Andesite ________ 10, 35, 39, 44, 45, 48, 58, 73, 7 4, 76 

Arequipa region, Peru ------------------------ 14, 28, 36, 58 

Ash, definition ---------------------------------------------------------- 3 

Ash, volcanic, definition -------------------------------- 3, 4, 7 

Ash fall, definition ----------------------------------------------------

Ash-fall materials, welding ------------------------------------ 5 

Ash-fall tuff, occurrence ---------------------------------------- 21 

recognition -------------------------------------------- 23, 37, 38 

Ash flow, definition ---------------------------------------------------- 3 

non welded ------------------------------------------------ 17, 18, 27 

study of ---------------------------------------------------- 1, 2, 15 

temperature -------------------------------------------------- 16, 21 

velocity ----------------------------------------------------------- ______ 41 

Ash-flow deposits, thickness -------------------- 2, 20, 22 

used as stratigraphic marker __ ______________ 14, 23 

Ash-flow deposits in foreign localities, 

Africa ---------------------------------------------------- 45 

Argentina ---------------------------------------- 2, 44, 45, 48 

Asia Minor ------------------------------------------------------------ 23 

Australia -------------------------------------------------- 13, 49, 72 

Queensland ---------------------------------------------------- 28 

Canada, Sudbury Basin ---------------------------------- 50 

central Europe ---------------------------------------- _______ 49 

Corsica __________ _ ---------------------------------------- 29, 45, 49 

Island of Scando:a --------------------------- --------- 29 

Costa Rica ---------------------------------------------------- 48, 7 4 

Ecuador ____ _ -------------------------------------------- 16 

El Salvador ---------------------------------------------- 47, 48 

England -------------------------------------------------------- 49 

Hungary ------------------------------------------------------------ __ _ 72 

Iceland ---------------------------------------------------- 2, 15, 56 

Indonesia ------------------------------- ------------------------------ 5 

Italy ---------------------------------------------------------------------- 9 

Phlegrean Fields ------------------------ ___ 7 

Japan _______ ------------------------ 2, 5, 7, 47, 48, 73 

Kuinimi, Koza -------------------------------- _____ ______ 64 

Kyushu ---------------------------------------------------- 3, 4 

Korea -------------------------------------------------------------------- 72 

Mexico -------- -------------------------------------------------------- 2 

Guerrera ------------------------------------------------ __ 29 

Morocco --------------------------------------- ____ 2, 37, 49, 77 

New Zealand 2, 13, 14, 23, 24, 28, 36, 41, 46 

North Sumatra, Lake Toba 

region ------------------------ 2, 28, 58, 62, 69 

Peru ----------------------------------------------- ____ 2, 5, 35, 36 

Russian Armenia -------------------- 8, 23, 28, 48, 73 

Siberia ------------------------------------------------ 49 

South Sumatra -------------------------------------- 14, 56, 57 

West Pasoemah region ___ __________ _ 

Sumatra -------------------------------------------------------- 23, 35 

Sweden -------------------------------------------- 2, 45, 50, 72 

Wales -------------------------------------------------------------------- 49 

Ash-flow materials, criteria for 

recognition ------------------------------------

dispersal by flowage __________________ _ 

2, 18 

41 

iron content ------------ --------------------------------- _ 31, 45 

nomenclature ____ ____ ___ --------------------------------- ____ __ 2- 3 

origin -------------------- ______ _ ----------------- 2, 12, 16, 27 

I NO EX 

Ash-flow materials-Continued 

welding ---------------------------------------------------- 5, 24-26 

Ash-flow tuff, areal 

extent ____ 2, 13, 14, 16, 17, 22-23, 29 

cooling history -------------------------------------------- 24, 26 

crystal content -------------------------------------- 8, 56, 57 

degree of sorting ------------------------------------------------ 19 

definition -------------------------------------------------------------- 3 

density ------------------------------------------------------------------ 24 

development of concept ---------------------------- 9, 75 

erosion patterns ____________________ 20, 21, 24, 29, 30 

gentle dips ---------------------------------------------------- 23, 24 

Chiricahua National Monument, Ariz ---------------- 30 

Collapse structures, linear ch:tracteristics ____ ____ 17 

Cotopaxi eruption ---------------------------------------------------- 16 

Crater Lake National Park ------------------------------------ 14 

Crater Lake region, volcanic 

rocks ---------------------------- 11, 13, 14, 23, 30 

Cristobalite ---------------- 27, 33, 36, 37, 41, 44, 45, 46, 

56, 66, 67. 68, 69, 70, 71, 77 

Crystallization, stages ------------------------------------------------ 10 

vapor-phase ______ ____ 5, 18, 21, 22, 24, 26, 30, 36 

Crystals, concentration ---------------------- 25, 26, 36, 73 

Curtis, G. H., quoted ------------------------------------------------ 5 

joint pattern ------------------------------------------------------ 21 D 

mapping ---------------------------------------------------------------- 2 Dacite ------------- ----------------------- 10, 11, 26, 30, 31, 45, 46 

mechanisms of deposition ---------- 15, 17, 38, 39 Devitrification _______ _ 9, 13, 18, 21, 24, 26- 28, 31, 33, 

mineralogy ---------------------------------------------------- 21 • 27 34, 36-38, 44- 45, 56, 59, GO, 62, 65, 66, 

origin ---------------------------------------------------------- 2, 8, 17 67, 68, 69, 70, 71, 72, 76, 77 

percentage of rock fragments ____ 8, 18, 21, 24 Dikes, rhyolite ---------------------------------------------------- 17, 2G 

permeability ---------------------------------------------------------- 40 Diller, J. S., and Patton, H. B., quoted __________ 11 

porosity ______ 24, 25, 26, 36, 37,46-47,57,62, 63,64 Directed-blast hypothesis ---------------------------------------- 17 

pyroclastic character ____________ 18- 19, 32, 33, 37 Dust, volcanic, definition ---------------------------------------- 4 

sorting ------------------------------------------------------------ 21• 22 volcanic, occurrence ------------------------------------ 18, 19 

stratification ------------------------------------ 16, 19• 20- 22 Dust clouds, definition ---------------------------------------------- 4 texture ------------------------------------------------------------------ 21 

thickness ------------------------------------------------ 13, 20, 26 

volume ------------------------------------------------------ 14, 17, 20 

welded, occurrence ------------------------------------ 17, 24 

recognition ------------------------------------ 18, 24, 30 welding ____ ____________ _____ T _____ __ 12, 17, 24-26, 33-35 

Aso caldera ------------------------------------------------------ 3, 4, 23 

Aso lava, areal extent ------------ ----------------------------------- 23 

definition ------------------------ --------------------------------- _ 3 

petrologic characteristics -------------------------------- 24 

use of term -------------------------------------------------- 4, 14 

Augite --------------------- ------------------------------------- 35, 39, 73 Auvergne region, France ___ ______ ____________ __ ___________ _____ 12 

Avalanche, volcanic, definition _________ _____ _____ 3, 6 

Avalanche depo-sits, glowing ____ __ __ __ 1, 4, 7, 14, 17 

Axiolite, definition ______________ --- -------------------------------- 4 

Axiolitic structure, description and 

definition ____________ _______ ________ 4, 9, 36, 71 

B 

E 

Ejecta, volcanic ---------------------------------------------------------- 12 

Eruptions, Cotopaxi, Ecuador -------------------------------- 16 

Katmaian ------------------------------------------------ 9, 13, 14 

observations ---------------------------- __ _ ----- ------------ 16, 31 

Pelean ---------------------------------------------------- 9, 17, 41 

producing ash-flow tuffs -------------------- 14, 16, 38 

Eutaxite, definition -------------------------------------------- 4, 10 

Exfoliation ----------------- ---------------------------------------- 30 

F 

Fabric --------------------------------------- ------------------------------------ 33 

Faults o~· rift zones ----------------------------------- __ __ __ 17 

Feldspar __ ----------- 27, 33, 35, 36, 37, 39, 44, 45, 56, 

58, 60, 66, 67' 68, 69, 70, 71, 73, 77 

Fenner, C. N., quoted -------------------- 5, 6, 8, 16, 39 

Fiamme, definition and description ____ 4, 7, 9, 24 

Flow banding ---------------------------------------- 10, 26, 30, 61 

Flow breccia, definition -------------------------- ________ ___ 12 

Bandelier formation 30 Flow structures ------------------------ 17, 24, 25, 34, 61, 72 

Fluidization ------------------------------------------------------------------ 3 Bare Mountain, Nev ------------------------------------------------ 14 

Battleship Rock, N. Mex __________ 19, 28, 46, 47, 63 

Biotite ---------------------------------------- 30, 35, 44, 58, 60, 62 

Bishop tuff ------------------------ 13, 14, 20, 23, 28, 42, 46 

Block and ash flow, use of term -------------------------- 7 

Blocks, definition ------------------------------------------------------- 4 

Blyth, F. G. H., quoted -------------------------------- 3, 4, 5 

Boyd, F. R., quoted -------------------------------------------------- 42 

Breccia ------------------------------------------------------------------ 9, 21 

cemented -------------------------------------------------------------- 9 

fused ---------------------------------------------------------------------- 26 

welded -------------------------------------------------------------------- 11 

Brisbane tuff ------------------------------------------ 13, 28, 49, 72 

Bubble fractures -------------------------------------------------------- 17 

c 

Calderas, Aso -------------------------------------------------------------- 14 

collapse, Krakatau type ------------------------------------ 14 

relation to welded-tuff formation __ 17, 18 

Canary Islands, Tenerife ---------------------------------------- 9 

Celadonite -------------------------------------------------------------------- 56 

Chalcedony ------------------------------------------------------ 20, 38, 70 

Fluorine, in late-"'tage minerals _______ __ __________________ 12 

Foreign materials __ ______________ ------------------------------------- 35 

40th parallel, exploration -------------------------------- 9, 76 

Fossil fumaroles _____ ___ _ ---------------------------- -------- ---- 30, 31 

Fusion, occurrence __ _______ ------------------------------------------- 26 

temperature _________ _ _______ ------------------------------------- 42 

See also tuff, fused. 

G 

Glass, brecciation ---------------------------------------- 11, 24, 26 

color ------------------------------------------------ 11, 24, 25, 45 

obsidianlike ---------------------------------------------------------- 26 

viscosity ---------------------------------------------------------------- 33 

Glowing clouds ---------------------------------------------- 4, 6, 39 

G lutwolken -------------------------------------------------------------- 4, 6 

Graded bedding ---------------------------------------------------------- 21 

Griggs, R. F., quoted ---------------------------------- ____ _ 7, 16 

H 

Heat conservation ---------------------------------------------- 41, 42 

Hibok-Hibok volcano ------------------------------------------ 17, 20 

Hornblende ------------------------------------------------------------ 35, 42 

79 
80 

Iddings, J. P., quoted ---------------------------------------- 4, 11 

Ignimbrite, age -------------------------------------------------- 49, 50 

definition -------------------------------------------------------------- 4 

microscopic characteristics ___ ___________ 34, 37, 45 

porosity ---------------------------------------------------------------- 46 

recognition in the field ------------------------------------ 23 

study of -------------------------------------------------------- 1, 14 

temperature -------------------------------------------------------- 41 

thickness -------------------------------------------------------------- 20 

use of term ------------------------'------------------------------- 13 

Incandescent tuff flow, definition ------------------------ 5 

description ------------------------------------------------------------ 14 

origin ---------------------------------------------------------------------- 14 

use of term ---------------------------------------------------------- 8 

Inclusions, clastic ------------------------------------------------------ 10 

Independence Pass region, Colorado ---------,---------- 17 

Indices of refraction ------------------------------------ 45-46, 71 

Iron content of ash-flow materials ---------------- 31, 45 

Iron Springs area, Utah ---------------------------- 34, 60, 62 

J 

INDEX 

Nuee ardente-Continued 

definition --------------------------------------------------------------

dual character ----------------------------------------------------

flowing properties ---------------------------------------- 15, 39 

occurrence ---------------------------------------------- 4, 17, 49 

processes producing ---------------------------------------- 17 

0 

Obsidian, colored bands ------------------------------------ 11, 24 

Obsidian Cliff rhyolite flow ------------------------------------ 11 

Obsidian spindles -------------------------------------------------------- 24 

Olivine -------------------------------------------------------------------------- 35 

Opal ------------------------------------------------------------------------ 20, 31 

p 

Pectinate structure -------------------------------------------- 4, 36 

Pelean cloud deposit ---------------------------- 12, 13; 17, 49 

Pelee, eruption ---------------- 5-6, 7, 12, 13, 15-16, 41 

Perret, F. A., quoted -------------------------------- 6, 16, 41 

Phenocrysts, composition ____________ 35, 39, 46, 58, 60, 

61, 64, 71, 72, 73, 77 

effect on distortion ---------------------------- 33, 34, 58 

number ____ 17, 22, 26, 33, 35, 39, 56, 57, 59, 61 

Rhyolite flows-Continued 

feeder dikes ------------------------------------------ 17, 18, 61 

sheared zones ------------------------------------------------------ 11 

Richards, H. C., and Bryan, W. H., quoted ______ 13 

Rickmers, W. R., quoted ------------------------------------------ 5 

Ring fractures ------------------------------------------------------------ 17 

Rittman, A., quoted -------------------------------------------------- 46 

River chan nels ---------------------------------------------------------- 9 

s 

Sand flow, areal extent -------··:·------------------------------- 23 

definition --------------------------------------------------------------

origin ------------------------------------------------ 12, 13, 16, 31 

use of term ---------------------------------------------------- 5, 7 

Sanidine ---------------------------------------------------------------- 22, 35 

San Juan Mountains, Colo -------------------------------------- 20 

Sedimentary deposits, characteristics ---------------- 1 

Shepherd, E. S., quoted -------------------------------------------- 40 

Sikhote-alin volcanic region ---------------------------------- 34 

Silica, percentage in ash-flow materials ____________ 31 

Joints 21, 24, 26, 28-29 Phonolite ---------------------------------------------------------------------- 10 

redeposition ------------------------------------------ 20, 41, 42 

Sillar, definition -------------------------------------------------------- 8 

origin ------------------------------------------------------------ 28, 36 

K use of term -------------------------------------------------- 5, 14 

Kaolin --------------------------------------------------------------------------- 31 

Katmaian eruption ------------------------ 13, 14, 16, 36, 39 

Komagatake volcano ---------------------------- 14, 17, 22, 31 

Krakatau caldera ------------------------------------------------------ 14 

I~ 

Lacroix, Alfred, quoted -------------------------------------- 6, 7 

Ladu deposits ------------------------------------------------------------ 17 

Lahar, definition ------------------------------------------------ 4, 5 

field characteristics ------------------------------------------ 21 

Lake Toba, Sumatra, pyroclastic 

deposits ____________________ 14, 28, 58, 62, 69 

Lamington volcano ---------------------------------------------------- 17 

Lapilli, composition in ash-flow tuffs --------------- 5 

definition -----------------------------------------------------·-------- 5 

deformation ---------------------------------------------------------- 25 

La Soufriere, eruption -------------------- 4, 5, 12, 13, 15 

La ti te ---------------------------------------------------------------------------- 10 

biotite-augite ------------------------------------------------ 10, 73 

quartz ------------------------------------------------------------ 30, 35 

Lithophysae, occurrence 19, 26, 27, 28, 37, 65, 66 

Load compaction ------------------------------------------ 24, 26, 28 

M 

Magmatic differentiation ---------------------------------------- 38 

Magnetite -------------------------------------------------------------- 35, 45 

Mansfield, G. R., and Ross, C. S., quoted ____________ 8 

Mapping, of ash-flow tuff. See ash-flow tuff. 

Marshall, Patrick, quoted ---------------------------- 4, 5, 37 

Matumoto, Tadaiti, quoted ------------------------------------

McKays, Calif ------------------------------------------------ 10, 35, 73 

Merapi volcano ---------------------------------------------------------- 17 

Microcline -------------------------------------------------------------------- 35 

Mineralogy ------------------------------------------------------------------ 21 

Minerals, devitrification -------------------- 27, 36, 44, 45 

late-stage -------------------------------------------------------- 12, 69 

vapor-phase ____ 14, 18, 21, 26-28, 30, 31, 33, 44 

Mount Katmai, eruptions -------------------------------- 12, 13 

Mount Pelee, eruptions ------------------------------------ 3, 12 

Mudflow, volcanic, definition -------------------------------- 5 

volcanic, field characteristics ------------------------ 21 

water and gas content ---------------------------- 12 

See also lahar. 

Mud Lake region, Idaho -------------------------------------------- 14 

Mud lava, definition -------------------------------------------------- 5 

Mud spates, description ------------------------------------------- 5 

N 

National Geographic Society Expedition ___ 12, 16 

Nomenclature, ash-flow materials ------------------- 2-3 

welded tuffs ------------------------------------------ 11 

Nomlaki tuff ---------------------------------------- 14, 23 

North Island, New Zealand, rhyolitic tuffs ______ 13 

Nuee ardente, cause of lahars ------------------------ 5 

Physicochemical factors ------------------------------------------ 38 

Piperno ------------------------------------------------------------------ 4, 14 

definition -------------------------------------------------------- 7, 10 

origin -------------------------------------------------------------- 9, 12 

Pipernoid structure -------------------------------------------------- 7 

Pirsson, L. V., quoted -------------------------------------------- 8 

Plasticity, glass ____ 8, 13, 33, 34, 41, 59, 60, 64, 65 

magma fragments -------------------------------------------- 11 

Pressure, effect on erupted materials ---------------- 39 

Pumice, collapsed -------------------- 8, 11, 19, 24, 27, 33, 

37' 45, 64, 73, 7 4, 77 

color -------------------------------------------------------- 24, 25, 45 

definition -------------------------------------------------------------- 7 

deformation during 

welding ---------------- 25, 26, 27, 28, 67, 77 

ejected -------------------------------------------------- 14, 16, 22 

glassy ------------------------------------------------------------- 12, 33 

microscopic characteristics ____________________ 33, 62 

origin -------------------------------------------------------------------- 13 

recrystallization ------------------------------------ 14, 18, 44 

size of particles ------------------------------------ 13, 25, 63 

sorting ------------------------------------------------------------------ 13 

viscosity ---------------------------------------------------------------- 41 

welded, definition ---------------------------------------------- 8 

recognition ---------------- 19, 21, 24-26, 63, 75 

use of term ---------------------------- 11, 12, 13, 75 

Pumice and scoria flows ---------------------------- 23, 30, 31 

Pumice fall, definition -------------------------------------------- 7 

Pumice-fall tuff, occurrence ---------------------------------- 21 

Pumice flow, banding ---------------------------------------------- 22 

definition ------------------------------------------------------ 7, 33 

lithologic characteristics 

14, 19, 22, 23, 30, 31, 33 

origin ------------------------------------------------ 12, 17, 30, 31 

thickness -------------------------------------------------------------- 22 

Sintering ---------------------------------------------------------------------- 43 

Slate, f elsi tic -------------------------------------------------------------- 12 

Snake River Plains area, Idaho ---------------------------- 14 

Snowdon, Wales, felsitic slates ---------------------------- 12 

rhyolites ---------------------------------------------------------------- 13 

Solovev, S. P., quoted ---------------------------------------------- 34 

Sosman, R. B., quoted ------------------------------------------- 41 

Specific gravity, rhyolitic tuffs ---------------------------- 46 

Spherulites ------------------- 14, 26, 27, 28, 37, 66, 69, 70 

Structure, axiolitic .. 4, 9, 36, 37, 45, 71, 72, 75, 77 

columnar ------------------------------------------------ 20, 28, 29 

eutaxitic ---------------------------- 9, 10, 24, 29, 30, 34 

rhyolitic ---------------------------------------------------------------- 9 

T 

Temperature, ash flows 16, 21, 26, 31, 39 

Ia va ---------------------------------------------------------------- 16, 3 9 

magma ------------------------------------------------------------------ 41 

mudflow ---------------------------------------------------------------- 12 

piperno ------------------------------------------------------------ 7, 9 

pumice -------------------------------------------------------------------- 7 

pyroclastic materials -------------------------- 4, 38, 39 

sand flow ------------------------------------------------------- 12, 39 

welded tuff -------------------------------------- 14, 26, 28, 34 

Tent rocks ------------------------------------------------------------------ 30 

Tephra, definition ---------------------------------------------------- 8 

Terminology, diversity --------------------------------------------

Thunder eggs ------------------------------------------------ 14, 38, 70 

Topography, effect on ash-flow 

deposition ------------------------ 20, 22, 23, 28 

Trachyte ----------------------------------------------------------------------- 10 

Tridymite ------------------ 27, 33, 36, 41, 42, 44, 45, 69 

Purpose of report ------------------------------------------------------ 2 Truckee Canyon region, Nevada ---------------------------- 36 

Pyroclastic, definition ---------------------------------------------- 7 

Pyroclastic materials, definition ---------------------------- 7 

nomenclature -------------------------------------------------- 2-3 

origin -------------------------------------------------------------------- 13 

size classification ---------------------------------------- 7, 19 

sorting ------------------------------------------------------------ 12, 19 

transporting mechanism -------------------------------- 12 

welding ------------------------------------------------------------------ 13 

Pyroclastic rocks, distribution ------------------------------ 1 

Pyroxene ---------------------------------------------------------------------- 45 

Q 

Quartz ------------------- 35, 39, 42, 45, 57, 60, 64, 72, 77 

R 

Ransome, F. L., quoted ----------------------------------------- 10 

Rattlesnake formation -------------------------------------------- 59 

Rhyodacite --------------------------------------------------------- 35, 46 

Rhyolite domes ------------------------------------------------------ 11 

Rhyolite flows -------- 9, 10, 11, 17, 31, 37, 39, 61, 75 

Tuff, crystal, definition ------------------------------------------- 8 

definition ---------------------------------------------------------- 3, 8 

devitrified -------------------------------------------------------------- 18 

distinguished from lava -------------------- 10, 18, 24 

felsite -------------------------------------------------------------------- 12 

fused -------------------------------------------------------------- 26, 27 

lithic, definition -------------------------------------------------- 8 

nonwelded, definition ---------------------------------------- 5 

erosion forms ------------------------------------ 20, 30 occurrence ______________________________ 14, 22, 24, 29 

origin ---------------------------------------------------- 14, 40 

similarity with sand flow --------------- 13, 31 

Pelean ------------------------------------------------- 12, 13 

recrystallization --------------------------------------------- 14 

rhyolitic, composition ----------- 13, 14, 17, 44, 46 

temperature ------------------------------------------- 42 

viscosity -------------------------------------------- 42 

vitric, definition ---------------------------------------------- 8 

recognition -------------------------- 18, 24, 25, 26 

weathering characteristics -------------- 21, 22, 24