Rocks composed of volcanic fragments and their classification

Earth-Science Reviews - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

ROCKS COMPOSED OF VOLCANIC F R A G M E N T S A N D THEIR CLASSIFICATION

RICHARD V. FISHER University of California at Santa Barbara, Santa Barbara, Calif. (U.S.A.)

SUMMARY WENTWORTH and WILLIAMS' (1932) and BLYTHE'S (1940) reviews of pyroclastic nomenclature are excellent accounts of pre-1940 terminology. Changes or additions to pyroclastic names since that time have come mainly from vigorous research on processes of pyroclastic flow and their resulting depositional product, ignimbrite. It has long been realized that pyroclastic fragments become mixed with other types of fragments, but only recently has this been recognized in classifications. Russian workers (BLOKHINAet al., 1959; VLODAVETSet al., 1962), PANT6 (1959), FISHER (1961) and T(JROK 0962) have attempted to unify pyroclastic and epiclastic rock terminology. Russian authors do this by classifying rock mixtures composed of pyroclastic and non-volcanic ("sedimentary") mixtures. FISHER (1961) attempts to unify the terminology by naming genetic terms for processes of fragmentation, viz., pyroclastic, epiclastic and autoclastic, to which may be added alloclastic (WRIGHTand BOWES, 1963) and hyaloclastic (RITTMANN, 1960), and by using the standard clast-size limits of the Wentworth Scale for all of the genetic types. Perhaps the most fundamental disagreements in volcaniclastic names arise from disagreement or misconceptions about some common terms such as "sedimentary", "volcanic", "epiclastic" and "pyroclastic". For example, the terms pyroclastic and epiclastic refer to different processes of fragmentation, not to different processes of deposition. Both types of particles may be deposited by streams or wind in any physiographic environment, but this does not alter their original mode of fragmentation. The terms sedimentary and volcanic are much broader in meaning than epiclastic and pyroclastic. International agreement on nomenclature of volcaniclastic sediments and rocks will very likely never come about until agreement is reached on the meaning and use of these four terms.

INTRODUCTION The present-day state of flux of classifications, and the continued additions to the list of names for volcanic rocks, indicate much the same state of vigor and conEarth-Sei. Rev., 1 (1966) 287-298

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fusion that prevailed when the literature of pyroclastic and related rocks was reviewed by WENTWORTH and WILLIAMS (1932). It also reflects the large amount of research on pyroclastic flows (such as nu6es ardentes) and their resultant rocks over the past two decades, and the attention that has been given in the past 5 years to rocks with mixtures of pyroclastic and epiclastic volcanic fragments. The decisive presentation of pyroclastic nomenclature by Wentworth and Williams, and the later discussion by BLYTHE(1940) give a clear picture of pre-1940 terminology. Recent additions to volcanic nomenclature are given in the GIossao, of Geology and Related Sciecy (AMERICANGEOLOGICALINSTITUTE,1957) and in the dictionary Geological Nomenclature (SCHIEFERDECKER, 1959). The latter contains definitions in English, French, German, and Dutch.

SOME GENERAL TERMS

Whether a worker accepts one classification or parts of it, or puts together his own, depends almost entirely upon his opinions concerning certain definitions. Most basic to this discussion are the general terms "volcanic", "pyroclastic", "epiclastic" and "sedimentary". Volcanic is defined in the following m a n n e r (AMERICAN GEOLOGICALINSTITUTE, 1960a, p.315): " . . . of, pertaining to, like, or characteristic of, a volcano; characterized by or composed of volcanoes, as a volcanic region, volcanic belt; produced, influenced, or changed by a volcano or by volcanic agencies; made of materials derived from volcanoes, as a volcanic cone". A fundamental difference of opinion over the meaning and use of the word "volcanic" is shown by the recent criticism by WRIGHT and BOWLS (1963) of the author's classification of volcanic breccias (FISHER, 1960). WRIGHT and BOWLS(1963, p.80) maintain that "volcanic" is an adjective referring only to the process of volcanism, whereas I hold that it is an adjective which can apply as well to sediments derived from volcanic sources as to those produced directly by volcanic explosion. This difference in opinion appears to be a minor argument in semantics, but application of these different meanings leads to large differences in classification. Wright and Bowes, for example, must reject any combined epiclastic-pyroclastic classification and nomenclature, even though in nature there is every gradation between pyroclastic and nonpyroclastic sediments. Pyroclastic was defined by WENTWORTH and WILLIAMS (1932) as " . . . an adjective applied to rocks produced by explosive or aerial ejection of material from a volcanic vent". A "pyroclastic" rock is, therefore, one that has been produced directly by volcanism. The words "pyroclastic" and "volcanic" have different meanings but sometimes are used synonymously. The dictionary of Geological Nomenclature (ScHIEFERDECKER, 1959), for example, uses the term "volcanic" synonymously with "pyroclastic", in the term "volcanic sand". "Sand", in this inEarth-Sci. Rev., l (1966) 287.-298

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stance refers to a fragment size range between 2 and 0.5 mm (between lapilli and ash), and volcanic refers to a process of breaking. If, however, "volcanic" means " o f volcanic composition" as well as "produced by a volcanic process", then a volcanic sand may be composed of volcanic, although non-pyroclastic, fragments eroded from lava flows. Sedimentary is defined (AMERICAN GEOLOGICAL INSTITUTE, 1960b, p.59) as: " . . . 1. Solid material settled from suspension in a liquid. 2. Solid material, both mineral and organic, that is in suspension, is being transported or has been moved from its site of origin by air, water or ice and has come to rest on the earth's surface either above or below sea level." This definition does not imply any special composition of fragments or any special way in which the fragments are formed. "Sedimentary" is often used synonymously with epiclastic, but epiclastic refers to "mechanically deposited sediments (gravel, sand, mud) consisting of weathered products of older rocks. Detrital material from pre-existent rocks." The substitution of "sedimentary" for "epiclastic" is muddy usage, but causes little confusion when a clastic deposit is composed of the weathered and eroded products of volcanic flows. Confusion may arise, however, when unconsolidated pyroclastic debris (i.e., fragments produced by a volcanic process) is transported by erosional agents (rivers, wind, ice), or when pyroclastic debris is deposited directly from an eruption into a basin where epiclastic particles (either volcanic or non-volcanic in composition) are accumulating. The important point is that pyroclastic and epiclastic refer to different processes of fragmentation, but not necessarily to different processes of deposition. Both pyroclastic and epiclastic particles, for example, may be deposited by streams or by the wind. "Ash", "lapilli", "blocks", "bombs", "tuff" and "agglomerate" are primarily size terms applied to fragments in unconsolidated deposits or in rocks of pyroclastic origin. Thus, there is no reason to prefix "pyroclastic" or "volcanic" to these terms, such as in "volcanic tuff" or "pyroclastic lapilli". The term "pyroclastic sandstone" is correct in the context presented here, but because it is synonymous with tuff (a rock composed of sand-size pyroclastic fragments), the term is unnecessary. "Sedimentary tuff", although correct, is unnecessary because the fragments of tuff, like all sediments, are deposited by "settling out" from water or air. "Epiclastic tuff", however, is a contradiction in terms because tuff is composed of pyroclastic fragments. They are not broken by weathering or erosion as the word "epiclastic" implies. Tephra, a recent addition to the nomenclature (THORARINSSON, 1954), is a collective term for deposits of volcanic material thrown through the air from a volcanic vent. Volcanic clastic (BLOKmNA et al., 1959) and volcaniclastic (FISHER, 1961) are used to encompass the entire field of clastic rocks composed in part of, or entirely of volcanic fragments. "Volcaniclastic" is used to pinpoint and include the entire spectrum of fragmental volcanic rocks formed by any mechanism or origin, eraEarth-Sci. Rev., 1 (1966) 287-298

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placed in any physiographic environment (on land, under water or under ice), or mixed with any other volcaniclastic type or with any non-volcanic fragment types in any proportion.

PYROCLASTIC ROCKS

Pyroclastic.[tows A great many terms and differing classifications have arisen from research on pyroclastic flows and their deposits. Because of continuing research, matters of terminology are not yet settled. Smith (In: ARAMAKI and YAMASAKI, 1963, p.90) defines pyroclastic flow as " . . . all fragmental flows or avalanches composed of pyroclastic material irrespective of temperature of emplacement", a definition implied in the discussion of pyroclastic flows by Waters (GILLULY et al., 1951, p.447), and accepted herein. This method of dispersal, whether on land or under the sea (cf. FISKE, 1963), may be regarded as one of the three main mechanisms of extrusive volcanic dispersal along with (1) lava flows and (2) aerial transportation. Ross and Smith (SMITH, 1960 a, b; Ross and SMm~, 1961) review the previous literature, the development of concepts, point out unsolved problems, and give the stratigraphic and petrographic characteristics of pyroclastic flows and their deposits. In this series of papers, they use the term "ash flow" synonymously with "pyroclastic flow". Rock types that result from pyroclastic flows are extremely diverse, ranging from loose unconsolidated ash to solid rock. This diversity is caused by initial differences in temperature, composition and volume of pyroclastic flows. MARSHALL(1935) introduced and defined the term "ignimbrite" (fire cloud rock) as the rock deposited by a "nu6e ardente" (one type of pyroclastic flow) without regard for degree or manner of solidification or for the size of the fragments. "Welded tuff" (IDDINGS, 1909, p.331) has been used synonymously with "ignimbrite" by many workers, although, as has been pointed out, a "welded tuff" may be neither "welded" nor always "tuff". It is regarded here as only one type of ignimbrite. WErE (1954) recognized three main ignimbrite rock types, namely, "lockerer Gluttuff" (uncemented "glowing" tuff), "Kristallisationstuff" (indurated by crystallization due to rise of hot gases during cooling of the pyroclastic flow; equivalent to "sillar" as defined by FENNER, 1948), and "Schmelztuff" (indurated by welding of glass shards; equivalent to "welded tuff"). PANT6 (1963) illustrates the great number of terms that are applied to rocks produced by pyroclastic flows. Examination of Table I shows a rather consistent grouping of nu6es ardentes into two main types, namely, avalanches and directed explosions (low pressure vertical or horizontal eruptions). WILLIAMS(1957) adds a third distinct group, the Earth-Sci. Rev., 1 (1966)287 298

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ROCKS COMPOSED OF VOLCANIC FRAGMENTS TABLE 1 CLASSIFICATIONS OF PYROCLASTIC FLOWS

LACROIX (1930) I

11 III

nu6es ardentes p616ennes (a) nu6es ardentes d'explosion dirig6e (b) nu6es ardentes d'avalanche nu6es ardentes d'explosion vulcanienne nu6es ardentes du massif du Katmai

ESCHER (1933) 1

I1

(a) gloedwolken van het Pel6e type (b) gloedwolken van het Merapi type gloedwolken van her St. Vincent type

NEUMANN VAN PADANG (1933) I

II

afstortingsgloedwolken (formed by avalanches) explosiegloedwolken (formed by explosions)

MACGREGOR (1952) I

II Ill IV V

avalanches of domal disintegration (Merapi) discharged domal avalanches (Pel6e) directed domal avalanches (Pel6e) vertically initiated domal nu6es ardentes (Pel6e) vertically initiated crateral nu6es ardentes (St. Vincent)

WILLIAMS (1957) l

II 11I

Pel6an type: explosions through flank of a dome or collapse of dome Krakatoan type: vertical low pressure explosions through craters fissure type: low-pressure upwelling of effervescing magma through fissures

fissure type, to include the process which forms thin widespread sheets of ignimbrite of the type described by MACKIN (1960) that cover vast tracts of land in the Great Basin province in the United States. Classifications of the processes and the resultant rock types have grown simultaneously and separately. The term pyroclastic flow is preferred here as an inclusive term for the various mechanisms of dispersal, and ignimbrite is preferred as the name for all of the various rock types produced by the emplacement of a pyroclastic flow. These terms do not imply specific fragment sizes or temperature of emplacement, but yet are genetic in the sense that they apply to a particular set of volcanic processes and products.

Volcaniclastic rocks formed by rapid quenching of hot lava Rocks formed by steam explosions and rapid hydration of hot lava flowing into Earth-Sci. Rev., I (1966) 287 298

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water and mud, or onto or beneath snow and ice have long been known. The process is not entirely pyroclastic nor is it an entirely volcanic process, but is included in this section for convenience. The literature of these rocks is partly reviewed by SILVESTRI (1963). MCBmNEY (1963) discusses aspects of submarine extrusions, showing that explosive disruption of basalt is not possible at depths greater than 500 m. Both of these papers, along with many others, were part of a 1961 symposium in Italy. CARLISLE(1963)gives an account of the origin of subaqueous shards in the formation of "aquagene tuff". FISKE (1963) describes evidence that many of the graded volcaniclastic rocks mapped by him in Mount Rainier National Park (FlSKE et al., 1963) originated as submarine pyroclastic flows. RITTMANN (1960, p.82; 1962, p.72-73) introduced the term "hyaloclastite" for rocks fragmented by rapid quenching of hot lava, and the term "hyaloclastic" for the process of producing a hyaloclastite. Thus, "hydrovolcanic breccias" (FISHER, 1960, p. 977) would be more aptly called "hyaloclastic breccias". Pyroclastic mixture names

Mixture terms for the various size grades of pyroclastic fragments is a necessary Blocks and bombs > 6 4 mm

Pynodostic breccia

Tuff-breccia

/Loo stone

LaPu k #

Tuff

6 4 - 2 mm Lapilli

< 2 mm Ash

Fig. l. Mixture terms and e n d - m e m b e r terms used for pyroclastic flagments.

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part of the language. The most commonly used mixture names in pyroclastic terminology are "tuff-breccia" and "lapilli-tuff". Tuff-breccia, a firmly entrenched name, was originally defined by NORTON (1917, p. 120) as " . . . made up of fragmental products of explosive eruptions. The matrix consists of the finer materials of the eruption and in some instances has been washed in by mudflows. Or the matrix may be formed by gangue and are stuffs deposited in the interstices by heated waters. Tuff breccias often include fragments of the country rock torn from the sides of the duct below the base of the volcanic cone." The term was restricted by WENTWORTH and WILLIAMS (1932, p.46) to volcanic breccias of pyroclastic origin with abundant tuff matrix. Most authors have'followed the definition of Wentworth and Williams, although the term "tuff" as used'in tuff-breccia, has in some instances been used synonymously with "pyroclastic". Lapilli-tuff is also a mixture term, but was defined by WENTWORTH and WILLIAMS (1932, p.52) as the indurated aggregate of lapi!li. FISHER(1961) proposed "lapillistone" as an end-member term. Mixture terms and end-member terms are shown in Fig.l. Precise percentage boundaries for the mixtures are not given, because they vary from perhaps as much as 40 % to as little as 15 ~ , depending upon the prejudices of the individual.

GENERAL CLASSIFICATIONS OF VOLCANICLASTIC ROCKS

Genetic categories Many different types of genetic terms may be used in the classifications of volcaniclastic rocks. Preferred terms in my classification (FISHER, 1960, 1961) refer to processes o~ fragmentation, namely, pyroclastic, epiclastic and autoclastic. WRIGHT and BOWES(1963) add alloclastic, and, because of recent attention given to hyaloclastites (cf. SILVESTRI, 1963), it may be advisable to add hyaloclastic to this list because it is a unique process of fragmentation. Such mercurial shifting or adding of genetic terms is a necessary adjunct of continued research and the consequent clarification of origins.

Grade size categories One of the earliest classifications of pyroclastic and non-pyroclastic rock mixtures, and the first application of precise size terms to pyroclastics was by BAILEY(1926, p.109, Table II). The terminology and definitions of pyroclastic and epiclastic mixtures were given by WENTWORTH and WILLIAMS (1932), but this class of rocks received scant attention in separate classifications until recently (BLOKHINAet al., 1959; PANT6, 1959; FISHER, 1961 ; TOROK, 1962; VLODAVETSet al., 1962). The classification by a Soviet committee (VLODAVETSet al., 1963) shows that agreement has

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FISHER

TABLE II SEDIMENTARY AND PYROCLASTIC TERMINALOGY

(Modified after BAILEY,1926) Component fragments diameter

percentage

> l mm 1/16-1 mm < 1/16 mm

> 50~ > 50~ > 50~

Pyroclastic rocks 1

Epiclastic volcanic rocks 2

Volcanic breccia Sand-tuff Dust or mud-tuff

Volcanic conglomerate Tuffaceous sandstone Tuffaceous shale

1 Show no effect of erosion. 2 Volcanic material transported by running water. not yet been reached in the Soviet Union. The bibliography of this translated Soviet paper demonstrates the continuing and active interest in the subject by Russian workers. Even though different grade scales have been used for fragment sizes (U.S. Bureau of Soils, Atterberg Scale, Wentworth Scale, and many others), different designations in grade sizes by different authors for pyroclastic fragments are not greatly different (Fig. 2). The lowest size limit dividing lapilli from blocks and bombs is set at 25 m m (TOROK, 1962), although the tendency has been to raise this limit from the 32 m m set by WEYTWORTH and WILLIAMS (1932). PANT6 (1959) uses 50 m m as does the dictionary of Geological Nomenclature (ScmEFERDECKER, 1959, p. 256). FISHER (1961) uses 64 m m to correspond to the limit between pebbles and boulders. The lower size limit of lapilli was placed at 4 m m by WENTWORTH and WILLIAMS (1932), but recent attempts to combine pyroclastic and epiclastic classifications have resulted in lower limits, such as 1 m m (PANT6, 1959), 2 m m (SO,EFERDECKER, 1959; FISHER, 1961), and 2.5 m m (TOR6K, 1962). Russian workers (VLODAVETS et al., 1962) have set 10 m m as the lower limit to lapilli but have inserted a size term "volcanic gravel" between lapilli and ash with its lower limit set at 2 ram. In this usage, however, "volcanic" is synonymous with "pyroclastic". Comparison of WENTWORTH and WILLIAMS(1932) grade size limits for pyroclastic fragments to those set by various authors shows that attempts to unify pyroclastic and non-pyroclastic terminology has led to the alteration of pyroclastic fragment size limits to conform with some standard scheme of epiclastic grade-size terminology. Discussion o f classifications

Russian workers have long been involved in pyroclastic-epiclastic rock terminology. Earth-Sci. Rev., 1 (1966) 287 298

ROCKS

COMPOSED

OF VOLCANIC

295

FRAGMENTS

GRADE SIZE LIMITS USED IN GRADE SIZE LIMITS USED BY AUTHORS RELATING PYROCLASTIC CLASSIFICATION~ PYROCLAST1CANDEPICLASTIC SEDIMENTS AND ROCKS

DICTIONARY DEFINITIONS

WENTWORTH~d WILLIAMS,1932 BLYTHE,1940

SCHIEFERDECKER) 1959

64 5O

PANTO,1959

Blocks

Blocks

Blocks and

and

and

bombs

bombs

bombs

Lapilli

Lapilli

etal, FISHER)1961 VLODAVET$ 1962

Blocks and

Blocks

bombs

and

TOROK,1962

Blocks

bombs

Blocks

end

and

bombs

bombs

~g 25

Lapilli Lo filli

Lapilli

10

Lopilli Lopilli

Volcanic gravel

2.5 2

L

Volcanic

IIJ o o

0.5

sand Sand

<

~.25 <

m

L

a 0

o

Ash Dust

0.1 < 0.06250.05 -

<

~:

Ash Dust

and

oP •-

-

-

Dust -

Dust

Dust

ash

o Fig. 2.

Grade size limits and terms for unconsolidated pyroclastic debris.

SHATALOV (1937) classified rock types produced by mixtures of pyroclastic and "sedimentary" (i.e., epiclastic) material. The procedure of classifying rock mixtures is followed by BLOKHINAet al. (1959). This classification includes (1) pyroclastic rocks (100% pyroclastic fragments), (2) tuffites (~> 50% pyroclastic, < 50% "sedimentary" [epiclastic] material), (3) tuffogenic rocks (:> 50% "sedimentary" [epiclastic] material, < 50% pyroclastic material). This practice of classifying mixtures is essentially followed by the 1962 Soviet proposal on pyroclastic classification (VLODAVETSet al., 1963). Rather than classify mixtures, the author's first consideration (FISHER, 1961) is to the process of breaking, and secondly to the size of the fragments. Genetic Earth-Sol. Rev.,

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categories are autoclastic, pyroclastic, and epiclastic (to which hyaloclastic should be added), thereby placing the emphasis on the manner of fragmentation. It is important to distinguish between fragments that are produced instantly such as the pyroclasts, and those that are produced over a longer period of time by the weathering of volcanic rocks, because these processes include fundamental energy differences. Pyroclastic fragments :may be dispersed and deposited by streams, but such dispersal does not alter the fact that the fragments are pyroclastic in origin. Thus, there may be "primary" (unreworked) as well as "secondary" (reworked) pyroclastic deposits. Epiclastic fragments that originate by weathering and erosion of volcanic rocks may, like the pyroclastic fragments, be later transported and form epiclastic deposits. Lithification of these deposits results in epiclastic volcanic rocks, and include epiclastic volcanic sandstone (a rock composed of sand size fragments eroded from volcanic rocks), or epiclastic volcanic siltstone. If desirable, admixtures of pyroclastic fragments in these epiclastic volcanic rocks may be indicated by using the word "tuffaceous" as a prefix. Such admixtures include tuflites and tuffogenic rocks of BLOKHINA et an. (1959) depending upon the percentages of the respective pyroclastic and epiclastic fragments.

CONCLUSIONS

Developments in pyroclastic classification and nomenclature since the WENTWORTH and WILLIAMS' review (1932) are involved mainly with processes of pyroclastic flows and the ignimbrites formed from the flows. Recent proposals to include all clastic rocks containing a significant percentage of volcanically derived fragments and/or minerals into a single classification is a second area where matters of nomenclature remain unsettled. Continued work on rocks with pyroclastic and epiclastic mixtures has forced critical examination and clarification of the language used to describe mixtures of pyroclastic debris with non-volcanic material, and also has caused a sharpening of the usage of pyroclastic terms. It should be said again that "volcanic" is not a synonym for "pyroclastic" and that "sedimentary" is not a synonym for "epiclastic". It is clear that international agreement on the classification and use of terms for volcaniclastic sediments and rocks will never be even closely approximated until agreement is reached on the meaning of these four terms.

REFERENCES AMERICAN GEOLOGICAL INSTITUTE, 1957. Glossary of Geology and Related Science, 1st ed. Washington, D.C., 325 pp.

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AMERICAN GEOLOGICALINSTITUTE, 1960a. Glossary of Geology and Related Science, 2nd ed. Washington, D.C., 352 pp. AMERICAN GEOLOGICAL INSTITUTE, 1960b. Glossary of Geology and Related Science, Suppl., 1st ed. Washington, D.C., 72 pp. ARAMAKI, S. and YAMASAKI, S., 1963. Pyroclastic flows in Japan. Bull. Volcanol., 26: 89-99. BAILEY, T. L., 1926. The Gueydon, a new Middle Tertiary formation from the southwestern coastal plain of Texas. Univ. Texas Bull., 2645:187 pp. BLOKHINA, L. 1., KOPTEV-DVORNIKOV, V.S., LOMIZE, M . G . , PETROVA, M. A., TIKHOMIROVA, E. I., FROLOVA, I. I. and YAKOVLEVA, E. B., 1959. Principles of classification and nomenclature of the ancient volcanic clastic rocks. Intern. Geol. Rev., 1(12): 56-61. BLYTHE, F. H., 1940. The nomenclature of pyroclastic deposits. Bull. Volcanol., 6: 145-156. CARLISLE, D., 1963. Pillow breccias and their aquagene tufts, Quadra Island, B. C. J. Geol., 71: 48-71. ESCHER, B. G., 1933. On a classification of central eruptions according to gas pressure of the magma and viscosity of the lava. Leidsche Geol. Mededel., 6: 4 5 4 9 . FENNER, C, N., 1948. Incandescent tuff-flows in southern Peru. Bull. Geol. Soc. Am., 59: 879893. FISHER, R. V., 1960. Classification of volcanic breccias. Bull. Geol. Soc. Am., 71:973 982. FISHER, R.V., 1961. Proposed classification of volcaniclastic sediments and rocks. Bull. Geol. Soc. Am., 72: 1409-1414. FISKE, R. S., 1963. Subaqueous pyroclastic flows in the Ohanapecosh Formation, Washington. Bull. Geol. Soc. Am., 74: 391406. FISKE, R. S., I~OPSON,C. A. and WATERS,A, C., 1963. Geology of Mount Rainier National Park, Washington. U.S., Geol. Surv., Prof. Papers, 444:93 pp. GILLULY, J., WATERS, A. C. and WOODFORD, A. O., 1951. Principles of Geology. Freeman, San Francisco, Calif., 631 pp. IDDINGS, J. P., 1909. Igneous Rocks. Wiley, New York, N.Y., 464 pp. LACRoIX, A., 1930. Remarques sur les matdriaux de projection des volcans et sur la gen+se des roches pyroclastiques qu'ils constituent. Livre Jubilaire Centenaire Soc. Gdol. France, 2: 431472. MACGREGOR, A. G., 1952. Eruptive mechanisms: Mt. Pel6e, the soufri6re of St. Vincent, and the Valley of the Ten Thousand Smokes. Bull. Volcanol., 12: 49-74. MACKIN, J, H., 1960. Structural significance of Tertiary volcanic rocks in southwestern Utah. Am. J. Sci., 258: 81-131. MARSaALL, P., 1935. Acid rocks of Taupo Rotorua volcanic district. Trans. Roy. Soc. New Zealand, 64: 323-366. McBIRNEY, A. R., 1963. Factors governing the nature of submarine volcanism. Bull. Volcanol., 26: 455469. NEUMANN VAN PADANG, M., 1933. De uitbarsting van de Merapi (Midden Java) in dejaren 19301931. Vulkanol. Seismol. MededeL, 12:135 pp. NORTON, W. H., 1917. Studies for students. A classification of breccias. J. Geol., 25: 160-194. PANTd, G., 1959. Vorschlage zur Schaffung einer einheitlichen Terminologie ffir vulkanische Gesteine. Z. Angew. Geol., 5: 373-376. PANTd, G., 1963. Ignimbrites of Hungary with regard to their genetics and classification. Bull. Volcano/., 25: 174-181. RITTMANN, A., 1960. Vulkane und ihre Tiitigkeit, 2. Aufl. Enke, Stuttgart, 335 pp. RITTMANN, A., 1962. Volcanoes and their Activity. Wiley, New York, N.Y., 305 pp. Ross, C. S. and SMITH, R. L., 1961, Ash-flow tufts; their origin, geologic relations and identification. U.S., Geol. Surv., Prof. Papers, 366: 1-77. SCHIEFERDECKER, A. A. G. (Editor), 1959. Geological Nomenclature. Noorduijn, Gorinchem, 523 pp. SHATALOV, YE. T., 1937. On a rational nomenclature for some sedimentary and pyroclastic rocks. Materialy po Izuch. Okhotsko-Kalymskogo Kraya, Ser. H. SILVESTRI, S. C., 1963. Proposal for a genetic classification of hyaloclastics. Bull. Volcanol., 25: 316-321. SMITH, R. L., 1960a. Ash flows. Bull. Geol. Soc. Am., 71: 795-842.

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