Short Notes

RICHARD V. FISHER

PROPOSED CLASSIFICATION OF VOLCANICLASTIC

SEDIMENTS AND ROCKS

Abstract: Volcaniclastic sediments and rocks aredivided here into autoclastic, pyroclastic, and epi-clastic types with grain-size limits the same as non-volcanic epiclastic rocks. Autoclastic rocks containfragments that are produced within (but not usuallyextruded from) volcanic vents, during movementof lava flows, or by gas explosions within flows thathave ceased to flow. Pyroclastic rocks contain frag-ments produced by volcanic explosion and extrudedas discrete particles from volcanic vents. Epiclastic

volcanic rocks contain fragments produced byweathering and erosion of solidified or lithified vol-canic rocks of any type. Volcaniclastic types maybe mixed in all proportions with each other or withnonvolcanic fragments, although these mixtures arenot designated within this classification. A non-genetic category, based only upon particle size andthe presence of volcanic material, is included forrocks with clasts of unknown origin.

Many classifications of pyroclastic rocks and/or pyroclastic fragments have been suggestedin the past and are reviewed by Wentworthand Williams (1932). Only recently, however,have attempts been made to group all volcanicclastic (termed Volcaniclastic in this paper) rocksinto a single system (Blokhina et al., 1959;Panto, 1959). There is a need for such compre-hensive systems, because rocks of pyroclasticorigin are only one category of Volcaniclasticrocks. "Pyroclastic" is clearly defined andwidely used as "an adjective applied to rocksproduced by explosive or aerial ejection of ma-terial from a volcanic vent. . . ." (Wentworthand Williams, 1932, p. 24-25). If this definitionis to be accepted and used, the following can-not be considered pyroclastic: (1) fragmentsproduced in volcanic vents but not ejectedfrom the vent; (2) fragments formed duringthe breaking up of a moving lava flow; (3)fragments that originate by the weathering anderosion of solidified lava flows or consolidatedpyroclastic deposits.

A simple working classification that incor-porates all the factors involved in the develop-ment of Volcaniclastic rocks is clearly impos-sible. The one developed here is based on (1)the primary origin of the fragments, and (2)their grain size. In addition, a purely descrip-tive category is included for the Volcaniclasticrocks of doubtful or unknown origin.

There is complete gradation between non-

volcanic sedimentary rocks and extrusiveigneous rocks, and all types may be includedwithin the Volcaniclastic rocks. The pigeonhol-ing of all types into separate categories is notpossible, nor is it advisable, because our knowl-edge of the Volcaniclastic rocks is somewhatlimited by our ability to recognize their variousorigins. Criteria of recognition have been givenby Pirsson (1915) on tuff, by C. S. Ross (1928)on reworked pyroclastic and/or epiclastic vol-canic rocks, and by Enlows (1955) and R. L.Smith (1960) on welded tuff. A superb con-tribution to our knowledge of marine volcani-clastic rocks has been made by W. R. Dickin-son (1958, Geology of the Izee area, GrantCounty, Oregon, unpublished Ph.D. thesis,Stanford University). Discussion of criteria forrecognizing the types of Volcaniclastic rocks isbeyond the scope of the present article.

Grabau (1924), in his masterful althoughlargely unaccepted scheme of rock classifica-tion, has in part solved the problem. His exo-genetic (clastic) subdivision of rocks includespyroclastic as one of five major types. Pyro-clastic is further subdivided by grain size usingthe terms rudyte (>2.5 mm), arenyte (0.5mm-2.5 mm), and lutyte (<0.5 mm). Thesesame terms (with "ite" used instead of "yte")are in use today (Dunbar and Rodgers, 1957),but with Grabau's size limits changed to agreewith the Wentworth scale of grade sizes.Grabau's pyrorudyte is a pyroclastic rock cor-

Geological Society of America Bulletin, v. 72, p. 1409-1414, September 1961

1409

1410 R. V. FISHER—CLASSIFICATION, VOLCANICLASTIC SEDIMENTS, ROCKS

responding in grain size to other rudytes (con- prefer indefinite comparisons of pyroclasticglomerate and breccia), pyrarenyte corresponds debris to pea and walnut sizes.in grain size to other arenytes (sandstone), and The limits of pyroclastic particles should bepyrolutyte corresponds in grain size to other reset because quantitative size analyses oflutytes (shale). He thus equated pyroclastic pyroclastic debris are made by sedimentarygrade sizes to epiclastic grade sizes. Further- petrographic methods, because pyroclastic sizemore, he clearly recognized that pyroclastic limits in use are not uniform, and, mostrocks may be reworked by erosional agents and importantly, because such a change would

TABLE 1. CLASSIFICATIONS OF PYROCLASTIC FRAGMENTSWan. twe . r lh andW i l l i a m s 1432.

B l o c k said

Bombs

L a p i

Coirit

F - n e

l i

*5h

T w e n V i o f el , 1950Bombs

Lapi 1 l i

Coarse

F m &

k s H

tmmons, et at. , 1955

B l o c k s , Bombs ,

U p i l l i , arvdt inders

A s h

Pant 6, i^$<j

B locksand

Bombs

L a p i 1 1 i

Oust

T h i s Repor t.01 tie R l n f U

and.Bombs

l a p i H i

C c a r s e

f i n e

A s h

prefixed hydro (water), anemo (wind), andatmo (weathering), giving hydropyrorudyte,anemopyrarenyte, etc.

In order to include all volcaniclastic rockswithin a single system, it is necessary to usesimilar limiting grade sizes. Studies of pyro-clastic debris by Moore (1934), Baak (1949),Segerstrom (1950), Thorarinsson (1954), andothers show the tendency for use of standards:diment-size terms. Baak uses the U. S.Bureau of Soils scale, Thorarinsson uses theAtterburg scale, Segerstrom and Moore use theWentworth scale.

Thorarinsson (1954, p. 1) points out thatsize groups of pyroclastic fragments are notprecisely defined. This is not strictly so, butdefined size limits of pyroclastic fragments vary(Table 1). Some authors (Holmes, 1946, p.451; Stearns and Macdonald, 1946, p. 16) still

relate pyroclastic, epiclastic, and pyroclastic-epiclastic mixtures within a single scheme. Thesize scale used here (Table 2) is the Wentworthscale as recommended by Lane et al. (1947),except the word "pebble" is substituted for"gravel."

Blocks and bombs are here equated to sedi-ment terminology by setting size limits ofcoarse blocks and bombs to correspond withboulder size (>256 mm), fine blocks andbombs to correspond with cobble sizes (64mm-256 mm), and lapilli to correspond withpebble sizes (2 mm-64 mm). Coarse ash isequivalent to sand sizes (2 mm-1/16 mm),and fine ash is equivalent to silt- and clay sizes.The wide range given to fine ash is more ap-parent than real because ash of clay size isvolumetrically unimportant (Baak, 1949, p.54-55).

SHORT NOTES 1411

Volcanic rocks are herein divided first on thebasis of origin of particles (autoclastic, pyro-clastic, epiclastic), and secondarily on particlesize (Table 3). A nongenetic category is in-cluded based only upon presence of volcanicparticles and their sizes. Secondary classifica-tions using different criteria may be set upunder the proposed classification. For example,

material of "fine-dust" size (Macdonald, 1953).Gas entrapped in lava that has ceased to flowor in pyroclastic flows after deposition maycontinue expansion and disruption of solidifiedor partially solidified material, thus producingclastic zones (Earl F. Cook, written communi-cation, 1961). Processes within volcanic pipesmay also produce fine- and coarse-grained frag-

TABLE 2. PROPOSED TERMINOLOGY AND GRAIN SIZE LIMITS FORPYROCLASTIC FRAGMENTS

Grade size(mm)

64

2

1 l\f\

- 1/256 -

Epiclastic fragments

Boulders (and "blocks")

Cobble

Pebble

Sand

Silt

Clay

Pyroclastic fragments

Coarse

Fine

Blocks

bombs

Lapilli

Coarse

Fine

Ash

further subdivision of tuff may be based upontype of transporting agent (fluvial, glacial,eolian), upon environment of deposition (la-custrine or marine), upon mode of origin ofpyroclastic fragments (essential, accidental, ac-cessory), upon composition (crystal, lithic orvitric), etc. The reader is referred to Went-worth and Williams (1932) for a review ofvarious classifications.

Autoclastic applies to all the possible waysfragmental rocks may be produced in volcanicvents, by friction in flowing lava or by internalgas explosions within flows after they haveceased to flow. In flowing lava that is cooling,friction produces large fragments, as well as

ments (Hans Cloos, 1941; Reynolds, 1954;Gates, 1959). Tuffisite is in use for clastic ma-terial produced in volcanic pipes, and it maybe well to restrict the term to ash-sized ma-terial. Such clastic material may be composedof any pre-existing rocks derived from the wallsof a volcanic pipe as well as clasts derived fromthe magma. It is possible therefore to have anyrock type, including any type of volcaniclasticrock, within the volcanic pipe. In addition,fragmental material produced in the volcanicpipe may flow onto the surface (Curtis, 1954).Hybrid rocks such as these cannot be neatlycategorized and should be fully describedrather than given a simple name.

1412 R. V. FISHER—CLASSIFICATION, VOLCANICLASTIC SEDIMENTS. ROCKS

Pyrodastic fragments are produced by vol-canic explosion and are extruded as discreteparticles from vents. "Primary" is used hereto indicate that the material has not beenmoved from its place of deposition before itslithification. For example, "primary tuff" isformed of lithified ash-fall material that has notbeen reworked by epigene geomorphic agentsor by gravity. "Reworked" indicates that ash-fall material has been moved from its original

use to "tuff breccia." This gives lapilli tuff asomewhat different meaning from that given itby Wentworth and Williams (1932, p. 47).Stevens (1959, p. 119) proposed lowering thesize limit of pyroclastic breccia to 2 mm whichwould eliminate the term lapilli tuff.

The upper size limit for ash in a lithified tuffis changed from 4 mm to 2 mm to agree withsediment terminology. This limit is loweredprimarily because situations may arise when

TABLI; 3. PROPOSED CLASSIFICATION OF VOLCANICLASTIC ROCKSPredominan tgrain 5i?e

(mm)

— 2 5 6 —

i

16

H

Autoclastic

Flowbre.c.c.ia,

kutci breccia,

Intrusionbraccia

TnHiiiU

Pyrocldst icPrimary or Reworked

P y r o c l a a t i tbre.ce.ia.,

Agglomerate

Lapil l s tone

Coarse

fiM

TuH

e p i c l a s l i c

Epic lasUc volcanicbreccia,

tpiclsst ic volcanicconglomerate

tpic last ic volcanic,sandstone

Epic last ic volcanics i l ts tone

Epic last ic 'vo lcanicclaystone

* +Equivalent nongenet ic

terms £

Volcan ic Breccia,

Volcanic conglomerate

Volcanic sandstone

Volcanic. s i l ta tone

Vo l can i c c laystone

* May be rti ix.ed w i t h norwoicanic- c las t ic ma te r i a l+ Add ad j a t t i ve "tuffaceout" to rocks con t a in ing p y r o c l a s t i c m a t e r i a l < a<nr r< in 5156

place of deposition and redeposited beforelithification, as for example "reworked tuff."Reworked tuff is to be distinguished fromrocks formed by epiclastic processes.

Terms for lithified aggregates of bombs andblocks, except for grain-size limits, are similarto Wentworth and Williams' definitions. Pyro-clastic breccia is used for lithified aggregates ofpyroclastic blocks rather than "volcanic brec-cia" as proposed by Wentworth and Williams.Agglomerate is a rock aggregate composedmainly of large (>64 mm) pyroclastic frag-ments rounded by volcanic processes. Coarseand fine denote various size limits of the Went-worth scale. Lapillistone is proposed here forrock aggregates composed predominantly oflapilli (2 mm-64 mm) because there is no termfor rocks composed dominantly of lapilli-sizedfragments. Lapilli tuff is formed of nearly equalamounts of lapilli and ash and is analogous in

one would be forced to decide whether frag-ments of a rock are pyroclastic or epiclastic inorder to name it (i.e., breccia vs. tuff, orbreccia vs. sandstone). If the limit of 2 mm isconsistent throughout the classification, thenongenetic name volcanic sandstone or volcanicbreccia may be used without having to de-termine the origin of fragments. The size limitsfor coarse and fine ash (and tuff) are changedfrom Wentworth and Williams' 1/4 mm to1/16 mm for consistency. Coarse tuff is equiva-lent in particle size to sandstone, and fine tuffcorresponds to siltstone and claystone. A three-fold subdivision into coarse, medium, and fineto correspond with sandstone, siltstone, andclaystone is unnecessary.

Epiclastic is defined as "a term applied tomechanically deposited sediments (gravel, sand,mud) consisting of weathered products of olderrocks. Detrital material from pre-existent

SHORT NOTES 1413

rocks" (American Geological Institute, 1957,p. 97). Epiclastic volcanic rocks are defined asthose derived by weathering and erosion oflithified or solidified volcanic rocks. In anearlier paper (1960, p. 978) the author used"epiclastic" in a sense that would include allvolcaniclastic material transported by epigenegeomorphic agents, including unconsolidatedpyroclastic debris. Upon examination of thewhole field of volcaniclastic rocks, however, itbecomes apparent that rocks containing parti-cles formed by pyroclastic processes and thoseformed by weathering processes should be,insofar as possible, in separate categories. Com-minution of magma or lava by volcanic ex-plosion is unique in the formation of clasticrocks, because liquid is converted directly tosolid clastic material. Weathering and erosionaccomplishes granulation at a much slower rate,and probably with a greater total expenditureof energy. It is important to distinguish, ifpossible, fragments produced by explosionfrom those produced by weathering processes.

Graywacke that contains volcanic fragments(volcanic wackes of Williams, Turner, andGilbert, 1954) may be included within theepiclastic volcanic rock category, and manyare in large part composed of pyroclastic frag-ments. The bulk of volcanic wackes observedby me in an area of lower Tertiary volcanicrocks in the Cascade Mountains near MountRainier, Washington (Fisher, p. 1395, thisbulletin), appear to be epiclastic. Pogue (1909),Edwards (1950), and Dickinson (1958, Geology

of the Izee area, Grant county, Oregon, un-published Ph.D. thesis, Stanford Univ.) discussvolcanic wackes formed in probable geo-synclinal environments. Williams, Turner, andGilbert (1954, p. 303-304, 308) give a generalaccount of epiclastic volcanic rocks and gray-wackes.

Nongenetic names based upon presence ofvolcanic fragments and minerals and upon theirsizes are needed for volcaniclastic rocks withfragments of unknown origin. Volcanic sand-stone, for example, is a rock containing volcanicparticles of sand size regardless of the originof the fragments.

Acknowledgments

Many of the author's ideas are the result ofcorrespondence and conversations with severalgeologists. The instructive hours spent in dis-cussion with Ray E. Wilcox, on the field ofvolcanic rocks in general and pyroclastic rocksin particular have greatly aided the writer informulating his ideas. Comments and sug-gestions on the classification by Richard L.Hay, and George I. Smith are appreciated. Thewriter wishes to thank Howel Williams, JulianD. Barksdale, and Robert M. Norris for valua-ble suggestions on the original manuscript, andBetty Skipp and Earl F. Cook for suggestionson the present manuscript. Discussion andcorrespondence with William R. Dickinson hasbeen very helpful. Responsibility for theclassification, of course, rests with the author.

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1414 R. V. FISHER—CLASSIFICATION, VOLCANICLASTIC SEDIMENTS, ROCKS

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UNIVERSITY OF CALIFORNIA AT SANTA BARBARA, GOLETA, CALIF.MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, DECEMBER 27, 1960