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Geology

Descriptive nomenclature and classification of pyroclastic deposits and fragments: Recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks

R. Schmid

Geology 1981;9;41-43 doi: 10.1130/0091-7613(1981)9<41:DNACOP>2.0.CO;2

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Notes

Geological Society of America Descriptive nomenclature and classification of pyroclastic deposits and fragments: Recommendations of the lUGS Subcommission on the Systematics of Igneous Rocks

R. Schmid Institut für Kristallographie und Pétrographie, ETH-Zentrum, 8092 Zürich, Switzerland

INTRODUCTION question of how broadly the terms admixtures. The extended meaning of During the last four years, a period "pyroclast" and "pyroclastic deposits" "pyroclast" does not contradict the when working meetings of the Inter- should be defined. One group, represented linguistic content of this term, "pyr" national Union of Geological Sciences by geologists whose chief concern is denoting fire and "clast" indicating (IUGS) Subcommission on the Systematics pyroclastic rocks, prefers to restrict breakage. of Igneous Rocks were dealing with pyro- "pyroclastic deposits" to subaerial fall, clastic deposits, six questionnaires on the flow, and surge deposits and to use the Terms "Agglomerate" and descriptive nomenclature and classification median grain diameter (of the nonballistic "Pyroclastic Breccia" of pyroclastic rocks were circulated to components) as a base of the granulo- Following the preference of many more than 150 geologists throughout the mere classification. Another group, volcanologists, "agglomerate" is applied world. One of the questionnaires which is composed mainly of paleovol- to coherent as well as to incoherent accompanied an issue of the Bulletin canologists and geologists dealing only materials, whereas "pyroclastic breccia" Volcanologique. The answers were temporarily with pyroclastic rocks, prefers refers to mainly consolidated materials carefully analyzed to obtain representa- to include within the term "pyroclastic because the term "breccia" is traditionally tive opinions, upon which the recommen- deposits" also lahars, subsurface and vent used for coherent materials. dations in this paper are based. The deposits (hyaloclastites, intrusion and recommendations were ratified by the extrusion breccias, tuff dikes, diatremes, Term "Tuff Subcommission at its Paris meeting in and so forth). Because experienced How broadly should the term "tuff" July 1980. volcanologists frequently cannot clearly be defined? The answers on this question From the beginning, the Subcommission recognize the specific genetic origin of a ranged from "consolidated ash" to "all aimed at a descriptive, rather than a volcaniclastic rock in the field (for consolidated pyroclastic deposits." Two genetic, classification suited for field example, to distinguish hyaloclastites advantages would result if "tuff" were use, including a minimum number of from other types of pyroclastic rocks), defined in the broad sense: (1) "Tuff terms and based mainly on the granulo- the Subcommission recommends that could be used as a complementary term to metrie properties of pyroclastic deposits. "pyroclastic deposit" be used in a broad "tephra." (2) Coming generations of sense. It defines "pyroclast" as "generated The Subcommission also intended to 1 earth scientists would be free to replace compile a separate glossary of common by disruption as a direct result of vol- "pyroclastic breccia" and "agglomerate" volcaniclastic rock terms, but this is canic action" instead of "generated by by the terms "block tuff" and "bomb still incomplete. disruption during volcanic eruptions," tuff," thus reducing the number of basic pyroclastic deposits being "assemblages descriptive pyroclastic rock terms and . . . of pyroclasts." Moreover, it allows COMMENTS ON RECOMMENDED using for polymodal or poorly sorted "pyroclastic deposits" to contain as much DEFINITIONS AND CLASSIFICATION pyroclastics self-explanatory composite as 25% by volume of epiclastic, organic, The following paragraphs outline the terms such as "ash-block tuff" or chemical sedimentary, and diagenetic reasons for our final choice of nomen- "bomb-lapilli tuff." The Subcommission clature and classification summarized decided to take only one step in this in the next section. direction by using "tuff" not only for ash-size materials but also, as "lapilli Term "Pyroclast" tuff," for coarser pyroclastics. If the Answers on the questionnaires 'The adjective "direct" excludes autobrec- term "tuff" is used alone it should ciation of lava flows, because the lava flow comprise, however, ash-size materials revealed a major difference among active itself is the direct result of volcanic action, not workers on pyroclastic rocks on the its brecciation. only.

GEOLOGY, v. 9, p. 41-43, JANUARY 1981 41 Granulometrìe Classification PUMICE, Term "Epiclast" GLASS In their definitions, the pyroclasts are The definitions of "epiclast," "epiclastic characterized by, in addition to other deposit," and "epiclastic rock" must properties, their size, using as a quantifier be regarded as provisional because they the "mean diameter." In very coarse and fall outside the purview of the Sub- in consolidated pyroclastic deposits the commission. They had to be given to "mean diameter" is usually estimated by clearly delineate pyroclasts and pyroclastic eye, whereas in incoherent materials it deposits from epiclasts and epiclastic can be determined by sieving. Because deposits. there does not exist a standard procedure that could be prescribed for measuring RECOMMENDED DEFINITIONS AND the mean diameter in all cases in the CLASSIFICATION same way, the Subcommission declines Pyroclasts to define this term. CRYSTALS- ROCK CRYSTAL FRAGMENTS Pyroclasts are the individual crystals, Instead of "median diameter,"2 the FRAGMENTS crystal fragments, glass fragments, and more generalized term "average diameter" Figure 1. Subdivision of tuffs and ashes accord- rock fragments generated by disruption has been used in the granulometrie classi- ing to their fragmental composition. as a direct result of volcanic action. The fication of pyroclastic deposits, taking shapes they assumed during disruption into account that granulometrie analyses or during subsequent transport to the pri- will rarely be carried out and that rocks is lacking. When in future such an mary deposit must not have been altered generally the grain size will be estimated agreement is achieved, it may be necessary by later redeposition processes. If they by eye. to modify the divisions so that they will were altered, the crystals or fragments The Subcommission, in deciding 011 fit appropriate sedimentary size limits. would be called "reworked pyroclasts" appropriate granulometrie size limits, Sedimentologists are invited to reinforce or "epiclasts" (if their pyroclastic origin would have preferred to divide the granu- their efforts to establish a unified is uncertain). lometrie scale at 50, 2, and 0.05 or granulometrie classification of sediments. A bomb is a pyroclast with a mean 0.1 mm. Because these numbers are rot diameter commonly exceeding 64 mm. Its even numbers on the scale widely used Genetic Prefixes shape (ellipsoidal, discoidal, or irregular) by sedimentologists, the 64, 2, and 1/16 The terms for pyroclastic deposits cited or its surface (for example, "bread-crust" mm limits were choosen. These numbers, below may be prefixed by terms denoting surface) indicates that during its forma- however, have to be regarded as provi- the specific genetic origin of the deposit tion and subsequent transport it was in sional as long as international agreement or the chemical composition of the parent a wholly or partly molten condition. on granulometrie divisions of sedimentary magma—for example, "air-fall tuff," A block is a pyroclast with a mean "lacustrine tuff," "laharic ash-lapilli diameter exceeding 64 mm, whose com- tuff," "rhyolitic crystal tuff," "vent monly angular to subangular shape indi- agglomerate." The terms may also be 2 cates that during its formation it was The median diameter of the grain popula- replaced by purely genetic terms, such as in a solid state. tion of a rock is the diameter by which the area below a weight-percentage frequency distribu- "hyaloclastite" and "base-surge deposits," Lapilli are pyroclasts of any shape, tion curve is divided into two equal parts. whenever it seems appropriate to do so. with mean diameters of 2 to 64 mm. Ash grains are pyroclasts with mean diameters smaller than 2 mm.

TABLE I. GRANULOMETRIC CLASSIFICATION OF PYROCLASTS AND Dust grains (or fine ash grains) are OF UNIMODAL, WELL-SORTED PYROCLASTIC DEPOSITS pyroclasts with mean diameters smaller than 1/16 mm.

Clast size Pyroclast Pyroclastic deposit (mm) Pyroclastic Deposits: General Terms Pyroclastic deposits (= "pyroclastics") Mainly unconsolidated: Mainly consolidated: include both consolidated3 and uncon- tephra pyroclastic rock solidated assemblages of pyroclasts. They Bomb, block Agglomerate, bed of Agglomerate, must contain more than 75% pyroclasts blocks pyroclastic breccia by volume. or borib, block tephra Pyroclastic rocks are predominantly 64 mm consolidated pyroclastic deposits. Lapillus Layer, bed of lapilli Tephra is a collective term for pyro- or clastic deposits that are predominantly Lapilli tuff lapilli tephra unconsolidated. 2 mm Coarse ash grain Coarse ash Coarse (ash) tuff 1/16 mm Fine ash grain Fi rie ash (dust) Fine (ash) tuff 3 "Consolidated" as used in the following (dust grain) (dust tuff) is thought to comprise adjectives such as "coherent," "cemented," and "indurated."

42 JANUARY 1981 TABLE 2. TERMS FOR MIXED PYROCLASTIC-EPICLASTIC ROCKS

Pyroclastic* Tuffites Epiclastic Avg (mixed pyroclastic-epiclastic) (volcanic and/or nonvolcanic) clast size (mm)

Agglomerate, agglutinate pyroclastic breccia Tuffaceous conglomerate, Conglomerate, breccia 64 tuffaceous breccia Lapilli tuff 2 coarse Tuffaceous sandstone Sandstone (Ash) tuff 1/16 fine Tuffaceous siltstone Siltstone 1/256 Tuffaceous mudstone, shale Mudstone, shale 100 75 25 0% by volume

•Pyroclasts •Volcanic + nonvolcanic epiclasts (+ minor amounts of biogenic, chemical sedimentary and authigenic constituents)

*Terms according to Table 1.

Pyroclastic Deposits: Terms for Unimodal one dominant size fraction should be Mixed Pyroclastic-Epiclastic Rocks and Well-Sorted Pyroclastic Deposits named by using an appropriate combi- (Table 2) (Table 1, Figure 1) nation of the terms cited in Table 1. "Tuffites" are rocks consisting of A pyroclastic breccia is a pyroclastic Examples: mixtures of pyroclasts and epiclasts rock whose average pyroclast size exceeds ash-lapilli tuff (lapilli > ash) (<75% pyroclasts, >25% epiclasts by 64 mm and in which angular pyroclasts lapilli-ash tuff (ash > lapilli) volume). predominate. lapilli tuff-breccia/-agglomerate An agglomerate is a pyroclastic rock (lapilli - blocks/bombs) ACKNOWLEDGMENTS or deposit whose average pyroclast size (ash) tuff-breccia/-agglomerate Reviewed by C. R. Bacon and P. C. Bateman. exceeds 64 mm and in which rounded (ash ~ blocks/bombs) The Subcommission members appreciate the pyroclasts predominate. ash-lapilli tuff-breccia/-agglomerate help of all colleagues who answered the ques- A lapilli tuff is a pyroclastic rock whose (lapilli > ash — blocks/bombs). tionnaires; we regret that space limitations preclude listing all of their names here. Discus- average pyroclast size is 2 to 64 mm. sions with H.-U. Schmincke on some critical A tuff (or ash tuff) is a pyroclastic Epiclasts and Epiclastic Deposits points were helpful. The following participated rock whose average pyroclast size is less Epiclasts are crystals, crystal fragments, in the working meetings during which answers than 2 mm. glass fragments, and rock fragments that to previous questionnaires were discussed and subsequent questionnaires were outlined: C. D. A dust tuff (or fine ash tuff) is a have been liberated from any type of pre- Branch (Australia), J. W. Cole (New Zealand), pyroclastic rock whose average pyroclast existing rock (volcanic or nonvolcanic) W. Duffield (United States), A. M. Goodwin size is less than 1/16 mm. by weathering or erosion and transported (Canada), H. Honnorez (United States), M. J. from their place of origin by gravity, air, Le Bas (Great Britain), V. Lorenz (BRD), water, or ice. V. E. Neal (New Zealand), R. Schmid (Switzer- Pyroclastic Deposits: Terms for land), V. Sziky-Fux (Hungary), M. E. Teruggi Polymodal or Poorly Sorted An epiclastic deposit is a consolidated (Argentina), P. M. Vincent (France). Pyroclastic Rocks or unconsolidated aggregate of epiclasts. Polymodal or poorly sorted pyroclastic An epiclastic rock is a mainly consoli- MANUSCRIPT RECEIVED OCT. 22, 1980 rocks containing pyroclasts of more than dated epiclastic deposit. MANUSCRIPT ACCEPTED OCT. 27, 1980

GEOLOGY PRINTED IN U S.A. 43