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BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 69, PP. 1071-1073 AUGUST 1953

DEFINITION OF VOLCANIC BRECCIA

BY RICHARD V. FISHER Common usage of the term breccia by most usefulness because of these various definitions' geologists limits it to a composed of not only in the matter of grade size but, far angular fragments. The lower-size limit of the more importantly, in the matter of genetic fragments is generally set at 2 mm, the limiting implications. Indeed, in the most recent size for granules and larger particles (Went- glossary of geologic terms (American Geological worth, 1922; 1935). Logically it should follow Institute, 1957), volcanic breccia has been that a volcanic breccia is a breccia composed of defined as a "more or less indurated pyroclastic angular volcanic fragments larger than 2 mm. rock (author's italics) consisting chiefly of Norton (1917, p. 162), in his classification of accessory and accidental angular 32 mm breccias, includes volcanic breccias under the or more in diameter lying in a fine ." heading of subaerial breccias, although he does It is commonly recognized that volcanic not set a size limit for the fragments. He breccias may originate in a variety of ways further subdivides volcanic breccias (p. 170) (Anderson, 1933, p. 215-276; Wentworth and into flow breccia, which forms by fragmentation Williams, 1932, p. 32-33; Gilluly, Waters, and of during its flow, and tuff breccia "made up Woodford, 1951, p. 606), and apparently most of fragmental products of explosive eruptions." geologists use the term in a broad sense, but in Reynolds (1928, p. 97-107) in a modified most classifications it is limited to pyroclastic version of Norton's classification includes rocks, regardless of common usage (Heinrich, volcanic breccias under the heading of breccias 1956, p. 53; Emmons, Theil, Stauffer, and of igneous origin and as genetic subtypes names Allison, 1955, p. 432; Williams, Turner, and explosion breccias, flow breccias, and intrusion Gilbert, 1954, p. 149; Lahee, 1952, p. 269; breccias, although the heading "volcanic Twenhofel, 1950, p. 319; Spock, 1953, p. 59; breccia" is not used in the classification. Pirrson and Knopf, 1947, p. 112; Johannsen, Wentworth and Williams (1932, p. 51-52) 1939, p. 7; Hatch, Rastall, and Black, 1950, define a volcanic breccia as a p. 258). There obviously is a confusing dis- composed of angular volcanic fragments ex- crepancy between classification and usage. ceeding 32 mm which were solidified before Anderson (1933, p. 246), following Lacroix, aerial flight and deposition, a definition which has reviewed the variety of ways in which apparently excludes volcanic breccias that volcanic breccias can originate and has ar- form by processes other than explosive, such as ranged their origins in the following manner: flow breccias, breccias formed by extrusions I. Volcanic breccias not transported by of solid lava through a volcanic vent (auto- water breccias of Curtis, 1954), and breccias formed A. Crumbling of a dome (breches by the subaerial action of water such as mud- d'ecroulcmeni) flow action (lahars; Van Bemmelen, 1949, p. B. Intrusion (massive peperites) 191) or by short transportation of volcanic C. Friction fragments by stream action. D. Crumbling of advancing lava flow Twenhofel (1950, p. 319) uses the term vol- (blocky lava) canic breccia in the same sense as Wentworth E. Eruptions and Williams—i.e., pyroclastic—but has modi- a. Vulcanian fied the size limits. He regards the lower limit b. Pelean (nuees ardentes) as 4 mm (fine-grained volcanic breccia). This c. Ultra-vulcanian (Bandai-san) size restriction is also used by Gilluly, Waters, F. Dry avalanche (Vesuvius) and Woodford (1951, p. 606). II. Volcanic breccias transported by water Kemp (1952, p. 36) considers 1 cm as the A. Eruptions lower-grade limit for particles in volcanic a. Through a crater lake breccias and is followed by Emmons et al. b. Melting of ice and snow (1955, p. 432). Some authors have used 1 mm c. Following heavy rains as the lower-grade limit. d. Accompanied by heavy rains Although many authors today follow Went- B. Not related to eruptions worth and Williams, the term loses much of its a. Collapse of the dam of a crater lake 1071

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b. Heavy rains falling on uncon- particles may be stuck together by adherence solidated ejecta of once-plastic surfaces, and thus be "open- c. Rapid melting of snow and ice structured" (Macdonald, 1953, p. 177). Also, (Mount Shasta) matrix may be formed during movement of a Since there can be great differences in the lava flow by mutual abrasion of particles, environments of deposition of the various thereby causing fine sand and dust to develop within the interstices of the fragments (Mac- TABLE 1.—PROPOSED GRADE-SIZE LIMITS FOR donald, 1953, p. 177; Curtis, 1954, p. 463). VOLCANIC BRECCIA The term volcanic breccia should be defined to include any type of volcanic breccia, with or Grade Pyroclastic -volcanic without a matrix. size terminology terminology Normally the fragments of a volcanic breccia (mm) are composed of volcanic material. Occasionally, however, loose rock of any origin may be in- >32 Pyroclastic breccia corporated within a brecciating flow as it and Agglomerate Volcanic breccia moves along the surface of the ground. Upon and Volcanic solidification, the flow breccia may be composed 32-4 Lapilli tuff in part of nonvolcanic fragments set in a volcanic matrix. This type of breccia should 4-2 Tuff <4 mm. also be included under the term volcanic breccia. types of extrusive coarse-grained fragmental A proposed definition for volcanic breccia volcanic rocks, and since the term volcanic which would enable a geologist to use it for breccia used in a broad sense is extremely any type of volcanic breccia, regardless of useful, it should be made clear that "volcanic origin, is suggested as follows: breccia" includes all types of volcanic breccias A volcanic breccia is a rock composed predomi- composed of large angular volcanic fragments, nantly of angular volcanic fragments greater than regardless of origin. The definition should in- 2 mm in size set in a subordinate matrix of any clude rocks of such diverse origins as mudflow composition and texture, or with no matrix; or breccias and block breccias, both types of which composed of fragments other than volcanic set in are volcanic breccias though certainly not a volcanic matrix. Pyroclastic in origin. If this, or a similar definition, were to be Discussion accepted, then a statement such as "a field of Since "volcanic breccias" formed by flu- volcanic breccias" would not imply "a field of viatile processes are "true" sedimentary rocks, volcanic rocks of explosive origin," but in- the grade-size limitations of a generalized term stead "a field of breccias composed of volcanic should be considered. It is herein proposed that fragments." To the author, the use of the term 2 mm be set for the lower-grade-size limit of in the latter sense is a far better representation fragments composing volcanic breccias, but of the facts, for many volcanic "fields" are that established grade sizes for the purely composed of volcanic breccias of all modes of Pyroclastic volcanic breccias be retained—i.e., origin, not just pyroclastic. 32 mm (Table 1). Further, it is proposed that Pyroclastic breccia (a term used by Macdonald, References Cited 1953, p. 177) be used for the "volcanic breccia" American Geological Institute, 1957, Glossary of of Wentworth and Williams. geology and related sciences: Nat. Acad. Sci.- In keeping with a broad meaning of the Nat. Research Council, 325 p. term volcanic breccia, the matter of matrix is Anderson, C. A., 1933, The Tuscan formation of important. Volcanic breccias may be sedi- northern California with a discussion of the origin of volcanic breccia: Univ. Calif. Pub., mentary iu origin, hence will have a "sandy" Bull. Dept. Geol. Sci., v. 23, no. 7, p. 215-276 or "muddy" matrix; they may be pyroclastic Curtis, G. H., 1954, Mode of origin of pyroclastic in origin with a tuffaceous matrix, or have a debris in the Mehrten formation of the Sierra vesicular and pumiceous matrix common to Nevada: Univ. Calif. Pub., Bull. Dept. Geol. Sci., v. 29, no. 9, p. 453-502 the products of glowing avalanches (Williams, Emmons, W. H., Thiel, G. A., Stauffer, C. R., 1949, p. 23); or they may be caused by frag- and Allison, I. R., 1955, Geology: New York, mentation of flowing lava, with the result that McGraw-Hill Book Co., Inc., 638 p.

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Gilluly, J., Waters, A. C., and Woodford, A. O., Spock, L. E., 1953, A guide to the study of rocks: 1951, Principles of geology: San Francisco, New York, Harper and Brothers, 256 p. Freeman and Company, 631 p. Twenhofel, W. H., 1950, Principles of sedimenta- Hatch, F. H., Rastall, R. H., and Black, M., 1950, tion: 2d ed., New York, McGraw-Hill Book Co., The petrology of the sedimentary rocks: London, Inc., 673 p. Thomas Murby and Company, 383 p. Van Bemmelen, R. W., 1949, The geology of Heinrich, E. W., 1956, Microscopic petrography: Indonesia: v. 1A, Hague, Martinus Nijhoff, New York, McGraw-Hill Book Co., Inc., 296 p. 732 p. Johannsen, A., 1939, A descriptive petrography of Wentworth, C. K., 1922, A scale of grade and class the igneous rocks: vol. I, Univ. Chicago Press, terms for clastic : Jour. Geology, v. 2d ed., 318 p. 30, p. 377-392 Kemp, J. F., 1952, A handbook of rocks, Grout, 1935, The terminology of coarse sediments: F. F. editor: 6th ed., New York, D. Van Nat. Research Council Bull. 98, p. 225-246 Nostrand Co., Inc., 300 p. Wentworth, C. K., and Williams, H., 1932, The Lahee, F. H., 1952, Field geology: 5th ed., New classification and terminology of the pyroclastic York, McGraw-Hill Book Co., Inc., 883 p. rocks: Nat. Research Council Bull. 89, p. 19-53 Macdonald, G. A., 1953, Pahoehoe, aa, and block Williams, H., 1949, Geology of the MacDoel lava: Am. Jour. Sci., v. 251, p. 169-191 quadrangle: Calif. State Bur. Mines and Geology, Norton, W. H., 1917, Studies for students: A Bull. 151, 78 p. classification of breccias: Jour. Geology, v. 25, Williams, H., Turner, F. J., and Gilbert, C. M., p. 160-194 1954, Petrography: San Francisco, Freeman and Pirsson, L. V., and Knopf, A., 1947, Rocks and Company, 406 p. rock : New York, John Wiley and Sons, UNIV. or CALIFORNIA, SANTA BARBARA COLLEGE, Inc., 349 p. GOLETA, CALIF. Reynolds, S. H., 1928, Breccias: Geol. Mag., v. MANUSCRIPT RECEIVED BY THE SECRETARY OF LXV, no. 3, p. 97-107 THE SOCIETY, FEBRUARY 3, 1958

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