P. W. FRANCIS Department of Earth Sciences, The Open University, Walton Hall, Walton, Bletchley, Buckinghamshire, England

Cannonball Bombs, A New Kind of Volcanic Bomb from the Pacaya ,

ABSTRACT fourth group of bombs, those which derive their shape mainly from post-impact mechani- Volcanic bombs with a distinctive shape are cal attrition while bouncing down the scoria produced by post-impact mechanical rounding cone of the active volcano. The other groups processes while traveling at high speed down of bombs may also show the effects of rolling the slopes of the scoria cone of the Pacaya or bouncing, but some of them, such as cowpat Volcano in Guatemala. The name "cannonball bombs, are so plastic that they generally do bombs" is proposed for bombs formed by this not travel far from their place of impact. mechanism. DISCUSSION INTRODUCTION Formation of the new kind of bomb was ob- Many names have been applied to different served by the author on the Pacaya Volcano in kinds of bombs ejected by volcanoes. Tsuya Guatemala (lat. 14°23'0", long. 90°36'2"), (1939) listed no less than 14 types, classified on about 40 km SSW. of . This the basis of shape, and he concluded that the 2,552-m-high volcano has had a history of shape of a bomb and its inner structure are activity dating back to 1565 (Mooser and "derived from a 'schlieren' that developed in others, 1958). After being dormant since 1846, the flowing within the crater from which a new episode of eruptive activity began in they were ejected, although subjected to defor- August 1965 (Rose, 1967) and has continued mation in their passage through the air." intermittently until the present. The recent More recently, authors (for example, Mac- activity has built up a young, steep, scoria cone Donald, 1967, 1972; Rittman, 1962) have de- 400 m high on the southwestern flanks of an scribed a variety of different bombs, some of older, higher cone. them with a number of colorful synonyms, and While the volcano was in a state of moderate most of them having characteristic shapes. All Strombolian activity in February 1970, it was these types, however, appear to fall into three observed that around the base of the young genetic groupings: (1) impact bombs, which scoria cone, there were many pale-gray, smooth- include cowpat, cowdung, and pancake bombs; ly rounded bombs, ranging in size from a few (2) "aerodynamic" bombs, which include rib- centimeters to more than 1 m, and in shape bon, rotational, spindle, fusiform, and spherical from subspherical to prolate ellipsoid. The bombs; and (3) expanding bombs, which in- presence of these distinctive bombs was also clude breadcrust and exploding bombs. noted by Rose (1967), who recorded sizes as The morphology of these bombs is well much as 5 m. The bombs were composed documented, but differences in opinion exist of basaltic lava with olivine and plagioclase between authors on the mechanisms by which phenocrysts and contained as much as about the various kinds of aerodynamic bombs ac- 30 percent of vesicles. There was no evidence, quire their shape. MacDonald (1967, 1972) within broken specimens of the bombs, of in- summarized some of the arguments. Bombs of ternal structures such as layering or cores of all three groups, however, are shaped primarily other rocks. by their impact, in-flight behavior, and subse- The volcano was periodically ejecting in quent cooling history. I propose a possible minor explosions red hot fragments of fairly

Geological Society of America Bulletin, v. 84, p. 2791-2794, 1 fig., August 1973 2791

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hot, but solid. The outer surfaces were pale gray with rock dust and covered with "braises," clearly the results of numerous impacts during the descent. Many of the large angular boulders at the base of the cone also showed much evidence of hits by descending bombs. Many bombs were shattered on impact, and many broken fragments were found.

SUMMARY The ellipsoidal shape of many of the bombs argues against a "ball-milling" process oper- ating within the vent itself, which would tend to produce bombs uniformly subspherical in shape. It is concluded that the bombs are shaped mainly by mechanical attrition during their descent of the volcano, and not by processes acting either within the volcanic vent or during the first "flight" of the bomb above the vent. It is also possible that the repeated blows sustained by the bombs during their descent may have "forged" them into shape, but it is not possible to assess how im- portant this effect may be. Viscous lava is essential for this kind of bomb to be produced; after a minor explosion in the summit crater. Puffs c£ dust on the slopes of the cone mark the sites of succes • more fluid lava would neither be capable of sive impacts of bombs bouncing their way down the: bouncing nor of sustaining a rapid spin without slopes. The ground at the base of the cone is littered breaking up. A long descent is also clearly with "spent" bombs and fragmented remains of essential. others. (Drawn from a color transparency.) Such bombs must be common on other volcanoes with similar properties. It is sug- viscous pasty lava. Some of this material fell gested that the term "eannonball bombs" is back around the vent in irregular sheets. Many appropriate to them. more smaller fragments, which were only twist- ing slowly when first ejected, fell back and ACKNOWLEDGMENTS bounced off at a glancing angle to the 35° slope I am grateful to G.P.L. Walker for reading of the scoria cone. As they did so, they ac- an early draft of this note: and to M. P. Coward quired an extremely rapid spin, and descended and P. R. Cobbold for help in the field. Most the flank of the cone in a large number of of the funds for the field work came from a bounces, taking only a few seconds of elapsed Grant-in-Aid from the Royal Society. time (Fig. 1). At each bounce, the descending bomb sent up a puff of dust; on still photo- REFERENCES CITED graphs taken at the time, dust clouds from MacDonaid, G. A., 1967, Forms and structures of more than 15 bounces by the same bomb can be extrusive basaltic rocks, in Hess, H. H., and identified on the lower slopes of the cone, anc it Poldervaart, A., eds., , The Polder- is likely that many more took place during the vaart treatise on rocks of basaltic composition: descent. New York, Interscier.ce, p. 1-61. It was noted from a secure site at the base 1972, Volcanoes: Englewood Cliffs, N.J., of the cone that after traveling the height of Prentice-Hall Ltd., p. 125. Mooser, F., Meyer-Abich, H., and McBirney, A. the cone, a large proportion of the bombs was R., 1958, Active volcanoes of Mexico, Central still traveling extremely fast and spinning very America, in Catalogue of the active volcanoes rapidly with a loud whirring noise. of the world including solfatara fields, Pt. 6: Inspection of newly arrived bombs at the Naples, Italy, Internat. Volcanol. Assoc., p. base of the cone revealed that they were red 1-36.

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Rittman, A., 1962, Volcanoes and their activity: origin of basaltic bombs from volcano Huzi New York, Interscience, p. 76-79. (Fuji): Tokyo Univ. Earthquake Research Rose, W. I., 1967, Notes on fumaroles and recent Inst. Bull., v. 27, no. 4, p. 809-825. activity of Volcan Pacaya, in Bonis, S., ed., Excursion guide book for Guatemala: [Guate- mala] Inst. Geog. Nac. Geol. Bull., no. 4, p. MANUSCRIPT RECEIVED BY THE SOCIETY JULY 25, 31-33. 1972 Tsuya, H., 1939, On the form and structure of REVISED MANUSCRIPT RECEIVED FEBRUARY 13, volcanic bombs with special reference to the 1973

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