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1 Polygenetic nature of a rhyolitic dome and implications for hazard assessment: 2 Cerro Pizarro , 3 by G. Carrasco-Núñez and N. Riggs 4 5 ABSTRACT 6 Rhyolitic domes are commonly regarded as monogenetic volcanoes associated with single, brief 7 eruptions. They are characterized by short-lived successions of pyroclastic and effusive activity 8 associated with a series of discrete eruptive events that apparently last on the order of years to 9 decades. Cerro Pizarro, a ~ 1.1 km3 rhyolitic dome in the eastern Mexican , shows 10 aspects of polygenetic including long-term repose periods (~ 50-80 ky) between 11 eruptions, chemical variations with time, and a complex evolution of alternating explosive and 12 effusive eruptions, a cryptodome phase, and . This eruptive behavior provides 13 new insights into how domes may evolve. A protracted, complex evolution bears 14 important implications for hazard assessment if reactivation of an apparently extinct rhyolitic 15 dome must be seriously considered. 16 17 Keywords: monogenetic volcanism, polygenetic volcanism, , dome growth, volcanic 18 hazards, Mexican Volcanic Belt 19 20 INTRODUCTION 21 Monogenetic volcanoes comprise a wide spectrum of relatively small volcanic structures 22 (generally less than a few km3 erupted material) that show a commonly simple evolution (one 23 eruption, or a few clearly related eruptions), short life span (commonly years to decades for 24 volcanoes, but possibly as much as a few centuries for rhyolitic domes), and minor 25 chemical composition changes. Monogenetic volcanoes are, in general, either or rhyolite, 26 while polygenetic volcanoes, which erupt repeatedly and have a large and persistent 27 storage chamber, are commonly andesitic or dacitic in composition. Although the most common 28 monogenetic volcanoes are cinder cones, cones and rings, and volcanoes of basaltic 29 composition, most rhyolite domes also fit the criteria of monogenetic volcanoes. Mafic 30 monogenetic volcanoes are common in small and large volcanic fields (e.g., Springerville 31 , Condit and Connor, 1996; Michoacán-Guanajuato volcanic field, Mexico, 32 Hasenaka and Carmichael, 1985). Rhyolitic domes occur either as isolated individual volcanoes 33 (e.g. , Mexico; Siebe et al., 1995; Cerro Pizarro, Riggs and Carrasco-Nuñez, 34 2004), as part of dome fields (e.g. Taylor Creek, New Mexico; Duffield et al., 1995) or on the 35 flanks of or within larger volcanoes (e.g., Mono-Inyo field-Long Valley ; Miller, 1985; 36 South Sister volcano, ; Scott, 1987; dome, Valley of Ten Thousand Smokes; 37 Hildreth and Fierstein, 2000). A “typical” rhyolitic dome, as modeled by Duffield et al. (1995) 38 for the Taylor Creek field, evolves from vent-clearing pyroclastic eruptions to dome extrusion 39 over a short time span perhaps only a few years in duration. No rhyolite domes have been 40 observed forming, and estimates of lifespans range from a few to several years (Mono-Inyo field, 41 Miller, 1985; Novarupta, Hildreth and Fierstein, 2000) to a maximum of a few centuries (e.g. 42 Taylor Creek Rhyolite, Duffield et al., 1995). Individual rhyolite domes or related groups of 43 domes do not have significant major-element compositional variations over their growth stages 44 (Scott, 1987; Duffield et al., 1995). Because of this relatively simple common evolutionary path, 45 rhyolite domes are not generally considered very hazardous, similar to monogenetic basaltic 46 systems. 47 Cerro Pizarro rhyolitic dome, located in the eastern Trans-Mexican Volcanic Belt (Fig. 1), 48 evolved through periods of effusive and highly explosive activity that were separated by 49 cryptodome intrusion, edifice sector-collapse, and prolonged erosional episodes (Riggs and 50 Carrasco-Nuñez, 2004). Chemistry of the eruptive products also changed over time. This 51 evolution, in addition to the ~ 50-80 ka repose period between the main eruptive episodes, 52 indicates that a model of short-lived, monogenetic activity does not characterize all rhyolite 53 domes. The purpose of this paper is to describe the polygenetic nature of Cerro Pizarro dome in 54 terms of timing, chemical variation, and eruptive behavior. These new insights have important 55 implications for hazard assessment of some young domes that otherwise may be 56 considered extinct volcanoes. 57 58 REGIONAL SETTING 59 Cerro Pizarro is located within the Serdán-Oriental basin in the easternmost Mexican 60 Volcanic Belt (Fig. 1). The Serdán–Oriental is a broad, intermontane, relatively flat 61 lacustrine/playa closed basin characterized by isolated small basaltic scoria cones, 62 and tuff rings and maar volcanoes of basaltic and rhyolitic composition, and somewhat larger

2 63 rhyolitic domes such as Cerro Pizarro (Riggs and Carrasco-Núñez, 2004), Cerro Pinto, and Las 64 Derrumbadas (Siebe et al., 1995). The regional basement comprises locally exposed Cretaceous 65 limestone and small intrusive rocks. 66 Volcanic deposits within the Serdán-Oriental basin are dominated by pyroclastic material 67 derived from Los Humeros caldera, located about 16 km north of Cerro Pizarro. Activity at Los 68 Humeros began with the emplacement of the ~115-km3 Xáltipan at ~460 ka (Ferriz 69 and Mahood, 1985). A highly explosive event occurred at about 100 ka, producing the 15-km3 70 Zaragoza ignimbrite (Ferriz and Mahood, 1984; Carrasco-Núñez and Branney, 2005). 71 72 EVOLUTION OF CERRO PIZARRO 73 The evolution of Cerro Pizarro took place in four main stages (after Riggs and Carrasco- 74 Núñez, 20004; Fig. 2). In the first stage, vent-clearing explosions incorporated of 75 basement rocks including vesicular from a nearby scoria cone, Cretaceous limestone, and 76 Xáltipan ignimbrite (Ferriz and Mahood, 1985). Subsequent eruptions produced surge and fallout 77 layers followed by passive, effusive dome growth. Oversteepened flanks of the dome collapsed 78 at times to produce block-and ash-flow deposits and, slightly later, an external vitrophiric 79 carapace developed (Fig. 2A). This early stage corresponds well to the model proposed by 80 Duffield et al. (1995). 81 During the second stage, a new pulse of magma caused the emplacement of a cryptodome, 82 which inflated the volcano and strongly deformed the vitrophyric carapace as well as the older 83 parts of the dome, producing subvertical orientations of the overlying pre-dome units. 84 Disintegration of this cryptodome caused a debris avalanche as the western flank of the volcano 85 collapsed (Fig. 2B) (cf. Mount St. Helens, 1980, Voight et al., 1981; Soufrière Hills, Voight et 86 al., 2002). 87 The third stage (Fig. 2C) was characterized by a prolonged period of of the dome 88 and passive magma intrusion. Erosion cut canyons as much as 30 m deep and produced 89 heterolithic debris- and hyperconcentrated-flow deposits by reworking the debris-avalanche 90 deposit deposited during Stage II. At approximately 116 ka (see below) magma intrusion caused 91 the collapse crater to fill and the present-day conical shape of the volcano was formed. No 92 evidence exists for pyroclastic or collapse-related deposits associated with this dome growth.

3 93 The fourth and final stage (Fig. 2D) includes both hiatus activity following dome growth at 94 116 ka and the final eruptions of Cerro Pizarro. Several pyroclastic successions were emplaced, 95 including the ~100-ka Zaragoza ignimbrite (Carrasco-Núñez and Branney, 2005), which overlies 96 the volcaniclastic deposits derived from the sector collapse of Cerro Pizarro (unit B in Fig. 3A). 97 Stage IV surge and fall deposits are widely dispersed on the apron around the volcano (Fig. 4). 98 The sequence includes two distinctive marker beds (Fig. 3B): the lower one (“a”) is rich 99 whereas the upper one (“c”), which has abundant lithic clasts. 100 101 CHEMISTRY OF THE CERRO PIZARRO PRODUCTS 102 XRF analysis of 14 samples shows that Cerro Pizarro eruptive products are high-silica 103 rhyolite, in contrast to products of the surrounding volcanoes, which are dacitic or andesitic in 104 composition. Rocks from Stages I, II, and III are very similar in major and trace element

105 chemistry, and vary from Stage IV only being slightly higher in TiO2 and Fe2O3,and lower in

106 MnO and Na2O. More consistent variations are observed for trace elements: Rb, Y, Nb, Ni, Zn 107 are higher for the Stage IV pyroclastic rocks, and Sr, Ba, and Zr are lower than the rest of the 108 Cerro Pizarro rocks. Detailed discussion of the will be presented elsewhere. 109 Even though marked inter-dome compositional variations are observed in rhyolitic fields, 110 individual domes generally show a more homogeneous composition (e.g. Taylor Creek rhyolite, 111 Duffield et al., 1995; Inyo volcanic chain, Sampson, 1987; of South Sister volcano, 112 Scott, 1987) (Fig. 5). The relatively large trace-elements changes between Stages I, II, and III 113 and Stage IV deposits of Cerro Pizarro (Fig. 5) may be due to differentiation processes such as 114 fractionation occurring over the long hiatus in eruptive activity. 115 116 LIFESPAN OF CERRO PIZARRO DOME 117 We have dated four samples of Cerro Pizarro rhyolite and one of underlying basalt by the 118 40Ar/39Ar method (Table 1). Stratigraphic relations preserved on the dome (Riggs and Carrasco- 119 Núñez, 2004) indicate that the basalt scoria cone was unconsolidated at the time of cryptodome 120 emplacement. This observation, combined with the ages of the basalt (190 ± 20 ka) and of 121 the first two stages (220 ± 60 ka and 180 ± 50 ka) strongly suggests that the three eruptive events 122 occurred in short succession without any significant interruption; we consider that all three 123 occurred at ~200 ka. The near-contemporaneity of the two rhyolitic eruptive events is also

4 124 supported by the homogeneous composition that all the associated eruptive products exhibit in 125 both major and trace elements. 126 Rebuilding of the dome (Stage III) occurred at 116 ± 12 ka. By analogy with the current 127 growth of the dome in Mount St. Helens, the cone may have been re-established quickly, once 128 magma began to be emplaced. The cone material falls well within the geochemical range of the 129 older dome material. Following a substantial hiatus that lasted, within errors, between 29 and 73 130 ky, a final explosive event occurred at 65 ± 10 ka (Stage IV) to produce the final pyroclastic 131 sequence. 132 133 CERRO PIZARRO DOME: MONOGENETIC OR POLYGENETIC MAGMATIC 134 SYSTEM 135 domes comprise compositions from high-silica and to high-silica 136 rhyolite. Composition may play an important role in controlling dome growth styles: while 137 rhyolitic domes tend to be simpler and form through endogenous growth, dacitic and andesitic 138 bodies more commonly have mixed endogenous and exogenous activity (Duffield et al., 1995). 139 Andesitic-dacitic domes are often associated with larger volcanic systems, either large central 140 composite volcanoes (e.g. Santiaguito: Harris et al., 2003; Showa-Shinzan: Mimatsu, 1995; 141 Mount St. Helens:, Swanson et al., 1987) or Pelean-type volcanoes (e.g. Merapi: Newhall et al., 142 2000; Mount Pelée: Lacroix, 1904; Soufrière Hills: Sparks and Young, 2002), and therefore are 143 associated with multiple effusive events that last for longer periods of time. These volcanoes and 144 volcanic systems are polygenetic, involving larger amounts of magma, variations in eruptive 145 activity, and longer life spans (Table 2). 146 The volume of Cerro Pizarro compares closely with other monogenetic volcanoes (~ 1.1 147 km3). The complexity of its eruptive activity, however, more closely resembles that of a 148 than an individual rhyolitic dome. For example, although cryptodomes are 149 common in the literature, they are generally confined to large magmatic systems. Likewise, 150 collapse of a major sector of the volcanic edifice is generally associated with stratovolcanoes like 151 Mount St. Helens (Voight et al., 1981) or Pelean-type volcanoes such as Soufrière Hills volcano 152 (Voight et al., 2002). Although Cerro Pizarro is rhyolitic and therefore more likely to have 153 behaved as a monogenetic system, by virtue of chemistry, activity, and lifespan it should be 154 classified as polygenetic. To the extent, however, that complex domes like Soufrière Hills or

5 155 Unzen Volcano are erupting as multiple-vent systems, with distinguishable, if undramatic 156 changes in chemistry, clearly the designation of polygenetic is likely for many silicic domes. 157 Cerro Pizarro can therefore be considered hybrid in terms of its magmatic activity, both in 158 chemistry, in eruptive style, and in lifespan, between monogenetic rhyolitic domes, which erupt 159 quickly in a predictable way, and polygenetic andesitic-dacitic domes, which often follow a far- 160 more complex evolutionary path. 161 162 IMPLICATIONS FOR HAZARD ASSESSMENTS 163 Reactivation of a rhyolitic dome after a long period of repose has not been previously 164 reported. Reactivation of a seemingly extinct volcano carries very important implications for 165 assessment of volcanic hazards, particularly considering that renewed activity might be explosive 166 or involve sector collapse of the volcanic edifice. Regardless of whether a dome like Cerro 167 Pizarro should be considered polygenetic or monogenetic, the combined stratigraphic, 168 geochemical, and geochronologic evidence from the volcano shows that a rhyolite dome has the 169 potential for renewed activity after a long hiatus. Future eruptions in the Mexican Volcanic Belt, 170 or any district where rhyolitic domes seem to erupt in isolation from other, larger systems, will 171 serve as an excellent test to assess the apparent severe hazards associated with these small 172 volcanoes. 173 174 ACKNOWLEDGMENTS 175 Funding was provided by PAPIIT grant No. IN 104401 and logistical support by Centro de 176 Geociencias, UNAM. R. Lozano performed the chemical analyses at LUGIS (UNAM). S. 177 Medina provided useful references. W. McIntosh and L. Peters performed the isotopic dating at 178 New Mexico Geochronological Laboratory and helped us resolve complex age data. Fruitful 179 discussions with W.A. Duffield, W.E. Scott, and C, Newhall are gratefully acknowledged. W.A 180 Duffield and B. Zimmer read an earlier version of the manuscript and provided comments that 181 strongly helped us improve the text. 182 183 REFERENCES CITED

6 184 Carrasco-Núñez G., and Branney, M. J., 2005, Progressive assembly of a massive layer of 185 ignimbrite with a normal-to-inverse compositional zoning: the Zaragoza ignimbrite of central 186 Mexico: Bulletin of , v. 68, p. 3-20. 187 Condit, C. D., and Connor, C. B., 1996, Recurrence rates of volcanism in basaltic volcanic fields; 188 an example from the Springerville volcanic field, Arizona: Geological Society of America 189 Bulletin, v. 108, p.1225-1241. 190 Duffield, W. A., Richter, D. H., and Priest, S. S., 1995, Physical volcanology of silicic 191 domes as exemplified by the Taylor Creek rhyolite, Catron and Sierra counties, New Mexico: 192 US Geological Survey Map I-2399, 1:50,000. 193 Duffield, W.A., and Ruiz, J., 1992, Compositional gradients in large reservoirs of silicic magma 194 as evidenced by versus Taylor Creek rhyolite lava domes: Contributions to 195 and Petrology, v. 110, p. 192-210. 196 Ferriz, H., and Mahood, G., 1985, Eruption rates and compositional trends at Los Humeros 197 Volcanic Center, Puebla, Mexico: Journal of Geophysical Research, v. 89, p.8511-8524. 198 Harris, A., Rose, W. I., and Flynn, L., 2003, Temporal trends in extrusion at 199 Santiaguito 1922-2000: Bulletin of Volcanology, v. 65, p. 77-89. 200 Hasenaka, T., and Carmichael, I. S. E., 1985, The cinder cones of Michoacán-Guanajuato, 201 central Mexico: their age, volume and distribution, and magma discharge rate: Journal of 202 Volcanology and Geothermal Research, v. 25, p. 105-124. 203 Hildreth, W., and Fierstein, J, 2000, Katmai volcanic cluster and the great eruption of 1912: 204 Geol. Soc. Am. Bulletin, v. 112, p. 1594-1620. 205 Luhr, J. T., and Simkin, T., 1993, Parícutin: the volcano born in a Mexican cornfield: Phoenix 206 Geoscience Press, pp. 427. 207 Miller, C. D., 1985, eruptions at the Inyo volcanic chain, California: implications for 208 possible eruptions in the : , v. 13, p. 14-17. 209 Mimatsu, M., 1995, Showa-Shinzan diary (English translation). Executive committee 50th 210 anniversary of Showa-Shinzan, Hokkaido, , pp.179. 211 Newhall, C. G., Bronto, S., Alloway, B., Banks N. G., Bahar, I., del Marmol, M. A., 212 Hadisantono, R. D., Holcomb, R.T., McGeehin, J. et al., 2000, 10,000 Years of explosive 213 eruptions of Merapi Volcano, Central : archaeological and modern implications: Journal 214 of Volcanology and Geothermal Research, v. 100, p. 9–50.

7 215 Riggs, N. F., and Carrasco-Núñez, G., 2004, Evolution of a complex, isolated dome system, 216 Cerro Pizarro, central México: Bulletin of Volcanology, v. 66, p. 322-335. 217 Sampson, D. E., 1987, Textural heterogeneities and vent area structures in the 600-year-old lavas 218 of the Inyo volcanic chain, eastern California, in Fink, J.H., ed., The Emplacement of silicic 219 domes and lava flows: Geological Society of America Special Paper 212, p. 89-102. 220 Sampson, D. E., and Cameron, K. L., 1987, The geochemistry of the Inyo volcanic chain: 221 multiple magma systems in the Long Valley region, eastern California. Journal of 222 Geophysical Research, v. 92, p. 10403-10421. 223 Scott, W. E., 1987, Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon, 224 in Fink, J.H., ed., The Emplacement of silicic domes and lava flows: Geological Society of 225 America Special Paper 212, p. 35-45. 226 Siebe, C., Macías, J. L., Abrams, M., Rodríguez, S., Castro, R., and Delgado, H., 1995, 227 Quaternary explosive volcanism and pyroclastic deposits in east central Mexico: implications 228 for future hazards: Geological Society of America Field Guide, p. 1-47. 229 Sparks, R. S. J., and Young, S. R., 2002, The eruption of Soufrière Hills volcano, Monserrat 230 (1995-199): overview of scientific results, in Druitt, T. and Kokelaar, B. P., eds., The eruption 231 of Soufrière Hills volcano, Monserrat (1995-199): Geological Society, London, Memoirs, v. 232 21, p. 45-69. 233 Swanson, D. A., Dzurisin, D., Holcomb, R. T., Iwatsubo, E.Y., Chadwick, W. W., Casadevall, T. 234 J., Ewert, J. W., and Heliker, C. C., 1987, Growth of the lava dome at Mount St. Helens, 235 Washington (USA), 1981-1983, in Fink, J.H., ed., The Emplacement of silicic domes and lava 236 flows: Geological Society of America Special Paper 212, p. 1-16. 237 Voight, B., Glicken, H., Janda, R. J., and Douglass, P. M., 1981, The rockslide avalanche of May 238 18, in Lipman, P. W., Mullineax, D. R., eds., The 1980 eruptions Mount St. Helens, 239 Washington: US Geological Survey Special Paper 1250, p. 347-377. 240 Voight, B., Komorowski, J. C., Norton, G. E., Belousov, A., B., Belousova, M., Boudon, G., 241 Francis, P. W., Franz, W., Heinrich, P., Sparks, R. S. J., and Young, S. R., 2002, The 26 242 December (Boxing Day) 1997 sector collapse and debris avalanche at Soufrière Hills 243 Volcano, Monserrat, in, Druit, T. H., Kokelaar, B. P., eds., The eruption of Soufrière Hills 244 Volcano, Monserrat, from 1995-1999: Geological Society of London Memoir, v. 21, p. 363- 245 407.

8 246 247 FIGURE CAPTIONS 248 Fig. 1. Geologic map of Cerro Pizarro (modified from Riggs and Carrasco-Núñez, 2004) Units 249 are grouped into four main stages of evolution of the volcano. Inset map shows location of C. 250 Pizarro within the Mexican Volcanic Belt. 251 Fig. 2. Summary of Cerro Pizarro evolution (modified from Riggs and Carrasco-Núñez, 2004). 252 A) Stage I, initial open-vent explosions and the growth of a rhyolitic dome with a glassy 253 carapace; B) Stage II, intrusion of a cryptodome and subsequent destabilization of the volcanic 254 edifice, causing sector collapse (note that the ages of Stages I and II are well within errors and 255 may have occurred within a few tens of years); C) Stage III, quiescence with intense erosion 256 and consequent reworking products, and intrusion of dome at ~116 ka; D) Stage IV, explosive 257 eruptions at ~65 ka producing a sequence of surge and fallout layers. 258 Fig. 3. Stratigraphic relations of Stage IV Cerro Pizarro deposits with other pyroclastic deposits. 259 A) Reworked debris-avalanche deposit (A) overlain by ~100 ka Zaragoza Ignimbrite (B) and a 260 pumice-fall deposit (C) from an unknown source. This succession is overlain by ~65-ka fallout 261 and surge deposits (D) from C. Pizarro, with a soil horizon at the top (E). Photo taken ~2 km 262 west of C. Pizarro. B) Layers “a” and “c” (see Fig. 4) separated by a thin, finely laminated 263 surge sequence. Units overlie Zaragoza Ignimbrite. Photo taken ~3 km northeast of C. Pizarro. 264 Fig. 4. Isopach maps of Stage IV fallout deposits (see Fig. 3). Layer "a"- dashes, layer "c" - dots 265 (see Fig. 3); thickness in cm. Different orientation of dispersal axes indicates changes in wind 266 direction during eruptions. 267 Fig. 5. Plots showing geochemical variations for the early (~ 200 - 100 ka) and late (~ 65 ka) 268 stages of Cerro Pizarro rocks in comparison with other rhyolitic domes. A) Rb/Sr versus Rb. 269 B) Nb versus Rb. Taylor Creek data from Duffield and Ruiz (1995), Inyo volcanic field from 270 Sampson and Cameron (1987). 271 272 TABLES 273 1. 40Ar/39Ar isotopic ages of rocks and pyroclastic deposits of Cerro Pizarro and underlying 274 basalt 275 2. Comparison of volcanic domes.

9 Qz 97o 27' 97o25' 2500 cp Qp 3 Qv 011a cp chb 02 o 3 Qv 19 31' chb * * 2800 cs2 Ks 2 cs Ks C026 3 cmb Pizarro cmb cp N 1 Ks * cp33 * 019* cs1 Qv 19o30' cp3

o o 20 97 2400 Mexico 0 1 2 2 km km City Citlaltépetl Qv Contour interval 100 m, 20 m contours in NW corner Younger deposits Citlaltépetl 100 km CERRO Qp lacustrine deposits, basalt lava, and alluvium PIZARRO Stage IV pyroclastic-flow, -surge, -fall Zaragoza ignimbrite and other cp 3 deposits (65±10 ka) Qz pyroclastic deposits Stage III heterolithogic ; sand matrix stony rhyolite (116±12 ka) chb cs3 Stage II debris-avalanche deposit monolithogic breccia; cryptodomal cmb cs2 (inferred boundary) ash matrix stony rhyolite (180±50 ka) Stage I

cs11 stony rhyolite and vitrophyre (220±60 ka); pyroclastic-flow, -surge, and -fall deposits Older rocks Contact, location Basaltic lava and breccia (190±20 ka) approximate in some cases Qv and rhyolitic ignimbrite 40 39 Ar/ Ar sample site; Cretaceous sedimentary rocks * see Table 1 Ks

Carrasco-Nuñez and Riggs, Fig. 1 A. Stage I: ~ 200 ka vitrophyre

pyroclastic deposits scoria cone lava flow pre-Cerro Pizarro volcanic and sedimentary substrate

B. Stage II: ~ 200 ka debris-avalanche deposits partial collapse cinder vitrophyre of cryptodome cone pyroclastic and carapace scoria cone deposits lava flow strata pre-Cerro Pizarro volcanic and sedimentary substrate

C. Stage III: ~200 - 115 ka eroded debris-avalanche erosion deposits cinder vitrophyre cone dome edifice rebuilt scoria cone lava flow pre-Cerro Pizarro volcanic and sedimentary substrate

D. Stage IV: ~115 - 65 ka Zaragoza Ignimbrite young pyroclastic deposits

vitrophyre scoria cone lava flow pre-Cerro Pizarro volcanic and sedimentary substrate

Carrasco-Núñez and Riggs, Fig. 2 E Layer c D Layer a C

1 m B

A A B

Fig. 3. Carrasco-Núñez and Riggs (2005) *.cdr 97º27´ 97º26´ 97º25´ Layer a Layer c

10 Guadalupe Sarabia 20 19º 31´ 30

15 19º 30´ 10 N

Juan Sarabia 5 (Pizarro) 0 1 2 3 km

Fig. 4. Carrasco-Núñez and Riggs (2005) *.cdr 100

) Cerro Pizarro

m (late stages) p p

( 10 Taylor

r Creek S /

b Inyo R 1 Cerro Pizarro (early stages) 0.1 0 100 200 300 400 500 Rb (ppm) 60

50 Taylor Creek

) 40 m

p Inyo volcanic p

( 30 field Cerro Pizarro b

N 20 (late stages)

10 Cerro Pizarro (early stages) 0 0 100 200 300 400 500 Rb (ppm)

Fig. 5. Carrasco-Núñez and Riggs (2005) *.cdr TABLE 1. 40Ar/39Ar AGES OF ROCKS AND DEPOSITS OF CERRO PIZARRO† Sample Stage Dated Rock type Age Geographic coordinates K/Ca # steps/ Age error material analysis Latitude Longitude (ka) 2 σ

01 IV ¥ pumice wt mean 19º 30.45´ 97º 27.3´ 200.1 14 65 10 26 III sanidine¥ rhyolite 19o30.31’ 97º 25.3’ 12.2 9 116 12 lava 019 II § rhyolite plateau 19º 30.4´ 97º 26.2´ 101.2 5 180 50 lava 02 I sanidine¥ rhyolite wt mean 19º 30.95´ 97º 26.6´ 49.2 11 220 60 vitrophyre 011a gm basaltic plateau 19º 31.05´ 97º 25.0´ 0.9 7 190 20 conc.§ lava †40Ar/39Ar dating performed at New Mexico Geochronological Laboratory using a MAP 215-50 mass spectrometer. ¥Ages for sanidine crystals were determined by total laser fusion, age analysis was obtained by weighted mean (wt mean). §Ages for biotite or groundmass concentrate (gm conc.) by furnace step-heating.

14 TABLE 2: COMPARISON OF VOLCANIC DOMES† Name Composition Eruptive activity Volume Life span Ref.¥ Merapi, Andesite Lava flow, block-and- ~20 km3 (total ~2000 yrs to 1 ash flow volume) present Santiaguito, Dacite , lava 1.1 – 1.3 km3 1920 to 2 flow present Novarupta, caldera Rhyolite Magma effusion 0.13 km3 <4 years 3 related, USA Taylor Creek, Rhyolite Near-vent surge and fall; <1 km3~10km3; 20 domes 4 dome field, USA magma effusion total 100 km3 within ~2 ky Cerro Pizarro, Rhyolite Pyroclastic flow, fall, ~1.1 km3 ~100 ka 5 isolated dome, surge, debris avalanche; Mexico magma effusion Parícutin, Mexico Basalt Scoria cone, lava flow ~1.1 km3 9 years 6 † Merapi, and Santiaguito not considered monogenetic, based on their compositional variations and life spans. Parícutin provided as example of monogenetic basaltic volcano for comparison. ¥References: 1: Newhall et al. (2000); Newhall, pers. comm. (2005); 2: Harris et al. (2003); 3: Hildreth and Fierstein (2000); Hildreth, pers. comm. (2005); 4: Duffield et al. (1995); 5: Riggs and Carrasco-Nuñez (2004); this study; 6: Luhr and Simkin (1993)

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