Polygenetic nature of a rhyolitic dome and implications for hazard assessment: 1

Cerro Pizarro volcano, Mexico 2

by G. Carrasco-Núñez and N. Riggs 3

4

ABSTRACT 5

Rhyolitic domes are commonly regarded as monogenetic volcanoes associated with single, brief 6

eruptions. They are characterized by short-lived successions of pyroclastic and effusive activity 7

associated with a series of discrete eruptive events that apparently last on the order of years to 8

decades. Cerro Pizarro, a ~ 1.1 km3 rhyolitic dome in the eastern Mexican Volcanic Belt, shows 9

aspects of polygenetic volcanism including long-term repose periods (~ 50-80 ky) between 10

eruptions, chemical variations with time, and a complex evolution of alternating explosive and 11

effusive eruptions, a cryptodome phase, and sector collapse. This eruptive behavior provides 12

new insights into how rhyolite domes may evolve. A protracted, complex evolution bears 13

important implications for hazard assessment if reactivation of an apparently extinct rhyolitic 14

dome must be seriously considered. 15

16

Keywords: monogenetic volcanism, polygenetic volcanism, rhyolites, dome growth, volcanic 17

hazards, Mexican Volcanic Belt 18

19

INTRODUCTION 20

Monogenetic volcanoes comprise a wide spectrum of relatively small volcanic structures 21

(generally less than a few km3 erupted material) that show a commonly simple evolution (one 22

eruption, or a few clearly related eruptions), short life span (commonly years to decades for 23

mafic volcanoes, but possibly as much as a few centuries for rhyolitic domes), and minor 24

chemical composition changes. Monogenetic volcanoes are, in general, either basalt or rhyolite, 25

while polygenetic volcanoes, which erupt repeatedly and have a large and persistent magma 26

storage chamber, are commonly andesitic or dacitic in composition. Although the most common 27

monogenetic volcanoes are cinder cones, tuff cones and rings, and maar volcanoes of basaltic 28

composition, most rhyolite domes also fit the criteria of monogenetic volcanoes. Mafic 29

monogenetic volcanoes are common in small and large volcanic fields (e.g., Springerville 30

Volcanic Field, Condit and Connor, 1996; Michoacán-Guanajuato volcanic field, Mexico, 31

2

Hasenaka and Carmichael, 1985). Rhyolitic domes occur either as isolated individual volcanoes 32

(e.g. Las Derrumbadas, Mexico; Siebe et al., 1995; Cerro Pizarro, Riggs and Carrasco-Nuñez, 33

2004), as part of dome fields (e.g. Taylor Creek, New Mexico; Duffield et al., 1995) or on the 34

flanks of or within larger volcanoes (e.g., Mono-Inyo field-Long Valley caldera; Miller, 1985; 35

South Sister volcano, Oregon; Scott, 1987; Novarupta dome, Valley of Ten Thousand Smokes; 36

Hildreth and Fierstein, 2000). A “typical” rhyolitic dome, as modeled by Duffield et al. (1995) 37

for the Taylor Creek field, evolves from vent-clearing pyroclastic eruptions to dome extrusion 38

over a short time span perhaps only a few years in duration. No rhyolite domes have been 39

observed forming, and estimates of lifespans range from a few to several years (Mono-Inyo field, 40

Miller, 1985; Novarupta, Hildreth and Fierstein, 2000) to a maximum of a few centuries (e.g. 41

Taylor Creek Rhyolite, Duffield et al., 1995). Individual rhyolite domes or related groups of 42

domes do not have significant major-element compositional variations over their growth stages 43

(Scott, 1987; Duffield et al., 1995). Because of this relatively simple common evolutionary path, 44

rhyolite domes are not generally considered very hazardous, similar to monogenetic basaltic 45

systems. 46

Cerro Pizarro rhyolitic dome, located in the eastern Trans-Mexican Volcanic Belt (Fig. 1), 47

evolved through periods of effusive and highly explosive activity that were separated by 48

cryptodome intrusion, edifice sector-collapse, and prolonged erosional episodes (Riggs and 49

Carrasco-Nuñez, 2004). Chemistry of the eruptive products also changed over time. This 50

evolution, in addition to the ~ 50-80 ka repose period between the main eruptive episodes, 51

indicates that a model of short-lived, monogenetic activity does not characterize all rhyolite 52

domes. The purpose of this paper is to describe the polygenetic nature of Cerro Pizarro dome in 53

terms of timing, chemical variation, and eruptive behavior. These new insights have important 54

implications for hazard assessment of some young silicic domes that otherwise may be 55

considered extinct volcanoes. 56

57

REGIONAL SETTING 58

Cerro Pizarro is located within the Serdán-Oriental basin in the easternmost Mexican 59

Volcanic Belt (Fig. 1). The Serdán–Oriental is a broad, intermontane, relatively flat 60

lacustrine/playa closed basin characterized by Pleistocene isolated small basaltic scoria cones, 61

and tuff rings and maar volcanoes of basaltic and rhyolitic composition, and somewhat larger 62

3

rhyolitic domes such as Cerro Pizarro (Riggs and Carrasco-Núñez, 2004), Cerro Pinto, and Las 63

Derrumbadas (Siebe et al., 1995). The regional basement comprises locally exposed Cretaceous 64

limestone and small Miocene intrusive rocks. 65

Volcanic deposits within the Serdán-Oriental basin are dominated by pyroclastic material 66

derived from Los Humeros caldera, located about 16 km north of Cerro Pizarro. Activity at Los 67

Humeros began with the emplacement of the ~115-km3 Xáltipan ignimbrite at ~460 ka (Ferriz 68

and Mahood, 1985). A highly explosive event occurred at about 100 ka, producing the 15-km3 69

Zaragoza ignimbrite (Ferriz and Mahood, 1984; Carrasco-Núñez and Branney, 2005). 70

71

EVOLUTION OF CERRO PIZARRO 72

The evolution of Cerro Pizarro took place in four main stages (after Riggs and Carrasco-73

Núñez, 20004; Fig. 2). In the first stage, vent-clearing explosions incorporated xenoliths of 74

basement rocks including vesicular basalts from a nearby scoria cone, Cretaceous limestone, and 75

Xáltipan ignimbrite (Ferriz and Mahood, 1985). Subsequent eruptions produced surge and fallout 76

layers followed by passive, effusive dome growth. Oversteepened flanks of the dome collapsed 77

at times to produce block-and ash-flow deposits and, slightly later, an external vitrophiric 78

carapace developed (Fig. 2A). This early stage corresponds well to the model proposed by 79

Duffield et al. (1995). 80

During the second stage, a new pulse of magma caused the emplacement of a cryptodome, 81

which inflated the volcano and strongly deformed the vitrophyric carapace as well as the older 82

parts of the dome, producing subvertical orientations of the overlying pre-dome units. 83

Disintegration of this cryptodome caused a debris avalanche as the western flank of the volcano 84

collapsed (Fig. 2B) (cf. Mount St. Helens, 1980, Voight et al., 1981; Soufrière Hills, Voight et 85

al., 2002). 86

The third stage (Fig. 2C) was characterized by a prolonged period of erosion of the dome 87

and passive magma intrusion. Erosion cut canyons as much as 30 m deep and produced 88

heterolithic debris- and hyperconcentrated-flow deposits by reworking the debris-avalanche 89

deposit deposited during Stage II. At approximately 116 ka (see below) magma intrusion caused 90

the collapse crater to fill and the present-day conical shape of the volcano was formed. No 91

evidence exists for pyroclastic or collapse-related deposits associated with this dome growth. 92

4

The fourth and final stage (Fig. 2D) includes both hiatus activity following dome growth at 93

116 ka and the final eruptions of Cerro Pizarro. Several pyroclastic successions were emplaced, 94

including the ~100-ka Zaragoza ignimbrite (Carrasco-Núñez and Branney, 2005), which overlies 95

the volcaniclastic deposits derived from the sector collapse of Cerro Pizarro (unit B in Fig. 3A). 96

Stage IV surge and fall deposits are widely dispersed on the apron around the volcano (Fig. 4). 97

The sequence includes two distinctive marker beds (Fig. 3B): the lower one (“a”) is pumice rich 98

whereas the upper one (“c”), which has abundant lithic clasts. 99

100

CHEMISTRY OF THE CERRO PIZARRO PRODUCTS 101

XRF analysis of 14 samples shows that Cerro Pizarro eruptive products are high-silica 102

rhyolite, in contrast to products of the surrounding volcanoes, which are dacitic or andesitic in 103

composition. Rocks from Stages I, II, and III are very similar in major and trace element 104

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

MnO and Na2O. More consistent variations are observed for trace elements: Rb, Y, Nb, Ni, Zn 106

are higher for the Stage IV pyroclastic rocks, and Sr, Ba, and Zr are lower than the rest of the 107

Cerro Pizarro rocks. Detailed discussion of the geochemistry will be presented elsewhere. 108

Even though marked inter-dome compositional variations are observed in rhyolitic fields, 109

individual domes generally show a more homogeneous composition (e.g. Taylor Creek rhyolite, 110

Duffield et al., 1995; Inyo volcanic chain, Sampson, 1987; rhyodacite of South Sister volcano, 111

Scott, 1987) (Fig. 5). The relatively large trace-elements changes between Stages I, II, and III 112

and Stage IV deposits of Cerro Pizarro (Fig. 5) may be due to differentiation processes such as 113

crystal fractionation occurring over the long hiatus in eruptive activity. 114

115

LIFESPAN OF CERRO PIZARRO DOME 116

We have dated four samples of Cerro Pizarro rhyolite and one of underlying basalt by the 117 40Ar/39Ar method (Table 1). Stratigraphic relations preserved on the dome (Riggs and Carrasco-118

Núñez, 2004) indicate that the basalt scoria cone was unconsolidated at the time of cryptodome 119

emplacement. This observation, combined with the ages of the basalt (190 ± 20 ka) and lavas of 120

the first two stages (220 ± 60 ka and 180 ± 50 ka) strongly suggests that the three eruptive events 121

occurred in short succession without any significant interruption; we consider that all three 122

occurred at ~200 ka. The near-contemporaneity of the two rhyolitic eruptive events is also 123

5

supported by the homogeneous composition that all the associated eruptive products exhibit in 124

both major and trace elements. 125

Rebuilding of the dome (Stage III) occurred at 116 ± 12 ka. By analogy with the current 126

growth of the dome in Mount St. Helens, the cone may have been re-established quickly, once 127

magma began to be emplaced. The cone material falls well within the geochemical range of the 128

older dome material. Following a substantial hiatus that lasted, within errors, between 29 and 73 129

ky, a final explosive event occurred at 65 ± 10 ka (Stage IV) to produce the final pyroclastic 130

sequence. 131

132

CERRO PIZARRO DOME: MONOGENETIC OR POLYGENETIC MAGMATIC 133

SYSTEM 134

Felsic domes comprise compositions from high-silica andesite and dacite to high-silica 135

rhyolite. Composition may play an important role in controlling dome growth styles: while 136

rhyolitic domes tend to be simpler and form through endogenous growth, dacitic and andesitic 137

bodies more commonly have mixed endogenous and exogenous activity (Duffield et al., 1995). 138

Andesitic-dacitic domes are often associated with larger volcanic systems, either large central 139

composite volcanoes (e.g. Santiaguito: Harris et al., 2003; Showa-Shinzan: Mimatsu, 1995; 140

Mount St. Helens:, Swanson et al., 1987) or Pelean-type volcanoes (e.g. Merapi: Newhall et al., 141

2000; Mount Pelée: Lacroix, 1904; Soufrière Hills: Sparks and Young, 2002), and therefore are 142

associated with multiple effusive events that last for longer periods of time. These volcanoes and 143

volcanic systems are polygenetic, involving larger amounts of magma, variations in eruptive 144

activity, and longer life spans (Table 2). 145

The volume of Cerro Pizarro compares closely with other monogenetic volcanoes (~ 1.1 146

km3). The complexity of its eruptive activity, however, more closely resembles that of a 147

stratovolcano than an individual rhyolitic dome. For example, although cryptodomes are 148

common in the literature, they are generally confined to large magmatic systems. Likewise, 149

collapse of a major sector of the volcanic edifice is generally associated with stratovolcanoes like 150

Mount St. Helens (Voight et al., 1981) or Pelean-type volcanoes such as Soufrière Hills volcano 151

(Voight et al., 2002). Although Cerro Pizarro is rhyolitic and therefore more likely to have 152

behaved as a monogenetic system, by virtue of chemistry, activity, and lifespan it should be 153

classified as polygenetic. To the extent, however, that complex domes like Soufrière Hills or 154

6

Unzen Volcano are erupting as multiple-vent systems, with distinguishable, if undramatic 155

changes in chemistry, clearly the designation of polygenetic is likely for many silicic domes. 156

Cerro Pizarro can therefore be considered hybrid in terms of its magmatic activity, both in 157

chemistry, in eruptive style, and in lifespan, between monogenetic rhyolitic domes, which erupt 158

quickly in a predictable way, and polygenetic andesitic-dacitic domes, which often follow a far-159

more complex evolutionary path. 160

161

IMPLICATIONS FOR HAZARD ASSESSMENTS 162

Reactivation of a rhyolitic dome after a long period of repose has not been previously 163

reported. Reactivation of a seemingly extinct volcano carries very important implications for 164

assessment of volcanic hazards, particularly considering that renewed activity might be explosive 165

or involve sector collapse of the volcanic edifice. Regardless of whether a dome like Cerro 166

Pizarro should be considered polygenetic or monogenetic, the combined stratigraphic, 167

geochemical, and geochronologic evidence from the volcano shows that a rhyolite dome has the 168

potential for renewed activity after a long hiatus. Future eruptions in the Mexican Volcanic Belt, 169

or any district where rhyolitic domes seem to erupt in isolation from other, larger systems, will 170

serve as an excellent test to assess the apparent severe hazards associated with these small 171

volcanoes. 172

173

ACKNOWLEDGMENTS 174

Funding was provided by PAPIIT grant No. IN 104401 and logistical support by Centro de 175

Geociencias, UNAM. R. Lozano performed the chemical analyses at LUGIS (UNAM). S. 176

Medina provided useful references. W. McIntosh and L. Peters performed the isotopic dating at 177

New Mexico Geochronological Laboratory and helped us resolve complex age data. Fruitful 178

discussions with W.A. Duffield, W.E. Scott, and C, Newhall are gratefully acknowledged. W.A 179

Duffield and B. Zimmer read an earlier version of the manuscript and provided comments that 180

strongly helped us improve the text. 181

182

REFERENCES CITED 183

7

Carrasco-Núñez G., and Branney, M. J., 2005, Progressive assembly of a massive layer of 184

ignimbrite with a normal-to-inverse compositional zoning: the Zaragoza ignimbrite of central 185

Mexico: Bulletin of Volcanology, v. 68, p. 3-20. 186

Condit, C. D., and Connor, C. B., 1996, Recurrence rates of volcanism in basaltic volcanic fields; 187

an example from the Springerville volcanic field, Arizona: Geological Society of America 188

Bulletin, v. 108, p.1225-1241. 189

Duffield, W. A., Richter, D. H., and Priest, S. S., 1995, Physical volcanology of silicic lava 190

domes as exemplified by the Taylor Creek rhyolite, Catron and Sierra counties, New Mexico: 191

US Geological Survey Map I-2399, 1:50,000. 192

Duffield, W.A., and Ruiz, J., 1992, Compositional gradients in large reservoirs of silicic magma 193

as evidenced by ignimbrites versus Taylor Creek rhyolite lava domes: Contributions to 194

Mineralogy and Petrology, v. 110, p. 192-210. 195

Ferriz, H., and Mahood, G., 1985, Eruption rates and compositional trends at Los Humeros 196

Volcanic Center, Puebla, Mexico: Journal of Geophysical Research, v. 89, p.8511-8524. 197

Harris, A., Rose, W. I., and Flynn, L., 2003, Temporal trends in lava dome extrusion at 198

Santiaguito 1922-2000: Bulletin of Volcanology, v. 65, p. 77-89. 199

Hasenaka, T., and Carmichael, I. S. E., 1985, The cinder cones of Michoacán-Guanajuato, 200

central Mexico: their age, volume and distribution, and magma discharge rate: Journal of 201

Volcanology and Geothermal Research, v. 25, p. 105-124. 202

Hildreth, W., and Fierstein, J, 2000, Katmai volcanic cluster and the great eruption of 1912: 203

Geol. Soc. Am. Bulletin, v. 112, p. 1594-1620. 204

Luhr, J. T., and Simkin, T., 1993, Parícutin: the volcano born in a Mexican cornfield: Phoenix 205

Geoscience Press, pp. 427. 206

Miller, C. D., 1985, Holocene eruptions at the Inyo volcanic chain, California: implications for 207

possible eruptions in the Long Valley caldera: Geology, v. 13, p. 14-17. 208

Mimatsu, M., 1995, Showa-Shinzan diary (English translation). Executive committee 50th 209

anniversary of Showa-Shinzan, Hokkaido, Japan, pp.179. 210

Newhall, C. G., Bronto, S., Alloway, B., Banks N. G., Bahar, I., del Marmol, M. A., 211

Hadisantono, R. D., Holcomb, R.T., McGeehin, J. et al., 2000, 10,000 Years of explosive 212

eruptions of Merapi Volcano, Central Java: archaeological and modern implications: Journal 213

of Volcanology and Geothermal Research, v. 100, p. 9–50. 214

8

Riggs, N. F., and Carrasco-Núñez, G., 2004, Evolution of a complex, isolated dome system, 215

Cerro Pizarro, central México: Bulletin of Volcanology, v. 66, p. 322-335. 216

Sampson, D. E., 1987, Textural heterogeneities and vent area structures in the 600-year-old lavas 217

of the Inyo volcanic chain, eastern California, in Fink, J.H., ed., The Emplacement of silicic 218

domes and lava flows: Geological Society of America Special Paper 212, p. 89-102. 219

Sampson, D. E., and Cameron, K. L., 1987, The geochemistry of the Inyo volcanic chain: 220

multiple magma systems in the Long Valley region, eastern California. Journal of 221

Geophysical Research, v. 92, p. 10403-10421. 222

Scott, W. E., 1987, Holocene rhyodacite eruptions on the flanks of South Sister volcano, Oregon, 223

in Fink, J.H., ed., The Emplacement of silicic domes and lava flows: Geological Society of 224

America Special Paper 212, p. 35-45. 225

Siebe, C., Macías, J. L., Abrams, M., Rodríguez, S., Castro, R., and Delgado, H., 1995, 226

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for future hazards: Geological Society of America Field Guide, p. 1-47. 228

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of Soufrière Hills volcano, Monserrat (1995-199): Geological Society, London, Memoirs, v. 231

21, p. 45-69. 232

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J., Ewert, J. W., and Heliker, C. C., 1987, Growth of the lava dome at Mount St. Helens, 234

Washington (USA), 1981-1983, in Fink, J.H., ed., The Emplacement of silicic domes and lava 235

flows: Geological Society of America Special Paper 212, p. 1-16. 236

Voight, B., Glicken, H., Janda, R. J., and Douglass, P. M., 1981, The rockslide avalanche of May 237

18, in Lipman, P. W., Mullineax, D. R., eds., The 1980 eruptions Mount St. Helens, 238

Washington: US Geological Survey Special Paper 1250, p. 347-377. 239

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Francis, P. W., Franz, W., Heinrich, P., Sparks, R. S. J., and Young, S. R., 2002, The 26 241

December (Boxing Day) 1997 sector collapse and debris avalanche at Soufrière Hills 242

Volcano, Monserrat, in, Druit, T. H., Kokelaar, B. P., eds., The eruption of Soufrière Hills 243

Volcano, Monserrat, from 1995-1999: Geological Society of London Memoir, v. 21, p. 363-244

407. 245

9

246

FIGURE CAPTIONS 247

Fig. 1. Geologic map of Cerro Pizarro (modified from Riggs and Carrasco-Núñez, 2004) Units 248

are grouped into four main stages of evolution of the volcano. Inset map shows location of C. 249

Pizarro within the Mexican Volcanic Belt. 250

Fig. 2. Summary of Cerro Pizarro evolution (modified from Riggs and Carrasco-Núñez, 2004). 251

A) Stage I, initial open-vent explosions and the growth of a rhyolitic dome with a glassy 252

carapace; B) Stage II, intrusion of a cryptodome and subsequent destabilization of the volcanic 253

edifice, causing sector collapse (note that the ages of Stages I and II are well within errors and 254

may have occurred within a few tens of years); C) Stage III, quiescence with intense erosion 255

and consequent reworking products, and intrusion of dome at ~116 ka; D) Stage IV, explosive 256

eruptions at ~65 ka producing a sequence of surge and fallout layers. 257

Fig. 3. Stratigraphic relations of Stage IV Cerro Pizarro deposits with other pyroclastic deposits. 258

A) Reworked debris-avalanche deposit (A) overlain by ~100 ka Zaragoza Ignimbrite (B) and a 259

pumice-fall deposit (C) from an unknown source. This succession is overlain by ~65-ka fallout 260

and surge deposits (D) from C. Pizarro, with a soil horizon at the top (E). Photo taken ~2 km 261

west of C. Pizarro. B) Layers “a” and “c” (see Fig. 4) separated by a thin, finely laminated 262

surge sequence. Units overlie Zaragoza Ignimbrite. Photo taken ~3 km northeast of C. Pizarro. 263

Fig. 4. Isopach maps of Stage IV fallout deposits (see Fig. 3). Layer "a"- dashes, layer "c" - dots 264

(see Fig. 3); thickness in cm. Different orientation of dispersal axes indicates changes in wind 265

direction during eruptions. 266

Fig. 5. Plots showing geochemical variations for the early (~ 200 - 100 ka) and late (~ 65 ka) 267

stages of Cerro Pizarro rocks in comparison with other rhyolitic domes. A) Rb/Sr versus Rb. 268

B) Nb versus Rb. Taylor Creek data from Duffield and Ruiz (1995), Inyo volcanic field from 269

Sampson and Cameron (1987). 270

271

TABLES 272

1. 40Ar/39Ar isotopic ages of rocks and pyroclastic deposits of Cerro Pizarro and underlying 273

basalt 274

2. Comparison of volcanic domes. 275

N

Qp

Qz

cp3

cp3

Qv

Qv2500

2400

chbchb

cs2cs2 cs3

2800

Ks

Qv

0 1 2 km0 1 2 km

cmbcmb

Qv

cp3

cs1

cp3cp3

* *

* **CPizarroC

Pizarro

19o31' 19o31'

19o30'19o30'

97o 27' 97o25'

Contour interval 100 m,20 m contours in NW cornerContour interval 100 m,20 m contours in NW corner

Citlaltépetl

0191

019

026026

011a011a0202

Ks Ks

Carrasco-Nuñez and Riggs, Fig. 1

debris-avalanche deposit (inferred boundary)

lacustrine deposits, basalt lava, and alluviumQp

Cretaceous sedimentary rocks

Zaragoza ignimbrite and other pyroclastic depositsQzQzQz

stony rhyolite (116±12 ka)cs3

cryptodomal stony rhyolite (180±50 ka)

cs2

stony rhyolite and vitrophyre (220±60 ka); pyroclastic-flow, -surge, and -fall deposits

cs1cs1

pyroclastic-flow, -surge, -falldeposits (65±10 ka)

cp3 

monolithogic breccia; ash matrix

Basaltic lava and breccia (190±20 ka) and rhyolitic ignimbrite

Qv

Younger deposits

Older rocksContact, location

approximate in some cases

Ks

cmbcmb

heterolithogic breccia; sand matrixchbchb

Stage I

Stage II

Stage IV

Stage III

* 40Ar/39Ar sample site;see Table 1

20o

100 km

Mexico City

CERRO PIZARRO

Popocatépetl Citlaltépetl

97o 97o

pre-Cerro Pizarro volcanic and sedimentary substratepre-Cerro Pizarro volcanic and sedimentary substrate

A. Stage I: ~ 200 ka vitrophyre

pyroclasticdeposits

lava flowscoria cone

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

pre-Cerro Pizarro volcanic and sedimentary substratepre-Cerro Pizarro volcanic and sedimentary substrate

pre-Cerro Pizarro volcanic and sedimentary substratepre-Cerro Pizarro volcanic and sedimentary substrate

pre-Cerro Pizarro volcanic and sedimentary substratepre-Cerro Pizarro volcanic and sedimentary substrate

scoria cone lava flow

vitrophyre

cinder cone

cinder cone

B. Stage II: ~ 200 ka

lava flowpyroclasticdeposits

pyroclasticdeposits

debris-avalanchedeposits

scoria cone

vitrophyre

cinder cone

cinder cone

C. Stage III: ~200 - 115 ka

lava flow

eroded debris-avalanchedeposits

scoria cone

vitrophyre

erosion

partial collapse of cryptodome and carapace

strata

dome edificerebuilt

Carrasco-Núñez and Riggs, Fig. 2

A

B

C

DE

1 m

A B

Fig. 3. Carrasco-Núñez and Riggs (2005) *.cdr

Layer a

Layer c

00 11 22 3 km3 km

Guadalupe Sarabia

N

30

1510

5

20

10

97º27´ 97º26´ 97º25´

19º31´

19º30´

Layer a Layer c

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

Juan Sarabia (Pizarro)

0 100 200 300 400 500

Rb (ppm)

Rb

/Sr

(p

pm

)

0.1

1

10

100

Taylor Creek

Inyo

Cerro Pizarro(early stages)

Cerro Pizarro(late stages)

0

10

20

30

40

50

60

0 100 200 300 400 500

Rb (ppm)

Nb

(p

pm

)

Taylor Creek

Inyo volcanic

field

Cerro Pizarro(early stages)

Cerro Pizarro(late stages)

Fig. 5. Carrasco-Núñez and Riggs (2005) *.cdr

14

TABLE 1. 40Ar/39Ar AGES OF ROCKS AND DEPOSITS OF CERRO PIZARRO† Sample Stage Dated

material Rock type Age

analysis Geographic coordinates Latitude Longitude

K/Ca # steps/ crystals

Age (ka)

error 2 σ

01 IV sanidine¥ pumice wt mean 19º 30.45´ 97º 27.3´ 200.1 14 65 10

26 III sanidine¥ rhyolite lava

plateau 19o30.31’ 97º 25.3’ 12.2 9 116 12

019 II biotite§ rhyolite lava

plateau 19º 30.4´ 97º 26.2´ 101.2 5 180 50

02 I sanidine¥ rhyolite vitrophyre

wt mean 19º 30.95´ 97º 26.6´ 49.2 11 220 60

011a gm conc.§

basaltic lava

plateau 19º 31.05´ 97º 25.0´ 0.9 7 190 20

†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.

15

TABLE 2: COMPARISON OF VOLCANIC DOMES† Name Composition Eruptive activity Volume Life span Ref.¥

Merapi, Indonesia Andesite Lava flow, block-and-ash flow

~20 km3 (total volume)

~2000 yrs to present

1

Santiaguito, Guatemala

Dacite Pyroclastic flow, lava flow

1.1 – 1.3 km3 1920 to present

2

Novarupta, caldera related, USA

Rhyolite Magma effusion 0.13 km3 <4 years 3

Taylor Creek, dome field, USA

Rhyolite Near-vent surge and fall; magma effusion

<1 km3~10km3; total 100 km3

20 domes within ~2 ky

4

Cerro Pizarro, isolated dome,

Mexico

Rhyolite Pyroclastic flow, fall, surge, debris avalanche;

magma effusion

~1.1 km3 ~100 ka 5

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)