The Origins of Late Quaternary Debris Avalanche and Debris Flow Deposits

Home , Cofre de Perote, Las Derrumbadas, Mount Shasta, Nevado de Toluca, Oriental Basin

The origins of Late Quaternary debris avalanche and debris ﬂ ow deposits from Cofre de Perote volcano, México

Rodolfo Díaz-Castellón1,*,†, Bernard E. Hubbard2,†, Gerardo Carrasco-Núñez1, and José Luis Rodríguez-Vargas1 1Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Campus Juriquilla, 76230 Querétaro, México 2U.S. Geological Survey, Eastern Mineral Resources, MS 954, 12201 Sunrise Valley Drive, Reston, Virginia 20192, USA

ABSTRACT on a larger scale. The younger Xico avalanche Scott et al., 1995; Vallance and Scott, 1997) and deposit contains abundant smectite, jarosite, the 16.4–16.6 ka Teteltzingo lahar from Citlalté- Cofre de Perote volcano is a compound, kaolinite, gypsum, and mixed-layered illite/ petl (Carrasco-Núñez et al., 1993, 2006), have shield-like volcano located in the northeast- smectite, which are either defi nitely or most left extensive deposits many tens of kilometers ern Trans-Mexican volcanic belt. Large likely of hydrothermal alteration origin. from their sources, signaling that population debris avalanche and lahar deposits are asso- Smectite in particular appears to be the most centers situated far from a volcanic edifi ce may ciated with the evolution of Cofre. The two abundant and spectrally dominant mineral still be at risk. Volcanic debris avalanches and best preserved of these debris-avalanche and in summit ground truth samples, ASTER debris fl ows are not always accompanied by debris-fl ow deposits are the ~42 ka “Los Pes- mapping results, Xico avalanche deposit, and eruptive activity. They can be triggered by a cados debris fl ow” deposit and the ~11–13 ka an older (pre-Xico avalanche) deposit derived variety of factors such as increased precipita- “Xico avalanche” deposit, both of which dis- from collapse(s) of ancestral Cofre de Perote tion, as exemplifi ed in 1998 by the Casita ava- play contrasting morphological and textural edifi ce. However, both Xico avalanche and lanche and lahar triggered by Hurricane Mitch characteristics, source materials, origins and Los Pescados debris flow deposits show (Sheridan et al., 1999; Scott et al., 2005); slope emplacement environments. Laboratory some evidence of secondary, postemplace- instability and over steepening caused by gla- X-ray diffraction and visible-infrared refl ec- ment weathering and induration, which cial erosion (Crowley et al., 2003); strong seis- tance spectroscopy were used to identify the is evident by the presence of gibbsite, and mic activity (Martinez et al., 1995; Scott et al., most abundant clay, sulfate, ferric-iron, and hydroxyl interlayered minerals, in addition 2001); or they can occur even without warning. silica minerals in the deposits, which were to recently formed halloysite and hydrous A number of studies have made interpre- either related to hydrothermal alteration or silica (i.e., indurating) cements. Field-based, tations about the origins of ancient volcanic chemical weathering processes. Cloud-free visible infrared image spectroscopy (VIS/IR) debris avalanches and fl ows based on the type Advanced Spaceborne Thermal Emission spectral measurements offer the possibility and distribution of clay, sulfate, and silica min- and Refl ection Radiometer (ASTER) remote of distinguishing primary minerals of hydro- erals indicative of either hydrothermal alteration sensing imagery, supporting EO-1 Hyperion thermal alteration origin in debris-avalanche and/or chemical weathering processes (e.g., image spectra, and fi eld ground truth samples and debris-fl ow deposits from those pro- Crandell, 1971; Carrasco-Núñez et al., 1993; were used to map the mineralogy and distri- duced either by in situ chemical weathering Vallance and Scott, 1997; Vallance, 1999; Capra bution of hydrothermally altered rocks on or bulked from weathered source materials. and Macias, 2000; Capra and Macias, 2002; Pul- the modern summit of Cofre de Perote. The garin et al., 2004; Carrasco-Núñez et al., 2006; results were then compared to minerals iden- INTRODUCTION Murcia et al., 2008). However, few studies have tifi ed in the two debris-avalanche and debris- mapped the distribution of clay-rich, hydrother- fl ow deposits in order to assess possible source Volcanic debris avalanches and debris fl ows mally altered rocks on volcanoes with down- materials and origins for the two deposits. (i.e., lahars) originate as slope failures high on stream populations at risk (e.g., Crowley and The older Los Pescados debris-fl ow deposit volcanic edifi ces and transport enormous vol- Zimbelman, 1997; Hubbard, 2001; Finn et al., contains mostly halloysite and hydrous silica umes of fl uidized rock and soil into surrounding 2001; Crowley et al., 2003; Finn et al., 2007) or minerals, which match the dominant miner- river valleys. In populated areas, these phenom- compared the mineralogy of actual debris-ava- alogy of soils and weathered volcanic deposit ena can be incredibly destructive and deadly, as lanche and debris-fl ow deposits with those of in the surrounding fl anks of Cofre de Perote. tragically exemplifi ed by the >22,000 people potential source rock areas on the volcano (e.g., Its source materials were most likely derived killed by lahars from the Nevado del Ruiz vol- Pevear et al., 1982; Frank, 1983; Hubbard, 2001; from initially noncohesive or clay-poor fl ows, cano, Colombia, in 1985, during a minor erup- Opfergelt et al., 2006), assuming that such rocks which subsequently bulked with clay-rich tion that melted glacial ice and bulked with indeed still remain on the edifi ce. For example, valley soils and alluvium in a manner similar clay-rich soils and alluvium from the surround- Pevear et al. (1982) notes that the 18 May 1980 to lahars from Nevado del Ruiz in 1985, but ing valleys (Lowe et al., 1986; Pierson et al., debris avalanche deposit from Mount St. Helens 1990). Lahars containing abundant hydrother- lacks acid-sulfate minerals such as kaolinite and *Corresponding author: [email protected]. mally produced clays, such as the 5.6 ka Osceola alunite, but contains abundant chlorite, mixed †These authors contributed equally to this work. mudfl ow from Mount Rainier (Crandell, 1971; layered chlorite/smectite (i.e., corrensite), and

Geosphere; August 2012; v. 8; no. 4; p. 950–971; doi:10.1130/GES00709.1; 16 ﬁ gures; 3 tables. Received 6 April 2011 ♦ Revision received 23 November 2011 ♦ Accepted 1 February 2012 ♦ Published online 16 July 2012

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saponite (an Mg/Fe2+ or trioctahedral smectite), and to assess the signifi cance of hydrothermal four months) dry season from January to April, indicative of a sealed hydrothermal system that alteration and/or soil-forming processes in gen- and receives >1400 mm of annual precipitation prevented acidic fl uids from reaching the sur- erating future debris avalanches and lahars from (Elsass et al., 2000). Soils on the eastern fl anks face or near surface oxidizing environments. Cofre de Perote volcano. are also andosols but are typically indurated with Minor amounts of acid-sulfate alteration min- hydrous silica minerals (i.e., silcrete) (Elsass erals such as kaolinite, alunite, cristobalite, BACKGROUND: GEOLOGIC AND et al., 2000). Land use on the east side of Cofre tridymite, and opal were known to exist locally GEOGRAPHIC SETTING de Perote volcano favors a dense vegetation can- around the vicinity of fumarolic and geothermal opy ranging from cloud forest to low jungle and areas on the pre-1980 Mount St. Helens summit Cofre de Perote is the northernmost and savannas with numerous scattered sugar cane dome and goat rocks dome (Pevear et al., 1982 second highest (~4220 masl) volcano of the and coffee plantations. On both sides of Cofre and references therein). These dome rocks were Citlaltépetl–Cofre de Perote volcanic range. de Perote volcano, the weathering zone extends subsequently removed during the 18 May 1980 It is situated at the eastern end of the Trans- to a maximum depth of ~4 m (Dubroeucq et al., rockslide, though (not surprisingly) their altera- Mexican volcanic belt (TMVB; Fig. 1). The 1998; Elsass et al., 2000), although this varies tion products are not evident in the resulting Trans-Mexican volcanic belt is a Neogene vol- with slope and often grades into saprolite on the debris-avalanche deposit as shown by Pevear canic arc characterized by its oblique geometry western side (Elsass et al., 2000). et al. (1982). In contrast, Hubbard (2001) used with respect to the Middle American subduc- Morphologically, Cofre de Perote volcano airborne visible–infrared imaging spectrometer tion zone trench. It contains a wide range of could be described as a compound, shield-like (AVIRIS) hyperspectral data to map a variety of volcanic structures, including large silicic cal- volcano with a broad and gently sloping profi le hydrothermal alteration minerals on the modern deras, andesitic stratovolcanoes, silicic domes, (Fig. 2) (Carrasco-Núñez et al., 2010). Consid- edifi ce of Citlaltépetl, as well as the remnants and large basaltic monogenetic fi elds (Demant, ering that Cofre de Perote volcano is probably of two ancestral edifi ces, and compared them 1978). The easternmost Trans-Mexican volcanic extinct because its last eruptive episode ended with the mineralogy of the debris-avalanche belt comprises the Citlaltépetl–Cofre de Perote ~200 ka (Carrasco-Núñez et al., 2006), detailed and debris-fl ow deposits that resulted from their volcanic range and the Serdán-Oriental Basin, geologic mapping and study of its potential haz- collapse . marked by a bimodal volcanism with numer- ards have received little attention until recent This study focuses on Cofre de Perote vol- ous maars, domes, and cinder cones compris- times. Nevertheless, the young-looking scarps cano, one of the main volcanoes of the Citlal- ing a scattered monogenetic fi eld. Poly genetic along the eastern portions of the summit (Fig. 3) tépetl–Cofre de Perote volcanic range, which volcanism is mostly limited to the Citlaltépetl– suggest collapse episodes at a time later than has produced two large (i.e., >108 m3) debris- Cofre de Perote volcanic range, where the only the cessation of its eruptive activity. This geo- avalanche and debris-fl ow deposits dated at remaining currently active volcano in the range morphological evidence, along with the two ~42 ka and ~11–13 ka, informally named and is Citlaltépetl (C, Fig. 1). Another important documented Los Pescados debris-fl ow and Xico referred to as Los Pescados debris fl ow and Xico structure to the evolution of the Cofre de Perote avalanche and debris fl ow deposits on the east- avalanche, respectively (Carrasco-Núñez et al., volcano and NW of it is the Los Humeros caldera ern lower slopes (Carrasco-Núñez et al., 2006, 2006). Both deposits contain abundant clay and (8, Fig. 1), which is actually composed of three 2010), are further evidence of postconstruc- related nonclay minerals and exhibit contrast- nested calderas, two of which (Los Humeros tional collapse and edifi ce instability. Cirques ing morphological and textural characteristics, and Los Potreros) are nested inside an older and U-shaped valleys dominate the western which provide clues to their different origins ancestral caldera. fl anks of Cofre de Perote volcano (Fig. 3) and and modes of emplacement. Cofre de Perote volcano is one of the largest provide evidence of late Pleistocene glacial Also, the modern-day summit area of Cofre structures of the Citlaltépetl–Cofre de Perote vol- erosion, similar to that of cirques found on the de Perote volcano contains visible collapse canic range. It represents a major physiographic western fl anks of Citlaltépetl and Las Cumbres scarps, evidence of glacial erosion, and hydro- and orographic divide between the Coastal Plain farther south along the Citlaltépetl–Cofre de thermally altered areas. Because chemical to the east and the basin of Serdan-Oriental to the Perote volcanic range (Lorenzo, 1964; White, weathering and soil-forming (i.e., pedogenic) west, the latter of which forms an altiplano pla- 1986; Heine, 1988; Siebe et al., 1993; Lachniet processes play a critical role in triggering land- teau. The difference in elevation between the two and Vázquez-Selem, 2005; Rodríguez, 2005). slides, debris avalanches, and debris fl ows at physiographic provinces is more than 1200 m, Carrasco-Núñez et al. (2010) propose that the tropical latitude volcanoes in particular, we which has been partly attributed to the tilting of geologic evolution of Cofre de Perote volcano discuss these processes further in the context of pre-volcanic basement rocks (Carrasco-Núñez can be divided into four different stages, three clay-mineral formation and the origins of the et al., 2006 and references therein). Differ- of which correspond to major effusive construc- Los Pescados debris-fl ow and Xico avalanche ences in climate, vegetation, and rock weath- tional stages, and a fourth corresponding to a deposits. In particular we examine the nature ering regimes between the two physiographic collapsing stage as depicted in Figure 4. Erup- and origin of clay, silica, sulfate, and ferric-iron provinces are also striking. The western side is tive activity at Cofre de Perote volcano varies minerals in the Los Pescados debris-fl ow and a semiarid desert with low (<400 mm) annual in age with the earliest activity dated at 1.3 ± Xico avalanche deposits by comparing them to precipitation, an eight-month dry period, and 0.12 Ma (Pleistocene) for lavas exposed east of minerals found within hydrothermally altered annual temperature variations up to 18 °C with the city of Coatepec on the eastern fl anks, while rocks collected from possible source areas an average between 11 and 14 °C (Dubroeucq on the western fl ank, lavas are dated at 0.51 Ma near the Cofre de Perote volcano summit and et al., 1998). Soils on the western side of Cofre (Fig. 4A). This fi rst-stage effusive activity is mapped imagery using ASTER. In the process, de Perote volcano are andosols modifi ed by followed by a second stage comprised of super- we present fi eld, laboratory, and remote-sensing aeolian processes and caliche (i.e., calcrete) imposing lava fl ows and domes, which change methods that can be used to study other debris- cementation (Dubroeucq et al., 1998). The east- from basaltic-andesitic to andesitic-dacitic in avalanche and debris-fl ow deposits of unknown ern side of Cofre de Perote volcano has a sub- composition. These eruptive products have been origin at other volcanoes around the world, tropical to tropical climate with a brief (less than dated within the range of ages from 0.31 to

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Figure 1. Location map for Cofre de Perote volcano and select regional, structural, topographic and volcanic features: 1—Pico de Orizaba; 2—Las Cumbres; 3—Cerro Desconocido; 4—Cerro Tecomales; 5—Cofre de Perote volcano; 6—Las Lajas; 7—Naolinco volcanic ﬁ eld; 8—Los Humeros caldera; 9—Laguna de Alchichica; 10—Laguna de Quechulac; 11—Laguna de Atexcac; 12—Caldera de Tecuitlapa; 13—Laguna de Aljojuca; 14—Cerro Pinto; 15—Las Derrumbadas; 16—Cerro Pizarro; 17—Sierra Negra; A—Los Pescados River; B—Atopa River; C— Gavilanes River; D—Limon River; E—Teocelo River; F—Río Chico River; G—upper Los Pescados River; H—Huitzilapan River.

0.42 Ma (Fig. 4B). The third and fi nal effusive ing to Los Pescados debris-fl ow DF and Xico deposits belong to catastrophic eruptions of Los stage lasted from ~0.25 Ma until eruptive activ- avalanche (Fig. 4D). Apparently there were no Humeros Caldera and tend to mantle some parts ity ceased at ~0.2 Ma (Fig. 4C) (Carrasco-Núñez explosive eruption products generated during of the lavas produced during the fi rst stage of and Nelson, 1998). Figure 5 shows the distribu- the construction of Cofre de Perote volcano Cofre de Perote volcano evolution, as depicted tion of the geologic units representing the main (Fig. 5); however, on the lower western fl anks of in Figure 4B with a thin line between these two effusive stages, as well as debris-avalanche and the volcano, there are some pyroclastic deposits units. However, these pyroclastic deposits lay debris-fl ow deposits representing the fourth that correspond to the Xaltipan ignimbrite dated stratigraphically below lava fl ows and breccias stage of Cofre de Perote volcano, correspond- at 0.45 Ma (Ferriz and Mahood, 1984). These comprising the third constructional stage of the

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been reported, such as one from Las Cum- bres volcano (Rodríguez, 2005), which is also exposed near the convergence of Los Pescados debris fl ow and Xico avalanche. At each location, we retrieved at least 1 kg of mostly matrix material, as our primary focus is on the most dominant minerals in the clay-sized fractions, except for hydrothermally altered rock samples from the Cofre de Perote volcano summit area, which is better exposed (Fig. 7). The Los Pes- cados debris-fl ow and Xico avalanche samples were collected from steep, vertical outcrops in order to avoid thick vegetation and soil cover. Despite this, several of our sample sites displayed <50-cm-thick, weathered, and indurated surfaces that were removed in order to expose underlying “fresher” deposit material. In some cases, this weathered surface included a 2- to Figure 2. Compound shield morphology of Cofre de Perote volcano 3-cm-thick organic layer. In order to distinguish as viewed from the west. Note the gentle profi le and low apparent between alteration minerals formed by hydro- slopes and lack of obvious collapse-related scarp features, which can thermal processes from those formed by chemi- lead to misleading conclusions about the stability of the edifi ce. cal weathering processes, we were careful to remove as much soil contamination as possible before sampling. All samples were collected volcano. Although the variable thickness of this and debris-fl ow deposits from nearby Citlalté- during the dry season in order to avoid the need pyroclastic layer is generally unknown, they can petl–Cofre de Perote volcanic range sources, for extensive drying in the laboratory, which be estimated in the range of tens of meters. The which were sampled for comparison and to clar- would alter certain clay minerals of interest. The fi nal episode of eruptive activity occurred along ify possible confusion between likely volcanic following are fi eld descriptions of the Los Pes- the outer fl anks (not shown) and is related to source areas. cados debris-fl ow and Xico avalanche deposits parasitic cinder cones located northeast of the A few samples (e.g., CP-0525, CP-0526, and their source area. Detailed descriptions of main Cofre de Perote volcano edifi ce. The most and CP-0527; Fig. 6) were collected at loca- other deposits shown in Figure 7 are provided recent of this late-stage activity corresponds to tions where other avalanche deposits have by Carrasco-Núñez et al. (2006). El Volcancillo, a cinder cone dated at 900 yr B.P. (Siebert and Carrasco-Núñez, 2002).

FIELD, LABORATORY, AND REMOTE-SENSING DATA AND ANALYTICAL METHODS

Fieldwork and Sample Collection

We limited most of our fi eldwork to the proximal parts (Fig. 3) of the studied deposits. Therefore, the Xico avalanche was the most thoroughly sampled of the two deposits (Fig. 6). Nonetheless, the Los Pescados debris fl ow was also sampled at a few locations along Los Pes- cados River, which is the main eastern-fl owing drainage of Cofre de Perote volcano and the northern sector of the Citlaltépetl–Cofre de Perote volcanic range (Fig. 6). For comparative purposes, Figure 6 shows the mapped extent of these and other major debris-avalanche and debris-fl ow deposits along the main drainage Figure 3. Digital elevation model showing the distribution of the Xico avalanche deposit and pathway along Los Pescados River and several arcuate (i.e., “horseshoe-shaped”) scarp on the eastern fl anks of Cofre de Perote volcano. Note of the tributary drainages that converge toward it the hummocky topography in the medial zone of the avalanche deposit. Also shown is the (Carrasco-Núñez et al., 2006). Samples include extent of Late Pleistocene glacial dissection on the western fl anks of Cofre de Perote volcano; those collected from the Cofre de Perote vol- these fl anks are sometimes mistaken for debris-avalanche scarps. Coordinates are based on cano summit area, Xico avalanche, Los Pesca- Universal Transverse Mercator (UTM) Zone 14, and topography is based on Instituto Nacional dos debris-fl ow, and adjacent debris-avalanche de Estadística, Geografía e Informática (INEGI) 1:50,000-scale elevation data.

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Cofre de Perote Volcano Summit Area The summit area shows spectacular horseshoe-shaped scarps (Fig. 3), which provide the most obvious evidence of major collapse episodes in the past. The shield-like morphology of Cofre de Perote volcano is truncated at the SE ﬂ ank (Figs. 5 and 7) by these fresh, steep-looking scarps, which show the volcano’s inner structure. These scarps expose a sequence of andesitic-dacitic lava ﬂ ow beds with large irregular areas showing the more permeable and hydrothermally altered rocks (Fig. 7). Several samples of hydrothermally altered rock material (e.g., CP-0515, CP-0519, and CP-0520; Fig. 6) were collected mainly from safely accessible, fractured, and brecciated bedrock areas near the main summit scarp (Fig. 7). Sample CP-0637 was retrieved from one of the most pervasively altered exposed zones (circled area 1, Fig 7).

Los Pescados Debris Flow Los Pescados debris-fl ow extends for a dis- tance of ~60 km with variable thickness up to 25 m. The deposit is massive and includes a mixture of boulders and gravels within a strongly cemented clayey-silty matrix; this matrix cement material was undetermined in the fi eld but is dominated by hydrous silica minerals based on laboratory XRD and infrared spectral measurements, both of which are further discussed in detail. Such minerals are quite com- Figure 4. Evolution of Cofre de Perote volcano after Díaz-Castellón (2009); also in Carrasco- mon in indurated soils and volcanic deposits Núñez et al. (2010). Section corresponds to line A–A′ of the geologic map (Fig. 5). (A) Emplace- exposed on the eastern fl anks of Cofre de Perote ment of basal shield edifi ce; (B) construction of a central dome complex; (C) fi nal edifi cation volcano (e.g., Elsass et al., 2000). Hubbard of the compound volcano; (D) sector collapse event producing debris-avalanche and debris- et al. (2007) were able to estimate a volume fl ow deposits. masl—meters above sea level. of 0.35 km3 for the Los Pescados debris-fl ow deposit based on deposit thickness, planimetric area, total runout, and geographic information The age of Los Pescados debris fl ow has been jigsaw-shaped fracturing in clasts. Fresh lithic system (GIS)–based, cross-sectional lahar map- estimated using 14C as 42 ka (Carrasco-Núñez fragments are mostly andesitic rocks contain- ping methods. et al., 2006), which occurred long after cessation ing abundant pyroxene and plagioclase pheno- At the convergence between Río Los Pescados of activity from Cofre de Perote volcano. crysts, which resemble summit area rocks from with Río Huitzilapan (Figs. 1 and 6), relatively the Cofre de Perote volcano third stage (Fig. 5). fl at terraces can be seen (“Río Pescados debris Xico Avalanche These same rocks are also found in Los Pesca- fl ow”; Hubbard et al., 2007). Along its fl ow path, The most recent deposit associated with the dos debris fl ow, though in lesser abundance. At it shows inverse gradation with an abundant collapsing stage of Cofre de Perote volcano is the town of Xico (Fig. 6), the average measured silty-clayey matrix. There are also some jigsaw- Xico avalanche, with a mapped area of ~73 km2 depth of the deposit was ~22 m above old, thick shaped blocks present, which are typical of (Fig. 6) and an average thickness of 30 m, which basaltic lava fl ows, where there is no contact avalanche deposits; however, no hummocks are yields a volume of ~2.19 km3 (Díaz-Castellón, (with the Los Pescados debris-fl ow deposit). observed. Lithic fragments inside the deposit are 2009). Xico avalanche age ranges from ~11– similar to those found at the summit of Cofre de 13 ka based on a 14C date derived from wood col- Laboratory X-ray Diffraction and Visible Perote volcano (Fig. 5); in fact, these fragments lected from the deposit (Carrasco-Núñez et al., to Short-Wave–Infrared (0.4–2.5 μm) display aphanitic texture comprised of large 2006). Stratigraphically, Xico avalanche is on top Refl ectance Spectroscopy crystals of plagioclase with abundant pyroxenes, of Los Pescados debris fl ow but only covers the Samples of hydrothermally altered rocks from and they are dominated by andesite and basalt latter at the proximal and medial extents because the Cofre de Perote volcano summit and matrix lava fl ows and breccias. Carrasco-Núñez et al. Los Pescados debris fl ow has a longer runout. material from the two debris-avalanche and (2006) proposed that the Los Pescados debris At proximal locations, Xico avalanche displays debris-fl ow deposits were analyzed using both fl ow was derived from the rapid transformation the distinctive hummocky topography of debris- X-ray diffraction (XRD) and visible to short- from a debris avalanche to debris fl ow, similar avalanche features (Fig. 3). Xico avalanche con- wave–infrared (VIS/IR herein; 0.4–2.5 μm) to the behavior of the Teteltzingo lahar at Cit- tains megablocks (26–32 m long) and is clast to refl ectance spectroscopic methods. In general, laltépetl volcano (Carrasco-Núñez et al., 1993). clast supported, heterolithologic, and displays both methods provide complementary mineral-

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For this study, we make use of laboratory- measured VIS/IR spectra to identify and char- acterize absorption features related to the most abundant (i.e., spectrally dominant) minerals. XRD provides additional information about the bulk mineralogy of each sample, including mineral phases, which are either not abundant enough to be detected using VIS/IR spectra, or masked by surface coatings dominated by other

′ minerals. Both methods are complementary and have advantages and disadvantages for fi eld and/or laboratory study of debris-avalanche and debris- fl ow deposits. For example, unlike XRD and other laboratory-based analytical methods, VIS/IR spectral-refl ectance measurements do not require elaborate sample preparation methods that can either alter the sample from the way it occurs naturally or require small or specifi c portions of the sample that may not be representative of the material from which it came (Crowley and Vergo, 1988a). However, VIS/IR spectra generally penetrate only the upper few tens of micrometers of the surface being measured (Buckingham and Sommer, 1983), unlike XRD, which yields volumetric-based mineral abundances. Using one of earliest portable fi eld spectrometers available, Marsh and McKeon (1983) note that dominance or interference of absorption features between mineral compo- ′ nents within intimate mixtures (Clark, 1999) sometimes necessitates further XRD analysis, which can only be done in the laboratory. We likewise solve this problem by measuring XRD on the same powdered samples we measured using VIS/IR spectra, although the results of each analytical method were interpreted inde- pendently of one another. For XRD and VIS/IR spectral analysis, all Figure 5. Geology of Cofre de Perote volcano. Note the higher distribution of fractured of our samples were mechanically sieved to bedrock around the summit area. Coordinates are based on Universal Transverse Mercator separate the coarser sand fractions dominated (UTM) Zone 14, North America 1983. Section A–A′ corresponds to a cross section of the by pyrogenic minerals (i.e., those of igneous or volcano described in Figure 4. volcanic origin) of little or no interest such as feldspars, quartz, pyroxenes, amphiboles, and unaltered vitric components, from the silt- and ogical information that is usually but not always The origin of VIS/IR spectral absorption fea- clay-size fractions using standard sieves with in agreement (e.g., Buckingham and Sommer, tures related to minerals characteristic of weath- meshes ranging in diameters from –6 to 4 φ. 1983). For example, Buckingham and Sommer ered and hydrothermally altered volcanic rocks Further, mineralogical analysis was conducted (1983) and Hunt and Ashley (1979) all note that is reviewed by Hunt (1977), Hunt and Ashley using powders containing the smallest size frac- hematite and goethite can occur as rock coatings (1979), and papers referenced therein. For exam- tion (i.e., very fi ne sand and smaller <4 φ) in that can spectrally mask a prominent absorption ple, aluminous clay minerals such as kaolinite order to concentrate minerals formed possibly feature at 0.43 μm (Fig. 8) related to jarosite. and smectite (Fig. 8) display diagnostic absorp- by either hydrothermal alteration or chemical As a result, XRD patterns can show prominent tion features at ~1.4 μm and 2.2 μm (Fig. 8) with weathering processes. Some samples of hydro- X-ray peaks related to jarosite, but weak to non- an optional feature ~1.9 μm, if water-expandable thermally altered rocks from the Cofre de Perote existent peaks related to hematite and goethite, layers are present. Broader absorption features volcano summit area were crushed and/or disag- despite often greater abundances of the latter at wavelengths <1.2 μm are characteristics of gregated prior to sieving. Notably, the intensity two minerals. Buckingham and Sommer (1983) hematite (not shown), goethite, and jarosite of pyrogenic mineral peaks in XRD data can be discuss other examples where VIS/IR refl ec- (Fig. 8), while the sulfate minerals, alunite and used as an inverse proxy measure of alteration tance spectra identifi ed hydroxyl-bearing clay jarosite, display prominent features at ~2.17 μm intensity, as we show in our results using feld- minerals that were not detected by XRD. and ~2.26 μm, respectively (Fig. 8). spar peaks.

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Figure 6. Distribution and location map of debris-avalanche and debris-fl ow deposits from a variety of volcanic source areas along the major valleys draining Cofre de Perote volcano along the northern sector of the Citlaltépetl–Cofre de Perote volcanic range (CCPVR). Also shown are sample locations and major nearby cities and towns. Grid units are Universal Transverse Mercator (UTM). The mapped distribution of Las Cumbres, the 1920 seismogenic debris fl ow, and Los Pescados debris-fl ow deposits have been modifi ed from Carrasco- Núñez et al. (2006). More detailed and precise locations for summit and proximal avalanche samples 0637, 0519, 0515, 0520, 0501, and 197 are shown in Figure 9. Coordinates are based on UTM Zone 14, Ellipsoid of North America 1983.

For XRD, we used a Scintag X-ray diffrac- erals. However, identifi cation of phyllosilicates including mixed-layered clays and various grain tometer using CuKa radiation with an intensity required reference to published patterns, mostly sizes of the same mineral (Crowley and Vergo, of 45 kV measuring a total 2-theta (i.e., 2θ) from Moore and Reynolds (1997) and references 1988a, 1988b; Clark et al., 1993). interval from 2° to 40° @ 0.02 steps per inter- therein. Feldspar peaks were identifi ed using val. Both the original random powdered sample our own powdered standards representing solid- ASTER and Hyperion Remote-Sensing and (crushed <4 φ) and clay-sized (<2 μm) fractions solution compositions typical for andesitic and Spectral Analysis were measured. Following the methods of Moore dacitic volcanic rocks exposed throughout the For this study, we map hydrothermally and Reynolds (1997), oriented clay mounts were Citlaltépetl–Cofre de Perote volcanic range (e.g., altered rocks at Cofre de Perote volcano using prepared by separating the clay-sized fraction Negendank et al., 1985). multispectral Advanced Spaceborne Thermal left in suspension from the coarser fractions set- Visible to short-wave infrared refl ectance Emission and Refl ection Radiometer (ASTER) tling in distilled water for ~4 h unless fl occula- spectra were measured using a fi eld-portable data acquired 29 March 2002 onboard the Terra tion occurred. Ethylene glycol saturation and analytical spectral device (ASD) measuring vis- satellite and hyperspectral imagery acquired heat treatments were done to test for expandable ible to short-wave infrared refl ected radiation 28 November 2001 by the EO-1 Hyperion and mixed-layered clays, chlorite and hydroxy- from 0.4 to 2.5 μm. All sample powders were sensor. Hyperion provides continuous (196 out of interlayered clays (Moore and Reynolds, 1997; measured against a dark carbon background, 242 channels) coverage from 0.4 µm to 2.4 µm Meunier, 2007), the latter of which is formed in using a quartz lamp as an artifi cial light source, at spectral resolution of ~10 nm (Folkman soil environments (Barnhisel, 1977). Identifi ca- and brightness values were measured relative et al., 2001). In particular, ASTER imagery was tion of many crystalline, nonclay mineral phases to “Spectralon,” a high-refl ectivity plastic stan- acquired most optimally under cloud-free con- were easily done using automated X-ray peak dard. Spectral absorption features were iden- ditions during the height of the dry season. matching software and Joint Committee on Pow- tifi ed and visually interpreted using spectral ASTER measures refl ected radiation in three der Diffraction Standards (JCPDS) for pure min- library and published data for various minerals, bands between 0.52 µm and 0.86 μm (visible

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Between 2000 m and 3500 m, clouds bank up against the Citlaltépetl–Cofre de Perote volcanic range to produce montane cloud forests (Lauer, 1973), which obscure the volcanic deposits and soils of much of Cofre de Perote volcano from remote-sensing mapping. Therefore, we spatially subsetted our nine-band ASTER refl ectance data in order to focus on the uppermost portions of the Cofre de Perote volcano summit cone, especially areas above the ~3500 m timber line (Lauer, 1973), where grasses gradu- ally thin out and are replaced by bare rock and tephra exposures (Figs. 5 and 9). Spectral end members representing the three most common (i.e., abundant) classes of altered rocks exposed on the Cofre de Perote volcano summit (Fig. 9) were derived using an automated pixel-purity analysis procedure (Board- Figure 7. Main scarp of Cofre de Perote volcano showing anomalously-colored and pervasively man et al., 1995), after masking out vegetated altered, hydrothermal alteration zones mapped using Advanced Spaceborne Thermal Emis- pixels on the lower slopes. Pixels with the stron- sion and Refl ection Radiometer (ASTER) as shown in corresponding Figure 9. Also shown are gest spectral absorption features, often diagnos- major stratigraphic and structural controls on the location of hydrothermally altered rocks tic of individual minerals and mineral mixtures preserved within the modern edifi ce. Circled area 1 contains ground truth sample CP-0637, characteristic of hydrothermally altered rocks, with sample CP-0519 collected at a location just above cliff face above the circled area. Circled were extracted and used as spectral end mem- areas 2 and 3 were not accessible, but are shown for cross-referencing purposes with Figure 9. bers based on principal components analysis– based, multidimensional image transformations described in detail elsewhere (e.g., Green et al., 1988; Boardman, 1993; Boardman et al., 1995). and near-infrared [VNIR]) and in six bands Although cloud-free ASTER coverage exists These spectral end members were then used as from 1.00 µm to 2.43 μm (short-wave infra- over the entire mapped extents of the Xico ava- reference image spectra for subsequent match- red [SWIR]), with 15 m and 30 m resolution, lanche and Los Pescados debris-fl ow deposits fi lter classifi cation (Harsanyi and Chang, 1994) respectively (Fujisada, 1995). Level 1B VNIR (Fig. 6), the medial and distal portions of the of the entire ASTER scene, the results of which and SWIR “at-sensor” radiance data were spa- deposits between 2000 m and 500 m, respec- are discussed in the subsequent section. tially resampled to the same 15 m resolution, tively, are covered by dense, mixed montane Because of Hyperion’s narrow swath cover- coregistered, atmospherically corrected, and oak forests, which grade into tropical rainforest, age (~7.7 km wide), limited spatial resolution calibrated to refl ectance using “ACORN” soft- open savannas, and tropical deciduous forest (30 m/pixel), and lower signal-to-noise (1/6 that ware (ImSpec LLC, 2004). toward the coast of Veracruz (Lauer, 1973). of ASTER SWIR; Hubbard et al., 2003), as well

Figure 8. Comparison of spectral bandpasses of Advanced Spaceborne Thermal Emis- sion and Reﬂ ection Radiometer (ASTER) and EO-1 Hyperion in relation to diagnostic spectral features of ferric-iron, clay, and sulfate minerals typically found in hydrothermally altered rocks. Modiﬁ ed after Hubbard et al. (2003).

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mapped as yellow, orange, and red, respectively: (1) areas dominated by smectite mixtures and characterized by strong smectite absorption features in both the ASTER and Hyperion data (e.g., circled yellow areas 1 and 3, Figs. 7, 9, and 10); (2) areas characterized by moderate to weak ferric-iron slopes between ASTER bands 2 and 1 and weaker Al-OH clay absorption features at 2.2 μm, which correspond to ASTER band 6 (e.g., area 3 spectral plots, Fig. 10); and (3) areas characterized by strong ferric-iron features between ASTER bands 2 and 1, but no resolvable Al-OH clay absorption features. The third and latter class of iron-oxide (e.g., hematite) altered rocks (red-colored areas, Fig. 9) could be related to rapid in situ degas- sing of edifi ce-building lava fl ows and tephra (e.g., Crowley and Zimbelman, 1997), or slower chemical weathering of magnetite and other iron-bearing phases within these rocks (e.g., Buckingham and Sommer, 1983) as opposed to hydrothermal alteration. In the case of Cofre de Perote volcano, these areas are more exposed and more extensively mapped on the western fl anks because it is more arid and has less vege- ta tion cover than the eastern fl anks (Fig. 9). Circled areas 1–3 (Fig. 9) are among the most “” intensely altered areas of the Cofre de Perote volcano summit; these areas are easily visible on the ground as colored zones distinct from the surrounding bedrock areas (corresponding circled areas 1–3, Fig. 7). Although small in size (<30 m for intensely altered bedrock areas), these areas plus their downslope talus aprons were well resolved spatially and spectrally using ASTER (Fig. 9). There are several rea- sons for this, including: ideal off-nadir viewing Figure 9. Pervasively altered (yellow-colored pixels), partially to incipiently altered (orange- conditions (i.e., –8.5° pointing angle), enhanced colored pixels), and ferric-iron oxidized (red-colored pixels) rocks mapped using calibrated SWIR signal-to-noise, effective calibration, nine-band Advanced Spaceborne Thermal Emission and Refl ection Radiometer (ASTER) 15-m resolution VNIR bands that are sensitive refl ectance imagery. Base image is a false-color composite of bands 6, 3, and 1 displayed as to ferric-iron minerals in altered areas, and talus red, green, and blue, respectively. Also shown are key geographic and vegetation-landcover derived from the alteration zones that exagger- features discussed in greater detail in text. Circled areas 1 through 3 correspond with those ates their mapped extents. Notably, these same shown on Figure 7. Circled area 1 contains ground-truth samples CP-0637 and CP-0519 as areas mark permeable ash and tephra layers, explained in the caption of Figure 7. Circled areas 2 and 3 were not accessible to sample. which are more susceptible to hydrothermal Circled area 4 contains ground-truth samples CP-0515 and CP-0520, which are off the view alteration (Watters et al., 2000). of Figure 7. Sample CP-197 is the most proximally collected sample of Xico avalanche, while Circled area 4 (Fig. 9) is located off the sample CP-0501 represents an older debris-avalanche deposit buried by overlying lava fl ows photo graph shown in Figure 7, but it is easily from Cofre de Perote volcano. These samples are also shown on Figure 6. accessible via the road leading to the summit area, and radio facilities (Fig. 7). Pixels defi n- ing the road were mapped because nearby as the persistent cloud cover over the Cofre de RESULTS rocks, including altered rocks, are used as local- Perote volcano summit area and limited expo- ized sources of pavement gravel material. Simi- sure of the most intensely altered rocks near the Hydrothermal Alteration Mapping of the larly, the most proximal exposures of the Xico summit (Fig. 7), the Hyperion data were used Cofre de Perote Volcano Summit Area avalanche deposit (sample sites CP-197 and for spectral analysis instead of for mapping CP-0501; Fig. 9) were mapped well along the purposes. However, the Hyperion data were Figure 9 shows ASTER spectral matched- roadside despite the dense vegetation cover on also calibrated using the same ACORN soft- fi ltering results displaying the distribution of the eastern fl anks for this same reason. At these ware (ImSpec LLC, 2004) used to calibrate the three most dominant classes of altered rocks two sample locations, it is quite possible that ASTER data. on the Cofre de Perote volcano summit and the road mapped using ASTER (Fig. 9), traces

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A mally altered rocks exposed at the Cofre de Perote volcano summit. In this case, both the XRD (Fig. 11A) and VIS/IR spectra (Fig. 11B) agree and display X-ray peaks and visible- infrared absorption features related to jarosite and a kaolinite-smectite mixture, though smectite appears to be slightly more abundant than kaolinite. The VIS/IR refl ectance spectrum (Fig. 11B) lacks the secondary jarosite absorption feature at 2.26 μm, perhaps due to masking by abundant clays (e.g., Hunt and Ashley, 1979). However, it does display the prominent 0.43 μm ferric-iron charge transfer feature diagnostic of jarosite (Figs. 8 and 11B), and 0.66 μm absorption, which is much stronger for goethite than it is for hematite. Notably, hematite and/or goethite were not abundant enough to be detected in the XRD (Fig. 11A), even though minute amounts (i.e., a few percent) of these minerals can be readily detected using VIS/IR spectroscopy (Buckingham and Sommer, 1983; Clark, 1999). Sample CP-0519 (Figs. 11E and 11F) displays a similar mineralogy to sample CP-0637 B collected nearby (both within circled area 1; Fig. 9), but its XRD pattern (Fig. 11E) shows that it contains alunite in addition to jarosite. Alunite displays a prominent absorption feature at 2.165 μm, which overlaps with part of the kaolinite doublet absorption at 2.2 μm (Fig. 8), and has a secondary absorption feature at 2.325 μm. Sample CP-0519 lacks these diagnostic alunite absorption features (Fig. 11F) due to masking by more abundant kaolinite (e.g., Hunt and Ashley, 1979), which displays diagnostic doublet absorption features at both 1.4 µm and 2.2 μm. A large interlayer water absorption feature is prominent at 1.9 μm (Fig. 11F), for which XRD patterns confi rm the presence of expandable smectite layers. Comparison of samples CP-0637 and CP-0519 with laboratory spectra (Figs. 11B and 11F, respectively), ASTER and Hyperion image spectra from the same location (Fig. 10A), and spectral library spectra of smectite, kaolinite, and various mixtures of the two minerals (Fig. 12) shows interesting results. Despite random Figure 10. Hyperion and Advanced Spaceborne Thermal Emission and Refl ection Radiometer and coherent noise, the 196-band Hyperion (ASTER) image spectra showing key spectral absorption features related to mapped hydro- spectrum (middle, Fig. 10A) resolves the smec- thermal alteration zones shown as circled areas in Figure 9. Each image spectrum represents tite singlet absorption well (e.g., Figs. 8 and an average of at least four adjacent pixels in our efforts to reduce spatially incoherent noise. 12), even after it is convolved to the nine-point band passes of ASTER for comparative purposes. However, seasonal differences between the imagery are noticeable when comparing the extent of outcropping exposure of smec- We collected and analyzed four representative the ASTER image spectrum (top, Fig. 10A) tite-rich matrix from both the Xico avalanche samples from the Cofre de Perote volcano sum- with the Hyperion convolved to ASTER image deposit and an underlying pre–Xico debris-ava- mit area for ground truth (Figs. 6, 7, and 9). Fig- spectrum (bottom, Fig. 10A), the latter of which lanche deposit discussed in a later section. This ure 11 shows XRD and VIS/IR spectral analy- underwent noise reduction in the convolution smectite clay-rich material could be a consider- sis results for the four Cofre de Perote volcano process. The ASTER spectrum shows a slight able hazard for driving along mapped portions summit samples. Sample CP-0637 represents chlorophyll absorption in band 2 (0.66 μm), of this road during the rainy wet season. the most intensely (i.e., pervasive), hydrother- which is related to subalpine grasses that grow

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Figure 11. X-ray diffraction (XRD) (left plots; A, C, and E) and corresponding visible to short-wave infrared (VIS/IR) reﬂ ectance spectra (right plots; B, D, and F) for hydrothermally altered rocks collected in the Cofre de Perote volcano summit area. Mineral abbreviations are discussed in detail in the text. For XRD plots, (A) represents an oriented glycolated pattern, while (C) and (E) represent random powder results. Other treatments not shown are summarized in Table 2. Interpretations of all spectral absorptions in (B), (D), and (F) are summarized in Table 3.

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incipient, where volumetrically smaller portions of the original rock (e.g., glassy groundmass) are altered; to partial, where major (e.g., tens of percent) portions of the original rock are altered; to pervasive, where the entire volume of original rock, including crystalline phases, are replaced by secondary clay and/or sulfate minerals. He also notes several modes of occurrence, which vary from replacement (the most common), to fracture-fi lling (e.g., vein and stock structures common in ore deposits) and encrustation (i.e., coating) types. The XRD patterns of area 4 samples CP-0515 (Fig. 11C) and CP-0520 (not shown) are dominated by plagioclase feldspar, suggest- ing incipient alteration, although their VIS/IR spectral-refl ectance patterns (Fig. 11D) display broad singlet absorptions at 2.2 μm indicative of smectite. Visible to short-wave infrared refl ectance spectroscopy is more sensitive to clay mineral coatings than XRD (Buckingham and Sommer, 1983), especially when they are present in low concentrations such as in incipiently altered rocks represented by samples CP-0515 and CP-0520. However, the smectite absorption features displayed by samples CP-0515 and CP-0520 are much broader than those displayed by a pure spectral library sample (Fig. 8). This broadening could be due to mixing with amor- phous hydrous silica minerals such as Opal-A (Hunt and Ashley, 1979), allophane (Cooper and Mustard, 1999), or grain-size variations related to particle-packing differences on the surface of smectite-bearing coating materials (Cooper and Mustard, 1999). Figure 12. Spectral library data (Clark et al., 1993) comparing a Samples CP-0515 and CP-0520 (circled variety of kaolinite-smectite mixtures with halloysite, smectite, well- area 4, Fig. 9) are perhaps the most representa- crystalline kaolinite, and poorly crystalline kaolinite. tive samples of the second and most widespread class of altered rocks mapped using ASTER (orange-colored areas, Fig. 9). These areas are at elevations as high as 4000 m (Barois et al., ferric-iron content at this location. Because of its characterized by moderate to weak ferric-iron 1998). This elevation contour corresponds to the higher signal-to-noise, ASTER can better sepa- slopes between ASTER bands 2 and 1, and level somewhat below the main avalanche-scarp rate aerial mixtures abundant in kaolinite from relatively weaker (than yellow-colored areas, cliff face shown in Figure 7. The corresponding aerial mixtures abundant in smectite, using fewer Fig. 9) yet still resolved Al-OH absorption fea- Hyperion spectrum shows a slight absorption bands than Hyperion (e.g., Hubbard et al., 2003). tures at 2.2 μm (ASTER band 6). We interpret feature near 2.3 μm, which is either due to dry In this case, image spectra derived from both the distribution pattern for this class of altered (or senescent) grass or noise. None of these veg- data sets (Fig. 10) lack clear evidence of kao- rock as coinciding with coatings, encrusta- etation-related features are found in the sample linite features, and are all dominated by smectite tions, and fi llings along the heavily fractured spectra (Figs. 11B and 11F). absorption features. This suggests that smectite bedrock areas of the Cofre de Perote volcano The ASTER and Hyperion image spec- is perhaps the most abundant alteration mineral summit (Fig. 5), which can best be character- tra (Fig. 10B) of circled area 3 (Figs. 7 and 9) at the scale of ASTER and Hyperion pixels for ized by incipient to partial in alteration inten- show similar spectral features in both data sets, these most intensely altered areas (yellow col- sity. In contrast, yellow-colored areas (Fig. 9) though not quite as well resolved as in the case ored, Fig. 9). Also, our samples from these areas represented by samples CP-0637 and CP-0519 of area 1. This is perhaps due to the smaller size contain smectite intimately mixed with other contain much less feldspar and more abundant of this feature and oversampling of its aerial minerals, which may not be fully representative clay, sulfate, and ferric-iron minerals, which are extent within the 30-m pixels of Hyperion and of the larger scale (i.e., 30 m), remote-sensing most indicative of pervasive alteration intensity. ASTER SWIR bands. Despite this problem, pixel areas from which they were derived. Additional areas mapped as the second class the ASTER spectrum (top, Fig. 10B) displays Zimbelman (1996) ranks the intensity of of altered rocks (orange areas, Fig. 9) are found the highest slope between the 15-m resolu- hydrothermally altered rocks at Mount Rainier on the lower southwestern fl anks of Cofre de tion bands 2 and 1, which is indicative of high and other Cascade volcanoes as ranging from Perote volcano. This includes the town of Los

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Altos (Fig. 9), which sits on top of alluvial either halloysite or kaolinite. Also, they display TABLE 1. GRANULOMETRICS ALONG DEBRIS deposits and soils near ~3000 m (Fig. 5). Soil interlayer water absorption features at 1.9 μm, AVALANCHES AND DEBRIS FLOWS Sample Gravel Sand Silt Clay profi les (i.e., pedons) (e.g., circled area 5, Fig. 9) which are diagnostic of halloysite, smectite, or CP-0514 48.47% 49.74% 1.58% 0.21% and chemical weathering rates in this area were other expandable clay mineral. The most diag- CP-0631 58.14% 39.69% 2.10% 0.07% characterized by Dubroeucq et al. (1998), who nostic absorption features for di-octahedral alu- CP-0626 37.39% 58.85% 3.36% 0.40% μ CP-0636 53.77% 44.63% 1.41% 0.19% report abundant 10-angstrom (Å) halloysite at minous clays are at or near 2.2 m, which, in CP-0715 86.02% 13.48% 0.50% 0.00% depth (<230 cm), which becomes progressively the case of Los Pescados debris-fl ow samples, CP-0716 66.37% 32.23% 1.39% 0.01% dehydrated as 7 Å halloysite toward the surface. are broad due to either mixing between clay Numerous other pedons characterized at this and/or hydrous silica minerals, or perhaps grain- (3100 m) and lower elevations around the east- size and particle-packing effects (Cooper and results (Fig. 16) than the Los Pescados debris- ern and western fl anks of Cofre de Perote vol- Mustard , 1999). fl ow samples (see Fig. 6 for sample locations). cano show halloysite to be the dominant chemi- For these samples, XRD provided more This is due in part to much stronger absorption cal weathering mineral in andosols derived from conclusive mineralogy results for the three features, and hence greater abundances of clay volcanic deposits in the area (Barois et al., 1998; Los Pescados debris-fl ow samples (Fig. 14). minerals in the Xico avalanche samples than Dubroeucq et al., 1998; Elsass et al., 2000; All three samples contain dehydrated and/or in the Los Pescados debris-fl ow. To facilitate Dubroeucq et al., 2002). hydrated forms of halloysite, and display large discussion, VIS/IR refl ectance spectra (Fig. Because kaolinite- and smectite-intimate peaks at 22.18° 2θ due to cristobalite peaks 15) have been grouped and/or stacked together mixtures often resemble halloysite, even when superimposed on one of the smaller but major to highlight samples containing similar clay using hyperspectral data (e.g., Crowley and peaks of plagioclase feldspar (Fig. 14). Cristo- mineral compositions. For example, samples Vergo, 1988a; Hauff et al., 1990; Fig. 12), it is balite and opal are common components of soils CP-0524, CP-0512, and CP-165 (Fig. 15A) possible and quite likely that during spectral and weathered volcanic rocks (Wilding et al., are each dominated by strong kaolinite doublet classifi cation of our nine-band ASTER data, 1977) and especially at Cofre de Perote volcano absorption features at 1.4 μm and the more halloysite-bearing soils and alluvial deposits (Barois et al., 1998; Dubroeucq et al., 1998; diagnostic 2.2 μm wavelength positions. Cor- exposed below timberline were confused with Elsass et al., 2000; Dubroeucq et al., 2002). responding XRD patterns (Figs. 16A, 16B, and altered rocks containing smectite and kao- 16C) confi rm the presence of strong 7 Å kaolin- linite mixtures above timberline (orange areas, Clay Mineralogy of the Xico Avalanche ite peaks in the d001 position (i.e., ~12° 2θ), Fig. 9). Notably, the town of Los Altos was and Similar Deposits with good preferred orientation in the d002 and mapped well in the imagery because its roads d003 positions (i.e., ~20° and ~35° 2θ, respec- and building roofs are constructed using local Xico avalanche samples generally show better tively). Sample CP-165 also displays a promi- sources of clay-rich building materials, such as agreement (i.e., less confusion) between VIS/IR nent water absorption feature at 1.9 μm (Fig. soils and weathered volcanic deposits. spectral-refl ectance results (Fig. 15) and XRD 15A). The corresponding XRD pattern indicates

Clay Mineralogy of the Los Pescados Debris-Flow Deposit Granulometric analyses on selected samples of the Los Pescados debris-fl ow were done to determine fi ne fraction distribution. Sample CP-0514 was initially analyzed for granulometric analysis, though further samples were needed. For this reason, we collected additional samples (Fig 6). Analysis show that the deposit varies in grain size from 86% to 32% of gravel clasts, 58% to 13% for sand clasts, 3.3% to 0.5% for silt clasts, and 0.4 to 0 for clay particles (Table 1). Grain size of particles below 4 φ indicates a low fi ne-grained content, whereas clay content is below 1%, confi rming that it is noncohesive in origin. Comparably, cohesive debris fl ows and lahars generally contain between 3% and 5% clay and have higher silt contents (Cran- dell, 1971; Scott et al., 1995). Figure 13 shows VIS/IR laboratory refl ectance spectra measured for the silt plus clay fractions of Los Pescados debris-fl ow samples CP-184, CP-0514, and LP-01. All three samples display similar spectral absorption features, which are not conclusive due to intimate mixing between clays and other hydrous minerals. For Figure 13. Visible to short-wave infrared (VIS/IR) refl ectance spectra of samples of Los Pes- example, they all display weak doublet absorp- cados debris fl ow (LPDF) showing key clay mineral absorption features discussed in text. tion features at 1.4 μm, which are diagnostic of Interpretations of all spectral absorption features are summarized in Table 3.

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Figure 14. X-ray diffraction (XRD) results for Los Pescados debris-ﬂ ow (LPDF) samples shown in Figure 13. All samples shown in (A), (B), and (C) represent oriented, air-dried pattern; the rest are plagioclase- feldspar peaks and/or noise. Min- eral abbreviations are discussed in detail in text. Other treatments not shown are summarized in Table 2.

that this is due to a hydroxy-interlayered smec- sample for which VIS/IR refl ectance measure- tance pattern dominated by smectite (Fig.16B), tite (HIM; Fig. 16C), which yields a 14 Å peak ments are most sensitive, while others may be but an XRD pattern (Fig. 16E) showing no evi- that subsequently collapses to 10 Å after heat- concentrated throughout the bulk volume of the dence of it. This may be the result of grain size, ing (Meunier, 2007). Aluminum (and/or iron) sample for which XRD is most sensitive (e.g., packing, and coating effects (e.g., Buckingham hydroxy-interlayering suggests pedogenic mod- Buckingham and Sommer, 1983). and Sommer, 1983; Cooper and Mustard, 1999) ifi cation of smectite, perhaps formed originally Xico avalanche samples CP-0523 and discussed previously with respect to other sam- as a result of hydrothermal alteration. Neither CP-0511 (Fig. 15B) are both dominated by ples showing disagreement between XRD and kaolinite nor smectite occurs abundantly in soils strong smectite-related absorption features at VIS/IR spectral data. However, unlike Cofre in and around Cofre de Perote volcano (Barois 1.4, 1.9, and 2.2 μm. The corresponding XRD de Perote volcano summit samples CP-0515 et al., 1998; Dubroeucq et al., 1998; Elsass et al., pattern for CP-0511 (Fig. 16D) confi rms that and CP-0520, Xico avalanche sample CP-0523 2000; Dubroeucq et al., 2002). expandable smectite is the most abundant min- does display relatively lower intensity plagio- Notably, the XRD patterns for samples eral in this sample; however, it also contains clase feldspar peaks, a strong cristobalite (i.e., CP-0524 and CP-0512 (Figs. 16A and 16B, smaller amounts of gypsum and jarosite, both hydrous opal-C?) peak, and weak hematite respectively) show additional strong peaks of which are the clearest evidence of hydrother- peaks (Fig. 16E). The latter two minerals agree related to hematite and cristobalite, which are mal alteration origin yet. Similar to XRD and with the corresponding VIS/IR spectral-refl ec- either weak or not apparent in their correspond- VIS/IR refl ectance patterns of Cofre de Perote tance pattern (Fig. 15B). ing VIS/IR spectral-refl ectance patterns (Fig. volcano summit samples CP-0515 and CP-0520 Xico avalanche samples CP-0521 and CP-197 15A). This can be explained by the concen- (Figs. 11C and 11D, respectively), Xico ava- (Fig. 15B) and CP-155 and AX-1 (Fig. 15C) all tration of some minerals on the surface of the lanche sample CP-0523 yields a VIS/IR refl ec- exhibit spectral absorption features similar to

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C D

F E

Figure 16. X-ray diffraction (XRD) results for Xico avalanche samples shown in Figure 15. Samples are arranged in the order in which they are discussed. Speciﬁ c sample preparation treatments associated with each pattern are stated below the sample number for clarity purposes. Mineral abbreviations are summarized in Table 2 and discussed in detail in text. The results of other treatments and samples not shown are summarized in Table 2 as well.

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G H

K L

Figure 16 (continued).

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each other and the three Los Pescados debris- stratigraphically below the Xico avalanche was Xico avalanche samples CP-0524 and fl ow samples (Fig. 13). However, the XRD pat- sampled along this same road (sample CP-0501; CP-0527 are two out of 12 Xico avalanche terns of CP-0521 and CP-197 (Figs. 16F and Fig. 9). However, preliminary XRD and VIS/IR samples that do not contain detectable amounts 16G, respectively) are dominated by hydrated spectral analysis results (not shown) both show of feldspar, quartz, and/or pyroxene within their and dehydrated forms of halloysite, while the a mixture of smectite and halloysite as the <4 φ sized fractions (Figs. 16A and 16L, respec- XRD patterns of CP-155 and AX-1A-X1 (Figs. dominant clay minerals in this older (pre–Xico tively). Notably, quartz is the most weathering 16H and 16I, respectively) are dominated by avalanche) debris-avalanche sample, the age resistant of three pyrogenic contaminant min- expandable smectite mixed with either kaolin- and extent of which has not yet been studied or erals contained in our samples. Feldspar and ite in the case of AX-1 (Fig. 16I) or halloysite mapped (see Figs. 6 and 9 for location). pyroxene minerals are less weathering resistant in the case of CP-155 (Fig. 16H). As discussed Xico avalanche samples CP-0510 and than quartz. However, samples CP-0524 and in a prior section, intimate mixtures of smectite CP-0501 both illustrate examples of VIS/IR CP-0527 differ from one another in that the for- plus kaolinite (and/or other types of kaolin min- spectra (Figs. 15C and 15D, respectively) domi- mer is dominated by abundant kaolinite (Fig. erals) are diffi cult to distinguish from halloysite nated by strong smectite absorption features at 16A), while the latter is dominated by abundant using VIS/IR refl ectance spectra (Fig. 12). Sam- 1.4 and 2.2 μm and/or weak halloysite-related halloysite (Fig. 16L). In particular, the XRD pat- ple AX-1 is particularly interesting because it absorption features at these same wavelengths. tern (Fig. 16L) and VIS/IR refl ectance pattern shows evidence of mixed-layered illite-smectite The XRD patterns of both samples (Figs. 16J (Fig. 15D) of sample CP-0527 both show X-ray (MIS), in addition to containing kaolinite (Fig. and 16K, respectively) confi rm that smectite peaks and spectral absorption features related to 16I). This particular clay-mineral assemblage is indeed the most dominant clay mineral in the presence of halloysite and gibbsite. Gibbsite (MIS plus kaolinite) has been documented in both samples. Also, both samples show XRD and aluminous hydroxy-interlayered 2:1 lay- hydrothermal alteration environments but is evidence of hydrated and/or dehydrated forms ered clays are both strong indicators of intense not common in near-surface (i.e., shallow) soil- of halloysite, which are most likely the result chemical weathering of volcanic deposits, and forming environments, even if K-rich sedimen- of postemplacement chemical weathering. The are also known to coexist in volcanic soils under tary and igneous rock substrates are involved XRD patterns show that both samples con- certain conditions (e.g., Ndayiragije and Del- (Moore and Reynolds, 1997, p. 172–183). Also, tain jarosite (Figs. 16J and 16K), with sample vaux, 2003). samples CP-155 and A-X1 both contain jarosite, CP-0510 exhibiting the most intense jarosite which is certainly of hydrothermal origin (Figs. peaks, and therefore the most abundant amounts SUMMARY AND DISCUSSION 16H and 16I, respectively), while samples of jarosite evident in all the Xico avalanche CP-0521 and CP-197 do not (Figs. 16F and samples collected. Jarosite abundance in sam- A summary of XRD results is provided in 16G, respectively ). ple CP-0510 is clearly enough to produce a Table 2 for all samples analyzed in this study, Notably, CP-197 is the proximal-most sample prominent 0.43 μm ferric-iron charge transfer including additional random powder, oriented, of the Xico avalanche collected (Fig. 6). It was feature diagnostic of jarosite in the correspond- and specially treated patterns not shown in Fig- collected along a road leading up to the ~3000 ing VIS/IR refl ectance spectra for this sample ures 11, 14, and 16. Table 3 summarizes the masl elevation mark along the eastern fl anks (Fig. 15C). However, other diagnostic jarosite dominant minerals found in each sample based of Cofre de Perote volcano (sample CP-197; absorption features are not well resolved due to on VIS/IR spectral analysis results. Results from Fig. 9). An older debris-avalanche deposit lying masking by more abundant clay minerals. XRD and VIS/IR spectral analysis of samples

TABLE 2. SUMMARY OF XRD ANALYSIS RESULTS OF ALTERED ROCK AND DEBRIS FLOW SAMPLES Sample ID Location plag qtz crist smec hall/mhall MIS HIM kao gyp hem al jar pyx gibb horn CP-0637 CP CP-0515 CP CP-0520 CP CP-0519 CP ? CP-0527 XA CP-197 XA CP-0512 XA CP-0521 XA CP-0524 XA CP-155 XA CP-0523 XA CP-0511 XA CP-0510 XA ? AX-1 XA CP-165 XA ? CP-0501 PXA ? ? CP-0514 LPDF ? CP-184 LPDF ? LP-01 LPDF CP-0528 LC CP-0525 UN CP-182 UN G-01 UN Note: Summary includes random powder and/or oriented patterns not shown in Figures 12, 13, and 15. Plag—Plagioclase Feldspar (undetermined solid solution series); qtz—quartz; crist—cristobalite; smec—smectite (ie. montmorillonite); hall—halloysite (10-angstrom hydrated variety); jar—jarosite and/or natrojarosite; Kao—Kaolinite; gyp—gypsum; hem—hematite; al—alunite and/or natroalunite; pyx—pyroxene (undetermined solid solution series); gibb—gibbsite; horn—hornblende (or similar amphibole); HIM—14-angstrom hydroxy-interlayered mineral; MIS—mixed-layered illite/smectite; mhall—meta- or dehydriated-halloysite; LPDF—Los Pescados debris ﬂow deposit; XA—Xico Avalanche deposit; PXA—pre-Xico (older) debris avalanche deposit; LC—Las Cumbres debris ﬂow/avalanche deposit; CP—Cofre de Perote volcano summit area; UN—Uncertain deposit. ?—samples with peaks of questionable intensity and mineral assignment relative to background noise.

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TABLE 3. SUMMARY OF DOMINANT MINERALOGY OF ALTERED ROCK AND DEBRIS FLOW SAMPLES volcano (Elsass et al., 2000, and references Ppectrally dominant Spectrally dominant therein). Unlike the Xico avalanche, miner- Sample ID Location ferric-iron mineral(s) clay* mineral(s) als in the matrix of the Los Pescados debris CP-0637 CP gth, jar k/s CP-0515 CP gth? smec, hsil? fl ow appear to be entirely of pedogenic origin, CP-0520 CP hem? smec, hsil? with little or no minerals formed evidently as a CP-0519 CP gth kao CP-0527 XA w/und hall, gibb result of hydrothermal alteration. Based on this CP-197 XA w/und hall evidence and grain-size analysis data, the Los CP-0512 XA hem? kao Pescados debris fl ow began as a noncohesive or CP-0521 XA w/und hall CP-0524 XA w/und kao “clay-poor” lahar (Vallance, 2000). The ~42 ka CP-155 XA w/und k/s age of the Los Pescados debris fl ow corresponds CP-0523 XA hem smec to a period in time in which the ancestral sum- CP-0511 XA gth? smec CP-0510 XA gth, jar smec, hall mit of Cofre de Perote volcano was most likely AX-1 XA w/und k/s covered by glaciers, as evidenced by glacial CP-165 XA w/und kao CP-0501 PXA w/und smec cirques on the western fl anks of the volcano CP-0514 LPDF w/und k/s or hall (Fig. 3). Another possible source of water could CP-184 LPDF w/und k/s or hall have been land-falling tropical cyclones like LP-01 LPDF w/und k/s or hall CP-0528 LC w/und kao, hall, amph those that often affect the Gulf Coast of Mexico CP-0525 UN w/und hall and Central America today. CP-182 UN w/und k/s or hall We propose two possible sources for the G-01 UN w/und k/s or hall Note: Summary based on interpretation of visible and near-infrared-short-wave-infrared (VNIR-SWIR) halloysite and cristobalite in the Los Pesca- (0.4–2.5 mm) reflectance spectra of samples (including spectral plots not shown in Figures 12, 14 and 15) of dos debris-fl ow deposit. One possibility is that XA—Xico Avalanche deposit; PXA—pre-Xico (older) Avalanche deposit; CP—Cofre de Perote volcano summit the halloysite and cristobalite formed in situ area; LC—Las Cumbres debris flow/avalanche deposit; LPDF—Los Pescados debris flow deposit; and UN— Uncertain deposits. Mineral abbreviations are as follows: smec—Smectite; hall—halloysite; kao—kaolinite; jar— through chemical weathering after the deposit jarosite and/or natrojarosite; hem—hematite; gth—goethite; k/s—kaolinite-smectite mixtures; hsil—hydrous silica was emplaced. However, if this was the case, we minerals (e.g., opal and/or chalcedony); gibb—gibbsite; amph—amphibole (e.g., hornblende); w/und—weak would see the following evidence in the fi eld: and/or undetermined spectral absorption features; ?—inconclusive or uncertain mineralogy. *Includes other notable hydrous minerals associated with clays in hydrothermal or pedogenic environments (1) numerous clasts with visible weathering and/or non-phyllosilicate minerals with spectral absorption features in the SWIR (see text for details). rinds, and (2) uniformly distributed induration of the deposit with weathering profi les as deep as 1–3 m beyond the exposed surface of the of debris-avalanche/fl ow deposits not related to hematite, goethite, cristobalite, and other types deposit (e.g., Elsass et al., 2000). In sampling Cofre de Perote volcano (see Fig. 6 for sample of hydrous silica, which can be either pyrogenic, the deposit, we were careful to avoid the most locations) are also shown. The origins of some hydrothermal alteration, or chemical weathering indurated exposures and collected matrix mate- of these deposits are uncertain (“UN” labeled related in origin; and (5) minerals that are clearly rial beyond the fi rst few meters at each outcrop. samples, Tables 2 and 3) because of their prox- of pedogenic (i.e., soil-forming) and chemical- Also, larger clasts and blocks of andesite within imity to more than one deposit and lack of dis- weathering origin such as halloysite, gibbsite, the deposit, including a few we collected along tinguishing fi eld characteristics, such as lack of and hydroxy-interlayered clay minerals. For with our matrix samples, contained no visible hornblende, a key indicator of volcaniclastic example, alunite was found in only one Cofre weathering rinds. We favor an alternative origin rocks from Las Cumbres volcano (Scuderi et al., de Perote volcano summit area sample (sample for the halloysite and cristobalite, such that the 2001; Rodríguez, 2005; Carrasco-Núñez et al., CP-0519, Table 2), though not abundant enough Los Pescados debris fl ow bulked (Scott et al., 2006). A more detailed description and char- to be detected using VIS/IR refl ectance spectra 1995) with clay-rich soils and alluvium prior to acterization of possible debris-avalanche and (Table 3). Jarosite was found in several Cofre de being emplaced. This is evidenced by numerous debris-fl ow source areas for all deposits exposed Perote volcano summit area and Xico avalanche rounded clasts within the Los Pescados debris- in the Huitzilapan and Pescados river valleys are samples (Table 2), two of which contained suf- fl ow deposit, indicative of stream transport long summarized elsewhere (e.g., Carrasco-Núñez fi cient amounts to yield diagnostic spectral distances from their source. et al., 2006). features suitable to identify them in the 0.4 to Based on remote-sensing mapping and XRD The minerals summarized in Tables 2 and 3 1.0 μm wavelength range (samples CP-0637 and VIS/IR reflectance analysis of ground- can best be grouped into fi ve classes: (1) pyro- and CP-0510, Table 3). Gypsum, though formed truth samples, smectite appears to be the most genic (i.e., mostly derived from porphyritic by brine concentration in arid evaporate-playa abundant and dominant mineral throughout volcanic rocks) contaminant minerals such environments (e.g., Eugster and Hardie, 1978), hydrothermally altered areas exposed high on as quartz, feldspar, pyroxene, and amphibole; suggests hydrothermal alteration origin for sam- the Cofre de Perote volcano summit. Similarly, (2) sulfate-bearing minerals such as alunite ples collected from the eastern fl anks of Cofre smectite clays such as montmorillinite-beidel- and jarosite, the presence of which provide the de Perote volcano—in contrast to the arid basin lite were also found to be dominant in the fail- strongest evidence of hydrothermal alteration of Serdan Oriental on the western side of Cofre ure scarp and resulting debris-avalanche deposit origin; (3) single- and mixed-layered clay min- de Perote volcano, where such gypsum-bearing at Casita volcano, Nicaragua (Opfergelt et al., erals such as smectite, kaolinite and illite, and saline lakes are numerous. 2006). However, in the case of Cofre de Perote gypsum, which are not common in the soils and The matrix of the Los Pescados debris-fl ow volcano, much of this smectite is either mixed weathered volcanic deposits exposed through- deposit is dominated by halloysite and cristo- with kaolinite and other minerals (e.g., alunite, out the Citlaltépetl–Cofre de Perote volcanic balite (Tables 2 and 3), which are the two most jarosite, and perhaps hydrous silica) in the most range and are therefore most likely of hydro- abundant mineral phases in indurated volcanic intensely altered ash and tephra layers, or occurs thermal alteration origin; (4) minerals such as soils on the eastern fl anks of Cofre de Perote as coating and fi lling material in the surrounding

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fractured and partially altered bedrock areas. avalanche and debris fl ow inundation today issues, cloud cover, and the timing of the plow- These same minerals (except for alunite) were for populated areas downstream than existed ing and planting season in cases where such data also found in Xico avalanche samples (Table 2). during the time of emplacement of the Los Pes- are used for soil-mineral mapping (e.g., Hubbard Compared to other debris-avalanche and debris- cados debris fl ow and Xico avalanche. How- et al., 2002). Despite these limitations, ASTER fl ow deposits we sampled, the VIS/IR spectra ever, additional studies using more advanced data validated by Hyperion spectra were suc- of most Xico avalanche samples are dominated geophysical methods (e.g., Finn et al., 2001, cessfully used to map small, pervasively altered by either kaolinite or smectite absorption fea- 2007) are needed to resolve the full extent of areas on the Cofre de Perote volcano summit, tures, although two samples contain a mixture both water-saturated and hydrothermally altered and corresponding clay-rich debris-avalanche/ of the two minerals that could otherwise be con- rocks buried at depth. Also, future hazards from fl ow deposits outcropping along the roads. This fused with halloysite without supporting XRD noncohesive debris fl ows such as the Los Pes- demonstrates the utility of the fi eld, laboratory, (Table 3). However, the Xico avalanche deposit cados debris fl ow cannot be ruled out, assuming and remote-sensing methods presented in this also contains halloysite, gibbsite, and hydroxy- the availability of a large enough water source paper for the study of debris-avalanche and interlayered clay minerals, which could only (e.g., an unusually large snowpack and/or a stalled debris-fl ow deposits of uncertain origin at Cofre have resulted from chemical weathering after its tropical cyclone) and/or additional triggering de Perote volcano and other volcanoes around emplacement. mechanisms discussed in related work sources the world. In contrast to the Los Pescados debris-fl ow (e.g., Díaz-Castellón, 2009; Díaz-Castellón et al., deposit, the Xico avalanche deposit preserves 2009; Carrasco-Núñez et al., 2010). ACKNOWLEDGMENTS the best mineralogical evidence of a hydro- Special thanks to Consejo Nacional de Ciencia y thermal alteration origin of all the deposits we CONCLUSIONS Tecnología (CONACYT) project no. 44549-F; partial sampled (Table 2). This evidence, together with support was provided by “Prospectiva de un ambiente the large debris-avalanche scarp on the mod- X-ray diffraction and VIS/IR refl ectance spec- para la formación professional”–Universidad Nacio- ern summit (Figs. 3 and 7), and the timing of troscopy provided complementary mineralogical nal Autónoma de México (PAPIIT-UNAM) grant # IN106810, National Aeronautics and Space Admin- the collapse event (~11–13 ka), suggests that information for studying debris-avalanche and istration (NASA) grant NAG-57579 and NASA con- hydrothermal alteration at Cofre de Perote vol- debris fl ow deposits from Cofre de Perote vol- tract S-10224-X with the U.S. Geological Survey. cano may have been related to the late Pleisto- cano, although both methods have their unique We also thank the State University of New York at cene glacial retreat that occurred throughout the advantages and disadvantages. Generally, min- Buffalo for allowing us to use their installations, and Citlaltépetl–Cofre de Perote volcanic range, and erals resulting from hydrothermal alteration, Dr. Michael F. Sheridan for his aid on XRD analysis. James Crowley, Jeff Wynn, and two anonymous is speculated to have been a major factor in the such as those contained in our Xico avalanche reviewers provided helpful comments that allowed us emplacement of the 16.1 ka Teteltzingo lahar samples, tend be more easily identifi able using to improve the manuscript. Gabriel Origel provided deposit from Citlaltépetl (Carrasco-Núñez et al., VIS/IR refl ectance because they occur in greater additional comments on our ASD analysis. Labora- 1993, 2006). Although the timing between the abundance and yield stronger, more diagnos- tory technician support at Centro de Geociencias was provided by M.Sc. Sara Solis Valdez. Lic. Teresa Xico avalanche collapse at Cofre de Perote vol- tic spectral absorption features than weathered Soledad Medina provided an essential review of docu- cano and the Teteltzingo collapse at Citlaltépetl rocks and soils. Field spectroscopic measure- mented gray literature references. Logistic support differs by 3000–6000 years, the existence of ments can be used to distinguish the weathered, was provided by Centro de Geociencias (UNAM). an older smectite-rich debris-avalanche deposit indurated surfaces of outcrops from fresher rock This work is dedicated in loving memory of my father, near proximal outcrops of the Xico avalanche and matrix material that may have been derived Rodolfo Senior. (PXA sample CP-0501, Tables 2 and 3) suggests from hydrothermal alteration processes. 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