Cofre De Perote Research Articles

A quaternary monogenetic volcanic field in the Xalapa region, eastern Trans-Mexican volcanic belt: Geology, distribution and morphology of the volcanic vents

The most abundant volcanic manifestations along the east–west trending Trans-Mexican Volcanic Belt (TMVB) are the scoria cones. These have been grouped by other authors in extended monogenetic volcanic fields such as Michoacán-Guanajuato, Chichinautzin, Apan and Los Tuxtlas. Here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the Cofre de Perote volcano (CP), around the city of Xalapa in the state of Veracruz, Mexico. Within an area of about 2400 km 2, the “Xalapa Monogenetic Volcanic Field” (XMVF) contains over 50 late Quaternary volcanoes. Most of them are scoria cones, but small shield volcanoes and tuff rings also occur. The lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of CP, between 3000 and 500 m a.s.l. The representative rocks of the different volcanic centers include olivine basalt, basaltic andesite with phenocrysts of plagioclase, pyroxene and minor olivine, and andesite with phenocrysts of plagioclase and pyroxene. SiO 2 and Al 2O 3 contents of the rocks vary between 45 and 62 wt% and 15 to 18 wt%, respectively. Most of the basaltic rocks have MgO contents between 4.2 and 9 wt%, Ni and Cr concentrations between 23 and 180 and 10 to 380 ppm, respectively, with a typical calc-alkaline behavior. Trace elements suggest two types of magmas; the most abundant are characterized by an enrichment of LILE and LREE with negative anomalies of Nb and Ti, which denote a calc-alkaline affinity. Others are LILE depleted and show high concentrations of MgO, Cr, and Ni, which is typical of primary calc-alkaline magmas. The mean scoria cone morphological values are: cone height (Hco) = 90.8 m, cone diameter (Wco) = 686.38 m, crater diameter (Wcr) = 208.49 m and 0.12 km 3 for the cone volume. We dated twelve different scoria cones using the 40Ar/ 39Ar method; for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter (Hco/Wco) and maximum slope angle. Additionally, vegetation type, and cone and lava flow morphologies were also considered in order to determine rough relative ages. In general, the ages fall into three groups, those older than 2.0 Ma, those between 0.25–0.40 Ma and those less than approximately 0.1 Ma.

BIOGEOGRAPHICAL ANALYSIS OF SCARABAEINAE AND GEOTRUPINAE ALONG A TRANSECT IN CENTRAL MEXICO (COLEOPTERA, SCARABAEOIDEA)

Scarabaeinae and Geotrupinae (Coleoptera, Scarabaeoidea) species composition is analyzed along a 150 km long altitudinal transect that runs S-NE in the Mexican Transition zone. The transect is located in the state of Hidalgo in central-eastern Mexico. The spatial unit of analysis is the landscape. The transect crosses five different landscapes. As terms of reference for studying the geographic distribution of the species, the entomofauna distribution patterns for the Mexican Transition zone were used. The transect includes all the patterns established by Halffter for this zone. Only genera with northern origins were found in landscape of the Pachuca Sierra (mountain range). The two landscapes of the High Plateau (temperate and arid) have one genus with a northern origin (<em>Onthophagus</em>), along with species belonging to genera with Neotropical origins that evolved on the High Plateau. For the landscapes of the zacualtipán Sierra and the slope down to the Gulf–Las Huastecas region genera of Neotropical affinity dominate, and there are also some species with a tropical distribution and of northern-Old World origin. The relationship between the mountains and the phyletic lineages or genera of northern origin and of recent entry into the Mexican Transition zone is confirmed, as is that between the tropical lowlands and the Neotropical lines or genera, also recent arrivals. Taxa that arrived a long time ago, of either origin, do not exhibit this geographic-ecological dependence. The Hidalgo Transect is compared with two other, similar transects sampled in the Mexican Transition zone: the Cofre de Perote–Gulf Coast transect (Veracruz) and that of Manantlán (Jalisco). In the mountain landscapes, High Plateau and Tropical Lowlands, there were no important differences in the species composition of the groups studied. In contrast, in the Transition landscape (zacualtipán in the Hidalgo Transect) there were very notable differences. In the Cofre de Perote transect, an important functional group is missing from the treeless habitats: the roller Scarabaeinae. For the same landscape, in Manantlán, lineages with Neotropical affinities are represented by a single species which completely dominates the beetles of northern affinities. This contrasts markedly with the Hidalgo and Cofre de Perote transects where, in the Transition landscape, Neotropical taxa are well represented. It appears that, unlike the tropical lowlands where (geologically recent) penetration of Neotropical taxa is massive in all three transects, in the transition landscapes (originally covered by cloud forest) the penetration of Neotropical taxa is highly variable, and depends on the mountain range in which they are found. In the Conclusions section, we analyze how the beetle fauna with different distribution patterns have contributed to the composition of the fauna of the Hidalgo Transect and in general that of the Mexican Transition zone, resulting in a mixture (genera with northern-Old World affinity, and genera with Neotropical affinities) that give the Mexican Transition zone its unique character.

A new species of Dinothenarus (Coleoptera: Staphylinidae) from Mexico

Dinothenarus (s. str.) amisadaiae Santiago-Jiménez, sp. n. is described based on specimens from Cofre de Perote Volcano, Veracruz. The habitus and aedeagus are illustrated and distributional and biological data provided. A key to species of Dinothenarus (s. str.) of America is also provided. The discovery of this species in the high mountains of central Mexico represents a significant range extension for the genus Dinothenarus, previously known in Mexico only from Baja California, and especially for the subgenus Dinothenarus (s. str.), previously known only from northern North America and Eurasia.

Mechanical instability quantification of slopes at Cofre de Perote volcano, eastern Mexico

Cofre de Perote (CP) volcano is located at the eastern end of the Trans-Mexican Volcanic Belt (TMVB) at 19°30’ Lat N, 97°10’ Long W. At a height of 4,282 m.a.s.l, it comprises one of the most massive structures within the Citlaltepetl – Cofre de Perote volcanic range (CCPVR), which constitutes an important physiographic barrier that separates the central altiplano, also known as Serdan Oriental, from the coastal plains of the Gulf of Mexico. This massive structure has repeatedly collapsed, and at least two of the collapse events occurred long after activity ceased, suggesting that even extinct volcanoes may pose an important hazard to nearby populated areas. In the present work, volcanic instability is approached through both quantitative and descriptive methods that include combined numerical analysis of limit equilibrium, calculated with Bishop’s modified method, and finite element analysis. The combined techniques were applied after attaining mechanical parameters in the laboratory, the field and through careful geological observations in order to obtain a model that approaches structural conditions prior to the destruction of the volcano Reconstruction of the ancient volcano was used to propose an instability model for the modern summit, where maximum stress/ strain relationships before failure were determined for the reconstructed section and actual volcano summit. Results obtained from these models indicate that the volcanic edifice is still unstable, could experience failure in the near future, and should be considered for future hazard assessment.

Ecological and Reproductive characteristics of Geotrupes(Halffterius)rufoclavatusJekel1865 (Coleoptera: Geotrupidae: Geotrupinae) on the Cofre de Perote Volcano (Veracruz, Mexico)

Abstract We investigated the dung beetle Geotrupes (Halffterius) rufoclavatus Jekel on the eastern slope of the Cofre de Perote Volcano, Veracruz, Mexico, with monthly trapping samples taken during the years 1999 to 2001. Monthly beetle abundances were related to habitat and collection period. The distributions of geotrupinae species on the Cofre de Perote are mapped, providing new data on geographical distribution in the State of Veracruz. The highest quantities of G. rufoclavatus were collected in the forest habitat. Above 3,000 m, more adults were collected in pastures and cultivated fields, showing a single activity peak per year, whereas at lower altitudes, there were lower numbers in those habitats, but several activity peaks per year. In Tonalaco, Veracruz, monthly samples were taken during 2005–2006 to assess the reproductive state of the population. Male and female reproductive systems are similar to those of other geotrupine species. All adults collected were in a maturing state; one female coll...

Exploring the effect of changes in land use on soil quality on the eastern slope of the Cofre de Perote Volcano (Mexico)

Changes in land use on the eastern slope of the Cofre de Perote Volcano (Mexico) appear to have a negative effect on soil quality. In this study, we use a multivariate data set to research whether the change in land use has affected soil quality and to identify the indicators that best represent variability of the original data set. An elevation transect was identified as follows: upper segment (US), middle segment (MS) and lower segment (LS). The following nondisturbed areas and farm fields were sampled: US: pinus forest (PF), corn cropland (CP), and abandoned cropland (AC); MS: tropical cloud forest (TF), corn cropland (CC), and grassland (GL); LS: oak forest (OF) and sugarcane (SG). Sixteen soil chemical, physical, and biological attributes were measured on each site of interest. It was shown that the change in land use caused a reduction in organic material content, especially on MS and LS. The highest acidity was recorded in nondisturbed soils and abandoned cropland. Microbial biomass- C ( C m) and the microbial quotient ( C m/ C) were altered the most on MS and LS. C m and mineralized N (Nm) decreased from US to LS. Bulk density (BD) increased with the change in land use, especially on LS. Principal component analysis was used to analyze soil quality overall. The first principal component (PC1) explained 46% of the total variance of the data set, and seven soil attributes had significant loadings. C, N, and total porosity (TP) were negatively weighted and were contrasted with Mg, ECEC, BD, and C m/ C. The second principal component (PC2) explained 16% and had significant positive loadings on Ca, inorganic nitrogen (Ni), and C m. LS soils had the highest PC1 scores and US soils the lowest. The positively high PC1 scores recorded for LS soils revealed a greater sensitivity to changes in land use. In US, there were no significant effects on PC1 caused by land use change, while in the MS, positively high PC1 scores obtained in the CC soil were related to greater microbial activity and a decrease in C and N. In LS, SG soil had significantly higher PC1 scores than OF soil, indicating an increase in microbial activity.

Diversity of Carabidae (Insecta, Coleoptera) in Epiphytic Bromeliaceae in Central Veracruz, Mexico

This paper documents the existence of carabid assemblages associated with bromeliads on the Cofre de Perote, Veracruz, Mexico. Based on bromeliads sampled over three altitudinal ranges, the assemblages included at least 26 species with an arboreal lifestyle and another 11 species that are not strictly arboreal. Seven species are new to science, urging us to pay attention to the arboreal fauna in forest conservation studies. Composition of carabid assemblages associated with bromeliads changes with altitude. In lowlands, it is comprised almost entirely of species of Lebiini, with the Platynini dominating assemblages found in bromeliads >1,000 m above sea level. Our data suggest that carabids use bromeliads to reduce stresses associated with drought periods, the exact timing of which depends on altitude. The unexpected low diversity of the carabid fauna associated with bromeliads at middle altitude is explained in terms of anthropogenic conversion of the original forest to pastureland. Given the importance of arboreal elements, further fragmentation of subtropical and tropical mountain forest significantly threatens overall carabid diversity.

From forest to pasture: an evaluation of the influence of environment and biogeography on the structure of beetle (Scarabaeinae) assemblages along three altitudinal gradients in the Neotropical region

The objective of this study is to evaluate the effect of environmental (associated with the expansion of cattle ranching) and biogeographical factors on the diversity of dung beetle (Scarabaeinae) assemblages along three altitudinal gradients in the Neotropical region. One gradient is located in the Mexican Transition Zone, on the Cofre de Perote mountain, the other two are in the northern Andes (the Chiles Volcano and the Río Cusiana Basin). For the three gradients, the number of species and of individuals was similar in both forest and pasture, while species composition was different between habitats. On this mountain, species turnover in pastures was characterized by the addition of new species as altitude increased. In the northern Andes, species diversity was always greater in the forest than in the pasture, and species turnover between habitats was notably influenced by species loss with increasing altitude. As such the pasture fauna of the northern Andes was an impoverished derivative of the fauna present in the forests at the same altitude characterized by species of Neotropical affinity with a limited capacity for colonizing open, sunnier habitats. The opposite occurs in the areas used by cattle on the Cofre de Perote. This habitat has its own fauna, which is mainly comprised of Holarctic and Afrotropical species adapted to the prevailing environmental conditions of areas lacking arboreal vegetation. These results suggest that the impact on beetle communities caused by human activities can differ depending on the geographic position of each mountain and, particularly, the biogeographical history of the species assemblage that lives there.

Response of soil surface CO 2–C flux to land use changes in a tropical cloud forest (Mexico)

It is well known that land use changes contribute to increased atmospheric CO 2 concentrations. In the middle segment of the eastern slope of the Cofre de Perote Volcano (Mexico), many areas of tropical cloud forest have been converted to grassland and corn cropland. I investigated the response of soil surface CO 2–C flux to land use change over a 1.2-year period (May 2003–June 2004). To do this, soil surface CO 2–C flux was measured monthly in a tropical cloud forest, a corn–potato–corn rotation plot and a grassland using the static chamber method. Average CO 2 flux varied from 0.54 to 2.21 g C m −2 day −1 in the tropical cloud forest, 0.43 to 3.07 g C m −2 day −1 in the corn–potato–corn rotation and 1.51 to 4.87 g C m −2 day −1 in the grassland. Soil surface CO 2–C flux increased significantly with the change from tropical cloud forest to managed ecosystems. The highest CO 2 flux occurred in summer, with peaks in September 2003 for tropical cloud forest and grassland sites and in July 2003 and May 2004 for corn–potato–corn rotation. In autumn, soil surface CO 2–C flux started to decrease, with the lowest flux occurring in winter. For the entire data set, there was a weak positive correlation between soil surface CO 2–C flux and soil temperature ( R 2 = 0.12, P < 0.0001) and a weak negative correlation between soil surface CO 2–C flux and air-filled pore space ( R 2 = 0.17, P < 0.0001). The results indicate that tropical cloud forest, corn–potato–corn rotation, and grassland growing under the same climatic and soil conditions have different soil surface CO 2–C flux rates.

Multiple edifice-collapse events in the Eastern Mexican Volcanic Belt: The role of sloping substrate and implications for hazard assessment

The Citlaltépetl–Cofre de Perote volcanic chain forms an important physiographic barrier that separates the Central Altiplano (2500 masl) from the Gulf Coastal Plain (GCP) (1300 masl). The abrupt eastward drop in relief between these provinces gives rise to unstable conditions and consequent gravitational collapse of large volcanic edifices built at the edge of the Altiplano. Eastward sloping substrate, caused by the irregular configuration of the basement rocks, is the dominant factor that controls the direction of collapsing sectors in all major volcanoes in the region to be preferentially towards the GCP. These collapses produced voluminous debris avalanches and lahars that inundated the well-developed drainages and clastic aprons that characterize the Coastal Plain. Large catastrophic collapses from Citlaltépetl, Las Cumbres, and Cofre de Perote volcanoes are well documented in the geologic record. Some of the avalanches and transformed flows have exceptionally long runouts and reach the Gulf of Mexico traveling more than 120 km from their source. So far, no direct evidence has been found for magmatic activity associated with the initiation of these catastrophic flank-collapses. Apparently, instability of the volcanic edifices has been strongly favored by very intense hydrothermal alteration, abrupt topographic change, and intense fracturing. In addition to the eastward slope of the substrate, the reactivation of pre-volcanic basement structures during the Late Tertiary, and the E–W to ENE–SSW oriented regional stress regimes may have played an important role in the preferential movement direction of the avalanches and flows. In addition to magmatic-hydrothermal processes, high amounts of rainfall in the area is another factor that enhances alteration and eventually weakens the rocks. It is very likely that seismic activity may be the principal triggering mechanism that caused the flank collapse of large volcanic edifices in the Eastern Mexican Volcanic Belt. However, critical pore water pressure from extraordinary amounts of rainfall associated with hurricanes or other meteorological perturbation cannot be ruled out, particularly for smaller volume collapses. There are examples in the area of small seismogenic debris flows that have occurred in historical times, showing that these processes are not uncommon. Assessing the stability conditions of major volcanic edifices that have experienced catastrophic sector collapses is crucial for forecasting future events. This is particularly true for the Eastern Mexican Volcanic Belt, where in many cases no magmatic activity was associated with the collapse. Therefore, edifice failure could occur again without any precursory warning.

GEOGRAPHIC DISTRIBUTION, TAXONOMY, AND CONSERVATION STATUS OF NELSON'S WOODRAT (NEOTOMA NELSONI) IN MEXICO

Abstract We examined a specimen of Neotoma nelsoni collected in the Mexican state of Puebla, which represents the third known record for the species and the first one for the state. The distribution of the species is restricted to the eastern slopes of the volcanoes Pico de Orizaba and Cofre de Perote, mainly within the cloud forest. The specimen of N. nelsoni has external and cranial characteristics that are different from other species of the genus. The distribution of the species is small, the sizes of populations are small, and the populations exist in geographic isolation. None are located within legally protected areas, which presents problems for the conservation of the species.

Geomorphological evidence of the influence of pre-volcanic basement structure on emplacement and deformation of volcanic edifices at the Cofre de Perote–Pico de Orizaba chain and implications for avalanche generation

Pre-volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices. Therefore, systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa. Based on a compilation of pre-existing data, interpretation of aerial photographs and satellite images, and a collection of structural data we analyzed morphological and structural features of the Cofre de Perote–Pico de Orizaba (CP–PO) volcanic chain and its basement. We have identified three sets of regional lineaments that are related to basement trends. (1) NW 55° SE fractures are parallel to anticline folds observed in Cretaceous rocks that originated during Laramide shortening. These folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain. (2) NE 55° SW fractures are parallel to normal faults at the basement. We infer that these basement faults confine the CP–PO chain within a stepped graben with a total normal displacement of about 400 m. These faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents. Notably, lineaments of monogenetic vents concentrate where the basement is relatively shallow. (3) Another set of faults, oriented N–S, has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits. Lineaments measured in the volcanic edifice of Pico de Orizaba correlate with the regional trends. In particular, the NE 55° SW alignment of monogenetic vents and fractures at Pico de Orizaba suggest that the same dike trend exists within the volcanic edifice. A normal fault with similar orientation was documented at the NE continuation of an alignment crossing the volcanic edifice along the Jamapa canyon. In the absence of magmatic activity related to collapses, the displacement of NE 55° SW faults represents a potential triggering mechanism for generating avalanches at Pico de Orizaba volcano. Instability is enhanced by the presence of N–S trending fractures crossing the entire volcanic edifice and E–W fractures affecting only the present day cone. We conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity, but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation.

Edible Wild Mushrooms of the Cofre De Perote Region, Veracruz, Mexico: An Ethnomycological Study of Common Names and Uses

A field study in the Cofre de Perote region found that edible mushrooms play an important role in the socio-economics activities of the local population. Several very old common names indicate a long-standing traditional knowledge about this regional resource. Recent changes in marketing mushrooms are evident.

Climacium dendroides from Cofre de Perote, A High-Elevation Tropical Montane Site in Veracruz, Mexico

Climacium dendroides (Hedw.) F. Weber & D. Mohr is reported new to Mexico and in the first known locality in the tropics, at the elevation of 3,000 m, on the ancient shield volcano of Cofre de Perote. Variation in branching of the costa is assessed in this plant and in material of C. dendroides worldwide. Additional taxa of interest from this locality on Cofre de Perote include Aulacomnium palustre and Polytrichum commune (both new to the state of Veracruz).

Effects of subsistence farming system on soil surface CO 2–C flux on Cofre de Perote volcano slopes, Veracruz (Mexico)

Mountainous parts of the eastern slope of the Mexican Cofre de Perote volcano have suffered great ecological disturbance due to the conversion of forest to subsistence corn ( Zea mays) cropland. This research was conducted in order to evaluate the response of surface soil CO 2–C flux to land use modification, specifically the adoption of subsistence farming in this mountainous forest–cropland setting. The treatments consisted of a corn plot (CP), recently abandoned cropland (RA), old abandoned cropland (OA), and coniferous ( Pinus species) forestland (CF). For an 8-month period (August 1999–June 2000), CO 2–C flux concentration was measured monthly using the static chamber method. CO 2–C flux ranged from 1.80 to 5.22 g C m −2/day in the CF treatment, 2.76–8.45 g C m −2/day in the CP treatment, 3.24–7.48 g C m −2/day in the RA treatment, to 2.99–8.84 g C m −2/day in the OA treatment. Results showed that the conversion of coniferous forest to corn cropland leads to a significant increase in CO 2–C flux concentration from the soil surface to the atmosphere. There were few differences between current and former agricultural plots, demonstrating that crop abandonment did not lower CO 2–C flux from the soil’s surface. Soil temperature and moisture content were poorly correlated with CO 2–C flux; however, soil moisture data did show greater scattering. This indicates that soil temperature had a major effect on CO 2–C flux concentration. To offset some of the organic carbon depletion from soil caused by land use change, it is recommended that the subsistence farming system subject corn crop residue to a composting process. This can improve short-term soil fertility and increase corn crop yield, playing a central role in converting abandoned cropland into permanent cultures that facilitate forest conservation.

Late-Pleistocene to precolumbian behind-the-arc mafic volcanism in the eastern Mexican Volcanic Belt; implications for future hazards

An area of widespread alkaline-to-subalkaline volcanism lies at the northern end of the Cofre de Perote–Citlaltépetl (Pico de Orizaba) volcanic chain in the eastern Mexican Volcanic Belt (MVB). Two principal areas were active. About a dozen latest-Pleistocene to precolumbian vents form the 11-km-wide, E–W-trending Cofre de Perote vent cluster (CPVC) at 2300–2800 m elevation on the flank of the largely Pleistocene Cofre de Perote shield volcano and produced an extensive lava field that covers >100 km 2. More widely dispersed vents form the Naolinco volcanic field (NVF) in the Sierra de Chiconquiaco north of the city of Jalapa (Xalapa). Three generations of flows are delineated by cone and lava-flow morphology, degree of vegetation and cultivation, and radiocarbon dating. The flows lie in the behind-the-arc portion of the northeastern part of the MVB and show major- and trace-element chemical patterns transitional between intraplate and subduction zone environments. Flows of the oldest group originated from La Joya cinder cone (radiocarbon ages ∼42 000 yr BP) at the eastern end of the CPVC. This cone fed an olivine-basaltic flow field of ∼20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of Jalapa, the capital city of the state of Veracruz. The Central Cone Group (CCG), of intermediate age, consists of four morphologically youthful cinder cones and associated vents that were the source of a lava field>27 km 2 of late-Pleistocene or Holocene age. The youngest group includes the westernmost flow, from Cerro Colorado, and a lava flow ∼2980 BP from the Rincón de Chapultepec scoria cone of the NVF. The latest eruption, from the compound El Volcancillo scoria cone, occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in México and resemble paired aa–pahoehoe flows from Mauna Loa volcano. The El Volcancillo eruption initially produced the high effusion rate, short-duration Toxtlacuaya alkaline aa lava flow from the southeastern crater. This 12-km-long hawaiite (average 50.5% SiO 2) flow was followed by extrusion of the calc-alkaline Rı́o Naolinco lava flow from the northwestern crater. This large-volume (∼1.3 km 3) tube-fed basaltic pahoehoe flow (average SiO 2 49%) traveled 50 km. Inferred effusion rates suggest emplacement over a decade-long period. Flows of all three age groups are transected by Highway 140 and the railway that form major transportation arteries between Jalapa and Puebla. This area has not previously been considered to be at volcanic risk, but volcanism here has continued into precolumbian time. Future eruptions of similar magnitude and location to those documented here could pose significant hazards to transportation corridors and to densely populated areas in and to the north of Jalapa. Slight variations in vent locations could produce future flows down one or more of more than a half dozen drainages with widely varying population densities.

The Quetzalapa Pumice: a voluminous late Pleistocene rhyolite deposit in the eastern Trans-Mexican Volcanic Belt

The study area is located in the east part of the Trans-Mexican Volcanic Belt, in the Las Cumbres Volcanic Complex (LCVC) which lies between two large stratovolcanoes: Pico de Orizaba (5700 m a.s.l.) to the south, and Cofre de Perote (4200 m a.s.l.) to the NNE. The most conspicuous structure of the LCVC is a 4-km-diameter circular crater with a dacitic dome in the center, which constitutes the remains of a destroyed stratovolcano. The Quetzalapa Pumice (QP) was produced by a plinian eruption that was dated by the 14C method at 20 000 yr. BP. The eruptive sequence consists predominantly of pumice fall deposits and scarce intra-plinian pyroclastic flow deposits, which crop out on the west flank of the LCVC. The absence of post-plinian ignimbrite deposits is striking. The deposits are well sorted, clast-supported with reverse grading at the base, with a medium to high accessory lithics content. The maximum average thickness of the deposit in the proximal areas is about 15 m and has been divided into three members: the Basal Member (BM), 2 m thick with four submembers (BMf 1, BMf 2, BMf 3, and BMafl), the Intermediate Member (IM), 10 m thick with two submembers (IMpf and IMaf), and the Upper Member (UM), 3 m thick with four submembers (UMpl, UMsdf, UMwaf, and UMpls). The predominant component of the fall deposits is a white, highly vesiculated pumice with 71% SiO 2 content. Plagioclase is the most abundant mineral followed by 1–3-mm-long biotite phenocrysts. The accessory lithics are lavas mostly of andesitic composition. Their abundance increases toward the uppermost levels of the sequence. We calculate a minimum volume of 8.4 km 3 (2.22 km 3 dense rock equivalent), for the entire QP deposit. Isopach and isopleth maps show that the IM deposit has an elongated distribution with a NNE–SSW direction, whereas the UM deposit has a circular distribution. We estimate a maximum eruptive column height for the IM of 20 km. Field studies and isopach and isopleth maps indicate that the eruptive column was affected by a strong wind. Previous studies located the QP source in the Las Cumbres crater. However, based on the isopach and isopleth distribution, and the lack of pumice fall deposits inside the Las Cumbres crater, we consider that the QP emission center is located on the west flank of the LCVC, and was buried by its own pumice fall deposits. It coincides with an explosion crater called La Capilla formed during the closing phase of the QP eruption. A ‘pumice fountain’ model is proposed to explain the observed sequence of deposits. According to this model, the material was emitted through a ‘hose-type’ conduit during a monogenetic eruption of rhyolitic composition. This kind of volcanic activity is not extensively reported in the literature.

Andosol-forming process linked with soil fauna under the perennial grass Mulhembergia macroura

On the Cofre de Perote volcano, Mexico, at an elevation of 3000 m, vegetation is dominated by pine trees and Mulhembergia macroura grass. The grass undergoes a specific decomposition process whereby dead leaves and roots at different stages of decomposition accumulate below the plant, making a mat and forming a hummock of soil. Soil thin sections were prepared from the plant necromass and underlying soil to examine biological features by optical microscopy. Small organic fragments and soil aggregates were observed under a scanning electron microscope, coupled with a microprobe. C and N contents were measured, soil samples were analyzed by infrared spectroscopy and preliminary quantitative sampling of meso and macrofauna was executed in the plant residues and the underlying soil strata. Observations and analyses showed that soil microaggregates were faecal pellets from fauna, mainly Enchytraeids and Acari, living in the rooted soil below the grass. These biological aggregates exhibited a concentric internal structure and a silicon-rich coating which presumably protects them against microbial decomposition. Organic matter showed a predominance of aliphatic components upon aromatic components and appeared to be stable within the whole soil profile. As a consequence of plant die-off and mesofaunal activity, organic products accumulate and soil thickness and carbon storage increase with time.

Keywords

In this study, dendrogeomorphic techniques are employed to analyse the temporal frequency and spatial distribution of rockfalls on a talus slope of La Teta valley, located on the NW slopes of Cofre de Perote volcano at ~ 4000 m above sea level. Based on the interpretation of disturbance signals in growth rings of old-growth Pinus hartwegii Lindl. trees, we identify 100 growth disturbances related with rockfall events dated between 1780 and 2011, with slightly more than half of these events being dated to the last 50 years. The sectors most susceptible to rockfall correspond with the young rock lobes located at the foot of scarps. Roughly three in ten events has been triggered by regional, M > 6 earthquakes, whereas half of the events activity coincides with periods characterized by severe, prolonged summer rainfalls such as the ones occurred in 1995, 1998, 2005 and 2011.

An Unusual New Species of Chusquea (Poaceae: Bambusoideae) from Mexico

Chusquea perotensis is described from Volcdn Cofre de Perote. Although its spikelets are typical of Chusquea, this species is anomalous in having the internodes hollow with well defined cavities. Based on molecular and morphological data, C. perotensis is classified within Chusquea sect. Swallenochloa, and is considered a sister species of C. bilimekii, also from Mexico. Both species are rare and probably threatened in their fragile montane habitats. Keys, based on vegetative only and vegetative and flowering characters, to the species of Chusqutea sect. Swallenochloa in Central America and Mexico are provided.