We assembled a petrological and geochemical database for México's Quaternary volcanic rocks as one component of an interactive CD-ROM titled Volcanoes of México. That original database was augmented to a total of 2180 records for whole-rock analyses published through May 2004 in peer-reviewed literature, supplemented by a few Ph.D. dissertations for otherwise uncovered areas. The Quaternary volcanic rocks of México can be divided geographically into three tectonic settings: the Northern Mexican Extensional Province, Pacific islands, and the Mexican Volcanic Belt. The rocks also largely fall into three magma series: (1) intraplate-type alkaline, (2) calc-alkaline, and (3) lamprophyre. Many transitional varieties also exist, but we have established compositional rules to classify all samples into these three series. Intraplate-type alkaline rocks account for 30.8% of the database. Mafic intra-plate-type rocks are particularly abundant at Northern Mexican Extensional Province and Pacific island volcanoes. They are characterized by strong enrichments in Ti-Ta-Nb, and many have nepheline in their CIPW norms (named for the four petrologists, Cross, Iddings, Pirsson and Washington, who devised it in 1931) and carry xenoliths of deep-crustal granulite and upper-mantle spinel and/or plagioclase peridotite. Available data indicate that significant compositional differences exist between intraplate-type mafic rocks from these two tectonic environments, with the Pacific island examples relatively depleted in Cs, Rb, Th, U, K, Pb, and Sr compared to Northern Mexican Extensional Province equivalents. Mafic intraplate-type rocks from the Camargo and San Quintín fields in the northern part of the Northern Mexican Extensional Province are relatively enriched in 206 Pb/ 204 Pb (19.1–19.6), indicating likely involvement of HIMU (high µ) mantle in their genesis. Differentiated intraplate-type rocks (trachytes) are common at the Pacific island volcanoes, but nearly absent at the Northern Mexican Extensional Province volcanoes. Intraplate-type mafic alkaline rocks are also found in many different parts of the Mexican Volcanic Belt; we believe that the latter occurrences reflect involvement of Northern Mexican Extensional Province–type mantle sources in magma generation beneath the Mexican Volcanic Belt, where subduction-modified mantle is the dominant source feeding calc-alkaline and minor lamprophyric magmas to the surface. The calc-alkaline series, which accounts for 62.5% of the database, ranges from basalts (and lesser trachybasalts) to rhyolites but is dominated by andesites. These rocks are most prevalent in the E-W–trending, subduction-related Mexican Volcanic Belt, but are also found in Baja California, part of the Northern Mexican Extensional Province. They are characterized by enrichments in K-Ba-Sr and depletions in Ti-Ta-Nb, the classic global-scale features of subduction-related rocks. About 8.3% of the rocks from the Mexican Volcanic Belt have corundum in their CIPW norms, evidence of a significant role for sediment involvement in their petrogenesis, through either sub-duction of seafloor clays or contamination by pelitic lithologies during ascent through the crust. Sr and Nd isotopic data for calc-alkaline rocks from the Mexican Volcanic Belt form an array that is shifted toward higher 87 Sr/ 86 Sr compared to the intraplate-type suites, consistent with incorporation of subducted marine Sr. Calc-alkaline and lamprophyric rocks from Colima volcano and nearby Cántaro mark the depleted end of the Mexican Volcanic Belt isotopic array (lowest 87 Sr/ 86 Sr and 206 Pb/ 204 Pb, highest ϵ Nd ); the enriched end is marked by various basaltic andesites to rhyolites from the east-central part of the Mexican Volcanic Belt, where México's continental crust reaches its maximum thickness of 40–50 km, a fact that favors crustal contamination during magma ascent. Lamprophyres account for only 6.7% of the database. True lamprophyres, with phlogopite or amphibole phenocrysts in the absence of feldspar phenocrysts, are found exclusively in the western part of the Mexican Volcanic Belt, but compositionally (not mineralogically) similar rocks are found in four volcanic fields in northern Baja California, where they have been termed bajaites, and likened to adakites. Lamprophyres have extreme subduction-related geochemical signatures, with strong enrichments in K-Ba-Sr, and equally strong relative depletions in Ti-Ta-Nb. We consider western Mexican lamprophyres to represent the “essence of subduction,” partial melts of phlogopite- and apatite-rich veinlets in the subarc mantle, which are usually diluted by partial melts of the surrounding depleted peridotitic wall rocks to produce “normal” calc-alkaline magmas. Lamprophyres reached the surface in the western Mexican Volcanic Belt relatively undiluted by wall-rock melts only because of the strong extension imposed on the region by the influence of nearby plate-boundary activity.

Colima volcano is situated at the western edge of the Mexican volcanic belt within the Colima rift zone. This contribution presents new geochemical and Sr-Nd-Pb-O isotope data for Colima volcano rocks and plutonic xenoliths found in prehistorical lava flows. Colima volcano magmas display strong subduction signatures (positive peaks of Ba, K, Pb, and Sr, and negative anomalies of Nb and Ti) and were generated in a depleted mantle source and emplaced at crustal levels (garnet-free zone), where they experienced fractional crystallization of plagioclase and pyroxene. Gabbroic and granitoid xenoliths found in prehistorical lava flows show evidence for partial melting and are considered to be representative of the basement beneath Colima volcano. At upper-crustal levels, Colima volcano magmas were contaminated by granitoids, like those of the nearby Cretaceous Manzanillo and Jilotlán Batholiths. Sr-Nd isotope ratios of these intrusives are nearly identical to those of Colima volcano lavas. For that reason assimilation of the granitic crust is not detectable in diagrams of these isotopic systems but can be clearly seen in a ϵ Nd versus δ 18 O plot. In comparison to other large Mexican volcanic belt stratovolcanoes, Colima volcano lavas display the least evolved geochemical and isotopic signatures of this arc.

This study focuses on two issues that are still a matter of debate in subduction zones, particularly in western México: (1) the close association within the same volcanic complex of typical amphibole-free andesites to rhyolites and amphibole-bearing andesites to rhyolites, characteristic of the hydrated front of the Mexican arc; and (2) the occurrence of bimodal magmatism without evidence for interaction between mafic and intermediate to silicic magmas, which are in addition characterized by different petrogenetic affinities. Our case study is the San Pedro–Cerro Grande volcanic complex, a Quaternary silicic to intermediate dome complex located in western Mexico. Volcanic activity has been divided into two periods. In the middle Pleistocene, andesitic to dacitic magmas were emplaced along WNW-trending faults in the southern portion of the complex. The Las Cuevas pyroclastic sequence (older than ca. 500 ka) was emplaced during this episode, most likely from a local source. This first period of activity ended before ca. 280 ka with the emplacement of the Cuastecomate Plinian deposit, which is related to the formation of the San Pedro caldera, an ∼4-km-wide subcircular depression that is today partially buried by younger volcanic products. During the second period of activity (ca. 280–30 ka), rhyolitic and dacitic domes were mostly emplaced along the caldera rim and inside the caldera. In addition, hawaiites and mugearites built the Amado Nervo shield volcano on the caldera rim. Intermediate- to high-silica lava and pyroclastic rocks are subalkaline, whereas the Amado Nervo mafic lavas are transitional toward the alkaline series (Na-alkaline). No genetic relationships have been found between subalkaline and transitional Na-alkaline rocks, which are thought to represent different batches of magma from different mantle sources. Petrographic, geochemical, and isotopic variations observed in the transitional Na-alkaline Amado Nervo lavas point to a parental magma from a mantle melt that underwent limited olivine separation during its ascent to the surface. Among subalkaline rocks, two groups showing contrasting petrographical and geochemical features are recognized based on the presence of amphibole. Amphibole-bearing intermediate to silicic rocks are characterized by lower Ce and other incompatible trace element contents and lower 87 Sr/ 86 Sr (0.70382–0.70401) compared to amphibole-free rocks (0.70411–0.70424). On the basis of petrological characteristics, the two groups of magmas are interpreted to have evolved in two different magmatic reservoirs under different pressures and water contents in the mid-upper crust. Both groups of magmas were differentiated by open-system processes. We propose that assimilation and equilibrium crystallization (AEC) processes account for the amphibole-bearing rocks. Hotter and less evolved magmas interacted to a higher degree with the crust than the more evolved and colder magmas. This produced the observed higher 87 Sr/ 86 Sr in the less differentiated rocks of the amphibole-bearing group. On the other hand, amphibole-free rocks have chemical and isotopic characteristics that can be modeled by assimilation and fractional crystallization (AFC) processes. All data suggest that the two groups of subalkaline rocks have been generated by a common parental hydrous magma, but evolved in two different reservoirs. Amphibole-bearing magmas underwent amphibole fractionation in a mid-upper crustal reservoir and show assimilation of two types of basement: one akin to Oaxaquia and another akin to the Guerrero terrane. Amphibole-free magma only shows assimilation of an Oaxaquia-type basement.

Surface volcanic gases may reflect volatile budgets in magma and forecast impending eruptions, and their release to the atmosphere may affect climate. The dynamics of magma degassing is complicated, however, by differences in the solubility, partitioning, and diffusion of the various volatiles, all of which can vary with pressure, temperature, and melt composition. To constrain possible gas outputs, we carried out experiments to determine how Cl partitions between water bubbles and silicate melt, and decompression experiments to examine how Cl behaves during closed- and open-system degassing. We incorporated our findings and those from the literature for CO 2 and S into a steady, isothermal, and homogeneous flow model to estimate fluxes of gases at the vent from ascending water-rich magma, assuming different scenarios for the onset and development of permeability in bubbly magma. We find that, for given permeability scenarios, total gas fluxes vary with magma flux, but ratios of gas species do not change. The S/Cl and SO 2 /CO 2 ratios do change, however, depending on whether the magma is oxidized or reduced. After magma fragments into a Plinian eruption column, gases continue to escape from cooling pumice in the plume, but here the rate of gas release is controlled by diffusion, which varies with temperature. Degassing of pumice and ash was modeled by linking a steady-state plume model, which gives the vertical variation of mean temperature and velocity of particles inside the plume, to a conductive cooling model of pumices, which controls diffusion of Cl, CO 2 , and S in pumice. We find that gas loss increases with column height (mass flux) and initial temperature, because in both cases pumices cool over a longer time period, allowing more gas to diffuse out of the matrix glass. The amount of gas released also depends on the size distribution of particles in the erupting mixture, with less being released for a finely skewed distribution.

The evolution of Popocatépetl volcano was determined through the definition of rock units following morphostratigraphic criteria and detailed geological sections. The primitive volcano, named Nexpayantla, probably contemporaneous with Pies volcano, a part of the Iztaccíhuatl volcanic complex to the north, grew beneath the site of today's cone. This volcano produced mainly andesitic to dacitic lava flows, presented flank activity in the form of several large dacitic lava domes, and was intruded by dacitic to rhyolitic dikes. The evolution of Nexpayantla volcano finished with a large collapse to the south that produced the Lower Tlayecac avalanche deposit. A new cone, Ventorrillo volcano, was built on the remains of Nexpayantla and was formed mostly by andesitic lava flows, but did not present any recognizable flank activity. Ventorrillo volcano collapsed in a large Bezymianny-type eruption toward the southwest, producing the Upper Tlayecac avalanche deposit and the Tochimilco pumice. The Calpan fan was derived from collapse and eruptions of Pies volcano. The present-day cone grew through the emission of many andesitic to dacitic lava flows, which were grouped into eroded or covered lava slopes (Malpaís, Las Mesas, Metepec, and San Pedro Benito Juarez lava flows), and glaciated (Fraile lava flows) and nonglaciated (Las Cruces, Buenavista, Quimichule, Atlimiyaya, Chiquipixle, and Nealtican lava flows) lava slopes with marked features, both from the central vent and from flank eruptions, mainly to the northeast and southwest of the cone. The Ecatzingo and Ombligo-Xalipilcáyatl flank vents formed two well-defined lineaments. The relative ages of the lava flows were determined through morphology, stratigraphic relations, and tephra cover. Two stages of growth were separated by a large Plinian eruption, which emplaced the Black and White (B&W) and Pumice with Andesite (PWA) fall deposits, which were used as stratigraphic markers. Another twelve Plinian pumice deposits are interstratified with the lava flows. Four large volcaniclastic fans and five valley fill deposits form the volcano's piedmont, and have resulted from the successive emplacement of pyroclastic flows, lahars, and fluvial deposits along several gullies that mark the lower slopes of the volcano. Glacier melting coincident with several of the Plinian eruptions could have been responsible for some of the extensive lahar deposits.

The Valle de Bravo Volcanic Field is one of four monogenetic volcanic fields identified in the central sector of the Mexican Volcanic Belt. Michoacán-Guanajuato, Jilotepec, and Chichinautzin are the other three. The Valle de Bravo Volcanic Field is located at the southern front of the belt where it covers the western flank of the Pleistocene-Holocene Nevado de Toluca volcano. It covers an area of 3703 km 2 and includes at least 120 cinder cones, one shield volcano, a few lava domes, and two lava dome complexes. It overlies a rough paleotopography of Mesozoic metamorphic rocks (schists, metalimestones, and pillow lavas), Paleocene-Eocene granitic rocks, and Eocene-Oligocene ignimbrites. Based upon morphometric parameters that were calibrated with reported isotopic ages, four groups of cinder cones were identified, older than 40 ka, 40–25 ka, 25–10 ka, and younger than 10 ka. Lava domes occur sporadically as high domes, low domes, and coulees, with ages between Pliocene to Pleistocene. We also observed several mafic lava flows that lack a cone source, suggesting that they erupted from fissures. A geomorphologic analysis of the cinder cones indicates a relatively young age for most of them, since craters are still evident and flanks are little eroded. Many lava flows still show levees and some of them are little vegetated and lack soil, which is significant for this densely forested and humid area. An analysis of the distribution of the cinder cones shows that most vents follow a NE alignment. By contrast, the domes tend to be aligned in a NW trend. This suggests that emplacement of cinder cones follows the maximum horizontal compressional stress direction, parallel to the Cocos–North America plate convergence (NE), whereas the lava domes are better developed along the minimum horizontal stress direction, perpendicular to convergence (i.e., NW).

Approximately 21,700 yr B.P., after a period of quiescence of ∼4800 yr, Nevado de Toluca volcano erupted, producing the Lower Toluca Pumice deposit. The activity generated a 24-km-high Plinian column that lasted ∼11–13 h and dispersed 2.3 km 3 (0.8 km 3 dense rock equivalent) of tephra toward the NE, blanketing the Lerma basin, an area occupied today by the city of Toluca, with up to 5 cm of ash. Subsequent eruptive pulses were sub-Plinian in style, accompanied by phreatomagmatic explosions that emplaced surge deposits. Finally, the column collapsed toward the NE with the emplacement of a pumice flow deposit. The high vesicularity of the pumice from the basal Plinian layer, up to 83% by volume, indicates that exsolution was dominantly magmatic, and that pressurization of the magma chamber was probably due to a magma mixing process. Evidence for this includes the compositional range of juvenile products (61–65 wt% SiO 2 ), as well as the presence of two types of plagioclase, one in equilibrium and the other one with disequilibrium textures and reverse zoning. This suggests input of an andesitic liquid into the dacitic magma chamber. Based on the eruptive record, the most likely future eruptive activity at Nevado de Toluca volcano will be Plinian. Although quiet for more than 3250 yr, Plinian activity could occur after a long period of quiescence, and it could represent a hazard for the entire Toluca basin, where more than one million people live today.

Ignimbrites are deposits resulting from the eruption of volatile-rich, silicic magma. Few geochemical or petrological studies have been done concerning the Tertiary Ignimbrite Province of Central America in Honduras and Nicaragua. Previous work, using ages and geographical proximity, suggested that tephra layers recovered during Ocean Drilling Program (ODP) Leg 165 in the western Caribbean Sea were associated with deposits from explosive eruptions in Central America. A total of 112 marine tephra and 79 terrestrial samples from Nicaragua and Honduras were analyzed during this study. An electron microprobe was used for major oxides, and laser inductively coupled plasma–mass spectrometry (ICP-MS) was used for trace elements. The rare earth elements (REEs), due to their resistance to weathering, were used to correlate the marine tephra with the terrestrial samples. Cluster analysis resulted in the division of the samples into 6 geochemical groups. Visual inspection of these groups resulted in the reclassification of these 6 into 14 distinct geochemical groups. Each one of these geochemical groups displays unique REE trends relative to each other. The trends vary, relative to enriched mid-ocean-ridge basalt (EMORB), from strongly light (L) REE enriched with moderate to large, negative Eu anomalies, to nearly equal EMORB values without any Eu anomalies, to enriched REE with positive Eu anomalies. All but two groups consist of both marine and terrestrial samples. The geochemical correlation is strengthened using factor analysis, in which the REE values for each sample were reduced to two factors and replotted. Each group plots in a distinct field. The Lesser Antilles can be ruled out as contributing to the western Caribbean tephra due to the lack of any large-volume ignimbrite sheets within the arc, as well as the distinctly different REE trends of the magmas. The Sierra Madre Igneous Province in México is also ruled out as a potential source, due mainly to a significant age difference between Sierra Madre ignimbrites and the Caribbean Sea tephra layers. Besides the strong similarities in REE patterns, several of the groups and subgroups are geographically limited in extent, which might imply a specific group source location.

Volcanic explosions expel fragments following ballistic trajectories. The volcanic ballistic projectiles represent a hazard due to their high velocities and temperatures. They may affect people, ecology, infrastructure, and aircraft. In order to avoid volcanic ballistic projectile-related hazards, a map can be constructed. A volcanic ballistic projectiles hazard map depicts the likely distribution and maximum range of ballistic projectiles under given explosive scenarios. Different level hazard zones shown on the map allow local inhabitants and concerned authorities to make development, protection, and mitigation plans, and to define restricted areas. In order to determine the potential areas where the ballistics may fall, it is necessary to estimate their maximum range under different explosive scenarios. Explosive magnitude scenarios are defined by their characteristic kinetic energy. Therefore, the ballistic projectiles reach maximum distance from the source according to the maximum energy for each scenario. The trajectories described for the ballistic projectiles are determined by gravity and drag forces. Drag force depends, among other factors, on the drag coefficient (a function of the geometry of the ballistics). The maximum range of the projectiles depends also on the initial kinetic energy, the “launching” angle, the ballistic diameter, and the wind velocity. Another relevant aspect is that drag force is proportional to the air density (which decreases with altitude), and so projectiles with a given velocity (or kinetic energy) reach larger distances and heights at volcanoes with higher altitudes. This fact is important in the case of Volcán de Fuego de Colima (México) where the crater altitude is 3860 m above sea level. This work presents a useful hazards map for Volcán de Fuego de Colima based on ballistic data from this and other volcanoes.

Volcaniclastic debris flows generated in drainage basins of the Apennine mountains of southern Campania in response to pyroclastic fall deposition from four Holocene eruptions of Somma-Vesuvius: Avellino (3.8 ka), A.D. 79, A.D. 472, and A.D. 1631. These syneruptive debris flows are lithologically homogeneous and contain more than 90% of material from the parental eruption. They differ from inter-eruptive debris flows recognized in the area, which contain mixed lithologies of juvenile material (i.e., volcanic material from different eruptions). Diffuse rill erosion generated fines-rich volcaniclastic flows (mudflows), whereas partial saturation of coarse ash and lapilli generated coarser-grained debris flows. Lithofacies analysis shows that debris flows predominate versus hyperconcentrated flows and normal stream-flow deposits. Debris-flow deposits are massive, matrix supported, and have a gravelly-sandy texture. Large blocks are scarce due to their absence in the pyroclastic source material. Lithofacies association indicates that volcaniclastic debris-flow deposits aggraded rapidly by superimposition of different surges that spontaneously developed within the flow. Bulk-flow density ranges from 1840 to 2260 kg/m 3 (mean 2035 ± 207 kg/m 3 ). Geological data supported some considerations of hazard assessment in the study area and indicate that the syneruptive volcaniclastic flows stopped distally on active alluvial fans.

A new numerical code for simulating flows of granular material, TITAN2D, is used to model the Merapi-type block and ash flows resulting from the 1991 eruption of Colima Volcano, México. The 1991 block and ash flows reached distances of up to 4 km from the vent with a total estimated volume of 8 × 10 5 m 3 . The block and ash flows were modeled using a digital elevation model (DEM) of the region and compared to field data using a quantitative center-line comparison method. Input parameter values, which dictate flow dynamics, were varied to demonstrate the effect these parameters have on the program output. Analysis showed that the TITAN2D model performs best in replicating the center lines of the 1991 Colima block and ash flows when using an initial volume value representative of a single flow event rather than a total deposit volume.

Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km 2 , and total erupted volumes range between 0.5 and 2 km 3 /cone. An average eruption rate for the entire Chichinautzin was estimated at ∼0.5 km 3 /1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract Tephrochronological studies carried out over the past decade in the area surrounding Mexico City have yielded a wealth of new radiocarbon ages from eruptions at Popocatépetl, Nevado de Toluca, and Jocotitlán stratovolcanoes and monogenetic scoria cones in the Sierra Chichinautzin Volcanic Field. These dates allow us to constrain the frequency and types of eruptions that have affected this area during the course of the past 25,000 yr. They have important implications for archaeology as well as future hazard evaluations. Late Pleistocene and Holocene volcanic activities at the stratovolcanoes are characterized by recurrent cataclysmic Plinian eruptions of considerable magnitude. They have affected vast areas, including zones that today are occupied by large population centers at Puebla, Toluca, and Mexico City. During Holocene time, Nevado de Toluca and Jocotitlán have each experienced only one Plinian eruption, ca. 10,500 yr B.P. and 9700 yr B.P. respectively. During the same period of time, Popocatépetl had at least four such eruptions, ca. 8000, 5000, 2100, and 1100 yr B.P. Therefore, the recurrence interval for Plinian eruptions is less than 2000 yr in this region. The last two Plinian eruptions at Popocatépetl are of particular interest because they destroyed several human settlements in the Basin of Puebla. Evidence for these disasters stems from pottery shards and other artifacts covered by Plinian pumice falls, ash-flow deposits, and lahars on the plains to the east and northeast of the volcanic edifice. Several monogenetic scoria cones located within the Sierra Chichinautzin Volcanic Field at the southern margin of Mexico City were also dated by the radiocarbon method in recent years. Most previous research in this area was concentrated on Xitle scoria cone, whose lavas destroyed and buried the pre-Hispanic town of Cuicuilco ca. 1665 ± 35 yr B.P. The new dates indicate that the recurrence interval for monogenetic eruptions in the close vicinity of Mexico City is also <2000 yr. The longest lava flow associated with a scoria cone was erupted by Guespalapa and reached 24 km from its source; total areas covered by lava flows from each monogenetic eruption typically range between 30 and 80 km2, and total erupted volumes range between 0.5 and 2 km3/cone. An average eruption rate for the entire Chichinautzin was estimated at ~0.5 km3/1000 yr. These findings are of great importance for archaeological as well as volcanic hazard studies in this heavily populated region.

Abstract The Cuicuilco pyramid was one of the first true urban centres in the Basin of Mexico. Its construction started a few centuries BC , during the Late Preclassic period. The pyramid is partially covered by a basaltic lava flow produced by the Xitle monogenetic volcano. New stratigraphic work around the pyramid and the volcano together with new radiocarbon dates indicate that the pyramid and nearby settlements were abandoned as a direct consequence of the volcanic activity of Xitle. The new dates, obtained from material which clearly is contemporaneous with the volcanic activity, suggest that the eruption took place around 1670 years BP , some 300 years later than previously thought.