Most of the largest volcanoes in México are located at the frontal part of the Trans-Mexican Volcanic Belt and in other isolated areas. This chapter considers some of these volcanoes: Colima, Nevado de Toluca, Popocatépetl, Pico de Orizaba (Citlaltépetl), and Tacaná. El Chichón volcano is also considered within this group because of its catastrophic eruption in 1982. The volcanic edifice of these volcanoes, or part of it, was constructed during the late Pleistocene or even during the Holocene: Colima 2500 yr ago, Pico de Orizaba (16,000 yr), Popocatépetl (23,000 yr), Tacaná (∼26,000 yr), and Nevado de Toluca (>45,000). The modern cones of Colima, Popocatépetl, Pico de Orizaba, and Tacaná are built inside or beside the remains of older caldera structures left by the collapse of ancestral cones. Colima, Popocatépetl, and Pico de Orizaba represent the youngest volcanoes of nearly N-S volcanic chains. Despite the repetitive history of cone collapse of these volcanoes, only Pico de Orizaba has been subjected to hydrothermal alteration and slope stability studies crucial to understand future potential events of this nature. The magmas that feed these volcanoes have a general chemical composition that varies from andesitic (Colima and Tacaná), andesitic-dacitic (Nevado de Toluca, Popocatépetl, and Pico de Orizaba) to trachyandesitic (Chichón). These magmas are the result of several magmatic processes that include partial melting of the mantle, crustal assimilation, magma mixing, and fractional crystallization. So far, we know very little about the deep processes that occurred between the upper mantle source and the lower crust. However, new data have been acquired on shallower processes between the upper crust and the surface. There is clear evidence that most of these magmas stagnated at shallow magma reservoirs prior to eruption; these depths vary from 3 to 4 km at Colima volcano, 4.5–6 km at Nevado de Toluca, and ∼6–8 km at Chichón volcano. Over the past 15 years, there has been a surge of studies dealing with the volcanic stratigraphy and eruptive history of these volcanoes. Up to the present, no efforts have achieved integration of the geological, geophysical, chemical, and petro logical information to produce conceptual models of these volcanoes. Therefore, we still have to assume the size and location of the magma chambers, magma ascent paths, and time intervals prior to an eruption. Today, only Colima and Popocatépetl have permanent monitoring networks, while Pico de Orizaba, Tacaná, and Chichón have a few seismic stations. Of these, Popocatépetl, Colima, and Pico de Orizaba have volcanic hazard maps that provide the basic information needed by the civil defense authorities to establish information programs for the population as well as evacuation plans in case of a future eruption.

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.

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.

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.