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.