Hazards in monogenetic volcanic fields include processes and events occurring prior to, during, and after an eruption. This contribution identifies hazards resulting from processes occurring prior to and after a volcanic eruption. From recent experiences in the Chichinautzin volcanic field, hazardous events associated with reports of potential impending eruptions have turned out to be three types of false alarms: fires or gas explosions in sanitary landfills, underground fires, and anthropogenic lava flows. Typically, people who live at monogenetic volcanic fields know that an eruption is a likely event, so when they observe deformation of the ground, heat flow, and explosions, they report these anomalous events to the authorities as volcanic. A methodology should be established to cope with reports of new volcanic activity and to handle the outcome, which could be volcanic or nonvolcanic hazards. The hazards related to events occurring after an eruption include the planning of cities and villages around tube systems, building hazards over lava tubes, pollution due to sewage release in lava tube systems, with consequences to public health and the environment, and endangering threatened species that live in the volcanic systems after the eruptions. Here, we propose a view of volcanic hazards that has not been made before and is distinct from the usual hazards evaluation during eruptions.

Recent advances in the knowledge of the Cenozoic structure and stratigraphy of southern México reveal a geological evolution characterized by Upper Cretaceous orogenic deformation, followed by truncation of the continental margin and gradual extinction of arc magmatism in the Sierra Madre del Sur, prior to the onset of magmatism in the Trans-Mexican Volcanic Belt. Orogenic deformation began in the Late Cretaceous and was coeval with the Laramide orogeny with structures of similar orientation. Deformation consisted of E-W shortening that migrated to the east with time and with a general easterly vergence. Models that relate the Laramide deformation to a decrease in the angle of subduction of the Farallon plate, which was converging in western México, cannot be applied in southern México because Paleocene to upper Eocene arc magmatism occurs near the inferred paleotrench. An alternative possible origin due to collision of an insular arc against the western margin of México suffers from an absence of features and petrogenetic associations indicating the closure of an oceanic basin. In light of recent geochronological data, the general pattern of magmatic extinction from Upper Cretaceous–Paleocene in Colima and Jalisco to the middle Miocene in central and southeastern Oaxaca presents some variations inconsistent with a simple pattern of extinction toward the E-SE. Maastrichtian to lower Paleocene plutonism recognized in the Jalisco block and Manzanillo areas is contemporaneous with a magmatic episode that has some documented adakitic affinities in the central part of the Sierra Madre del Sur. Magmatism from the Paleocene to middle Eocene seems to be concentrated in the Presa del Infiernillo area, although isolated centers existed in areas such as Taxco or the eastern Jalisco block. Finally, the main axis of magmatism between the middle Eocene and Oligocene developed along what is the present-day continental margin and extends 200 km inland as a broad band. In general, the geochemical characteristics of this magmatism indicate a low degree of continental crustal assimilation. Two episodes of principally sinistral lateral faulting that activated NW-SE– and later N-S–oriented faults, with variations in time and space, have been documented during the Eocene and lower Oligocene. The N-S set of faults was active only in the north of the Sierra Madre del Sur, whereas the activity of the NW-SE set continued during the Oligocene along the Oaxaca continental margin. The recognition of these deformational episodes suggests that extensional directions related to lateral faulting changed from NNW-SSE to NE-SW, and locally produced normal displacements on preexisting discontinuities. Fundamental problems still exist in the interpretation of the plate tectonic processes that produced the stress regimes acting on the different sets of faults, as well as in the determination of the factors influencing the migration of magmatism. Some of the arguments used to postulate the presence of the Chortis block off the southwestern Mexican continental margin during the early Cenozoic are uncertain. On the other hand, models that explain restricted displacements of the Chortis block with respect to the Maya block—without juxtaposition with the southwestern margin of México—suggest that continental truncation was essentially caused by subduction erosion and leave open the interpretation of the observed magmatic migration.

The magmatic diversity of the Trans-Mexican Volcanic Belt is directly or indirectly controlled by two independent oceanic plates with differing geophysical and compositional parameters; by an extensional tectonic regime that operates with different intensities over the upper plate; by a continental basement with a diversity of ages, thick nesses, and compositions; and by a compositionally heterogeneous mantle wedge that has been modified to various extents by the slab-derived chemical agents. The convergent margin and the magmatic arc have not remained static throughout their geologic histories, but instead have shown significant changes in position, geometry, and composition. For these reasons, the Trans-Mexican Volcanic Belt is the result of one of the most complex convergent margins on the planet, the subject of more than a century of scientific investigations, and at the core of the most notorious debates on Mexican geology.

The Trans-Mexican Volcanic Belt has been recognized as a major volcanic arc, which crosses México from the Pacific Coast to the Gulf of México, that has displayed normal faulting and volcanism since the Miocene. In this work we present the deformation events that have been recorded N and S of the belt in order to establish when the crustal discontinuity originated and also to determine the deformation field precursor of the volcanic arc emplacement. In Mesa Central, the post-Laramide deformation occurred in three extensional events during the Eocene, Oligocene, and Miocene-Recent. The three events produced extension in two horizontal directions and shortening in a vertical direction. The direction of the principal extension in the Eocene is not well known. A 20% extension in an ∼ENE-WSW direction is recorded for the Oligocene event. The most recent event, active since the middle Miocene, has developed in the Trans-Mexican Volcanic Belt and along its northern boundary. In the Sierra Madre Oriental, Cenozoic deformation has been minimal. In the Taxco region, there were two post-Laramide deformation events, mainly a result of NW-SE and N-S lateral faults. The first one occurred in the late Eocene with a NNW-SSE horizontal extension direction. The second event was early Oligocene with a maximum extension to the NE-SW. It is concluded that since the Eocene, the deforma tion style has been different in Mesa Central and in the Sierra Madre del Sur, which implies the presence of a detachment zone between these provinces.

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.