José Luis Arce

The 10.5 ka Plinian eruption of Nevado de Toluca volcano, Mexico: Stratigraphy and hazard implications

During the late Pleistocene, a large Plinian eruption from Nevado de Toluca volcano produced a complex sequence of pyroclastic deposits known as the Upper Toluca Pumice. This eruption began with a phreatomagmatic phase that emplaced a hot pyroclastic flow (F0) on the east and northern flanks of the volcano. Eruption decompressed the magmatic system, almost immediately allowing the formation of a 25-km-high Plinian column that was dispersed by winds predominantly 70° to the northeast (PC0). Next, three other Plinian columns were dispersed in a northeast to east direction, reaching heights of 39, 42, and 28 km, resulting in fall layers (PC1, PC2, and PC3), respectively. These Plinian phases were interrupted several times by phreatomagmatic and collapse events that emplaced pyroclastic flows (F1, F2, and F3) and surges (S1 and S2), mainly on the eastern and northern flanks of the volcano. The eruption ended with the extrusion of a crystal-rich dacitic dome at the vent. The juvenile components of the Upper Toluca Pumice sequence are white, gray, and banded pumice, and gray lithic clasts of dacitic composition (63%–66% SiO2) and minor accidental lithic fragments. The fall deposits (PC1 and PC2) covered a minimum area of 2000 km2 and constitute a total estimated volume of 14 km3 (∼6 km3 DRE [dense-rock equivalent]). The mass eruption rate ranged from 3 × 107 to 5 × 108 kg/s, and total mass was 1.26 × 1013 kg. Charcoal found within Upper Toluca Pumice yielded an age of 10,500 14C yr B.P. (12,800–12,100 14C calibrated yr B.P.), somewhat younger than the earlier reported age of ca. 11,600 14C yr B.P. This new age for the pumice falls within the Younger Dryas cooling event. The eruption emplaced 1.5 m of pebble-sized pumice in the City of Toluca region and ∼50 cm of medium to fine sand in the Mexico City region. Distal lahar deposits derived from the Upper Toluca Pumice event incorporated mammoth bones and other mammals in the basin of Mexico. A future event of this magnitude would disrupt the lives of 30 million people now living in these cities and their surroundings.

Petrology of two contrasting Mexican volcanoes, the Chiapanecan (El Chichón) and Central American (Tacaná) volcanic belts: the result of rift- versus subduction-related volcanism

The alkaline El Chichón and calc-alkaline Tacaná volcanoes, located in southern Mexico, form parts of the Chiapanecan Volcanic Belt and Central American Volcanic Arc, respectively. El Chichón has emitted potassium-, sulphur-, and phosphorus-rich trachyandesites and trachybasalts (as mafic enclaves), whereas Tacaná has erupted basalts to dacites with moderate potassium contents, and minor high-Ti magmas (1.5–1.8 wt.% TiO2). The magmatic evolution in the two volcanoes has involved similar fractionating assemblages: Fe-Ti oxides, olivine, plagioclase, pyroxenes, amphibole, and apatite. K2O/P2O5 ratios and isotopic signatures indicate that magmas from both El Chichón and Tacaná have undergone significant crustal contamination. The volcanism at both Tacaná and El Chichón was previously related to northeastward subduction of the Cocos Plate, representing the main arc and the backarc, respectively. Although such an origin is in accord with Tacaná occurring 100 km above the Cocos Benioff Zone, it is inconsistent with: (a) the absence of a calc-alkaline belt between El Chichón and the Middle America Trench; and (b) truncation of the subducted Cocos Plate by the southwesterly dipping Yucatan slab near the Middle America Trench (i.e. the Cocos Plate does not presently underlie El Chichón). On the other hand, El Chichón and the Chiapanecan Volcanic Belt are located on the sinistral Veracruz fault zone that forms the northern boundary of the Southern Mexico block, which has been migrating relatively to the east since ca. 5 Ma. In this context, the anomalous high potassium, sulphur, and phosphorus levels in the El Chichón magmas are explicable in terms of rifting in a pull-apart system with the weak subduction fingerprint inherited from the Yucatan slab.

Eruptive History of the Tacaná Volcanic Complex

Tacana is the northernmost volcano of the Central American Volcanic Arc, and one of the four volcanic structures of the Tacana Volcanic Complex (TVC), from oldest to youngest: Chichuj, Tacana, and San Antonio volcanoes, and Las Ardillas dome. Geologic and radiometric data show that volcanic activity of the TVC began around 225 ka with the construction of Chichuj volcano within the 2 Ma old San Rafael Caldera. The edifice of Tacana began its construction west of Chichuj volcano around 50 ka. San Antonio volcano, and Las Ardillas Dome formed southwest of Tacana volcano during Late Pleistocene. Effusive and explosive eruptive activity has alternated from all eruptive centers of the complex. Flank collapses of Chichuj, Tacana, and San Antonio edifices have generated debris-avalanches. At least four plinian -subplinian events—two of which rank ~5 on the Volcanic Explosivity Index (VEI)—and nine other smaller explosive eruptions occurred at Tacana during the Holocene, the most recent one around 150 year BP. The 1949 and 1986 phreatic explosions from Tacana attracted scientific and public attention to the complex. At present, Tacana represents the second most dangerous volcano in Mexico after Popocatepetl.

Petrology and Geochemistry of El Chichón and Tacaná: Two Active, yet Contrasting Mexican Volcanoes

El Chichon and Tacana have been widely considered subduction-related volcanoes, although they show differences in mineral assemblage and magma composition. El Chichon emitted potassium- and sulfur-rich trachyandesites and trachybasalts during its eruptive history, whereas Tacana erupted basalts to dacites with moderate potassium contents, and minor high-Ti magmas. The magmatic evolution in both volcanoes involved similar fractionating assemblages of Fe-Ti oxides, olivine, plagioclase, pyroxenes, amphibole and apatite. Both K2O/P2O5 ratios and isotopic signatures, indicate that the melts of El Chichon and Tacana experienced significant crustal contamination. Magma genesis for both volcanoes has been related to the northeastward subduction of the Cocos Plate. Even if such origin agrees with the location of Tacana, situated 100 km above the Cocos Benioff Zone, a subduction origin is at odds with recent tectonic and geophysical data obtained for southern Mexico for El Chichon, located about 400 km from the trench. In this chapter we review the existing petrographic and geochemical data for El Chichon and Tacana volcanoes, in order to understand their magma genesis and evolution.

Geology and Stratigraphy of the Cerro Prieto Volcanic Complex, Baja California Norte, México

New mapping and stratigraphy of the Cerro Prieto Volcanic Complex indicates that it was constructed by several events: (1) The beginning was characterized by the emission of dacitic lavas that interacted with water-saturated sandstones, producing brecciated lavas; (2) then, effusive activity formed a lava dome that was destroyed by a phreatic eruption, producing a lithic fallout and a 300-m-wide summit crater; (3) afterwards, the volcano recorded effusive activity with the emplacement of three domes and a fissural lava flow; and (4) subsequent erosion of the volcanic complex resulted in the emplacement of debris flows around the complex. These magmatic events occurred at around 80 ka over a relatively short period of time. All rocks of this complex are composed of porphyritic lavas with phenocrysts of opx + plg + qtz + iron oxides ≫ ghost of amphibole, embedded in a groundmass with plagioclase microlites and glass, with a dacitic chemical composition (67–69 wt % silica).