R. Saucedo

Deposit temperature of pyroclastic density currents emplaced during the El Chichón 1982 and Colima 1913 eruptions

Abstract New data on the pyroclastic density current (PDC) deposit temperature (Tdep) are provided for two prominent eruptions of Mexican volcanoes of the twentieth century: the 1982 eruption of El Chichón and the 1913 eruption of Colima. In spite of similar lithofacies, magma composition and pre-eruptive conditions, the Tdep of the PDCs from the 1982 (El Chichón) and 1913 (Colima) eruptions differ significantly, with intervals of Tdep of 360–420 °C and 250–330 °C, respectively. These new data emphasize that a full understanding of the physical mechanisms responsible for equilibrium temperature attainment within a pyroclastic deposit has not yet been realized. The Tdep measured for El Chichón PDC deposits confirm the preliminary data published elsewhere, while Colima magnetic temperatures provide different values to those published previously. supplementary-material: Tdep measurements for the different sites at El Chichon volcano and Colima volcano are available at: http://www.geolsoc.org.uk/SUP18695.

Mixed magmatic–phreatomagmatic explosions during the formation of the Joya Honda maar, San Luis Potosí, Mexico

Abstract The Joya Honda maar (JHm) is located in central Mexico, 35 km NNE of the city of San Luis Potosí. It lies in the Plio-Quaternary alkaline Ventura-Espíritu Santo Volcanic Field located in the eastern part of Mesa Central province. The JHm eruption occurred at 311±19 ka (40Ar/39Ar) along a fissure that formed an elliptical crater (c. 1.3×0.9 km wide and c. 270 m deep) with a major axis oriented to the ENE–WSW. The eruption generated pyroclastic surge deposits that preferentially extended up to a distance of 7 km to the NW–NE of the crater, with a very limited distribution to the south. At the crater rim, the sequence is 60–80 m thick on the NE–NW wall and 1–15 m thick on the south–SW rim. The JHm sequence is divided into five units with different structures, textures, granulometry and components. The juvenile basanite clasts of these units display differences in vesicularity, density and morphology under scanning electron microscopy. These units correspond to the same number of eruptive phases as follows: Phase 1 occurred as a series of alternating strombolian and phreatomagmatic explosions that dispersed fall deposits and base surges; Phase 2 began with strombolian activity that emplaced basanite scoria with low contents of mantle xenoliths; Phase 3 continued with phreatomagmatic explosions that emplaced wet and dry pyroclastic surges; Phase 4 generated strombolian explosions rich in mantle xenoliths; and Phase 5 produced a violent strombolian phase that dispersed fallouts rich in mantle xenoliths and intermixed with discrete phreatomagmatic explosions that emplaced pyroclastic surges. These eruptive fluctuations during the genesis of JHm are a response to the relative proportions of magma–water interaction through time and complex faulting of the calcareous rocks underneath the volcano. The distribution and textural characteristics of the deposits suggest that simultaneous or alternating vents were active during the eruption, possibly following a fissure. These variations may have been subordinated to factors such as the availability of groundwater, the velocity of magma ascent, the discharge rate and degassing.

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.

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).