Mixed magmatic–phreatomagmatic explosions during the formation of the Joya Honda maar, San Luis Potosí, Mexico
R. Saucedo, J. L. Macías, Y. Z. E. Ocampo-Díaz, W. Gómez-Villa, E. Rivera-Olguýn, R. Castro-Govea, J. M. Sánchez-Núñez, P. W. Layer, J. R. Torres Hernández, G. Carrasco-Núñez, 2017. "Mixed magmatic–phreatomagmatic explosions during the formation of the Joya Honda maar, San Luis Potosí, Mexico", Monogenetic Volcanism, K. Németh, G. Carrasco-Núñez, J. J. Aranda-Gómez, I. E. M. Smith
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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.
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The nature and origin of the small-scale volcanic systems, generally referred to as “monogenetic”, have enjoyed an elevated level of interest during the past decade. There has been recognition that their ostensibly simple volcano types are a window into the nature of explosive volcanism, landscape evolution and the processes of magma generation in the Earth’s upper mantle. In the past few years, major conferences have offered specialized technical sessions dealing with monogenetic volcanism and there have been thematic conferences, such as the IAVCEI International Maar Conference series, which have provided a focus for discussion of volcanological and geochemical aspects of small-scale basaltic volcanism. Many new aspects of monogenetic volcanism have emerged and have clearly demonstrated that this volcanism can be very complex on a fine scale. This book is a collection of papers arising from two recent Maar Conferences (the fifth in Queretaro Mexico and the sixth in Changchun, China) and serves as a snapshot of current research on monogenetic volcanism.