ALBERT

Description: 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 ( 40 Ar/ 39 Ar) along a fissure that formed an elliptical crater ( c. 1.3 x 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.

Description: Despite a number of published papers focusing on the geodynamic implications of the recent Southern Puna mafic magmatism, there have been fewer studies of the volcanology and stratigraphy of this outstanding volcanism. This paper presents a detailed map of two well-preserved Quaternary scoria cones showing their complex stratigraphy. Complementary morphometric, morpho-structural, petrographic and geochemical data were used to reconstruct the evolution of both volcanoes. The occurrence of more than one eruption at each volcano was inferred by the recognition of temporal hiatuses using morpho-stratigraphic criteria. The polycyclic nature of both scoria cones could be related to a combination of a high input magma in response to lithospheric delamination, a favourable regional stress field and the interaction of rising magma with pre-existing faults. The youngest eruptions in both volcanoes were complex, with shifts in the eruptive style from violent strombolian to hawaiian/strombolian phases, and probably lasted for a few years. The explosive activity was accompanied by the emission of lava flows from lateral vents. Phreatomagmatic activity was triggered during the waning stages of the eruptions. The occurrence of more than one eruption in a single scoria cone and the changes in the eruptive style during long-lasting eruptions are important topics for volcanic hazard assessment in the Southern Puna.