Geological and environmental controls on the change of eruptive style (phreatomagmatic to Strombolian-effusive) of Late Pleistocene El Caracol tuff cone and its comparison with adjacent volcanoes around the Zacapu basin (Michoacán, México)

Abstract

The 28,300year BP (cal 32,300 BP) El Caracol tuff cone complex is one of the few phreatomagmatic volcanoes in the scoria-cone dominated Plio-Quaternary Michoacán-Guanajuato Volcanic Field (MGVF). It displays a shallow circular crater of ~1km in diameter that is filled with several meter-thick lava flows and is located between two NE–SW trending normal faults dipping NW. It lies directly on top of Pliocene lavas of the San Lorenzo shield volcano that forms part of a tectonic horst (topographic high) separating the Zacapu lake basin (1980m) in the south from the Río Angulo river valley (1760m) to the north. Detailed study of the tephra sequence indicates that the eruption occurred in two stages: 1) Weak phreatomagmatic, in which about 0.1–0.5km3 dense rock equivalent (DRE) of magma was issued within ~1 to 3months at the rate of 4–40m3/s, and 2) purely magmatic (Strombolian-effusive) during which the vent shifted slightly its position toward the NW, forming a small scoria cone (~100m high) on the crater rim of the tuff cone. From this scoria cone lava flows were issued, first into the tuff cone crater occupying its bottom, and subsequently toward the NW, down the outer flank of the tuff cone and into the plain, where they reached a distance of ~3.5km. During this stage ~0.6km3 DRE of magma was produced at the rate of ~4m3/s in a period of ~5months.Although El Caracol displays many features that are characteristic for a phreatomagmatic vent, its morphology, types of deposits, and its complex process of formation makes it strikingly different from the more typical case of the ~21,000year BP (cal 25,300 BP) Alberca de Guadalupe maar volcano, situated not far at the SE margin of the Zacapu basin. The latter was solely phreatomagmatic during the course of its eruption and is formed in its entirety by surge and fallout breccias consisting largely of xenolithic material. In contrast, at El Caracol the hydrogeological environment (namely the low groundwater flow gradient) did not allow for sustained water/magma interactions at ideal conditions (ratio of 0.2). Hence, the system soon dried out due to insufficient groundwater flow toward the eruption site and the eruption turned purely magmatic. This study shows that despite of its proximity to a large water reservoir (Zacapu lake basin) and similarities with Alberca de Guadalupe maar in regard to age, climate, magma composition, and tectonic environment, several other specific parameters play a crucial role in shaping a monogenetic phreatomagmatic volcano. The case of El Caracol (phreatomagmatic style followed by magmatic during the course of a monogenetic eruption) has been reported from other volcanic fields around the globe where it might have similar causes. Furthermore, the general scarcity of this type of volcanoes in the Michoacán–Guanajuato Volcanic Field implies that the conditions required for their formation are rarely met in this region.