One eruption, three separate vent areas: The case of the La Alberca-Joya Blanca monogenetic complex, Valle de Santiago area, Michoacán-Guanajuato Volcanic Field (Mexico)

The La Alberca-Joya Blanca monogenetic complex is located in the Valle de Santiago area of the Michoacán-Guanajuato Volcanic Field, within the Trans-Mexican Volcanic Belt. Field, stratigraphic, geochronological, petrographic, and geochemical evidence reveals that the complex formed ∼22,000 yr B.P. during a brief single eruptive period that involved magmatic, phreatic, and phreatomagmatic activity at three distinct vent areas. The erupted magma was basaltic trachy-andesitic and ascended through secondary fractures associated with the NNW-SSE-trending Tzitzio-Valle de Santiago fault zone. The eruption began at two vent areas (Vents #1 and #2), aligned E-W and located 1.6 km apart. Vent #1 produced Hawaiian-Strombolian activity that formed scoria-spatter mounds and lava flows (La Alberca mounds). At Vent #2, the eruption began with a phreatic phase that generated a mud deposit, which was followed by Hawaiian-Strombolian eruptions that built a scoria-spatter cone and emitted lava flows (La Alberca cone). While both vent areas remained active, new activity took place at a third vent (Vent #3) located 1.6 km south of Vent #1. At this site, phreatomagmatic explosions formed a tuff cone (Joya Blanca), driven by interactions between ascending magma and a shallow perched aquifer hosted within Middle Pleistocene lava flows. As the external water source was depleted, the eruption shifted to a Strombolian phase that produced a scoria mound located within the tuff cone's crater. Then, activity at Vents #1 and #3 ceased, and eruptive focus moved back to Vent #2, where the initially magmatic eruption that was building the La Alberca cone transitioned into a brief but intense phreatomagmatic phase, leading to the formation of a maar (Joya La Alberca) in approximately 4 to 10 days. This phreatomagmatic activity was sustained by groundwater sourced from a highly permeable fractured aquifer hosted within the same Middle Pleistocene lava flows. This final phreatomagmatic phase involved a relatively small magma volume (1.2–1.9 × 10 6 m 3 ) and required a groundwater flux ranging from 0.6 to 1.8 m 3 /s to achieve a water/magma ratio of 0.1–0.3. Such a flux correlates with cold and humid climatic conditions prevailing during the Late Pleistocene in the Valle de Santiago region, when annual precipitation was over 1000 mm. • The La Alberca-Joya Blanca complex is a multi-vent monogenetic eruption • The eruption involved phreatic, phreatomagmatic, and magmatic activity • The eruption occurred during the Late Pleistocene (∼22,000 yr ago) • Phreatomagmatic activity occurred in a fractured aquifer hosted in shield volcanoes