Eruptive activity of enigmatic medium-sized volcanoes in the Michoacán-Guanajuato Volcanic Field (MGVF), Central Mexico: The case of El Metate

Abstract:

The MGVF has a total area of ca. 40,000 km² and is well known for being the host of the only two monogenetic volcanoes in Mexico that were born in historical times: Jorullo (1759–1774) and Paricutin (1943–1952). Another particularity of the MGVF is its high number of eruptive vents with over 1000 small monogenetic cones and associated lava flows (average vol. of 0.021 km³) and ca. 400 medium-sized volcanoes (average vol. from 0.5 to 50 km³). Most of these medium-sized volcanoes may be characterized as shields that were produced dominantly by effusive activity as opposed to the small cones formed also by explosive phases of activity. The products of the small cones range from olivine basalts to andesites whereas the medium-sized volcanoes are restricted to a smaller compositional range in the andesitic domain. Although the medium-sized volcanoes are more sparsely distributed in time and space and less abundant than the small cones, the risks associated with renewal of this type of activity should not be neglected. This study focuses on El Metate which is probably the youngest shield of the MGVF (< 3,700 y. BP). Unlike a typical shield volcano composed of a succession of thin fluid basaltic flows, El Metate consists of well-preserved >60 m thick andesite flows distributed radially around a summit dome. Detailed mapping and sampling allowed us to reconstruct its eruptive activity and the time sequence of lava flow emplacement. We have identified 13 individual lava flows with lengths ranging between 3 and 15 km covering 103 km² and average thicknesses between 60 and 150 m. Individual volumes range between 0.5 and 3.5 km³ for a total of 11 to 15 km³. Estimates of flow emplacement parameters indicate maximum average effusion rates ranging between 15 and 100 m³.s^-1 and a cumulative duration from 15 to 30 years. Such a short emplacement time is comparable to the historical monogenetic eruption of nearby Paricutin volcano (9 years) but the erupted volume of lava is considerably larger (>11 km³ vs. ca. 0.8 km³). The young age of this eruption bears important implications, especially for evaluating future hazards in the MGVF. Furthermore, this study widens the use of the term “shield” and “monogenetic”, usually attributed to low-viscosity basaltic volcanoes, to more silicic compositions and a radically different flow emplacement style.