Las Cabras volcano, Michoacán-Guanajuato Volcanic Field, México: Topographic, climatic, and shallow magmatic controls on scoria cone eruptions

Abstract

Scoria cones are abundant in most volcanic fields on Earth, such as the Michoacán-Guanajuato Volcanic Field, in the central-western sector of the Trans-Mexican Volcanic Belt. However, there are few in-depth studies on their eruptive style and controlling factors, despite of their diversity in shape and composition which implies a wide range of hazards. Here, we present results of morphologic, stratigraphic, sedimentary, petrographic, and geochemical studies of the prominent Las Cabras scoria cone located west of the Zacapu lacustrine basin in the center of the Michoacán-Guanajuato Volcanic Field. This basaltic andesitic to andesitic volcano formed between 27 and 26 kyrs BP on the steep slopes (>10º) of the lava shield of El Tule volcano. Over time, its dominant eruptive style changed from Strombolian to effusive. Initial explosive activity built a 170-m-high scoria cone and deposited thick tephra fallout on the surrounding sloping terrain. Structures in the deposits indicate that early friable fine-grained tephra underwent significant erosion due to syn-eruptive heavy rain coupled with the sloping nature of the underlying ground. This erosion generated lahars that very likely reached the Zacapu lake based on the pre-eruptive topography. As the explosivity dropped, lava was emitted from the base of the cone first to the S and SE, forming a thick, viscous lobe that filled a pre-existing E-W valley. The flow direction then deviated to the N and NE, to form thinner, less-viscous lobes fed from the vent by an open-channel. The lavas are covered by hummocks made of agglutinates and bombs that indicate that the eruption terminated by catastrophic collapse of the SE sector of the cone, possibly triggered by the intrusion of magma within the cone, which destabilized its downslope segment. The sudden flank failure was potentially associated with a late effusive event and the hummocks may have been carried away by the lava surge. Whole-rock chemical variations and crystal disequilibrium textures point toward a complex magma feeding system, involving mixing and mingling between different magma batches. This study shows that the formation of scoria cones on a terrain with a marked slope (>10°) has profound impacts on the eruption dynamics and related hazards due to its effect on cone stability and ash erosion. It also evidences the erosive effect of syn-eruptive rain on fine-grained tephra, especially when deposited on a slope. Finally, it reveals the complex magmatic processes that may occur in the shallow plumbing system of monogenetic andesitic volcanoes, which could be particularly important in inland areas of continental arcs.

Holocene volcanic eruptions of the Malpaís de Zacapu and its pre-Hispanic settlement history

“…were reported from Cueva de los Hacheros (Punzo-Díaz and Martínez-Vázquez 2018, 2021) at the southern limit of the MGVF (Figure 2). Recent radiometric dating efforts in the MGVF (e.g., Chevrel et al 2016a; Guilbaud et al 2012, 2021; Kshirsagar et al 2015, 2016; Mahgoub et al 2017; Osorio-Ocampo et al 2018; Pola et al 2014; Ramírez-Uribe et al 2019; Reyes Guzmán et al 2018) have revealed numerous Late Pleistocene/Holocene monogenetic eruptions, and it seems safe to say that the average time interval between them is <400 years. Many of these must have affected ancient population groups since the first arrival of early man in this part of the American continent, although evidence for the direct impact of eruptions on early pre-Hispanic dwellers remains sparse.…”

“…Geological map of the Zacapu lacustrine basin (color-coded according to ages), modified after Guilbaud et al 2021. Black quadrangles delimit the areas previously studied in detail: Western quadrangle (shown in Figure 4) originally mapped by Reyes-Guzmán et al (2018); southern quadrangle by Ramírez-Uribe et al (2019); eastern quadrangle by Kshirsagar et al (2015); and northern quadrangle by Kshirsagar et al (2016).…”