Integrated sonar-drone geomorphological mapping of maar lakes: Insights into compound maar eruptive histories

Morphological analysis of maar volcanoes provides compelling evidence for understanding their geological evolution. Especially when their craters exhibit compound shapes resulting from multiple spatially shifting explosions throughout their eruptive histories. In such cases, the spatial relationship of the overlapping explosive footprints reveals the eruptive sequence. Moreover, after their eruptive origin, maar craters are exposed to sedimentary processes that modify their original shapes. A challenge in studying the morphology of maar volcanoes arises when they host volcanic lakes that obscure their inner-crater features. This study presents a morphological analysis of maar lakes with compound shapes through high-resolution Digital Elevation Models (DEMs) that integrate topographic and bathymetric data. Two Quaternary maar lakes located in the Serdán-Oriental basin (Mexico) were selected for this analysis: La Preciosa and Quechulac. Both depict pristine compound shapes with overlapping explosive footprints that record the migration of the eruptive loci during their eruptive history. Moreover, each maar has an unusual geological feature that cannot be explained without a comprehensive morphological analysis: In the case of La Preciosa, the northern half of its crater presents a step and pristine tephra ring that thins and disappears southwards, where only the pre-maar sequence is exposed. The Quechulac maar presents an irregularly shaped island made of tephra near the center of its lake. The origin and temporality of this island remain unclear, and its presence challenges the explosive nature of maar volcanoes, which tend to dig and deepen near the center of their craters. The morphological inspection based on the resulting DEMs led to insightful geological interpretations that explain these unusual geological features. In La Preciosa, the northern part of the present crater was formed by multiple explosions that migrated spatially in a trending west-to-east direction. In contrast, the southern part resulted from the gravitational collapse of the country rock, which was facilitated by an explosion-induced subrosion process. The Quechulac maar also records an east-to-west migration of the explosion loci. The latest explosions formed a nested crater rim in the west, from which the highest topography emerged as an island. This methodology, introduced for the first time in maar lakes, has potential applications in other lacustrine environments to enhance geological interpretations.