Two Small Volcanoes, One Inside the Other: Geophysical and Drilling Investigation of Bazina Maar in Western Eger Rift

Maar-diatreme volcanoes are small volcanic structures with a funnel-shaped crater surrounded by a tephra-ring. They are usually formed by the explosive phreatomagmatic eruptions when groundwater comes into the contact with magma. We focus on such a structure in the geodynamically active western Eger Rift (Czech Republic) and present results from multidisciplinary geophysical investigation calibrated by drilling in the newly discovered Pleistocene Bazina maar. We evaluated morphological (LiDAR-based DEM) data and confirmed the existence of a maar-diatreme structure by combined geophysical methods. In the map view, they revealed circular negative gravity anomaly, funnel-shape low-resistivity anomaly, and strong magnetic anomaly. These results allowed for the optimal location of two boreholes in the maar crater, which evinced its contact with country rocks and lithologies of the maar-diatreme filling. The drilling revealed coherent volcanic rocks and volcaniclastic deposits, moreover, it revealed a presence of a pyroclastic cone with the olivine nephelinite feeding conduit. Further investigations disclosed maar structure and subsequent pyroclastic cone(s) with several generations of eruptions and systematic decrease of water influence on the eruption style. Different eruption styles suggest a unique evolution of two volcanoes, one inside the other. The age of the Bazina maar eruption, estimated from the reverse polarity of the detected magnetic anomaly, implies that the effusion and solidification of the lava during the eruption must be older than 0.78 Ma (Pleistocene). This points to an active volcanism in the western Eger Rift in a span of similar to 0.5 Ma, where Bazina represents the oldest (maybe opening) phase. Plain Language Summary Maars are small volcanoes formed by the explosive eruptions when water comes into the contact with magma. They can form topographic depressions, however, their discovery is not easy and they can be easily missed in the fields. We focused on a newly discovered depression Bazina in the western Eger Rift (Czech Republic), which was supposed to be such a maar. Combination of geophysical surveys and drilling revealed small volcano with a complex structure. Geophysical results proved its magmatic origin and allowed for the optimal location of two exploration boreholes. They evinced the maar contact with the country rocks, coherent volcanic rocks and volcaniclastic deposits with basaltic conduit. Further investigations disclosed several subsequent eruptions within the maar with different eruption styles, from maar-forming eruption to Surtseyan style eruption and finally to Strombolian style eruption. These different eruption styles suggest the unique evolution of two volcanoes, one inside the other. The schematized animated history of this volcano is accessible at https://www.youtube.com/watch?v=VxhFrM6WR8c.