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

Abstract

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 Bažina 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 Bažina 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 ∼0.5 Ma, where Bažina represents the oldest (maybe opening) phase.