Did Forefather Czech know he was standing on a fossil lava lake? Revisiting the Oligocene Říp Hill volcano, Bohemian Massif

Říp Hill, a volcanic structure near the Eger Rift of the European Cenozoic Rift System, holds deep cultural significance for the Czech nation. According to legend, Forefather Czech (Bohemus) and his Slavic tribe declared the surrounding land their new home around the 6th or 7th century, marking the symbolic beginning of Czech history. Despite its prominence, the geology of Říp Hill remains underexplored. The complete Bouguer anomaly map reveals intersecting NE–SW negative and NW–SE positive lineaments. The NW–SE lineament is interpreted as a dense nephelinite dike parallel to the Elbe tectonovolcanic zone emplaced during N–S horizontal extension, which induced sinistral shearing and segmented the dike into an E–W en-echelon array. These inferred segmented dikes, identified by magnetic anomalies, may have served as volcanic feeders. The nephelinite body above the plumbing system represents a lava lake as shown by columnar joints forming an inverted fan, overlying entablature, and shallow concentric magnetic foliations. We speculate that the eroded crater was of a phreatomagmatic maar-type. Indirect evidence includes the large aerial extent of the nephelinite body, a circular gravity low around the hill, underlying groundwater-saturated Cretaceous sedimentary rocks, and analogies with similar Cenozoic maar-diatreme volcanoes in the Bohemian Massif.