Jan Valenta

The article summarises the results of a pilot project that investigated the riverine cultural landscape of the Jizera River, primarily focusing on the correlation between archaeological structures and the river’s watercourse. The authors have demonstrated a novel integration of archaeology, hydrology and some geophysical prospection methods, which together have enabled the delineation of potential areas for identifying sites of significant archaeological significance, both directly within the watercourse and its immediate environs. As well as archaeological structures, this approach has also facilitated an interpretation of the relationships between anthropogenic and natural phenomena.

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.

Ground fissures, especially if they open due to a sudden collapse of the surface, is a serious risk for populated areas. Their common occurrence in unconsolidated sediments of the Main Ethiopian Rift was found to be mostly a result of piping.The fissures start by piping in linear sub‐horizontal underground voids, which often propagate upwards resulting in ceiling collapse and formation of deep and long ground fissures with vertical walls. In the southern and central Main Ethiopian Rift the fissures pose a serious risk to infrastructure and settlements. The ground fissures are often linear (up to several kilometres long and often tens of metres deep) and accompanied by sinkholes (along the length). A detailed field mapping of the geological (rock composition, orientation and character of lithological boundaries, primary fabrics and brittle structures) and geomorphological features (especially a length, width and depth of fissures, sinkholes and gullies) followed by in situ seismic anisotropy measurements and a laboratory determination of the geomechanical properties of volcanoclastic deposits was carried out to investigate the ground fissures' origin. The conditions and factors leading to the formation of the ground fissures have been linked to: (a) the presence of regional normal faults and the associated extensional joints and (b) the alternation of lithological units with contrasting hydraulic permeability. The latter corresponds to a sequence of less permeable hard rocks (e.g., rhyolitic ignimbrites) overlain by heterogeneous, soft and permeable, unconsolidated volcaniclastic deposits with a low amount of clay (less than 10%). The ground fissures' occurrence has shown affiliation to areas which have a significantly high seismic anisotropy (more than 20% at the study sites), which can be used as a proxy to map out high risk areas prone to piping and ground fissure formation.

Comment by Bábek et al. (2020, https://doi.org/10.1029/2020JF005916) is concerning the interpretation of realized geophysical measurements. Since, to our knowledge, Klanica et al. (2020, https://doi.org/10.1029/2020JF005663) conducted the deepest geophysical surveys in the area, a scale of work was probably misunderstood within the interpretation of our results. Second, a large number of objections arise from the comment about the post‐Badenian geological development at the eastern margin of the Bohemian Massif. However, evidence for the time when particular geological processes took place is not definite and our conclusions of our study can be related to early Badenian age.

Extremely deep freshwater filled cave systems are common in karst systems globally. The origin and evolution of such caves are usually attributed to hypogenic (bottom‐up) processes, in which acidic groundwater dissolves limestone from below. However, these deep cave systems can form by epigenic (top‐down) processes, with meteoric waters descending from the surface underground. The Hranice Abyss (Czech Republic), with a reached depth of 473.5 m, is the deepest mapped extent of such a system globally, although its maximum depth is unknown. Multiple geophysical data sets (gravity, electrical resistivity tomography, audiomagnetotellurics, and seismic refraction and reflection) are used to investigate the extent and formation of the Hranice Abyss. The geophysical results suggest the Hranice Abyss extends to depths of ~1 km. Further, we identify structures within the karst, including buried cockpit karst towers with several NW‐SE‐oriented valleys. The new geophysical results from the Hranice Abyss, considered in combination with geological constraints of the region (tectonic evolution and morphology of karst structure), suggest an epigenic formation process, rather than the traditionally invoked hypogenic origin. Formation by epigenic rather than hypogenic processes has implications for local and regional karst history associated with areas hosting deep karst systems.

The Zebín Volcano, Czech Republic, is a quarried tuff cone that offers the opportunity to understand emplacement processes responsible for magma transport through a pyroclastic cone and magmatic processes involved in its development. A high‐resolution ground magnetometry survey, electrical resistivity tomography, and refraction seismic were conducted to supplement field mapping of the size, shape, and inner structure of the volcanic system. The Zebín Volcano yields normal polarity rocks, and the cone is underlain by a complex and branching magma feeder system with several diverging dikes. Samples were collected at 24 sites from feeder dikes and the main conduit for anisotropy of magnetic susceptibility and paleomagnetic analysis. Anisotropy of magnetic susceptibility results yield inferred magma flow directions indicating subhorizontal magma flow toward and away from the axial conduit as well as both upward and downward flow. Paleomagnetic data from all sites are normal polarity with statistically distinct remanence directions between some sites that indicate that significant time passed or deformation occurred during the growth of this volcanic system. ⁴⁰Ar/³⁹Ar isochron ages of groundmass provide emplacement ages of 18.38 ± 0.03 and 18.45 ± 0.03 Ma, whereas a weighted mean date of 18.52 ± 0.03 Ma for hornblende provides an eruption age. We argue that the Zebín Volcano evolved from a polyphase feeding system and through a complex feeder network. This detailed study shows that many monogenetic volcanoes deserve highly detailed study, as their subtleties can provide insight into broader crustal and magmatic environment during magmatism.