Eruptive and Magma Feeding System Evolution of Sośnica Hill Volcano (lower Silesia, SW Poland) Revealed from Volcanological, Geophysical, and Rock Magnetic Data

The Sośnica Hill volcano is part of the Oligocene to Miocene (30.9–20.0 Ma) Strzelin volcanic field. It is located 100 km east of the Ohře Rift in the eastern part of the Fore-Sudetic Block, south of the town of Strzelin, Poland. Modern quarrying has exposed the sub-volcanic magma feeder system of the central part of the volcano and an extrusive volcanic succession that includes a 40 m thick sequence of lava flows and pyroclastic deposits that collectively form a complex scoria cone. Geophysical data (ground magnetometry and electric resistivity tomography (ERT)) reveal sharp linear anomalies that are interpreted to reflect normal faults dissecting the volcano. The ERT data map both high and low resistivity bodies, likely representing coherent clay-free dry rocks and partly argilized volcaniclastic deposits, respectively. Paleomagnetic data from 20 intrusive sites reveal two populations of reverse polarity site mean data; 11 sites are of low dispersion and yield a group mean direction that is discordant to the expected field direction, while six sites are highly scattered. Three sites did not yield interpretable results. We interpret the 11 sites as time-averaged field directions that are discordant to the expected field. The high dispersion of the remaining six sites are interpreted to indicate sub-volcanic deformation associated with the growth of the volcanic construct or multiple magma pulses over an extended period of time relative to secular variation. AMS data from 35 sites show a range of flow directions that vary across the quarry without an orderly pattern of fabric orientations. The flow pattern identified from dike paired margin data exhibits sub-vertical upward flow, sub-vertical downward, and moderately inclined northwest flow. Field observations and mapping indicate a complex development of the system in terms of styles of eruptive activity and structure of the final volcanic edifice. The activity included Strombolian and effusive phases, followed by phreatomagmatic, Hawaiian and again effusive eruptions. Such diversity of eruptive styles shows that the origin of the volcano is more complex than a simple, ‘textbook’ monogenetic scoria cone. Palaesoil on top of Strombolian deposits document a longer break in activity, after which eruptions resumed with different style; this is also not typical of monogenetic cones. The lateral variation in the volcanic succession suggests eruptions from several smaller, local vents. The complex subvolcanic magma flow patterns recorded in dikes match the variation of surface eruptive products and documents dynamically changing magma distribution paths in the near-surface and intra-cone part of the feeding system of the volcano.