Evidences of interactions between Velký Roudný volcanic activity and its hydrous environments
Authors: Rapprich Vladislav , Skácelová Zuzana 
Published: February 2023

Velký Roudný scoria cone represents the most prominent volcanic edifice of the Plio-Pleistocene Bruntál Volcanic Field (according to Cajz et al. 2012). Various types of volcaniclastic deposits and volcanic features were interpreted in various ways in the past. However, several features remained overlooked, and the investigated ones required critical revision. Velký Roudný is an asymmetric horseshoe-shaped cone, with a crater open towards the southeast (Fig. 1a). The lava flows emitted from this cone flew southwards and also to the east/ southeast, where the lava filled a paleo-canyon of the Moravice River. In the paleo-canyon, the lava (up to 60 m thick) buried earlier fluvial sediments (Horský et al. 1972; Fig. 2). The best exposure of the lava flow is accessible in the active Bílčice quarry. The two levels of columnar jointing separated by a platy- to blocky-jointed domain represent the upper and lower colonnade of a single lava flow (Fig. 3a, b). The columnar jointing domains are interrupted by subvertically platy-jointed, piston-like features filled with breccia (Fig. 3c, d). These monomictic breccias are dominated by basaltic clasts with signs of abrupt quenching, like cauliflower structures (Fig. 3e, f ). The shapes of these breccia bodies, together with signs of magma/water interactions, suggest their origin from rootless explosions. Their extension through the complete thickness of the lava, as well as their distribution in the lava flow were earlier documented by geophysical survey (Štainbruch 2019), whose results are re-interpreted in this contribution (Fig. 4). Another effect of the lava/water interaction is represented by “sunburn” decomposition (spheroidal decomposition of the rock; Fig. 5a). The development of the spheroidal cracks seems to be restricted to the glass-containing groundmass (Fig. 5b). As the sunburn decomposition develops mainly at the base, around the rootless craters, and less frequently on the top of the lava. It seems that these irregular features reflect gentle hydration of glass-containing rock, similarly to perlitic cracks in silicic lavas (e. g., McPhie et al. 1993). Subsequently to cracking, the glass tends to analcime recrystallization (Fig. 5c, d), enhancing the decomposition of the rock. Pyroclastic deposits exposed in two small abandoned quarries near Razová and Karlovec, respectively, were interpreted as tuffites (mixed volcano-sedimentary deposits) in the past. Despite that these deposits contain abundant fragments of basement greywackes, the fragments display no signs of rounding or sorting (Fig. 6). Poorlysorted clast-supported deposits well correspond to the near-vent ballistic accumulation of phreatomagmatic eruption. At Karlovec, predominance of the finer-grained facies with diagonal bedding suggests that these rocks originated by base-surge deposition. The potential source of the phreatomagmatic eruption can be found in the circular depression underneath Velký Roudný (Fig. 1b, c), as supported by a magnetic anomaly (Fig. 7). The phreatomagmatic character of the volcanic activity possibly results from an eruption through a lake dammed by the Bílčice lava of the Velký Roudný scoria cone (Fig. 8).