Evidences of interactions between Velký Roudný volcanic activity and its hydrous environments

Abstract

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