The role of external environmental factors in changing eruption styles of monogenetic volcanoes in a Mio/Pleistocene continental volcanic field in western Hungary

The occurrence, shape, structure and eruption style of monogenetic volcanoes, such as maars, tuff rings, tuff cones and scoria cones, are generally governed by several internal (composition of the magma, magmatic flux, ascent rate, viscosity, volatile contents) and external conditions (regional and local tectonics, topography, and the presence of surfacial, ground and meteoric water). These controlling factors are together responsible for the eruption style, distribution pattern, volcanic facies architecture and morphology of the monogenetic volcanic landforms. The Late Miocene to Pleistocene Bakony-Balaton Highland Volcanic Field (BBHVF) in western Hungary is a typical small sized (< 50 eruption centres), basaltic, intraplate monogenetic volcanic field. Generally, initial eruptions of the BBHVF were phreatomagmatic (n = similar to 28); however, a lesser number (n = similar to 14) of predominantly scoria cone forming eruptions are also inferred. The temporal distribution of the Strombolian style scoria cones was concentrated mostly between 3 and 2.5 Ma. A detailed study of the changes in eruption styles recorded in the pyroclastic sequences suggested a change from a conventional phreatomagmatic to a magmatic fragmentation style during the activity of the volcanic field. A clear correlation has been identified between the long-term environmental changes of the region that resulted in a gradual shift from a more phreatomagmatic eruption style to a more magmatic eruption style. Detailed examination of the temporal distribution of K-Ar and Ar-Ar radiometric data, Digital Elevation Model and Dense Rock Equivalent-based volume calculations of eruptive products and origin of pyroclastic rocks (e.g. phreatomagmatic or magmatic) preserved in variously eroded monogenetic volcanoes were utilized to integrate available volcanological and climatological data to identify potential links between external and internal controlling parameters that responsible for long-term eruption style changes. At least 6 volcanic cycles have been identified by cluster analysis. Time gap between the cycles were vary from 1.66 up to 0.06 Ma, while the average eruption recurrence rate was similar to 0.1078 Ma/event. The time-volume diagram of the volcanism of BBHVF have shown time-predictive behavior combined with low magma-flux (total preserved volume -2.867 km(3)) and output rates (0.53 km(3)/Ma for the entire volcanic field and 0.90 km(3)/Ma for the last 5 cycles), suggesting that volcanism was largely tectonically-controlled and not magmatically-controlled. Furthermore, the topographic differences between the northern (elevated) and southern (basin-like) parts of the volcanic field, are also important in local differences in dominant fragmentation style, because the elevated part of the field was prone to host large, more magmatically-evolved volcanoes, than on the lower, water-saturated, unconsolidated sediments, which favored to the magma/water interaction driven phreatomagmatic fragmentation. The third controlling parameter, which seems to play an important role in controlling the eruptive style of monogenetic volcanism at BBHVF, was the paleoclimate fluctuation, especially during the time interval of 3.0 to 2.5 Ma. Thus, mainly the long-term environmental changes (e.g. aridification) have been response the shifting fragmentation style from phreatomagmatic to more magmatic ones. (C) 2010 Elsevier B.V. All rights reserved.