INFLUENCE OF HYDROGEOLOGY OF PRE-VOLCANIC ROCK UNITS ON THE FORMATION OF DIFFERENT TYPE OF MONOGENETIC VOLCANOES IN A MIO/PLIOCENE FLUVIO-LACUSTRINE BASIN: WESTERN HUNGARY

Volcanic fields in western Hungary are Late Miocene/Pliocene alkaline basaltic intracontinental fields and consist of erosional remnants of maars, tuff ring, scoria cones, and lava flows. At the onset of the eruption, magma began to interact with groundwater in the water-saturated Neogene fluvio-lacustrine sand beds. As the eruption continued, the craters grew and the phreatomagmatic blasts fractured the deeper (consolidated) rocks around the down-migrating explosion loci, giving the karst water free access to the explosion chamber. The appearance of maar volcanoes and their deposits of western Hungary are strongly dependent on the hydrological conditions of the fracture-controlled aquifer, which vary seasonally. Maar volcanoes formed due to phreatomagmatic explosions of mixing magma with water saturated clastic sediments in areas where thick Neogene silicilcastic units build up the immediate pre-volcanic strata. Such volcanoes have often formed magmatic infill. These vents named summer vents represent low water input from the lower karst level. Unusual maar (Tihany-type) had a special combination of water source from both the porous media aquifer and fracture-controlled aquifer, with the latter probably the dominant supply. Such maars developed in areas, where relatively thin Neogene fluvio-lacustrine units resting on the Mesozoic or Paleozoic fracture-controlled, karstwater-bearing aquifer. These maars most likely generated during spring (spring vents). In the northern part of the field scoria cones and shield volcanoes give evidence for a smaller impact of the ground and surface water in control the volcanic eruptions. Diatreme-filling rocks with sedimentary grains or mineral phases that derived from already eroded Neogene shallow marine to fluvio-lacustrine sedimentary units are the evidence that such a sedimentary cover were intact in syn-volcanic time. The abundance of such clasts in the pyroclastic rocks also indicates the importance of soft rock environment to where phreatomagmatic volcanoes erupted. The presence of intravent/crater peperite, subaqueous dome, shallow intrusions as well as hyaloclastite indicate that these volcanoes have been quickly flooded by ground and/or surface water, suggesting that they were erupted close to the level of paleoground water table.