HYDROVOLCANIC PROCESSES FORMING BASALTIC TUFF RINGS AND CONES ON CHEJU ISLAND, KOREA

Tuff rings and tuff cones are small volcanoes produced by explosive ma gma-water interactions and have been regarded as resulting from relati vely dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on fo ur Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, s hows that there are dry and wet types in both tuff rings and tuff cone s, and their variations are not satisfactorily explained by the prevai ling model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge-domin ated in tuff rings and fallout-dominated in tuff cones), irrespective of water-magma mixing ratios. The depositional processes are interpret ed to be in turn controlled by a number of fundamental controls, which include depositional settings, type, level, and lithology of aquifers , strength of country rocks, ground-water behavior, and properties and behavior of magma, These controls determine the explosion depth, cond uit geometry, mode of magma-water interaction, magnitude of explosion, eruption-column behavior, and subsequent depositional processes. The Suwolbong and Songaksan tuff rings, which formed almost entirely on la nd above fragile and permeable sediments and granites with some aquicl ude beds, were produced by contact-surface steam explosivity at depth because of the fragility of country rocks, insufficient and inhibited supply of shallow-level external water into the vents, and interaction of nonvesiculated magma with interstitial water. These conditions led to generation of buoyancy-dominated eruption columns and pyroclastic surges, resulting in tuff rings. On the other hand, the Ilchulbong and Udo tuff cones formed in shallow seas above extremely permeable but r igid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country reeks, sustained supply of shallow-level external water into the vent s, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the fo rmation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes a re more successfully explained by the fundamental controls rather than solely by the water-magma ratio. It is suggested that the water-magma ratio can explain the evolution of a single volcano or a group of vol canoes under otherwise identical conditions, but cannot explain the va riability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundam ental controls.