Hydrovolcanic processes forming basaltic tuff rings and cones on Cheju Island, Korea

Abstract

Tuff rings and tuff cones are small volcanoes produced by explosive magma-water interactions and have been regarded as resulting from relatively dry and wet eruptions, respectively, which are related to low and high mixing ratios of water to magma. However, comparative work on four Pleistocene basaltic tuff rings and cones on Cheju Island, Korea, shows that there are dry and wet types in both tuff rings and tuff cones, and their variations are not satisfactorily explained by the prevailing model. Instead, it is inferred that the morphological variations are directly caused by depositional processes (pyroclastic surge–dominated in tuff rings and fallout-dominated in tuff cones), irrespective of water-magma mixing ratios. The depositional processes are interpreted 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, conduit 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 land above fragile and permeable sediments and granites with some aquiclude 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 rigid basalt lavas. The explosions occurred at shallow depths mainly by bulk-interaction steam explosivity because of the rigidity of country rocks, sustained supply of shallow-level external water into the vents, and interaction of vesiculated magma with free water. This process resulted in the generation of dense, inertia-dominated jets and the formation of tuff cones mainly by fallout processes. It is thought that the morphological and sedimentological variations of these volcanoes are 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 volcanoes under otherwise identical conditions, but cannot explain the variability of tuff rings and cones in different hydrogeologic settings because the nature of hydroeruptions is governed by a number of fundamental controls.