Composite tuff ring/cone complexes in Jeju Island, Korea: possible consequences of substrate collapse and vent migration

Abstract

Hydromagmatic volcanoes (tuff rings and tuff cones) in Jeju Island, Korea, are divided into Holocene volcanoes that were constructed upon rigid, plateau-forming lavas and early to middle Pleistocene volcanoes that underlie the lavas and were constructed upon fragile, unconsolidated sediments. The older volcanoes are distinguished from the Holocene volcanoes in that the former have irregular morphologies. The irregular morphologies resulted from superposition or juxtaposition of multiple rim deposits of partial tuff rings and/or tuff cones that have contrasting lithofacies characteristics, bed attitudes, and paleoflow directions. The rim deposits are bounded by laterally persistent truncation surfaces, which suggest large-scale collapse of the substrate together with the overlying tephra piles and/or erosion during a break in eruptive activity. Collapse was probably caused by the instability of the friable sedimentary substrate, removal of lateral support because of downward quarrying of volcanic conduits, and liquefaction of the water-saturated substrate by volcanic seismicity. The path of magma supply was diverted in some cases after collapse, giving rise to migration of the active vent. In other cases, the eruption style changed abruptly from cone-forming (fallout-dominated) to ring-forming (surge-dominated) eruptions after collapse. The resultant volcanic edifices thus consist of multiple rim beds of partial tuff rings and/or tuff cones commonly with non-circular morphologies. Substrate collapse and lateral vent migration, as observed in Jeju Island, appears to be an important phenomenon in hydromagmatic volcanoes, because they are commonly constructed upon unstable sedimentary substrates. Substrate collapse and vent migration needs to be taken into account as an important control on volcano morphology and structures and as a means of lateral enlargement of terrestrial, submarine, or extraterrestrial volcanoes.