Yongmeori is a tuff ring formed by accumulation of volcanic ashes resulted from explosive magma-water interactions (hydrovolcanic activity). It underwent three collapses along the volcanic edifice. As a result, deposits of the tuff ring were transported in different directions forming three stratal packages, or layers. The overall thin stratification (layers of sedimentary rocks parallel to each other and visible in cross-section) with undulatory or mega-ripple bedforms suggests that it was deposited mostly by pyroclastic surges (turbulent, ground-hugging flows of volcanic gases, steam, and pyroclastic materials). Yongmeori resulted from superposition of multiple tuff-ring deposits that have contrasting bed attitudes and paleoflow directions. The laterally persistent truncation surface is interpreted to have formed during a break in eruptive activity after large-scale collapse of the substrate. Collapse was probably caused by the instability of the friable sedimentary substrate (the U Formation), removal of lateral support due to downward quarrying of volcanic conduits, and liquefaction of the water-saturated substrate by volcanic seismicity. The path of magma supply was probably diverted in some cases after the collapse, giving rise to migration of the active vent. The resultant volcanic edifice thus became non-circular or irregular. All these processes were possible because Yongmeori was built upon unconsolidated shelf sediment named the U Formation before the effusion of the shield-forming lavas of Jeju Island. Yongmeori is also a historic site where a Dutch merchant ship was shipwrecked in 1653. The survivors of the ship were detained in Korea for 13 years.