Tephra Studies on Quaternary Explosive Eruptions in the Japanese Islands

Abstract

Japanese tephra studies have developed steadily in the last five decades. First two decades until 1976 brought fundamental ideas and techniques for correlation and for recognition of widespread co-ignimbrite ash falls. Geochemical and physical characterization of tephras advanced greatly in the third decade in 1980s. This enabled identification and distinction of many individual tephra layers and then compilation of tephra catalogs for solid tephrostratigraphic framework. VEI=7 classed large-scale eruptions had occurred at several calderas in Kyushu and Hokkaido Islands during the period from Middle Pleistocene to Holocene. Estimated average frequency is once in every a few ten thousand years. Their impacts on human society and ecosystem were analyzed through many disciplines of Quaternary research and turned out to have been very severe. Late Pleistocene and Holocene eruptive histories of medium-scale Plinian eruptions (VEI=4-6) are mostly documented and included in catalogs. Studies on their impacts on humankind during both pre-historical and historical ages confirmed severe volcanic hazards. The knowledge makes long-term prediction and volcanic hazard mitigation more reliable. Detailed stratgraphic and chronological studies on Holocene pyroclastic deposits helped to establish precise eruptive histories of some active volcanoes. Many recent works focused on Pliocene to Middle Pleistocene widespread tephras. Middle Pleistocene tephrochronology has provided information on regional changes of the frequency and magnitude of eruptions and magma discharge rate as a function of time. Studies of older distal tephras preserved in non-volcanic sediments let us know the occurrence of large-scale eruptions without any volcanic edifice preserved. The 105 to 106 year scale tephrochronology is revealing very long-term evolution of volcanic activity and magma discharge.