GEOMORPHOLOGICAL DEVELOPMENTS AND CLASSIFICATION OF THE QUATERNARY VOLCANOES IN JAPAN

Abstract

The evolutions of 140 Quaternary volcanoes in Japan have been studied mainly with the aid of air-photo interpretation. The study has revealed that these volcanoes have been evolved along three definite courses. On the basis of the result, the Quaternary volcanoes in Japan are classified into three types: A-, B and C-type volcanoes. A-type volcanoes generally take an evolutional course as follows : The 1 st stage : A gently sloping large cone with simple structure is constructed. The cone is composed of mafic thin lava flows and scoria falls (olivine-pyroxene basalt-andesite, SiO2 56% in average). The 2nd stage: Thick lava flows are added on the upper slope of the large cone. Therefore, the surfaces of the cone show stepped slopes. In some volcanoes, the cone iss greatly destructed by the Bandaian eruption just after the maximum growth of it, forming a horseshoe-shaped caldera on the summit. The 3rd stage: Explosive eruptions eruptions of pyroclastic flows and falls (pumiceous, hornblende-pyroxene andesite-dacite, SiO2 62%) are repeated, causing destruction of the older cone and construction of pyroclastic cones and pyroclastic flow surfaces. The 4th stage: A small caldera is formed on the summit of the cone, constructing lava domes within it. Typical cone volcanoes with simple structure, such as Fuji and Yotei Volcanoes, have developed only to the 1st stage. The volcanoes with horsehoe-shaped calderas, such as Chokai and Bandai Volcanoes, have evolved to the 2nd stage. Oshima-Komagatake and Asama Volcanoes frequently erupting pumice flows and falls, have developed to the 3rd stage. Truncated volcanoes with small calderas on the summits such as Akagi and Haruna Volcanoes, have evolved to the 4th stage. Small Krakatoan caldera volcanoes, such as Nigorigawa and Hijiori Volcanoes would be considered as A-type volcanoes which have developed to the 3rd or 4th stage without passing through the 1st and 2nd stages, because the properties and volumes of the eruptive materials are similar to those of typical A-type volcanoes. A-type volcanoes are subdivided into three-Al, A2, A3 type volcanoes-according to the difference of the developmental stage of each volcano. A1-type volcanoes are typical, large cone volcanoes with simple structure, having developed to the earlier stage (the 1st and/or 2nd stage). A2-type volcanoes are dissected or truncated composite volcanoes, having developed to the later stage (the 3rd and/or 4th stage). A3-type volcanoes are small Krakatoan caldera volcanoes, having developed to the later stage without passing through the earlier stage. B-type volcanoes are large Krakatoan caldera volcanoes which are characterised by voluminous pumice eruptions, resulting in the formations of large calderas, such as Aso, Ata, Towada and so forth. Their evolutions may be simplified as follows: Caldera stage Voluminous acidic pumice flows (ash flows) and falls violently issue out, resulting in formation of a large caldera and extensive pyroclastic flow surfaces. Post caldera stage Within and around the large caldera a-type volcanoes (small stratovolcanoes), histories and landforms of which are similar to those of A-type volcanoes, are constructed. They aree also subdivided into three-a1, a2 and a3 type volcanoes. Classification and definition of a1, a2 and a3-type volcanoes are same to those of A1, A2 and A3 type volcanoes. Sakurajima Volcano standing at the rim of Aira Caldera Volcano is regarded as a1-type volcano. Examples of a2-type volcanoes are Mashu Volcano rising at the eastern rim of Kutcharo Caldera Volcano, and Usu Volcano formed at the southern rim of Toya Caldera Volcano. Tarumai Volcano built up at the southern rim of Shikotsu Caldera Vocano is a3-type volcano.