火山体とその発達

抄録

Outlines of the studies on the fundamental unit landforms (of lava flows, lava domes, pyroclastic flows, pyroclastic cones, debris avalanches and debris flows), the evolutions of individual volcanoes and temporal and regional changes of the volcano types, are briefiy described. Some of them are summarized as follows : (1) Some stratovolcanoes such as Shasta volcano are different from those of Japan in morphology and evolution, accompanied with extrusion of rhyolite domes and effusion of basalt lava flows almost simultaneously. (2) It should be reinvestigated whether co-magmatic small to medium scaled volcanisms prior to catastrophic pyroclastic erupttions associated with caldera formations existed in Japanese caldera volcanoes such as Kutcharo, Aso etc. (3) Vulsini volcano near Rome, Italy, seems to be an intermediate type between shield and caldera volcanoes. The gentle slopes outside the caldera are composed of pyroclastic flows and thin fluidal lava flows. (4) Between shield and stratovolcanoes there are intermediate type ones surrounded by very gentle and extensive lower slopes composed of basalt lava flows such as Newberry, Medicine Lake Highland, Klyuchevskoy volcanoes, etc. (5) The evolutions of the most Quaternary stratovolcanoes in Japan can be volcanostratigraphically explained by a hypothesis that they have been probably provided from individual magma reservoirs composed of parental basalt magma rising as diapirs from the upper mantle. (6) The volcanism in Japanese Islands since 2-3 Ma has changed from acidic to basic in nine regions such as Tokachi, Sengan, Aizu-Shirakawa, etc., suggesting a process different from that of rising diapirs of basalt magmas, e.g., that partial melting of crust by heating of basaltic diapirs from upper mantle occurred firstly, forming subsequently the caldera volcanoes such as Onikobe, Hakkoda, etc. and that basalt magmas as the diapirs rose secondly up, resulting in the formation of the stratovolcanoes such as Taisetsu, Iwate, Nasu volcanoes, etc.