Dependence Of Volcanic Systems On Tectonic Stress Conditions As Revealed By Features Of Volcanoes Near Izu Peninsula, Japan

摘要

Five volcanoes, Asama, Fuji, Izu-Tobu, Izu-Oshima and Miyakejima, which are aligned along the same volcanic chain in central Japan, are subjected to different degrees to the stress conditions associated with the continental collision and subduction along wedge shaped trenches near Izu peninsula. At a great distance from the collision boundary, Asama is characterized by an axially symmetric edifice with a unique summit crater for magma effusion and Miyakejima has a similar symmetric structure accompanied by radial eruptive fissures. At the opposite extreme Izu-Tobu, which is the closest to the collision boundary, consists of many small monogenetic volcanoes that lack a central vent. In intermediate locations, Fuji and Izu-Oshima exhibit elongated volcanic edifices with both summit vents and parallel fissures on their flanks for magma effusion. To understand this systematic trend of volcanic features, growth of volcanic systems is analyzed based on magma transport in cylindrical conduits and planar fissures available in individual stress conditions. It is assumed in this analysis that fissures are produced with extensional stresses or high magma pressures enough to overcome compressive stresses and that cylindrical conduits that have arisen from localization of fissures can be persistently used for repeated magma supply. The analysis predicts that volcanoes are classified into the following three types that represent their growth processes in different stress conditions. (1) A polygenetic volcano having an axially symmetric edifice grows with unique magma supply through the central conduit under compressive or neutral stresses, (2) a polygenetic volcano having an elongated edifice grows with a summit crater and eruptive fissures parallel to the elongation under weak extensions, and (3) a group of small monogenetic volcanoes is constructed along many parallel fissures under strong extensions. This classification is examined for the five volcanoes and some other volcanoes in the world. The alternate occurrences of summit and flank eruptions observed at some polygenetic volcanoes are interpreted to be controlled by the summit height, magma densities and stress conditions that influence the pressure of a common magma source.