2013 Volume 58 Issue 1 Pages 91-102
Spatial distribution of parasitic vents is closely related to movements of magmas at a certain depths of main conduit of the central volcano. A simple method for studying distribution of parasitic vents is presented: That is numbers of parasites per unit area according to radial distances from the central vent. On and around Sakurajima volcano, two peaks of the distribution diagram are found at roughly 2.5 and 8.5 km in radial distances. These can be interpreted into that the magmas branched away at different depths of the main vent. The branching mechanism is discussed from a standpoint of material mechanics. In this case, magmatic forces are assumed to be due to point dilatations that have proved effective in interpretation of surface deformations observed at various volcanoes. To interpret formation of parasitic vents, or outward fractures, on the flanks of a polygenetic volcano, the theory of maximum shear stress is adopted. As a result, a parasitic vent branches off from the main conduit at a depth that is related to the radial distance of the parasitic vent from the center of the volcano, and theoretically we may expect twin parasites symmetrically with respect to the center of the volcano. Whether new magmas outburst at the main crater or a new parasitic-vent fractures at the flank may depend on conditions of the main crater, the relative strengths of both the sites, and mechanism of branching. The three largest parasitic eruptions on Sakurajima volcano in historical times, the 1471～76, the 1779～80, and the 1914 eruptions, are examined: Each of these eruptions opened two vents on the opposite flanks of the central summit with a partly exception in the 1779～80 eruption. The exceptional case is suggestive for formation mechanism of twin parasitic cones. Formation of such twin vents is mechanically normal but empirically odd. An empirical fact that parasitic volcanoes only erupt once is hypothetically interpreted: Surroundings of parasitic conduits are probably strengthened mechanically by intrusion of magmas, and the sub-conduit may be tightly choked with lavas. We may say that the next eruption of Sakurajima volcano may take place at the summit crater, and otherwise, statistically, parasitic eruptions may burst probably on the flank and rarely at the sea. The parasitic vents would open at a region of “parasite-gap” on the flank, and would twin at the opposite sides of the summit. To improve the forecast, we need to clarify the formation mechanism of parasites in more detail.
