Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 91-102

ArticleParasitic†Eruptions on Sakurajima Volcano
Izumi YOKOYAMA＊(Received September 29, 2011 ; Accepted April 19, 2012)
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 isnumbers 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 beinterpreted into that the magmas branched away at different depths of the main vent. The branching mechanism isdiscussed 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 ofmaximum shear stress is adopted. As a result, a parasitic vent branches off from the main conduit at a depth that isrelated to the radial distance of the parasitic vent from the center of the volcano, and theoretically we may expect twinparasites 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 thesites, and mechanism of branching. The three largest parasitic eruptions on Sakurajima volcano in historical times, the1471〜76, the 1779〜80, and the 1914 eruptions, are examined: Each of these eruptions opened two vents on theopposite flanks of the central summit with a partly exception in the 1779〜80 eruption. The exceptional case issuggestive 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: Surroundingsof parasitic conduits are probably strengthened mechanically by intrusion of magmas, and the sub-conduit may betightly 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 thesummit. To improve the forecast, we need to clarify the formation mechanism of parasites in more detail.
Key words : parasitic eruptions, monogenetic volcanoes, maximum shear stress, Sakurajima volcano, twin parasiticvents.

1．Introduction
Parasitic volcanisms have been deemed as secondary
though they are actually an important manifestation of volcanic forces. Parasitic eruptions on a polygenetic volcano
occur when magmas branch off from the central conduit ata certain depth and erupt on the flanks forming cones.
These parasitic cones are different from monogenetic ones
formed in volcanic fields or volcano groups. The latter
involves with the ascent of individual magma batches fromgreat depths. Parasitic volcanoes are nevertheless considered to be monogenetic because their vents are
generally not expected to experience additional activity
when once an eruption cycle is over. Understanding thecauses of parasitic eruptions, rather than eruptions from the†In the present paper, the author prefers “parasitic” to“lateral” because the former expresses the meaning oforigination better. P.O.D. says that “parasite” is an animal
or a plant, living in or on another & drawing nutrimentfrom it.

central vent of a polygenetic volcano, is important becausesuch vents may bring volcanic activity closer to humansettlements, and because lava flows erupted from parasitic
vents may more readily inundate areas low on the flanks ofthe volcano. Therefore, it is important to examine thedistribution of parasitic vents and interpret the mechanismsthat give rise to this distribution.
Of many historical records of eruptions on Sakurajima
volcano, the three biggest eruptions all produced two ventson the opposite flanks of the central summit. Suchcharacteristics of the Sakurajima eruptions should be themost suitable subject in the discussion of parasitic volcanisms.

＊Higashi 1-17-7-1304, Kunitachi, Tokyo 186-0002, Japan.
Corresponding author: Izumi Yokoyama
e-mail: yokoizu@aol.jp