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 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 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 the
opposite 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 parasitic
vents.

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 the
distribution 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

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Izumi YOKOYAMA

Fig. 2. Radial density distribution of parasitic vents (perkm2) over Sakurajima and Ooshima volcanoes.
Fig. 1. Distribution of parasitic vents over Sakurajima
volcano after Yamaguchi (1975) and Kobayashi (1988).
“Tw” and “Te” indicate the series of vent (5 each)
formed by the 1914 eruption, and “Ts” does the lava
flow from land in the 1914 eruption. “Ai” and “As” arethe lava flows of submarine origin and the lava flows
from land, respectively and both were issued by the1779 eruption. “SC” is “Showa Crater”.

2．Spatial Distribution of Parasitic Cones on Sakurajima Volcano
The spatial distribution of volcanic cones is an important subject in studying the origins of volcano groups.
Hasenaka and Carmichael (1985) discussed distribution ofvents in the Michoacán-Guanajuato volcanic field in
Mexico (1016 cinder cones over an area of 6×104 km2).
The cinder cones in the field are usually random-spaced
and indicate no preferred orientation. They estimated thedensity of vents in various areas and found that roughly 75% of the volcanoes are located between 200 and 300 km in
the distance from the Middle America Trench. To thesame volcanic field, Connor (1987, 1990) applied cluster
analyses and two-dimensional Fourier analyses and foundthat the vent alignment data imply the stress field on aregional scale. In this field, there are so many monogenetic cones that his methods are successful.
In the following, spatial distribution of parasitic cones
on some polygenetic volcanoes shall be examined from a
consideration of that there are usually not so numerous
parasites there.
2-1 Sakurajima volcano
The island of Sakurajima volcano was originated at thesea bottom in Holocene. The island was connected with
the Oosumi Peninsula by lava flows of the 1914 eruption.
The present Sakurajima volcano is composed of three maincones on a line from N to S. Parasitic eruptions haveoccurred frequently on this polygenetic volcano. The
rocks of the ejecta from the volcano are all andesitic. The
historical records of eruptions on Sakurajima volcano dateback to 708 A.D. At present, it is one of the most active
volcanoes in Japan.
On the volcano, there are many cones formed in historical times, but it is rather difficult to distinguish allparasitic cones exactly because the older ones are covered
with lava flows of the later periods. In the present discussion, identification of parasitic cones is fundamentally
important. The author can count 25 parasitic cones on andaround Sakurajima volcano on geologic maps provided by
Yamaguchi (1975) and Kobayashi (1988). The distribution of parasitic cones on and around the volcano is shownin Fig. 1. Parasitic eruptions were historically recorded as
in 764, 1471〜76, 1779〜80, and 1914. In 1939, a crater(“SC” in Fig. 1) was formed at a height of about 750 m
a.s.l. and about 500 m distant from the center of thesummit crater of South-Peak (1040 m a.s.l.). This isnamed “Showa Crater” (formed in the Showa era). In thepresent discussion, this crater is excluded from theparasitic vents because it is vey close to the main craterand may be deemed as an auxiliary crater: Its vent may
have branched from the main vent of South-Peak at ashallow depth due to some local structures.
Numbers of parasitic vents per unit area against theradial distance from the center of the volcano (the middle
point between N-Peak and S-Peak in Fig. 1) are obtained
as shown in Fig. 2 where the 1914 vents at the eastern andwestern flanks are represented by those of the highest
altitudes in respective group because the lava flows issued
first from the highest vents and later from the lower ones
on both the flanks: This means that their origins are thesame in respective group. The distribution in Fig. 2 shows
double-peaked densities around 2.5 and 8.5 km in radial
distance; the former is the mode presenting the flank vents

Parasitic Eruptions on Sakurajima Volcano

Fig. 3. Distribution of parasitic cones over Ooshima volcano after Nakamura (1961).

around a height of 300 m a.s.l. and the latter presents theislets in NE off the volcano island, formed in 1779〜1780by submarine eruptions.
In the following, for reference, in comparison with
Sakurajima volcano, the similar discussion shall be extended to the other polygenetic and monogenetic volcanoes.
2-2 Izu-Ooshima volcano
Izu-Ooshima volcano belongs to the Shichito (SevenIzu-Islands)-Mariana arc. In the following, we call this
volcano simply Ooshima volcano. The distribution ofparasitic cones on this volcano is shown in Fig. 3 after
Nakamura (1961). We have no historical records ofparasitic eruptions on this volcano. The radial density
distribution of parasitic cones is also shown in Fig. 2. The
cones within the caldera and on the caldera rim are
excluded from the data because they are closely related tothe main conduit of the main cone (Mihara-yama): They
are auxiliary to the main cone. The diagram for Ooshima
volcano in Fig. 2 shows double-peaked frequencies around
3.5 and 5.5 km in radial distance from the center of the
volcano: The former is the mode corresponding to theparasitic cones around a height of 300 m a.s.l. and thelatter does to the cones at the N and S ends of the island.
Nakamura (1961, 1977) discussed the activity of parasitic cones in stratigraphic studies of Ooshima volcano.

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He measured trends of parasitic cones with the eye, andfound that the distribution of whole parasitic cones isrestricted to two narrow zones running nearly parallel tothe long axis of the island, and proposed that these fracturezones were produced mainly by regional stress rather thanthat they were produced independently within the island by
up-thrusting stresses of a magma reservoir. His viewpoint
is different from that of the present author, and the
distribution of the parasites on Ooshima volcano may
exhibit dual patterns for different viewpoints, i.e. linear
arrangements and circumferential ones.
Even if a linear trend of the parasitic cones is recognizedon Ooshima volcano, not the same on Miyake volcano(Nakamura, 1984). The latter is located at a distance ofabout 70 km S, both the volcano-islands belonging to theSeven-Izu-Islands and similarly originated from basaltic
magmas. This means that the tendency proposed on
Ooshima volcano is rather local not regional. It is problematical whether a volcano body is a simple accretion on
the earth crust or a derivative of the crustal structure.
Here, in order to recognize characteristics in distributionof parasitic cones on polygenetic volcanoes, the similar
method shall be applied to a monogenetic volcano group.
2-3 The East-Izu monogenetic volcano group
This volcano group is located at the eastern part of the
Izu Peninsula and off the eastern coast. Aramaki and
Hamuro (1977) studied geology of the group. There areabout 70 cones on land and 80 in the sea as shown in Fig. 4(a). In March〜May 1930, earthquakes swarms were felt
around Ito City. And in November of the same year, theNorth-Izu Earthquake (Ms 7.3) occurred. Further, both the
1978〜1979 earthquake swarms and an earthquake of Ms
6.7 were located at the E of the Izu Peninsula. In July
1989, earthquake swarms started again: On July 13, a
small submarine eruption occurred at the northernmost
part of the group, about 3 km N of Ito City (a star symbolin Fig. 4 (a)). The eruption site coincides with theepicentral area of the 1930 earthquake swarms (Ueki,1992). This is the first historic eruption of this group.
Among the group, there is no central volcano. To
examine a trend in the distribution of monogenetic conesin this area, if any, tentatively the center of the group is set
at the cross mark in Fig. 4 (a). Along the E shore of thepeninsula, some cones may have been eroded. Then the
distribution of the cones is plotted against the radial
distance from the cross as shown in Fig. 4 (b). In
comparison with polygenetic volcanoes, Sakurajima andOoshima, the diagram has not so prominent peaks. Thismeans that the distribution does not largely depend on
position of the assumed center, or there is no principal ventforming a central cone. An interpretation for the origin of
this monogenetic group shall be discussed later.
3．Formation of Parasitic Vents
Formation of parasitic vents on polygenetic volcanoes

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Izumi YOKOYAMA

Fig. 4. The East-Izu monogenetic volcano group.
(a) Distribution of monogenetic cones after Aramaki
and Hamuro (1977). A cross symbol is assumed as thecenter of the group.
(b) Density distribution of cones (per km2) against theradial distance from the assumed center of the group.

shall be discussed from viewpoint of their magmatic force
and crustal strength. Here we assume magma conduit ofmain crater to be pipe-shaped for the purpose of general
discussion admitting planar dikes as an alternative path tothe crater. The magma proceeds upward due to buoyancy
or magma pressure in excess of lithostatic pressure, andreaches a certain depth. Hereafter, the magma movement
depends on the condition of pre-existing central vent. If
the conduit to the main vent is available, or magmatic force
can form a new conduit to the main crater, the magma willreach there, and if the main conduit is tightly chokedagainst the ascending magma, the magma should takeanother way aiming at the highly stressed points.
3-1 Models of magmatic forces at the origin
In principle we deal with magmatic forces acting
upwards and pipe-shaped conduits beneath an active
polygenetic volcano. Here, for the sake of analyticaltreatments, we assume a pressure source embedded in asemi-infinite elastic body. The source can be at the middleof the conduit, and not necessary at a magma reservoir.
We adopt a coordinate system for a spherical pressuresource of radius a, and depth D and pressure distribution at
the origin is expressed by spherical harmonics. Two

simple and fundamental models are considered:a) A uniform expansion of the source (point dilatation):
Constant pressure P = P0 and intensity a3 P0 . This is amodel of a magma reservoir exerting pressure uniformly
and radially around the source. The deformation at thesurface caused by this model was analytically calculated
first by Sezawa (1931). On the other hand, Anderson(1936) used the same model to discuss the formation ofvarious dykes, sills and cone-sheets.
b) We may assume another similar model: sourcepressure P=P0 cos θ, in which all the components of theforce are upward parallel to Z-axis. This may better
approximate force acting at the uppermost part of a magma
conduit. Soeda (1944) calculated deformation caused byforce of this type, analytically but with some approximations.
These models are simple and have amply proved to be
successful in interpretation of volcanic deformations invarious cases. It is reasonable for us to start with thesemodels. Deformation at the ground surface caused by theabove two models are given in detail by Yamashina (1986)
assuming radius a to be much smaller than depth D. The
results obtained by the two models are substantiallysimilar. In the following discussion, the present author
adopts the point dilatation model for convenience.
3-2 Criteria of fracture
Criteria of fracture in material mechanics (e.g. Jaeger,1964) are applicable to the discussion of fractures on
volcanoes caused by subterranean force. Formation ofcentral vents at the summits of polygenetic volcanoes canbe explained by the maximum principal stress theory
while formation of parasitic vents may be a subject of
discussion by the maximum shear stress theory (or the
maximum stress difference theory).
When the pre-existing central crater or vent, right over
the origin, is strongly solidified and blockaded, magma
canʼt break out at this point. Instead, the magma at depths
should proceed obliquely toward the mechanically moststressed point under various conditions. In this case, thetheory of maximum shear stress is applicable as alreadyreviewed by De la Cruz-Reyna and Yokoyama (2011).
According to the theory of maximum shear stress, thecritical stress is equal to a half of the horizontal differential
stress and is represented as:
1/2(σ x−σ z)(1)where σx and σz denote the greatest and least principal
stresses, respectively. And the maximum shear stress occurs
across a plane whose normal bisects the angle between σx
and σz. In the present discussion, we do not know how the
principal stresses are distributed on each volcano. However,some general results may be appropriate to explain theformation of parasitic vents.
In the following, we assume a simple source model
beneath a horizontal plane: point dilatation, P=P0 at depthParasitic Eruptions on Sakurajima Volcano

95

D and with intensity a3P0. The free surface displacements
and the corresponding strain tensor are:u z=C 0 D/R 3, and εzr=−3C 0 Dr/R 5,
u r=C 0 r/R 3, and εrr=C 0(D 2−2r 2)/R 5,
u =0,(2)and ε=C 0 /R 3 ,where C0 = (λ+2 μ) a3 P0 / {2 μ (λ+μ)}, R= D 2 + r2,
and μ and λ denote the Lameʼs parameters.
At the ground surface, σzz=0, and the maximum shear
stress (σx−σz) is expressed in polar coordinates (r, φ, θ )as:
σ rr−σ =−6μ C 0 r 2/R 5.

(3)

The value of (3) takes positive or negative maximum at:
r=± 6 D/3=±0.82D, or D=1.22r(4)

In other words, the medium receives the maximum shear
stress at a radial distance r=±0.82 D, or where the dip
angle of the pressure source from the fracture point at thesurface is 51°. In the equation, we expect the maximum
horizontal differential stress at two points, r=±0.82 D, oron the opposite sides of the center of volcanoes and these
are independent on declination θ though the maximum
shear stress acts in the limited plane. Possibility of such apair of fracturing points may depend on particularconditions on each volcano. When one of the parasitic
vents is opened, the stresses may concentrate there and theother vent probably cannot be opened. Therefore, twinparasitic vents are odd in general. And furthermore, as
shall be mentioned later, parasitic vents are usuallymonogenetic, and so parasitic vents cannot be repeatedly
formed at the same spots. The relation (4) is shown in Fig.
5 agrees to the result by Anderson (1936, Fig. 8). In the
figure, a branch point from the main conduit may be a
singular point in the volcanic plumbing system such as amagma reservoir and a kind of knots along the conduit.
On actual volcanoes, flank surfaces are not horizontal,
and the volcano structure is heterogeneous, and so theabove results should be considered as rather approximative.
Whether magma breaks out from main crater or fromparasitic vents depends on the condition of existing main
crater and strength of the medium concerned, or on thebalance between magma pressure and rock strength at thesites concerned. Main craters at the summits suffer fromcompressive stress, and parasitic sites do from bothcompressive and shear stresses. Thus, rock strength plays
an important role in the above determination. Compressive rock strength is roughly in order of 100 MPa and shear
strength roughly in order of 10 MPa (Jaeger, 1964, Tables
III and IV). The place of fracture depends on balance
between upward compressive force against the strong
compressive strength and shear force against the weakFig. 5. Schematic model for formation of parasitic vents.
(a) Geometrical distribution of the magma paths.
(b) A simplified elevation showing small branchesfrom sub-conduit. Numerals 1, 2, 3 - - - denote theorder of outburst in usual cases.

shear strength. At present the balance cannot be estimatedquantitatively because some parameters in the above
discussion are not accurately determinable. Strict
exclusiveness between summit eruption and parasitic
eruption has not been recognized in many historicaleruptions. On some volcanoes, parasitic eruptions haveoccurred simultaneously with summit eruptions.
3-3 Examples on some volcanoes
We apply the above interpretation to some volcanoes;
Sakurajima, Ooshima, Usu volcanoes and the East-Izu
monogenetic volcano group.
Sakurajima volcano: As mentioned above, in Fig. 2,
radial distribution of parasitic cones shows clearly separate
two peaks, at 2. 5 and 8. 5 km from the central summit
crater. Future parasitic eruptions may statistically takeplace around a circle of 2.5 km in radius. The branching
points of the two groups from the main conduit are roughly
3 and 10 km deep, respectively by the relation (4). These

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Izumi YOKOYAMA

are the branch points of the oblique magma paths, and insuch cases, the magma pressure is not enough to break out
at the summit crater through the main vent and the magma
may search weak points at lateral sides. Locations ofparasitic cones depend on the position of the branch pointfrom the main vent.
If we assume that the volcano structure has remained tobe the same since the past parasitic eruptions, the above
two branch points may be correlated with any characteristics of the actual subsurface structure: Kamo et al.
(1980) located an attenuating zone of seismic shear waves
at a depth of 3〜6 km beneath the volcano. Ishihara(1990) deduced existence of a magmatic and pressuresource zone at depths of 4〜6 km that caused the surface
deformation, and Iguchi (2007) clarified another magmaticzone at depths of 6〜15 km that extruded dykes toward
SW causing A-type earthquakes. The above two branch
points from the main vent are located in such particularzones. The zones may be attributable to magma batches
and magma reservoirs.
Ooshima volcano: In Fig. 2, the two groups of parasitic cones, 3.5 and 5.5 km distance from the center of the
volcano, may have branched from the main conduit at
depths of about 4 and 7 km, respectively. The shallow one
may correspond to a pressure source or a magma batch at a
depth of 4 km that was determined by Ida et al. (1988)
who interpreted the deformations occurring in November
1987 after the 1986 eruption. On the other hand, the deeper
one may be related to the top of scattering body at a depthof 8〜10 km beneath the caldera located by Mikada (1994)
who applied tomographic methods using seismic waves.
Usu volcano: Magmas in the historical activities of this
volcano are dacites, and have produced lava domes within
the summit crater and on the flanks, and occasionallypyroclastic flows. The three historical eruptions, in 1910,
1943 and 2000, producing parasitic vents were observedwith the instruments of those days. They are distributed
all along a contour of roughly 200 m a.s.l. or a circle ofroughly 2. 5 km from the center of the main volcano asshown in Fig. 6. According to the relation (4), the depth of
the branch may be roughly 3 km beneath the volcano.
Hereupon, Onizawa et al. (2002) accurately determined
the hypocenters of the earthquakes in the 2000 eruption at
a depths 2〜4 km beneath the volcano using the threedimensional P- and S-wave velocity structures. This
offers a suggestion regarding the character of the branches.
In Fig. 6, “HM” denotes a round mound measuring about
100 m in relative height, formed in prehistoric period. The
1910 eruption formed more than 40 vents of various sizes
and also mound MS -Hill measuring about 100 m inrelative height, accompanying phreatomagmatic and phreatic explosions. Such a group of vents may have branchedfrom the sub-conduit at a certain depths as the case of the1914 eruption of Sakurajima volcano (Fig. 5 (b)). In the1944 eruption, first “YH” point in Fig. 6 gradually lifted

about 30 m without explosions and then the upheaval migrated towards the N. There, lava dome SS , ShowaShinzan (new mountain in the Showa era) grew up to about
200 m in relative height accompanying explosions for 21
months. The 2000 eruption was phreatic and phreatomagmatic, and outburst at more than 60 vents, tiny and small indiameter. It is noticeable that the vents of the 1910 eruption and the 2000 eruption did not overlap each other evenat their contact area ( KP in Fig. 6). This means thatparasitic cones are monogenetic. Concerning the future
eruptions of Usu volcano, we may expect that it should
occur at the summit, or otherwise at “parasite-gap” aroundthe volcano, probably along the contour of 200 m a.s.l.
As for the seismic activities in the last four eruptions of
Usu volcano, the largest magnitude of the precursory
earthquakes were determined as follows:
The 1910 eruption (parasitic, phreatic and phreatomagmatic): Ms 5.1
The 1944 “ (parasitic, lava dome): Ms 5.0,
The 1977 “ (summit, magmatic): Ms 4.3,
The 2000 “ (parasitic, phreatic and phreatomagmatic):
Ms 4.6 (14 hrs after the outburst).
Roughly speaking, parasitic eruptions were accompanied with larger earthquakes than those of the summit
eruptions. These earthquakes may have been related to theformation of the parasitic conduits.
The East-Izu monogenetic volcano group: As discussed by De la Cruz-Reyna and Yokoyama (2011), theMichoacán-Guanajuato volcanic field occupies a large
area, roughly 100 km×200 km, and its monogenetic cones,e. g. Jorullo and Paricutin, prove to have their origins
independently around 30〜60 km deep. On the contrary,
the East-Izu group occupies a rather small area, roughly 30
km × 40 km, and according to Kuno (1954), the monogenetic cones derive from a magma batch at rather shallow
depth (around 10 km deep) in the granitic layer. From thisviewpoint, the monogenetic volcano groups in East-Izu
and Michoacán-Guanajuato are of different types in their
origins. As discussed previously on the East-Izu group,
density distribution of cones around the tentative centerhas no prominent peaks: This means that there is no main
vent in the group. To such a geological condition, themaximum shear stress theory is applicable to explain
formation of the East-Izu group. In this case, at thesurface, compressive rock strength must be much higher
than shear strength, and hence the magma proceeds
obliquely toward a point of maximum shear stress (r=0.82
D) to form a monogenetic vent that is not parasitic. If
there are similar magma batches in the area, a monogenetic
volcano group finally may be formed. This is a possible
interpretation upon formation of the East-Izu monogenetic
volcano group under particular conditions.

Parasitic Eruptions on Sakurajima Volcano

97

Fig. 6. Parasitic vents on Usu volcano.
Red marks denote the 1910 vents, black ones the 2000 vents, and green ones the 1977
vents in the summit crater.. KU and OU lava domes were formed in 1769 and
1853, respectively. MS mound was formed in 1910 and SS lava dome in 1944.
“HM” mound is pre-historic.

4．Parasitic Vents formed in Historical Times on Sakurajima Volcano
The eruptions of Sakurajima volcano have been historically recorded since 708, small or large, more than 40
in total number, and the three large eruptions occurring in1471〜76, 1779〜80, and 1914, were all parasitic andproduced twin vents in each eruption. In the following,
the three cases shall be briefly commented based on thereport of Yamaguchi (1975).
4-1 The 1471〜76 eruption (the Bunmei era)
The volcano burst into an eruption at a height of 500 m
a. s. l. on the NE flank of N-Peak, and the lava flows
reached the E shore. Identification of the vents is notalways easy because the ejecta of later eruptions thickly
cover them. On the SW flank of S-Peak, a vent opened ata height of 400 m a.s.l. and two vents followed at the lower
heights. The lava flows from these vents reached the SW
shore. The two groups of eruption vents are roughly
symmetrical with respect to the center of the volcano, andmarked with “Bn” and “Bs” in Fig. 7 where the starsymbols denote the vents of the first eruptions. The radial
distances of the parasitic vents from the center of the
volcano are roughly 3 and 2 km, and 2.5 km on the average.
As discussed above, their branching points were roughly 3
km beneath the volcano. The magma may have branched

Fig. 7. Twin parasitic vents on Sakurajima volcano after
Kobayashi (1988). “Ai” denotes the vents of submarine origin, formed in the 1779 eruption.

into two directions, NE and SW.
4-2 The 1779〜80 eruption (the An-ei era)
The eruption took place at the vents of the two groups,“An” and “As” in Fig. 7. The “An” vents were located ata height of 650〜750 m a.s.l. on the flank of N-Peak, and98

Izumi YOKOYAMA

the “As” vents at a height of 680 m a.s.l. on the flank of SPeak. Considering that the summit craters of the two
peaks were not active, we may suppose that the twinparasitic vents were not auxiliary to the main crater but
may have been formed by the fore-mentioned mechanism. The depth of the branch point was roughly 1.2 km
beneath the volcano.
According to Kobayashi (1988), explosions began atboth the NE and S flanks of the volcano, and the lava flows
from the NE vents reached the sea (“As” in Fig. 1). At thelast stage, almost simultaneously with the flank eruptions,submarine eruptions occurred at the NE sea bottom
forming a few islets (“Ai” in Figs. 1 and 7). It isinterpreted that the magmas branched into only the NE
direction at a larger depth. The submarine eruption sites
are about 8 km in radial distance from the center of the
volcano. Then we estimate the depth of a branching point
roughly as 10 km by the relation (4). This point is situatedin a magmatic zone suggested by Iguchi (2007). Both thelavas from the flanks and the sea bottom are analogous inchemical components (Yamaguchi, 1975).
Here, it is noticeable that “Ai” vents were notaccompanied by their counterparts at the opposite side of
the island. This should arouse a question: This may
suggest a difference between shallow (1.2 km) and deep
(10 km) tectonic structures beneath Sakurajima volcano.
4-3 The 1914 eruption (the Taisho era)
Its sequence was as follows:[16 30 h, Jan. 9] The first precursory earthquake wasregistered at the Nagasaki observatory located at 150 km
distance from Sakurajima volcano.
[08 00 h, Jan. 12] A column of white smoke wassuddenly shot up in the form of a pine tree from the top of
S-Peak. The present author doubts that some gases leaked
to the main conduit from ascending magma and subconduits had not reached the flank surface yet.
[10 00 h, Jan. 12] Explosions began from two vents
formed at heights of 550 and 400 m a.s.l. on the W slopeof S-Peak. And 10 min. later, on the eastern flank, explosions occurred from a few vents around height of 380 ma.s.l. The explosions gradually intensified and the explosion clouds reached a height of about 8 km before 11 00 h.
According to Kotô (1916) and Yamaguchi (1975),
finally 5 vents were formed on each flank as shown in Fig.
8 (a) where the highest vents on both the flanks wereformed at first. Radial distances of the both parasitic vents
are about 2 km on average, and the branch depths are about
2.5 km beneath the volcano.
[18 28, Jan. 12] During the explosions, an earthquake ofMs 7. 1 occurred. It was re-examined by Abe (1981)
applying modern seismological methods to the data fromthe worldwide stations of those days, and determined the
hypocenter at shallow part at the S of Sakurajima volcano.
This earthquake may have no relation to formation of the
sub-conduits.

Fig. 8. Parasitic vents formed on Sakurajima volcano in
the 1914 eruption after Kotô (1916).
(a) Distribution of parasitic vents (numerals 1〜5
indicate the order of outbursts of each group).
(b) A bilateral injection-chamber model of the parasitic
vents formed by the eruption.

[20 00 h approx., Jan. 13] Lavas began to flow out fromboth the flanks, eastern and western vents, and continued
for about 25 days. It took about 65 hours from the firstprecursory earthquakes to the outburst, and about 100
hours to the beginning of lava effusions. These timeintervals may suggest that the sub-conduits were completed in a few days.
[Early February 1914] The main lava flows stopped on
both sides. The lavas from the vents on the western andeastern flanks covered areas of 11. 0 and 12. 7 km2,
respectively. The total volume of the lavas amounted to1.56 km3 in about 25 days. Thus, the discharge rate of asingle vent amounts to roughly 400 m3/s, that is somewhat
larger than those of Kilauea and Etna for the similar
duration of activity (Wadge, 1981). Originally discharge
rates of lavas depend on position and size of vents,
viscosity of lavas and driving forces. Considering that thelavas of Kilauea and Etna are basaltic and more fluidal
than the lavas of Sakurajima, we may suppose the highdischarge rate of the 1914 Sakurajima eruption is due toParasitic Eruptions on Sakurajima Volcano

the low altitudes of the vents and strong explosivity.
4-4 Eruption models of twin parasites
There is a hypothesis that the vents on both sides,
eastern and western flanks are on the same vertical planar
dike passing the center of the volcano. The present author
doubts that the central vent should erupt more possibly inthis hypothesis. For reference, Kotô (1916) proposed amodel for the mechanism of parasitic eruptions assuming
bilateral injection-chamber shown in Fig. 8 (b). He may
have assumed the equilibrium of magma-heads. However,as above-mentioned, at the first phase of the eruption, theexplosion clouds issued from the parasitic vents reached aheight of about 8 km: It is rather difficult to interpret suchactivities by the above injection-chamber. On the other
hand, we need to explain why the central vent did not work
during the parasitic eruptions.
An alternative model extended from the present
discussion is as follows: The sub-vents of the both groupsof flank vents, “Te” and “Tw” in Fig. 7, are located atabout 2 km from the central summit. Then, the branching
points of both vents from the main conduit are estimated atabout 2. 5 km deep by the relation (4), as schematicallyillustrated in Fig. 5 (b). And on both the slopes, the vent at
higher altitude erupted earlier with high explosivity. The
summit craters remained quiet during the 1914 eruption
except ejection of a column of white smoke from thesummit crater of S-Peak in the morning of the first day ofthe eruption. This fact does not support the assumptive
intrusion of a vertical, radial and planar dyke from thecenter of the volcano. Lava effusions were not fissure
ones, but migrated from the top vent to the bottom one
probably due to hydraulic magma-pressure.
We know a few examples of flank eruptions that
occurred simultaneously at two opposite sides on the slope,symmetrically with respect to the central summit: e.g. the1970 eruption of Hekla, Iceland. At Hekla, two fissure
eruptions at the SW and NE flanks occurred within only 70
minutes. The vents of the eruptions were located at about
3〜4 km distances from the central crater of Hekla.
Thorarinsson and Sigvaldason (1972) interpreted these
eruptions from petrologocal discussion as to have been
deep-fed separately by magma chambers different fromthat of Hekla. Anyhow it is a confronted problem why
twin parasitic vents have been repeatedly produced on
Sakurajima volcano.
Petrological compositions of the lavas issued in thethree eruptions: According to Yamaguchi (1975), theSiO2 contents of the lavas from the three eruptions
changed from 66, 64 to 60 % in chronological order. As
mentioned above, the branch depths of the three parasitic
eruptions are 2. 5 km, 1. 2 and 10 km, and 2. 5 km,
respectively. At present, we do not find any clear
correlation between the SiO2 contents of the lavas and thebranch depths.

4-5

99

Activities of Sakurajima volcano after the 1914
eruption
The 1935 eruption: 21 years after the 1914 eruption,
Sakurajima volcano first erupted in a minor magnitude
ejecting ashes for a few days. The eruption formed a ventwithin the crater of S-Peak.
The 1939 eruption: “Showa Crater” (“SC” in Figs. 1
and 7) opened at a height of 700〜750 m a.s.l. on theeastern flank of S-Peak of which crater rim is 1060 m a.s.l.
The dimension of the crater was 50 m in diameter and 100
m in depth. The crater issued pyroclastics, not lava flows.
As mentioned before, this crater is regarded as an auxiliary
to the main crater.
The 1946 eruption: After 1939, minor explosions wererepeated in 1941, 1942, 1943 and 1945 at Showa Crater.
In 1946, the crater was activated and extruded lavas of
0.15 km3 (DRE) of which SiO2 content is 61 %.
The explosion activities after the 1946 eruption: Small
explosions were observed at Showa Crater in 1947 and1948. In 1955, explosive activities began at the summit
crater of S-Peak, and the eruptive activity declined near theend of the century. Then the activity migrated to Showa
Crater in 2006.
Iguchi et al. (2010) compared the activities of Showa
Crater during 2006 to 2010 with the previous activity inthe crater of S-Peak, and examined 4 parameters: (1)moments of long-period phase of explosion earthquakes,(2) amplitudes of air-shock caused by explosions, (3)intensity of pressure source causing the ground deformation associated with explosions, (4) weights of volcanic
ash issued by explosions. Finally they concluded that
explosive eruptions at the summit crater are larger by
10〜100 times than those at Showa Crater. This conclusion does not totally contradict with the above presumption that Showa Crater is auxiliary to the S-Peak crater.
4-6 “Parasitic volcanoes only erupt once.”
The three twin parasitic eruptions occurring in thehistorical period on the flank of Sakurajima volcano afford
us suggestive information: They donʼt overlap each otheras well as on Usu volcano (Fig. 6) though both thevolcanoes are relatively small, 10 and 5 km respectively inbasal diameter. In this sense, parasitic volcanoes behave
as monogenetic volcanoes although the latter originatesfrom deep sources and usually cluster in rather wide area.
The following is a hypothetical explanation of the fact thatparasitic volcanoes only erupt once:
During a cycle of volcanic activity, the surroundings ofparasitic vents must be compacted and strengthened
mechanically by magma intrusions (piling effects). The
central vents of polygenetic volcanoes, after an eruptive
activity stops, may be loosely choked with lavas and ejecta
to a certain depth due to their own weight. On the other
hand, sub-conduits of parasitic volcanoes probably remain
almost the same as the beginning because the volcanoes do
not always repeatedly explode to enlarge the conduits and100

Izumi YOKOYAMA

end the activity after one cycle, and also may be tightly
choked with lavas due to oblique slope of the sub-conduit.
When magma re-activates at the depths of volcanoes, it
begins to ascend for the summit crater through the main
conduit. If upward magmatic pressure is strong enough toclear the main conduit, the summit crater should reopen.
On the other hand, under the same situation, the magma
reaches the branch point for sub-conduit that is chokedtightly. In this case, oblique pressure is unfavorable toclear sub-conduit leading to a parasitic vent. If particularconditions are satisfied, a parasitic vent may be newly
formed as discussed in the previous chapter.
5．Conclusion
A simple method to find a trend, if any, in spatial
distribution of parasitic cones is proposed: Estimations of
numbers of the cones per unit area with increasing radial
distance from the centre of volcano. And its application is
exemplified on Sakurajima and Izu-Ooshima volcanoes.
By simple mechanical assumption, the radial distances ofdense distribution of cones are related to the depth of the
branching point from the main conduit. In this paper the
branching points are assumed to correlate with magma
reservoirs and some singular points though the mechanism
is not yet resolved. As far as we cannot clarify themechanism of branching from the main conduit, it isdifficult for us to predict the exact site of the next parasitic
eruption on the flanks.
The parasitic eruptions on and around Sakurajima
volcano in historical ages are briefly described from theviewpoint of their formation mechanisms. The twinparasitic vents are normal on Sakurajima volcano, and oddon the other volcanoes. In the later stage of the 1779〜80
eruption, the submarine vent (“Ai”) branched from 10 km
depth, but the vent were not accompanied by its
counterpart at the opposite side of the island. This may
suggest a difference in tectonic structure between shallow
and deep parts (3 and 10 km) beneath Sakurajima volcano.
Future eruptions of Sakurajima volcano probably may
take place at the summit craters, or otherwise statisticallyparasitic vents may be formed at circumferential zones ofradial distances 2. 5 or 8. 5 km from the center of the
volcano, on the flank and in the surrounding sea area,
respectively. And surely the vents on the flanks may
appear at “parasite-gaps”. In case of parasitic eruptions on
the flanks, twin vents may be formed very probably. It isdifficult to comment further on future eruptions of this
volcano as far as the plumbing system of magma reservoir,
main-conduit, and sub-conduits are not clarified.
As to the theory of parasite formation, there is much to
be done for its completion. A satisfactory theory would
only be completed after field observations of parasitic
eruptions over a long period.

Acknowledgements
The author is grateful to the late Prof. K. Yamaguchi
and the late Prof. B. Kotô for their important reports on
geology of Sakurajima volcano. The author wishes tothank sincerely T. Kobayashi who kindly provided theauthor with a geological map of Sakurajima volcano
indicating the distribution of parasitic vents on and aroundthe volcano. The author is grateful to C. B. Connor for
his valuable comments on an earlier version of themanuscript. The author benefited by critical comments of
T. Hashimoto during preparation of the manuscript. Valuable comments given by anonymous referees improved themanuscript very much: The author is cordially thankful tothem.
References
Abe, K. (1981) Seismometrical re-evaluation of the Sakurajima earthquake of January 12, 1914. Geophys. Bull. Hokkaido
Univ., 39, 57-62 (in Japanese with English abstract).
Anderson, E. M. (1936) The dynamics of the formation ofcone-sheets, ring-dykes, and caldron-subsidences. Proc. Roy.
Soc. Edin., 56, 128-157.
Aramaki, S. and Hamuro, K. (1977) Geology of the HigashiIzu monogenetic volcano group. Bull. Earthq. Res. Inst., 52,
235-278 (in Japanese with English abstract).
Connor, C. B. (1987) Cluster analysis and two-dimensional
Fourier analysis of cinder cone distribution; Central Mexicoand SE Guatemala (abstract). Eos Trans. AGU, 68, 1526.
Connor, C. B. (1990) Cinder cone clustering in the TransMexican Volcanic Belt: Implications for structural andpetrologic models. J. Geophys. Res., 95, 19,395-19,403.
De la Cruz-Reyna, S. and Yokoyama, I. (2011) A geophysicalcharacterization of monogenetic volcanism. Geofís. Internac.,
50, 255-270.
Hasenaka, T. and Carmichael, I. S. E. (1985) The cinder conesof Michoacán-Guanajuato, central Mexico: Their age,volume and distribution, and magma discharge rate. J.
Volcanol. Geotherm. Res., 25, 105-124.
Ida, Y.,Yamaoka, K. and Watanabe, H. (1988) Model of volcanic activity associated with magma frain-back -Imlication for eruptions of Izu-Oshima volcano after December ,1986-. Bull. Earthq. Res. Inst., 63, 183-200 ( in Japanesewith English abstract).
Iguchi, M. (2007) Structure of Sakurajima volcano revealed
by geophysical observation and significance of its survey.
BUTSURI-TANSA, 60, 145-154 (in Japanese with English
abstract).
Iguchi, M., Yokoo, A. and Tameguri, T. (2010) Intensity of
volcanic explosions at Showa Crater of Sakurajima Volcano.
Bull. Disas. Prev. Res. Inst., Kyoto Univ., 53B, 233-240 (inJapanese with English abstract).
Ishihara, K. (1990) Pressure sources and induced grounddeformation associated with explosive eruptions at anandesitic volcano: Sakurajima volcano, Japan. In Magma
Transport and Storage (Ryan, M. P. ed.), John Wiley &Sons Ltd, 335-356.

Parasitic Eruptions on Sakurajima Volcano
Jaeger, J. C. (1964) Elasticity, fracture and flow. London,
Methuen & Co. Ltd, 212 p.
Kamo, K., Nishi, K., Takayama, T. and Ueki, S. (1980)
Seismicity in the region south of Sakurajima and the regionof abnormal propagation of seismic waves. In: 3rd Joint
Observation of Sakurajima Volcano (Kamo, K. ed.), Sakurajima Volcanol. Obs., 11-15 (in Japanese).
Kobayashi, T. (1988) Geological map of Sakurajima volcano.
In A guidebook for Sakurajima volcano (Aramaki, S.,
Kamo, K., Kamada, M., and Kagoshima Prefectural Government eds), Kagoshima Internat. Conf. on Volcanoes, 88 p.
Kotô, B. (1916) The great eruption of Sakurajima in 1914. J.
Col. Sci., Imp. Univ. Tokyo, 38, 1-237.
Kuno, H. (1954) Geology and petrology of Omuroyama
Volcano Group, North Izu. J. Fac. Sci. , Univ. Tokyo, (Sec.
2), 9, 241-265.
Mikada, H. (1994) An elastic scattering theory and its
application to the understanding of subsurface structure of
lzu-Oshima volcano. Ph. D. thesis, Univ. of Tokyo, 226 p.
Nakamura, K. (1961) Stratigraphic studies of the pyroclastics
of Oshima volcano, Izu, deposited during the last fifteen
centuries, II. Activity of parasitic volcanoes. Sci. Pap. Coll.
Gen. Educ., Univ. Tokyo, 11, 281-319.
Nakamura, K. (1977) Volcanoes as possible indicators of
tectonic stress orientation. J. Volcanol. Geotherm. Res., 2,
1-16.
Nakamura, K. (1984) Distribution of flank craters of Miyakezima volcano and the nature of the ambient crustal stressfield. Bull. Volcanol. Soc. Japan, 29, Spec. Issue, S16-S23

101(in Japanese with English abstract).
Onizawa, S., Oshima, H., Mori, H. Y., Maekawa, T., Suzuki,
A., Ichiyanagi, M. and Okada, H. (2002) Three-dimensional seismic velocity structure around Usu volcano,Japan. Bull. Volcanol. Soc. Japan, 47, 495-506 (in Japanesewith English abstract).
Sezawa, K. (1931) The plastico-elastic deformation of a semiinfinite solid body due to an internal force. Bull. Earthq.
Res. Inst., 9, 398-406.
Soeda, K. (1944) On the deformations produced in a semiinfinite elastic solid by an interior source of stress. Q. J.
Seismol., 13, 263-291 (in Japanese).
Thorarinsson, S. and Sigvaldason, G. E. (1972) The Hekla
eruption of 1970. Bull. Volcanol., 36, 269-288.
Ueki, S. (1992) Seismological research on the 1989 submarine
eruption off Ito, Japan. Ph. D. Thesis, Tohoku Univ. 160 p.
(in Japanese).
Wadge, G. (1981) The variation of magma discharge during
basaltic eruptions. J. Volcanol. Geotherm. Res., 11, 139168.
Yamaguchi, K. (1975) Research on Sakurajima Volcano-Geological and Petrological Researches of the Surrounding
Area of Kagoshima Bay and Sakurajima Volcano-. Educa.
Soc. Earth Sci. Japan, 128 p. (in Japanese).
Yamashina, K. (1986) Stress fields and volcanic eruptions.
Bull. Volcanol. Soc. Japan, 30, Spec. Issue, S101-S 119 (inJapanese with English abstract).

(Editorial handling Masato Iguchi)

桜島火山における寄生火口の噴火
横山泉

桜島火山には多くの寄生火口が地質及び地形の面から認められている．また，その噴火史において寄生火口の噴火がしばしば記録されている．寄生火口の分布パターンを調べるのに，色んな方法が提案されてきた2が，ここでは，火山中心から半径方向の密度分布（km 当たり）を調べた．
一般論として，火山の下に点力源を仮定して，地表面で直応力の分布と水平差応力の分布を考え，岩石の強度を考慮すると，寄生火口の生ずる地点の見当がつく．それは，地表で力源を伏角 51°で見る山腹の地点
で，火山中心に対して対称な 2 点である．多くの火山では，対で生ずることは少ない．桜島火山の寄生火口の火道が主火道から分岐する深さを求めると，深さが 3 km と 10 km の 2 群となる．これらの深さと既に推定されているマグマ溜まりとの関連につぃて触れた. 桜島火山の歴史時代の（1471 年以降の 3）回の大噴火
は総て，山頂に対称的に対をなして形成された．このことは力学的には正常であるが，事例としては例外である．ただ，分岐の深さが 10 km の場合（1779〜80 年噴火），山頂に対して対称位置に寄生火口が生じていない．この例外的な事例は，桜島地下で，浅部と深部で地殻構造が異なることに起因するのかも知れない．
更に，寄生火口が再噴火しない機構について仮説を述べた．
次の桜島火山の噴火地点は何処であろうか．山頂火口か，それでなければ，寄生噴火である．その場所は統計的に，山体の中心軸から約 2.5 km 或は 8.5 km の円環上で，かって噴火したことのない地点が考えられる．寄生火口の火道が主火道から分岐する機構が未解明である限り，これ以上のことは言えない．

Izumi YOKOYAMA102

Appendix:
After this paper was accepted in April 19, 2012 by the Bulletin, a paper discussing the 1888 eruption of Bandai
volcano was published on this Bulletin ---- Hamaguchi, H. and Ueki, S. (2012) Notes on the 1888 Phreatic explosion at
Bandai volcano (1) The re-examination of the location of explosive source and direction of outbursts. Bull. Volcanol.
Soc. Jpn, 57, 111-123 (in Japanese with English abstract). They newly interpreted the 1888 eruption of Bandai into twolateral ones around the summit and proposed a mechanical model for producing the two vents by tensional stressexerted from a pressurized spherical source. Their results are similar to those of the present paper.