Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 1, pp. 17-33

ArticleGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido,
Japan) Inferred from Aerial Laser Mapping and Geological Field Surveys
Yoshihiko GOTO＊, Satoru MATSUZUKA＊＊, Seiji KAMEYAMA＊＊ and Tohru DANHARA＊＊＊(Received June 12, 2014 ; Accepted January 10, 2015)
Aerial high-resolution laser-scanner mapping and geological field surveys were performed over the Nakajima
Islands within Toya caldera, southwestern Hokkaido, Japan, to study the evolutional history of the islands. The laserscanner mapping survey covered the entirety of the Nakajima Islands, an area of 3×3 km. A three-dimensional digital
map produced from the laser-scanning data revealed detailed topographic features of the islands. Geological fieldsurveys were carried out over the whole area of the Nakajima Islands, to determine how their topography relates to thegeological and lithological features. These surveys suggest that the islands consist of a tuff cone, eight lava domes, anda cryptodome. The tuff cone has a low profile and wide crater and consists of dacitic pyroclastic deposits, suggestingthat the cone was produced by explosive eruptions resulting from the interaction of dacitic magma and groundwater.
The lava domes are conical or pancake-shaped and composed of dacitic to andesitic lavas, suggesting that the domes
formed by extrusions of high-viscosity, dacitic to andesitic magmas. The cryptodome consists of coherent dacite
overlain by mudstone and sandstone, suggesting that the dome formed by the uplift of caldera-floor deposits following
the intrusion of high-viscosity dacitic magma. Mudstones and sandstones are present along the northeastern andsouthwestern parts of the islands, implying that the caldera floor was uplifted during or prior to the volcanism, forming
a bulge (small resurgent dome) at the center of the caldera. We infer that the Nakajima Islands have evolved from
caldera resurgence related to the ascent of voluminous dacitic to andesitic magma, followed by subsequent formation ofmultiple dacitic to andesitic domes and phreatomagmatic eruptions on the resurgent dome.
Key words : laser-scanner mapping, geology, Nakajima Islands, Toya caldera, evolutional history

1．Introduction
LIDAR (light detection and ranging) is a powerful tool
for those studying the morphological features of volcanoes(e.g., Chiba et al., 2007a, 2007b ; Hunter et al., 2003 ; Pesci
et al., 2007). Three-dimensional digital mapping based onhigh-resolution laser-scanning data provides invaluable
information on the distribution and morphology of craters,
lavas, domes, and pyroclastic deposits, as well as reworked
deposits. Laser-scanner mapping is particularly useful forsurveying topographic features in thickly vegetated areas,
for which ‘tree-removing’ data filtering can be used toreveal the topography of the ground surface (Chiba et al.,2007b ; Goto et al., 2011).
We conducted aerial laser-scanner mapping and geological field surveys over the Nakajima Islands within
Toya caldera, southwestern Hokkaido, Japan (Fig. 1), tostudy the morphological and geological features of theislands. The Nakajima Islands are a silicic volcanic complex
produced by post-caldera volcanism (Katsui, 1990 ; Ota,＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.
＊＊
Tanaka Consultant Co. Ltd, Shinkai-cho 2-1-3, Tomakomai,
Hokkaido 053-0052, Japan.

1956 ; Soya et al., 2007 ; Ui et al., 2013 ; Yokoyama et al.,1973), but their detailed geology remains unknown
because it is obscured by the thick vegetation cover (Fig.
2). Three-dimensional digital mapping based on the laserscanning data allowed us to study the detailed topographic
features of the islands, and use them as the basis forunderstanding their geology. This paper describes the
topographic and geological features of the Nakajima
Islands and discusses the evolutional history of the islands.
2．Nakajima Islands
The Nakajima Islands are located in the central part ofToya caldera (Fig. 1). The caldera covers an area of 10×
11 km, and was formed by violent silicic eruptions atapproximately 110 ka (Ganzawa et al., 2007 ; Machida andArai, 2003 ; Okumura and Sangawa, 1984 ; Takashima etal., 1992 ; Yokoyama et al., 1973). The caldera-forming
eruptions generated large-volume, rhyolitic, pyroclastic
fall (Machida and Arai, 2003), and flow (Yokoyama et al.,＊＊＊

Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,
Kita-ku, Kyoto 603-8832, Japan.

Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp

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Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

Fig. 1. Location map of the Nakajima Islands in Toya caldera, southwestern Hokkaido, Japan. The survey
area (see Fig. 6) is shown by the rectangle. The location of Usu volcano is also shown. The base map was
taken from the 1 : 50, 000 scale topographic map Toyako-onsen issued by the Geospatial Information
Authority of Japan (GSI). The topographic contour interval is 20 m.

1973) deposits around the caldera. The Toya caldera holds
a freshwater lake (Lake Toya), which occupies the entire
caldera floor (i.e., 10×11 km) and has a maximum depthof 180 m. The water level of Lake Toya is 84 m above sea
level (a.s.l.).
The Nakajima Islands comprise four islands : Oshima(2.5×2.8 km), Kannon-jima (380×430 m), Benten-jima(300 × 450 m), and Manju-jima (180 × 230 m ; Fig. 1).
Oshima Island is the largest, and its highest point is 370 m
above the lake level (454 m a.s.l.). Thermoluminescence(TL) dating of lavas on the islands suggests that they
formed 40-45 ka (Takashima et al., 1992). The Nakajima
Islands host no active fumaroles, and their surfaces arewidely covered with thick vegetation, mainly broadleaf

trees (Fig. 2A).
A bathymetric map of Lake Toya (Fig. 3) suggests that
the Nakajima Islands lie on a submerged rise that has anirregular morphology. A reef located 750 m northeast of
Oshima Island (Fig. 3) consists of dacitic lava blocks,
which suggests that it is a submerged lava dome (Ota,
1956 ; Tanakadate, 1918).
3．Laser-scanner mapping survey
The laser-scanner mapping survey covered the whole of
the Nakajima Islands, encompassing an area extending 3
km N-S and 3 km E-W (Fig. 1). The survey was carriedout by Tanaka Consultant Co. Ltd, using a Develo LISA3
instrument (Fig. 4). Under typical conditions, the scanner

Geology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys

19

Fig. 2. (A) View of the Nakajima Islands from the south. (B) Oblique aerial photograph of OshimaIsland from the west. The photograph was taken from a helicopter during the laser-scanner
survey on 13 March 2011, when the islands had a thin covering of snow (<30 cm thick).

is able to measure distances of up to 1000 m with anaccuracy of ±10 mm (Table 1). The lightweight instrument (Fig. 4B) can be mounted on an airplane, a manned
helicopter, or a radio-controlled helicopter. In the present
survey, the LISA3 was mounted on a manned Robinson
R44 helicopter (Fig. 4A).
The survey was performed on 13 March 2011. The considerations involved in setting the date of the survey were :(1) the timing (between the end of leaf fall and the onset ofleaf budding to minimize tree noise) ; (2) good groundconditions (little or no snow cover) ; (3) the number ofavailable GPS NAVISTAR satellites (to minimize positioning error) ; and (4) good flight conditions. On 13
March, the broadleaf trees on the Nakajima Islands werebare, and the weather was fair. The ground was thinly

covered with snow (<30 cm ; Fig. 2B).
Fig. 5 shows the helicopter flight route of the laserscanner mapping survey. The helicopter flew at a height
between 400 and 750 m above the lake level (i.e., 484-834
m a. s. l.), and remained about 400 m above the groundsurface over the Nakajima Islands. The measurement
duration was 1 h (11 : 30-12 : 30 local time). The positionof the helicopter was recorded at 1-s intervals using a
Global Positioning System (GPS) and at 0.01-s intervals
using an inertial measurement unit (IMU). After the flight,
the three-dimensional flight route was reconstructed fromthe GPS and IMU data.
The laser-scanning data were cleaned to remove noise
and then filtered to produce a digital terrain model (DTM),
which shows the bare ground surface. As part of the20

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

Fig. 3. Bathymetric map around the Nakajima Islands. The uplifted area is outlined by a dashed line. The location of asubmerged lava dome (reef) is also shown. The base map was taken from the 1 : 25,000 scale topographic map Toya
issued by the Geospatial Information Authority of Japan (GSI). The topographic contour interval is 10 m.

filtering process, trees and buildings were carefully
removed using the TerraScan application (Terrasolid Co.
Ltd). Both automatic and handpicking methods were usedduring this process to avoid the unintentional removal ofuseful information. The automatic method was applied to
the whole survey area, whereas the handpicking method
was applied to local bumpy areas. No correction was madefor the snow on the ground ; therefore, the DTM includes
the thickness of the snow. As the snow was <30 cm thick,it did not adversely influence the mapping results.
After filtering, a triangulated irregular network (TIN)
was produced from the ground data. The size of the TIN(i.e., one side of the triangle) was 1-2 m. The TIN was thenconverted to grid elevation data (grid size of 1×1 m) using
the linear-approximation interpolation method. The
applications TerraScan and Terramodel (Terrasolid Co.
Ltd) were used for this conversion. Finally, a threedimensional, digital topographic map was produced fromthe grid elevation data (Fig. 6).
The map (Fig. 6) has contour lines with 2-m intervals,
which allows the angle of inclination of the ground surface
to be expressed by shading : gentle slopes appear palegrey, whereas steep slopes appear dark grey. Comparedwith previous topographic maps at scales of 1 : 50, 000(Fig. 1) or 1 : 25,000 (Fig. 3), this new digital topographic
map (Fig. 6) has a much higher resolution and allows
detailed investigation of the topography to aid geologicalinterpretations (Fig. 7).
The digital topographic map (Fig. 7) shows that thenorthern slopes of volcanoes on the islands generally haverough surfaces, whereas the southern slopes have smoothsurfaces (e. g., the Higashiyma and Nishiyama domes).
This difference reflects real topography. Field surveys
revealed that the northern slopes of the volcanoes commonly have small U-shaped gullies (5-20 m wide, 20-40 m
long, 5-10 m deep), whereas the southern slopes are covered

Geology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys
Table 1.

21

Specifications of the Develo LISA3 laser scanner.

The laser class is based on the safety classification of laser
devices according to Japan Industrial Standard (JIS) C6802.
Note that the beam divergence of the LISA3 is 0.25 mrad
(i.e., a beam diameter of 25 mm at a distance of 100 m).

Fig. 4. (A) Photograph of the LISA3 instrument
mounted on a manned helicopter. (B) Close-up
photograph of the LISA3 instrument.

with thick surface soil and lack such gullies. We infer thatthe gullies on the northern slopes formed in response toerosion by snow (or ice).
Oblique, three-dimensional images of the islands fromvarious viewpoints were also produced from the gridelevation data (Fig. 8). These images are visually superior
to the topographic map, and helped to improve our understanding of the three-dimensional morphology of volcanic
edifices.
4．Geological field surveys
Geological field surveys of the Nakajima Islands (Oshima,
Kannon-jima, Benten-jima, and Manju-jima) were performed from October 2010 to September 2014. Detailed
field investigations were conducted along the lakeshore,
ridges, and valleys, as well as on the peaks of the islands.
We found several outcrops of dacitic to andesitic lavas,
dacitic pyroclastic deposits, and mudstones from the islands(Fig. 9 ; locations are shown in Fig. 7). The sizes of theseoutcrops vary from 1 × 1 m to 5 × 20 m. Rock samples
were collected from these outcrops. For the field sites
where no outcrops were found (e. g., the Hokusei andKitayama domes ; Fig. 7), rock samples were collectedfrom relatively large (>50 cm in size) angular rocks scattered on the surface. Petrographical observations andFig. 5. Flight route of the helicopter for the laserscanner mapping survey. The arrows indicate theflight directions of the helicopter.

whole-rock major-element chemical analysis were also
performed on the rock samples (Tables 2 and 3).
A lake-bottom geological field survey was also carriedout 150 m southeast of Kannon-jima Island (Ukimi-do ;Fig. 3) in June 2012, using a small manned boat. We foundan outcrop of volcanic sandstone and mudstone (larger
than 10 × 20 m in area) at a water depth of 1 m. Rocksamples of the volcanic sandstone and mudstone (each10-20 cm in size) were collected from the lake bottom
using a hand rake (described in section 13).

22

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

Fig. 6. Three-dimensional digital topographic map of the Nakajima Islands generated from the aerial laser-scanner mapping
survey. The topographic contour interval is 2 m.

5．Topographic and geological features of the Nakajima
Islands
The three-dimensional digital topographic map (Fig. 6)
and images (Fig. 8) reveal the detailed topographic featuresof the Nakajima Islands (Fig. 7). The notable features are apyroclastic cone (Higashiyama), eight lava domes (Higashiyama, Nishiyama, Hokusei, Kitayama, Nansei, Kannonjima, Benten-jima, and Manju-jima), and a cryptodome(Hokutou-misaki). These features, and their interpretations,along with the results of the geological field survey, aredescribed in the following sections and summarized inTable 2. We named each of the volcanic edifices identified
in this study following local geographical names.

6．Higashiyama pyroclastic cone
The Higashiyama pyroclastic cone, located in thesoutheast of Oshima Island (Fig. 7), is elliptical in planview, ranging in basal diameter from 1500 m (N-S) to
2200 m (E-W). In east-west cross section, it has a low
profile with the highest point being 206 m above the lakelevel (Fig. 8A). The cone has a large-diameter crater,which is 1400-2000 m across (elongated E-W) and open
to the east. Its crater floor is flat and mostly occupied by alava dome (Higashiyama dome). The cone largely retains
its primary morphology except for a missing section of theeastern part. The outer slope of the cone is slightly erodedand has radial gullies and a horizontal terrace (Figs. 7 andGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys

23

Fig. 7. Interpretation of the map shown in Fig. 6. Bold lines show the boundaries of the volcanic edifices.
Dashed lines indicate the location of the horizontal terrace (56 m above the lake level). The locations ofFig. 9A‒H, and an outcrop of pyroclastic deposit at the Higashiyama pyroclastic cone are also shown.

8A). The horizontal terrace occurs at 56 m above the lakelevel (140 m a.s.l.), and overprints the radial gullies (Fig.
7). The northern slope of the cone partially covers the
Hokutou-misaki dome, suggesting the cone formed after
the dome. A lobate mound at the southeastern tip ofOshima Island (pyroclastic deposit in Fig. 7) is inferred to
be a remnant of the crater floor of the Higashiyama
pyroclastic cone, because the mound has similar morphological features observed along the southern part of theHigashiyama pyroclastic cone (both have a well-developed terrace at 56 m above the lake level).
Field surveys suggest that the outer slope of theHigashiyama pyroclastic cone has no outcrops. The outerslope of the cone is covered with humus soil, and scattered
dacitic lava blocks, which are up to 2 m in size. The dacitic
lava blocks are composed of grey, poorly vesicular dacite
and minor pale grey, vesicular dacite. The poorly vesicular
dacite contains phenocrysts of plagioclase, hornblende,
hypersthene, quartz, opaque minerals, and minor augite,whereas the vesicular dacite contains phenocrysts of plagioclase, hornblende, hypersthene, quartz, and opaqueminerals. The horizontal terrace is 5-20 m wide, andcovered with a thick soil.
The field surveys also suggest that the crater floor of the24

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

Fig. 8. Oblique three-dimensional digital images, produced by laser-scanner survey, of the Nakajima Islands viewed fromthe south (A) and east (B).

Higashiyama pyroclastic cone consists of pyroclastic deposits. A dacitic pyroclastic deposit occurs at the lakeshoreof the lobate mound (pyroclastic deposit in Fig. 7). The
deposit is pale reddish grey and composed of pale grey,
vesicular dacite clasts (<50 cm in size) and minor grey,

poorly vesicular dacite clasts (<50 cm in size) set in acoarse- to fine-grained matrix. The vesicular dacite(probably juvenile pyroclasts) contains phenocrysts ofplagioclase, hornblende, hypersthene, quartz, and opaqueminerals (65-66 wt.% SiO2 ; Table 3). The matrix of theGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys
Table 2.

25

Properties of volcanic edifices on the Nakajima Islands.

The height is relative to the lake level. The volume of each volcanic edifice was calculated by approximating each to apolyhedron above the lake level. The height and volume of Manju-jima dome is relative to the lake bottom. Abbreviations : Pl,plagioclase ; Ho, hornblende ; Opx, orthopyroxene ; Qz, quartz ; Cpx, clinopyroxene ; Ol, olivine ; Opq, opaque minerals.

Table 3.

Whole-rock major-element chemical compositions of volcanic rocks from the Nakajima Islands.

Compositions were determined by X-ray fluorescence (Rigaku RIX-2000) at Shimane University, Japan, following
Kimura and Yamada (1996). Fe2O3*=total iron as Fe2O3. L. O. I.=loss on ignition.

deposit is inhomogeneous and consists of crystals ofplagioclase, hornblende, hypersthene, quartz, opaqueminerals, and blocky-shaped volcanic glass. This deposit
may be debris from primary dacitic pyroclastic deposits.

The morphology and components of the Higashiyama
pyroclastic cone suggest that it is a tuff cone or tuff ring(Cas and Wright, 1987 ; Vespermann and Schmincke, 2000)
produced by phreatomagmatic explosions resulting from

26

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

Fig. 9. Field photographs showing the lithofacies of volcanic domes on the Nakajima Islands. (A) Dacite lava of theHigashiyama dome. The hammer is 33 cm long. (B) Large dacite lava block (20 m in size) of the Nishiyama dome. The
scale ruler (in the white circle) is 1 m long. (C) Coherent dacite of the Hokutou-misaki dome. The scale ruler is 1 m long.
(D) Indurated mudstone of the Hokutou-misaki dome. The red and white segments of the scale ruler are 10 cm long. (E)Indurated mudstone showing slickensides from the Hokutou-misaki dome. (F) Flow-banded dacite lava of the Nansei
dome. The red and white segments of the scale ruler are 10 cm long. (G) Polyhedral-jointed dacite lava of the Bentenjima dome. The hammer is 33 cm long. (H) Flow-banded andesite lava of the Manju-jima dome. The scale ruler is 1 mlong. The locations of Fig. 9A‒H are shown in Fig. 7.

Geology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys

27

the interaction of magma and groundwater. Such activity is
to be expected because the Nakajima Islands are locatedwithin the Toya caldera, which is filled with water. We
infer that the Higashiyama pyroclastic cone formed during
the late stages of activity on the Nakajima Islands, as thecone largely retains its primary morphology. The horizontal terrace on the cone may have formed by erosion
following a rise in the lake water level after formation of
the cone.

The morphology and chemical composition of the
Nishiyama dome suggest it was produced by the extrusionof high-viscosity dacitic magma. The dome differs inmorphology from the Higashiyama dome in terms oflacking a flat top. This difference may be attributed to thehigher viscosity and/or smaller volume of lava at the
Nishiyama dome. The low degree of erosion suggests that
the Nishiyama dome was emplaced during the late stagesof activity at the Nakajima Islands.

7．Higashiyama dome
The Higashiyama dome, located within the Higashiyama
pyroclastic cone (Fig. 7), is elliptical in plan view, ranging
in basal diameter from 1000 m (N-S) to 1350 m (E-W). In
east-west cross section, the dome has a flat top withsteeply sloping sides, and rises 292 m above the lake level(Fig. 8A). The dome retains its primary morphology
except for the presence of several narrow gullies on thesides, and an indistinct horizontal terrace at 56 m above thelake level (Fig. 7).
Field surveys revealed that the dome consists of massive
dacite with platy joints spaced at intervals of 5-30 cm (Fig.
9A). The dacite is grey, non-vesicular, and contains phenocrysts of plagioclase, hornblende, hypersthene, opaqueminerals, and minor augite (63-64 wt.% SiO2 ; Table 3).
The morphology and chemical composition of theHigashiyama dome suggest that it was produced by
extrusion of high-viscosity dacitic magma. The low degreeof erosion suggests that the dome was emplaced during thelate stages of activity at the Nakajima Islands. The dome
and the Higashiyama pyroclastic cone probably comprise a‘tuff cone-dome complex’ that was produced by progressive activity that began with phreatomagmatic explosions
and ended with the effusive emplacement of the lava
dome, as commonly reported in water-rich environments(Zimmer et al., 2010).

9．Hokusei dome
The Hokusei dome, located in the northwest of OshimaIsland (Fig. 7), is elliptical in plan view, ranging in basal
diameter from 500 m (E-W) to >800 m (N-S). In northsouth cross section, it has a ridged top (elongated N-S)
with steeply sloping sides, and rises 256 m above the lakelevel (Fig. 7). The dome has been moderately eroded, andgullies are developed on its surface (Fig. 7). It is unclear
whether or not a horizontal terrace occurs at 56 m above
the lake level. The southern part of the Hokusei dome ispartly covered by the Nishiyama dome (Fig. 8B).
Field surveys suggest that the Hokusei dome has no
outcrops. The surface of the dome is covered with humus
soil and a scattering of angular to subangular, dacite lava
blocks up to 1 m across. The dacite is reddish grey, poorly
vesicular, and contains phenocrysts of plagioclase, hornblende, hypersthene, quartz, and opaque minerals (63 wt.%
SiO2 ; Table 3).
The morphology and chemical composition of the
Hokusei dome suggest it was formed by the extrusion ofhigh-viscosity dacitic magma. Judging from the degree of
erosion, the Hokusei dome was emplaced during the earlystages of activity at the Nakajima Islands. The Hokuseidome differs from the Nishiyama dome in phenocryst
assemblage (e.g., quartz is present ; Table 2) and degree of
erosion, suggesting that they are independent domes thatwere emplaced during different stages of activity.

8．Nishiyama dome
The Nishiyama dome, located at the western end of
Oshima Island (Fig. 7), is almost circular in plan view,
ranging in basal diameter from 900 m (N-S) to 1050 m(E-W). In cross section, it has a pointed top with steeply
sloping sides, and rises 370 m above the lake level (Figs.
2B and 8A). The dome largely retains its primary morphology, except for the presence of several narrow gullies (Fig.
7) and an indistinct horizontal terrace at 56 m above thelake level. The Nishiyama dome partly covers the Hokuseidome and the Nansei dome (Fig. 7).
Field surveys suggest that the Nishiyama dome has no
exposures. The surface of the dome is covered with humus
soil and a scattering of angular dacite lava blocks up to 10
m in size (mostly <2 m across ; Fig. 9B). The dacite is
reddish grey, poorly vesicular, and contains phenocrysts ofplagioclase, hornblende, hypersthene, and opaque minerals(64 wt.% SiO2 ; Table 3).

10．Kitayama dome
The Kitayama dome, located in the northern part ofOshima Island (Fig. 7), is elliptical in plan view, ranging inbasal diameter from 600 m (E-W) to 900 m (N-S). In
north-south cross section, it has a ridged top (elongatedN-S) with steeply sloping sides, and rises 279 m above thelake level (Fig. 7). The dome has been moderately eroded,
and radial gullies are developed on its surface (Fig. 7). The
eastern and western slopes of the dome possess different
morphologies. The eastern slope has a gentle foot with ahorizontal terrace at 56 m above the lake level, whereas thewestern slope does not have a gentle foot. The eastern footof the Kitayama dome grades to the Hokutou-misaki dome
(Fig. 7).
Field surveys suggest that the Kitayama dome has no
outcrops. The surface of the dome is covered with humus
soil and angular to subangular dacite lava blocks up to 1 m

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Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

across. The dacite is grey, poorly vesicular, and contains
phenocrysts of plagioclase, hornblende, hypersthene,
opaque minerals, and minor quartz (65 wt.% SiO2 ; Table
3).
The morphology and chemical composition of the
Kitayama dome suggest it was produced by the extrusionof high-viscosity dacitic magma. The eastern slope of thedome has a gentle foot with a horizontal terrace, implyingthat the eastern foot consists of relatively soft sediments.
Therefore, the dome could be a partly extrusive cryptodome (McPhie et al., 1993) formed by the partial extrusionof dacite magma from a cryptodome. Judging from thedegree of erosion, the Kitayama dome was emplaced
during the early stages of activity at the Nakajima Islands.
The Kitayama dome resembles the Hokusei dome in its
degree of erosion and phenocryst assemblage, suggestingthat these domes were emplaced during a similar stage ofactivity.
11．Hokutou-misaki dome
The Hokutou-misaki dome, located in the northeast of
Oshima Island, has a unique morphology (Fig. 7), being
amoeboid or semi-rectangular in plan view, ranging inbasal diameter from 550 m (NW-SE) to > 750 m (NESW). In north-south cross section, it gently rises 101 m
above the lake level (Fig. 7). In detail, the dome comprises
two broad ridges, both of which trend NE-SW (Fig. 7).
The southeastern ridge is 650 m long, 300 m wide, andrises 101 m above the lake level. The ridge consists of agentle northwestern slope that dips 5-20° (interpreted toform part of the original dome surface) and a steep
southeastern slope that dips 40-45° (interpreted to be afault surface). The southwestern part of this ridge iscovered by the Higashiyama pyroclastic cone. The northwestern ridge is 450 m long, 150 m wide, and rises 91 m
above the lake level. The ridge resembles the southeasternridge in morphology, and consists of a gentle northwestern
slope and steep southeastern slope. The western part of thisridge grades into the Kitayama dome. Both the southeastern and northwestern ridges have a horizontal terrace(<20 m wide) at 56 m above the lake level.
Field surveys revealed that the Hokutou-misaki dome
consists of coherent dacite and mudstone. The dacite is
exposed on the southeastern slope of the southeastern ridge(Fig. 9C ; see the location in Fig. 7). Here, the dacite contains platy joints spaced at intervals of 10-80 cm (Fig. 9C).
The dacite is grey and contains phenocrysts of plagioclase,
hornblende, hypersthene, and opaque minerals (64 wt. %SiO2 ; Table 3). The mudstone is exposed on the northwestern slope of the southeastern ridge (Fig. 9D ; see thelocation in Fig. 7). Here, the mudstone (> 5 m thick)overlies the coherent dacite (>2 m thick). The mudstone isreddish grey, massive, indurated, and in places contains
slickensides on sheared planes (Fig. 9E). The mudstone
consists of lithic fragments, crystals of plagioclase, quartz,

hypersthene, trace hornblende, opaque minerals, andvolcanic glass. The refractive index of the volcanic glass
ranges from 1.4930 to 1.4952 (mean, 1.4941 ; mode, 1.
494 ; number of grains measured, 30 ; as determined using
an RIMS2000 instrument at Kyoto Fission-Track Co. Ltd),suggesting that the volcanic glass originated from the Toya
pyroclastic flow deposits (Machida and Arai, 2003 ;Yokoyama et al., 1973). The mudstone is thus interpreted
to have originated from the uplifted floor deposits of theToya caldera. We have also found many pebbles of red
indurated sandstone along the lakeshore. Therefore, the
Hokutou-misaki dome probably consists of coherent dacite
overlain by mudstone and sandstone.
The morphology and components of the Hokutoumisaki dome suggest it is a cryptodome (Minakami et al.,1951) formed by the uplifting of caldera-floor deposits
following the intrusion of dacitic magma, or a partially
extrusive cryptodome (McPhie et al., 1993) formed by thepartial extrusion of dacite magma from a cryptodome. The
indurated mudstone with slickensides (Fig. 9E) is interpreted to have formed by heating and shearing of thecaldera-floor deposits caused by the intrusion of hot dacitic
magma. Similar indurated mudstone with slickensides has
been reported from the Showa-Shinzan cryptodome at Usu
volcano, Japan (‘natural brick’ ; Mimatsu, 1962). The
asymmetric ridge morphology of the Hokutou-misaki
dome could reflect hinge-like uplifting of the overlying
sediment during dome growth. We infer that the faults(steep southeastern slopes) of the Hokutou-misaki dome
were produced during the hinge-like dome growth. Similar
hinge-like dome growth has been reported from the
Usu-Shinzan cryptodome at Usu volcano, Japan (Katsui etal., 1985). The Hokutou-misaki dome probably formed
during the early stages of activity, because it does notdeform the Higashiyama pyroclastic cone.
12．Nansei dome
The Nansei dome, located in the southwest of OshimaIsland (Fig. 7), exhibits a narrow ridge extending NW-SE(200 m wide, 900 m long, highest point 94 m above thelake level). The dome is severely eroded, and its primarymorphology is poorly preserved. The highest point of thedome is located in its northwestern part. It is possible that
the Nansei dome occurs above a feeder dyke oriented
NW-SE. It is also possible, however, that the Nansei dome
is a thick lava flow, whose flow direction was from
northwest to southeast.
Field surveys revealed that the Nansei dome consists ofa lower, flow-banded unit (Fig. 9F) and an upper, massive(non-flow-banded) unit. The lower flow-banded unit (Fig.
9F) is reddish grey and comprises alternating bands ofhighly vesicular dacite and poorly vesicular dacite, both ofwhich contain phenocrysts of plagioclase, hornblende,
hypersthene, quartz, and opaque minerals (66 wt.% SiO2 ;Table 3). The upper unit consists of grey, poorly vesicularGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys

dacite that contains phenocrysts of plagioclase, hornblende, hypersthene, quartz, opaque minerals, and minor
olivine and augite. The contact between the lower andupper units is not exposed. Field surveys on the ridge of
the Nansei dome revealed that the dome has a small
horizontal terrace at 56 m above the lake level (140 m a.s.l.).
The morphology and chemical composition of the
Nansei dome suggest it was produced by the extrusion ofhigh-viscosity dacitic magma. Judging from the degree of
erosion, the Nansei dome was emplaced during the earlystages of activity at the Nakajima Islands. The Nansei
dome has been TL dated to 45±13 ka (Takashima et al.,1992). Ui et al. (2013) interpreted the Nansei dome to bea remnant pyroclastic cone that consists of pyroclastic
surge deposits. However, they misinterpreted the lower
flow-banded unit (Fig. 9F) as surge deposits ; no pyroclastic deposits were observed at Nansei dome during ourfield surveys.
13．Kannon-jima dome
The Kannon-jima dome on Kannon-jima Island (Fig. 7)
is elliptical in plan view, ranging in basal diameter from
380 m (N-S) to 430 m (E-W). In east-west cross section,
the dome has a pointed top with gently sloping sides, andrises 88 m above the lake level (Fig. 8A). The dome has
been severely eroded, and its primary morphology ispoorly preserved. The dome has several subhorizontal
terraces at 20-56 m above the lake level (Fig. 7), suggesting that the lake level rose after emplacement of thedome, causing erosion of the lower half of the dome. Field
surveys revealed that the Kannon-jima dome comprises
massive andesite with platy joints spaced at intervals of
50-200 cm. The andesite is grey, and contains phenocrystsof plagioclase, hornblende, hypersthene, and opaqueminerals (60 wt.% SiO2 ; Table 3).
The bathymetric map (Fig. 3) suggests that the Kannonjima dome lies on a submerged rise that extends NW-SE.
This rise has a flat top (450×900 m in area) located 1-5 m
below the surface of the lake. On-ship field surveys 150 m
southeast of the Kannon-jima dome (Ukimi-do ; Fig. 3)
revealed that the rise consists of volcanic sandstone andmudstone. The volcanic sandstone is brown, thinly bedded,
and consists of mineral fragments (<3 mm) of plagioclase,quartz, augite, opaque minerals, and minor hornblende.
The mudstone is brown, laminated, and possesses identicalcomponents to the sandstone. The sandstone and mudstone
are gently dipping, although we were unable to accurately
measure dips and strikes as these units were underwater.
Previous studies have noted the presence of volcanic
sandstone at the Kannon-jima dome (Katsui, 1990), whichis consistent with our results.
The morphology and chemical composition of the
Kannon-jima dome suggest it was produced by the extrusion of high-viscosity andesitic magma. The existenceof volcanic sandstone and mudstone at the submerged rise29

near the dome suggests that the caldera floor was uplifted
during or prior to formation of the lava dome. Judging
from the degree of erosion, the Kannon-jima dome was
emplaced during the early stages of activity at the
Nakajima Islands.
14．Benten-jima dome
The Benten-jima dome, adjacent to the Kannon-jima
dome, is elliptical in plan view, ranging in basal diameterfrom 250 m (N-S) to 500 m (E-W ; Fig. 7). In east-west
cross section, the dome has a flat top with steeply sloping
sides, and rises 42 m above the lake level (Fig. 2A). The
dome has been severely eroded, and its primary morphology is poorly preserved. The flat top is probably a
horizontal terrace produced by erosion.
Field surveys revealed that the dome comprises massive
dacite (64 wt.% SiO2 ; Table 3) with platy joints spaced atintervals of 20-80 cm. The dacite is grey, non-vesicular,
and contains phenocrysts of plagioclase, hornblende,
hypersthene, opaque minerals, and minor quartz. The
dacite characteristically contains abundant mafic enclaves(< 10 cm in size). At the lakeshore, the dacite contains
small polyhedral joints (10-20 cm across) within masterjoints (Fig. 9G), suggesting that the lower part of the lava
was quenched by contact with water. Such polyhedral
joints were not found in the upper part of the lava.
The bathymetric map (Fig. 3) suggests that the Bentenjima dome stands on a submerged rise that extends
ENE-WSW and is joined to the submerged rise beneath
the Kannon-jima dome. The geology of the submerged risebeneath the Benten-jima dome is unknown.
The morphology and chemical composition of the
Benten-jima dome suggest it was produced by the extrusion of high-viscosity dacitic magma. The dome differsfrom the Kannon-jima dome in terms of mineral assemblage(e.g., quartz is present ; Table 2) and chemical composition(e. g., higher silica content ; Table 3), suggesting thesedomes formed during different stages of activity. Judging
from the degree of erosion, the Benten-jima dome was
emplaced during the early stages of activity at the
Nakajima Islands. The dome has been TL dated to 40±6
and 41±8 ka (Takashima et al., 1992).
15．Manju-jima dome
The Manju-jima dome, located on Manju-jima Island(Fig. 7), is elliptical in plan view, ranging in basal diameterfrom 180 m (N-S) to 230 m (E-W). In cross section, it has
a flat top with steeply sloping sides, and rises 32 m abovelake level (Fig. 2A). The dome has been severely eroded,
and its primary morphology is poorly preserved. The flat
top is probably a horizontal terrace produced by erosion.
Field surveys revealed that the dome comprises massive,
partially flow-banded andesite lava (Fig. 9H). The andesite
is reddish grey, vesicular, and contains phenocrysts ofplagioclase, hornblende, hypersthene, quartz, augite, opaque

30

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

minerals, and minor olivine (59 wt.% SiO2 ; Table 3).
The bathymetric map (Fig. 3) suggests that the Manjujima dome lies on a submerged mound that is subcircularin plan view. The mound is interpreted as a lower part ofthe lava dome (i.e., andesite) or a cryptodome (i.e., uplifted
caldera floor). The geology of this submerged mound has
not been studied, and it is unknown whether the moundcomprises volcanic rocks or sediments. The whole
volcanic edifice of the Manju-jima dome, including thesubmerged lower part, is subcircular in plan view, ranging
in basal diameter from 350 m (N-S) to 450 m (E-W), andrising 82 m above the lake floor.
The morphology and chemical composition of the
Manju-jima dome suggest it is a lava dome or a partly
extruded cryptodome produced by the extrusion of andesite magma. Judging from the degree of erosion, the Manjujima dome was emplaced during the early stages of activity
at the Nakajima Islands.
16．Discussion
The Nakajima Islands consist of a tuff cone, dacitic toandesitic lava domes, and a dacitic cryptodome, suggestingthat the islands have evolved in response to complex
volcanic activity involving multiple intrusions/extrusionsof dacitic to andesitic magmas and phreatomagmatic
explosions. These volcanic edifices consist of various
types of dacites and andesites with varying degrees of
erosion, suggesting they formed over several stages of
volcanic activity. We infer that the Hokutou-misaki,
Hokusei, Kitayama, Nansei, Kannon-jima, Benten-jima,
and Manju-jima domes formed in the early stages, whereas
the Higashiyama pyroclastic cone, and the Higashiyama
and Nishiyama domes, formed during the late stages ofactivity on the islands.
The Higashiyama pyroclastic cone is the only volcanic
cone on the Nakajima Islands (Fig. 7). We infer that thecone is the source crater for the Nj-Os tephra (Kasugai etal., 1990 ; Machida and Arai, 2003), because the Nj-Os
tephra is the only large-volume pyroclastic fall deposit
erupted from the islands (Kasugai et al., 1990 ; Machida
and Arai, 2003). Although Miyabuchi et al. (2014) estimated that the Us-Ka tephra (Goto et al., 2013) was
erupted from the Nakajima Islands, the tephra is characterized by low-K chemical composition, indicating it was
erupted from Usu volcano (Goto et al., 2013 ; Tomiya andGoto, 2014). The vesicular dacite of the Higashiyama
pyroclastic cone is identical in phenocryst assemblage to
the Nj-Os tephra, consistent with our inference. We
therefore infer that the activity of the Higashiyama
pyroclastic cone began with phreatomagmatic explosions,shifted to magmatic explosions (which produced the
Nj-Os tephra), and ended with magmatic effusions (which
produced the Higashiyama dome).
The environment at the time of the volcanic eruptions
that produced the Nakajima Islands cannot be easily

determined, but it is constrained by several observations.
We found no hyaloclastite on the Nakajima Islands, suggesting the eruptions mostly occurred in a subaerial environment. However, the presence of a tuff cone (Higashiyama
pyroclastic cone) and quench texture in the Benten-jimadome (small polyhedral joints ; Fig. 9G) suggest that the
volcanic eruptions occurred in a water-rich environment.
We therefore infer that the volcanic eruptions of the
Nakajima Islands occurred in a water-rich subaerial
environment or a shallow-water environment. We presume
that the water level of the lake at the time of volcanic
eruptions was similar to the present-day level, as thequench texture in the lava of Benten-jima dome (Fig. 9G)
was found only at the lakeshore, and not in the upper partof the dome. The timing of the lake formation is unknown.
We suppose that the lake formed just after the caldera
collapse (at least until 100 ka).
The presence of mudstone and sandstone at the
Hokutou-misaki dome (Fig. 9D) and at the Kannon-jimadome suggests that the caldera-floor deposits of the Toya
caldera were uplifted during or prior to the volcanism,
forming a bulge at the center of the caldera. This inferenceis supported by the fact that the Nakajima Islands lie on asubmerged rise with highly irregular morphology (Fig. 3).
This bulging was most probably caused by local uplift ofpoorly consolidated lake-floor deposits caused by theascent of voluminous dacitic to andesitic magma. The
bulge is thus interpreted to be a resurgent dome (Cole etal., 2005 ; Lipman, 2000).
The resurgent dome of the Nakajima Islands is muchsmaller than that of Valles caldera (Smith and Bailey,1968). The small-scale resurgent doming at the Nakajima
Islands may be attributed to the small size and/or subspherical morphology of the underlying magma chamber.
Acocella et al. (2001) modeled caldera resurgence using
layered sands to represent the crust and silicone torepresent magma, and their experiments indicated twomodes of resurgence that depended on the aspect ratio(magma chamber width to crustal roof thickness). At an
aspect ratio of approximately one, a domical uplift with acentral uplift formed in the roof. However, at low aspectratios (<0.4 ; i.e., shallow and wide magma chamber), a
domical uplift with a central depression formed ; in thecase of an extensional setting, a graben formed. The
laboratory experiments by Acocella et al. (2001), combined with the morphology of the Nakajima Islands (nocentral depression or graben), suggest that the resurgent
dome of the Nakajima Islands was produced by the
inflation of a magma chamber with a high (approximatelyone) aspect ratio.
The small-scale resurgent doming at the Nakajima
Islands may also be attributed to the relatively soft calderafloor deposits of the Toya caldera. Valles caldera may havea rigid caldera floor composed of densely welded tuff,
which is likely to be broadly uplifted by the inflation ofGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido, Japan)
Inferred from Aerial Laser Mapping and Geological Field Surveys

31

underlying magma. In contrast, the Toya caldera has asofter, non-rigid caldera floor composed of non-welded
pyroclastic deposits that are unlikely to be broadly
uplifted. This inference is supported by the fact that the
Toya pyroclastic flow deposits (Yokoyama et al., 1973)
are non-welded.
The existence of a horizontal terrace on the volcanic
edifices on the Nakajima Islands (at 56 m above the lakelevel) suggests that the water level of the lake rose after the
volcanism of the Nakajima Islands. The terrace is welldeveloped on the Higashiyama pyroclastic cone and
Hokutou-misaki dome, suggesting that relatively soft
pyroclastic deposits and sediments were selectively
eroded. The terrace is indistinct on the harder lava domes,
such as the Higashiyama dome and the Nishiyama dome.
A terrace at the same elevation has also been found on the
inner caldera wall of Lake Toya (Ota, 1956). Ota (1956)
reported four terraces (10, 25, 40, and 57 m above the lakelevel) from the inner caldera wall. The terrace on the
volcanic edifices on the Nakajima Islands (56 m above thelake level) may correspond to the uppermost terrace (57 m)of Ota (1956). As the Nakajima Islands formed at 45-40 ka(Takashima et al., 1992), the rise in water level must haveoccurred after 40 ka.
The rise in water level might have been driven by either
climate change, such as global warming, or alternatively, ageological event in the Toya caldera area. We prefer thelatter because climate change cannot readily explain such alarge change in water level (the height of the terrace is 56
m above the present lake level). The largest geologicalevent after 40 ka in this area was the formation of Usu
volcano at the southern rim of the caldera at approximately
19 ka (Goto et al., 2013). During this period, the caldera
floor around the volcano was significantly uplifted (see thebathymetry of Lake Toya around Usu volcano in Fig. 1),
and basaltic lavas entered the caldera floor, invading thecaldera rim (Yokoyama et al., 1973). The rise in waterlevel can be explained by the uplifting of the caldera floor
around Usu volcano, and by the basaltic lavas entering thecaldera floor. Thus, we infer that the terrace was formed bya rise in the lake water level that resulted from theformation of Usu volcano at around 19 ka.

Fig. 10. Evolution of the Nakajima Islands. (A) Toya
caldera formed at 110 ka by violent silicic explosive
eruptions. (B) The caldera floor was filled withfresh water, forming a caldera lake. (C) A bulge(small resurgent dome) formed on the caldera floor
at 45-40 ka following intrusion of silicic magma.
(D) Volcanic eruptions occurred on the bulge at
45-40 ka, forming the Nakajima Islands. (E) A rise inlake level occurred at 19 ka as a result of the creationof the Usu volcano. (F) The lake level decreased tothe present lake level. The ages are approximate.

32

Yoshihiko GOTO, Satoru MATSUZUKA, Seiji KAMEYAMA and Tohru DANHARA

The lake water level decreased to the present level after
the building of Usu volcano. The timing and cause of thelowering are unknown. One explanation is that the increasein water level at 19 ka caused the lake to overflow, forming
a river that gradually incised the caldera wall while
lowering the lake water level. This drain model is consistent with the fact that a small river (the Sobetsu ; Fig. 1)
has its source in Lake Toya and cuts the caldera rim.
Therefore, the evolution of the Nakajima Islands can besummarized as follows (Fig. 10). (1) Toya caldera wasformed at approximately 110 ka by violent silicic eruptions
associated with pyroclastic flows (Fig. 10A). (2) The
caldera then filled with fresh water, forming a caldera lake
(Fig. 10B). (3) The caldera floor was uplifted at about
45-40 ka by an intrusion of silicic magma, which formed abulge (small resurgent dome ; Fig. 10C). (4) Volcanism
subsequently occurred on the bulge, producing a numberof silicic lava domes, a cryptodome, and a tuff cone (Fig.
10D). (5) A rise in the lake water level caused by thecreation of Usu volcano occurred at around 19 ka, and thisproduced a terrace on the volcanic edifices at 56 m above
the present lake level (Fig. 10E). (6) The lake water level
then decreased to the present level (Fig. 10F).
17．Conclusions
The aerial laser-scanner mapping and geological fieldsurveys reported here have provided invaluable information that has improved our understanding of the geology
and evolution of the Nakajima Islands. These surveys
suggest that the islands consist of a tuff cone, eight lava
domes, and a cryptodome. The uplifted caldera floor(mudstone and sandstone) is exposed on the northeasternand southwestern parts of the islands. The Nakajima
Islands are inferred to have evolved from caldera
resurgence driven by the ascent of voluminous dacitic toandesitic magma, and subsequent extrusions of the magmaon the resurgent dome.
Acknowledgements
This research was sponsored by the Ministry of
Education, Culture, Sports, Science and Technology ofJapan (MEXT), and was supported financially by the
Hokkaido Bureau of Economy, Trade and Industry (METI)
and the Muroran Institute of Technology. We thank T.
Nitta and M. Kiriki (Tanaka Consultant Co., Ltd) for
assistance with the mapping survey. We also thank E.
Kawaminami and K. Kawaminami (Kawanami Kanko Co.,Ltd) for help with the geological survey. XRF analysis of
the rock samples was performed by Y. Sawada of Shimane
University. Comments by R. Furukawa (AIST), an
anonymous referee, and T. Watanabe (University of
Toyama) significantly improved the manuscript.
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(Editional handling Toru Watanabe)

北海道洞爺カルデラ中島火山の火山地質と形成史：
空中レーザーマッピングと地表地質調査による推定
後藤芳彦・松塚33

悟・亀山聖二・檀原徹

我々は，北海道洞爺カルデラ中島火山の火山地質を明らかにするため，ヘリコプター搭載型の高分解能レーザースキャナを用いたレーザーマッピングと，地表地質調査を行った．レーザーマッピングは中島の全域を含む 3×3 km で行い，計測データから 3 次元のデジタル地形図を作成した．地表地質調査は 3 次元デジ
タル地形図を用いて中島の全域で行い，詳細な産状記載と岩石記載を行った．デジタル地形図と地表地質調
査から，中島の詳細な火山地質と形成史が明らかになった．中島は，デイサイトおよび安山岩質マグマが噴出して形成した 8 個の溶岩ドーム（東山ドーム，西山ドーム，北西ドーム，北山ドーム，南西ドーム，観音島ドーム，弁天島ドーム，饅頭島ドーム），デイサイトマグマが湖底堆積物を押し上げて形成した潜在ドーム（北東岬ドーム），およびデイサイト質のマグマ水蒸気噴火により形成したタフコーン（東山火砕丘）からなる．中島の北東部と南西部には，泥岩と砂岩からなる湖底堆積物が分布しており，中島の火山活動がカルデ
ラ底の隆起を伴ったことを示す．中島は，洞爺カルデラ中央部のリサージェントドームの形成と，それに伴うデイサイト〜安山岩質マグマの噴出により形成されたと考えられる．高分解能レーザースキャナによる地形計測と 3 次元デジタル地形図を用いた地質調査は，火山地質の解明に極めて有効である．