Joumalofvolcanology and geotbennalrese3rdt

ELSEVIER Journal of Volcanology and Geothennal Research 66 (1995) 137-148

Ash eruption of the Naka-dake crater, Aso volcano, southwestern Japan

Koji Ono a, Kazunori Watanabe b, Hideo Hoshizumi c, Shin-ichiro Ikebe d

a OYO Corporation. Kudan-kita 4-2-6. Chiyoda. Tokyo 102. Japan b Faculty of Education. Kumamoto University. Kumamoto. Kumamoto 860. Japan

C Department of Geology. Geological Survey of Japan. Tsukuba. Ibaraki 305. Japan d Aso Volcano Museum. Kusasenri-ga-hama. Aso. Kumamoto 869-22. Japan

Received 7 January 1992; accepted 7 July 1994

Abstract

Naka-dake is the only active central cone in the Aso caldera. central Kyushu. Japan. During active periods it erupts charac­teristically black ash, and the ash has been the dominant eruptive product during the past 6,000 years at Naka-dake volcano. The ash is formed by brittle fracturing of the solid glassy top of the magma column, and is transported by a gas stream from under the solid top. Occasionally, frothy scoria, Pele's hair and glass spheres are also ejected from the underlying liquid part. This type of ash eruption is not peculiar to Aso volcano but it is an important, fairly commonly occurring process in andesite volcanism.

1. Introduction

Emissions of black ash characterize the present activ­ity of Naka-dake, the only active central cone in the Aso caldera. southwestern Japan. Aso has the longest documented volcanic history of any volcano in Japan with records dating from the sixth century. We know. by deciphering old documents. of frequent pyroclastic eruptions, including ash emissions and strombolian eruptions. and occasionally more explosive, probably phreatomagmatic. activity and mudflows or pyroclastic flows. Crater lakes or pools seemed to have been long­lived in the historic past. No record of lava outflow exists. The geology around the crater also indicates that the young Naka-dake pyroclastic cones erupted mostly black ash during the past 6000 years as well as small amounts of lava flow ( Watanabe et al.. 1991).

Our purpose here is to summarize the geology and the recent activity of Naka-dake volcano and. particu­larly, to discuss the ash-producing eruptions. Little is

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known about the long-term. ash-producing smoking activity compared to the more conspicuous. but short­lived. strombolian or vulcanian activity of andesitic volcanism. Smoking activity may contribute as much to the eruptive volume as more well-documented activ­ities.

2. Geology of Naka-dake volcano

Aso volcano. located in the central Kyushu volcanic field in southwestern Japan, is a large caldera volcano. The caldera, 24 km north-south and 18 km east-west in diameter, resulted from outflow of four voluminous pyroclastic flows from ca. 0.3 to 0.09 Ma. More than seventeen post-caldera volcanoes. ranging from basalt to rhyolite. are clustered near the center of the caldera (Ono et aI., 1981; Ono and Watanabe. 1985) (Fig. lA).

138 K. Ono et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148

Fig. I. Sketch map around Naka-dake volcano, Aso caldera, Kyushu, Japan. (A) The margin of the caldera and central cones (stippled). ~ indicates volcanic vent. (B) Sketch map around Naka-dake vol­cano. 0 = old volcanic edifice; Y= young volcanic edifice; A = youngest pyroclastic cone; I = active, northernmost craterlet.

; ",;"

N13

Humus (1,200 ± 110 yBP)

N14

--f;i;m,m! Scoria from Kishima-dake

N7

N8

N9

Naka-dake is a stratovolcano of basaltic andesite to basalt. The vent area of Naka-dake volcano has a com­plex, threefold structure (Fig. 1B). The old volcanic edifice (0) is the main cone of this volcano, and rises about 900 m from the caldera floor. The upper half of the cone is a steep-sided (25-30°) complex cone mainly made of welded driblets with subordinate amounts of other pyroclastic materials. The lower half of the cone is gently sloping, at about 10°, and is mainly composed of lava flows. The western half of the upper part of the old edifice was destroyed by a northwest­ward collapse, leaving a 250-m-high collapsed crater wall. The collapse probably occurred coeval with the outflow of a basaltic pyroclastic flow, ca. 14,500 yr B.P. (Fig. 2). The wall embraces a low cone of the young volcanic edifice (Y) which is a gently sloped

Mudflow deposit

• Ash with Humus

D Brown weathered ash

I;.~;.~;.~I Phreatomagmatic ejecta

1 ••• 1 Scoria

em

Horizon of lava flows --mrTTTT.ri. from Naka-dake -0', 2J 1'0

\l '

'~:, Pyroclastic flow deposit ~;:;~ (l4,520±2IOyBP) 0"0 ,- , a,<>

Fig. 2. A columnar section of the ejecta mostly from Naka-dake volcano which erupted for the past ca. 15,000 yr at 4 km north-northeast of the Naka-dake crater (Watanabe. 1991).

K. Ono et al. / Journal o/Volcanology and Geothermal Research 66 (1995) 137-148 139

Fig. 3. Ash-fall layers mostly from Naka-dake volcano. The bottom of the I-m-Iong scale points the "K-Ah" ash of ca. 6300 yr B.P. from the Kikai caldera, off southern Kyushu (Fig. 2). Roadcut 4 km southwest of the Naka-dake crater.

cone, mainly made oflava flows and capped by a pyro­clastic-surge cone. The lavas in the NIl interval of Fig. 2 are of pyroxene andesite which flowed down south­ward from this cone. The lavas are a little younger than 6300 yr B.P. because a widespread tephra "K-Ah" (Machida and Arai, 1978) is intercalated in the under­lying N13 interval (Fig. 2). The youngest pyroclastic cone (A) is a low cone and is mostly enclosed in the wide crater of the pyroclastic-surge cone of the young volcanic edifice. The acti ve crater of the youngest cone is a composite of seven craterlets aligned in N-S direc­tion. The northernmost (Fig. I B) is the only one that has been active in the last 60 years, although some others also were active until the eruption of 1933.

In the stratigraphic section (Fig. 2), above the lava of Nil, most of the deposits is yellowish brown or reddish brown ashes which are weathered products of black ash similar to the ejecta of the present activity (Fig. 3). Intercalated in these ashes are a few layers of lithic-rich lapilli and an ash bed of phreatomagmatic origin from the Naka-dake crater, and basaltic scoria beds from Kishima-dake, Ojo-dake and Kome-tsuka, other central cones.

The wall of the active, northernmost craterlet, ca. 140 m high, consists of a pile of thin layers of ash with

scattered coarse clasts (Fig. 4), while the columnar section inside the southernmost crateriet of the young­est cone is mainly composed of a thick driblet layer (Fig. 5,A, more than 20 m), that is densely welded and is overlain by a layer of explosion breccia (Fig. 5, B, 5-15 m), mainly made of altered, accessory lithics and layers of recent black ash (Fig. 5, C, 3-5 m). The thick beds at the crateriet, welded driblet and explosion brec­cia, rapidly thin outward and can hardly be traced even in a column 1.5 km distant from the craterlet. On the other hand, thin layers of ash, except coarse clasts, extensively cover the surrounding area and compose most thicknesses there as in a column 4 km north­northeast of the crater (Fig. 2). Evidently, the layers of welded driblet and explosion breccia are the products of cone-building activities while the ash layers are products of a sheet-forming-type activity (Walker, 1973).

The estimated volume of ash-fall deposits of the past 6000 years is 8-10 km3

, recalculated as dense rock (DRE) 5-6 km3

, whereas the volume of lava flows of the same period is less than 0.05 km3 and the volume of coarse clasts near the vent is maximally 0.02 km3

• It shows that lava flows and coarse clasts constitute only

140 K. Ona et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148

Fig. 4. Nearly continuous smoking in the northernmost craterlet on October 15. 1989. The vent radius is ca. 30 m. The smoke is con­vective and visible from just above the vent orifice indicating the condensation of water vapor. The wall of the craterlet behind the smoke consists of a pile of thin layers of ash.

1 % of the total eruption products from Naka-dake in these 6000 years and that ash is the major product.

3. Activity of the Naka-dake crater

Recently, Aso volcano has been active for periods ranging from a few months to three years with inter­vening periods of dormancy as long as a few years. Active periods in the last two decades are 1974, 1979, 1984-1985 and 1989-1991. A light -green-colored hot water pool occupies the bottom of the crater during dormant periods. The surface temperature of the pool water fluctuates around 40-50°C and rises to 70°C as the next active period approaches (Asosan Weather

Station, pers. commun., 1991). The following account is the sequence of the best recorded activity of 1989-1991 (lkebe and Watanabe, 1989, 1990; Japan Mete­orological Agency, 1990).

Just after the dry-up of the pool, the crater bottom was perforated by innumerable fumaroles emitting blue-white, high-temperature gas that glowed at night. The surface temperature of the crater bottom, at its highest point, rose to 586°C (Asosan Weather Station, pers. commun., 1991). Then a vent, 15 x20 m wide, opened near the center of the crater bottom and emitted almost continuously, ash-laden brown smoke (Fig. 4). The emission of smoke was accompanied by rumbling and body-felt tremor, both probably caused by ajetting gas stream. The temperature of the crater bottom mark­edly dropped soon after the opening of the vent, leaving behind a hot part restricted to the area around the vent. Besides black ash, sometimes incandescent scoria blocks were ejected. The vent widened and subse­quently changed shape by collapse of its wall, accom­panied with an increase of white ash from altered materials.

In the later half of the active period, explosive or phreatomagmatic events occurred frequently and a hot water pool reappeared at the crater bottom. A large phreatomagmatic eruption took place in the pool. In the early part of this activity, very wet blocks were ejected and were followed by discharge of dry ash, indicating the dry-up of the pool. During the last stage of the active period, mud eruptions frequently occurred in the pool.

The active periods of 1974, 1979 and 1984-1985 differed in duration, but followed sequences similar to those of 1989-1991. The following descriptions are common features of these periods.

3.1. Ash/aUout/rom smoking

The most characteristic volcanic activity of Aso vol­cano is the fallout of black ash from dark smoke (Fig. 6). The ash is of juvenile basaltic andesite and is the dominant component of the eruption products through each eruption period lasting from a few months to as long as three years. The smoking is continuous, blasting tens of minutes to hours, or intermittent, as small puffs with intervening pauses ofless than one minute to more than ten minutes. The height of the smoke plume usu­ally does not exceed 1000 m, resulting in almost con-

K. Ono et al. / Journal o/Volcanology and Geothermal Research 66 (1995) 137-148 141

C B

A

Fig. 5. Northeast side of the wall of the southernmost craterlet. The lower horizon is a thick (ca. 20 m) bed of driblet (A), mostly densely welded and columnar jointed (lighter-colored part) with unwelded top (dark part). Overlying are a layer of explosion breccia (B), which thickens to the left, and a dark-colored, recent black ash bed at the top (C).

centric distribution of ash around the source due to the varying directions of low-altitude wind (Fig. 7 A,B).

During the active period when the ash discharge rate is high, the smoke near the opening of the vent glows red, even during daytime, from incandescent ash. Flames from gas combustion are sometimes seen around the smoke column.

3.2. Strombolian activity

During periods of intense activity, often at its later stage, ballistic ejection of red-hot scoria blocks occurs either nearly continuously or with successive small explosions. The time interval between small explosions and vesicularity of scoria varies. Most parts of the coarse ejecta, however, fall around the vent orifice on the crater bottom, and those that cleared the rim of the craterlet leave only scattered clasts on the ground with­out forming a continuous layer. This type of activity does not last long and amounts to be less than ten days in total in one active period. The volume of ejecta outside the crater, therefore, is negligible compared with the ash from smoking.

3.3. Phreatomagmatic eruption

Occasionally, ph rea to magmatic eruptions take place. Successive small explosions at short intervals

build higher eruption columns than usual for ash emis­sions, but rarely exceed 2000 m. Mixed black and white smoke and ballistic ejection, represented by tephra fin­ger jets, are common characteristics.

Small-scale phreatomagmatic activity occurred fre­quently in 1989 along with topographic changes around the vents. A similar sequence of events was described at White Island volcano, New Zealand (Houghton and Nairn, 1989). Larger phreatomagmatic events tend to occur when the ash discharge from the vent is choked by a thick cover of ejecta or water from heavy precip­itation. Products of phreatomagmatic eruptions are bal­listic ejecta, ash fallout and sometimes low-temperature block and ash flow and pyroclastic surge.

4. Eruption products

The ash and scoria erupted during the recent activity are in general olivine-augite basaltic andesite. Modal and chemical compositions are given in Table 1. Recent eruption products from the Naka-dakecraterfrom 1929 until 1990 are practically the same in mineralogy and chemistry with only minor fluctuation through this period. Phenocrysts are about 30 volume % in total and consist, in decreasing order, of plagioclase (20% or more), augite, olivine and iron-oxide.

142 K. Dno et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148

Fig. 6. Smoking from the Naka-dake crater and ash fall on November 10, 1974. The plume height is ca. 500 m.

B lOkm

IOkm

Fig. 7. Distribution of fallout materials from the smoking activity of the Naka-dake crater. Contour in cm. (A) From June to August, 1979 (modified from Asosan Weather Station, 1980). (B) From November 21,1989 to October I, 1990 (modified from Watanabe, 1991).

4.1. Scoria

Scoria varies in porosity and crystallinity of the groundmass. Scoria blocks are usually less than a few centimeters in diameter but some exceed 15 cm. Some are so frothy as to be reticulite. Most of them are lustrous, dark-brown in color and the groundmass, under the microscope, is clear, pale-brown glass free of microlites. Others are denser, black in color, with a groundmass carrying microlites of plagioclase and pyroxene. Cored bombs containing lithic clasts are found. Lithic clasts, fresh or altered to white, are basal­tic andesite, perhaps derived from the older edifice of this volcano.

4.2. Ash

Black, mostly sand-sized ashes are classified into four groups: (I) block-type ash; (2) splash-type ash; (3) crystals; and (4) rough-surface ash.

Block-type ash, the most common type, consists of angular and nearly equant polyhedral ash surrounded by smooth, near-planar surfaces (Figs. 8,9, 10). Con­choidal fracturing is observed on some surface planes (Fig. 10). Surface planes often cut vesicles (Figs. 9,

Table 1 Selected analyses (wt. %) of essential products from the Naka-dake crater, Aso volcano

SI S2 S3 G Modal composition of scoria

(XRF) (XRF) (wet) (EPMA) (15 October 1974)

Si02 54.71 54.51 53.8756.9 Phenocrysts: (27) Ti02 0.94 0.88 0.95 1.2 Plagioclase 22 AI20 3 18.21 18.54 18.45 14.8 Augite 3 FeO· 8.43 8.38 8.39 10.0 Olivine 1.5 MnO 0.15 0.15 0.16 0.2 Magnetite 0.5 MgO 3.54 3.48 3.78 3.1 Orthopyroxene tr CaO 8.72 8.79 9.14 6.3 Na20 3.01 2.96 3.07 3.5 Groundmass 73 K20 2.00 2.03 1.92 3.1 P20S 0.29 0.29 0.27 -Total 100.00 100.00 100.00 99.1

Analysts: SI, S2-T. Soya; S3-T. Ohmori; G-T. Soya and K. Ono. S 1-S3 = scoria blocks of 2 November 1989, 26 November 1979, and 15 October 1974, respectively; G=groundmass of scoria of IS August 1974. aTotai Fe as FeO.

K. Ono et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148 143

Fig. 8. Photomicrograph of ash of November 14, 1989. Angular ashes of clear brown glass (center and center left) and dark devitrified groundmass (lower left and upper right comer). Elongated vesicles in the glass are seen (left). A free crystal (top) and one mantled by dark groundmass (lower right) are plagioclase. Photo width is 0.8 mm.

Fig. 9. Photomicrograph of the ash of May 16, 1990. Angular ash of clear brown glass with round vesicles (top), glass with microlites (lower left), glass with dusty aggregates of devitrification products (upper right) and dark devitrified ground mass (lower right). A rod­like crystal (bottom) is orthopyroxene and a round crystal induded in the ash (lower left) is clinopyroxene. Photo width is 0.8 mm.

10). These features indicate that the ash was shaped by brittle fracturing of the groundmass. Crystallinity of the groundmass is variable ranging from clear pale­brown glass, glass with aggregates of acicular quench

crystals, to completely devitrified, dark-brown and nearly opaque groundmass (Figs. 8,9).

Splash-type ash, commonly found in products of strombolian eruptions, includes bubble walls, string- or irregular-shaped ashes, fibrous ashes or Pele's hairs (Fig. 11) and minute glass spheres (Fig. 12). Glass spheres were first detected in the ash of the 1974 Aso eruption (Ono and Watanabe, 1975). Under the bin­ocular microscope, the color of spherical ashes varies, according to the crystallinity of their groundmass, from

Fig. 10. SEM photo of block-type ash, surrounded by smooth fracture planes with pits of bubbles. Conchoidal fracture is seen near the center. Rough-surface ashes are around the block-type ashes. Photo width is 0.23 mm. Ash of April 20, 1990.

Fig. 11. SEM photo of a scoria fragment with micro-Pele's hair. Ash with surface grooves or tubular vesicles along the elongation at lower right. Photo width is 0.6 mm. Ash of December 14, 1990.

144 K. Ono et al. / Journal o/Volcanology and Geothermal Research 66 (1995) 137-148

Fig. 12. SEM photo of glass spheres. They are associated with Pele's hairs. Large and smaller spheres are stuck by collisions to each other to form composite droplets (Heiken and Wohletz, 1985, p. 9). The surface is smooth but with small amount of dust adhered to it. Photo width is 0.12 mm. Ash of December 14,1990.

yellow in clear glass, silver gray in clear glass with a thin de vitrified skin, to black in completely devitrified ones.

Crystals are separate phenocrystic minerals (Figs. 8, 9). Rough-surface ashes are included in most eruption products and are common in products of phreatomag­matic eruptions. They are irregular-shaped fragments, surrounded by etch-pitted surfaces, of accessory rock fragments or recycled ash altered by fumarolic action at the crater bottom. White ash of altered material is an aggregate of alunite and anhydrite.

5. Erupted mass and discharge rate

Only two figures are available for the total amount of ejecta of one active period of the Naka-dake crater: ca. 0.9 X 107 t for the activity of 1979 (McClelland et aI., 1989, table 8-25) and ca. 1.2 X 107 t for that of 1989-1991 (Watanabe, 1991) (Table 2). The distri­bution of fallout material from the smoking activity from June to August 1979 is depicted in Fig. 7A and from November 21, 1989 to October 1, 1990 in Fig. 7B. They represent nearly half of the total ejecta of the active periods of 1979 and 1989-1991, respectively.

Average daily rates of ash discharge are calculated from the data of the total ejecta of the activity of 1979 and 1989-1991 (Table 2). Hayakawa and Imura

Table 2 Discharge rate of the ash from Naka-dake, Aso volcano

Date Duration Weight of Rate of ejecta discharge

(m, d, h) (xlift) (X 10" tId)

1979 June 18 d 142- 7.9 1979 July 31 d 162- 5.2 1979 Aug. 26d 159- 6.2 1979 Oct. 31 d 97- 3.1 1979 Nov. 28 d 327- 11.7 (1979 (134 d) (8840) (6.6) total) 1989- 10mb 1200" 4.0 1991 1989 Oct. 27 d 5.4d

9-Nov.4 1989 Nov. 13 h 5.Qd 5-{)

Duration: m = month; d = day; h = hour. -McClelland et aI., 1989, p. 271.

Volumetric eruption rate (m3 /s)

12 8

10 5

18 (10)

8

8

6

bDuration of smoking phases (Watanabe and Ikebe. unpubl. data). CWatanabe,I991. dHayakawa and Imura, 1991.

50

• Aso Naka-dake 40 0 Other volcanoes

E ~

30 .., L: Ol 'v L: 20

OJ E ;j

c:: 10

• 0 10' 10' 10' 10' 10' 10

Volumetric eruption rate (m-3/s)

Fig. 13. The smoking activities of Aso on the diagram of the rela­tionship between plume height and volumetric eruption rate (Wilson et aI., 1978). The Aso smokings are plotted near the lower left comer of the diagram.

( 1991) gave the daily discharge rate for two time spans in the activity of 1989 (Table 2). The result from these sources agree reasonably with each other in the range of (3.1-11.7) X 104 t/day.

The smoking of Aso volcano is nearly continuous. Based on a few observations, assuming that the real duration of smoke discharge is 5% of -the whole time span of the smoking phases and that the density of the

K. Ona et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148 145

ash is 1.5 g/ cm3, the volumetric eruption rate of smok­

ing itself is calculated from the daily discharge rate as 4.8-18 m3

/ s. The plume height, usually lower than 1000 m, is reasonable with respect of volumetric erup­tion rate vs. plume height (cf. Wilson et aI., 1978). The Aso plumes are so small in rate of the volumetric eruption that they are plotted near the lower left corner of the figure (Fig. 13).

6. Ash eruption

6.1. Essential origin o/the ash

The splash-type ashes such as Pele's hairs and glass spheres are evidently essential, having originated from liquid magma. Some ashes, especially those of phrea­tomagmatic eruptions (namely fragments of altered sediments or solid lava), are evidently accessory. The origin of polyhedral and devitrified ash, however, may be questioned. Some ashes, especially of phreatomag­matic eruptions, are encrusted with sublimate minerals on vesicle walls, indicating a recycled origin. Acces­sory lithic ashes are surrounded by fractured planes of crystallized groundmass of lavas and are much lighter colored compared with the black ash. Petrographically, most ashes of the block-type seem to be essential, due to the similar appearance of their groundmass glass relative to that of splash-type ash. SEM observations show that surfaces of most ashes appear fresh with much less adhered dust compared with that of recycled ashes. It is difficult to estimate the ratio of recycled or accessory versus essential ash in smoking activity. Dur­ing the stage of vent opening or widening, the percent­age sometimes increases up to 30% or more but in usual smoking it is probably less than 10%.

The volume of ejecta outside the crater of one par­ticular explosion during an active period, of September 9, 1979 for instance, was nearly identical to the volume of the pit produced by the event (Wada et aI., 1980). This is conformable to the observation that the ejecta of this eruption mostly consist of accessory lithics and some recycled ashes but without obvious essential ash (Ono et aI., 1982). In contrast, total ejecta of the active period from 1989 to 1991 is 1.2 X 107 t (Watanabe, 1991 ), and this vol ume corresponds to a depth of 200 m for the area of the crater bottom or 50 m for that of the northernmost craterlet. In reality, the topography

after an active period is essentially the same as that before. This indirectly indicates that most of the volume of the ejecta during the period was supplied from a deeper source and the contribution of the recycled or accessory ashes is unimportant.

6.2. Origin o/the block-type ash discharged during smoking activity

The most common and characteristic in the eruption products of Naka-dake is the block-type ash discharged during smoking activity. Two possible causes for the origin of the ash are considered: ( 1) hydroclastic; and (2) break-up of the glassy top part of the magma col­umn. Formation of fracture-bounded block-type ash is commonly attributed to chilling by water (Fisher and Schmincke, 1978). The presence of the crater pool during non-active periods, or sometimes even in active periods, and occasional phreatomagmatic activity sug­gest that groundwater plays some role in the activity of the Naka-dake crater. The quasi-steady and non-explo­sive nature of the continuous smoking, however, requires also a quasi-steady and balanced supply of both magma and water. That is a rather difficult con­dition to be realized and would indicate that direct contact of magma with groundwater is not prevailing under the vent. During the continuous and quasi-steady smoking from the established vent, the wall of the vent around and above the magma column is heated to form a hot and dry sheath for the magma which prevent the magma from direct contact with groundwater. Occa­sional phreatomagmatic explosions, resulting from contact of groundwater with magma, occurred when a sudden increase in the supply of magma or groundwater or both happened, e.g. accidental opening of a crack.

The main role of the groundwater in the crater bottom probably is to cool the top part of the magma column. The rapid drop of the crater bottom temperature after the opening of the central vent, except for a hot part around the vent as observed in the activity of 1989-1991 (see p. 139), indicates prevalence of groundwater in the crater-fill sediments. The groundwater surround­ing the magma column will effectively cool its top part to lead it into the subsolidus glassy region. A mound­shaped, semi-solid lava on the crater bottom at the top of the magma column was observed at the final stage of the activity of 1933 (Aoki et aI., 1940). Rapidly cooling semi-solid lava near the top of the column is

146 K. Dno et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148

highly thermally stressed and will break off when high­strain velocity was applied, in a mechanically unstable condition (see p. 139) in the vent with an emitting gas stream. Almost spontaneous break up of a hot lava block of dacite to produce brittle-fractured, block-type ash was frequently observed at the generation of block and ash flows of Fugen-dake, Un zen volcano during its activity of 1991. The ash was formed when the block was torn apart from the front of the lava dome and before its crash on to the ground surface. Presently, we conclude that the main cause to form the block-type ash at Naka-dake is "dry" brittlefracturing of the cool­ing glassy top of the magma column.

6.3. Other examples of ash eruption: Sakurajima and Suwanosejima

Recent eruptions of some other Japanese volcanoes are also examples of ash eruptions. Sakurajima vol­cano, located on southern Kyushu, is presently the most active volcano in Japan. Recent activity of the volcano since 1955 is characterized by frequent explosions var­ying in size and ash smoking. The smoking often fol­lows explosions but non-explosive emission of ash smoke also commonly occurs. The height of the erup­tion clouds rarely exceeds 4000 m for large explosions but is less than 2000 m for common explosions and much less for smoking. Fallout ash from the smoke clouds is the main eruption product. Ballistic ejecta of incandescent blocks, though remarkable in size, is neg­ligible in volume compared with the ash.

Historic large eruptions of this volcano, as in 1471-1479, 1779 and 1914, started with a plinian eruption to generate a high eruption column and were followed by lava outflow. The main phase of the activity to dis­charge magmatic material lasted less than ten days. This type of activity has contributed to build the edifice of the volcano. The eruption since 1955 differs from the large eruptions of the past in that it lasts tens of years and that most of the recent ejecta has been dispersed as ash fallon the surrounding area and has not contributed to build the cone.

The ash of recent eruptions of Sakurajima consists of black, sand-sized, polyhedral, glassy fragments of pyroxene andesite, similar in shape to the block-type ash of Aso volcano. The crystallinity of the groundmass ranges from glassy, brown to pilotaxitic, micro-crys­talline, but most commonly it is partly to completely

devitrified and full of crystallites. The petrographic similarity in texture of both ash and lava flows makes it difficult to distinguish accessory ash from essential ash. As in the case of Aso, however, the eruptive vol­ume of the ash suggests that the Sakurajima ash mainly is of magmatic origin. The total ejecta since 1955 is (2-3) X 108 t, nearly a tenth of that of the large cone­building eruption of 1914. The total ejecta roughly corresponds to the volume ofthe top 300 m of Minami­dake, the active cone of Sakurajima volcano, but the topography of the volcano has not changed essentially since the beginning of the activity. This, again, is indi­rect evidence of the mostly magmatic origin of Saku­rajima ash (Kobayashi, 1986).

The recent eruption, since 1949, of Suwanosejima, an insular volcano off southern Kyushu, is nearly the same as that of Sakurajima. Typical eruptive activity, described as strombolian, lasts from one to a few days and consists of series of small explosions with intervals of seconds to hours. Plume height rarely attains to 4000 m and is commonly less than 1500 m (McClelland et aI., 1989). Fallout ash from the eruption cloud is the most voluminous product in the total ejecta and coarser ejecta fallen around the vent is less in volume than ash (J. Hirabayashi and R. Imura, pers. commun., 1991). The ash is sand sized, polyhedral and consists of pyrox­ene andesite and is very similar in appearance to that of Aso and Sakurajima.

6.4. Ash eruption in andesite volcanism

The smoking and its products, fallout ash, character­ize the recent activity of Aso volcano. The eruption cloud is low, usually not exceeding 1000 m height, and the ejecta volume from one plume is very small. The ashes are plotted near the upper margin of the D-F diagram ( Walker, 1973) expressing their overall fine­ness (Fig. 14). They are located in the phreatoplinian sector (Self and Sparks, 1978) of the diagram. Large D values of the phreatoplinian examples of Self and Sparks (1978) are thought to be due to high eruption columns of single or of a small number of large events with a high discharge rate as fine-grained counterparts of plinian to subplinian deposits. Eruption products, illustrating the wide variety in eruption mode, that are plotted in the phreatoplinian field, are given in the fol­lowing examples.

K. Ono et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148 147

100.-----------------',.maa.r--------------, : PHREA rOPUVIAN

F%

50

SURTSEYAN I I I I

0.05 5 500 50000

Dkml

Fig. 14. Dispersal-fragmentation diagram (Walker, 1973; Wright et ai., 1980). The fallout materials from the smoking of Aso are plotted on or near the upper margin of the diagram.

Wright et al. (1980) plot a few examples of vulca­nian products near the upper margin in the field of phreatoplinian on the D-F diagram. They describe vul­canian activity as characterized by successive small explosions and by a small volume of the products of one plume. The ash cloud of the 1971 eruption of Fuego, Guatemala, reached 8000 m above the crater (Volcanological Society of Japan, 1973) and the max­imum column height of the eruption of Ngauruhoe, New Zealand, in January and March 1974, was 4700 m and highly explosive events were followed by con­tinuous ash discharge with the column lowered with time (Nairn et aI., 1976). On the other hand, the prod­ucts of the 1963 eruption of Irazu, Costa Rica (Murata et aI., 1966), were described by Walker (1973) as the result of "explosions so weak or the crater is so deep that only the finer grained debris clear the crater rim".

These facts indicate that deposits from various erup­tion modes are plotted in the same field, phreatoplinian, in the D-F diagram. Walker's classification of airfall deposits (Walker, 1973) has merit to define uniquely, using their characteristics without concern for phenom­enological definition, which is often ambiguous, and thus extending the scheme to geologic deposits. Although it is useful and successful for the deposits of his trend-line I from hawaiian to plinian orultraplinian, for eruptions of high F values, the division of fields in the D-F diagram does not seem to correspond uniquely with a single mode of eruption. Phreatoplinian or any other single term, defined in the D-F diagram is per­haps not appropriate for a term which covers these wide variety of eruption modes.

The mode of the smoking activity of Aso volcano, having high F values in the D-F diagram, is, possibly, similar to that of 1963 of Irazu but it is quite different from normal vulcanian eruptions as defined by Wright et al. (1980). Sakurajima and Suwanosejima erupt sometimes in vulcanian or strombolian mode but at other times by smoking. We use the term ash eruption, as its dominant product is fallout ash from smoking, but the eruption may be sometimes accompanied by strombolian, vulcanian or phreatomagmatic activity. Ash eruption is not only common in the activity of Aso, Sakurajima and Suwanosejima but, perhaps, more common in andesite volcanism of mild intensity. This kind of activity is characterized by a small rate of magma discharge but may last tens of years. Smoking events and resulting ash fall at these volcanoes have not been studied as much as strombolian or vulcanian activities ejecting incandescent blocks. Considering the total volume of ejecta in long-lasting activities, how­ever, ash eruptions are an important activity for those volcanoes in terms of magma output.

7. Conclusion

(1) Black ash is the dominant component of eruption products from the Naka-dake crater, Aso volcano. This is derived from the solid, glassy top of the magma column, and is transported by a gas stream from under the solid top. Some frothy scoria, Pele's hair and glass spheres are also ejected from the underlying liquid part.

(2) The term ash eruption is used when the principal product is fallout ash from smoking regardless of the eruption is accompanied by products of other modes such as strombolian, vulcan ian or phreatomagmatic.

(3) Ash eruption is not peculiar to Aso, Sakurajima and Suwanosejima, but is a fairly common and impor­tant type of activity of andesite volcanoes of mild inten­sity.

Acknowledgements

In the course of this work, in the field and laborato­ries, we have benefited by receiving information, help, cooperation and discussion from the following insti­tutions and their staff and colleagues: Office of Vol­canic Observations and Asosan Weather Station, both

148 K. Dno et al. / Journal of Volcanology and Geothermal Research 66 (1995) 137-148

of Japan Meteorological Agency; Aso Volcanological Laboratory of Kyoto University and Y. Tanaka; Aso Volcano Museum of Kyushu Sanko Co.; 1. Hirabayashi of Kusatsu-Shirane Volcano Observatory, Tokyo Insti­tute of Technology; R. Imura of Tokyo Metropolitan University (now at Geological Survey of Japan); T. Soya, K. Uto and K. Kano of the Geological Survey of Japan. Prof. G.D. Stanley Jr., University of Montana, kindly reviewed the draft. The early version of this paper was greatly revised by the reviews by Prof. A.L. Grunder, Oregon State University, and Prof. T. Koya­guchi, University of Tokyo. We are very grateful to all of them.

References

Aoki, S., Honda, T. and Hayamizu, L, 1940. Report on the activity of Aso volcano, February, 1933. Kenshinjihou, II: 133-163 (in Japanese).

Fisher, R.V. and Schmincke, H.-V., 1978. Pyroclastic Rocks. Springer-Verlag, Berlin, 472 pp.

Hayakawa, Y. and Imura, R., 1991. Eruptive history of the past 80,000 years of Aso volcano and the 1989 eruption. BUll. Vol­cano!. Soc. Jpn., 36: 25-35 (in Japanese with English abstract).

Houghton, B.F. and Nairn, LA., 1989. The phreatomagmatic and strombolian eruption events at White Island volcano 1976-82: Eruption narrative. In: B.F. Houghton and I.A. Nairn (Editors), The 1976-82 Eruption Sequence at White Island Volcano (Wha­kauri), Bay of Plenty, New Zealand. N. Z. Geol. Surv. Bull., 103: 13-22.

Ikebe, S. and Watanabe, K., 1989. The 1989 eruption of Naka-dake, Aso volcano. Bull. Volcano!. Soc. Jpn., 34: 389-390 (in Japa­nese).

Ikebe, S. and Watanabe, K., 1990. Recent activity of Naka-dake, Aso volcano: March, I 988-November, 1989. Chishitsu News, 426: 6-14 (in Japanese).

Japan Meteorological Agency, 1990. Volcanic activity at Aso. Rep. Coord. Comm. Predict. Volcanic Erupt., 47: 61-70 (in Japa­nese).

Kobayashi, T., 1986. Volcanic ash deposits formed by the intermit­tent eruptions of the acti ve volcano Sakurajima. Rep. Ref. Cent. Sci. Res. Southwest Pacific Area, Kagoshima Vniv., Spec. Vol., I: 1-12 (in Japanese with English abstract).

Machida, H. and Arai, F., 1978. Akahoya ash-a Holocene wide­spread tephra erupted from the Kikai caldera, South Kyushu,

Japan. Quat. Res., 17: 143-163 (in Japanese with English abstract).

McClelland, L., Simkin, T., Summers, M., Nielsen, E. and Stein, T.e. (Editors), 1989. Global Volcanism 1975-1985. Prentice­Hall, Englewood Cliffs, NJ, 655 pp.

Murata, K.J., Dondoli, C. and Saenz, R., 1966. The 1963--65 eruption of Irazu volcano Costa Rica (The period from March 1963-October 1964). Bull. Volcano!., 29: 765-796.

Nairn, I.A., Hewson, C.A.Y., Latter, J.H. and Wood, C.P., 1976. Pyroclastic eruptions of Ngauruhoe volcano, central North Island, New Zealand, 1974 January and March. In: R.W. Johnson (Editor), Volcanism in Australia. Elsevier, Amsterdam, pp. 383-405.

Ono, K. and Watanabe, K., 1975. Volcanic ash of 1974 eruption of Aso volcano. Bull. Volcano!. Soc. Jpn., 20: 121-122 (abstract, in Japanese) .

Ono, K. and Watanabe, K., 1985. Geological Map of Aso Volcano. Geological Map of Volcanoes 4. Geo!. Surv. Japan (in Japanese with English abstract).

Ono, K., Kubotera, A. and Ota, K., 1981. Aso volcano. In: A. Kubot­era (Editor), Field Excursion Guide to Sakurajima, Kirishima and Aso Volcanoes. Volcanol. Soc. Japan, pp. 33-52.

Ono, K., Shimokawa, K., Soya, T. and Watanabe, K., 1982. Geolog­ical and petrological study on the ejecta of 1979 eruption of Naka­dake, Aso volcano, Japan. Report on an urgent study of 1979 eruption of Ontake and Aso volcano. Agency Sci. Technol., pp. 167-189 (in Japanese).

Self, S. and Sparks, R.J.S., 1978. Characteristics of widespread pyro­clastic deposits formed by the interaction of silicic magma and water. Bull. VolcanoL, 41: 196-212.

Volcanological Society of Japan, 1973. Volcan de Fuego. Bull. Vol­canic Erupt., II: 35-36.

Wada, T., Kikuchi, S. and Ono, H., 1980. The explosion of Naka­dake, Volcano Aso on the 6th of September, 1979. Bull. Vol­canol. Soc. Jpn., 25: 245-253 (in Japanese with English abstract).

Walker, G.P.L., 1973. Explosive volcanic eruptions-a new classi­fication scheme. Geol. Rundsch., 62: 431-446.

Watanabe, K., 1991. On the ejecta of Aso volcano. Proc. 23rd Symp. of .. Sabo Gakkai" ,pp. 1-9 (in Japanese).

Watanabe, K., Kawano, T., Tanoue, T., Hirae, M. and Takada, S., 1991. Activity history of Naka-dake in recent 15 kilo-years, Aso volcano. Abstr. Spring Meet. Volcanol. Soc. Japan, 1991, p. 39 (in Japanese).

Wilson, L., Sparks, R.S.J., Huang, T.C. and Watkins, N.D., 1978. The control of volcanic column heights by eruption energetics and dynamics. J. Geophys. Res., 83: 1829-1836.

Wright, J.V., Smith, A.L. and Self, S., 1980. A working terminology of pyroclastic deposits. J. Volcanol. Geothenn. Res., 8: 315-336.