Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows

.hmrpml of I'olcanology and Geothermal Research, 54 ( 1993 ) 310-333

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Elscx ier Science Publishers B.V., A m s t e r d a m

Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991" Dacite lava domes and pyroclastic flows Setsuya Nakada a and Toshitsugu Fujii b ~Deparlment of Earth and t~lam'ta~3•Scienee.~, Ix)'u.~hu ~:niver~ity, l'ul~uoka 5;l 2, .lap,In bEarlhquake Research Institute, Univcr~tty
ABSTRACT Nakada, S. and Fujii, T., 1993. Preliminary report on the activity at Unzen Volcano (Japan I. Nox ember 10~)()-No\ cmb~,'l 1991 : Dacite lava domes and pyroclastic flows..I. ~blcanol. Geotherm Rcs., 54:319-333. The eruption of Unzen Volcano commenced on 17 November 1990. Phreatic and phreatomagmatic eruptions occurred by early May 1991. No large-scale explosive eruptions preceded the extrusion of lava domes. Laxa domes appeared in a summit crater on 20 May 1991, and they grew on the steep slope of Mr. Fugen at Unzen Volcano. Rockfalls from the margins of the domes frequently generated pyroclastic flows. Major pyroclaslic flows occurred on 3 June. 8 June, and 1 September 199 I. The 3 June pyroclastic flow killed forty-three persons. Man> of the pyroclastic flo~,s seem to ha~ e resulted from the simple rockfalls, except one flow on 8 June, which was accompanied by an explosion from the crater. Many of the rockfalls that generated pyroclastic flows were witnessed. As of November 1991, Unzen Volcano was still actix e with a nearly constant magma-supply rate of about 0.3 X 106 m ~/d. The total magma output exceeded 45 X I(V' m ~ b~ the beginn i n g o f N o v e m b e r 1991. The volume of the lava domes is more lhan 23× 1()6 m~.

Introduction

After 198 years of dormancy, Unzen (Unzendake) Volcano in the Shimabara Peninsula, Kyushu, Japan, erupted on 17 November 1990 (Fig. 1). Preceded by phreatic and phreatomagmatic eruptions, new lava domes formed at the summit of Mr. Fugen (Fugendake) on 20 May 1991. Continuous growth of lava domes and falls of lava blocks from the margins frequently generated pyroclastic flows. In this paper we describe the chronology of eruptive events and the nature of pyroclastic flows, mainly the major flows, which occurred ( orrespondence to: S. Nakada, D e p a r t m e n t o f Ea'rlh and P l a n e t a w Sciences, Kyushu University. Fukuoka 812. Japan.

on 3 June, 8 June and 15 September 1991. Unzen Volcano was formed about 500 ka in the Unzen volcanic graben (e.g., Ohta, 1984; Nakada and Tanaka, 1991 ) where N-S tensile stress is dominant. Mainly dacite magma erupted throughout the Unzen volcanic activity. Thick lava flows and domes together with their collapse deposits (debris avalanches, debris flows, block-and-ash flows, and talus) characterize the eruption products (Tanaka and Nakada, 1988: Ozeki and Kobayashi, 1991 ). Most of the Unzen volcanic rocks are porphyritic and contain hornblende and biotite phenocrysts. Mt. Fugendake's summit cone probably collapsed 20,000 yrs B.P., resulting in the formation of a horseshoe-shaped depression (ca. 1.5 km wide; Myoken caldera) (Figs. 1 and 2a).

(t377-0273/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserxed,

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killed by this disaster (e.g., Katayama, [964; Ui, 1983 ). Before the 1991 eruption, three craters (Jigokuato, Kujukushima, and Fugen-ike craters) had opened near the eastern rim of the Myoken caldera (Fig. 1 ).

Chronology of 1990-1991 eruption November 1990-May 1991

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Several domes formed within the caldera after the caldera formation. Historical eruptions occurred in 1663, 1792, and 1990. In 1663, andesite lava (57.5 wt.% SiO2) emerged from a crater within the Myoken caldera and flowed down the northern slope through a notch cutting the caldera rim (Homma, 1935; Ohta, 1984). The volume of this lava flow is about 2 × 106 m 3. In 1792, dacite magma (66.5 wt.% SIO2) erupted on the flank of the Mt. Fugen, and flowed as block lava flow. The eruption of magma continued for about 50 days. There is no record of pyroclastic flows during the 1792 eruption. The total volume of the 1792 lava flow is 22-30X 106 m 3. Just after the 1792 eruption, a large collapse of the Mayuyama lava dome (east of Mt. Fugen; Fig. 2a) took place. The collapse may have been triggered by strong earthquakes that had occurred during the lava eruption, resulting in the generation of a largescale debris avalanche. The avalanche poured into the Ariake Sea and caused a tsunami to strike the opposite shore (Higo district ). About 15,000 persons in Shimabara and Higo were

Following a group of earthquakes below the summit, a phreatic eruption took place at two summit craters (Jigokuato and Kujukushima) on 17 November 1990 (Umakoshi et al., 1991 ). This eruption has continued but the degree of activity has diminished by February 1991. On 12 February 1991, a phreatomagmatic eruption occurred along a fissure (Byobuiwa craters; Fig. 1 ). The volcanic activity had become more active, such that eruptions took place at three craters. An eruption column with cock's tail plumes was observed on 9 April. Fragments as much as 20-30 cm across were ejected during the end of April and early May. The vertical eruption stopped on 12 May. The Jigokuato Crater had enlarged as much as 50 m in diameter and 30 m in depth. Very shallow volcanic earthquakes began to occur just under the eruptive site on 13 May (Fig. 3 ). At the same time, the summit was swelling: EDM (electron-optical distance measurement) indicated a rapid inflation of the summit area (Saito et al., 1991). NW-trending parallel cracks formed around the Jigokuato Crater, which mirrored the graben structure of crater area. A lava spine within the Jigokuato Crater was first recognized by a press aircraft on 20 May. The lava spine was capped by black-colored crater-fill sediments. The next day the lava spine was broken into several large blocks showing wide fracture surfaces. The blocks were uplifted by magma supplied from beneath them, resulting in the formation of a lava dome (dome 1 ) (Fig. 4a). A large, but crude,

PRELIMINARY REPORT ON THE~CTIVITYATLINZENVOLC~N()(JAP~N),N()VEMBER 19c,~0 NO\"EMB~R 19tll

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Fig. 2. (a) View of Mr. Fugen ( F u g e n d a k c ) from the west. Old lava domes were formed within Myoken ('aldera, ~ h i c h is breached southeastward. The 1991 lava d o m e s formed in the eastern periphery of the caldera. Mr. M a > u ) a m a ~ h i c h collapsed in 1792 is in the background. ( b ) Eastern view of Mr. Fugen (17 N o v e m b e r 1991 ). Visible arc two courses along which pyroclastic flows descended from the lava domes on the top: f l - o n t = M i z u n a s h i Rixcr, r i g h t = O s h i g a d a n i Valley. The K i t a - K a m i k o b a area is the center of this photo where the Oshigadani Valley joins wilh lllc Mizunashi River.

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petal structure within the lava dome appeared in the E - W direction, implying the intrusion of magma with E - W elongation. The number of summit earthquakes decreased with time (Fig. 3 ). On 23 May, the crater was filled with lava blocks, which began to fall down the steep outer slope of the crater. The first pyroclastic flows were observed descending the steep eastern slope on 24 May. A burst of volcanic tremor accompanied each pyroclastic flow discharge (Shimizu et al., 1991 ). Thick rain clouds prevented visual observation of the summit area, including the lava dome, but on 26 May small pyroclastic flows were clearly observed descending the eastern valley. The travel distance of pyroclastic flows increased day by day. On 26 May, one pyroclastic flow reached 3 km from the vent, injuring one person. Repeated collapse of the eastern margin of the dome produced pyroclastic flows. The lava dome grew unstably over the eastern edge of Jigokuato Crater, and increased in length, width, and thickness day by day.

June-July 1991 Multiple pyroclastic flows descended the eastern slope during the afternoon of 3 June. On this day, a major pyroclastic flow at 16h9 JST ( G M T + 9 hours) killed 43 people (Fig. 5). About 50 houses were broken and burnt

down. The volume of the pyroclastic flow deposit was roughly estimated at 0.5-0.7× 106 m 3. This eruption destroyed the eastern half of the lava dome and the eastern rim of Jigokuato Crater, and created a horseshoe-shaped depression with a width of 150 m that opened eastward. The volume of the remaining lava dome was about 0.48 × 106 m 3. By 8 June a new lava had appeared within the horseshoe-shaped depression and grew unstably (dome 1: 4 June8 June). The lava domes attained a volume similar to that of the pre-3 June dome (dome 1: 20 May-3 June). The upper waterfall, whose original height was about 100 m (waterfall 1 ), had been buried by block-and-ash flow deposits by 8 June. On 8 June, pyroclastic flows appeared again and at 19h48 began to continuously cascade down the eastern slope reaching a point 5.5 km from the vent. The 8 June pyroclastic flows consist of three main flows. An infrared camera recorded a strong explosion from the summit at 20h06, the m o m e n t at which the third major pyroclastic flow pulse was apparently descending the slope. In this explosion, breadcrusted bombs as much as 5 cm in diameter were ejected to a m a x i m u m of distance of 4 km northeast of the crater. The volume of the third pyroclastic-flow deposit was 0.5-1.0 × 106 m 3. About 70 houses were burnt but no casualties were reported. About 2/3 of the lava domes, including the dome created after 3 June (dome

PREI IM1NARY REPORT ON THE ACTIVITY AT U N Z E N \ O L ( ' A N O (JAP~.N), NOVEMBER 1990-NOVIc!M BFP, I~lCai

l : 4 June-8 June), were destroyed; as a result, the horseshoe-shaped depression widened up to 200 m in width; the volume of the remaining part of dome 1 was about 1.5× 105 m 3 (Fig. 6 ). Bad weather prevented the inspection of the summit area until 14 June, when a new dome (dome 2 ), about 100 m across and 50 m high, was found within the depression. A vulcanian-type eruption occurred at 23h59 on 11 June, and pumice with a breadcrusted or cauliflower surfaces showered the northeastern foot of the volcano. The maximum diameter of pumice was 46 cm at a distance of 3 km from the crater. No pyroclastic flows were documented during this eruption, which was accompanied by strong tremor and a sharp deflation of the summit area (Ueki et al., 1991). The summit area began to reinflate 2 hours after the explosion and continued to swell for 10 hours before reaching a constant level. As a result, a small depression of 20-30 m diameter appeared within the horseshoe-shaped crater; its location was just above the former Jigokuato Crater (Fig. 6). On 17 June, for the first time since the extrusion of the lava, a tephraladen plume was observed to erupt continuously from Jigokuato Crater through a small space in the lava blocks in the northwestern part of dome 1 Dome 2 filled the 8 June depression and grew eastward, its front repeatedly collapsing (Fig. 6). A steep and deep-valley headwater of the Mizunashi River filled with pyroclastic-flow deposits and talus, and the front of the dome flowed over these deposits; the surface on which the lava dome proceeded was much less steep (ca. 20 ° ) than the original bottom of the valley (ca. 35 ° ). The surface texture of the dome is petal-like with two lobes created near the extrusion center at the top of the dome (Fig. 4b). Although the lava surface flowed southeastward at a rate of about 50 m / d after mid-June, the dome hardly lengthened ( 10 m / d as maximum; Figs. 6 and 7) due to the constant collapse at the front. From late June, the horseshoe-shaped

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depression filled with dome material, and lava blocks began to overflow (Fig. 4c). The resultant rockfalls filled the eastern floor of the Myoken Caldera and, then, the northeastern part of the caldera rim was destroyed by their weight. As a result, the course followed by the pyroclastic flows shifted northeastward until late July. Heavy rains on 30 June caused a lahar in the Mizunashi River. The lahar originated in the headwaters of the Oshigadani and Akamatsudani valleys (north and south of the Mizunashi River, respectively). The lahar deeply eroded the pyroclastic-flow deposits at Kitakamikoba (Fig. 5), overflowed northward from the Mizunashi River and entered the sea. About 90 houses along the downstream of the Mizunashi River were demolished by this lahar.

August-November 1991 In mid-August a group of shallow earthquakes occurred beneath the former Jigokuato Crater (Fig. 3). Following this event new magma overrode dome 2 just above the former Jigokuato Crater (Figs. 6 and 7); growth of dome 2 stopped. A small-scale explosion took place through the northern base of dome I on 11 August, resulting in the formation of a small (20 m diameter) explosion pit with a small tuff cone around it. By the end of August, the new dome (dome 3) had grown to a length of 400 m, a width of' 300 m, and a height of 80 m (Figs. 4d, 5 and 6 ). kava blocks from the margin of dome 3 fell down into the Oshigadani Valley, forming pyroclastic flows. By late August, the pyroclastic flows had traveled about 3 km from the crater. The talus-covered crater wall of the former Jigokuato Crater (old reddish lavas and pyroclastics) had been bulldozed northeastward by the intruded magma until 15 September; it arrived at the caldera rim in late August. A group of earthquakes started to take place in early September (Fig. 3). Red-colored ash was ob-

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the formation o f the largest pyroclastic flows at 18h57, which ran about 5.5 km. About 190 houses in the O n o k o b a area (Fig. 5 ) were burnt by the ash-cloud surge that accompanied the 18h57 pyroclastic flows. The area had been already been evacuated and there were no injuries.

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PRELIMINARY REPORT ON THE ACTIVITY AT UNZEN V( )L(TANO (JAPAN). NOVEMBER 1990-NOVEMBER 1991

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After the collapse, a large horseshoe-shaped depression (ca. 300 m wide, 300 m long, and 150 m m a x i m u m depth) was left behind (Fig. 6). The estimated volume of dome lavas and the basement rocks removed was about 3 . 0 X 10 6 m 3, ca. 2/3 of which was dome lavas. A new lava dome (dome 4) grew within the horseshoe-shaped depression (Fig. 4e). This dome continued to grow both eastward arid upward in October and November, and became the largest of the four domes by Novernber (500 m long, 350 m wide, and 200 m high; Fig. 4f). The headwater of the Oshigadani

Valley was filled with dome materials by midOctober (Fig. 2b). The growth of dome 4 continuously pushed former domes southward, causing small-scale collapses of domes 1,2, and 3. The rockfalls generated low-temperature pyroclastic flows: the ash clouds rose only weakly, and the deposits were mainly yellowish-colored ash.

Magma-supply rate and petrography The eruption rate ranged from 0.08 to 0.1X 10 6 m 3 / d within the first week after 20

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TABLE 1 XRF analyses of the 1991 lavas at Unzen Volcano

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SiO2 TiO2 A]203 Fe20~ MnO MgO CaO Na20 K20 P205

64.61 0.66 16.06 4.76 0.07 2.49 4.95 3.86 2.37 0.16

64,66 0.67 16.04 4.74 0.07 2.45 4.92 3.87 2.42 0. t7

64.65 0.67 16.11 4.83 0.07 2.39 4.90 3.77 2.44 0.16

64.71 0.67 15.97 4.78 0.07 2.39 4.88 3.95 2.42 0.17

64.70 0.66 16.17 4.72 0.07 2.37 4.87 3.87 2.42 0.16

64,74 0,66 16,21 4.62 (i)./)7 2.28 4.93 3.93 2,4!) 0.16

(~4.96 ().O~ ~5,91 46 ~) (L07 2,39 4.86 3.88 2.42 (Lib

*Total iron as Fe203. The sum of 10 elements was recalculated to 100%. N = n umber of analyses averaged. D m = dome lava, Blk = lava blocks m blockand-ash flow deposits, B m b = b o m b . Analyzed at Kyushu Universit3.

May, and was nearly constant during J u n e October (about 0.3X106 m 3 / d ) . The total volume of extruded magma by mid-October (for 148 days) was estimated to be about 43 × 106 m 3 on the basis of digital mapping of the lava domes and pyroclastic flow and talus deposits (dense-rock-equivalent value) by the Geographical Survey Institute, Japan. This implies an average magma-supply rate of about 0.29 × 106 m3/d. The daily eruption rate may have varied, but the averaged rate for each week or longer period is nearly constant. The lava domes had a total volume of about 23× l06 m 3 by mid-November. The ratio of lava in intact domes to lava in rock falls is variable. Therefore, the growth rate of lava domes was not uniform. Lava domes grew at the rate of about 0.2 X 106 m3/d from June to mid-July; the value decreased to about 0.1 X 106 m3/d from late July to mid-August. The volume of dome 4 was about 12 × 106 m 3 in mid-November. The growth rate of dome 4 was about 0 . 2 × 106 m3/d. The growth rate range (0.04 to 0.07 km3/yr) for the 1991 lava domes is close to that for lava domes for 27 years at Bezymianny, Kamchatka (0.06 km3/yr), and for the first 3 years

at Santiaguito, Guatemala (0.06 km3/yr), according to the summary of Swanson et al. ( 1987, table 2 ). The growth rate of the Mount St. Helens dome (0.014 km3/yr) is less than these values. Preceding the appearance of a new dome, a group of shallow earthquakes always took place in or below the lava domes except by the birth of dome 2 (Fig. 3). Dome 2 appeared within a horseshoe-shaped depression formed by a landslide on 8 June. The landslide destroyed the top of the magma conduit, and, then, magma may have emerged smoothly into the depression. Thus far, erupted magmas are dacite with a small compositional range (Table 1 ). They are less silicic than the 1792 lava (66.5 wt% SiO2 ). The 1991 dacite is porphyritic and contains about 20 vol.% of plagioclase, amphibole, biotite, and quartz phenocrysts. Microphenocrysts of orthopyroxene, augite, and Ti-rich magnetite are embedded in a glassy groundmass. The peripheral parts of some plagioclase phenocrysts have a dusty appearance. Amphibole phenocrysts consist of hornblende cores mantled by pargasite, as well as aggregates of

PRELIMINARY REPORT ON THE ACTIVITY AT UNZEN VI)LCANO (JAPAN), NOVEMBER 1990-NOVEMBER 19t~l

fine-grained pyroxenes. Biotite phenocrysts are also mantled by pargasite and pyroxenes.

Major pyroclastic flows 3 June Based on helicopter observations, pyroclastic-flow deposits of the 1991 eruption can be subdivided into a block-and-ash flow (the main part), and an ash-cloud surge. The latter was surrounded by a sheared zone, where deposits were too thin to be identified from the helicopter, and leaves and barks of trees were burnt but not toppled (Fig. 5). The boundary between surge deposits and the shear zone was not sharp. The block-and-ash flows of 3 June extended about 3.2 km from the vent while the tip of the surge reached about 4.0 km. The tip of the block-and-ash flows, which was easily identified as they contained large lava blocks, was around 2 m in thickness. In the surge area, all the houses were completely broken and partly burnt, and cars were toppled over or pushed sideways. The thickness of ash-cloud surge deposits at Kita-Kamikoba area was around 20 cm according to the witness of the Ground Self-Defense Force who entered this area on 4 June. Based on its distribution, the ash-cloud surge seems to have spread from the waterfalls (Fig. 5 ); the direction of fallen trees points toward the two falls. Forty-three people were killed by the pyroclastic flows. All the victims were found inside the declared danger zone and beyond the police line. Most of the casualties occurred in the surge area (Kita-Kamikoba area), where bodies of Maurice and Katia Krafft and Harry Glicken were found among the other victims (site A of Fig. 5); most casualties were massmedia reporters. Death was caused by sew~re burns and inhalation of hot gas. 8 June An extensive area of trees was burnt by the ash clouds that accompanied the 8 June pyro-

329

clastic flows. The area affected by the new ashcloud surge deposits could not be clearly delineated by helicopter, because the area covered by the 3 June surge deposits was overprinted by the 8 June surge. However, it is clear that trees on slopes along the headwaters of the Mizunashi River were mowed down (Fig. 5). Large lava blocks (up to 5 m ) dammed up the flow front in the Mizunashi River. Cars burnt and toppled by the 3 June flows had been moved as much as 10 m. Houses near the front of the flow were burnt, but had not been pushed over, such that roof tiles laid on the debris of structures they had once protected. Characteristics of the 8 June pyroclastic flow are: ( 1 ) juvenile lava fragments were contained in the block-and-ash flows; (2) the seared zone is relatively wide, compared with the seared zone from 3 June pyroclastic flows; (3) the blockand-ash flow nearly reached the terminus of seared zone; and (4) the ash-cloud surge was restricted to the proximal area. These facts imply that the horizontal component of the 8 June explosive eruption was small, so that the pyroclastic surge may have lost force beyond about 4 km from the crater, due to the long travel distance and gentle slope (about 5 :~). 15 September The 15 September pyroclastic flow moved through the Oshigadani Valley, changing its course along the Taruki Height, and then joined the Mizunashi River (Fig. 5). Until 15 September, the Oshigadani Valley had resembled a bobsled course in which the southwestern slope of the Taruki Height was smooth and gentle. However, the pyroclastic flows did not jump straight over the Height, but traveled along the deepest course. The ash-cloud surge that accompanied the last pyroclastic flow rushed straight out of the narrow mouth of the Oshigadani Valley, while the main flows changed their courses due to the topographic control. Large blocks more than 10 m across were present in the deposits about 500 m

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Fig. 8. Ash-cloud surge deposits of 3 and 8 June at Kita-Kamikoba (site A in Fig. 5 ), covering the basement soil, ~ lower layer of 15 cm thick (just behind scale plate ) is from 3 June, and an upper layer of 3 cm (around the top of scale ) is from 8 June. The lower layer grades in color from dark gray through light brown to reddish brown.

Fig. 9. Block-and-ash flow deposits of 15 September at Kita-kamikoba covering the basement soil. A thin layer of ashcloud surge deposit (darkest waved layer of about 5 cm thick near bottom of this photo; layer I of Sparks et al.. !(~73), thinning down toward left of this poto, is present under a bottom layer (dark layer up to about 5 cm thick: layer 2a ) of block-and-ash flow deposits. The boundary between layers 2a and 2b (thickest layer) is not clear, Burnt trees pointing the upstream of flow (right) are included near the layer 2b bottom.

PRH.IMIN,~RYREPORT()NTHEACT1VITYATUNZENVOL(~.NO(JAPANI.

downstream of the mouth of the Oshigadani Valley. An eyewitness pointed out that the ashcloud column rose up from near the m o u t h of the Oshigadani. A car previously burnt by June pyroclastic flows (site A in Fig. 5 ) was thrown about 60 m by the surge. At Onokoba, burnt trees were not mowed down but leaned slightly downwind. Although it appeared that the surge rapidly lost force when it detached from the rest of the pyroclastic flow, its temperature remained high. The detachment of ash-cloud surge from the pyroclastic flow was also described by Fisher and Heiken (1982) in the 1902 eruption at Mount Pelee. Preliminary inspections at Kita-Kamikoba during September and October showed that the surge deposits in the toppled cars are only about 5 cm thick. The ash-cloud surge deposils at Kita-kamikoba vary in thickness from 40 to 5 cm, and are composed of two layers whose thicknesses vary from place by place: (1) a lower layer of coarse-grained (fines-depleted) sand, which grades in color from dark gray to reddish brown upward, and (2) an upper graycolored sandy layer (Fig. 8). The boundary between the two layers is sharp. This set is c o m m o n in the surge deposits around KitaKamikoba. Places untouched by the 8 June ashcloud surge lack the upper layer. R o o f tile and lava fragments are rarely included in the middle of the lower layer. At a former tobacco field, the reddish-brown part of the lower layer is replaced by burnt leaves of tobacco. The lower layer should be of the 3 June ashcloud surge. They may correspond to layer 1 of Sparks et al. (1973). Preliminary grain-size analysis for samples collected from these layers shows well-sorted grains with median diameters around 0.15 m m and similar cumulative patterns among different colored parts in the lower layer. This may indicate that the reddish-brown colored part was oxidized after deposition, whereas the bottom and the part near burnt tobacco were not oxidized due to oxygen deficiency. As this color grading may correspond to one ash-cloud surge, the upper layer

NOVEMBER 1990 N()VEMBER l t)'41

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at Kita-Kamikoba might be from 8 June; the upper reddish-brown-colored part may have been eroded away. Lava blocks up to 50 cm across with breadcrusted or cauliflower surfaces were contained in the block-and-ash flow deposits, mostly those of 8 June. The percentage of such juvenile blocks relative to massive blocks of dome lava is small, probably less than 10 %. Late September field inspections at KitaKamikoba at places near the block-and-ash flow deposits that were affected by multiple surges on 15 September, showed that the surge deposits consist of multiple layers of sand and fine ash. A lower sandy layer ( 10 cm thick) is covered with layers of fine ash (individually a few cm thick and collectively about 5 cm thick ), one of which contains accretionary lapilli. These layers are, in turn, furthermore overlain by alternating thin layers of sand and ash with a dune structure (as much as 5 cm thick). The surge deposits at the Onokoba area consist simply of a lower gray-colored sand layer and an upper reddish-brown-colored ash layer, which are different from the assemblage for the 3 June surge deposits at Kita-Kamikoba. The total thickness is most c o m m o n l y about 5 cm, but drifts as much as about 20 cm thick are present. The upper ash layer may correspond to layer 3 of Sparks et al. (1973). The block-and-ash flow deposits of 15 September at Kita-Kamikoba are about 5 m thick, and the interior was still heated 3 weeks after deposition. Lava blocks in the deposits are massive, without the breadcrusted or cauliflower surfaces seen in the 8 June pyroclasticflow deposits. The block-and-ash flow deposits are covered with reddish, fine-ash layer about 10 cm thick (layer 3 of Sparks et al., 1973). Field inspections in 1992 revealed that the block-and-ash flow deposits show neither grading nor layering except the lowermost part ( < 5 cm thick) where a fine-grained laver without large lithics (layer 2a of Sparks et al., 1973) is present (Fig. 9). Gas-pipe structure was rarely observed in the block-and-ash flow

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deposits, layer 2b. Matrix of the upper 1/31/2 of layer 2b is reddish-brown-colored and that of the lower 1/3 part is darkest-colored, implying oxidation of the upper part after deposition. In some places, a thin layer (ca. 5 cm) of ash-cloud surge deposits is present under the layer 2a (Fig. 9); the surge deposits include burnt twigs and bark chips. Mechanism of pyroclastic flows At Unzen, many pyroclastic flows descending the slopes were witnessed and recorded on videotapes by both our observation group and the mass media. Many pyroclastic flows formed due to rock falls at the edge of the lava domes. This suggests that they were Merapi-type pyroclastic flows of Macdonald ( 1972, p. 149) or those resulting from gravitational collapse of a dome (Cas and Wright, 1987). The moment of generation of the 3 and 8 June, and 15 September flows, however, was not witnessed due to poor weather conditions (rain clouds or smog from the preceding rockfalls ). The last of the 8 June pyroclastic flows was accompanied by an explosion directly from Jigokuato Crater. The explosive eruption may have been triggered by a landslide (collapse of large amount of dome lava). There is no evidence of a phreatomagmatic eruption on 8 June. The lava blocks fell from the toe of the domes due to their instability, and were broken into small pieces in an instant when they landed on the slope. The pieces descended along the valley, smoking, and left behind ash-clouds rising above the landing points. None of the pyroclastic flows including those of 15 September, were associated with an ash-laden column that originated at the crater. Study of videotapes shows that ash-laden columns rose upwards as flows passed over the waterfalls (Fig. 5 ), and that pyroclastic flows which cascaded over waterfalls seem to have recovered their energy so that they increased speed from there. Trees toppled in the surge deposits pointed toward the waterfalls. These facts imply that the mix-

s N A K A D A A N D F F[Llll

ture of block and ash expanded at the waterfalls, probably due to the fragmentation of incandescent lava blocks. Reactivation of pyroclastic flows by the shock of fall was described in 1929-1932 eruption at Mt. Pelee (Perret, 1937, pp. 100-101 ). The maximum travel distance of pyroclastic flows was about 5.5 km for the 8 June and 15 September flows. Pyroclastic flows which descended on the bare surface of solid rocks (irregular original surface of the valley) traveled a larger distance than did those on depositfilled valley. Major pyroclastic flows occurred when the valley was not yet filled with talus and pyroclastic-flow deposits. Pyroclastic flows which took place both during early June-midAugust and during October-November became small in scale; the travel distance was almost less than 1 km. These facts may reflect the cushioning effect of thick talus and the disappearance of the irregular original surface, both of which would soften the shock of the falling blocks and prevent their fragmentation. The average speeds of some of the pyroclastic flows were estimated using the relation between the travel distances, observed using Doppler radar of the Ground Self-Defense Force, and the durations of seismicity which are generated by flows. The higher the average speed ofa pyroclastic flow, the longer its travel distance: about 100 k m / h r (28 m / s ) for the flow that reached 3 km, and about 50 k m / h r ( 14 m / s ) for 1 km. The average speed ofa pyroclastic flow which occurred in late August and traveled about 3 km was also estimated at about 93 k m / h r (26 m / s ) using the time lag between the start of the seismic tremor and the signal cessation of the seismometer which was broken by the flow. The average speed of the last flow on 15 September was estimated at about 95 k m / h r (26 m / s ) , using the interval between the start of the flow's seismic signal and its cessation when the flow destroyed the telemeter cable at Onokoba. There is no method to calculate the speed for the last flow of the 8 June pyroclastic flows.

PRELIMINARY REPORT ON TIlE ACTIVITY AT UNZEN VOI.CANO (JAPAN), NOVEMBER 1990-NOVEMBER 1991

Acknowledgements This is a part of work carried out by the Joint University Research Group at Unzen Volcano. We thank the members, especially, K. Ohta and K. Kamo who helped us throughout this study. R.P. Hoblitt and R.V. Fisher provided helpful reviews. T. Kobayashi, N. Matsuwo, T. Yanagi, and S. Aramaki collected some rock samples analyzed in this study. T. Ui, S. Aramaki, and Y. Miyake joined preliminary inspection of pyroclastic flow deposits at Kita-Kamikoba. S. Maeda, K. Yamashita, and Y. Motomura carried out XRF and micro,probe analyses at Kyushu University. M. Sumita kindly did grain size analysis for ash-cloud surge deposits. The Ground Self-Defense Force provided the convenience of helicopter surveys and information from their observation systems, and helped us to collect samples from pyroclastic-flow deposits. Some expenses of this study were defrayed by the Grant-in-Aid of the Ministry of Education, Culture, and Science. Japan. References Cas, R.A.F. and Wright, J.V., 1987. Volcanic Successions, Modern and Ancient. Allen and Uniwin, London, 528 PP Fisher. R.V. and Heiken G., 1982. Mt. Pelee, Martinique: May 8 and 20, 1902, pyroclastic flows and surges. ,I. Volcanol. Geolherm. Res., 13: 339-371. Hon3ma, F.. 1935. Unzendake. Bull. Volcanol. Soc. Jpn., Set. I, 3:71-123 (in Japanese). Kalasama, N., 1964. Old records of natural phenomena concerning the "Shimabara Catastrophe". Sci. Rcp. Shimabara Volcano Obs., Kyushu Univ., 9:1-41 (in Japanese with English abstract). Macdonald, G.A.. 1972. Volcanoes. Prentice-Hall, London, 510 pp.

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Nakada, S. and Tanaka, M., 1991. Magmatic processes of Unzen Volcano. Bull. Volcanol. Soc. Jpn.. 33:113-121 (in Japanese with English abstract ). Ohta, K., 1984. Unzen Volcano. Geography, Geology and Volcanic Phenomena. Nagasaki Pret~'cture, 98 pp. ! in Japanese ). Ozeki. N. and Kobayashi, F., 1991. P?roclaslic flow ticposits distributed around the base of Unzendake Volcano. Programme Abstr., Volcanol. Soc. Jpn., 2:33 im Japanese). Perret, F.A.. 1937. The eruption of MI. t'elee, 1927-19 ~2. Carnegie Inst. Washington, Publ., 458, 126 pp. Saito, E.. Watanabe, K., Suto, S., Hoshizumi. tt., Endo. H., Kazahaya, K , Kawanabe, 5'., Sakaguchi, K., Takarada. S., Yamamoto, T. and Takada, A., 1991. Ground deformation of Unzen volcano while 1990-91 eruption measured by electro-optical distance measurement. Programme Abstr., Volcanol. Soc. Jpn., 2:20 ~in Japanese ). Shimizu, H., Umakoshi, K. and Matsuwo. N., 1991. Generating mechanisms of seismic ~aves at IJnzen "~olcano ( I ) - - Comparison between seismic e',ents and surface phenomena. Programme 4bstr., Volcanol. Soc. Jpn., 2:148 (in Japanese). Sparks, R.S.L Self. S. and Walker, G.P.L., 1973. Products ofignimbrite eruption. Geolog}. 1: ] 15-118. Swanson, D.A., Dzurisin, 1)., Holcomb, R.T.. lwaisubo, W.Y., Chadwick, W.W. Jr., Casade~all, T.J., Ewcrt, J.W. and Heliker, C.C., 1987. Growth of the lava dome at Mount St. Helens, Washington, ([IS:~,). 1981-1983. Geol. Soc. Am., Spec. Pap.. 212: 1-16. Tanaka, M. and Nakada, S., 1988. Geology of eastern part of Unzcn Volcano. Sci. Rep, Shimabara Earthquake Volcano Obs., K~ushu Uni\.. 14:1 11 (in Japanese with English abslract ). Ueki, T., Yamashina, K., Ono, H., lshihara. K., Iguchi, M.. Shimizu, H. and Mixamachi, H., 1991. Tilt changes accompanied wilh an explosion and a pyroclaslic tlow of Unzen Volcano. Programme ~kbslr., Volcanol. Soc. Jpn.. 2:22 (in Japanese). U i, T., 1983. Volcanic dr}' avalanche deposits - - idenlification and comparison with nonvolcanic debris strcarn deposits. J. Volcanol. Oeotherm Res., 18:135-15(i) Umakoshi, K., Shimizu, H., Matsuo, N.. Fukui, R. and Ohta. K., 1991. Forcrunning and follo~ ing seismic activit} o]'the 1990 Unzen eruption. Programme Abstr., Volcanol. Soc. Jpn., 1:5 (in Japanese).