Discharge rates of fallout tephra and frequency of plinian eruptions during the last 400,000 years in the southern Northeast Japan Arc

~

QuaternaryInternational, Vols 34-36, pp. 79-87,1996.

) Pergamon 1040--6182(95)00071-2

Copyright © 1996 INQUA/Elsevier Science Ltd Printed in Great Britain. All rights reserved. 1040-6182/96 $32.00

DISCHARGE RATES OF FALLOUT TEPHRA AND FREQUENCY OF PLINIAN ERUPTIONS DURING THE LAST 400,000 YEARS IN THE SOUTHERN NORTHEAST JAPAN ARC Takehiko Suzuki

Department of Geography, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-03, Japan (Received 4 October 1994; accepted in revised form 20 April 1995) Tephrochronology is one of the most useful tools for reconstructing the record of past explosive eruptions of the last several hundred thousand years. In order to estimate discharge rates of fallout tephra and frequency of plinian eruptions (including sub-plinian eruptions) in the south Northeast Japan Arc, tephra eruptions have been identified by cataloging all known plinian and sub-plinian deposits. One hundred and fifty-two plinian and sub-plinian deposits derived from twelve polygenetic volcanoes, from Asama volcano in the south to Adatara volcano in the north, are identified. The Volume of the largest plinian deposit is 25.9 km3 (DRE: 6.2 kin3), and 41% of the deposits have volumes >1 km 3. Most volcanoes have one or two periods characterized by repeated plinian eruptions, named in this study as a 'plinian stage'. The lengths of plinian stages for these volcanoes are variable, i.e. from 20,000 to 270,000 years. The frequency of plinian eruptions during the plinian stage varies from 0.04 times per 1000 years to 1.3 times per 1000 years. In contrast, discharge rates do not vary so greatly, and average 0.78-0.04 (DRE: 0.19-0.01) km 3 per 1000 years. Copyright © 1996 INQUA/Elsevier Science Ltd

twelve volcanoes in this area during the last few hundreds of thousand years are evaluated. All investigated volcanoes (Fig. 1) are polygenetic. Eleven of them are andesitic stratovolcanoes, including Asama (most southerly), Haruna, Kusatsu, Azumaya, Akagi, Nantai, Nyoho, Takahara, Nasu, Bandai, and Adatara (most northerly). One is a small dacitic caldera named Numazawa. In this paper, changes of discharge rates and frequency of plinian eruptions in the life span of each volcano are discussed.

INTRODUCTION Rates of volcanic output as long term eruption rates have been estimated by many previous studies (Fujii, 1975; Francis and Rundle, 1976; Crisp, 1984; Tsukui et al., 1986). These studies determined rates of volcanic output by dividing the volumes of volcanic edifices, occasionally including lava flows or pyroclastic flow deposits, or both, by the total duration of volcanism. However, it is impossible to evaluate the rate of volcanic output as a function of time by this method, because it indicates an accumulated volume of eruptive material associated with discriminative eruptive events occurring over periods as long as the life time of the volcano. Moreover, such studies have not focussed on the frequency of eruptions nor their variation. In fact, studies discussing changes of both rate and frequency are limited to only a few (e.g. Houghton et al., 1995). In contrast, we can estimate discharge rates of pyroclastic fall deposits as a function of time over a variable time span because it is possible to estimate volume and age for each pyroclastic fall deposit by the tephrochronological method. Also, we can determine changes in the frequency of explosive eruptions that produce pyroelastic fall deposits. The Northeast Japan Arc, subducted by the Pacific Plate, is an island arc with many polygenetic Quaternary volcanoes characterized by andesitic or dacitic rocks, or both (Fig. 1). In the southern part of the Northeast Japan Arc, pyroclastic fall deposits formed during the last several hundreds of thousands of years are well preserved and have been well studied tephrochronologicaUy (Suzuki, 1992). In this study, volumes and discharge rates of plinian deposits and frequency of explosive eruptions for

METHODS OF STUDY Under favourable conditions tephrochronology is a good tool for monitoring changes of frequency and magnitude of eruptions more explosive than an eruption classified as VEI = 2 (volcanic explosivity index; Simkin and Siebert, 1994) without loss of accuracy during the last few hundreds of thousands of years. The stratigraphy of tephra layers in the investigated area has been well established (Fig. 2). Firstly, the volume of each plinian deposit must be determined. Several methods have been used to calculate volumes of plinian deposits, and the accuracy of estimates is dependent on the method used (Pyle, 1989). In this study it is most important to determine volumes using one method for all deposits because a purpose of this study is to compare volumes to indicate relative magnitudes of eruptions. Consequently, the most favourable method for estimating the volume is using isopach maps and an empirical formula proposed by Hayakawa (1985). This formula is V = 12.2TS where V is the volume, T is a certain thickness, and S is the area enclosed within the 79

80

T. Suzuki

135"E

140"E

145* E

IILo ;gr

'

I

I'

I"

j-- 35"N

11

FIG. I. Index map. Volcanoes referred to in this study: 1, Asama; 2, Haruna; 3, Kusatsu; 4, Azumaya; 5, Akagi; 6, Nantai; 7, Nyoho; 8, Takaharn; 9, Nasu; 10, Numazawa (caldera); 11, Bandai; 12, Adatara.

Tephra (29.08 km3), and three other tephras. The method was used to estimate volumes of pyroclastic fall deposits ranging from 10-~ to 102 km3 by Hayakawa (1985). By

isopach of thickness T. This formula was obtained on the basis of the volumes estimated by the crystal concentration method for the Nambu tephra (2.16 km3), Waimihia

i

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.

0 I,A/I YP

D'AT

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----Ym 5

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S ~---=--Ym 6

NY~

:

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FM~

T B _ 13 me-----TAm - 1

Kt MO-a~ Nmm

TB- 8 ~ • T B - I -=

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FIG. 2. Time-space diagram of Late Quaternary tephras in the southern part of the Northeast Japan Arc. Every line with a dot indicates a plinian deposit (occasionally sub-plinian deposit) or a eo-ignimbrite ash-fall deposit formed by a single explosive eruptive event. Volcano names are along the top of the diagram.

81

Discharge Rates of Fallout T e p i d and Frequency of Plinian Eruptions TABLE 1. Cat~ogue ofplinian and mb-pliniml_deposiWin ~

[Irt of N ~ / a l m a .

References for each named volcano: 1. Maclfida and Arm

(1992),Nakazawa et al.(1984)'Soda (1990)'Anti (1993);2. Mschida md Anti (1992),Soda (1969);3. Soda et al.(1988)'Hayskawa and Yui (1989);6. Machida and Anti (1992)'I~yalmwa (1985),Mnlncto (1992); 10. ~ md Anti (1992); 11. Chunma and Ymhida (1982); 12. Machida and Arai (1992)' Soda and Saijo (1987)'Dmt for vckmma 4, 5, 7, S, & 9 axe from the author (u~md~v/Jed) Name of tephra

Volume of tephra (kmS) l

Deposit density ($/cmS) 2

Volume (DRE; 2.5 g/cm 3)

Age

1. Aroma 1 AS-A 2 AS-A' 3 AS-B 4 AS-C 5 AS-D 6 AS-D2 7 AS-Kn 8 AS-F 9 AS-G l0 AS-Sj l l YPk 12 YP-2 13 YP-1 14 OkP-2 15 OkP-1 16 SP 17 BP-9 18 BP-8 19 BP-7 20 BP-6 21 BP-5 22 BP-4 23 BP-3 24 BP-2 25 BP-1

0.7 n.d. 1.7 0.6 n.d. n.d. n.d. n.d. n.d. n.d. 2.8 n.d. 1.6 0.5 0.5 2.6 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 2.9

(0.6)

0.17

A.D. 1783

(0.6) (0.6)

0.41 0.14

A.D. 1108 4th cert. 4.5 ka

2. Haruna 1 FP 2 FA 3 AA 4 HA 5 HP 6-7--

5.1 0.3 n.d. n.d. 2.6 n.d. n.d.

8--

n.d.

9-10 - 11 - 12 - 13 - -

n.d.

5.4 ka

0.67

(0.6) (0.6) (0.6) (0.6)

0.38 0.12 0.12 0.62

(0.6)

0.70

(0.6) (O.6)

1.22 0.O7

(0.6)

0.62

(O.6) (0.6)

0.0OO5 0.O24O

5ka 300-350 430-520 430-520 430-520 430-520

(0.6)

0.84

250 ka >600 ka >700 ka

n.d. n.d.

n.d. n.d.

3. Kmmtsu 1 KS-Ku 2 3P 3 Ng-6 4 Ng-4

0.0 0.1 n.d. n.d.

5 Ng-1

n.d.

6 Kurumi

n.d.

4. A n m a y a 1 MiP 2 Ykm 30yk

3.5 n.d. n.d.

l0 ka 13-14 ka 13-14 ka 13-14 ka 14 ka 14 ka 15-20 ka ca. 20 ka ca. 20 ka ca. 20 ka ca. 20 ka ca. 20 ka ca. 20 ka ca. 20 ka ca. 20 ka >20 ka

(0.6)

6th cen. 6th een. 5--4th een. 30 ka 40--42 ka ca. 250 ka ca. 250 ka ca. 250 ka >130 ka 300-520 ka 300-520 ka 300--520 ka >520 ka

ka ka ka ka ka

T. Suzuki

82

TABLE 1. (Continued). Name of tephra

Volume of tephra (km3) l

5. Akagi 1 K1P 2 C1P 3 KP 4 UP 5 Nm-1 6 Nm-2 7 MzP-1 8 MzP-2 9 MzP-3 10 OP 11 MzP-4 12 MzP-5 13 MzP-6 14 MzP-7 15 MzP-8a 16 MzP-8b 17 MzP-9 18 MzP-10 19 MzP-11 20 MzP-12 21 MzP-13 22 MzP-14 23 MoP

n.d. n.d. 25.9 5.1 2.0 2.9 2.6 1.2 0.1 3.9 0.7 n.d. 3.1 n.d. n.d. n.d. c.b. 2.0 n.d. n.d. n.d. n.d. 7.3

6. Nantal 1 SP 2 IP 3 Ku 1 4 KR-1 5 KR-2 6 KR-3 7 KR-4 8 Ku 2 9 KR-5 10 KR-6 11 KR-7 12 KR-8 13 KR-9 14 KR-10 15 KR-11 16 KR-12 17 KR-13 18 KR-14 19 KR-15 20 KR-16 21 KR-17 22 KR-18 23 KR-19 24 Ku 3 25 Ku 4 26 OgS

2.5 3.8 1.3 n.d. n.d. n.d. n.d. <1.0 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. <1.0 <1.0 1.2

Deposit density (g/cm3) 2

Volume (DRE; 2.5 g/cm 3)

Age

0.2 (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6)

2.22 1.23 0.48 0.70 0.62 0.38 0.02 0.94 0.17

(0.6)

0.75

(0.6)

0.48

20-30 ka ca. 30 ka 32 ka 45-50 ka 45-50 ka 45-50 ka 56-59 ka 60-70 ka 60-70 ka ca. 70 ka ca. 80 ka ca. 85 ka ca. 100 ka 100--130 ka ca. 130 ka ca. 130 ka ca. 130 ka ca. 130 ka

(0.6)

1.75

350--400 ka

(0.6) (0.6) (0.6)

0.60 0.91 0.31

12-13 ka 12-13 ka

(0.6)

<0.24

(0.6) (0.6) (0.6)

<0.24 <0.24 0.29

20 ka

Discharge Rates of Fallout Tephra and Frequency of Plinian Eruptions

83

TABLE 1. (Continued). Name of tephra

Volume of tephra (km3) 1

Deposit density (g/cm3)2

Volume (DRE; 2.5 g/cm3)

7. Nyoho 1 Hg 1 2 Hg 2 3 MaS 4 Ok 5 Ym 1 6 Ym 2 7 Ym 3 8 Ym 4 9 Ym 5 10 So 11 Ym 6 12 Nm 13 Yt 14 Ym 7 15 Ym 8 16 Ym 9 17 Ym 10 18 Si 1 19 Si 3 20 Si 4 21 Yg

c.b. 3.3 5.5 1.2 n.d. <0.5 <2.3 <2.3 <0.5 3.0 0.7 2.4 2.3 1.2 0.4 0.6 1.3 1.6 1.0 0.6 1.0

8. Tukahara 1 Kb 2 Kt 3 Ub 4 Si 2

Age

(0.6) (0.6) (0.6)

0.79 1.32 0.29

(0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6) (0.6)

<0.12 <0.55 <0.55 <0.12 0.72 0.17 0.58 0.55 0.29 0.10 0.14 0.31 0.38 0.24 0.14 0.24

ca. 85 ka ca. 85 ka ca. 100 ka 100-130 ka 130-190 ka 130-190 ka 130-190 ka 130-190 ka 130-190 ka 130-190 ka 130-190 ka 190-250 ka 190-250 ka 190-250 ka 190-250 ka 190--250 ka 190-250 ka 250-350 ka 250-350 ka 250-350 ka 300-350 ka

0.9 3.7 2.4 0.8

(0.6) (0.6) (0.6) (0.6)

0.22 0.89 0.58 0.19

ca. 190 ka 190 ka 250 ka 250-300 ka

9. NIIm 10s 1 20s 3 30s 4 4 Kr 5 Sr 1 6 Sr 2 7 Sr 3 8 Sr 4 9 Sr 5 10 Sr 6 11 Sr 7 12 Sr 8 13 Sr 9 14 Sr 10 15 Sr 11 16 Sr 12

0.1 <0.1 0.2 <0.1 0.8 0.1 0.6 0.2 0.4 0.8 2.0 1.1 2.0 1.9 0.7 0.7

(0.5) (0.6) (0.5) (0.5) (0.6) (0.6) 0.61 (0.6) (0.6) 0.69 0.51 0.15 0.45 0.56 (0.6) (0.6)

0.02 <0.02 0.04 <0.02 0.19 0.02 0.15 0.05 0.10 0.22 0.41 0.07 0.36 0.43 0.17 0.17

16-25 ka 16-25 ka 40-50 ka 50-55 ka 200-350 ka 200-350 ka 200-350 ka 200-350 ka 200-350 ka 200-350 ka 190-350 ka 190-350 ka 190-350 ka 190-350 ka 300-350 ka 300-350 ka

10. Nnmmmwa 1 Nm-NM 2 Nm-KN 3 SbP 4 TgP

1.5 1.4 n.d. n.d.

(0.6) (0.6)

0.36 0.34

5ka 50-55 ka 150-200 ka 200-300 ka

11. Bamld~ 1 HP1 2 HP2

n.d. n.d.

30-46 ka 72-83 ka

T. Suzuki

84

Name of tephra

Volume of tephra

12. Adatara 1 N1

n.d.

2 N2 3 N3 4 SS 5 Mt 6D 7 MS 8 AP 9 FP

n.d. n.d. n.d. n.d. 1.7 n.d. n.d. n.d.

(knl3) 1

TABLE 1. (Continued). Deposit density (g/cm3)2

Volume (DRE; 2.5 g/cm3)

(0.6)

Age

30 ka 40 ka 55 ka 105 ka 110 ka 130 ka 150 ka

0.41

~n.d., not determined; c.b., contained below (e.g. volume of Hgl plus Hg2 = 3.3.km3). 2Values in parentheses are assumed.

using this method, it is possible to determine the volumes of half the plinian deposits treated in this study because they have an isopach map published, or newly acquired. Secondly, the ages of all plinian deposits were collected from published age data based on tephrochronology, ]4C, and fission track methods. When geochronologic information was unavailable, the ages of plinian deposits in tephric loess were used as calibrators under the assumption that the accumulation rate of tephric loess has been constant.

VOLUMES AND AGES OF PLINIAN DEPOSITS

Catalogue of plinian and sub-plinian deposits Plinian and sub-plinian deposits are listed in Table I in order of age for each volcano. One hundred and fifty-two deposits have been identified. The volumes of half of them were determined using isopach maps. Volumes are presented not only in terms of pumice but also as dense rock equivalent (2.5 g/cm3). Volumes in dense rock equivalent are determined by assuming that pumice fragments have densities of 0.6 g/cm 3. However, when the density of the deposit was determined directly, volumes were calculated using this measured density, and they are presented in Table 1. For the other half of the deposits it was impossible to construct isopach maps because exposures to measure the thicknesses of deposits are restricted. This results from very local distributions of the tephra that were associated with small scale eruptions. Volumes of deposits unable to be calculated (58 deposits) are most likely <1 km 3. A volume frequency histogram for all deposits listed is shown in Fig. 3. There are three deposits of volumes < 0.1 km 3, which make up 2% of total deposits. Fifty-seven percent of listed deposits have volumes from 0.1 to 1.0 km 3, and 33% from 1 to 3 km 3. Twelve deposits derived from six volcanoes out of twelve have volumes >3 km 3, eight percent of the listed deposits. Deposits with volumes >10 km 3 occur in only one case, i.e. Kanuma tephra (32 ka), derived from Akagi volcano. The frequency distribution of these volumes is not

similar to that of earthquake magnitudes. This is possibly caused by the following: (1) there were many unrecognized small-scale subplinian eruptions; (2) small amounts of volcanic materials were effused; however, they were not associated with sub-plinian eruptions, but with vulcanian eruptions, steam explosions and/or strombolian eruptions; (3) small-scale eruptions rarely occurred. First, we can exclude case (1) because the observations for fallout tephras in this study were carried out at less than 10 km distance from the vent for most volcanoes. It seems that there are many volcanoes explained by case (2). At least three volcanoes; Asama, Kusatsu, and Nasu, have many records of vulcanian eruptions and/or steam explosions in historical times. One volcano is explained by case (3), namely Numazawa volcano, which has no geological evidence of small scale eruptions in the vicinity of its vent.

DISCUSSION

Plinian stage Eruptive histories of the investigated volcanoes during the last 400,000 years are shown in Fig. 4. A closed circle indicates an eruption, and its size shows the volume of associated deposits. These eruptive histories indicate that most volcanoes have one or two specific periods characterized by repeated plinian (including sub-plinian) 87

9O 80

n=152

7O 60

50 E 30 20 10 0

11

--1

3 j

--

V
0.1~ V<1,0

1~;V<:3 3~V<10

1

lOn;V

Volume (krn3: not DRE)

FIG. 3. Volume-frequency histograms.

Discharge Rates of Fallout Tephra and Frequency of Plinian Eruptions i 12. Adatara

O0

•

85

coo?

11. Bandai 10.Nemazawa

(~dera) _

9. Nasu

0+

o +

. . . . . . o

8. Takahara

• ~ll~

•

*

•

II 7. Nyoho

•

6. Nantai

"'rr" r"

•

r"

r

""

plinian

.1=

5. A~gi

stage 4mmnOelo

• 0

eoooO

•

pliniandeposit

*

3>km3>l l > k n d aO.1 0.1>kin3

4. Azumaya 3. Kasatsu 2. Harena mm

1. Asarna 0

I 100

I 200

I 300

I 4OO

Age (ka) FIG. 4. Eruptive histories o f twelve volcanoes in southern Northeast J a p a n A r c during the last 4 0 0 , 0 0 0 years.

eruptions. In this study, such a period is named a 'plinian stage'. The ages of the beginning and end of a plinian stage are defined as the ages of the oldest and youngest deposits associated with repeated sub- and normal plinian eruptions. No duration is defined for recognizing a plinian stage. Results axe shown in Fig. 4 in which a plinian stage is indicated with a bar. Consequently, all the volcanoes (except Kusatsu, Azumaya, and Numazawa) have one or two plinian stages.

Comparison of characteristics of plinian stages The length of plinian stages is variable, ranging from 20,000 years to 265,000 years (Table 2). Those of Akagi, Nyoho, and the older Nasu volcanoes are 118,000, 265,000, and 100,000 years, respectively. In contrast, the plinian stages of Asama and Nantai are shorter, i.e. 20,000 years long. This is most likely because both are in early plinian stages. Moreover, it appears that Haruna and younger Nasu volcanoes are currently in plinian stages. Nyoho volcano has the longest plinian stage of 265,000 years which ended at 85 ka. The frequency of plinian eruptions during the plinian stages varies greatly from 0.04 times per 1000 years to 1.3 times per 1000 years. In particular, Asama and Nantai volcanoes have the two largest frequencies, of the order of once every 1000 years. Moreover, they have specific periods of higher frequency in plinian stages. For example, Nantai volcano has a high frequency of 3.7 times per 1000 years between 20 ka and 13 ka. This means that the average interval of explosive eruptions (plinian and sub-plinian eruptions) is about 270 years. Haruna, Akagi, and Nasu volcanoes have frequencies in the order of 0.1 times per 1000 years, and Nyoho, Takahara, Bandal, and Adatara in the order of 0.01 times per 1000 years. In contrast to the frequency of eruptions, discharge

rates of plinian deposits in plinian stages show little variation among volcanoes, i.e. from 0.78 to 0.04 (DRE: 0.19-0.01) km 3 per 1000 years. Asama, Haruna, Akagi, Nantai, Nyoho, and the older Nasu volcanoes have rates in the order of 0.1 km 3 per 1000 years. Bandai volcano has a lesser rate of 0.04 (DRE: 0.01) km 3 per 1000 years (Table 2). It is necessary to consider the meaning of discharge rate and frequency in plinian stages. Each seems to be an independent indice of volcanic activity. They both seem to be important for discussion of the volcanism. However, the discharge rates estimated by this study are not equivalent to the true discharge rates of total volcanic products. This is because, in the case of stratovolcanoes, the cumulative volume of all pyroclastic fall deposits does not generally contribute much to the volume of total volcanic output. The volumes of lava flows and pyroclastic flow deposits constructing edifices of stratovolcanoes are considerably larger than those of pyroclastic fall deposits. The ratio of volcanic material, excluding pyroclastic fall deposits, increases with decreasing SiO2 ratio and percentage of volatiles. In order to discuss the factors that affect total eruption rates, and which have not been discussed here, it is necessary to estimate discharge rates of all fallout tephra, lava flows, pyroclastic flow deposits, and so on.

CONCLUSIONS The Northeast Japan Arc, formed by subduction of the Pacific Plate, is an island arc with many polygenetic Quaternary volcanoes characterized by andesitic or dacitic rocks. The modes of eruptions that have occurred at these volcanoes are generally explosive rather than effusive, as shown by the presence of many plinian deposits. The purpose of this study was to establish the 400,000 year-long stratigraphy of fallout tephras derived

Rate of plinian stage in DRE Rate of plinian stage

Discharge rate (Ima3/l~}O years)

Cemntttive volume (kin3) Total fallouttephra volume in DRE Total fallouttephra volume Volume not in plinian stage in DRE Volume in plinian stage in D R E

Times/1000 years

Duration (x 1000 years) Number of eruptions

Volcano

0.19 0.78

3.71

15.5 0

3.71

1.25

20 25

1 Asama

0.05 0.20

1.97

9 0.19

2.16

0.12

42 5

2 Haruna

4.1

0.98 5.5

1.32

3 Kusatsu 4 Azumaya

0.07 0.43

8.15

57.9 1.752

9.9

0.19

118 22

5 Akagi

0.13 0.55

2.64

11 0

2.64

1.30

20 26

6 Nantai

0.03 0.11

6.82

28.4 0

6.82

0.08

265 21

7 Nyoho

0.02 0.07

1.88

7.8 0

1.88

0.04

110 4

8 Taka-hara

0.02 0.11

2.24

11.3 0.06

2.3

0.12

100 12

9 Nasu (older)

0.00 0.00

0.06

0.05

55 4

10 Nasu (younger)

4.9

1.18

11 Numazawa

0.01 0.04

0.48

2 0

0.48

0.04

50 2

0.02 0.07

1.61

9.7 0.72

2.33

0.06

100 6

12 Bandai 13 Adatara

TABLE 2. Characteristics of plinian stages. Volumes presented are not only in pumice equivalent but also in dense rock equivalent (2.5 g/cm3). Volumes in dense rock equivalent were determined by assuming that pumice fragments have densities of 0.6 g/cm3

N

OO

Discharge Rates of Fallout Tephra and Frequency of Plinian Eruptions from the volcanoes which are distributed in the southern part of the Northeast Japan Arc. From this stratigraphical study, 152 sub- to normal plinian eruptions at twelve volcanoes have been recognized. The ages and volumes of these deposits were determined, and then the eruption rates of fallout tephras, and the frequency of explosive eruptions, were estimated. Eruptive histories indicate that most volcanoes have one or two specific periods characterized by repeated plinian eruptions. In this study, this period has been named a 'plinian stage'. Thus, all the volcanoes have one or two plinian stages, except Numazawa (caldera), Kusatsu, and Azurnaya volcanoes. The length of the plinian stage is variable, the frequency of plinian eruptions during this stage varying greatly from 0.04 times to 1.3 times per 1000 years. In contrast to frequency of eruptions, discharge rates of plinian deposits show less variation among volcanoes, i.e. from 0.78 to 0.04 km 3 per 1000 years. ACKNOWLEDGEMENTS Part of this work was financially supported by Grant Aid No. 05780347 through the Ministry of the Education, Science and Culture, Japan. Many points in this study were clarified by comprehensive discussions with Hiroshi Machida (Tokyo Metropolitan University) and Yukio Hayakawa (Gunma University). I would like to record my appreciation to Jim Begdt and Koji Okumura who reviewed the manuscript.

REFERENCES Arai, F. (1993). Eruptive history in Joshu. ln: Aral, F. (ed.), Kazanbai Kokogaku, pp. 30-53. Kokonshoin, Tokyo. Chuman, N. and Yoshida, T. (1982). On the geology of the southern foot of Mt. Bandm. Annual Report of Synthetic Study of Fukushima University: the Nature of Lake lnawashiro, 3, 21-32. Crisp, J.A. (1984). Rates of magma emplacement and volcanic output. Journal of Volcanology and Geothermal Research, 20, 177-211.

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Fujfi, N. (1975). Material and energy production from volcanoes - - a review from plate tectonics. Bulletin of the Volcanological Society of Japan, 20, 197-204. Francis, P.W. and Rundle, C.C. (1976). Rates of production of the main magma types in the cenWal Andes. Geological Society of America Bulletin, 87, 474-480. Hayakawa, Y. (1985). Pyroclastic geology of Towada volcano. The

Bulletin of the Earthquake Research Institute Universityof Tokyo, 60, 507-592. Hayakawa, Y. and Yui, M. (1989). Eruptive history of the Knsatsu Shirane Volcano. Quaternary Research (Japan), 28, 1-17. Houghton, B.F., Wilson, C.J.N., McWilliams, M.O., Lanphere, M.A., Weaver, S.D., Briggs, R.M. and Pringle, M.S. (1995). Chronology and dynamics of a large silicic magrnatic system: central Taupo Volcanic Zone, New Zealand. Geology, 23, 13-16. Machida, H. and Arai, F. (1992). Atlas of Tephra In and Around Japan. University of Tokyo Press, Tokyo (in Japanese), 276 pp. Muramoto, Y. (1992). Middle-Late Pleistocene pyroclastic air-fall deposits in the eastern region of the Nikko volcano group, northeastern Japan - - eruption history of the Nikko volcano group. Geoscience Reports of Shizuoka University, 18, 59-91. Nakazawa, H., Arai, F. and Endo, K. (1984). Tephrostratigraphy of Asama volcano during Kurofu-Maekake period. Progranune and Abstracts. Japan Association for Quaternary Research, 14, 69-70. Pyle, D.M. (1989). The thickness, volume and grainsize of tephra fail deposits. Bulletin of Volcanology, 51, 1-15. Simkin, T. and Siebert, L. (1994). Volcanoes of the World (2nd edn.), pp. 23-25. Smithonian Institution, Washington, D.C. Soda, T. (1989). Two 6th century eruptions of Haruna Volcano, central Japan. Quaternary Research (Japan), 27, 297-312. Soda, T. (1990). Nature and climate of Gnnma Prefecture. In: Editorial Board of History of Gunma Prefecture (eds), History of Gunma Prefecture, pp. 37-129. Gunma Prefecture, Maebashi. Soda, T. and Saijo, K. (1987). Tephras distributed around Adatara volcano. Annals of the Tohoku Geographical Association, 39, 205-. Soda, T., Noto, K. and Arai, F. (1988). Age of the Kumakura pumice fall deposit erupted from Kusatsu-shirane volcano, central Japan. Annals of the Tohoku Geographical Association, 40, 272-275. Suzuki, T. (1992). Stratigraphy of tephra layers from the latter half of middle Pleistocene to late Pleistocene in the Chubu-Kanto area, central Japan. Geographical Reports of Tokyo Metropolitan University, 27, 29-53. Tsukui, M., Saknyama, M., Koyagnchi, T. and Ozawa, K. (1986). Longterm eruption rates and dimensions of magma reservoirs beneath Quaternary polygenetic volcanoes in Japan. Journal of Volcanology and Geothermal Research, 29, 189-202.