natural radionuclides in volcanic activity

Applied Radiation and Isotopes 58 (2003) 393–399

Natural radionuclides in volcanic activity$

Jun Sato*

Department of Industrial Chemistry, School of Science and Technology, Meiji University, Higashi-mita, Tama-ku, Kawasaki, Kanagawa

214-8571, Japan

Received 16 October 2002; accepted 16 November 2002

Abstract

Natural radionuclides of 222Ra, 210Pb and 212Pb present in the magma are emitted during the eruption of volcanoes.

Depletion of 222Rn in pumices and in lava showed that significant amounts of 222Rn were released from erupting

magmas. Atmospheric 210Pb originating from the 1991 eruption of Mt. Pinatubo was detected in Japan and in Korea as

a temporal increase in the atmospheric concentration after the eruption. Atmospheric 212Pb originating from the 2000

eruption of Mt. Miyake-jima was also detected as an abrupt rise in atmospheric concentration after the event.

r 2003 Elsevier Science Ltd. All rights reserved.

Keywords: 222Rn; 210Pb; 212Pb; Volcanic eruption; Releasing efficiency; Atmospheric concentration

1. Introduction

When a volcano erupts explosively, the erupting

magma will emit a large volume of volatile components

as volcanic gas, including H2O vapor and CO2, which is

suggested by the occurrence of the vesicular pumices.

The large amount of volcanic gas, which is emitted at

once, will carry the fragments of magma into the upper

part of the atmosphere. Magma remains for a long time

in the magma reservoir until its eruption, and the

natural radionuclides of 222Rn, 220Rn, 210Pb and 212Pb

present in the magma are probably in radioactive

equilibrium with their parent radionuclides of the U-

and Th-series. These daughter radionuclides will also be

emitted from the magma during the eruption.

This paper discusses (1) the efficiency of the release of222Rn from volcanic products and (2) the observation of

atmospheric 210Pb and 212Pb discharged by eruptions.

2. The efficiency of release of 222Rn from volcanic

products

2.1. Radon-222 degassing from pumices

As a portion of volatile components is emitted into

the atmosphere at the eruption, and the estimation of

the proportion of the released volatiles by an eruption

can offer one criterion, or parameter, for assessing the

violence of the eruption. However, it may be impossible

to send any measuring device safely into the ascending

volcanic plume. As a portion of the 222Rn (half-life:

3.8 d) will also be emitted into the atmosphere and a part

of 222Rn will be lost from the magma, a temporal

disequilibrium will appear between 222Rn and the

remaining 226Ra (half-life: 1.6� 103 yr) in the volcanic

products, including pumices, when they solidify imme-

diately after the eruption. Measurement of the growth of222Rn in the pumice collected immediately after an

eruption is expected to furnish some quantitative

information on the proportion of the volatile compo-

nents lost and the utilization of 222Rn as the tracer can

be expected to meet the purpose.

The 222Rn lost from the magma can be estimated by

use of the growth curves prepared from an erupted

fragment.

$This paper was presented at the 4th International Con-

ference on Isotopes (March 10–14, 2002, Cape Town, South

Africa).

*Corresponding author. Fax: +81-44-934-7906.

E-mail address: [email protected] (J. Sato).

0969-8043/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0969-8043(02)00317-2

The growth curves of 222Rn is expressed as the ratio of

the 222Rn concentration to that of the coexisting 220Rn

(half-life: 55.6 s), produced by the decay of the thorium-

series nuclide 224Ra, because the short lived 220Rn

returns to equilibrium immediately after the erupted

material cools and solidifies:

Að222RnÞ=Að220RnÞ

¼ ½Aeqð222RnÞ=Aeqð

220RnÞ� ½l2=ðl2 � l1Þ�

� ½expð � l1tÞ � ð1� aÞ expð � l2tÞ�; ð1Þ

where l1 and l2 are the decay constants of 226Ra and222Rn, respectively, a (0 p a p 1) is the proportion of222Rn remaining in the erupted magma, A terms are the

radioactivities and Aeq0s denote the radioactivities in

equilibrium of 222Rn and 220Rn, and t is the time elapsed

from the end of the 222Rn emission.

Fig. 1 shows the 222Rn/220Rn ratios for pumices from

the Sakura-jima volcano (321N, 1311E), located south-

west Japan, 1000 km away from Tokyo (Sato and Sato,

1977). Ejected andesitic pumice was collected immedi-

ately after the eruption and was sent to Tokyo for

analysis and non-destructive g-ray spectrometry was

started within 2 d. The 222Rn build-up, A (222Rn), was

estimated by the amount of 214Bi (half-life: 19.9m) in

equilibrium. Measurements were continued for 1 week.

The points in Fig. 1 show an increase in the amount of222Rn. The 226Ra content in the pumice, Aeq (

222Rn), can

be obtained by the same measurement performed 1

month later, after 222Rn has reached equilibrium with226Ra. The growth curves were calculated using Eq. (1).

In the case of the eruption in 1976, 40% of the 222Rn

remained in the pumice, i.e. 60% of the 222Rn was

emitted from the magma.

Similar measurements were applied to some other

eruptions of the Sakura-jima volcano. The results shown

in Fig. 1 indicate that the percentage of emission is

different from eruption to eruption, possibly reflecting

the degree of degassing or the intensity of the explosion.

However, in the case of the Usu volcano (431N,

1411E), located in northeast Japan, in spite of an

explosive eruption, almost all of 222Rn remained in the

collected pumices (Sato et al., 1979).

2.2. Radon-222 degassing from lava flows

Eruptive activity that forms a lava flow is not so

explosive as the ejection of pumice. The volcanic islands

Miyake-jima (341N, 1401E) and Izu-Oshima (351N,

1391E), located south of Tokyo, erupted forming

basaltic lava flows in 1983 and 1986, respectively.

‘‘Red-hot’’ lava samples were obtained from the inner

part of the flow where the lavas were still ‘‘red colored’’

and at high temperature, but were not fluid. Fig. 2 shows

the growth curves of 222Rn obtained with the red-hot

lavas (Takahashi et al., 1984; Sato et al., 1990).

The dashed lines in Fig. 2 represent the complete

emission of 222Rn at the moment of eruption. All of the

observed points for the growth 222Rn lie below the

dashed lines. The solid lines were drawn by assuming

that the red-hot lava was still emitting 222Rn until it was

cooled by being picked up. The two lava flows showed

that the lavas continued degassing of 222Rn effectively

until the time of collection.

Almost complete release of 222Rn has also been

reported in lavas from the Arenal (101N, 851W), Costa

Rica, and Kilauea (191N, 1551W), Hawaii Island,

volcanoes (Gill et al., 1985).

2.3. Radon-222 degassing from pyroclastic flow

The Unzen volcano (331N, 1301E), located southern

Kyushu, Japan, made a decitic lava dome. The lava

0

0.2

0.4

0.6

0.8

0 10 20 30

0

0.2

0.4

0.6

0.8

0 10 20 30

0

0.2

0.4

0.6

0.8

0 10 20 30 90

150

Time after eruption (d)



A (

22

2R

n )

A (

22

0R

n )

A (

22

2R

n )

A (

220R

n )

A (

22

2R

n )

A (

22

0R

n )

Dec. 21, 1978

Jul. 20, 1977

May 13, 1976

0

0.4

0.8

0.80.4

0

0.8

0

0.4

Fig. 1. Variations in the activity ratios of the uranium and

thorium series observed in pumices from Mt. Sakura-jima at

various times after the eruptions. Growth curves are based on

the assumption that a 226Ra–222Rn equilibrium was established

in the magma before the eruption. The parameters in the figures

are the proportions of 222Rn retained in the erupting magmas.

Error bars are based on the counting statistics (1s).

J. Sato / Applied Radiation and Isotopes 58 (2003) 393–399394

dome frequently collapsed to produce pyroclastic flows,

referred to ‘‘Melapi-type pyroclastic flow’’. Airborne

materials were collected some distance away from the

dangerous flows. The 222Rn degassing patterns from

these airborne materials are shown in Fig. 3 (Takahashi

et al., 1993). The release efficiency was generally small,

although some of the data are scattered widely.

2.4. Radon-222 degassing experiment

The degree of degassing of rare-gases, including222Rn, was studied with basaltic volcanic rock by

heating stepwise in an electric furnace. Fig. 4 shows

the results for the JB-1 basalt, the geochemical reference

rock sample issued by the National Institute of

Advanced Industrial Science and Technology, Japan

(Geological Survey of Japan). Helium is released quickly

at low temperature, while the degassing of 222Rn began

at 10501C and 60% of the 222Rn was released at 13001C

(Sato et al., 1980).

As the rock-type of the lava from the Miyake-jima

and Izu-Oshima volcanoes was basalt, the release

efficiency obtained in the heating experiment can be

compared with the observations with the actual lava

flow samples. The temperature of the lava flow samples

was estimated to be less than 10001C, although there

were still ‘‘red-hot’’ lavas in fissure of flows. In

comparison with the experimental 222Rn releasing

efficiency of 20% at 10501C, the releasing efficiencies

of the basaltic lava flows were obviously larger,

indicating that the emission of volatile components

possibly promoted the emanation of 222Rn as a carrier

gas.

0

0.1

0.2

0.3

0.4

0.5

0.6

0 10 20 30 40Days after cooling

214B

i (

10

-2 d

ps/

g )

10 20 30 5 10 205Oct. Nov.

100 %

SamplingEruption

80

60

20

0

40

0

0.1

0.2

0.3

-10 0 10 20 30 40Days after cooling

214B

i (

10

-2 d

ps/

g )

Eru

pti

on

20 30 10 20 30 101Nov. Dec. Jan.

100 %

80

40

060

1983 Miyake-jima lava

1986 Izu-Oshima lava

Fig. 2. Growth of 222Rn observed in the 1983 Miyake-jima and

in the 1986 Izu-Oshima lavas. The specific radioactivity of 214Bi

in equilibrium with 222Rn was measured. The growth curves,

indicated by solid lines, are based on the assumption that the

release of 222Rn had ceased at the time of sampling. The

numbers given on the curves are the postulated percentages of

retained 222Rn. When the release is assumed to have ceased at

the time of eruption, the growth curve for 0% retention, for

example, shifts to a position indicated by dotted lines. Error

bars are based on the counting statistics (1s).

0.6

0.8

1.0

0.6

0.8

1.0

0.6

0.8

1.0

0.6

0.8

1.0

0.6

0.8

1.0

0 5 10 15 20 25 30

0.9

0.9

0.9

0.9

0.9

0.5

0.5

0.0

A (

222R

n )

/ A

(220R

n )

1.0

1.0

1.0

1.0

1.0

Days after collapse

Apr. 01, 1993

Dec. 23, 1991

Sep. 15, 1991

Sep. 06, 1991

Jul. 25, 1991

Fig. 3. Growth of 222Rn observed in the 1991–93 products

from the Unzen volcano. The specific radioactivity of 214Bi in

equilibrium with 222Rn was measured and the activity ratios of214Bi to 208Tl are plotted. The activity ratio in equilibrium [Aeq

(214Bi)/Aeq (208Tl)] in Eq. (1), is normalized to unity. The

numbers given on the curves are postulated fractions of retained222Rn, ‘‘a’’ in Eq. (1). Error bars are based on the counting

statistics (1s).

J. Sato / Applied Radiation and Isotopes 58 (2003) 393–399 395

The small releasing efficiency found with the dacitic

pumice from the Usu volcano and the dacitic pyroclastic

flow from the Unzen volcano may be partly due to the

lower temperature of dacitic magma as compared to

basaltic and andesitic magmas.

3. Observations of atmospheric 210Pb and 212Pb

discharged by eruptions

3.1. Lead-210 originating from volcanic activity

Atmospheric 210Pb (half-life: 22.3 yr) is one of the

progeny of 222Rn generated from the earth’s crust, and

exists in the atmosphere attached to aerosol particles.

Atmospheric concentrations of 210Pb have long been

observed as a tracer for atmospheric transport and

mixing, and of the deposition processes, which affect the

distribution and the residence time of aerosol particles.

The majority of atmospheric 210Pb concentrations were

observed to range from o1mBq to several mBqm3.

Aerosol particles were collected almost daily at

Tsukuba Science City (361N, 1401E), Ibaraki, Japan,

50 km northeast of Tokyo. Lead-210 can be determined

in the collected aerosol samples together with cosmic-ray

induced 7Be. The concentration of atmospheric 210Pb

was approximately 0.5mBq/m3, and showed a seasonal

variation as shown in Fig. 5 (Sato et al., 1994). Atmo-

spheric concentrations of 210Pb increased in spring and

fall, and decreased in summer. The concentration of 7Be

also exhibited a similar pattern of variation.

Some of the atmospheric 222Rn emitted from the

ground surface may be transported into the upper part

of the troposphere or into the lower part of the

stratosphere, decaying with the half-life of 3.8 d. Long-

lived 210Pb that is produced from 222Rn may stay and

accumulate there for 1 yr or 2 yr. Some of the 210Pb

accumulated there can behave similarly with the

cosmogenic 7Be, and may exhibit similar variation

pattern.

0

0.5

1

0 500 1000 1500

Rem

ain

ed f

ract

ion

Temperature (ÞC)

222Rn

4He

40Ar

Fig. 4. Release patterns of radiogenic He, Ar and 222Rn from JB-1 (Kita-matsuura basalt). The sample was heated for 60min at each

temperature. The radon-222 release pattern was obtained under atmospheric pressure.

0.0

0.5

1.0

0

5

J J A S O N D J F M A M

210P

b (

mB

q•m

-3)

7B

e (m

Bq•m

-3)

Fig. 5. Comparison of the 210Pb and 7Be concentrations

observed at Tsukuba Science City after the 1991 eruption of

Pinatubo volcano (June, 1991) with those observed in the

period 1988–1990 prior to the eruptive event. The bars

represent the scatter of the data for the 3 yr. (K): 1991–1992

(after the eruption).


Five days after the 1976 eruption of Mt. Etna volcano

(381N, 151E), Sicily, the high atmospheric concentration

of 210Pb on the island was reported to range from 40 to

120mBq/m3 (Lambert et al., 1976). This observation

implies that, on a large eruptive event, a part of the210Pb along with the parent 222Rn in the erupting

magma may possibly be released into the atmosphere,

and the 210Pb may exist in the atmosphere together with

the 222Rn daughters. The fact that the high concentra-

tions of 210Pb (10mBq/m3) observed around the Sakura-

jima volcano, when it was active, suggested the emission

of 222Rn and 210Pb associated with the eruptive activity

(Komura et al., 1992). A temporal (2–3 d) increase of

0.3mBq/m3 in the atmospheric concentrations of 210Pb

and of volatile elements was reported to have been

observed at Tsukuba Science City, 2 weeks after the

eruption of Mt. St. Helens (461N, 1221W), Washington,

USA, in 1980 (Hirose et al., 1982). An increase in the

amount of 210Pb fallout was observed at Sakai (351N,

1351E) and Kumatori (341N, 1351E), southern Osaka,

Japan, from June 1980, to early 1981 after the eruption

of Mt. St. Helens and from 1982 to 1983 after the

eruption of the El Chich !on volcano (171N, 93W),

Chiapas, Mexico, in 1982 (Matsunami, Megumi, 1992).

On the 1980 eruption of Mt. St. Helens, observations at

Fayetteville (361N, 941W), Arkansas, USA, showed an

increase in the amount of 210Po in the rain

samples collected during the winter season of

1980–1981 (from December to February) as the fallout

of the volcanic products, while no obvious increase

in the amount of 210Pb was observed (Kuroda et al.,

1984).

The Pinatubo volcano (151N, 1201E) on the central

Luzon in the Philippines produced a large explosion in

1991. The top of the volcanic plume reached the

stratosphere, higher than 29 km above the sea level.

The total amount of aerosol particles discharged into the

stratosphere by the eruption was estimated to be

between 15 and 30Mtons.

Fig. 5 shows the monthly average atmospheric con-

centrations of 210Pb and 7Be at Tsukuba Science City for

the period from June 1991 to May 1992. The concentra-

tions of 210Pb for the period from June to November

1991 and from February to May 1992 are in the usual

range of variation of the average value for the period

from 1988 to 1992. A temporal increase was observed in

December 1991, and January 1992, which suggests that

unusual phenomena took place during this period (Sato

et al., 1994).

The total amount of aerosol particles in the strato-

sphere over Tsukuba Science City began to increase

from October 1991, and reached a maximum in

December (Hayashida and Sasano, 1993). It can be

inferred that some of the 210Pb in the stratosphere,

associated with the aerosol particles injected by the

eruption of the Pinatubo volcano, reached Japan at this

time, 6 months after the eruption, and transported into

troposphere.

A similar temporal increase in the atmospheric

concentration of 210Pb was also observed at Seoul

(381N, 1271E), Republic of Korea. The variation of the

concentration is shown in Fig. 6 (Sato et al., 1999). This

temporal increase is coincident with that observed at

Tsukuba Science City (Sato et al., 1994), and this

increase was also estimated to be due to the fallout of the

stratospheric 210Pb originating from the eruption of

Pinatubo volcano.

These observations imply that over a period of 6

months the aerosol particles traveled 3000 km through

the stratosphere from the Philippines to Japan and

Korea.

3.2. Lead-212 originating from volcanic activity

Atmospheric 212Pb (half-life: 10.6 h) is one of the

progeny of 220Rn that is emitted from the ground

surface into the atmosphere, and exists in the atmo-

sphere attached to aerosol particles. Lead-212, the

longest-lived decay product of 220Rn, has also been

used as a tracer to estimate the degree of vertical mixing

in the atmosphere within the surface air layer (Assaf and

Biscaye, 1972). As the main source of the atmospheric

0

1

2

3

4

5

6

0

0.2

0.4

0.6

0.8

J J A S O N D J F M A M

1991 1992

(a)

(b)

Con

cent

rati

on o

f 21

0 Pb

(mB

q •

m3 )

Con

cent

rati

on o

f 21

0 Pb

(mB

q •

m3 )

Fig. 6. Variation of atmospheric concentrations (’) observed

at Seoul (a) and at Tsukuba Science City (b) after the 1991

eruption of the Pinatubo volcano (June, 1991) with the data

from April 1989 to March 1990 (prior to the eruption) at Seoul

and with monthly averaged data for the 3 yr (1988–1990: prior

to the eruption) at Tsukuba Science City.


220Rn is the ground surface, the atmospheric concentra-

tion of 212Pb in the surface air reflects the geological and

meteorological background of the observed locality and

the neighboring area. The atmospheric concentration of212Pb is negligible in maritime air as well as even in the

air above continental areas when the ground surface is

covered with snow (Assaf and Biscaye, 1972).

Erupting magma contains 220Rn and 212Pb along with222Rn and 210Pb, and when a volcano bursts into a large

eruption, the volcanic plume stands as high as the upper

part of atmosphere, and some of the eruptive products,

possibly containing 212Pb from the erupting magma, will

be emitted into the atmosphere.

The Miyake-jima volcano began erupting in July

2000, and was still active in March 2002. As the

ascending magma in the 2000 eruption of Miyake-jima

contained 220Rn and 212Pb along with 222Rn and its

decay products, the atmospheric concentration of 212Pb

around the volcano may possibly have been increased by

the eruptive event. This eruption did not emit magma,

but large amounts of SO2, as much as 2–5� 104 ton/d,

were released during the highly active period. Two

explosive eruptions took place on August 18 and 29 with

tall volcanic plumes. At the eruption on August 29, the

top of the plume reached a height of approximately

8000m above the crater and a few thousands tons of

SO2 were emitted. Although the two eruptive events

were not as large as the 1991 eruption of the Pinatubo

volcano, it was estimated that the 212Pb emitted into the

atmosphere was transported 170 km, the distance from

the Miyake-jima volcano to Kawasaki (361N, 1401E), a

neighboring city of Tokyo, since aerosol particles were

collected, within the time corresponding to the half-life

of 212Pb carries on a southerly wind with a velocity of

several meters per second.

Fig. 7 shows the variation in the atmospheric con-

centration of 212Pb observed during the eruptions of the

Miyake-jima volcano from July to September. The

overlapping open circles show the variation of the

atmospheric concentration of SO2, which was observed

at the neighboring air-monitoring station (Kawasaki

City Monitoring Center); the solid triangles denote

eruptive events.

While the usual concentrations are in the range of 4–

55mBq/m3, a high concentration of 8772mBq/m3 was

observed on August 30, one day after the August 29

event. The atmospheric concentration of 212Pb after the

eruption was two or three times larger than the mean

value for this season. The high concentration of SO2 was

also observed on August 29. As they are roughly

coincident with each other, the temporal increase in

the atmospheric 212Pb observed was inferred to result

from the eruption, being transported on the southerly

wind (Koike et al., 2001). The appearance of the slight

time lag in the arrival time between 212Pb and SO2 may

possibly be partly due to the different durations of

detection of 212Pb and SO2 and partly due to the

difference in the diffusion velocity between SO2 gas and

aerosol particles.

4. Conclusion

These observations suggest that the use of natural

radionuclides can be a useful tool for monitoring

magnamatic activities.

0

20

40

60

Con

cent

ratio

n of

SO

2 (p

pb)

0

30

60

90

120

Con

cent

ratio

n of

212 Pb

(m

Bq.

m- 3

)

8 13 18 23 28 2 7 12 17 22 27 1 6 11 16 21 26 1

Sampling date

Jul. Sep.Aug.

Fig. 7. Variation in the atmospheric concentration of 212Pb along with SO2 (-J-). ‘‘m’’ denotes eruptive events.


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