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Radon and volcanic activity: Recent advances
0735-245X/91 $3.00 + .00 Pergamon Press plc
Nucl. Tracks Radiat. Meas., Vol. 19, Nos 1-4, pp. 409--413, 1991 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
RADON AND VOLCANIC ACTIVITY:
RECENT ADVANCES
N. SEGOVIA ININ, Ap. Post 18-1027,
11801, Mexico D.F., Mexico.
ABSTRACT Radon is a naturally occurring radioactive gas present in all of the underground fluids that have been in contact with uranium bearing rocks. Radon fluctuations in soil and ground water associated with minor volcanic activity are often obscured by anomalies of non-volcanic origin. However, when large volumes of volatile rich magmas accumulate beneath a volcano, the changes of radon levels in soil or water may be much higher than those produced by other causes even at a large distance as compared with the horizontal dimensions of the volcano. When a large explosive eruption occurs, the measurement of radon and thoron daughters' activities in the ejected tephra provides the means for estimating the amount of magma undergoing degassing. After comparison with the ejected volume at a given stage of the eruptive episode, it also offers clues about the possibility of further eruptive activity. KEYWORDS
Environmental
radon;
volcanic
activity;
radon anomalies.
INTRODUCTION Volcanic activity is caused by the movement of magma towards the earth's surface, giving rise to different phenomena such as earthquakes, volcanic tremors, fumarolic and hydrothermal activity and anomalies in some geophysical and geochemical parameters and other geological manifestations (Iwasaki et al., 1975). Degassing, melting, crystallisation, fusion and dissolution of materials can occur with the associated alterations in rocks, soils and ground waters. The intensity of those phenomena varies as a function of time. A volcanic eruption occurs when magma, magmatic emanations and their reaction products break through the earth's crust emerging violently to the surface. The solid components of magma have low transport capability as compared to the gaseous products which can move easily even through small fractures. The particular characteristics of an eruption are a function of several parameters, among the most important ones we can mention magma viscosity, gas content, and the physical characteristics and geometry of the rock matrix. The two first are particularly important since, as pressure decreases during the transport of the magma to the surface, the gas present in the liquid is exsolved giving place to the generation of bubbles. If magma viscosity is low, bubbles evolve compensating pressure differences; however for high viscosities, bubbles move slowly, accumulating pressure that will be liberated by fragmentation of the magma, therefore, producing an explosive eruption. The explosive potential of a volcanic eruption is essentially determined by the pressure from the magmatic volatile products.
409
410
N. SEGOVIA
The action of the gases in volcanic manifestations represents one of the recognized hazards associated with volcanism. As an example we can mention, among the high risk phenomena, the "nuee ardente", a type of pyroclastic flow composed by gases and pyroclastic materials at high temperatures and velocities slipping down from a volcanic cone during an explosive eruption (as it occurred during the eruptions of Montagne Pelee in 1902 in Martinica and of E1 Chichon in 1982, in Mexico). An example of sudden degassing is g i v e n by the 1986 Lake Nyos catastrophe occurred in Cameroon, where rich CO2 gas of magmatic origin was liberated, producing the death by asphyxia of 1746 people and a large amount of animals (Shenker and Dietrich, 1986). Evolution of volcanic gases is thus, for several reasons, an important field of study. RADON MONITORING
IN ACTIVE VOLCANOES
222Rn is a natural radioactive gas present in all geofluids in contact with uranium bearing rocks. Since 1927, with the work reported by Shiratoi, radon is known to be associated with gases in fumaroles and thermal springs. Being chemically inert, radon incorporation to geofluids is dependent upon molecular diffusion. Fluids flow is then the dominant mechanism by which radon is transported; the limiting factor in the transport length being its half life (01guin et al., 1990). In this sense radon can be considered as a natural radioactive tracer giving information on recent geofluids fluctuations. The evidence that radon concentration undergoes significant changes with volcanic activity is growing rapidly, however the complexity of the physical and chemical processes participating in a volcanic eruption results in a wide spectrum of radon fluctuation patterns that has its own peculiarities for each active volcano. Several authors had reported radon studies in fumaroles, springs and soil and also, when a large explosive eruption occurs, the measurement of radon and thoron daughters activities in the ejected tephra has proved useful for estimating the amount of magma undergoing degassing. The first paper concerning the correlation between radon fluctuations and a volcanic eruption was published by Chirkov (1976) from several years observations of radon concentrations in spring water associated to the Karimsky volcano. Since that time anomalous radon changes have been observed in different volcanoes. Among the reported active volcanoes under radon monitoring we can mention: Karimsky (URSS), Usu and Sakurajima (Japan), Chichon (Mexico), Piton de la Fournaise (France), Etna and Vulcano (Italy), Mount St Helens and Kilauea (USA), Krafla (Iceland), Poas and Irazu (Costa Rica), etc.. Those volcanoes have experience eruptive periods of different explosivity in the last 15 years. MONITORING
METHODOLOGIES
Radon monitoring in active volcanoes have been reported essentially in soil, ground water and fumaroles. The monitoring methods include mostly track detectors, scintillation and semiconductor detection procedures. Track detectors are used for soil radon determination; techniques such as liquid scintillation are employed for radon in water determination and semiconductors and solid scintillation spectrometry have also being used for water, soil and fumarole studies in volcanoes. The necessity for continuous monitoring in order to obtain anomalous data for short term gases outbursts, requires active electronic devices or automatic passive detectors exchangers for short exposure times. In volcanoes like Kilauea, Usu, Sakurajima, observations are performed with track, semiconductor or scintillation alpha detectors, while in Iceland, Mexico, Costa Rica, Etna or Piton de la Fournaise, track detectors for soil radon determinations are commonly used. MECHANISMS
PROPOSED TO EXPLAIN RADON ANOMALIES
Even if anomalies have been observed as related to volcanic eruptions the number of correlations recognised are few. Radon fluctuations in soil and ground water associated with minor volcanic activity can often be obscured
R A D O N A N D VOLCANIC ACTIVITY
411
by anomalies of non volcanic origin (Shapiro et al., 1984/85; Segovla st al., 1987a). However, when large volumes of volatile rich magmas accumulate beneath a volcano, the changes of radon levels in soil or water may be much higher than those produced by any other cause even at distances large compared with the dimensions of the volcano. Radon anomalies observed in soil and ground water have been estimated to originate in magma degassing or proceeding from the crustal rocks under stress around the volcano. Among the mechanisms proposed to explain radon evolution in soil gases, the pore collapse proposed by King (1986) suggests that pressure generates an upflow of soil gas. This mechanism was proposed to explain radon anomalies associated to seisms, but it has been proved useful when high temperature and pressure conditions are reached, like in the magma uplift case. Other authors have suggested that an important volume of magma at high pressure and temperature produces important alterations in the underground gas fluence and that anomalous signals can reach distances from the volcanic cone as large as 10-15 km (De la Cruz-Reyna et al., 1985). Mechanisms concerning the microfracturing of rocks induced by stress have been proposed as associated to intrusive events enhancing the radon degassing from rocks near the surface (Thomas et al., 1986) as well as the radon flux in shallow ground water systems (Chirkov, 1976). The radon anomalies are expected to have a correlation with hydrothermal activity. Intrusive and eruptive episodes have been reported to generate radon in soil and ground water anomalies (Seidel et al., 1988; Notsu et al., 1983). From studies at the Karimsky volcano, Flerov (1986) proposed that ultrasonic vibrations generated in rocks due to magma upflow could enhance rock emanation. This last mechanism is sustended by laboratory experiments. Enhancement of soil radon by hidden faults has also been proposed (Kusakabe and Hirabayashi, 1988; Aubert and Baubron, 1988). Other authors indicate the possibility that a flux of magmatic rich radon gas penetrate directly into the shallow ground water system (Gasparini and Mantovanni, 1978). This mechanism requires of a rapid transit in order to significative radon quantities could survive during its transport towards the surface. In this case anomalous concentrations of other volatiles should be observed and higher concentrations of carbonates, sulfates, halogen acids or temperature increase should be expected. If those species are not found it is not probable that radon can be considered of magmatic origin. In Tacana, Mexico, changes in sulfates and carbonates in a spring have been observed prior to a phreatic eruption (De la Cruz-Reyna et al., 1989), while radon in soil did not showed any anomaly except that its level was twice as compared with the concentration measured one year later, but no sudden radon outburst was observed. In fumaroles radon variations have been attributed to changes in the transport time of radon enriched steam from its source in ground water till the discharche point at the surface. Hauksson (1981) suggested this mechanism from observation of simultaneous increase of the phreatic level, while Cioni et al. (1981) interpret their results concerning an inverse anomaly of radon in fumaroles as due to a mixing of gases between a shallow radon rich steam source and a deep one, radon depletted, but rich in magmatic gas. In some cases, long and short-lived radon and thoron decay products activities in the ejected tephra has provided a mean for estimating the amount of magma undergoing degassing. After comparison with the ejected volume at a given stage in the eruptive episode, the possibility of further eruptive activity can be estimated (De la Cruz-Reyna et al., 1985; L e Cloarec et al., 1986). FUTURE WORK Earthquakes are routinely reported with the Richter scale. For volcanism no analogous system is commonly used, even if several proposals have been published to estimate a magnitude scale for eruptions. Newhall and Self (1982) have proposed a scale for measuring the explosivity character of an
412
N. SEGOVIA
eruption based on statistics of volcanological data. They define the Volcanic Explosivity Index (VEI) taking into account several parameters such as magnitude, intensity, destructiveness, dispersive power, violence, energy, and associate to eruptions a VEI value with a scale from zero to 8. A study of observed radon anomalies as related with the explosivity of an eruption has been initiated (Segovia et al., 1987b). Defining the anomaly magnitude from the maximum and minimun radon values during the observation time, some results can lead to a oriterium for quantifying the radon anomalies as a function of explosivity. Very few data are available at present, but at least from Karimsky, Chichon, Colima and Tacana eruptive episodes, a preliminary estimation suggests that a VEI of 4 would be associated to a factor of ten in the radon signal. It is desirable that published radon data in active volcanoes will be available in the near future in order to correlate with simultaneous changes obtained from other geochemical components and with geophysical data such as continuous microseismicity, deformation and fracturation monitoring. This will lead to a better understanding of active volcanoes evolution. ACKNOWLEDGEMENTS The author acknowledges
financial support from CONACyT,
Mexico.
REFERENCES Aubert, M. and J.C. Baubron (1988). Identification of a Hidden Thermal Fissure in a Volcanic Terrain Using a Combination of Hydrothermal Convection Indicators and Soil-atmosphere Analysis. J. Volcanol. Geotherm.Res., 35, 217-225. Chirkov, A.M. (1976). Radon as a Possible Criterion for Predicting Eruptions as Observed at Karimsky Volcano. Bull. Volcanol., 39, 126-131. Cioni, R., E. Corazza, F. D'Amore (1981). Radon in Fumarolic Gases from Vulcano Island (Sicily, Italy). Geothermics, 13, 385-388. De la Cruz-Reyna, S., M. Mena, N. Segovia, J.F. Chalot, J.L. Seidel, M. Monnin (1985). Radon Emanometry in Soil Gases and Activity in Ashes from E1 Chichon Volcano. PAGEOPH., 123, 407-421. De la Cruz-Reyna, S., M.A. Armienta, V. Zamora, F. Juarez (1989). Chemical Changes in Spring Waters at Tacana Volcano, Chiapas, Mexico: A Possible Precursor of the May 1986 Seismic Crisis and Phreatic Explosion. J. Volcanol. Geotherm. Res., 38, 345-353. Flerov, G.N., A.M. Chirkov, S.P. Tretyakova, L.V. Dzholos, K.I. Merkina (1986). The Use of Radon as an Indicator of Volcanic Processes. Izvestiya, Earth Physics. 22, 213-216. Gasparini, P. and M.S.M. Mantovani (1978). Radon Anomalies and Volcanic Eruptions. J. Volcanol. Geotherm. Res., ~, 325-341. Hauksson, E. (1981). Episodic Rifting and Volcanism at Krafla in North Iceland: Radon(222) Emission from Fumaroles near Leirhnjukur. J. Geophys. Res., 86 (B12), 11806-11814. Iwasaki, I., T. Ozawa, M. Yoshida, M. Kamada (1975). Forecast of Volcanic Eruptions by Chemical Methods. Bull. Volcanol., 39 (1), 91-103. King, C.¥. (1986). Gas Geochemistry applied to Earthquake Prediction: an Overview. J. Geophys. Res., 91 (BI2), 12269-12281. Kusakabe, M. and J. Hirabayashi (1988). Distribution of Rn Concentrations in Soil Gases and Continuous Observations of Reductive Gas at Sakurajima Volcano. Rep. 6th Joint Obs. Sakurajima Volcano. 81-88. Le Cloarec, M.F., G. Lambert, J.C. Le Roulley, B. Ardouin (1986). Long Lived Radon Decay Products in Mount St. Helens Emissions: An Estimation of the Magma Reservoir Volume. J. Volcanol. Geotherm. Res., 28, 85-89. Newhall, C.G. and S. Self (1982). The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism. J. Geophys. Res., 87 (C2), 1231-1238. Notsu, K., T. Abiko, H. Wakita (1983). Radon Concentrations in Groundwater Related to the Volcanic Activities of Usu Volcano. Bull. Volcanol. Soc. Japan (Ser. 2), 28, 305-308.
RADON AND VOLCANIC ACTIVITY
413
Olguin, H.T., N. Segovia, J. Carrillo, J.L. Iturbe, E. Ordoflez, S. Bulbulian (1990). ~ R n C o n t e n t and 234U/~sU Activity Ratio in Groundwaters. J. Radlanal. Nucl. Chem., 141, 17-23. Schenker, F. and V.J. Dietrich (1986). The Lake Nyos Gas Catastrophe (Cameroon). A Magmatological Interpretation. Schweiz. Mineral. Petrogr. MICC., 66, 343-384. Seidel, J.L., A. Bonneville, J.F. Lenat (1988). Mesures de Radon en Relation avec l'Activite du Piton de la Fournaise (Reunion), de 1983 a 1987. C.R. Rcad. Sci. Paris, 306, Serie II, 89-92. Segovia, N., J.L.Seidel, M.Monnin (1987a). Variations of Radon in Soils Induced by External Factors. J. Radioanal. Nucl. Chem. Left., 119, 199-209. Segovia, N., S. De la Cruz-Reyna, M. Mena (1987b). Caracter Cuantitatlvo del Valor Predictivo de las Anomalias de Radon Observadas en Volcanes. Technical Report, AI-87-39, ININ, (Mexico). Shapiro, M.H., A. Rice, M.H. Mendenhall, J.D. Melvin, T.A. Tombrello 1984/85) Recognition of Environmentally Caused Variations in Radon Time Series. PRGEOPH., 122, 309-326. Shiratoi, K. (1927). The Variation of Radon Activity of Hot Spring. Sci. Rep. Tohoku Imp. Univ., Ser. 3, 16, 614-621. Thomas, D.M., K.E. Cuff, M.E. Cox (1986). The Association Between Ground Gas Radon Variations and Geologic Activity in Hawaii. J. Geophys. Res. 91 (BI2), 12186-12198.
Nucl. Tracks Radiat. Meas., Vol. 19, Nos 1-4, pp. 409--413, 1991 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
RADON AND VOLCANIC ACTIVITY:
RECENT ADVANCES
N. SEGOVIA ININ, Ap. Post 18-1027,
11801, Mexico D.F., Mexico.
ABSTRACT Radon is a naturally occurring radioactive gas present in all of the underground fluids that have been in contact with uranium bearing rocks. Radon fluctuations in soil and ground water associated with minor volcanic activity are often obscured by anomalies of non-volcanic origin. However, when large volumes of volatile rich magmas accumulate beneath a volcano, the changes of radon levels in soil or water may be much higher than those produced by other causes even at a large distance as compared with the horizontal dimensions of the volcano. When a large explosive eruption occurs, the measurement of radon and thoron daughters' activities in the ejected tephra provides the means for estimating the amount of magma undergoing degassing. After comparison with the ejected volume at a given stage of the eruptive episode, it also offers clues about the possibility of further eruptive activity. KEYWORDS
Environmental
radon;
volcanic
activity;
radon anomalies.
INTRODUCTION Volcanic activity is caused by the movement of magma towards the earth's surface, giving rise to different phenomena such as earthquakes, volcanic tremors, fumarolic and hydrothermal activity and anomalies in some geophysical and geochemical parameters and other geological manifestations (Iwasaki et al., 1975). Degassing, melting, crystallisation, fusion and dissolution of materials can occur with the associated alterations in rocks, soils and ground waters. The intensity of those phenomena varies as a function of time. A volcanic eruption occurs when magma, magmatic emanations and their reaction products break through the earth's crust emerging violently to the surface. The solid components of magma have low transport capability as compared to the gaseous products which can move easily even through small fractures. The particular characteristics of an eruption are a function of several parameters, among the most important ones we can mention magma viscosity, gas content, and the physical characteristics and geometry of the rock matrix. The two first are particularly important since, as pressure decreases during the transport of the magma to the surface, the gas present in the liquid is exsolved giving place to the generation of bubbles. If magma viscosity is low, bubbles evolve compensating pressure differences; however for high viscosities, bubbles move slowly, accumulating pressure that will be liberated by fragmentation of the magma, therefore, producing an explosive eruption. The explosive potential of a volcanic eruption is essentially determined by the pressure from the magmatic volatile products.
409
410
N. SEGOVIA
The action of the gases in volcanic manifestations represents one of the recognized hazards associated with volcanism. As an example we can mention, among the high risk phenomena, the "nuee ardente", a type of pyroclastic flow composed by gases and pyroclastic materials at high temperatures and velocities slipping down from a volcanic cone during an explosive eruption (as it occurred during the eruptions of Montagne Pelee in 1902 in Martinica and of E1 Chichon in 1982, in Mexico). An example of sudden degassing is g i v e n by the 1986 Lake Nyos catastrophe occurred in Cameroon, where rich CO2 gas of magmatic origin was liberated, producing the death by asphyxia of 1746 people and a large amount of animals (Shenker and Dietrich, 1986). Evolution of volcanic gases is thus, for several reasons, an important field of study. RADON MONITORING
IN ACTIVE VOLCANOES
222Rn is a natural radioactive gas present in all geofluids in contact with uranium bearing rocks. Since 1927, with the work reported by Shiratoi, radon is known to be associated with gases in fumaroles and thermal springs. Being chemically inert, radon incorporation to geofluids is dependent upon molecular diffusion. Fluids flow is then the dominant mechanism by which radon is transported; the limiting factor in the transport length being its half life (01guin et al., 1990). In this sense radon can be considered as a natural radioactive tracer giving information on recent geofluids fluctuations. The evidence that radon concentration undergoes significant changes with volcanic activity is growing rapidly, however the complexity of the physical and chemical processes participating in a volcanic eruption results in a wide spectrum of radon fluctuation patterns that has its own peculiarities for each active volcano. Several authors had reported radon studies in fumaroles, springs and soil and also, when a large explosive eruption occurs, the measurement of radon and thoron daughters activities in the ejected tephra has proved useful for estimating the amount of magma undergoing degassing. The first paper concerning the correlation between radon fluctuations and a volcanic eruption was published by Chirkov (1976) from several years observations of radon concentrations in spring water associated to the Karimsky volcano. Since that time anomalous radon changes have been observed in different volcanoes. Among the reported active volcanoes under radon monitoring we can mention: Karimsky (URSS), Usu and Sakurajima (Japan), Chichon (Mexico), Piton de la Fournaise (France), Etna and Vulcano (Italy), Mount St Helens and Kilauea (USA), Krafla (Iceland), Poas and Irazu (Costa Rica), etc.. Those volcanoes have experience eruptive periods of different explosivity in the last 15 years. MONITORING
METHODOLOGIES
Radon monitoring in active volcanoes have been reported essentially in soil, ground water and fumaroles. The monitoring methods include mostly track detectors, scintillation and semiconductor detection procedures. Track detectors are used for soil radon determination; techniques such as liquid scintillation are employed for radon in water determination and semiconductors and solid scintillation spectrometry have also being used for water, soil and fumarole studies in volcanoes. The necessity for continuous monitoring in order to obtain anomalous data for short term gases outbursts, requires active electronic devices or automatic passive detectors exchangers for short exposure times. In volcanoes like Kilauea, Usu, Sakurajima, observations are performed with track, semiconductor or scintillation alpha detectors, while in Iceland, Mexico, Costa Rica, Etna or Piton de la Fournaise, track detectors for soil radon determinations are commonly used. MECHANISMS
PROPOSED TO EXPLAIN RADON ANOMALIES
Even if anomalies have been observed as related to volcanic eruptions the number of correlations recognised are few. Radon fluctuations in soil and ground water associated with minor volcanic activity can often be obscured
R A D O N A N D VOLCANIC ACTIVITY
411
by anomalies of non volcanic origin (Shapiro et al., 1984/85; Segovla st al., 1987a). However, when large volumes of volatile rich magmas accumulate beneath a volcano, the changes of radon levels in soil or water may be much higher than those produced by any other cause even at distances large compared with the dimensions of the volcano. Radon anomalies observed in soil and ground water have been estimated to originate in magma degassing or proceeding from the crustal rocks under stress around the volcano. Among the mechanisms proposed to explain radon evolution in soil gases, the pore collapse proposed by King (1986) suggests that pressure generates an upflow of soil gas. This mechanism was proposed to explain radon anomalies associated to seisms, but it has been proved useful when high temperature and pressure conditions are reached, like in the magma uplift case. Other authors have suggested that an important volume of magma at high pressure and temperature produces important alterations in the underground gas fluence and that anomalous signals can reach distances from the volcanic cone as large as 10-15 km (De la Cruz-Reyna et al., 1985). Mechanisms concerning the microfracturing of rocks induced by stress have been proposed as associated to intrusive events enhancing the radon degassing from rocks near the surface (Thomas et al., 1986) as well as the radon flux in shallow ground water systems (Chirkov, 1976). The radon anomalies are expected to have a correlation with hydrothermal activity. Intrusive and eruptive episodes have been reported to generate radon in soil and ground water anomalies (Seidel et al., 1988; Notsu et al., 1983). From studies at the Karimsky volcano, Flerov (1986) proposed that ultrasonic vibrations generated in rocks due to magma upflow could enhance rock emanation. This last mechanism is sustended by laboratory experiments. Enhancement of soil radon by hidden faults has also been proposed (Kusakabe and Hirabayashi, 1988; Aubert and Baubron, 1988). Other authors indicate the possibility that a flux of magmatic rich radon gas penetrate directly into the shallow ground water system (Gasparini and Mantovanni, 1978). This mechanism requires of a rapid transit in order to significative radon quantities could survive during its transport towards the surface. In this case anomalous concentrations of other volatiles should be observed and higher concentrations of carbonates, sulfates, halogen acids or temperature increase should be expected. If those species are not found it is not probable that radon can be considered of magmatic origin. In Tacana, Mexico, changes in sulfates and carbonates in a spring have been observed prior to a phreatic eruption (De la Cruz-Reyna et al., 1989), while radon in soil did not showed any anomaly except that its level was twice as compared with the concentration measured one year later, but no sudden radon outburst was observed. In fumaroles radon variations have been attributed to changes in the transport time of radon enriched steam from its source in ground water till the discharche point at the surface. Hauksson (1981) suggested this mechanism from observation of simultaneous increase of the phreatic level, while Cioni et al. (1981) interpret their results concerning an inverse anomaly of radon in fumaroles as due to a mixing of gases between a shallow radon rich steam source and a deep one, radon depletted, but rich in magmatic gas. In some cases, long and short-lived radon and thoron decay products activities in the ejected tephra has provided a mean for estimating the amount of magma undergoing degassing. After comparison with the ejected volume at a given stage in the eruptive episode, the possibility of further eruptive activity can be estimated (De la Cruz-Reyna et al., 1985; L e Cloarec et al., 1986). FUTURE WORK Earthquakes are routinely reported with the Richter scale. For volcanism no analogous system is commonly used, even if several proposals have been published to estimate a magnitude scale for eruptions. Newhall and Self (1982) have proposed a scale for measuring the explosivity character of an
412
N. SEGOVIA
eruption based on statistics of volcanological data. They define the Volcanic Explosivity Index (VEI) taking into account several parameters such as magnitude, intensity, destructiveness, dispersive power, violence, energy, and associate to eruptions a VEI value with a scale from zero to 8. A study of observed radon anomalies as related with the explosivity of an eruption has been initiated (Segovia et al., 1987b). Defining the anomaly magnitude from the maximum and minimun radon values during the observation time, some results can lead to a oriterium for quantifying the radon anomalies as a function of explosivity. Very few data are available at present, but at least from Karimsky, Chichon, Colima and Tacana eruptive episodes, a preliminary estimation suggests that a VEI of 4 would be associated to a factor of ten in the radon signal. It is desirable that published radon data in active volcanoes will be available in the near future in order to correlate with simultaneous changes obtained from other geochemical components and with geophysical data such as continuous microseismicity, deformation and fracturation monitoring. This will lead to a better understanding of active volcanoes evolution. ACKNOWLEDGEMENTS The author acknowledges
financial support from CONACyT,
Mexico.
REFERENCES Aubert, M. and J.C. Baubron (1988). Identification of a Hidden Thermal Fissure in a Volcanic Terrain Using a Combination of Hydrothermal Convection Indicators and Soil-atmosphere Analysis. J. Volcanol. Geotherm.Res., 35, 217-225. Chirkov, A.M. (1976). Radon as a Possible Criterion for Predicting Eruptions as Observed at Karimsky Volcano. Bull. Volcanol., 39, 126-131. Cioni, R., E. Corazza, F. D'Amore (1981). Radon in Fumarolic Gases from Vulcano Island (Sicily, Italy). Geothermics, 13, 385-388. De la Cruz-Reyna, S., M. Mena, N. Segovia, J.F. Chalot, J.L. Seidel, M. Monnin (1985). Radon Emanometry in Soil Gases and Activity in Ashes from E1 Chichon Volcano. PAGEOPH., 123, 407-421. De la Cruz-Reyna, S., M.A. Armienta, V. Zamora, F. Juarez (1989). Chemical Changes in Spring Waters at Tacana Volcano, Chiapas, Mexico: A Possible Precursor of the May 1986 Seismic Crisis and Phreatic Explosion. J. Volcanol. Geotherm. Res., 38, 345-353. Flerov, G.N., A.M. Chirkov, S.P. Tretyakova, L.V. Dzholos, K.I. Merkina (1986). The Use of Radon as an Indicator of Volcanic Processes. Izvestiya, Earth Physics. 22, 213-216. Gasparini, P. and M.S.M. Mantovani (1978). Radon Anomalies and Volcanic Eruptions. J. Volcanol. Geotherm. Res., ~, 325-341. Hauksson, E. (1981). Episodic Rifting and Volcanism at Krafla in North Iceland: Radon(222) Emission from Fumaroles near Leirhnjukur. J. Geophys. Res., 86 (B12), 11806-11814. Iwasaki, I., T. Ozawa, M. Yoshida, M. Kamada (1975). Forecast of Volcanic Eruptions by Chemical Methods. Bull. Volcanol., 39 (1), 91-103. King, C.¥. (1986). Gas Geochemistry applied to Earthquake Prediction: an Overview. J. Geophys. Res., 91 (BI2), 12269-12281. Kusakabe, M. and J. Hirabayashi (1988). Distribution of Rn Concentrations in Soil Gases and Continuous Observations of Reductive Gas at Sakurajima Volcano. Rep. 6th Joint Obs. Sakurajima Volcano. 81-88. Le Cloarec, M.F., G. Lambert, J.C. Le Roulley, B. Ardouin (1986). Long Lived Radon Decay Products in Mount St. Helens Emissions: An Estimation of the Magma Reservoir Volume. J. Volcanol. Geotherm. Res., 28, 85-89. Newhall, C.G. and S. Self (1982). The Volcanic Explosivity Index (VEI): An Estimate of Explosive Magnitude for Historical Volcanism. J. Geophys. Res., 87 (C2), 1231-1238. Notsu, K., T. Abiko, H. Wakita (1983). Radon Concentrations in Groundwater Related to the Volcanic Activities of Usu Volcano. Bull. Volcanol. Soc. Japan (Ser. 2), 28, 305-308.
RADON AND VOLCANIC ACTIVITY
413
Olguin, H.T., N. Segovia, J. Carrillo, J.L. Iturbe, E. Ordoflez, S. Bulbulian (1990). ~ R n C o n t e n t and 234U/~sU Activity Ratio in Groundwaters. J. Radlanal. Nucl. Chem., 141, 17-23. Schenker, F. and V.J. Dietrich (1986). The Lake Nyos Gas Catastrophe (Cameroon). A Magmatological Interpretation. Schweiz. Mineral. Petrogr. MICC., 66, 343-384. Seidel, J.L., A. Bonneville, J.F. Lenat (1988). Mesures de Radon en Relation avec l'Activite du Piton de la Fournaise (Reunion), de 1983 a 1987. C.R. Rcad. Sci. Paris, 306, Serie II, 89-92. Segovia, N., J.L.Seidel, M.Monnin (1987a). Variations of Radon in Soils Induced by External Factors. J. Radioanal. Nucl. Chem. Left., 119, 199-209. Segovia, N., S. De la Cruz-Reyna, M. Mena (1987b). Caracter Cuantitatlvo del Valor Predictivo de las Anomalias de Radon Observadas en Volcanes. Technical Report, AI-87-39, ININ, (Mexico). Shapiro, M.H., A. Rice, M.H. Mendenhall, J.D. Melvin, T.A. Tombrello 1984/85) Recognition of Environmentally Caused Variations in Radon Time Series. PRGEOPH., 122, 309-326. Shiratoi, K. (1927). The Variation of Radon Activity of Hot Spring. Sci. Rep. Tohoku Imp. Univ., Ser. 3, 16, 614-621. Thomas, D.M., K.E. Cuff, M.E. Cox (1986). The Association Between Ground Gas Radon Variations and Geologic Activity in Hawaii. J. Geophys. Res. 91 (BI2), 12186-12198.