Geochemical Journal, Vol. 28, pp. 137 to 138, 1994

Special Issue Geochemistry of Crater Lakes Preface

MINORU KUSAKABE

Institute for Study.of the Earth's Interior, Okayama University, Misasa, Tottori-ken 682-01, Japan

Crater lakes usually sit on top of disaster, the Misasa meeting covered a wider conduits, and act as condensers of magmatic vapor. spectrum of subjects related the geochemistry of The chemistry of active crater lakes represents the volcanic lakes. This special issue consists of 10 results of integrated volcano-fluid interactions papers dealing with the water chemistry taking place within the shallow regions of the including proposals on ways to prevent future gas volcano. Studies of crater lakes can therefore disasters, results of the search for gas accumulation provide information on both deep magmatic ac in other crater lakes, geochemical monitoring of tivity and variations in the degassing state of a active, hot crater lakes, fluid-volcano interaction shallow magmatic body. The Lake Nyos gas di in terms of chemical and isotopic variations, and saster of August 1986 and a similar event two an evaluation of past volcanic activity recorded in years earlier (August 1984) at Lake Monoun, both sediment cores from volcanic lakes. in , resulted from the accumulation of Evans et al. evaluate the evolution of the wa magmatic CO2 in the bottom layers of these two ter-column structure of Lake Nyos after the 1986 lakes. Geochemical monitoring of crater lakes is gas disaster and estimate the pre-release structure thus a promising tool not only for forecasting of the lake. On the basis of the pre-release struc volcanic events but also for the mitigation of the ture, they present a scenario of the catastrophic Nyos-type gas disasters. Acid-mineralized waters gas release which explains the observations and formed by condensation of hot magmatic volatiles testimonies at the time of the gas release. At Lake in crater lakes are thought to bear some resem Nyos, a large amount of CO2 still remains blance to hydrothermal fluids acting in the genesis undegassed and it is increasing at an alarming rate of acid-sulfate alteration and mineralization of due to continuous input of CO2 from the lake volcanic-hosted metal deposits. bottom. Therefore, the necessity of urgent degas This special issue of the Geochemical Journal sing was adopted as the resolution of the first is a collection of some of the papers which were IWGCL meeting. Along this line, a safe way to presented at an international symposium on haz degas Lake Nyos is proposed by Freeth. Stimulated ardous crater lakes held in Misasa, Japan, from by gas accumulation at lakes Nyos and Monoun, 19-24 October 1992 as part of the second meeting geochemical surveys were carried . out for some of the International Working Group on Crater crater lakes in Italy and Azores by Martini et al. Lakes (IWGCL). The IWGCL, formed soon after Their results show that Lake Albano in Italy has the 1986 Lake Nyos disaster, aims at promoting some geochemical similarities to the Cameroonian (1) communication between researchers with gassy lakes, although its CO2 concentration is common interests, (2) organization of international much lower than that of Lake Nyos. Christenson programs; meetings and field workshops, (3) concludes that Nyos-type gas release is unlikely standardization for reporting data, and (4) joint to occur at Crater Lake of Mt. Ruapehu, New development of instruments for crater lake survey. Zealand, after evaluating convection and stratifi While the first IWGCL meeting held in Nancy cation of the lake. (France) in 1990 focused mainly on the Lake Nyos Molten pool is often found at the bottom

137 138 M. Kusakabe

of active crater lakes, and the floating sulfur slicks lake system, Rowe presents hydrogen, oxygen and are common in such environments. According to sulfur isotopic ratios of various materials available Takano et al., the chemistry, morphology and gas from the system to constrain the hydrologic regime content of sulfur slicks reflect the activity of sub and the sources of sulfur which is abundant at aqueous fumaroles, and can be used for volcano Pods. Volcanic lakes are often fed by hot springs monitoring. Using a box model, Ohba et al. at the bottom. At Lake Mashu, a caldera lake in evaluate heat, water and chloride budgets of Hokkaido, Japan, the occurrence of hot springs at Yugama crater lake of Mt. Kusatsu-Shirane, Japan. the bottom has been identified. Tanaka et al. show They conclude that the increased chloride flux into that the variation in and manganese concen the lake coupled with an increased level of seis trations of sediments cored at several points of micity resulted from fracturing of the surrounding the lake bottom represents the changes in activity region of a shallow, cooling magma. The crater of the bottom hot springs. The changes were dated lake at Mt. Kelut in Indonesia is notorious for its by the 210Pb method and also by tephrochronology. lahars generally associated with eruptions of the From this wide range of subjects, it is expected volcano. Badrudin reports the recent results of that this special issue provides baseline informa geochemical monitoring of the Kelut crater lake tion on the processes occurring beneath volcanoes including significant increases of sulfate, chloride, and associated lakes. Such basic information would boron, magnesium concentrations and of Mg/Cl certainly contribute to better understanding of the ratio in the lake water just prior to the 1990 fluid-volcano interaction and mitigation of natural eruption, as well as past records of the lahar di disasters related to hazardous crater lakes such as sasters at Kelut. Pasternack and Varekamp studied Nyos and Kelut. three crater lakes sitting on top of the dormant I thank the following journal referees, whose volcano Keli Mutu in Flores (Indonesia) in terms constructive comments improved the quality of the of rock-water interaction and mass balance. They papers published here. They include A. Arribas, stress that the lakes are not closed reaction vessels, B. W. Christenson, J. Cornwell, W. C. Evans, S. but represent part of a dynamic system through J. Freeth, M. Halbwachs, S. Kanari, H.-J. Kumpel, which a large amount of chemical elements are T. Masuzawa, E. Matsumoto, Y. Nojiri, T. Ohba, transferred by acid brines, implying modern ana G. L. Rowe, R. Rye, H. Shinohara, B. Takano, K. logs of mineralization of volcano-hosted metal Tietze and J. C. Varekamp. I also thank Y. deposits. Such a recognition is also true of the Matsuhisa, Executive Editor of the Geochemical very active crater lake system at Pods, Costa Rica. Journal, and G. Tanyileke for their support in ed After a series of publications on the Pods crater iting the manuscripts in this issue.