by Setsuya Nakada, Kozo Uto, John C. Eichelberger

Volcanic eruptions of felsic magma cause large disasters. More than 50 % of the recent disasters are due to events related to mass flows, such as pyroclastic/debris flows, debris avalanches and tsunamis, the latter resulting from entry of mass flows into the sea or lakes. In more explosive events, volcanic ash and gas cause other serious problems, such as global-scale environmental change and failure of jet engines of airplanes flying over the volcanoes. Although all these events are destructive, they are destructive in different ways. Thus it is vitally important to understand not just when a volcano will erupt, but how it will erupt. Felsic volcanoes are abundant in island arcs and continental margins, where cities with large populations are concentrated. Unzen Volcano is such a volcano, where a huge volcanic disaster occurred about 200 years ago and the latest eruption occurred in 1990-95. During the latter period, about ten thousand pyroclastic flows were produced due to partial collapses of a growing lava dome, threatening the lives of people around the volcano. Eruptions of felsic magmas are not always explosive. Three major recent eruptions, Pinatubo in 1991, Mount St. Helens in 1980, and Unzen in 1990-95, were caused by felsic magmas with chemical compositions similar to each other, including similar concentrations of the volatile components that account for explosive activity. However, styles of eruption among the three are very different. Nearly all of Pinatubo’s magma was ejected explosively as ash falls and pyroclastic flows. Mount St. Helens’ eruptions began explosively, but produced more and more dome lavas as the episode progressed. Unzen was least explosive, with almost no ash eruptions and with nearly all pyroclastic flows being due to disintegration upon collapse of already extruded lava, not to fragmentation within the conduit. Evidently, a degassing process where the volatile component escapes effectively from magma during its ascent is the major factor controlling whether volcanism is explosive or non-explosive, and hence the kind of damage that an eruption will produce. The upper part of the magmatic conduit is believed to be the place where most degassing occurs, based upon what is known about the pressure-dependent solubility of volatiles in magma and on the source location of geophysical signals during eruption. Such signals as volcanic tremor may be direct evidence of the degassing process. However, there is much to learn about how so much gas can escape from magma so rapidly. In-situ inspection of the conduit and its wall rock is the most effective approach to understand the mechanism. Scientific drilling at Inyo Domes, at Long Valley Caldera, is the only case to date where a young volcanic conduit was penetrated and sampled, but the 600-year-age of eruption was too old for there to be complementary geophysical monitoring data and the conduit had cooled completely. Despite these limitations, an important model of degassing was proposed, based on the results. The results have been extended by examination of geologic exposures of roots of old lava domes such as Mule Creek Dome in New Mexico, which revealed structures similar to those found in drilling, and by development of numerical models for degassing. A necessary next step in developing our understanding of the causes of explosive versus non-explosive eruption is to drill into a conduit whose eruptive activity was actually observed. Drilling into the conduit of the latest lava dome at Unzen is a most desirable goal, because the lava that solidified in the conduit is still hot and the location of conduit is well defined by geophysical signals recorded intensively and precisely during eruption. With this challenging scientific project, the combination of direct information on degassing obtained by drilling with geophysical signals obtained during the eruption can solve the problem of eruption mechanisms of felsic magma. In addition, the meanings of geophysical signals can be more correctly interpreted for predicting eruptions and the development of hydrothermal systems after eruption can be elucidated. Two pre-proposals of Unzen scientific drilling were evaluated by the International Continental Scientific Drilling Program (ICDP). The importance of scientific drilling and the technical problems of drilling itself were discussed in international workshops in 1997 and 2000. Following the first meeting, the Science and Technology Agency of Japan initiated funding of the Unzen Scientific Drilling Project (USDP), as a one-year feasibility study. During the next three years, drilling of two flank holes and one pilot hole for the conduit drilling were completed or are in progress. Questions of the tectonic and magmatic development history of Unzen are being solved through synthesis of geologic data from the surface and the flank holes, and design of the conduit drilling and selection of the drill site has been undertaken. Slant drilling from the northern upper slope of the volcano was found to be an option able to penetrate the conduit above sea level. The permitting process is easier in this site and the cost of drilling is less expensive than a site proposed initially that is within a special area of nature preservation, close to the dome. It is proposed that conduit drilling will start in 2002 jointly with STA and ICDP, and continue until 2005. Japanese principal investigators are asking STA to defray the major portion of the conduit drilling cost, estimated to be 8.8 MD in total. Fund for a portion of drilling operation (total 3 MD) is requested from ICDP. Foreign scientists, including Co-investigators, will request their own research resources from their respective national science foundations. Five principal investigators have the responsibility of science handling of the conduit drilling, including samples description-partition and logging experiments. Over 60 scientists comprise an international team of the conduit drilling project at Unzen.

• To understand magmatic processes in the upper conduit, we describe the scientific objectives as follows:
• 1. Penetration of the conduit at multiple elevations from zero to ~700m above sea level (corresponding to depth beneath the summit of 1500 m to 800 m). At least, one penetration should be at as shallow a level as drilling technology permits.
• 2. Collecting rock samples down to, in and around the conduit 3. Collecting fluid samples in and around the conduit 4. Logging as dense as possible down to and through the conduit (images and physical parameters).
• 5. Core diameter larger than 60mm in the conduit. Fragmentation experiments to be conducted at high pressures and temperatures require this value.
• 6. Installation of equipment for geophysical and geochemical monitoring in upper levels of the well, and electrodes for resistivity and thermal probes for temperature at deeper levels.
• 7. Measuring gas permeability across the region of the conduit. (Pack off high-temperature portion and inject gas.)

Active Volcano, Conduit, Directional Drilling, Explosion, Hazards, ICDP-2001/01, Japan, Kyushu, Shimabara, Thermal Regimes, UNZEN, USDP, Volcanic Systems

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