地学雑誌 Journal of Geography（Chigaku Zasshi） 127（2）103⊖105 2018 doi:10.5026/jgeography.127.103 Overview of the Special Issue “Progress of Studies on Caldera-forming Eruptions and Future Problems” ＊ ＊＊ Nobuo GESHI and Mitsuhiro NAKAGAWA Large-scale pyroclastic eruptions（ LSPE） can on the Earth every 10,000 years. eject 10s⊖100s km3 of silicic magma onto the Recently, the possibility of LSPE and related Earth’s surface as pumice falls and ignimbrites. potential disasters occurring have been high- The evacuation of voluminous magma from a lighted with a consensus about the risk of low- shallow magma chamber may result in the frequency natural disasters（ e.g., Tatsumi and collapse of a caldera. The frequency of LSPE is Suzuki-Kamata, 2014）. There have been many “low”, with less than 10 eruptions of VEI 7 or volcanological studies, taking various approach- more recorded during the last 10,000 years on es, on the mechanisms of LSPE and caldera the Earth. However, LSPE can cause fatal di- collapses. Given these trends, the Commission sasters both near the eruption site and at a dis- on Collapse Calderas（ CCC） was established in tance from it, while also seriously impacting the 2008 as a scientific commission of the Interna- global climate. Pyroclastic fall-outs, ignimbrite tional Association of Volcanology and Chemis- flows, and tsunamis during the 1815 eruption try of the Earth’s Interior（ IAVCEI）, to encour- of Tambora （Self et al., 1984） and the 1883 age and promote a wide spectrum of research eruption of Krakatau（ Self and Rampino, 1981） on collapsed calderas and LSPEs, from the caused destructive damage in areas surround- perspectives of geology, geophysics, numerical ing the volcanos, although magmas ejected dur- and analogue modelling, hazard assessment, ing these eruptions totaled less than 100 km3. and risk management, as well as economics Large volumes of volcanic ash in the atmosphe- and the environment. The first workshop was re also resulted in a climatic aberration, known held at the Las Cañadas Caldera Complex on as the Year Without a Summer, in Europe Tenerife Island, Spain, to discuss issues to be （Robock, 2000）. Although these eruptions are clarified relating to the origins and behaviors the largest known in history, many geological of calderas. Subsequently, the commission has records indicate that some pre-historic LSPE held international workshops every two years. discharged magmas in volumes of an order one All of the workshops were held in caldera- or two times larger than these Indonesian ex- volcanic fields to permit on-site field discus- amples. Many collapsed calderas surrounded sions as well as indoor lectures. The 6th work- by massive ignimbrite deposits are distributed shop was held in September 2016 at the Kita- in subduction zones and continental hot spots. yuzawa hot-spring site, Hokkaido, located in A detailed analysis of deposits surrounding the Shikotsu-Toya-Kuttara caldera field, which these collapsed caldera reveals that on average is one of the most active caldera volcanic fields more than one mega-eruption VEI > 8 occurs in Japan. Forty-seven researchers from ten ＊ Research Institute of Earthquake and Volcano Geology, National Institute of Advanced Science and Technology, Tsukuba, 305-8567, Japan ＊＊ Graduate School of Science, Hokkaido University, Sapporo, 060-0810, Japan ― 103 ― countries attended the workshop and discussed the physical properties and structures of the the mechanisms of caldera-forming eruptions rock hosting an active magma body is one of and associated topics. Prior to the workshop, a the challenging targets for understanding the training course on caldera volcanism was also activities of caldera volcanoes. To understand held at the Otaki Seminar House of Hokkaido magma chamber collapse and caldera collapse University, at the Kitayuzawa hot-spring site. formation, it is crucial to investigate deforma- The course particularly targeted early-career tion and faulting structures inside collapse researchers. The course included some high- calderas. Goto and Danhara （2018） provide level lectures by leading researchers and field- interesting data on the subsurface electrical re- indoor training to reconstruct the eruption se- sistivity of Toya caldera. quence from geological records. It is also important to construct a general This special issue presents topics and asso- view of the activities of caldera volcanoes based ciated research results discussed at the work- on individual examples, in order to understand shop. The workshop highlighted the importance caldera volcanism（ Geshi, 2018）. In particular, of geological approaches to caldera volcanism. the relationships among the driving mechanism In particular, it is still critical to collect indi- of LSPE, caldera collapses, and massive ignim- vidual examples, in order to obtain a general brite eruptions can be understood based on a understanding of LSPE and caldera-forming physical model with many examples accumu- eruptions, because modern volcanology has lated of eruption sequences and development almost no real-time observations of LSPE with of magmatic systems. Geological investiga- caldera-formation, except for the case of the tions into the eruption sequences of individual 1991 Pinatubo eruption, which formed a very caldera-forming eruptions are also important. small caldera. In Japan, there have been more Goto et al. （2018）, Miyasaka and Nakagawa than 10 examples of caldera-forming LSPE （2018）, and Nakagawa et al. （2018） provide within the last 100,000 years in Hokkaido and detailed reconstructions of eruption sequences Kyushu. Research to reconstruct detailed erup- of these important caldera-related eruptions in tion sequences, to precisely date eruptions, and Hokkaido. The results clarify that these calde- to perform petrological investigations of their ra-forming eruptions can be divided into sev- magma systems has been encouraged. Based eral stages with clear time gaps. This provides on these case studies, a generalized model of important constraints on the dynamics of the the sequence of caldera-forming eruptions and magma plumbing system of a caldera volcano. their controlling mechanisms is expected to be Simultaneous eruptions from discrete magma constructed. systems are proposed for several caldera- The process whereby large volumes of magma related eruptions based on detailed petrologi- accumulate in the crust is also fundamental cal investigations（ e.g., Shadai eruption of the to LSPE studies. Mechanical and tectonic con- Shikotsu caldera, Nakagawa et al., 2018）. The trols on the accumulation of magma at shallow resurgence of a magmatic system after a cal- levels in the Earth’s crust are among the main dera collapse is also an important target. Goto topics of recent caldera research. In particular, and Wada（ 2018） re-construct the post-caldera ground deformation in and around the magma eruption sequence of the Kucharo caldera based chamber provides direct signals associated with on a detailed tephra chronology. the accumulation of magma beneath the poten- Hasegawa et al. （2018a） also attempt to tial eruption sites of LSPE. To reconstruct the evaluate the time scale of an eruption using underground activities of a magma body, we the paleo-magnetic method. Understanding the should know the physical properties of the host control mechanism for stopping and re-opening rock surrounding the magma body（ Yamasaki, large-scale eruptions is one of the main targets 2018）, as well as the magma itself. Identifying of volcanology. These studies are based on field ― 104 ― observations of erupted materials. To improve Tephrochronology of Nakajima pumice. Journal methods of analyzing field deposits, detailed of Geography（ Chigaku Zasshi）, 127, 157⊖173.（ in Japanese with English abstract） observations of outcrops are fundamental. Goto, Y., Suzuki, K., Shinya, T., Yamauchi, A., Mi yo- Hasegawa et al.（ 2018b） describe several key shi, M., Danhara, T. and Tomoya, A.（ 2018）: Stra- outcrops of the calderas investigated at the tigraphy and lithofacies of the Toya Ignimbrite in workshop. southwestern Hokkaido, Japan: Insights into the The long-term development of the magma caldera-forming eruption at Toya caldera. Journal of Geography（ Chigaku Zasshi）, 127, 191⊖227. plumbing systems of caldera-forming LSPE is Hasegawa, T., Mochizuki, N. and Oiwane, H.（ 2018a）: also an important topic for caldera volcanology, Methods of estimating the durations of super large because LSPE need an accumulation of magma eruptions based on pyroclastic deposits. Journal within the plumbing system before the onset of of Geography（ Chigaku Zasshi）, 127, 273⊖288.（ in Japanese with English abstract） an eruption. The time scale of the maturation Hasegawa, T., Matsumoto, A., Tomiya, A. and Naka- of a magma plumbing system and expected pre- gawa, M.（ 2018b）: Large-scale caldera-forming eru- cursory signals for LSPE are also interesting ption and active post-caldera volcano: General geol- targets not only for volcanology but also for risk ogy and representative outcrops of Toya caldera and management associated with volcanic disas- Usu volcano, southwestern Hokkaido, Japan. Jour- nal of Geography（ Chigaku Zasshi）, 127, 289⊖301. ters caused by LSPE. The potential impacts of （in Japanese with English abstract） caldera-forming LSPE on local and global en- Amma-Miyasaka, M. and Nakagawa, M.（ 2018）: Erup- vironments are also important targets of re- tion sequence of 60 ka Shadai eruption, Shikotsu search using geological approaches. volcano: Reexamination from trench and boring