By observing the massive energy of volcanic eruptions and seismic activity, we can realize that the Earth is alive. Specifically, magmas and geothermal fluids cause characteristic dynamism and diversity in the solid Earth. Living in Japan, one of the most well-studied island arc systems in the world, we are motivated to clarify the dynamics of volcanic eruptions and the activity of supercritical fluids in deep-seated rocks. Our location enables us to conduct detailed geology- and stratigraphy-based petrology. However, there is an increasing need for trained researchers that are familiar with volcanoes and erupted materials.
 Volcanic eruptions have a wide range of temporal and spatial scales and diverse modes of activity, from massive caldera eruptions that can destroy civilization to small-scale (but often high-risk) eruptions, such as phreatic eruptions. Moreover, eruption styles can change even within a single event. This diversity results from various factors. The tectonic setting primarily controls the initial and boundary conditions, such as the magma composition and production rate, stress fields, and thermal/lithological structure of the crust and mantle. Meanwhile, eruption explosivity is directly controlled by bubble growth, magma degassing, crystallization, fragmentation during magma storage, and ascent (Fig. 2).
 The primary purpose of our research is to identify the mechanisms that cause the diversity of volcanic eruptions and propose a method for predicting eruption styles (Fig. 1).
Volcanology is a typical multidisciplinary science. To accomplish this, we adopted the following approaches: 1) erupted material analyses based on detailed geology and stratigraphy, 2) laboratory experiments to simulate volcanic processes at high temperatures (and pressures), and 3) conduit flow numerical models. Because the intervals of large-scale eruptions are longer than the history of scientific eruption monitoring, the study of past eruptions is essential for volcanology. In front of outcrops, only well-trained volcanologists can imagine the events of the past. Imagination is also helpful for identifying surface processes immediately and correctly at the time of eruption.
 Eruptions occur in characteristically short timescales compared with other geological phenomena. This enables the use of laboratory simulations of various critical processes without significant timescale extrapolations, such as bubble formation, foaming, outgassing, fragmentation, crystal nucleation, growth, and dissolution (Movie 1).

 

 The behaviors of volatile components, such as water and carbon dioxide, in the Earth are important. Recent magnetotelluric observations have revealed that the supercritical CHO-fluid phase is widely distributed in the crust and upper mantle, challenging the conventional view of fluid undersaturation. Furthermore, fluids are thought to significantly influence various phenomena, including the generation of earthquakes.
 Moreover, supercritical fluid, which is supplied from a subducting slab to a mantle wedge, plays a crucial role in intra-slab earthquakes, magma genesis, and metasomatism. The amount of fluid carried deep into the Earth is another important issue (Fig. 2). However, the mantle under the stable continent is one of the most poorly understood geochemical reservoirs. Therefore, elucidating the distribution, abundance, and chemical composition of geological fluids is another research goal. As fluid continuously reacts with surrounding rocks during its travel to the surface, it is difficult to obtain quenched samples from high temperatures and pressures. As a result, the principal properties of supercritical geological fluids are not yet fully understood. Thus, multiple approaches, such as research on fluid inclusions in mantle xenoliths, in-situ spectroscopy, and microstructural experiments of fluid-rock systems under high pressures and temperatures were employed.
 By scientifically elucidating the mechanism of volcanic eruptions and the behavior of geofluids, we aim to make an essential societal contribution, such as volcanic disaster prevention.



Fig. 1 We study magmatic processes that produce diversity of eruption types based on petrologic analyses of pyroclasts and laboratory experiments.


Movie 1 In-situ FE-SEM observation of the growth behaviors of Fe particles at magmatic temperatures


Fig. 2 Our model for migration of slab-derived aqueous fluids in subduction zones