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Petrology and Volcanology

  • Atsushi Toramaru, Professor
  • Takeshi Ikeda, Associate Professor
  • Tomoharu Miyamoto, Assistant Professor
  1. Study of dynamics of volcanic eruptions, formation mechanism of rock pattern, vesiculation, crystallization and flow of magmas.
  2. Study of physico-chemical processes in the deep crust and upper mantle using metamorphic rocks.
  3. Study of the origin of rocks on the basis of the geochemical theory.

1. Study on mechanism of volcanic eruptions

Figure 1

Volcanic eruptions includes all processes resulting from melt generation, migration and accumulation at the mantle, crystallization and convection in magma reservoirs and magma ascent in the crust.

  1. We carry out the chemical and textural analyses on samples of lavas and pyroclasts such as pumice and ash, and estimate ascent velocities in the conduit or cooling rates of magmas.
  2. We study the fundamental processes of crystallization and vesiculation of magmas, which are still poorly understood, to establish the rigid framework by laboratory experiments and numerical simulations.
  3. Magmas show the complex behavior as a multi-phase flow system including bubbles and crystals in addition to silicate melts, which in turn, links to the variety in eruption styles in volcanic eruptions. We study such complex systems by analog experiments using dairy used materials and numerical simulations.
  4. Integrating results from above approaches, we try to understand what happen in the depth under volcanoes. Our final goals include revealing factors controlling the temporal shift of eruption styles and diversity volcanism.

2. Study on rock pattern

Figure 2

There are various types of rock patterns, e.g., Liesegang bands formed by the diffusion-reaction process in solidified magma bodies, columnar joints by fracturing and layering by stream line mixing such as marbling or suminagashi (Chinese ink marbling) as well as rock textures consisting of poly-minerals and glass. We study the formation processes of rock patterns by the qualitative and quantitative descriptions of natural samples, numerical and laboratory experiments.

3. Evolution of microstructure in metamorphic rocks

Figure 3
Reaction microstructure between green (upper left) and colorless (lower right) minerals to form aggregates of three minerals.

In the crust and mantle of the Earth, materials are composed of aggregates of mineral grains that undergo deformation and chemical reaction. The features of metamorphic rocks can be accounted for in terms of metamorphic reaction and plastic deformation in the rocks.

We describe mineral assemblage and chemical composition of constituent minerals, which is employed to determine physical condition such as pressure and temperature under which the system was equilibrated. Microstructural features such as grain-size distribution of mineral, spatial distribution and arrangement of minerals would provide information on strain and strain rate of deformation, mechanism of grain growth, duration time of high-temperatures (metamorphism) and temporal change of the physical condition. Quantitative characterization of the microstructures leads us to the understanding of these features.

4. Origin of rocks on the basis of the geochemical theory

Figure 4
Plastic flow of crustal materials at depth.

Common rock is composed from 10 major elements and other trace elements. The compositions are characterized by natural rule, which is restricted by origin of the rock and physical and chemical conditions of rock forming process; compositions of igneous rocks are discriminated by varieties of original mantle and process of differentiation during igneous activities from the mantle, and mass transfer and isotope homogenization during metamorphism are recognized from compositional research for metamorphic rocks, and these affect assemblages and quantities of constituent minerals in it. Although such phenomena and process could not be observed directly since they are caused in the deep basement under the ground, they are able to be considered from elemental behaviors during compositional evolutions on igneous and metamorphic rocks. Petrographic evidences are also important for considering the rock origin, since decoding of petrographic textures gives us qualitative information, which is often removed by geochemical analyses. Additionally, chronological research by radiometric dating give us information for understanding geological evolutions, and earth's history as their accumulations. Effective considering about the petrographic textures, the compositional characters, and the chronological evidences, without any contradiction, finally concludes petrologic research for revealing rock forming process. And then, we clarify the history of all geological units on the earth after compiling such evidences.

Figure 5
Plastic flow of crustal materials at depth.