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[SVC36-13] Deformation of a rhyolite lava below the Curie temperature: the Sanukayam lava in Kouzusima Island, Japan
Keywords:Rhyolite lava, Obsidian, Lava deformation, Kozushima, Paleomagnetism
High-viscosity rhyolite lava may have a vertical lithofacies variation with more than one hundred meter thickness. Because the interior is kept warm by its thickness and the breccia on the surface during flowing, the internal fluid lava penetrates the tip (breakout) after the flow stopped Breakout of such a time difference may be developed into a disaster. Therefore, it is necessary to understand the rheological characteristics of the rhyolite lava; however, rhyolite lava eruption has few opportunities to be observed directly. In this study, we aimed to clarify the made of lava advancing during cooling by conducting a magnetic study on Kozushima sandbank lava which is thought to have been erupted several thousands of years ago.
Paleomagnetic samples were taken from 4 pumice sites (29 samples) and an obsidian site (10 samples) in the upper glassy part. In addition, crystalline rhyolite (7 samples) and obsidian overlying the clastic layer (4 samples) were sampled. We call obsidian and pumice sites as glassy part. Analysis of thermoremanent magnetization (TRM) of the inner crystalline part and obsidian overlying the clastic layer shows a single TRM component in the orthogonal diagrams. The TRM results of the 3 pumice sites and an obsidian site shows up to two sharp demarcations in remanence directions in the orthogonal diagrams. The remaining pumice site did not provide interpretable remanence magnetization component during thermal demagnetization. The glassy part is, therefore, considered to have been deformed at least two times below 580 C due to the advance of the inner part. The sampling sites located at greater altitudes show lower temperatures at which components are demarcated. Since this is interpreted as the temperature difference inside the lava, each deformation occurred simultaneously .Mode of each deflection seems to be similar among 3 pumice sites; however, this is not the case for the obsidian site. The deformation of the thick glassy part of the rhyolite lava is not completely uniformly distributed. The obsidian site overlying the clastic layer gave a single TRM component, likely due to thermal annealing associated with abundant spherulite formation in the clastic layer.
Paleomagnetic samples were taken from 4 pumice sites (29 samples) and an obsidian site (10 samples) in the upper glassy part. In addition, crystalline rhyolite (7 samples) and obsidian overlying the clastic layer (4 samples) were sampled. We call obsidian and pumice sites as glassy part. Analysis of thermoremanent magnetization (TRM) of the inner crystalline part and obsidian overlying the clastic layer shows a single TRM component in the orthogonal diagrams. The TRM results of the 3 pumice sites and an obsidian site shows up to two sharp demarcations in remanence directions in the orthogonal diagrams. The remaining pumice site did not provide interpretable remanence magnetization component during thermal demagnetization. The glassy part is, therefore, considered to have been deformed at least two times below 580 C due to the advance of the inner part. The sampling sites located at greater altitudes show lower temperatures at which components are demarcated. Since this is interpreted as the temperature difference inside the lava, each deformation occurred simultaneously .Mode of each deflection seems to be similar among 3 pumice sites; however, this is not the case for the obsidian site. The deformation of the thick glassy part of the rhyolite lava is not completely uniformly distributed. The obsidian site overlying the clastic layer gave a single TRM component, likely due to thermal annealing associated with abundant spherulite formation in the clastic layer.