At Aso volcano, phreatic and phreatomagmatic eruptions have repeatedly caused volcanic disasters with fatalities near the crater. While it is important to conduct research on eruption forecasting for reducing the risk to tourists and climbers of this volcano, it is also important to improve disaster prevention literacy regarding the volcanic eruptions. To improve disaster prevention literacy, it is important to elucidate the mechanisms of volcanic disasters based on case studies. In this study, the author chronologically summarizes the incident of the phreatic eruption that occurred in 2021 at Aso volcano. Although the Japan Meteorological Agency issued volcanic activity information sequentially as the volcanic activity increased, this information did not lead to prompt and effective closure of mountain trails. There was also a problem with the collection of volcanic information by climbers. It became clear that information on volcanic activity was not used effectively by either those regulating the mountain trails or those using them. Efforts should be made to resolve these issues in the future.

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