Geological and geomorphological characteristics of a crater lake breakout lahar at Akita-Yakeyama Volcano, NE Japan.

Lahars are rapidly flowing debris–water mixtures that pose significant hazards to populations proximal to active volcanoes. It is important to understand the mechanisms that initiate these dangerous volcanmic phenomena. In this study, the author defined the water source of lahar. To mitigate lahars effectively, a comprehensive understanding of their scale, distribution, flow behavior, and causes is essential, highlighting the significance of detailed studies on past lahars for volcanic hazard mitigation. Although there are many studies that have evaluated the water sources of historic (observed) lahars, there are few studies that have investigated water sources based solely on geological records. This study was conducted in order to investigate the products of lava and water interaction that provide a record of the paleo-water level of a crater lake on the summit area of an active volcano. The field area for this study is Akita-Yakeyama Volcano, which is an active stratovolcano located in the north-eastern part of Honshu Island, Japan. We conducted geomorphological analysis, field surveys, sedimentary facies analysis, and XRD analysis of lahar deposits, hyaloclastite, peperite, and lava to understand the flow characteristics of a lahar, its water source, and the volcanic activity that triggered its onset. Based on a comparison between the inferred past water level and the current water level of the summit crater lake, the minimum discharge of the lahar was estimated to be 1.6 x 10 − 3 km 3 and traveled at least 4 km. The resultant laharic deposits consist of altered material and show clear features indicating temporal flow transformation. This case study has provided an example of how geological records can be used to define the water sources, flow characteristics of pre-historic crater lake breakout lahars, and relation between flow behavior and components.