Landform development process of a volcanic fan at the eastern foot of Mt. Sakurajima

Abstract

This study aims to clarify the landform development process ongoing since 1946 at the eastern foot of Mt. Sakurajima. It is the most active volcano in Japan. Pyroclastic materials such as volcanic ash and rock, related to volcanic activities, are removed to downstream areas through debris flow or traction, forming a volcanic fan at the foot of the mountain. By analyzing aerial photographs, satellite images, and laser profiler (LP) data, we confirmed that a large number of sediments were deposited on the Showa lava. Based on a field survey, the characteristics of landforms and vegetation differences depending on them were confirmed. Because of partial erosion at the same time as sedimentation being predominant, the volcanic fan area can be classified into four categories of landforms in the descending order of relative elevation: residual hill, Terrace I, Terrace II, and transportation surface. This order is also the order of the emergence of landform surfaces. The volcanic fan area expansion and landform development process were interpreted using aerial orthophotographs. The landform distribution could be mapped based on the interpretation of the land cover classification using satellite images. The distribution of elevation change was mapped using the LP data for different measurement periods. Because the sedimentation effect was larger in the upstream area, the elevation of the volcanic fan gradually increased. Alternatively, because the sediment erosion effect was larger in the downstream area, earlier-emerging landforms were found. This development process was closely related to the amount of volcanic ash associated with volcanic activities. These landform features were very similar to ordinary dissected fans that take a long time to develop.