Abstract
The asymmetrical distribution of the welded Ata large-scale pyroclastic flow deposit in Southern Kyushu, Japan was identified. This distribution pattern was defined as depositional ramps. Depositional ramps can be identified in valleys wider than 1 km and become smaller-scale with increasing distance from the source. Upslope directions of depositional ramps are generally radially away from the source caldera, suggesting that the structure was formed by the flow of pyroclastic material radially away from the source. The original depositional surface was reconstructed based on field mapping and density measurements of the pyroclastic flow deposit. Depositional ramps having a dip angle of more than 9° were reconstructed on the vent-facing slopes of the topography underlying the valley-filling deposits in the area within 10 km of the caldera rim. Such a dip angle is much larger than previously described dip angles. The size and gradient of the depositional ramps decreases with increasing distance from the source. Depositional ramps are recognized commonly in densely welded pyroclastic flow deposits. A high emplacement temperature is required to form the depositional ramps. This suggests that the pyroclastic flow was transported as a dense, fluidized layer to minimize heat loss.
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Suzuki-Kamata, K., Ui, T. Depositional ramps: asymmetrical distribution structure in the Ata pyroclastic flow deposit, Japan. Bull Volcanol 50, 26–34 (1988). https://doi.org/10.1007/BF01047506
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DOI: https://doi.org/10.1007/BF01047506