Longitudinal downsizing of hummocks by the freely-spreading volcanic debris avalanches in Japan

DOI Web Site 5 Citations 72 References

Bibliographic Information

Other Title
  • 山麓に拡散した火山性岩屑なだれが形成する流れ山の縦断分布特性

Search this article

Description

Hummocks of debris avalanches distributed at the skirt of volcanoes have been investigated mainly to prove the occurrence of catastrophic sector collapse and to discuss the flow mechanism. The hummocky topography, however, is expected to show some other geomorphological aspects about the collapse and debris avalanche. This study tries to find some new geomorphological meanings of debris avalanche hummocks on the basis of their longitudinal distribution pattern. Four debris avalanches, which are typical cases of freely spreading debris avalanches, were selected from four different volcanoes in Japan. Numbers of the debris avalanche hummocks investigated in this study are 172 at Shiribetsu volcano, 262 at Usu volcano, 200 at Iwaki volcano, and 643 at Nasu volcano. Morphological data of hummocks were measured using aerial photographs and GIS techniques.<BR>Deposition of hummocks starts at the lower part of volcanic piedmont, forming a gently sloping depositional surface, and terminated in the plain with rolling hilly landforms. The longitudinal distribution patterns of hummocks along the course of debris avalanche vary with avalanches, showing that hummocks are in clusters. Sizes of hummocks, however, generally decrease downstream for each of the four avalanches, and regression analysis of size-distance relationship shows that the relationship can be described by exponential functions with high correlation. The regression function indicates that the size of hummocks at source area should be determined by the volume of collapsed volcanic body. Namely, the cracks created in the collapsed volcanic body are potentially controlled by the magnitude of collapse. On the other hand, the downsizing rate of hummock is indicated to be controlled by the fluidity of debris avalanche which is expressed by the reversed values of equivalent coefficient of friction of debris avalanche. When avalanche is terminated in short distance, size of hummocks rapidly decreases downstream, while when avalanche flows down for a long distance, size slowly decreases. Namely, the higher in fluidity the avalanche is, the lower in downsizing the hummock is. As mentioned above, the size-distance relationship for the hummocks of volcanic debris avalanches in Japan shows the physical properties of volcanic body and of flow of avalanche.

Journal

Citations (5)*help

See more

References(72)*help

See more

Keywords

Details 詳細情報について

Report a problem

Back to top