Development of Mountains and Alluvial Fans in Japan, Taiwan, and the Philippines
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概要
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In Japan, where uplift and denudation have been active during the Quaternary, the growing stage of mountains has been divided into the earliest, early, younger-middle, older- middle, and later subslages. In this study, relationships have been discussed quantitatively between ihc development stage of mountains and alluvial-fan existence and size in Lectonically active Japan, Taiwan, and the Philippines. Among relief ratio, drainage-basin area, temperalure, and precipitation, relief ratio was determined to be the most important variable for predicting the existence of alluvial fans for rivers with source areas greater than 100 k. As mountains pass through their development stages from earliest to later, their relief ratios increase and the number and size of alluvial fans increase, especially from the older-middle substage. In drainage basins over 100 k, the relief ratio of 30% is estimated to be an important threshold for the existence of an alluvial fan. In the older-middle substage al most all rivers have high relief ratios greater than 30%, while only 63% of Ihe rivers show such high relief ratios in the younger-middle substage. In addition, Ihe size of alluvial fans in drainage basins with relief ratios greater than 60% is larger than the size with relief ratios lower than 60％. The percentage of the rivers with relief ratios over 60% in the older-middle substage is markedly higher than that in the younger-middle substage. In this way, the increase in relief ratios (steeper riverbeds) beginning in the older-middle substage provides good conditions for the development and increased size of alluvial fans. Furthermore, the tropical climate in the Philippines, which seems to be disadvantageous for the development of alluvial fans in general, does not necessarily result in smaller alluvial fans when alluvial fans do develop.
- 日本地理学会の論文
- 1999-11-01