台灣高山凍融作用與冰緣地形分布之研究

Abstract

寒凍為高山與冰緣氣候中，常見的環境因子，凍融作用亦是其中最重要的地形營力。台灣高山地區易發生凍融風化與寒凍潛移等作用，而對其地形演育甚為重要。本研究蒐集南湖大山氣溫及地溫資料，利用經驗與統計模型，推估凍融作用的規模、頻率與發生的時間，以及應用野外調查的結果，繪製高山地區之冰緣地貌圖。 由2001年至2005年南湖大山地溫觀測站的資料顯示，凍融循環在地表下2、10與20㎝處，平均每年凍融循環次數分別為55、8.75、1.25次，顯示出凍融作用在地表下2㎝處較其它深度活躍；發生的時間主要在秋季與春季，而冬季因為地表受到覆雪與結凍之影響，凍融作用較不明顯，而且地表下10㎝以上的結凍時間達66至71天。利用經驗統計之模型，模擬地表溫度及觀測值的相關係數達0.7875，並且凍融次數與實際觀測值相符。利用模型推估凍融作用於空間中的分布顯示出，海拔高度變化與凍融次數之間的關係呈現對數曲線變化，而凍融作用頻率與森林線高度的關係較不明顯。在南湖大山與合歡山皆可觀察到碎石坡、碎石堆與草原土趾階地等冰緣地形，且冰緣地形皆分布於森林線以上的裸露地或高山箭竹草原之中，顯示凍融在高山地區作用和冰緣地形區相當活躍，台灣北部高山冰緣垂直分布帶約在海拔3300至4000公尺。

Frost action is one of the important environmental factors in alpine and periglacial climate. Among them, freeze-thaw process is the most influential geomorphologic process in the area. Freeze-thaw weathering and gelifluction movement shapes the landform in high mountain area of Taiwan. In this study, the air and ground temperature data of 2, 10 and 20 cm depth of Mt. Nanhu was used to model the magnitude, frequency and distribution of freeze-thaw process. Besides, field investigation was also conducted to depict the geomorphological map in Mt. Nanhu area. According to ground temperature data in Mt. Nanhuta collected from 2001 to 2005, frequency of freeze-thaw processes at depth of 2 , 10 and 20cm was 55, 8.75 and 1.25 times per year respectively. The freeze-thaw process was weak at depth of 20cm. It was most frequent in autumn and spring but not few in winter for freeze and snow cover in that period. At the depth of 10 cm in the ground, there are about 2 months of frozen period per year in average. The measured and estimated ground temperature data are highly correlated, the R-square value is 0.7875, and the numbers of freeze-thaw from measurement and estimation by the model are identical. The relationship between elevation and freeze-thaw cycle is in logarithm regression by the estimation of the model. However, the relationship between freeze-thaw cycle and forest line in Mt. Nanhuta is not clear. According to field investigation, there are some periglacial landforms in the Mt. Nanhuta and Mt. Hehuan, including turf-banked lobes terraces, talus accumulation and talus slopes. Turf-banked lobes terrace are crescent shape terrace developed in Mt. Sheimajuei and the saddle between main peak and east peak of Mt. Nanhuta. Talus accumulation is developed in the saddle between main peak and east peak of Mt. Nanhuta. Talus slope is accumulated a lot of frost-wedging rock which is located at east of upper-cirque. It shows that freeze-thaw processes are active in the Mt. Nanhuta, so there are many periglacial landscapes between 3200-4000m above sea level.