LITHOLOGICAL CONTROL ON RIDGE TOPOGRAPHY IN ALPINE REGION OF THE JAPANESE ALPS

A clear correspondence between lithology and ridge topography is observed in alpine regions of Japanese Alps. Regardless any geological and petrographical differences, protruded portions correspond to the area of less jointed rocks, while the cols to that of densely jointed rocks.<br> The authors measured two parameters in field: mean joint interval (J) and bedrock tensile strength (S_f), in order to give quantitative criteria to evaluate the resistance of bedrocks aganist erosion. The value of Sf is calculated by<br> S_f=(V_f/V₁)²•S₁<br> where V_f and V₁ are the longitudinal wave velocity in field and in laboratory, respectively, and S₁ is the tensile strength in laboratory. For jointed rock mass, the value of Sf is practically regarded to be equal to the tensile strength of joint plane.<br> The Shirouma Range, Northern Japanese Alps, and the Akaishi Range, Southern Japanese Alps are investigated and the results are as follows:<br> 1. Tn southern part of Shirouma Range (Fig. 1), where the main ridge is composed of only one kind of rock (A type), peaks and any protruded points correspond to the rocks whose values of J and S_f are large (Fig. 3-a, b), while cols correspond to the small values of J and S_f. Fault zones with extremely small values of J and S_f form deeply cutted cols (e.g. Kaerazu Kiretto in Fig. 3-a).<br> 2. In northern part of Shirouma Range (Fig. 1), where main ridge is composed of various types of sedimentary and igneous rocks (B type), some cols correspond to the exposure of well-jointed serpentinite or fault-shattered rocks (Fig. 3-d, e). No good correspondence is recognized between the relief and any rock types (e. g. limestone, shale, rhyolite), because of their similar values of J and Sf.<br> 3. In Akaishi Range (Fig, 2), where main ridge is composed of various types of sedimentary rocks (C type), clear correspondence between the relief and rock types is recognized. Peaks correspond to chert or sandstone with large values of J and S_f, while cols to shale with small J and Sf. (Figs. 4 and 7).<br> According to the results mentioned above, it is concluded that the joint development (represented by the values of J or S_f) should be most important for the evolution of ridge topography regardless of the difference in rock types.<br> The authors discuss also the exogenic agent responsible for relief formation as follows:<br> Frost shattering is one of the prevailing weathering process in present alpine environments of the Japanese Alps. In spring and autumn, frequent alternation of freezing and thawing is observed on the bedrock surface of Mt. Ainodake, northern part of Akaishi Range (Fig. 8) Rock fall activity concentrates in these two seasons. Therefore the difference of resistance to the frost shattering is presumed to play an important role in relief formation. The laboratory tests show that most rocks of the Japanese Alps have low porosity (Table 1) and are disintegrated only along the joint plane through alternation of freezing and thawing. It can be said that the resistance against the frost shattering depends firstly on joint density and strength of joint plane. This was ascertained by the fact that the rate of debris yield (R_S) measured on a rock cliff of Akaishi Range is closely related to the values of J and S_f (Fig. 9). It should be concluded, therefore, that the local difference in frost shattering due to the difference in joint development of rocks is one of the principle factors controlling the ridge topography.