Abstract
This paper is an attempt to try to express the erosional features of volcanic cones in some quantitative indices and also to show the relations between the indices and the duration of erosion (T). All measurements to lead the indices are made by using topographic contour maps on the scale of 1 : 50, 000. Thirty four, monotonous, conical, isolated stratovolcanoes in Japan are selected for the measurements in order to eliminate various factors, other than age, affecting the present state of erosion, as far as possible. Firstly, as to the erosional features of the volcanic cones as a whole, two indices, e.g. erosion ratio R, (Ve/Vo)×100, and mean denudation depth Dd, (Ve/Ao), are examined. Here, Vo is the original volume of cone, Ve is the total eroded volume (Vo-Vp), Vp is the present volume of cone, and Ao is the basal area of cone. Secondly, concerning the erosional features of main radial valleys, five indices are examined as follows : 1. Form factor of drainage basin F, (A/L2, where A is drainage area, and L is basin length). 2. Sinuosity index φ, (l/L, where l is length of main stream). 3. D2, D3, and D4. These are defined as a distance from the head of a basin to a junction where the stream-order number of main stream changes into the second-order, the third-, or the forth-, respectively. 4. P1, P2, and P3. These are defined as the mean stream-order number of main radial streams at each of the points located at one, two, or three kilometers distance from the head of basin. 5. Furrow index Ie, (2S/L・h, where S is the area of projected longitudinal section of valley, and h is the relative height of the basin). The results, shown in Figs. 4～11 and in Tables 1 and 3, indicate that most of the indices are logarithmic functions of T, that is, from the points of view of various indices (x) except φ and Dn, the rate of erosion (dx/dT) are expressed in form of 1/T.
