硫黄島の面積の長期変動 Long-term Variation of the Area of Iwo Jima
Iwo Jima is small volcanic island located isolate in the Western Pacific Ocean about 1, 250 km south of Tokyo. The island with the area about 23 km<SUP>2</SUP> is covered with considerably dense vegetation and no rivers exist in this island. Thus, the source of sediments to nourish the beach seems to be quite limited in this island. Further, the coast is always subjected to rough seas because it is fully exposed to the open sea. Nevertheless, the coast is mostly rimmed by sandy beaches with about 200 m in width and the shoreline is still advancing toward the sea which increases the beach area year by year. Many scientists have pointed out that this increase might be caused by an unusual upheaval of this island exceeding 30 cm per year. However, no quantitative analyses have been done yet on this unique phenomenon that prevents from constructing any port facilities in this island. Thus, the author intends to investigate the features of the long-term variation of the area which have occurred in Iwo Jima during the past seven decades and to examine its variation mechanism.<BR>To investigate the features of the long-term variation, analyses are conducted on the seven maps and charts of Iwo Jima which have been published in the period from 1911 to 1981. The analysis reveals that the island has kept increasing its area over the past seven decades although the increasing rate has changed drastically in 1952 from 30, 000 m<SUP>2</SUP> to 90, 000m <SUP>2</SUP> peryear and that the increased area has reached roughly 4.3 km<SUP>2</SUP> in total during the sevendecades. This change of the increasing rate well corresponds to the fact that the upheaval rate of the island has changed in 1952 from roughly 10 cm to 30 cm per year which has been found by Kosaka <I>et al</I>. in 1979.<BR>To examine the effect of the upheaval on the features of the long-term variation of the area, calculation is carried out by giving the various values of imaginary upheaval to the bathymetric data shown on the chart covering the sea around Iwo Jima published by the Hydrographic Department of Japan Maritime Safety Agency in 1981. The result of the calculation indicates that the upheaval of 10 cm per year corresponds to the increasing rate of the beach area roughly 43, 000 m<SUP>2</SUP> per year and the upheaval of 30 cm per year corresponds to the increasing rate of the beach area roughly 130, 000 m<SUP>2</SUP> per year when the foreshore slope is assumed to be 1/20. These values are surely the ones before the beach is transformed by the action of the ocean waves. However, comparison of these values with the ones found through the former analyses indicates that the calculation has provided quite a reasonable estimation of the increase of the beach area. These findings clearly indicate that the continuous upheaval is the governing factor to cause the increase of the beach area in Iwo Jima.
地學雜誌 105(4), 448-458, 1996-08-25
Tokyo Geographical Society