VOLCANIC ASHES OF THE BANTAI VOLCANOES

Abstract

The volcanic ashes in the Bantai volcanic area are classified into two main volcanic ashes based upon the soil colors. One of them is A-series (black soil color), and the other is B-series volcanic ash (brown soil color). The B-series volcanic ash is underlain by lava, the Okinajima mudflow and higher lake terrace etc., and is widely spread and accumulated to a thickness of 0-2 meters in the area. Sometimes, brown ash is deposited with mudflow materials as a matrix. On the other hand, the A-series volcanic ash of 0-80 centimeters in thickness is found in the southern area of Bantai-san and Nekoma-dake. However, the A-series volcanic ash is either absent or only thinly accumulated in the area north of Bantai-san. The soil color of the black volcanic ash is presumed to be due to the colored mineral fragments and humus. The particles of the B-series volcanic ash are progressed in leaching to only a slight degree compared with the A-series volcanic ash, but the mineral fragments included can still be seen. The weathering of volcanic ashes occurs in and around the individual particles of the ashes. Such as particles that have been leached, also iron which is included in the particles of the ashes as a chemical composition has been changed by oxidation, and thus the ash will exhibits a brown soil color. The black parting layer in the B-series volcanic ash layer formed mainly by the difference in degree of leaching under the environment of the cold climate. The parting layer may have been deposited during the cold climate of Preboreal and Boreal period.