Abstract
The refractive indices of volcanic glass and orthopyroxene in the tephra layers mainly erupted from Iwate Volcano overlaying the Toya Ash around 100 ka activity were determined by means of the temperature variable type refractive index measuring device, MAIOT. Because MAIOT enables us to measure precise and rapid refractive indices on the above minerals, it is useful to distinguish each tephra layer around the studied area. The color, shape and amount of microphenocrysts of volcanic glass were also classified by microscopic identification. On the basis of these petrographic characteristics, identification of each tephra layer has been clarified. In particular, the Yukiura Pumice layer, which is thought to be one homogeneous layer, could be divided into three different pumice layers showing different pumice layers showing different refractive indices of glass. One unknown tephra layer situated on the Toya Ash is identified as one of the tephra originated in the Iwate Volcano by the comparison of refractive indices with the lists in this paper. On the foot of Iwate Volcano, the Aira Tn volcanic ash layer (AT) originated Aira caldera, southern Kyushu was intercalated in the Takizawa 1 Scoria in the Shibutami Volcanic Ash member. The Towada Chuzeri pumice was also interbeded in the humus bed just below the Sugo Scoria in the Wakare Volcanic Ash member.
