鳥取沖海底表層堆積物に見られる三瓶火山起源のテフラとその地質学的意義

その他

Volcanic ashes in the subsurface deposits of Japan sea, off the coast of Tottori, are composed of Utsuryo-Oki ash (U-Oki), Aira Tn ash (AT), Akahoya ash (Ah) and two kind of ashes originated from the eruption of Sanbe volcano. One is Sanbe-Unnan fall pumice (SUn) and its absolute ages of eruption show 40,000 to 50,000Y.B.P.. This tephra is probably the earliest deposit in the second period of volcanic activity of Sanbe volcano. The pumice of SUn is one of hornblende biotite rhyolite, and it shows that K_2O is generally more than <Na>_2O in glass. Other is Sanbe-Ikeda fall pumice (SIP). This tephra is another fall pumice of the later stage of the second eruption period of Sanbe volcano. This is abundant of heavy minerals than SUn. The deposit of SIP was observed to be covered with Aira Tn ash (AT) dated 22,000 Y,B.P..
The present finding bear new meaning that both SUn and SIP are distributed as far as 180km NE direct in from Sanbe volcano. On the basis of this evidence, we have suggested the posibility that these important tephras as key bed are distributed in and around Hyogo prefecture.