2004 Volume 110 Issue 1 Pages 19-37
Tephric-loess sequences are distributed widely in Japan. We attempt to distinguish volcanic events from the tephric-loess sequences using a newly developed ’RIPL Method (Refractive Index Physical Lobas Method)’. The RIPL Method is a technique of sampling a few centimeter thick strata continuously from the vertical section of a tephric-loess sequence, measuring all the refractive indices of minerals (mainly volcanic glass, orthopyroxene, and hornblende) included in each sample, and identifying tephra horizones in the tephric-loess from the characteristic feature of changes in the refractive index and mineral compositions.
We selected two outcrops around Yufudake Volcano, Kyusyu Island, Japan. Point A is on the southern foot of Yufudake Volcano, and point Y is between Yufudake and Kuju Volcanoes. Tephric-loess is well preserved at both points and the deposits include four widespread tephra layers, Aso-4 (86-90 ka), Kj-P1 (age unknown), AT (29 ka) and K-Ah (5330BC). Using the RIPL Method, we distinguished twelve eruptive event horizons (Event horizons 1 to 12) in the tephric loess sequences above the Aso-4, We indicated the possibility that Kuju and Yufudake Volcanoes are eruptive sources of these events as well as Aso Volcano southwest of the two points. Among the events, the Event horizon 9 recorded the Shirani pyroclastic flow deposites erupted from Kuju Volcano and ACP4 airfall pumice from Aso Volcano. We calculated ages of the twelve events as base levels of individual event horizons, with the assumption that depositional rate of the tephric-loess sequence of point A has been constant since 86-90 ka. The Event horizons 1 (74 ka), 2 (72 ka), 4 and 5a (67 ka) recorded the tephras erupted from Kuju Volcano and the Event horizons 3 (69 ka), 6 (64 ka), 7 (60 ka), 8 (58ka) and 10 (47 ka) recorded the tephras erupted from Yufudake or it’s near volcanoes because the similarity of each petrographic characteristics. The Event horizon 11 (21 ka) recorded the tephra from Onimoniyama Volcano and the Event horizon 12 (15 ka) recorded the tephra erupted Yufudake Volcano. When using the RIPL Method in the area located outside of isopaches of the tephras, the improvement of the grasping accuracy of the volcanic activity can be expected.
