[SVC47-P09] Reconsideration of Late Quaternary tephrostratigraphy on Mikurajima Volcanic Island, Izu Islands.
Keywords:Mikurajima Volcano, tephrostratigraphy, Kozushima-Tenjohsan tephra , NiijimaｰShikinejima tephra
In this study, we aimed to identify and correlate tephras distributed and established the tephrostratigraphy on Mikurajima Is. The result can contribute to not only establishing the activity history of Mikurajima volcano, but also to constructing the chronological framework comprehending the Izu Island.
We conducted outcrop observation, a simple drilling survey using a hand-operated auger and analysis of volcanic glass (the morphology, refractive index, and major element components) and revealed several new knowledgments as below.
We totally identified nine tephras on Mikurajima Is. These are Niijima-Mukaiyama tephra (Iz-Nm, AD886), Kozushima-Tenjohsan tephra (Iz-Kt, AD838), Mk-3 tephra, the latest eruption of Mikurajima volcano, (6.3 ka, Geological Survey of Japan 2017), two newly discovered tephras: Mk-2.2, Mk-2.1, Kikai-Akahoya tephra (K-Ah, 7.3 ka), Niijima-Shikinejima tephra (Iz-Sk, about 8 ka), Mk-2 and Mk-1 tephra, originated from Mikurajima volcano tephra, in descending order. Iz-Nm and Iz-Sk are newly recognized in Mikurajima Is. and provide new information on the southern limit of the distribution of each tephra. The stratigraphic relationships of these tephras are consistent with the age of Mk-3 (6.3 ka; on calibrated) reported by Sugihara and Shimada (1999) and Sekiguchi et al. (2001). It was also found that the eruption ages of Mk-2, Mk-1 are older than Iz-Sk (about 8 ka). Between Iz-Kt and Mk-3, we found volcanic glass as cryptotephra. That could be identified Amagi-Kawagodaira tephra (KGP, 3.1 ka).On the other hand, Mk-G, rhyolitic tephra of unknown origin, intercalated between Mk-2 and Mk-1 reported by Sugihara and Shimada (1999) ,could not be discovered in this study.
It was revealed that the Mk-3 eruption change from pyroclastic surge, lava outflow, eruption of lapilli and ash fall, and finally to ejection of pumice fall. This means that the final phase of Mk-3 eruption changed from of all out of the volcanic lapilli and ash to the Plinian eruption causing pumice fall without a geological time gap. As reported by Sugihara and Shimada (1999), it is considered that the vent of Mk-3 located around the lava dome in the southeastern part of the island based on its distribution. On the other hand, we could not find the sources for Mk-2 and Mk-1.
Two newly discovered tephras, Mk-2.1 and Mk-2.2, fell Mk-3 and K-Ah tephras. In this study, we could not determine the source of these two tephras, but if they are originated from Mikurajima volcano, Mikurajima volcano erupted three times during the period K-Ah and Mk-3 for about 1,000 years. It means that the occurrence of explosive eruption of Mikurajima volcano was more frequent than previous estimation.