11:00 〜 11:15
[SVC36-07] Long-term change of the eruption activities of Sakurajima volcano inferred from the fallout tephra deposits
キーワード：桜島火山、テフラ、14C年代
Geological and stratigraphic analyses of fallout tephra deposits from a volcano are the cues in unraveling the characteristics of individual explosive pumice eruptions and the long-term temporal change of volcanic activities in the volcano. Sakurajima volcano in SW Japan is a complex stratovolcano composed of two major edifices and has caused at least seventeen explosive pumice eruptions. Herein, we re-examined the distribution, volume, and eruption age of fallout tephra deposits from Sakurajima volcano to clarify the geological characteristics of each deposit and reconstruct the temporal change of the volcanic activities combined with the tephra deposits and other volcanic products. The seventeen tephra deposits from Sakurajima volcano were named P17 to P1 from lower to higher levels. Reconstruction of the distribution and stratigraphy of the tephra deposits were conducted based on our original field survey and compiling previous datasets, as the tephra sequence of Sakurajima has been studied from geological and archeological interests.
We define three eruption stages of Stage 1 (30–24 ka), Stage 2 (12.8–4.8 ka), and Stage 3 (4.5 ka–present), based on a major hiatus and shifting of the volcanic centers. Stages 2 and 3 are further subdivided according to the predominant mode of volcanic activity; Stage 2a (13–8 ka) and 2b (8–4.8 ka), and Stage 3a (4.5–1.6 ka) and 3b (1.6 ka–present), respectively. The revised distribution of tephra deposits indicates that a total of 14.5 km3 (5.8 km3 dense rock equivalent) of fallout tephra has erupted from Sakurajima. Among them, Stage 2a (13–8 ka) produced the largest volume of tephra fallout deposits, suggesting that Sakurajima peaked in magma discharge during Stage 2a (2.9 km3/kyr) and then decreased rapidly toward Stage 2b (8–4.8 ka; 0.07 km3/kyr). The eruption of large-volume tephra deposits in Stage 2a, followed by the development of a thick volcanic ash layer in Stage 2b, indicates the eruption style shifted from explosive pumice eruptions in Stage 2a to ash-producing Vulcanian activity in Stage 2b, with decreasing magma discharge rate. Thick volcanic ash deposits covered by several fallout tephra deposits during Stage 3 also reflect a shift in activity from repeated Vulcanian explosions with lava effusions in Stage 3a (4.5–1.6 ka) to predominantly pumice eruptions during the historical period (Stage 3b) with an increasing magma discharge rate. The case study of Sakurajima presented here demonstrates that the combined analysis of the distribution, stratigraphy, and age of pumice fall layers with the lava and pyroclastic cone deposits on the volcanic edifice is a useful tool for deciphering the growth history of complex stratovolcanoes.
We define three eruption stages of Stage 1 (30–24 ka), Stage 2 (12.8–4.8 ka), and Stage 3 (4.5 ka–present), based on a major hiatus and shifting of the volcanic centers. Stages 2 and 3 are further subdivided according to the predominant mode of volcanic activity; Stage 2a (13–8 ka) and 2b (8–4.8 ka), and Stage 3a (4.5–1.6 ka) and 3b (1.6 ka–present), respectively. The revised distribution of tephra deposits indicates that a total of 14.5 km3 (5.8 km3 dense rock equivalent) of fallout tephra has erupted from Sakurajima. Among them, Stage 2a (13–8 ka) produced the largest volume of tephra fallout deposits, suggesting that Sakurajima peaked in magma discharge during Stage 2a (2.9 km3/kyr) and then decreased rapidly toward Stage 2b (8–4.8 ka; 0.07 km3/kyr). The eruption of large-volume tephra deposits in Stage 2a, followed by the development of a thick volcanic ash layer in Stage 2b, indicates the eruption style shifted from explosive pumice eruptions in Stage 2a to ash-producing Vulcanian activity in Stage 2b, with decreasing magma discharge rate. Thick volcanic ash deposits covered by several fallout tephra deposits during Stage 3 also reflect a shift in activity from repeated Vulcanian explosions with lava effusions in Stage 3a (4.5–1.6 ka) to predominantly pumice eruptions during the historical period (Stage 3b) with an increasing magma discharge rate. The case study of Sakurajima presented here demonstrates that the combined analysis of the distribution, stratigraphy, and age of pumice fall layers with the lava and pyroclastic cone deposits on the volcanic edifice is a useful tool for deciphering the growth history of complex stratovolcanoes.