1:45 PM - 2:00 PM
[SVC30-01] Eruptive history of Atosanupuri volcano, eastern Hokkaido: the structure of the Atosanupuri caldera and its related pyroclastic flows
Keywords:Atosanupuri volcano, Atosanupuri caldera, correlation of tephra, composition of volcanic glass
Atosanupuri volcano, which is one of the post-caldera volcanoes of Kutcharo caldera volcano, exhibits fumarolic activity now around Atosanupuri lava dome. The volcano started the eruptive activity after the 40 ka Kutcharo Pumice flow eruption (Kp I), building the stratovolcano with andesitic and dacitic magmas. After that, the rhyolitic explosive eruptions occurred, forming the Atosanupuri caldera. And then, the multiple rhyolitic lava domes were created, at first out of the caldera and next within the caldera during the Holocene. In the last few thousand years, the phreatic eruptions occurred around the Atosanupuri lava dome. Although the outline of the eruption history of Atosanupuri volcano has been revealed, the eruption sequence of the Atosanupuri pyroclastic eruptions is still unknown: the correlation between the proximal and distal tephra of the Atosanupuri pyroclastic eruptions is still not performed. In addition, the recent research proposed that “Atosanupuri caldera” was the graben related to the formation of resurgent lava dome (Goto and McPhie, 2018). Therefore, to reveal the whole sequence of the Atosanupuri pyroclastic eruptions, we carried out the geological investigation, boring and trench surveys, and petrological study.
Based on the distribution and petrological features of the juvenile materials, the proximal deposits of the Atosanupuri pyroclastic eruptions can be divided into four types: Ishikaribetsu pyroclastic flow (Is-pfl) in the central area, Sunayu pyroclastic flow (Sn-pfl) in the northwestern area, Ikenoyu pyroclastic flow (Ik-pfl) in the southwestern area, and Biruwa pyroclasts (Bi) in the eastern area. The three pyroclastic flows show the graded and cross-stratified facies. Also, Bi is the weakly-welded pyroclastic rocks, constituting the uppermost part of the somma lavas. Ik-pfl is interfingered with the pyroclastic flow compositionally correlative with Is-pfl. The juvenile materials of three pyroclastic flows are the two-pyroxene rhyolite, whereas those of Bi are the two-pyroxene dacite. The distal tephras of the Atosanupuri pyroclastic eruptions are distributed in the eastern and northern regions: Nu-g (TyP), Nu-e, Nu-c, Nu-a, and Ch-c in ascending order, during the certain period between 32 and 12 ka (Hasegawa et al., 2009). We obtained the new 14C date (ca. 19 ka) from the soil just beneath Ch-c. According to the petrological features of the juveniles in these tephra, Nu-g, Nu-a and Ch-c can be correlated with Bi, Ik-pfl and Is-pfl, respectively. Also, Nu-e and Nu-c are similar to Ik-pfl in glass chemical trends.
Considering these results, it is interpreted that the Atosanupuri pyroclastic eruptions were composed of the multiple pyroclastic eruptions generated from at least four types of magmas. The first eruption was occurred by Bi magma at the end of the somma lava activity. After that, Ik-pfl magma was active, resulting in the three pyroclastic eruptions (Nu-e, Nu-c and Nu-a). During Nu-a eruption, Is-pfl magma also started its eruptive activity. At ca. 19 ka, this magma induced the large pyroclastic flow eruption (Ch-c). Although the eruption age of Sn-pfl is still unknown, it is suggested that Sn-pfl eruption was occurred at the same period as (or younger than) the Ch-c one based on the 14C date (ca. 11 cal. ka) from the soil above the Sn-pfl.
The results of our re-examination revealed that the Atosanupuri pyroclastic eruptions were the multiple eruptions generated from multiple types of magmas during the 10 thousand years. At present, there are the multiple depressions like “caldera (or crater)” at the Atosanupuri volcano and in the Lake Kutcharo. Also, these pyroclastic flow deposits exhibit the facies features of subaqueous pyroclastic flow, suggesting a lake was existed around the volcano at the Atosanupuri pyroclastic eruptions. Considering these evidence, it is reasonable that “Atosanupuri caldera” is “crater” rather than the “graben” related to the formation of the resurgent dome.
Based on the distribution and petrological features of the juvenile materials, the proximal deposits of the Atosanupuri pyroclastic eruptions can be divided into four types: Ishikaribetsu pyroclastic flow (Is-pfl) in the central area, Sunayu pyroclastic flow (Sn-pfl) in the northwestern area, Ikenoyu pyroclastic flow (Ik-pfl) in the southwestern area, and Biruwa pyroclasts (Bi) in the eastern area. The three pyroclastic flows show the graded and cross-stratified facies. Also, Bi is the weakly-welded pyroclastic rocks, constituting the uppermost part of the somma lavas. Ik-pfl is interfingered with the pyroclastic flow compositionally correlative with Is-pfl. The juvenile materials of three pyroclastic flows are the two-pyroxene rhyolite, whereas those of Bi are the two-pyroxene dacite. The distal tephras of the Atosanupuri pyroclastic eruptions are distributed in the eastern and northern regions: Nu-g (TyP), Nu-e, Nu-c, Nu-a, and Ch-c in ascending order, during the certain period between 32 and 12 ka (Hasegawa et al., 2009). We obtained the new 14C date (ca. 19 ka) from the soil just beneath Ch-c. According to the petrological features of the juveniles in these tephra, Nu-g, Nu-a and Ch-c can be correlated with Bi, Ik-pfl and Is-pfl, respectively. Also, Nu-e and Nu-c are similar to Ik-pfl in glass chemical trends.
Considering these results, it is interpreted that the Atosanupuri pyroclastic eruptions were composed of the multiple pyroclastic eruptions generated from at least four types of magmas. The first eruption was occurred by Bi magma at the end of the somma lava activity. After that, Ik-pfl magma was active, resulting in the three pyroclastic eruptions (Nu-e, Nu-c and Nu-a). During Nu-a eruption, Is-pfl magma also started its eruptive activity. At ca. 19 ka, this magma induced the large pyroclastic flow eruption (Ch-c). Although the eruption age of Sn-pfl is still unknown, it is suggested that Sn-pfl eruption was occurred at the same period as (or younger than) the Ch-c one based on the 14C date (ca. 11 cal. ka) from the soil above the Sn-pfl.
The results of our re-examination revealed that the Atosanupuri pyroclastic eruptions were the multiple eruptions generated from multiple types of magmas during the 10 thousand years. At present, there are the multiple depressions like “caldera (or crater)” at the Atosanupuri volcano and in the Lake Kutcharo. Also, these pyroclastic flow deposits exhibit the facies features of subaqueous pyroclastic flow, suggesting a lake was existed around the volcano at the Atosanupuri pyroclastic eruptions. Considering these evidence, it is reasonable that “Atosanupuri caldera” is “crater” rather than the “graben” related to the formation of the resurgent dome.