2009年気象庁阿蘇山ボーリングコアの層序

抄録

The Asosan core (JMA-V40) was drilled at a site 1.2km WSW of Nakadake crater, Aso Volcano in 2009. The core consists of the following nine units: 1) alternating beds of ash-fall deposits and buried soil layers (0-3.49m in depth); 2) alternating beds of ash including Nakadake N2 scoria (N2S; 1.5ka) (3.49-4.46m); 3) scoria-fall deposits from Kishimadake scoria cone (4ka; 4.46-9.00m); 4) a basaltic andesite lava (lava 1, 9.00-23.72m); 5) alternating beds of ash, soils and lahar deposits including the Kikai Akahoya tephra (K-Ah; 7.3ka) (23.72-31.00m); 6) a basaltic andesite lava (lava 2, 31.00-76.11m); 7) lahar deposits (76.11-86.47m); 8) a basaltic andesite lava (lava 3, 86.47-91.64m); 9) lahar deposits (91.64-100.00m). The three lavas recognized in units 4, 6 and 8 are quite similar in lithofacies, petrography and major element composition (SiO₂=52.1-53.4wt.%). This suggests that at least two compositionally homogeneous basaltic andesite lava flows were erupted from Nakadake Volcano around 8-5 cal ka and another similar one erupted somewhat earlier, but still in Holocene time.