1:45 PM - 3:15 PM
[SVC36-P06] Re-examination of the eruption age of Shimohorizawa lava, Yakedake volcano, Japan
Keywords:Yakedake Volcano, Paleomagnetic dating, Thermoluminescence dating
Yakedake volcano, located on the border between Nagano and Gifu prefectures, is an active volcano belonging to the Younger Yakedake volcano groups, which includes, from north to south, Yakedake volcano, Shirataniyama volcano, and Akandana volcano (Harayama, 1990; Oikawa, 2002). Yakedake volcano has been active since about 25 ka and the latest magmatic eruption was occurred at about 2.3 ka, effused summit dome lava accompanied with Nakao pyroclastic flow deposits (Oikawa, 2002). Recent activities were repeated phreatic eruptions, which occurred in 1907-1939 and 1962-1963.
Shimohorizawa lava flow, distributed at the eastern and southern foot of the volcano, is the largest lava flow, occupied almost 40-60% volume of the volcano (Oikawa, 2002). The eruption age of the Shimohorizawa lava flow is estimated to be 3,000-4,000 yBP, based on radiocarbon dates of wood chips in the Nakahorizawa mudflow sediments deposited directly above the lava flow at the eastern foot (Kawachi and Kobayashi, 1966; Ministry of Construction, 1993). There is no age directly obtained from the lava itself, so there is room for re-examination of the eruption age. In this study, we carried out eruption age determination of the Shimohorizawa lava flow using paleomagnetic and thermoluminescence (TL) dating methods.
Seventy-seven and fifteen lava blocks were collected from the southern lobe and the eastern lobe of the flow, respectively. We conducted thermal demagnetization experiments and paleointensity experiments using the IZZI method (Yu and Tauxe, 2005).
As a result, mean directions of Dec=-5.8°, Inc=47.9°, α95= 3.1° was obtained. Compared with the PSV data obtained from Lake Biwa sediments (Hayashida et al., 2007), ages of 5.0-6.3 ka, 7.0-7.2 ka, 11.0-11.4 ka, and 11.9-12.0 ka were estimated. Paleointensity is estimated to be 39.7 ± 4.0 µT. Compared with PSV data from the GEOMAGIA50 database (Knudsen et al., 2008), ages of 4.8-8.0 ka and 11.2-12.3 ka were estimated. Combining paleodirection age and paleointensity age, paleomagnetic ages of 5.0-6.3 ka, 7.0-7.2 ka, 11.2-11.4 ka, and 11.9-12.0 ka were obtained.
TL dating was measured using quartz of ca. 10 - 50 µm in size and TL age of 11 ± 1.3 ka was obtained.
The results obtained using paleomagnetic and TL methods in this study are summarized in 11.2-11.4 ka or 11.9-12.0 ka, which are considerably older than the 3,000-4,000 yBP of the previous values. The ages in the previous study were obtained from wood chips in the sediment directly above the lava flow that flowed to the east of the Shimohorizawa lava, while our data was obtained mainly from the southern lobe, suggesting that the ages of lava that flowed eastward and southward may be different. In addition, although the number of samples is small, paleomagnetic direction from the eastern lobe shows deeper inclination than the southern lobes, suggesting that this difference may be due to differences in eruption ages.
Shimohorizawa lava flow, distributed at the eastern and southern foot of the volcano, is the largest lava flow, occupied almost 40-60% volume of the volcano (Oikawa, 2002). The eruption age of the Shimohorizawa lava flow is estimated to be 3,000-4,000 yBP, based on radiocarbon dates of wood chips in the Nakahorizawa mudflow sediments deposited directly above the lava flow at the eastern foot (Kawachi and Kobayashi, 1966; Ministry of Construction, 1993). There is no age directly obtained from the lava itself, so there is room for re-examination of the eruption age. In this study, we carried out eruption age determination of the Shimohorizawa lava flow using paleomagnetic and thermoluminescence (TL) dating methods.
Seventy-seven and fifteen lava blocks were collected from the southern lobe and the eastern lobe of the flow, respectively. We conducted thermal demagnetization experiments and paleointensity experiments using the IZZI method (Yu and Tauxe, 2005).
As a result, mean directions of Dec=-5.8°, Inc=47.9°, α95= 3.1° was obtained. Compared with the PSV data obtained from Lake Biwa sediments (Hayashida et al., 2007), ages of 5.0-6.3 ka, 7.0-7.2 ka, 11.0-11.4 ka, and 11.9-12.0 ka were estimated. Paleointensity is estimated to be 39.7 ± 4.0 µT. Compared with PSV data from the GEOMAGIA50 database (Knudsen et al., 2008), ages of 4.8-8.0 ka and 11.2-12.3 ka were estimated. Combining paleodirection age and paleointensity age, paleomagnetic ages of 5.0-6.3 ka, 7.0-7.2 ka, 11.2-11.4 ka, and 11.9-12.0 ka were obtained.
TL dating was measured using quartz of ca. 10 - 50 µm in size and TL age of 11 ± 1.3 ka was obtained.
The results obtained using paleomagnetic and TL methods in this study are summarized in 11.2-11.4 ka or 11.9-12.0 ka, which are considerably older than the 3,000-4,000 yBP of the previous values. The ages in the previous study were obtained from wood chips in the sediment directly above the lava flow that flowed to the east of the Shimohorizawa lava, while our data was obtained mainly from the southern lobe, suggesting that the ages of lava that flowed eastward and southward may be different. In addition, although the number of samples is small, paleomagnetic direction from the eastern lobe shows deeper inclination than the southern lobes, suggesting that this difference may be due to differences in eruption ages.