12:00 PM - 12:15 PM
[SVC48-08] Eruptions during 6000 years at Nikko-shirane volcano, Central Japan
Keywords:Nikko-shirane volcano, Tephra stratigraphy, Holocene
At the foot of Mt. Nikko-shirane, four tephra layers from the Nikko-shirane volcano and three alien tephra layers were recognized. The Nikko-shirane tephra layers are named Nks-1, 2, 3, 4 downward (Okuno, 1993). We subdivide the Nks-1 tephra deposit into Nks-1a, 1b, 1c, 1d downward based on the color and grain size differences at the sampling point. The Nks-1a-d tephra contain vesicular and transparent–light-colored glass (pumice), non-vesicular and light-colored glass, non-vesicular and dark-colored glass, lithic clast, altered clast, and plagioclase, quartz, clinopyroxene and orthopyroxene crystals. The Nks-1a-c also contain a small amount of vesicular and dark-colored glass (scoria). The Nks-1 tephra deposit contains 16% pumice in maximum.
Two of three alien tephra layers are the Asama-B and Haruna-ikaho tephra deposits downward, and both are intercalated between the Nks-1 and 2 (Okuno, 1993; Tsutsui et al., 2005). We found an alien tephra layer just beneath the Nks-3 tephra. It is brownish silty ash with altered orangey pumice at the sampling point. The tephra contains 15% vesicular and colorless glass and they have similar major element composition to the Asama-D tephra.
At the summit of Mt. Nikko-shirane, we found a 0.17 m thick tephra layer beneath 0.07 m thick surface soil. Lower part of the tephra shows grayish white and the upper part shows yellowish brown ash. More than 80% components of the tephra are lithic clast, altered clast, and colorless minerals. Radioactive carbon dating of black soil beneath the tephra indicates 1686-1731 and 1808-1927 cal AD (14C dating: 110 ± 20 yrBP). Based on the 14C dating, the tephra layer would be produced by 1872-73 or 1889-90 eruption.
We propose that the Nks-1 might be magmatic eruption due to the Nks-1 tephra deposit contains 16% pumice in maximum. On the other hand, the tephra at the summit of Mt. Nikko-shirane should be the product of a phreatic eruption because of absence of pumice or scoria.