2006 Fiscal Year Final Research Report Summary
Understanding of magmatic eruptions and prediction of future eruptions in an active volcano, Zao
| Project/Area Number |
17540451
|
|---|
| Research Category |
Grant-in-Aid for Scientific Research (C)
|
| Allocation Type | Single-year Grants |
|---|
| Section | 一般 |
|---|
| Research Field |
Petrology/Mineralogy/Science of ore deposit
|
|---|
| Research Institution | Yamagata University |
|---|
Principal Investigator |
BAN Masao Yamagata University, Faculty of Science, Associate Professor, 理学部, 助教授 (50208724)
|
|---|
| Co-Investigator(Kenkyū-buntansha) |
YAGI Hiroshi Yamagata University, Faculty of Education, Art, and Science, Professor, 地域教育文化学部, 教授 (40292403)
KAGASHIMA Shin-ichi Yamagata University, Faculty of Science, Assistant Professor, 理学部, 助手 (70361243)
|
|---|
| Project Period (FY) |
2005 – 2006
|
| Keywords | Zao volcano / Active volcano / Volcanic unrest / Magma feeding system / Magma chamber / NE Japan / Tephra layers / Carbon 14 age |
|---|
| Research Abstract |
In the youngest stage (30 ka to present) of the Zao volcano, three active periods (ca. 31 to 29 ka, 7.5 to 4.1 ka, and 2.0 to present) can be seen. Piles of pyroclastic rocks by numerous small to medium sized eruptions are main products of the activities. In this study we examined the magma feeding system in the three periods, based on the petrologic features of the products.
Rocks erupted in the three periods are olivine+-pyroxene basaltic andesite to andesite, and these were formed by mixing of two end-member magmas, judged from the petrographic and mineralogic features. The estimated felsic end-members are similar among the periods, andesite (ca.60% in silica content) with orthopyroxene (Mg#=ca.64), clinopyroxene (Mg#=ca.68), plagioclase (An=ca.65) phenocrysts. The estimated mafic end-members are basalt with olivine (Fo=ca.80) and plagioclase (An=ca.90) phenocrysts in all periods, however, the bulk MgO, Cr and Ni contents of the erupted rocks are higher in the second period than in the other two periods. During the second and third periods, silica contents of the rocks decreased temporally from 58 to 55-56 % and recovered up to 58 %, and these variations can be explained by the different percentages of the basaltic magma involved in the mixing. Those features are suggesting that the mafic end-member magmas are distinct among periods, and may have been stored in the deeper part of the crust for ca.3.5 to 2.0 ky.
Looking at the chemical compositions of rocks in the past ca.0.8-ky eruptions closely, gradual decrease in Zr (and increase in Cr) contents toward upper part can be seen at least twice, which may correspond to the progressive injection of the basaltic magma to the shallower andesitic magma chamber, and it is estimated that the duration of each injection is less than 0.2 ky.
|
Research Products
(5 results)
