关键词: 四海火山灰, 粒度特征, 次布里尼式喷发

Abstract: The Sihai Tephra Sheet(STS)was formed by the youngest eruption of Longgang volcanic area.The basaltic air fall deposit of this eruption composed the Jinlongdingzi scoria cone and the fallout tephra sheet east of the volcano,near Hongqilinchang in Huinan county and Sihailinchang in Jingyu county.This eruption is considered as Sub-Plinian eruption by using Walker G P L's method.It suggests that the strength of Jinlongdingzi volcano eruption was large.The cumulative frequency plots area shows a good coordination with its concentrated area,as well as a straight right-hand uprising tendency,which indicates a similar sorting process due to gravity during its transportation and setting down.The proximal samples have more coarse grains while the distal part becomes fine-rich as the distance from the vent increased.Although the samples from the other scoria cones are taken from near the vent,the enrichment tendency of the fine material becomes more gentle and have a bigger mass fraction during φ<0,which suggests that the eruption strength of the other volcanoes are obviously smaller than that of the STS.Comparing the distribution feature of the STS with that of Tianchi volcano one can see that the slopes of the Tianchi fallout changes less while the slopes of STS have an obvious variation.The averaged maximum diameter of the STS is smaller than that of Tianchi but the coarse particle proportion of the STS is bigger than that of Tianchi.The up-rise tendency of the fine particles from the STS is not as high as that the samples from the Tianchi volcano,which indicates that the explosivity of the STS is less than that of Tianchi.

Key words: Sihai Tephra Sheet, grain-size characteristics, Sub-Plinian eruption