IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3C-P24

Date/Time: July 23 Poster

Formation of Qixiangzhan Eruption of Changbaishan Tianchi Volcano, China
Bo Pan1 , Jiandong Xu1 , Guangpei Zhong2 , Guoming Liu2 , Bo Zhao1
1Institute of Geology, China Earthquake Administration, China, 2 Changbaishan Volcano Observatory,Jilin, China
E-mail: dujushi1981@163.com

To comprehensively understand the eruption characteristics and history of active volcano is crucial for predicting
its future eruptions and hazard. Changbaishan Tianchi Volcano (CHTV) is one of the most dangerous active
volcanoes in Northeast Asia. It had experienced three periods of large-scale eruptions since the Holocene, i.e.
the Tianwenfeng period at about 5,000 years ago, the Qixiangzhan (QXZ) period at about 4,000 years ago andthe Millennium eruption at about 1,100 years ago, respectively. The type of Tianwenfeng and Millennium eruptions
is commonly accepted to be a typical Plinian eruption. However, there arises a considerable debate about thetype of QXZ eruption as to whether it is effusive or explosive. In high-resolution remote-sensing images, themorphology of the products of QXZ eruption looks like a lava flow, which flows along the northern slope of thevolcanic cone about 5.4 km in length and 400-800 m in width. However, the recent research work by the author
has revealed that the QXZ eruption should be a small-scale pulsed explosive eruption. The main evidence is asfollows: 1) The bulk-rock composition of QXZ eruption products is characterized by high SiO2 >71% and Na2 O+K2 O>10% contents representative of alkaline magma, which has high viscosity, low flowing ability and extremely highpotential of explosive eruption; 2) Field observations show that the QXZ eruption products appear as thin layers
about 2-5cm in thickness, significantly different from the massive or slaggy structures of lava flow; 3) Microscopic
observation reveals that most of the phenocrysts in the QXZ eruption products were severely broken by explosion
to form angular grains with well developed micro-cracks. The vesicles in the QXZ eruption products are irregularin shape and have rough margin, different significantly from the elliptical and smooth margin vesicles commonlyobserved in lava flow; 4) Stereomicroscopic observation shows that the QXZ eruption products are composed ofclastic particles and exhibit grain-supported texture with well developed irregular vesicles. Further study by using
SEM indicates that most of the clastic particles are fine volcanic ash and tiny pumice with rough surfaces. Basing
on the above analyses, we may conclude that the QXZ eruption can be assigned to a small-scale pulsed explosiveeruption. During the explosive eruption, a large number of fine pyroclastic particles flowed down the mountainslope as a high speed pyroclasstic flow to form thin layer of ignimbrite. Over many times of explosive eruptions,layer upon layer of ignimbrite were accumulated, resulting in a shape just like lava flow. Therefore, all the threelarge eruptions of CHTV in Holocene can be assigned to explosive eruption, rather than the previously proposedmodel of explosive-effusive-explosive explosions.

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