A DISCUSSION ON THE TYPE OF THE QIXIANGZHAN ERUPTION OF CHANGBAISHAN TIANCHI VOLCANO,NORTHEAST CHINA

Changbaishan Tianchi volcano is one of the most dangerous active volcanoes in Northeast Asia.It experienced three periods of large-scale eruptions since the Holocene,i.e.the Tianwenfeng period at about 5,000 years ago,the Qixiangzhan period at about 4,000 years ago and the 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 the type of Qixiangzhan eruption as whether it is effusive or explosive.In high-resolution remote-sensing images,the morphology of the products of Qixiangzhan eruption looks like a lava flow,which erupts from Qixiangzhan parasitic crater,and flows along the northern slope of the volcanic cone about 5.4km in length and 400~800m in width.However,the recent researches by the author have revealed that the Qixiangzhan eruption should be a small-scale pulsed explosive eruption.The main evidence is as follows:1) The bulk-rock composition of Qixiangzhan eruption products is characterized by high SiO2(≥71%) and Na2O + K2O(≥10%) contents representative of alkaline magma,which has high viscosity,low flowing ability and extremely high potential of explosive eruption; 2) Field observations show that the Qixinagzhan eruption products appear as thin layers about 2~5cm in thickness,the central part of which is welded stronger than the edge,significantly different from the massive or slaggy structures of lava flow.In addition,flame structures indicative of explosive eruption are well developed in the volcanic deposits around parasitic crater; 3) Microscopic observation reveals that most of the phenocrysts in the Qixiangzhan eruption products were severely broken by explosion to form angular grains with well developed micro-cracks.The vesicles in the Qixiangzhan eruption products are irregular in shape and have rough margin,different significantly from the elliptical and smooth margin vesicles commonly observed in lava flow; 4) Stereomicroscopic observation shows that the Qixiangzhan eruption products are composed of clastic particles and exhibit grain-supported texture with well developed irregular vesicles.Based on the above analyses,we may conclude that the Qixiangzhan eruption can be assigned to a small-scale pulsed explosive eruption.During the explosive eruption,a large number of fine pyroclastic particles flowed down the mountain slope as a high speed pyroclasstic flow to form thin layer of ignimbrite under the action of high temperature and strong shear forces.Over many times of explosive eruptions,layer upon layer of ignimbrite were accumulated,resulting in a shape just like lava flow.Therefore,all the three large eruptions of Changbaishan Tianchi volcano in Holocene can be assigned to explosive eruption,rather than the previously proposed model of explosive-effusive-explosive explosions.