Study On The Characteristics Of Fallout Pumices Deposits From The Millennium Eruption In Tianchi Volcano Changbaishan Area

The Tianchi Volcano in Changbaishan area is the largest active volcano in China with the most potential eruption risk.According to historical records,Tianchi Volcano have ever erupted in946,1168-1702,1903 and other years,caused huge impact on the natural environment and human civilization.From 2002 to 2006,Tianchi Volcano showed to be very active,which represent great possibility of volcanic eruption.A lot of research work has been carried out on the Tianchi Volcano and a series of progress has been made.The eruption history and ejection sequence of Tianchi Volcano is the focus of attention.There is much controversy over the stratigraphic division of airborne deposits over the Millennium Eruption,especially the coarse and heterogeneous layers of airborne pumice above the gray airborne pumice.Through detailed exploration of the field strata,we compare the two layers of airborne pumice,and conclude that the two phases of airborne pumice should be grouped into the Millennium eruption.In Chifeng period,the gray airborne pumice is relatively homogeneous,with the petrological characteristics of angular shape,shallow color,positive grain sequence,good separation and uniform particle size.The eruption pattern of Yuanchi period is pulsed eruption.Ochre pumice and black pumice interbeded with the petrological characteristics of angular shape,negative grain sequence,bad separation and mixed particle size.According to the field work and previous research data,the distribution range of two-stage floating rocks is roughly divided.The particle size characteristic parameters of the two periods of pumice rocks fall in the airborne accumulation area.The sorting coefficient of Chifeng period is relatively small,which suggests good separation.The ochre and black airborne pumice of Yuanchi period are mixed,which makes the separation not as good as that of Chifeng period.In terms of particle size,the gray pumice in Chifeng period are generally larger than the variegated pumice in Yuanchi period at the same place,which reflects the greater blasting performance during eruption.The composition of pumice in Chifeng period is rhyolite,while that in Yuanchi period is trachytic.Rhyolite magma possesses higher SiO₂ content,hence the magma viscous coefficient is higher and will save more energy before eruption.So in the same field point,gray rhyolite pumice of Chifeng period has larger granularity.The Chifeng airborne pumice has larger continuous internal porosity and thin pore wall with stomatal stretched.The Yuanchi ochre airborne pumice has continuous internal porosity and thick pore wall.And the Yuanchi black airborne pumice has larger discontinuous internal porosity and thick pore wall which can be twice than the pore.The difference of porosity and grain size reflects the difference of volcanic explosivity index between two periods.The phenocryst mainly includes feldspar and pyroxene.The feldspar in Chifeng gray pumice and Yuanchi ochre pumice experienced about the same degree of damage,while part of the feldspar in the black part of Yuanchi period is fully corroded.All kinds of pumice also contains a small amount of olivine,which indicate a deeper source of magma.Unbalanced composition and olivine from the deep show that magma experienced magma mingling.The magma chamber is disturbed bu the deep source magma,lava pouring out quickly,the unbalanced composition experienced corrosion but did not completely corroded.The airborne pumice in Chifeng period is rhyolite,which is consistent with previous data.The ochre airborne pumice in the Yuanchi period is trachytic,which is the same composition as the Baguamiao period.The black pumice is between trachytic and trachydacite,which is not corresponded with the Baguamiao stratum.Major and trace elements of the three types of pumice show that they are in the same magmatic evolution series.The ochre pumice of Yuanchi period has the same characteristic with the black pumice of Baguamiao period in the composition,trace elements,field stratigraphic position.Therefore,they are from the same issue.The eruptive strength of Yuanchi period is weaker than that of chifeng.Various magma components indicate that the complex magma system under the Tianchi Volcano may exist in stratified form.