Dynamical Processes Of Magma Mixing During Ca.1000 AD Eruption Of Tianchi Volcano In Changbaishan Area

On the basis of geological investigation in Tianchi Volcano, Changbaishan area and analyses of rock samples, the magma mixing and its dynamical processes have been studied through the application of geological fluid mechanics. The following conclusions have been come to:1. The magma mixing in Thianchi Volcano during ca. 1000 AD eruption definitely exists. In the different eruptive stages, the end member magmas of magma mixing are different. The mixing between the alkaline rhyolitic magma and alkaline trachytic magma is the main reason for the pumice with the light and dark bands.2. According to the study of petrography, petrochemistry and mineral chemistry of pumices, the white pumice consists of the phenocrysts of mainly anorthoclase, aegirine-augite, fayalite, Ti-magnetite and rarely quartz as well as the groundmass of light volcanic glass. The content of its phenocrysts is very low (<5%). However, the blakish-brown pumice is made up of the phenocrysts of mainly Na-oligoclase, rarely anorthoclase, aegirine-augite, Ti-magnetite and rarely fayalite as well as the groundmass of brown volcanic glass. The content of its phenocrysts is a little high (10-20%). Na-oligoclases with strong resorption show sieved-texture core, whereas the anorthoclases have no resorption.3. The pumice with the light and dark bands is the important evidence. Some of boundaries between the light parts and the dark parts are sharp, the another are ambiguous, but they all show the flow phenomena. The pumice with the light and dark bands is glassy and porphyritic, but different parts contain different phenocrysts. The light one has the characteristics of white pumice and its glass is rhyolitic, whereas the dark one has the features of blackish-brown pumice and its glass is trachyitic. Because of magma mixing, some parts of volcanic glass shows the transition from the rhyolitic to the trachyitic.4. Through the application of double diffusive convection, the magma mixingand dynamic processes of magma chamber in Thianchi Volcano during ca. 1000 AD eruption have been discussed and some possible stable dynamic processes of magma mixing have been speculated.The magma mixing in Thianchi Volcano has great effect for the ca. 1000 AD eruption. During the eruption, there are different types of magma mixing, but the further researches need to be done for the reason that magma mixing causes this eruption on earth.