Study on the characteristics of Quaternary volcanic pyroclasts from Nanlongwan volcano,Longgang volcanic cluster,Jilin Province.

The particle size,separation,surface texture,and internal structure of pyroclasts produced by explosive volcanic eruption bear a close relation to the mechanism of volcanic eruption,the reaction degree between the magma and water,the emplacement processes,and so on.In the light of this recognition,a profile of Nanlongwan volcano,Longgang volcanic cluster was studied in this paper to gain an insight into the eruptive type of the volcano.Samples of pyroclasts from each layer of this profile have been collected, and microscopic observation of pyroclastic morphology,analysis of particle size,and scanning electron microscopic(SEM)observation of pyroclastic surface textures have been carried out on all these samples.Microscopic observation makes clear that the pyroclasts from phreatic,phreatomagmatic and magmatic explosive eruption have different compositions and morphology.Particle size analysis indicates that particle size distribution feature bears a good relation to eruptive type,and different eruptive type has different fractal dimension scope.SEM analysis may provide information on the effect of the characteristics of the volcanic eruption on the features of pyroclasts. The results of this study show that the eruption of Nanlongwan volcano can be assigned to explosive volcanic eruption,which includes the early phreatomagmatic eruption advancing to magmatic eruption,and the late phreatomagmatic eruption,but dominated mainly by phreatic eruption.The reaction degree between the magma and water is different at different eruptive stages.