关键词: 火山碎屑, 分维数, 吉林龙岗

Abstract: The field investigation shows that the volcanism in Longgang,Jilin has three distinct eruption ways,i.e. phreatic eruption,phreatomagmatic eruption and magmatic eruption. The materials of phreatic eruption are rock fragments from basement or volcanic channel with a grain size less than 5mm and nearly isoaxisal shape. The products of phreatomagmatic eruption include basaltic pyroclasts and rock fragments of basement with greatly different grain sizes and isoaxial structure. The magmatic eruption generates blowhole-shaped pyroclasts which are of grain sizes 8～0.5mm,mostly nearly equi-axial structured and some being non-equiaxial. Based on the fractal theory,we analyze the fractal features of the grain size of pyroclasts from the Longgang volcanoes,Jilin Province. Our results show that the fractal dimension of the pyroclasts of phreatomagmatic eruption is smaller than that of the phreatic eruption,but greater than that of the magmatic eruption. The values of fractal dimension can be used as parameters to distinguish different types of volcanic eruption. For the Longgang volcanoes,the fractal dimension of pyroclasts at Jinglongdingzi on the upper profile over the water resisting layer is greater than 2,and those at Xiaojinglongdingzi on the lower profile is less than 2. Fractal dimension values can be regarded as an indicator to differentiate various eruptive sources and to classify stratum sequences associated with volcanoes. Our research demonstrates that the pyroclasts with fractal dimension less than 2 contain anisometric grains of various content,and their values of fractal dimension are negatively correlated with the grain content. This is the first time to study the fractal dimension of the grain sizes of pyroclasts for this area. It suggests a new approach to distinguishing eruptive types,strata sequence and eruption intensity of volcanoes in a quantitative manner.

Key words: pyroclasts, fractal dimension, Longgang Jinlin