The Study on Fallout Tephra Deposits in the Laoheishan Volcano

Key words: 
• volcano fallout /
• tephra /
• dopositscoria cone /
• tephra sheet

Abstract: Wudalianchi volcanic group is located in Heilongjiang Province in northeastern China. There are 14volcanoes, 12 of which erupted in the Middle Pleistocene and another two，the Laoheishan volcano andHuoshaoshan volcano，erupted between 1 719 and 1 721 in historic record. Both Laoheishan volcano andHuoshaoshan volcano consist of airfall tephra which is mainly composed of basaltic scoria, bomb, vokan-is cake and a few and lithic fragments. Based on the authors'study, the Laoheishan airborn tephra forms a scoria cone and a fallout sheet,respectively. 1) The Laoheishan basaltic scoria cone is a round haped cone with oilt top, its altitude is 515. 9m，higher than surrounding ground(162 m)，and its base area is 1. 25 km²，the diameter of crater takinga funnel shape is 370 -400 m. The Laoheishan scoria cone consists of loose, semiconsolidated to consoli-dated tephra. The grain size of the scoria cone is much bigger than that of the tephra sheet. The averagemaximum diameter of basaltic scoriais 11. 3一22. 3 cm at different points on the scoria cone. The grainsize of basaltic bomb and driblet, in general, ranges from 0. 2 to 2 m，the diameter of the biggest oneis over 3 m and the average maximum diameter of lithic clasts including the granite and basalt fragmentseffused at the early stage is 1. 8-3. 8 cm at different points on the scoria cone. 2) The distribution pattern of fallout tephra of the tephra sheet is fan haped. the apex of the fan islocated at the crater of the Laoheishan volcano. The airfall tephra is mainly settled down in SSE of thescoria cone, which reveals a strong NNW wind while the Laoheishan volcano erupted. The dispersal ofthe fallout tephra is not far away from the source, the distribution area of the tephrais limited, only 16kmz, which shows the eruption column is not too high. The thickness of the tephra decreases rapidlyoutwards from the source and the average maximum scoria size, the average maximum lithic size and themedian diameter of tephra within the tephra sheet deposit decrease exponentially away from the eruptivesource. The airfall tephra deposits of the tephra sheet can be devided into two parts, either of which hasstratigraphical sequences from the layer of lithic fragments to the l from the base of the Laoheishan scoria cone, which shows a pulsating eruption of the Laoheishan vol- cano. Meanwhile, it is supposed that the two tage explosions of the Laoheishan volcano are in accordante with 1 719 and 1 720 eruptions, respectively.3)According to the isopach map of the Laoheishan tephra sheet deposit,and by the Froggatt volume-thickness plot method,the volume 0.0063km³ of the tephra sheet deposit and by the Froggatt volume-thickness plot method[8]，the volume 0. 0063 km³of the tephra sheet deposit is calculated and it ismuch less than that of the scoria cone of the Laoheishan volcano (0.、 68km³), showing the tephra of theLaoheishan volcano belongs to a cone-building deposit. with the aid of the Walker[4]and Fisher [6]plotmethods, the Laoheishan volcanic eruption is considerde to belong to the Strombolian type, and basedon a higher value of F%，the author suggests that the Iaoheishan volcano erupts with strong explosiveforte. 4) The Laoheishan volcano and Huoshaoshan volcano are dorment ones and there is a possibility forboth of them to erupt again, for there are frequent microseisms and small seisms in the Wu dalianchi areanow adays.