The Study on Fallout Tephra Deposits in the Laoheishan Volcano
摘要: 由中心式喷发形成的老黑山火山碎屑降落堆积物，主要是玄武质火山渣、火山弹、火山饼和少量岩屑。在火山爆发自身能量和北北西风的影响下，形成高耸的火山渣锥和向南南东展布的低缓的火山碎屑席地貌景观。区域上系统测量了火山碎屑席中火山碎屑的厚度、最大平均粒径、分选系数和中值粒径，均呈现规律性变化；剖面层序则反映老黑山火山经过两个大喷发阶段，多次脉动式喷发。经估算，老黑山火山碎屑席中火山碎屑量比火山渣锥中要少得多，老黑山火山碎屑属“建造火山锥”火山碎屑沉积。据投点，老黑山火山喷发属斯通博里型。老黑山、火烧山火山仍有再次爆发的可能。未来火山喷发的潜在危险主要来自火山喷发空落堆积物。老黑山、火烧山火山的南东、东和南，包括五大连池市在内有必要对未来火山爆发加以防范。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，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 Walkerand Fisher 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.
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