大椅子山火山碎屑降落堆积物研究

STUDY ON FALLOUT TEPHRA DEPOSITS IN DAYIZISHAN

由中心式喷发形成的大椅子山火山碎屑降落堆积物,主要为玄武质火山渣和浮石,其次为火山弹、超镁铁质岩包体和岩屑。在强劲西风的影响下,该火山经过110多次脉动式喷发,形成了耸立的火山渣锥和低缓的火山碎屑席。在平面和剖面上,降落火山碎屑的组成、厚度、最大粒径、分选系数和中值粒径都呈有规律变化。构成碎屑席的火山渣分下、中、上三层,主要发育平面平行层理和递变层理。降落火山碎屑的各种特征表明,大椅子山火山的喷发过程非常复杂。

Located in Huinan County, Jilin province, the Dayizishan volcano belongs to Longgang volcano swarm.It was largely formed in the period from the late Pleistocene to early middle Pleistocene.The Dayizishan fallout tephra deposits consist of basaltic scoria,pumice, bomb.ultramafic xenolith and debris. The tephra deposits formed a scoria cone and a fallout tephra sheet in geomophological character. The authors, for first time, discovered that dispersed pattern of fallout tephra composing the tephra sheeet is fan -shaped, the apex of the fan is located in the west of the fan, which shows strong west wind was blowing while the volcano erupted.The tephra deposits composing the tephra sheet can be devided into three parts: lower, middle and upper. All the three parts were formed by pulsating explosions over 110 times. These deposits occur in the form of plane parallel beding and graded beding. Components, thickness, average maximum diameter,median diameter and sorting coefficient of the deposits are changed with the distance away from vent and the lapse of time orderly. The study is of significance on researching the history of atmosphere cycle and studying mechanism of volcanic activity.