Abstract:

Pyroclastic rock is the most direct object of physical volcanology and the most important topic of identifying the volcanic explosive fragmentation processes. Some particular species of pyroclastic rocks and equivalents can indicate key characteristics of the volcanism process, which is the basis to estimate the eruptive risks. Volcanic hazard is potential risk related to volcanic eruption, and it is one of the most important types of disasters that human beings face in nature. Volcanic disasters are directly related to the types of volcanic eruptions, among which explosive volcanic eruptions can cause the deadly intensive volcanic risks. The direct product of explosive volcanic eruption is to form various pyroclastic rocks, which represent the different types and intensities of volcanic hazards caused by the eruption process. The primary pyroclasts and secondary fragments reflect the difference of volcanic surface processes during eruptive or intermittent periods, while the distinguish of magmatic, phreatomagmatic and phreatic eruptive deposits marks the systematic development of modern volcanology, which is the leading work in the study of volcanic hazards. 1)Pyroclastic rocks are formed directly by transporting, accumulating and diagenesis of the expelled materials during the eruption. They usually consist of the primary materials such as broken magma, accidental fragments trapped by the volcanic conduit, as well as the epiclasts captured by the volcanic fluid flowing on the surface. Pyroclastic rock, as a direct product of explosive volcanism, has naturally becomes the most important research object in volcanology. The volcanic tephra laminae preserved by fine airfall volcanic ash in basins has been attracted attention because of their good isochron and environmental indication, and the associated rocks may need to be distinguished from different types of volcanic sedimentation such as bedded tuff, sedimentary tuff and tuffaceous mudstone. The autoclastic breccia produced by lava emplacement and the hyaloclatite formed by the quenching of lava under water represent fragmentation that is closely related to the lava flow, rather than those from explosive volcanism. 2)Pyroclast is mainly the product of explosive volcanism, but it can contain a certain amount of normal sedimentation and a small amount of rock fragment near the volcanic channel and the magma chamber roof. Pyroclats are generally defined as the direct products of explosive eruption behavior, while volcaniclastics are formed by volcanic degradation such as slope displacement, avalanche, lahar, and the autoclast generated by lava flowage and quenching. This classification not only emphasizes the difference in the forming process of different volcanic products, but also helps to distinguish the different mechanism in volcanological research and hazard estimation. Different types of pyroclastic rocks are formed with different fragment mechanisms and diagenetic ways, and some specific pyroclastic rocks represent various special types and scales of volcanic hazards. Although they are usually classified as primary clastics, the hazard caused by autoclastic breccia is significantly different. Cryptoexplosive breccia, although we have employed a rock name from pyroclastic rocks, is actually more concerned with its resource economics. 3)When we study the genetic types of pyroclastic rocks, the most important basis for identification is the forming mode of the materials, that is, the type of fragmentation, which include primary volcanism and secondary volcanism. Primary clasts are divided into pyroclast, which is formed by the direct action of volcanic eruption, and autoclast, which is produced by the flow process of lava flows, While secondary(exogenous)volcanism includes various kinds of exogenous clasts(epiclast)formed by volcanic surface processes. According to the proportion of magma and water content at eruptive environment, explosive eruption can be divided into three types: magmatic eruption, phreatomagmatic eruption and phreatic eruption, which represent the most basic process of explosive eruption, and are also the problems of genetic classification and identification often faced in the study of pyroclastic rocks.

Key words: Particular pyroclastic rocks, fragmentation processes, genetic nomenclature, hazard analysis

摘要：

火山碎屑岩是火山物理学研究的最直接对象, 也是鉴别碎屑化过程最重要的研究对象。 一些特殊类型的火山碎屑岩及相关岩石往往可以反映出某些特征的火山岩浆作用过程, 这正是甄别不同类型火山灾害发生概率的基本依据。 火山灾害是与火山和火山喷发有关的灾害, 它是人类在自然界面临的最重要的灾害类型之一。 火山灾害与火山喷发类型直接相关, 其中的爆破性火山喷发则可以形成最为致命的强烈火山灾害。 爆破性火山喷发的直接产物是形成各式各样的火山碎屑岩, 它们代表了喷发过程所带来的不同类型与强度的火山灾害。 原生火山碎屑与次生碎屑反映了火山喷发期与喷发间歇期火山表面过程的差异, 而岩浆喷发、 射汽岩浆喷发和射汽喷发堆积物的鉴定则标志了现代火山学研究的系统性深化, 它们是从事火山灾害研究的先导性工作。 细粒空降火山灰在水盆地里保存的火山灰纹层因其良好的等时面与环境指示意义而受到人们的重视, 与其伴生的岩石则可能需要区分层凝灰岩、 沉凝灰岩、 凝灰质泥岩等各种不同火山沉积作用类型。 熔岩流动碎屑化产生的自碎角砾岩和熔岩流水下淬火形成的碎玻熔岩代表了与熔岩流直接相关的碎屑化, 而不是与爆破性火山作用相关的碎屑化。 通常把它们划归为原生火山作用碎屑, 但其灾害意义却完全不同。 不同类型的火山碎屑岩形成于不同的碎屑化机理与成岩方式, 其中一些特定的火山碎屑岩更是代表了极为特殊的火山灾害类型与规模。 对于隐爆角砾岩, 尽管它是一种火山碎屑岩的岩石名称, 但其实我们更关心它的资源经济学意义。

关键词: 特殊类型火山碎屑岩, 碎屑化过程, 成因分类命名, 灾害性分析